transmission media laboratories - optolab
TRANSCRIPT
FAKULTA ELEKTROTECHNIKY A KOMUNIKAČNÍCH TECHNOLOGIÍ
VYSOKÉ UČENÍ TECHNICKÉ V BRNĚ
Transmission media – laboratories
Author:
prof. Ing. Miloslav Filka, CSc.
Komplexní inovace studijních programů a zvyšování kvality výuky na FEKT VUT v Brně OP VK CZ.1.07/2.2.00/28.0193
2 FEKT VUT v Brně
Content
1 INTRODUCTION.............................................................................................................. 8
1.1 MEASUREMENT OF TRANSMITTING QUANTITIES ........................................................... 10 1.2 MUTUAL CONVERSIONS OF NP – DB UNITS AND TRANSMISSION QUANTITIES. ............. 11
2 LOOP RESISTANCE MEASUREMENT & UNBALANCE OF RESISTANCE ..... 12
2.1 MEASUREMENT OF THE LOOP RESISTANCE AND UNBALANCE OF RESISTANCE. .............. 12
3 LOCALIZATION OF CABLE FAULTS ...................................................................... 15
3.1 INSTRUCTIONS: .............................................................................................................. 15 3.2 MEASURING PROCEDURE: .............................................................................................. 15
3.2.1 Measurement of loop resistance rab ......................................................... 15 3.2.2 Murray’s method ....................................................................................... 15 3.2.3 Varley’s method ......................................................................................... 15 3.2.4 Balancing the bridge ................................................................................. 15
3.2.5 Calculation of the resistance to failure: .................................................... 16
4 IMPULSE METHOD OF MEASURING ...................................................................... 18
4.1 INSTRUCTIONS: .............................................................................................................. 18 4.2 INTRODUCTION:............................................................................................................. 18
5 LOCATION OF THE FIBRE OPTIC CABLES USING ELECTRONIC
MARKERS AND GPS. .................................................................................................... 22
5.1 TARGETS OF LABORATORY EXERCISE ........................................................................... 22 5.2 INSTRUCTIONS FOR THE LESSON: .................................................................................. 22
5.3 THEORETICAL INTRODUCTION....................................................................................... 22 5.4 PRINCIPLE OF LOCALISATION......................................................................................... 22 5.5 DESCRIPTION OF THE MARKER LOCATOR DYNATEL 1420 EMS-ID ............................... 23
5.6 SIGNAL RESPONSE OF THE MARKER ............................................................................... 24 5.7 ELECTRONIC MARKS – EMS MARKERS .......................................................................... 25
5.8 THE TYPES OF MARKERS ................................................................................................ 25 5.8.1 Ball marker ................................................................................................ 25 5.8.2 Mini marker ............................................................................................... 26 5.8.3 Near surface marker .................................................................................. 26
5.8.4 Full range marker ..................................................................................... 26
5.8.5 Disc marker ............................................................................................... 27
5.9 DESCRIPTION OF THE NAVIGATION USING MOBILE PHONE NOKIA 700 AND PROGRAM
OPENWIG ..................................................................................................................... 27 5.9.1 Starting of navigation ................................................................................ 27
5.10 PROCESSING OF THE LABORATORY EXERCISE ................................................................ 28 5.11 LOCALISATION OF THE MARKER .................................................................................... 28
5.12 DATA READING FROM ID MARKER ................................................................................. 28 5.13 DATA RECORD INTO ID MARKER ................................................................................... 29 5.14 MEASUREMENT OF THE DEPTH OF THE MARKER ............................................................ 30 5.15 LOCALISATION OF MORE MARKERS ............................................................................... 31 5.16 FORMULATION OF THE LABORATORY EXERCISE ............................................................ 31
5.17 USED DEVICE ................................................................................................................. 33
5.18 CONCLUSION: ................................................................................................................ 33
6 LAN STRUCTURED CABLING TESTING ................................................................ 34
Transmission media – laboratories 3
6.1 THEORETICAL INTRODUCTION ....................................................................................... 34 6.1.1 T568A and T568B Specifications ............................................................... 34
6.2 NEXT ............................................................................................................................ 35 6.3 FEXT ............................................................................................................................ 35
6.4 PSNEXT ....................................................................................................................... 36 6.5 ELFEXT ....................................................................................................................... 36 6.6 PSELFEXT ................................................................................................................... 36 6.7 RETURN LOSS ................................................................................................................. 36 6.8 ATTENUATION ............................................................................................................... 37
6.9 LENGTH ......................................................................................................................... 37 6.10 PROPAGATION DELAY ................................................................................................... 37 6.11 ACR .............................................................................................................................. 38
6.12 PS ACR ......................................................................................................................... 38 6.13 DELAY SKEW ................................................................................................................. 38
7 TRANSMISSION PROPERTIES OF OPTICAL FIBRES ........................................ 39
8 MEASURING METHODS IN OPTICAL COMMUNICATIONS ............................ 41
8.1 METHODS OF OPTICAL FIBRE EXCITATION FOR MEASURING PURPOSES ........................... 42
9 MEASUREMENT OF BACKSCATTER ..................................................................... 54
10 MEASURING OF ATTENUATION BY OTDR .......................................................... 60
10.1 INSTRUCTION .................................................................................................................. 60
10.2 INTRODUCTION ............................................................................................................... 60
11 SPLICING OF OPTICAL FIBERS AND MEASURING OF THE ATTENUATION65
11.1 INSTRUCTIONS ............................................................................................................... 65 11.2 INTRODUCTION .............................................................................................................. 65
11.3 SPLICING PROCEDURE .................................................................................................... 68 11.4 SPLICING PROCES ON A S122C ....................................................................................... 71
11.5 MEASURING OF ATTENUATION ....................................................................................... 71
12 OPTICAL WIRELESS TRANSMITTION IN LABORATORY ............................... 73
12.1 MODIFICATION THE RONJA IN LABORATORY ................................................................. 73
12.2 MEASURING DEVICE ...................................................................................................... 73 12.3 MODIFICATION OF THE TRANSMITTER ............................................................................ 75 12.4 VOLTMETER AT THE RECEIVER ...................................................................................... 75
12.5 MEASURING PROGRAM IN COMPUTER ............................................................................ 75 12.6 MECHANICAL CONSTRUCTION ....................................................................................... 76
13 EDFA MEASUREMENT ............................................................................................... 78
13.1 ASSIGNMENT ................................................................................................................. 78
13.2 THEORETICAL INTRODUCTION ....................................................................................... 78 13.2.1 Optical source LS 420 ................................................................................ 83 13.2.2 Optical Power Meter PM 420 .................................................................... 83 13.2.3 SFT-TAP coupler ....................................................................................... 84
13.3 AMPLIFIER EDFA CLA-P(B)-01F ................................................................................. 84
13.4 INSTRUCTIONS RS232.................................................................................................... 85 13.5 PROCEDURE ................................................................................................................... 85
13.5.1 Working-out of protocol ............................................................................. 86 13.6 CONCLUSION ................................................................................................................. 87
4 FEKT VUT v Brně
14 MEASUREMENT OF CHROMATIC DISPERSION ................................................. 88
14.1 METHOD OF PHASE SHIFT AND DIFFERENTIAL PHASE SHIFT ........................................... 88 14.2 METHOD OF DELAYED PULSES IN THE TIME DOMAIN ..................................................... 88 14.3 MEASUREMENT OF POLARIZATION MODE DISPERSION (PMD) ....................................... 89
14.3.1 Method of scanning the wavelength .......................................................... 90 14.3.2 Method of POTDR ..................................................................................... 91
15 REFERENCES ................................................................................................................. 94
Transmission media – laboratories 5
List of figures
FIG. 1: CONNECTION FOR MEASURING BY THE MURRAY‟S METHOD. ......................................... 16 FIG. 2: CONNECTION FOR MEASURING BY THE METHOD. ............................................................ 17 FIG. 3: LINE WITH SHORT CIRCUIT. ............................................................................................ 19 FIG. 4: LINE WITH OPEN CIRCUIT. ............................................................................................... 19
FIG. 5: CAPACITY OF THE LINE. .................................................................................................. 20 FIG. 6: INDUCTANCE ON THE LINE. ............................................................................................. 20 FIG. 7: MULTIPLE REFLECTION OF THE SAME PULSE................................................................... 21 FIG. 8: THE PRINCIPLE OF MARKER LOCALISATION. ................................................................... 23 FIG. 9: DESCRIPTION OF THE FRONT PANEL. ............................................................................... 24
FIG. 10: LOCALISATION OF THE MARKER AND ITS INDICATION. ................................................. 24 FIG. 11: MINI MARKER. .............................................................................................................. 26
FIG. 12: NEAR SURFACE MARKER. ............................................................................................. 26 FIG. 13: FULL RANGE MARKER. ................................................................................................. 27 FIG. 14: DISC MARKER. .............................................................................................................. 27 FIG. 15: LOCALISATION OF THE MARKER. .................................................................................. 28 FIG. 16: LOCALISATION OF THE MARKER. .................................................................................. 29
FIG. 17: DATA RECORD INTO THE MARKER. ............................................................................... 29
FIG. 18: DATA RECORD INTO THE MARKER – PROCEDURE. ......................................................... 30 FIG. 19: DEPTH OF THE MARKER. ............................................................................................... 30 FIG. 20: DEPTH OF THE MARKER. ............................................................................................... 31
FIG. 21: MAP OF LOCALISATION AREAS. .................................................................................... 32
FIG. 22: T568A VS. T568B. ....................................................................................................... 34
FIG. 23: MEASUREMENT OF NEXT. ........................................................................................... 35 FIG. 24: MEASUREMENT OF FEXT. ........................................................................................... 35
FIG. 25: MEASUREMENT OF PS NEXT....................................................................................... 36 FIG. 26: MEASUREMENT OF RETURN LOSS. ............................................................................... 37 FIG. 27: MEASUREMENT OF ATTENUATION. ............................................................................... 37
FIG. 28: MEASUREMENT OF ACR. ............................................................................................. 38 FIG. 29: MEASUREMENT OF DELAY SKEW. ................................................................................. 38
FIG. 30: MULTIMODE STEP-INDEX FIBRE. ................................................................................... 39 FIG. 31: MULTIMODE FIBRE WITH VARYING REFRACTIVE INDEX. ............................................... 39 FIG. 32: SINGLE-MODE STEP-INDEX FIBRE. ................................................................................ 40 FIG. 33: EXAMPLES OF DIFFERENT REFRACTIVE INDEX PROFILES. .............................................. 40
FIG. 34: COUPLERS A) CLASSICAL, B) FIBRE COUPLER. ............................................................. 43 FIG. 35: MODE FILTER. .............................................................................................................. 43
FIG. 36: EFFICIENCY OF MODE FILTERING. ................................................................................. 44 FIG. 37: MECHANICAL MODE SCRAMBLERS. .............................................................................. 45 FIG. 38: FIBRE-TYPE MODE SCRAMBLERS. ................................................................................. 45 FIG. 39: STABILIZED OPTICAL SOURCE CIRCUIT. ........................................................................ 46 FIG. 40: MEASUREMENT OF OPTICAL POWER. ............................................................................ 48
FIG. 41: BLOCK DIAGRAM OF OPTICAL POWER METER. .............................................................. 48 FIG. 42: MEASUREMENT OF ATTENUATION BY CUT-BACK METHOD. .......................................... 49 FIG. 43: MEASUREMENT OF ATTENUATION BY INSERTION LOSS METHOD. ................................. 50 FIG. 44: OPERATIONAL MEASUREMENT OF ATTENUATION, USING INSERTION LOSS METHOD. .... 51 FIG. 45: MEASUREMENT OF ATTENUATION IN A SPLICE (FIBRE VALUES ARE KNOWN). ............... 52
FIG. 46: MEASUREMENT OF ATTENUATION IN A SPLICE (FIBRE VALUES ARE NOT KNOWN). ....... 53 FIG. 47: ELEMENT OF OPTICAL FIBRE. ........................................................................................ 55
6 FEKT VUT v Brně
FIG. 48: BLOCK DIAGRAM OF PULSE REFLECTOMETER. ............................................................. 56 FIG. 49: WAVEFORM OF RECEIVED BACKSCATTER POWER SHOWN ON A DISPLAY. .................... 58 FIG. 50: MEASUREMENT PROTOCOL OF OTDR REFLECTOMETER. ............................................. 59 FIG. 51: BLOCK DIAGRAM OF OTDR. ........................................................................................ 61
FIG. 52: IDEAL CURVE OF BACKSCATTER FOR LONGITUDINALLY HOMOGENEOUS FIBER. ........... 62 FIG. 53: TYPICAL EXAMPLES OF POSSIBLE FAILURES IN BACKSCATTER CURVE. ......................... 63 FIG. 54: DETAILS OF SPLICER. ................................................................................................... 66 FIG. 55: LCD DISPLAY WITH DETAILS. ...................................................................................... 66 FIG. 56: CONTROL BUTTENS OF EQUIPMENT. ............................................................................. 67
FIG. 57: EXAMPLE OF USING. ..................................................................................................... 68 FIG. 58: ONE OF THE EQUIPMENTS WHICH IS USED. ................................................................... 68 FIG. 59: EQUIPMENT AND MANUAL FOR THE STRIPPED FIBER. ................................................... 68
FIG. 60: EQUIPMENT FOR THE CLEAVE THE BARE FIBER. ........................................................... 69 FIG. 61: INSERTING OF FIBER. .................................................................................................... 69 FIG. 62: LCD MONITOR WITH DETAILS OF PROCES. ................................................................... 70 FIG. 63: SEQUENCE OF SPLICING PROCES. .................................................................................. 71
FIG. 64: FINALIZATION OF THE SPLICING PROCES. ..................................................................... 71 FIG. 65: GENERAL SCHEME OF MEASURING OF ATTENUATION. .................................................. 72 FIG. 66: WIRING DIAGRAMMEASURING DEVICE TO LABORATORY NEEDS. ................................. 74 FIG. 67: EXAMPLE OF DATA IN HYPERTERMINAL WINDOW........................................................ 76
FIG. 68: OVERALL LOOK TO ASSEMBLED MECHANICAL CONSTRUCTION. ................................... 77 FIG. 69: PRINCIPLE OF OPTICAL EDFA AMPLIFIER. ................................................................... 79
FIG. 70: OPTICAL RAMAN AMPLIFIER. ....................................................................................... 79 FIG. 71: WDM – WAVELENGTH DIVISION MULTIPLEXER. .......................................................... 80
FIG. 72: OLS 806 IN “RING APPLICATION”. ............................................................................... 81 FIG. 73: SPECTRUM OF WAVELENGTH DIVISION MULTIPLEXER. ................................................. 82
FIG. 74: IMPLEMENTATION AND CONNECTION OF WDM IN DT NETWORK. ............................... 82 FIG. 75: SCHEME OF THE WORKPLACE CONFIGURATION. ........................................................... 83 FIG. 76: BLOCK DIAGRAM OF THE SPLITTER. ............................................................................. 84
FIG. 77: METHOD OF PHASE SHIFT. ............................................................................................ 88 FIG. 78: METHOD OF DELAYED PULSES. .................................................................................... 88
FIG. 79: METHOD OF DELAYED PULSE, WITH A CASCADE OF BRAGG GRATINGS. ....................... 89
FIG. 80: WAVEFORM OF CHROMATIC DISPERSION. .................................................................... 89 FIG. 81: MEASUREMENT OF PMD BY INTERFEROMETRIC METHOD. ........................................... 89
FIG. 82: EXAMPLE OF PMD PLOT OF OPTICAL FIBRE, OBTAINED BY INTERFEROMETRIC METHOD.
90
FIG. 83: METHOD OF SCANNING THE WAVELENGTH. ................................................................. 90 FIG. 84: MEASUREMENT OF PMD BY THE METHOD OF DOP ANALYSIS. .................................... 91
Transmission media – laboratories 7
List of tables
TAB. 1: THE DIAMETER OF THE CU CORE AND SPECIFIC RESISTENCE. ........................................ 14 TAB. 2: CALCULATION OF THE RESULTED UNBALANCES. ........................................................... 14 TAB. 3: THE SPEED OF THE MEASURING PULSE. .......................................................................... 20 TAB. 4: TABLE OF THE BALL MARKER TYPES AS TO ITS USE. ...................................................... 25
TAB. 5: EXAMPLE OF THE TABLE FOR RECORDS OF MARKER INFORMATION. .............................. 31 TAB. 6: EXAMPLE OF THE TABLE FOR RECORDS OF ID MARKER INFORMATION. ......................... 32 TAB. 7: DATA TO BE RECORDED INTO THE MARKER. .................................................................. 32 TAB. 8: DESCRIPTIONS OF BUTTONS. ......................................................................................... 67 TAB. 9: TABLE OF COMMAND FOR SETTINGS. ............................................................................. 85
TAB. 10: SETTING OF HYPERTERMINAL. .................................................................................... 86
TAB. 11: THE TABLE OF MEASURED VALUES. ............................................................................. 86
8 FEKT VUT v Brně
1 Introduction
Introduction for Laboratory Training, Security of Labour, Transmitting Quantities:
The laboratory of transmitting media is in the building of FEKT, Technick 12, C5/52
The student is obliged to change his shoes by his own slippers. His/her shoes coat will be
laid under the hatrack which may be used for coats too.
The credit, scored by reached points will be given after completion of all measuring tasks
and hand-over of protocols.
The security in the laboratory of “Transmitting media” is in conformity with the exam of
security, passed through in the first year of study. Respecting the work with fibre optics there
is necessary to be very careful for fragments of fibres. The fragments may stick into the
finger, eye or may be breath in. There is imperative to collect all fragments into the “fragment
box”. Despite there are not used power lasers in the laboratory, there is not recommended to
watch output connectors directly while the retina of your eye may be endangered. (Attention –
infrared band!)
There is essential to become familiar with transmitting quantities.
Transmitting quantities are able to express transmitting features of the telecommunication
equipment by evaluation of two values of specific physical unit. The main transmitting
quantities used in telecommunications are attenuation (loss), gain and level are the ratio
quantity using unit decibel – dB.
Decibel is dimensionless unit of transmission, based on the decimal logarithm.
The unit of dB doesn‟t belong into the SI system.
The loss (attenuation) is transmitting quantity expressing the ratio of the power (voltage)
in the input to the power (voltage) in the output of the transmitting system.
The gain is transmitting quantity expressing the ratio of the power (voltage) in the output
to the power (voltage) in the input of the transmitting system.
The level is transmitting quantity expressing the ratio of the powers (effective values of
voltages) in two arbitrary points of transmitting system.
- P1, U1, …. quantities in the input of the transmitting system,
- P2, U2, …. quantities in the output of the transmitting system,
- Px, Ux, …. quantities in the arbitrary point of the transmitting system compared always
in the numerator.
Name of
quatity Symbol of quantity Definition
Mark
of
unit
Power loss a | |
| | dB
Voltage loss a | |
| | dB
Transmission media – laboratories 9
Power gain z | |
| | dB
Voltage gain z | |
| | dB
Absolute power
level um
| |
| |
| |
| |
dBm
Absolute
voltage level uu
| |
| | dBm
Relative gain
level ur
| |
| | dBm
Relative
voltage level uru
| |
| | dBm
- P0,= 1 mW, U0 ... basic relative quantities, where U0 = 0,775 V
- The symbols of units dBm, dBu, dBr, dBru are determined for usage in tables and
graphs first of all.
Absolute level of power is transmitting quantity expressing the ratio Px/ Po, where Px,is
the power (complex or resistive) in arbitrary point of specific system and Po is basic power,
equal 1 mVA or 1 mW defining absolute zero level of power. Absolute level of power may be
expressed in units of the transmission (the mark dBm).
Absolute level of voltage is transmitting quantity expressing the ratio Ux/ Uo, where Ux
performs the effective value of voltage in arbitrary point of specific system and Uo is
reference value of voltage, equal to 0,775 V. Relative level of voltage is defined by gaining
the power 1 mW on the real resistance 600 Ω may be expressed in units of the transmission
(the mark dBm).
.
Absolute voltage level may be expressed in units of transmission (mark dBu)
Relative power level is transmitting quantity, expressing the ratio Px/P1, where Px is the
power in the defined point of system and Px is the power in the point chosen as the input of
the transmitting system. Relative power level is expressed in the units of transmission (mark
dBr).
Relative level of voltage is transmitting quantity expressing the ratio Ux/ U1, where Ux
performs the effective value of the voltage in arbitrary point of specific system and U1 is
effective value of voltage in the arbitrary point chosen as the input of the transmitting system.
Relative voltage level is expressed in units of the transmission (the mark dBr).
Apart of quantities mentioned above are used another quantities expressing transmitting
characteristics of the circuits, elements or couplings. There is possible to express them by a
10 FEKT VUT v Brně
ratio of equal quantities. They are named as loss (attenuation) and may be described also in
decibel (operational loss, reflection loss, crosstalk loss, connection loss etc.
The transmission quantities defining characteristics of disturbing effects in the output or
receiver point of the transmitting system are also described in decibel (signal /noise ratio,
equal level of crosstalk e.g.)
There are used following characteristics for detailed expression of various levels:
dBm0: absolute power level related to the point of relative level zero,
dBmp: absolute power level measured by psophometrically
dBm0p: absolute power level measured by psophometrically related to the point
of relative level zero
The point of relative level zero is the reference point of transmission, which is usually
identified with transmitting point of four-wire circuit (channel). For the point with the relative
level zero may be written:
| | | | | | | |,
where
P1 is the power in the point of relative level zero,
U, is the voltage in the point of relative level zero
Absolute level of power related to the point of relative level zero is defined numerically
by difference of absolute and relative power level in the defined point (x) in decibel. The
relative level is related to the point of relative level zero.
| |
| |
| |
Absolute level of power measured psophometrically and related to the point of relative
level zero is the level of noise power in defined band of transmission weighted by the filter in
accordance with the ITU recommendation; expressed numerically is defined by the difference
of the absolute and relative noise level in the defined point (x) in decibel. Relative noise level
is related to the point of relative level zero. The probability of noise amplitude distribution is
drawing to the law of normal distribution.
1.1 Measurement of Transmitting Quantities
The measurement of transmitting quantities may be described as an activity comparing
two values of the equal physical phenomenon.
The measurement is based on the effective value of the alternative voltage.
The basic measuring method for measurement of transmitting quantities is performed by
the measurement of absolute level of signal or noise.
Level meter – dB meter is calibrated in absolute voltage levels expressed in decibels.
Zero point of the dB meter scale is relative level zero.
Transmission media – laboratories 11
The power level meter is calibrated in dBm. It points the value of absolute voltage level
directly enlarged by coefficient Δz expressing the ratio of the 600 Ω to the impedance for the
measurement of |Zx|, value of which is:
| |.
Psophometer – the meter of the noise is calibrated in dBm. It points directly the value of
the absolute noise level.
Its frequency band as well as amplitudes of their noise elements are limited by the filter
with amplitude characteristic of which express the sensitivity of human ear together with
electro-acoustic converter. The scale of the psophometer expresses also the objective value of
the power (voltage) of the noise and the subjective estimation of the disturbance rate of the
average listener.
The meter of power level related to the point of relative level zero is calibrated in dBm0;
it points directly the amount of specific power level reduced by the amount of relative level in
this point.
1.2 Mutual Conversions of Np – dB Units and Transmission Quantities.
The conversion of decibel to Neper units and vice versa is linear conversion of the
decimal logarithm with the base 10 to the natural logarithm with the base e = 2,71828
following formulas:
| |
| |
| |
| |
| |
| |
| |
| |
where M1, M2 are modules of the conversion with the values M1 = 2,30259 = ln 10 and
M2 = 0,43429 = log e
Examples:
1. The voltage of input of the line was measured as U1 = 60 V on the impedance of
Z1 = 600Ω. At the end of line was measured current I2 = 10 mA. What is the loss of the line?
2. The loss of line is 18 dB. At the end is the impedance Z1 = 600Ω and the current is
I2 = 10 mA. There was measured the voltage U11 = 49 V in the distance 1 km from the
beginning of line. Calculate the length of the line!
12 FEKT VUT v Brně
2 Loop Resistance Measurement & Unbalance of
Resistance
Assignment:
Accomplish the measurement of the loop resistance of pairs (I, II) and the unbalance of
their resistance inside the quad!
b) Theoretical introduction:
The aim of measuring of telecommunication cables is to determine values of electrical
characteristics of assembled cable line. Cable line is terminated on the clamps of headend for
the purpose of various measurements. Acceptance tests are provided after completing of
assembly.
The results of measurements are completed in protocols.
The needed values are determined in accordance with the ITU-T Recommednations.
The test of continuity
The test of continuity confirms if the tested wire is alongside whole cable line well
connected as well as uninterrupted and if the sequence of individual wires is identical in both
headends of the cable line section.
2.1 Measurement of the loop resistance and unbalance of resistance.
This measurement verifies the resistance of the wire loop to be documented for the
calculation of the unbalances inside the quads. This measurement is provided by the cable
bridge.
There are measured five values or the loop resistance of individual wires of the quad,
which are on the far end mutually connected. There are measured following loop resistances:
Resistance unbalance of the first pair is defined by subtraction of values of 2nd
and 3rd
measurement:
( ) ( )
Resistance unbalance of the second pair is defined by subtraction of values of 3rd
and 4th
measurement:
( ) ( )
Resistance unbalance of the combined (phantom) circuit is defined by subtraction of
values of 1st and 5
th measurement:
( ) ( )
The results of measurements for all cable elements are recorded into measuring protocol
”Measurement of Loop Resistance & Unbalance of Resistance”.
Transmission media – laboratories 13
In case of only single pair or unpaired number of pairs the individual wire of arbitrary
element, which is assigned as the wire c in the protocol. Then are measured only loop
resistances:
and resistance unbalance is defined by subtraction of 2nd
and 3rd
measurement:
( ) ( )
The resistance measured by direct current of individual wires is dependent to the
temperature in accordance with the formula:
( )
where Rt …. resistance of the wire [Ω] by the temperature t [oC]
R20 .. resistance of the wire [Ω] by the temperature t = 20 oC
ɑCu …. temperature coeficient of resistance [1/oC], for copper is 0,00393 1/
oC
t ..... temperature of wire [oC]
The ratio:
( ),
The temperature recount constants for resistance are plotted in the Table (by the
laboratory task) for t temperatures of copper cores of wires insulated by air – paper spanned
between –5 and +30 oC related to the temperatures +20
oC; +15
oC; +10
oC.
The specific resistance of the wire core in accordance with the formula:
where R ….. specific resistance of the core [Ω/km]
Rs …. resistance of the measured loop [Ω]
l …. length of the line [km].
Calculated values of the specific resistance (always maximal value for each type of line
– different coiling, diameter of wires etc. – are recorded into measuring protocol to the
column “Specific wire”.
Specific resistance of the copper core without resistance of Pupin coiles should not be
over values of the Table 2.1 by the temperature 20 oC.
There is necessary to add resistance measured by direct current for coiled elements.
Eventual overrun of the tabled values shall be justified in the measurement report, e.g. by the
higher specific resistance of used core material or higher temperature than 20 oC during
measuring etc.
Maximal permissible resistance unbalance among wire cores of each pair or combined
circuit for sections of the length over 35 km may be calculated by the multiplication of
following values by the length of the cable in km for cores:
14 FEKT VUT v Brně
Tab. 1: The diameter of the Cu core and specific resistence.
Diameter of the Cu
core Specific resistance
[mm] [Ω/km]
0,9 28,47
1,2 15,95
1,3 13,64
a) up to Ǿ 1 mm 0,04 [Ω/km]
b) over Ǿ 1 mm 0,027 [Ω/km]
For cable sections shorter than 35 km the resistance unbalance shall be for cores
a) up to Ǿ 1 mm less than 1,5 Ω
b) over Ǿ 1 mm less than 1 Ω
c) The procedure:
Use the manual of the measuring bridge
Measure in order with enclosed Tab. 1
Measurement of the loop resistance and resistance unbalance
Cable …….…….…….…….……. Measured from: ………….…….
Section …..…….…….…….……. Measuring device ………..…….
Length ….. …….…….…….……. Measured by ……………..…….
Tab. 2: Calculation of the resulted unbalances.
Quad/ Pair
Loop resistance *Ω+ Unbalance [Ω]
Specific
resistance
[Ω/km]
Notes
a + b a + c b + c b + d c + d a – b c – d
Transmission media – laboratories 15
3 Localization of cable faults
3.1 Instructions:
1. Familiarize with the cable bridge M1T 450.
2. Focus simulated fault (short circuit) on the line of length l = 500 m.
3. For focusing use and verify the accuracy of two methods:
MURRAY‟s METHOD
VARLEY„s METHOD
4. Compare the results.
3.2 Measuring procedure:
For measurements, use the following connection:
- Goog wire – RED – connect to terminal X1.
- Bad wire – BLUE – connect to terminal X2.
- Ground – WHITE – connect to terminal Z
3.2.1 Measurement of loop resistance rab
Function switch (left of the measuring instrument)put to the position „Rx“
The value is obtained by subtracting the value of resistance of the decade after
balancing the bridge, and multiplying by the scale.
3.2.2 Murray’s method
Function switch (left of the measuring instrument)put to the position „M“
Range switch (right of the measuring instrument put to the position) „M“
RANGES IN THE GREEN FIELD!!!
3.2.3 Varley’s method
Function switch (left of the measuring instrument)put to the position „V“
Range switch (right of the measuring instrument put to the position) „Rx V“
RANGES IN THE BLUE FIELD!!!
3.2.4 Balancing the bridge
Switch on the device – put the switch in the upper right corner to the position „ZAP“
and the side switch put to the position „10 V=“.
Gradualy increase the sensitivity of 1 to 5 and search for the best range and
simultaneously adjust the decade resistor to the full balance of bridge (the bridge will show
"0" in the middle of gaude).
16 FEKT VUT v Brně
3.2.5 Calculation of the resistance to failure:
Murray’s method:
For Murray‟s method is used connection shown on fig. 1. The measured and set values
M and R are applied in the relation for calculating the fault resistance:
Where is the value of loop resistance (measured in 1.), R is the value of resistance
subtracted on a decade and M je is the value of set range.
Murray‟s method (Fig. 1) is used to localization a cable faults on the far end.
Fig. 1: Connection for measuring by the Murray‟s method.
Varley’s method:
For Varley‟s method is used connection shown on fig. 2. The measured and set values
M and R are applied in the relation for calculating the fault resistance:
Transmission media – laboratories 17
Where is the value of loop resistance (measured in 1.),
is the value set on range
RxV and V is the value of resistance subtracted on a decade.
Varley‟s method (Fig. 2) is used to localization a cable faults on the near end..
Fig. 2: Connection for measuring by the method.
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4 Impulse method of measuring
4.1 Instructions:
1) Familiarize with the impulse locator of faults MEGGER TDR1000/2.
2) Find the appropriate time of duration of pulse for wiring with length l = 500 m.
3) For submitted samples of wiring measure distance and the character of the fault (short
circuit, disconnection).
4) Connect the supplied capacitors and resistors, and characterize the measured changes.
4.2 Introduction:
Impulse measurement technique is used to quick localization of failure (broken wire or
short circuit), especially in line with the spread of high speed (structured cabling, coaxial
cables). They are based on the fact that the voltage pulse propagating along line is partially
reflected at the point where the rapid impedance transition is. Short circuit or disconnection
causes complete reflection and the progressive impulse is reflected back to the source. This is
used for targeting faults.
Impulse methods are fast and comfortable, but it is not possible to measure broken
isolation of the cable. This is possible only in the case, when the impedance of the cable is
changed so much, that it cause reflection of the pulse.
Impulse sighting device converts measuremnt of the distance (to the point of failure)
to measuremnt of the time. Real distance is calculate from the velocity of the transmitted
pulse and from the half of time which elapses between sending and receiving the pulse.
To achieve maximum accuracy of focus is need to be established as accurately as
possible the impulse propagation velocity of the line. If this speed is measured, we use
reflections from inhomogeneity, whose distance from the point of measurement is known
(end of line is open, short circuit, connectors, etc.), Fig. 3 and Fig. 4. The speed of the v/2 is
calculated from the length of the line la to the known inhomogeneity and from the time that
elapses between sending the pulse and receiving the reflected pulse by the relation:
Distance to the place of the failure is calculated by the relation:
where tx is the propagation time corresponding to the distance lx. If velocity is not
known or cannot be determined from the reflection of the known inhomogeneity, we use
informative data from tab..
Transmission media – laboratories 19
Fig. 3: Line with short circuit.
Fig. 4: Line with open circuit.
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Tab. 3: The speed of the measuring pulse.
speed of spread
v/2 (m/µs)
wireless line 146
plastic cables 100
cable with air-paper izolation of wires with metal shield 112–118
A sudden increase in operational capacity of pair respectively in capacity of the line-
ground is caused either by water intrusion into the cable coating or by distortion of the cable.
The shape of the pulse is reflected in .
Fig. 5: Capacity of the line.
If the change of longitudinal inductance is occurred in the line, the shape of reflected
pulse is as in Fig. 6. This reflect is caused by the couplings, or the transition to a line with
lower operating capacity.
Fig. 6: Inductance on the line.
Transmission media – laboratories 21
Multiple reflection of the same pulse (Fig. 7) occurs when the output impedance of the
device is not adapted to the wave impedance of the line. Pulse reflected from the point of
failure at the beginning of the line partially reflects back and causes not only their own views
and views of other secondary pulses at times 2tx, 3tx, etc.
Fig. 7: Multiple reflection of the same pulse.
If the place of defects is near the beginning or end of line, reading is complicated by the
presence of the transmitted pulse or pulse reflected from the end of line.
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5 Location of the Fibre Optic Cables Using Electronic
Markers and GPS.
5.1 Targets of Laboratory Exercise
Learning of fibre-optics cables location using EMS markers supported by the GPS
technology. 3M marker locator Dynatel 1420 EMS iD produced and patented by 3M
company. Learning of the program OpenWIG supported by the mobile phone Nokia 700.
5.2 Instructions for the Lesson:
1) Determine initial point of the line entered by the lecturer using mobile phone
Nokia 700 and the program OpenWIG. Let the GPS switched on during your
passage along located line. It will support the navigation. GPS will generate the
code by reaching of target point, which is to be recorded into protocol!
2) Utilise 3M location system during passage which is able to localise EMS
markers. Each route is equipped with various types of markers for
telecommunication, power cables and water ducts!
3) Locate markers indicating the route of optical cable as well as other lines and
draw them into the map!
4) The figures of individual gains, signal response and depth of laid markers!
5) Record information loaded in iD markers if they are applied!
6) In case of no iD marker applied, locate the marker close to the entrance of
Technická 10 building and try to program gained information into the marker.
Data to be used for programming are in the Table titled: Data to be loaded into
the marker.
5.3 Theoretical Introduction
There is necessary to use special techniques for the localisation of fibre-optic cables due
to their special properties. This cable has no metallic elements and therefore the generating of
any magnetic or electric field for localisation is not available. The information is transmitted
by electrically neutral photons. There are several methods utilisable for localisation of these
dielectric cables: By-laying metallic wire alongside the cable route, the non-dielectric optical
cable using metallic wire inside or EMS markers utilising resonant circuit. This circuit is
initialised through the locator and therefore the markers need not any power supply. EMS
markers are delivered in various shapes and colours in accordance with their application.
5.4 Principle of localisation
The locator Dynatel 3M is used for transmitting of HF signal into the marker, which is
buried underground. Marker reflects the signal back to the locator, which indicates
acoustically or graphically the position of the marker.
Transmission media – laboratories 23
Fig. 8: The principle of marker localisation.
5.5 Description of the marker locator Dynatel 1420 EMS-iD
Basic control of the locator is possible by functional keys under its display. There are four
SW keys. The function of the key is displayed over the key. These functions are variable
depending on the operational regime of the locator.
Description of the functional panel of the marker locator Dynatel 1420 EMS-iD:
1. Switch receiver sensitivity.
2. Trace mode for locating markers – setup entries OK.
3. Menu – configuration of the device: clock, language, depth units and marker data.
4. Choice of the display backlight.
5. Icon of the loudspeaker volume.
6. Icon of the battery level.
7. Magnitude of the marker signal – reading of receiver signal.
8. Column indicator – graphical display of the received signal.
9. Gain value – on/off.
10. Volume of the receiver – off, low, medium, high.
11. Choice of the display contrast.
12. Choice of the indicates sensitivity of receiver.
13. Soft keys of the device, menu orientation, choices.
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Fig. 9: Description of the front panel.
5.6 Signal response of the marker
As to the reached signal response from the marker is possible to define exactly its
position. The position of the locator against position of the marker is seen in the Fig. 10
together with its signal response.
Fig. 10: Localisation of the marker and its indication.
Transmission media – laboratories 25
5.7 Electronic marks – EMS markers
The passive LC circuit is laid inside the marker. Markers perform passive antennas
without internal power sources to be charged. The marker locator transmits electromagnetic
field, which excites the chip being touched by the locator, which transmits the information of
its position. Outer covering of the marker is made from the polythene shell. They are
delivered in various types and colours specific for their usage.
The markers may be also provided with the iD accessory. This marker is labelled with
information, where iD code is typed. This code performs a number of 10 digits and a special
chip inside the marker together with the resonant LC circuit. The chip is determined for the
record of additional information concerning the line as about the owner, date of the origin,
applied technologies etc. This chip needs no power supply too. The information is loaded into
chip using locator of EMS markers.
5.8 The types of markers
Markers are differentiated during the manufacturing not only by the colour, but also in
accordance with the depth and mode of their burying.
5.8.1 Ball marker
This marker performs the ball of diameter about 11 cm. There is possible to bury its into
the depth of 150 cm. It is used in the narrow trenches. Ball marker is constructed in such a
way to be able to balance horizontal position of the chip not regarding its actual position after
burying. Inside is the mixture of the polypropylenglycol and water, in which flows the case of
LC circuit. Ball marker is produced in various colour modifications in accordance with its
usage.
Tab. 4: Table of the ball marker types as to its use.
Type of the marker Function Colour
Telecommunications Cable routes, extensions,
joints, bends, covers, crossing
Power supply
Cable routes, extensions, joints, bends, covers,
crossing, buried transformers, Loops, street
light
Cable TV Cable routes, extensions,
joints, bends, optical fibers
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Water ducts
Routes of tubes, valves,
distribution system, Tube
crossings, adapters, cleaning
outputs
5.8.2 Mini marker
This marker is of special construction enabling possible burying into the depth up to
180 cm also in the hard approachable soil. It is of the spoken form with diameter about 20 cm.
This form is helpful for the burying (see Fig. 11).
Fig. 11: Mini marker.
5.8.3 Near surface marker
This type of marker is used for optical cable line tracing under roadways or badly
accessible surface (Fig. 12). There is possible to bury its directly into asphalt or concrete. It
can be buried into the depth up to 60 cm. Near surface marker is of the cylindrical form of the
length 15 cm.
Fig. 12: Near surface marker.
5.8.4 Full range marker
This marker is utilised as a protection against damaging (Fig. 13) of the optical cable by
digging. It may be buried into the depth up to 240 cm and its diameter is 38 cm.
Transmission media – laboratories 27
Fig. 13: Full range marker.
5.8.5 Disc marker
Disc marker (Fig. 14) is not laid under the surface; it is designed for laying on the badly
accessible places into the dense vegetation, on the fillings etc.
Fig. 14: Disc marker.
5.9 Description of the navigation using mobile phone Nokia 700 and
program OpenWIG
GPS navigation is used for the navigation to the outgoing point of simulated
engineering network and also for the tracing of the route. It provides additional information
about areas where the markers are applied and prevents the unwished abandoning of the trace.
There is necessary to consider certain inaccuracy of GPS.
5.9.1 Starting of navigation
Switch on mobile phone Nokia 700,
Execute the program OpenWIG, which is on the main display,
Permit the usage of data of the application Position by the program OpenWIG,
Chose Start,
Permit the administration of user data,
Choose the trace entered by the lecturer (TraceA.gwc, TraceB.gwc, TraceC.gwc)
Follow the instructions from the display, for the control of the OpenWIG program use
the displayed soft keys (pointer, menu)
Record the code generated by the program by the tracing the route.
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Don‟t load the trace,
After exercise is completed, check the battery and if it is well, switch off the mobile
phone; consult the lecturer in opposite case.
5.10 Processing of the laboratory exercise
The manual for the locator Dynatel 1420 EMS-iD is available in the workplace. Study
the device before action outside and check the signal response of ball markers in laboratory.
Check following functions: Localisation mode of one marker, the data reading from the iD
marker, localisation mode of two markers and the measuring of the depth of marker. Check
also the work with mobile phone Nokia 700, in which the program for GPS localisation is
installed. Start up the application whereigo and become familiar with its operation. Load the
trace, which will be told by the lecturer. After these steps is possible to leave laboratory to the
terrain. Area for the localisation is located in front of the entrance of the building Technická
10 - dean's office of the Faculty of electro and communication technologies.
5.11 Localisation of the marker
There is necessary to choose the type of localised marker. You may choose several
types of them, but you will use in your exercise first of all telecommunications markers
labelled TEL. These markers are orange. Next are utilised markers for power supply. These
markers are coloured red and labelled PWR. Your locator is switched on for choose of
engineer route. Push button Locate and next button Markr and you will choose type of trace.
By localisation of single marker the mark Markr 2 shall be in position OFF. You lower the
gain by pushbuttons Gain Adjust until this moment, when the column indicator is opened.
After identification of the marker for assigned engineer network will be this column indicator
closed, acoustic signal will be single tone and the maximal signal is displayed.
Fig. 15: Localisation of the marker.
5.12 Data reading from iD marker
Check the reading of information from iD marker. If your route has not iD marker, you
will find them in the grass by the entrance to the Technická 10 - dean's office of the Faculty of
electro and communication technologies. There is necessary to push button Locate for data
reading, choose cyclically Marker 1 and choose the type of engineer network (see in Fig. 16).
The mark Marker 2 is switched off – OFF. Pus button Read and loaded data from iD will be
Transmission media – laboratories 29
red. Record this data into the table. Using pointers is possible to go through the menu. Data
from marker are loaded also in the locator, where it is possible to find them in Read history as
well as in the file named equally. There is loaded date and time of reading, information
concerning the owner eg. etc.
Fig. 16: Localisation of the marker.
5.13 Data record into iD marker
Thanks to the functionality of information record is possible to load important
information, these may be useful for next localisation or by the trouble shooting of optical or
another route. Check the data recording into the marker in front of the entrance of the building
Technická 10. Marker is to be found in the localisation mode of locator, it means Markr 1 Tel
and Markr 2 OFF. Push button Menu and next Write mode after marker is found. You may go
through menu using pointers up and down and you will choose the template BPMR and then
confirm by the pushbutton Veiw/Edit. Information of the marker will be displayed. Next you
will go through all information and paste them in accordance with the table: Data to be
recorded into the marker. Push button Modify and choose the User entry in the modification
window. Insert your login into the field Company and the number of the route, which you
went through into the field Job. Let rest settings unchanged. Next push the button OK.
Confirm it by push button Write marker finally after programming all data. There is necessary
to choose the type of marker, which the data are programmed into. There is necessary to hold
locator directly over the marker after uploading of all information and push Start write. Next
is the confirmation of steady blocking of data in the marker. You choose No Fig. 17 for data
will stay rewriteable.
In case of data blocking in the marker, your credit evaluation for this exercise will
be lowered and you are obliged to notify it to the lecturer.
Fig. 17: Data record into the marker.
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Fig. 18: Data record into the marker – procedure.
5.14 Measurement of the depth of the marker
There is necessary to keep the point of locater closely on the surface over the measured
marker. Then push the button Depth. The receiver will undergo “Looking for iD Marker(s)”
=> “No iD Marker Found” and next “Calculating signal, please wait”,,. Next is displayed
order to lift up the locator up to inch (15 cm) (see in Fig. 19 and Fig. 20). Next after locator is
elevated push the button Depth again. Then is displayed the depth of the marker in several
moments. Pushing button Locate you will return into the regime localisation of the marker.
Fig. 19: Depth of the marker.
Transmission media – laboratories 31
Fig. 20: Depth of the marker.
5.15 Localisation of more markers
This regime is necessary to choose by soft key Marker 2. Then it is necessary to choose
the needed type of engineer network. Using buttons Gain Adjust the gain is lowered until the
column indicator is open. After the marker is detected the indicator will be closed. We try to
reach the largest amplification of signal. After detection of the first marker we will push the
button of the type of another detected engineer network as PWR only eg. Locator is switched
into the regime of single marker localisation. For return to the regime of two markers
localisation is necessary to push soft key button Marker 2.
5.16 Formulation of the laboratory exercise
Use Tab. 5 for the records of gain, response and depth of the marker.
Tab. 5: Example of the table for records of marker information.
Marker – type (TEL, PWR …)
Signal response [dB]
Gain [dB]
Depth [cm] Notice
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Map (Fig. 21)of localisation areas in the facilities of the FEKT VUT Brno.
Fig. 21: Map of localisation areas.
Following table (Tab. 6) is for the recording of iD markers information.
Tab. 6: Example of the table for records of iD marker information.
Marker – type (TEL, PWR …)
Signal response [dB]
Gain [dB]
Depth [cm] Data
Company
Job
Location
Description
Company
Job
Location
Description
Company
Job
Location
Description
Company
Job
Location
Description
Next table (Tab. 7) is for data uploaded into iD marker.
Tab. 7: Data to be recorded into the marker.
Upload of data into iD ball marker.
Company Your login
Job Number of the route
Location PARK
Description BPRM
Transmission media – laboratories 33
5.17 Used device
Locator Dynatel 1420 EMS-iD, 3M, v.č. 09470018
5.18 Conclusion:
The aim of exercise is the processing of protocol, in which the measured values of
markers, code generated by GPS and drawing of approximate route of simulated optical or
power cable or water duct.
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6 LAN structured cabling testing
Make the following tests for the two enclosed (S) UTP cables. Specify the type of cable.
Test results for both cables evaluate and compare each other.
Make the following tests (SINGLE TEST, the manual from page 28):
Wire Map, NEXT, PS NEXT, ELFEXT, PS ELFEXT, REMOTE ELFEXT PS, PS
ELFEXT PLOT SCREEN, Return Loss, Attenuation, ACR, PS ACR, Length, Delay Skew,
Propagation Delay.
6.1 Theoretical introduction
Measurements for the correct functionality of structured cabling is essential. Precision
instruments can measure installed components and determine whether they met all the
requirements defined in international standards to ensure reliable operation of the
applications. In the case of CAT5e and CAT6 are measured following main parameters:
Wire Map
This parameter controls the correct termination of pairs is the patch panel, including the
shielding in STP cabling. It also checks the signal along the cable,
In problem:
• check the connection of the wires
6.1.1 T568A and T568B Specifications
T568A T568B
1 white and green 1 white and orange
2 green 2 orange
3 white and orange 3 white and green
4 blue 4 blue
5 white blue 5 white-blue
6 orange 6 green
7 white and brown 7 white and brown
8 brown 8 brown
Fig. 22: T568A vs. T568B.
Transmission media – laboratories 35
6.2 NEXT
Near End Cross Talk (Fig. 23) is a value that expresses how much signal gets from one
pair to another pair. Measurement of crosstalk at the near end is from the same end of the
cable as the source. All combinations of pairs within one cable are measured - ie 12-36, 12-
45, 12-78, 36-45, 36- 78, and 45-78 - on both ends.
Fig. 23: Measurement of NEXT.
In problem:
determine at which end of the cable shows NEXT error
check the maximum allowable conductor bifurcation chambers - that is 13 mm.
a frequent source of problems in cross-talk can also be the connectors or
couplings
6.3 FEXT
Far End Cross Talk (Fig. 24) expresses the crosstalk signal from one pair to another pair
measured at the far end. It is the same as NEXT, the parameter with the only difference being
that in the case FEXT is measured at different ends of the cable. Again, all combinations of
measured pairs within one cable - ie 12-36, 12-45, 12-78, 36-45, 36-78, 45-78. FEXT is an
important basis for parameter ELFEXT.
Fig. 24: Measurement of FEXT.
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6.4 PSNEXT
Power Sum NEXT (Fig. 25) is the theoretical value calculated from the previously
measured NEXT. PSNEXT parameter is particularly important for protocols that are used to
transmit all four pairs (eg Gigabit Ethernet). Power sum near end crosstalk expresses how
much signal in one cable gets from three pairs to the remaining fourth pair. Source of signal
and crosstalk measurements is done on the same end of the cable.
Fig. 25: Measurement of PS NEXT.
6.5 ELFEXT
EqualLevel Far End Cross Talk is better when transferring data than FEXT parameter.
Crosstalk inside the cable is reduced with increased attenuation. As in the ACR it is a
theoretical parameter (ie not measured but calculated from other previously measured values)
ELFEXT [dB] = FEXT [dB] – A [dB]. Far end crosstalk FEXT is reduced by the attenuation.
6.6 PSELFEXT
Power Sum ELFEXT is calculated from the values ELFEXT. As PSNEXT this
parameter is important for protocols that use the signal all four pairs. PSELFEXT shows how
much signal in the same cable gets from three pairs to the remaining pair. Source of signal
and crosstalk measurements carried out on opposite ends of the cable.
6.7 Return loss
Return Loss (Fig. 26) shows the reflected signal due to the different impedance. Due to
these imbalances, the part of the energy goes back to the transmitter, which can cause the
signal noise.
Transmission media – laboratories 37
Fig. 26: Measurement of Return Loss.
6.8 Attenuation
Attenuation shows the difference between the input signal and the signal at the end of
the wire. This is mainly due to resistance of the wire and is usually larger for higher
frequencies. Attenuation also increases with decreasing the diameter of the cable.
Fig. 27: Measurement of attenuation.
6.9 Length
There is a direct correlation between length and attenuation (ie the longer cable has the
higher attenuation). Measuring instruments used to measure the length of the TDR (Time
Domain Reflectometry), which means that the cable pulse is sent to the remote unit to bounce
back and is then recorded the time at which the pulse travels the whole track. Based on NRC
(Nominal Velocity of Propagation = percentage ratio of the speed signal cable to the speed of
light in vacuum) is then calculated by measuring the length of the segment. It is important to
realize that this is a length of twisted pairs (the electrical length), not "untangled" cable (so-
called physical length). To 85 m can be a difference between the electrical and physical length
to 5 m depending on the twisting of each pair.
6.10 Propagation Delay
This value expresses the signal delay from one end of the cable to the other. Typical
signal delay for category 5e cable is about 5 ns to 1 m, the limit is 5.7 ns to 1 m - which is
570 ns to 100 m PropagationDelay also serves as the basis for determining the value
DelaySkew.
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6.11 ACR
Attenuation (Fig. 28) to Crosstalk Ratio is a theoretical parameter (ie not measured but
is derived from two already measured values), which expresses the difference between NEXT
and attenuation: ACR [dB] = NEXT [dB] - A [dB]. If the level of attenuation meets or
approaches the level of crosstalk, signal is lost. The interval between NEXT and attenuation
must be at least 10 dB.
Fig. 28: Measurement of ACR.
6.12 PS ACR
This parameter is calculated from the value PSNEXT and attenuation. PSACR (f) =
PSNEXT (f) – Attenuation (f).
6.13 Delay Skew
DelaySkew (Fig. 29) determines the signal delay difference between the fastest and
slowest pair. The parameter affects DelaySkew - (1) pairs of different length, (2) differences
in the material (resistance, impedance, etc.) (3) the effect of ambient noise. If the difference is
too large, may be an incorrect interpretation of data in the active element. As with PSNEXT
and PSELFEXT DelaySkew parameter is critical to the protocols used to transmit signals all
four pairs.
Fig. 29: Measurement of delay skew.
Transmission media – laboratories 39
7 Transmission properties of optical fibres
The transmission properties of optical fibres depend in the first place on the type of
fibre design. In this respect, three types of fibre are distinguished:
multimode fibres with constant refractive index of the core and step refractive
index of the jacket; these fibres are simple to manufacture and handle, and of
a comparatively simple design but their drawback is greater attenuation and
dispersion, and small transmission capacity. They feature large core and jacket
diameters. An example of this type of fibre can be seen in Fig. 30.
Fig. 30: Multimode step-index fibre.
Some characteristics of this type of fibre: Dj = 50-200 µm, Dp = 120-300 µm,
dispersion 50 ns∙km-1
, attenuation 5-20 dB∙km-1
, and bandwidth 60 MHz.
Fibres of this type are mostly used in short-haul links, in particular for automation
purposes, short data transmissions, local networks, etc.
Multimode fibres with varying refractive index in transversal section of the core, which
have feature lower dispersion, lower attenuation, somewhat more complicated manufacture
and thus more complicated fibre design and splicing. The fibre has been standardized
according to the ITU-T recommendation: Dj = 50 µm, Dp = 125 µm. An illustration of the
refractive index pattern is given in Fig. 31.
Fig. 31: Multimode fibre with varying refractive index.
Some selected characteristics of this type of fibre are: dispersion at 0.85 µm is
1 ns∙km-1
, attenuation 2.5-5 dB∙km-1
, transmitted bandwidth 600 MHz.
In view of the above parameters, this type of fibre is particularly suited for
telecommunication purposes, namely for short-haul links.
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Single-mode fibres with constant refractive index of the core and step refractive index
of the jacket, which feature low dispersion, very low attenuation, and large transmission
capacity. They mainly find application in long-haul transmissions. In this case there is only
one mode propagating in the fibre, in the direction of the axis. To be able to achieve this state
it is necessary to reduce the core diameter to a value equal to only a few light wavelengths.
The diameter range is Dj = 7-9 µm, Dp = 125 µm, as shown in Fig. 32.
Fig. 32: Single-mode step-index fibre.
Fibre characteristics: dispersion ca. 0.3 ns∙km-1, attenuation below 0.2 dB∙km
-1 at
a wavelength of 1.55 µm, and a bandwidth of 10 GHz.
In cases when the refractive index changes stepwise, the term layered lightguides is
often used. In these cases the transmission is based on the principle of total refraction at the
core-jacket interface. In the second type, the lightguide with continuously varying refractive
index (so-called gradient-index lightguide), the path of the ray has the form of elliptic or
circular helix.
Since the transmission properties of optical fibres depend on the pattern of refractive
index distribution, various manufacturers apply further variants of different refractive index
profiles.
Frequently occurring are two-layer lightguides and gradient-index lightguides with the
curve of refractive index close to the parabolic curve. As will be given below, a more
complicated pattern of refractive index can yield a shift in the dispersion characteristic, etc..
Fig. 33: Examples of different refractive index profiles.
Transmission media – laboratories 41
8 Measuring methods in optical communications
The development of optoelectronic telecommunication systems has entailed the
development of new measuring methods and instruments for the measurement of the
parameters of fibres, cables and other optoelectronic elements. At first glance it might seem
that the measuring methods are analogous to the methods used for metallic lines but in fact
they are markedly different. Take, for example, the measurement of attenuation: the name is
the same in both cases, the theoretical foundation is the same but the approach is completely
different. The difference in measuring methods is given by the specific properties and
behaviour of light.
Leading manufacturers of optoelectronics currently offer a whole range of measuring
devices that feature simple operation and rapid measurement. The instruments are in most
cases provided with standardized connectors for optical radiation input and output.
For the area of optical measurement numerous recommendations have been worked out
in IEC, ITU-T and DIN, others are in the stage of preparation and a lot has still to be done, in
metrology in particular.
Optical measuring methods can be divided from various points of view; in the following
we will stick to this division:
Measurement on optical fibres;
optical measurement,
mechanical measurement.
Transmission measurement on optical fibres:
measurement of attenuation,
measurement of dispersion,
measurement of polarization mode dispersion,
measurement of backscatter;
measurement of bandwidth.
Measurement of optoelectronic components.
Special measuring methods.
Measuring instruments.
Another division can be from the viewpoint of application: for example, measuring
methods for the manufacture of optical fibres, measuring methods for the manufacture of
optical cables, measuring methods for the installation of cables, and measuring methods for
operational measurement.
From the viewpoint of building an optical track the following measurement must be
taken into account:
preparatory measurement (prior to acceptance inspection and prior to
installation),
measurement during cable installation,
measurement on an installed route,
final measurement (acceptance inspection after installation),
measurement during device installation,
regular check measurement of operation,
localization of possible failures on optical track,
check measurement of time stability of devices,
permanent check of the working of the whole optical system,
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experimental measurement.
Most of the measurement consists in establishing the parameters of optical fibre and the
properties of optoelectronic modules of the system. The measurement set includes
a permanent check of the working of the system, which consists in electrical measurement of
error rate and its evaluation.
8.1 Methods of optical fibre excitation for measuring purposes
Since the lightguide is never ideally straight and cylindrical but exhibits random bends
and cross-section ellipticity, rays propagating in it are also liable to random changes. New
modes can be formed on fibre inhomogeneities that were not present in the mode distribution
before this inhomogeneity and, on the contrary, modes can disappear (by emission into the
jacket or by absorption) that till then participated in the transmission. This event is known as
mode overflow, the so-called mode conversion.
After a certain passage through optical fibre (of several units to hundreds of metres)
there is a certain balance in the distribution of power into individual modes, so-called stable
mode distribution. This state is highly desirable for the conditions of correct measurement of
optical fibre parameters.
When this requirement is not respected the measurement can, due to the existence of
differential attenuation, carry an error that completely distorts the results. When measuring
close to the source in particular, the mode distribution changes and does not guarantee
measurement reproducibility.
Conditions of stable mode distribution also for verification on short sections can be
approximated by using special devices inserted between the source and the fibre being
measured. It is possible to use:
couplers,
mode scramblers,
mode filters.
Couplers are used for specific coupling of optical power to the fibre being measured, i.e.
with exactly determined numerical aperture and with the required size of beam trace. The
couplers can be of the classical type, i.e. made up of lenses and diaphragms or of the fibre
type, with a fibre of suitable parameters.
In the mode scrambler heavy decoupling (mixing) of modes takes place, which leads to
a homogeneous distribution of power on its output.
The mode filter is used to remove undesirable higher modes before the input to the unit
being measured.
Coupler realizations for a fibre of 50 μm in diameter and theoretical numerical aperture
NA = 0.2, inclusive of the coupling condition, are standardized in IEC. According to this
recommendation, the mode distribution can be considered stable when after the passage of
radiation through a fibre 2 m long the beam half-width measured in the near region is 26 ± 2
μm and the numerical half-aperture measured in the far region is NA = 0.11 ± 0.02.
According to the American association EIA, excitation approximates well the stable mode
distribution when the diameter of excitation beam is 70% ± 5% of the core diameter of the
fibre being measured, and the numerical aperture is 70% ± 5% of its numerical aperture. For
the fibre under consideration, the diameter of the beam trace should be 35 μm and its
numerical aperture 0.14. The above examples of coupler are illustrated in Fig. 34.
Transmission media – laboratories 43
Fig. 34: Couplers a) classical, b) fibre coupler.
Another approach consists in overexciting the measured fibre by an optical beam, which
means that the fibre is being excited by a beam whose diameter and numerical aperture are
larger than the diameter and numerical aperture of the fibre being measured. For the
telecommunication gradient fibre 50/125 μm the excitation beam should (according to IEC)
have a diameter larger than 140 μm and a numerical aperture larger than 0.3. The excitation
beam axis must be identical to the fibre axis. To remove the higher modes and to approximate
stable mode distribution a mode filter is connected to the input of the fibre being measured.
The filter can be obtained by winding the fibre onto a smooth cylinder (mandrel wrap filter).
For the 50/125 μm fibres a filter is recommended that consists of 5 fibre turns on a smooth
cylinder of 18 to 22 mm in diameter (see Fig. 35). Using such a mode filter a measuring
accuracy of ± 0.05 dB . km-1
can be obtained.
Fig. 35: Mode filter.
44 FEKT VUT v Brně
Perfect stable mode distribution can be obtained in such a way that a 1000 m section of
so-called dummy fibre (pre-fibre) is inserted between the transmitter and the fibre being
measured, as shown in Fig. 36.
Fig. 36: Efficiency of mode filtering.
The filter quality can be assessed using the filter efficiency criterion ΔΘ according to
the relation
%,1002
12
Θ
ΘΘΘ
where Θ1 is the angle of a beam exiting a fibre element 1 km long,
Θ2 is the angle of a beam exiting a fibre 2 m long, coupled to a mode filter
(see Fig. 36).
Mode scramblers can also act as mode filters: because of the coupling of higher modes
in mode scramblers, they act as filters. Examples of a mechanical scrambler and a serpentine
scrambler (the fibre is intertwined with seven cylinders of 1 cm in diameter, their centres are
spaced 1.3 cm) are given in Fig. 37. Mode scramblers made from different types of fibre are
shown in Fig. 38. The first of them, made up of different bits of fibre, is frequently used in
practice. The assembly is usually composed of ca. 1 m of SI fibre, a GI fibre of the same
length, and a third, SI fibre of 1 m in length. After the mode scrambler in the above
composition a certain fibre section (ca. 500 m) is connected to stimulate stable mode
distribution. Only an output prepared in this way can be connected to the fibre being
measured. In the other case given in Fig. 38, a section (ca. 2 m) of poor-quality fibre of
different dimensions and inhomogeneities is used for mode scrambling.
Transmission media – laboratories 45
Fig. 37: Mechanical mode scramblers.
Fig. 38: Fibre-type mode scramblers.
The results of attenuation measurement depend, to a considerable extent, on excitation
conditions of the fibres being measured. With the development of optoelectronics, its specific
parts, instruments, and measuring methods this dependence gradually decreases.
46 FEKT VUT v Brně
Couplers that need to be adjusted are of no use for practical purposes; they are better
suited to laboratory measurement. On the other hand, however, instruments designed for field
measurement require much attention and great care in repeated measurement. The reason lies
in the special effect of losses in the splices (connectors) of the measuring apparatus on the
fibre being measured. While in electrical measurement the possibility of losses in connectors
and splices can usually be neglected, in optical measurement these losses play an important
role. Because of the difficulty of setting accurately the same measuring conditions, the
definition and measurement of the input (output) power are very difficult. Losses in optical
connectors are comparable with the losses in tens to hundreds of metres of optical fibre.
Moreover, even in types of the highest quality there are certain fluctuations in losses in
repeated installation, which are difficult to define generally.
In the first place, they are losses due to connector design tolerances, in particular axial
misalignment, imperfect contact of fibre edges, and faulty fibre cleavage. In practice this
means the impossibility to measure with a greater accuracy than these uncertain losses in
connectors. Since connectors are deployed on the transmitting as well as the receiving side,
we are faced here with a fundamental limitation of the potential measuring accuracy (above
all the measurement of optical powers). For these reasons the measurement of optical fibres is
conducted at both ends and the average value is then established.
Formation of optical flux
Laser diodes (LD) or light-emitting diodes (LED) are used as stabilized sources. The
output level of LD depends on temperature and reflected light. To remove changes caused by
this dependence the output level is monitored and, using the feedback loop, the output level is
stabilized.
A stabilized source operating on the 1.3 μm wavelength requires the surrounding
temperature to be strictly stabilized. This stabilization is provided using a feedback circuit and
the Peltier cooling element, see Fig. 39.
Fig. 39: Stabilized optical source circuit.
The temperature characteristic of the output level of stabilized optical source with LD has
in the temperature range 5 - 50 ºC a deviation of less than 0.05 dB.
Transmission media – laboratories 47
At a constant temperature, LEDs have a stable output level for a long time and with
extremely high reliability. But output level is very sensitive to effects of surrounding
temperature. If a surrounding temperature sensor with a diode for temperature compensation
is used, adequate stability can be obtained. The temperature characteristic of the output level
of stabilized optical source with LED has in the temperature range 5 – 50 ºC a deviation of
less than 0.5 dB for the 0.85 μm and 1.3 μm bands.
The wavelength of the spectral width of light emitted by stabilized source must also be
stabilized. For the sake of measurement accuracy the emitted average wavelength must be
around 5 nm.
Some instruments are equipped with temperature control, which eliminates changes in
wavelength oscillation caused by changes in surrounding temperature. The light-emitting
elements used are LED. Due to their high temperature stability they are also suitable for the
measurement of transmission bandwidth.
The source of visible light consists of a He-Ne laser and an optical fibre connector. The
connector is designed such that it forms a clearly visible optical flux in optical fibre. The
mutual position of the He-Ne laser tube and the connector is stable, and to couple the optical
flux to the optical fibre a lens is used most frequently.
The radiation emitted from the optical fibre connector into space is immediately
diffused in order to protect workers against any health risk. For increased safety a protective
device is provided, which interrupts the emission of light when the optical fibre connector is
removed.
Sources of visible light are used in testing the optical fibre from the viewpoint of
damage and for the identification of individual fibres.
Optical detectors
The optical receiver detector is usually formed by the avalanche photodiode or the PIN
photodiode. The choice of sensor material (Si or Ge) depends on the required region of the
measurement wavelength:
Si material 0.5-1.1 μm
Ge material 1.1-1.6 μm
PIN diodes are used for highly sensitive detectors:
PIN Si, input power -90 dBm,
PIN Ge, input power -75 dBm.
APD for current measurement of higher levels:
APD Si, input power -60 dBm,
APD Ge, input power -40 dBm.
The range of levels measured is up to +10 dBm.
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Measurement of optical power
The measurement of optical power is one of the basic types in the area of optical
measurement. This measurement is performed via converting the power of optical beam,
which is emitted directly by different optical sources or emanates from optical fibre, to
electric signal by means of an optoelectronic (O/E) converter.
An optical power meter consists of three parts: indicator, sensor, and adaptor, see Fig.
40. The adaptor adapts the light flux from a source or fibre such that it is best adapted to the
sensor dimension, and thus maximum optical power can be delivered to the sensor. The
sensor converts optical power to electric power. The indicator serves to display the electric
signal on a screen.
Fig. 40: Measurement of optical power.
These parts of the optical power meter can be exchangeable so that the device can be
used for measuring the required wavelength range of the power being measured, and for
various types of signal reception.
The measurement enables establishing
operation of opto/electronic converters and modules (or transmitter and receiver),
losses in fibres (fibre attenuation),
losses in the splices of parts of optical route,
function of passive optical elements (attenuation networks, connectors, optical
switching arrays).
The measurement is conducted on the respective wavelength, which can be set in
advance. The choice depends on the electro/optical and opto/electronic converters used, which
are fundamental elements of sources and receivers.
The block diagram of an optical power meter is given in Fig. 41. To increase the
receiver sensitivity, an optical signal chopper with subsequent synchronous detection is used
in addition to the generally known blocks (CU stand for control unit). The sensitivity is thus
increased by ca. 20 dB.
Fig. 41: Block diagram of optical power meter.
Transmission media – laboratories 49
Measurement of attenuation
Attenuation represents the basic and most important transmission parameter and is an
overall measure of optical power losses in the optical signal propagation through the fibre.
Using the well-known definition, the attenuation of optical fibre between two points (1, 2 –
input, output) is determined from the relation
1
2
10 log dB ,P
AP
where P1, P2 are the optical powers (W) for wavelength λ.
In the case of stable mode distribution in the fibre, specific fibre attenuation can be
defined for wavelength λ
-1dB.km ,
A
where (km) is the distance between point 1 and point 2.
The measurement of attenuation is mostly performed only for discrete wavelengths of
850 nm, 1300 nm, 1310 nm or 1550 nm. The spectral characteristic of attenuation is mainly
important to fibre manufacturers.
IEC recommends three methods for the measurement of fibre attenuation:
- cut-back method,
- insertion loss method,
- backscattering method.
Because of its high sensitivity, the cut-back method is recommended as a reference
method (although it is a destructive method). After coupling the optical power from
a stabilized optical source (with connected internal or external transmit unit T.J. – coupler,
filter) to the measured fibre (Fig. 42) of length l, the power is measured at point 2 at the end
of the fibre (power meter). With the coupling conditions unchanged, the fibre is cleaved ca.
2 m from the beginning (at point l) and output P1 is measured. Attenuation and specific
attenuation of the fibre are calculated using relations. An accuracy of 0.01 dB . km-1
can be
achieved by this method.
Fig. 42: Measurement of attenuation by cut-back method.
The insertion loss method also requires measuring in two steps. This is an operational
method and is particularly suitable in the case of connectored fibres and cables. In the first
50 FEKT VUT v Brně
place, the measuring equipment must be calibrated via interconnecting the source and the
detector (see Fig. 43). After measuring we obtain the value of power P1. The fibre being
measured is then connected between the optical transmitter and the power meter, and the
value of power P2 is obtained. Attenuation and specific attenuation of the fibre are again
determined using relations. In this case, the attenuation measured consists of fibre attenuation
and attenuation of the splice of the fibre being measured. In the measurement of connectored
cables, the measurement precision is a function of the connector used and is usually worse
than 0.2 dB.
Fig. 43: Measurement of attenuation by insertion loss method.
Transmission media – laboratories 51
Fig. 44: Operational measurement of attenuation, using insertion loss method.
The method is used in practice also in such a way that on each side of the route (A, B)
both the source (transmitter) and the power meter (receiver) are located. The method proceeds
in four steps: two measurements are first made on each side via connecting the power meters
to sources (calibration), and two measurements with the fibre connected in both directions.
A simplified fundamental schematic is given in Fig. 44. The four values of optical power
obtained, P11, P12, P21, P22, are used to calculate the operational attenuation of fibre according
to the relation
2211
2112
PP
PPA
The specific attenuation can be calculated by substituting previous relations.
If in the above method we want to establish losses in the splices, we use for calibration
fibres of 2 to 3 m in length and of the same kind as the fibre being measured. On the
assumption that the coupling losses of connecting the reference cable and the cable being
measured are the same and the attenuation of reference cable can be neglected, then the
attenuation obtained in this way corresponds to losses in the cable alone.
The backscattering method (the principle is described below) for the measurement of
fibre attenuation is based on a completely different principle. In the two preceding methods
optical power was measured after passage through the fibre while in this method the time
dependence of backscattered optical power P (or the level) during pulse propagation in the
fibre is evaluated, which yields information on the quality of the whole fibre in dependence
on its length (see Fig. 45). From this dependence we can establish the attenuation of
a homogeneous section of fibre according to the relation:
dBlog52
1
P
PA
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Fig. 45: Measurement of attenuation in a splice (fibre values are known).
The meter, an optical reflectometer, is much more complicated than the power meter
and, consequently, also more expensive. However, using a display and recorder, the method
provides information not only about fibre attenuation but also about the fibre quality (failures,
defects) along the whole of its length.
The measurement of attenuation in a splice belongs to important measurements in the
construction of optical routes. Immediately after fusing (or otherwise joining) sections of
optical fibre it is absolutely necessary to measure and check the quality of the splice made. If
the splice is found to be of poor quality, the splice must be made again.
The measurement of attenuation in a splice is laborious and time-consuming, and
requires much care. There are two ways how to proceed in practice:
using the manufacturer‟s parameters of the fibres being joined,
without knowing the attenuation of the fibres being joined.
In the first case, the sections of the two fibres to be joined by splice S are from the
viewpoint of attenuation known from the measurement protocols provided by the
manufacturer. Prior to making the splice S, the level P1 is measured on the side of the first
fibre section (see Fig. 45). Subsequent to making the splice S, the level P2 is measured on the
output of the section being joined. The splice attenuation is determined from the relation
dB221S aPPA ,
where a2 dB is the attenuation from factory protocols for a specific length and
transmission wavelength.
In the second case, when the attenuation of the sections being joined is not known, the
procedure is as follows. Prior to fusing, the level P1 is measured on the output of the first
section. A provisional splice PS is made (see Fig. 46) and the level is measured on the output
of the spliced section P2. After the provisional splice, in the direction from the input, the fibre
is interrupted and the level P3 is measured. The definite splice DS is made and the value of
power level P4 is found. Attenuation V2 of the section being joined is then
2 3 2 dB .A P P
Attenuation in the definite splice is
DS 1 4 2 dB .A P P A
Transmission media – laboratories 53
Fig. 46: Measurement of attenuation in a splice (fibre values are not known).
In the measurement of connector attenuation a special reference fibre of 2-3 m in length
is used. It is connected between the transmitter and the receiver, and the level of optical power
P1 is measured. The reference fibre is replaced with a fibre obtained by joining two short
sections by a connector (the fibre must be of the same length and type as the reference fibre).
The level P2 is measured and the connector attenuation is established from the relation
K 1 2 dB .A P P
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9 Measurement of backscatter
The backscattering method is an effective means of diagnosing optical fibres. The
method provides a detailed picture of the attenuation and possible fluctuations in geometrical
and physical parameters along the fibre, inclusive of failure localization.
The method is based on evaluating the time dependence of backscattered power of
a narrow optical pulse coupled to the fibre. Backscattered light detected at its input comes
from the Fresnel reflections from refractive index discontinuity and from Rayleigh‟s
scattering on microscopic fluctuations of glass refractive index. The amount of backscattered
light is directly proportional to the passing optical power. Changing the intensity of
backscattered light enables measuring the fibre attenuation.
The fibre length can be established from the time delay of the reflection from the rear
end of fiber with respect to the reflection from the input end.
An analysis of backscattered light provides a picture of the fibre homogeneity
(inhomogeneity), and allows monitoring whether the mode distribution has become stable.
Great advantages of the method are its non-destructive nature and the possibility of
measuring from one fibre end.
To get a mathematical representation of backscatter signal it is necessary to perform an
analysis of light energy on an element of optical fibre, then to consider light scattering on a
length, then to determine the power returning back to the fibre, and finally to define the
relation for backscatter as a function of time.
Consider an optical pulse of energy E0 sent at time t = 0 from the fibre beginning x = 0. At
a distance x from fibre beginning the radiant energy of pulse Ei(x) will be
i O
0
exp ,
w
E x E d
where is the attenuation. For a certain constant value it will hold
i O e .
xE x E
Now consider scattering in x, x + dx, then
w
d O d
0
d x x exp d d ,E E x
where d (x) is the scattering coefficient at point x. Only part of this energy can
propagate in opposite direction in the fibre (S(x)). Thus
w
p O d
o
d x exp dE E S x x d x.
When viewed from the input side, it holds
Transmission media – laboratories 55
w x
O d
o o
d x exp d d d ,E E S x x x
where is the attenuation of reverse direction. In the case that ,
then
2 α x
O dd x e dE E S x.
Symbols for the calculation are given in Fig. 47.
Fig. 47: Element of optical fibre.
By interchanging the variables E and x we obtain the dependence of power on time
O OE P t.
It holds
sk2 . ,x v t
skd d ,2
vx t
where Δ t is the pulse width, and vsk is the group velocity.
Then we get
.eS5,0t tvskdO
vtPP
It is clear from the result that backscatter power is dependent on input power Po, pulse
width Δt and on the fibre parameters S and αd (αd are losses due to Raileigh‟s scattering per
unit of length).
The coefficient of backscattering S for a fibre with step refractive index is calculated
according to the relation
2
2
1
3,
8S
n
for a fibre with parabolic refractive index
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2
2
1
,4
Sn
where
2 2
1 2 .n n
The resultant exponential curvature depends on the size of attenuation and on group
velocity.
When choosing the pulse width it is necessary to accept a compromise between the
requirement for photodiode sensitivity and the resolution power. With increasing pulse length
the maximum measurable total attenuation increases but the accuracy of length measurement
decreases. The power of reflected radiation is ca. 50 to 60 dB lower than a passing optical
pulse and this makes heavy demands on the device sensitivity.
For reasons given above, the design of optical reflectometer is continuously being
perfected. Principles known from information theory (problems of signal and noise reception)
are applied such as correlation methods and frequency synthesis methods. Also, memories are
employed, in which certain values of attenuation of reflected pulses are stored and their
average values are evaluated (this reduces measurement errors caused by receiver noise). A
more detailed discussion of these methods would be beyond the scope of this publication; this
is rather a matter for specialists in the area of measurement in telecommunications.
The fundamental principle and method of backscatter measurement are obvious from
the block diagram shown in Fig. 48. For the measurement itself it is sufficient to connect the
fibre being measured to the input connector of the reflectometer.
Fig. 48: Block diagram of pulse reflectometer.
Transmission media – laboratories 57
The reflectometer is composed of a source of optical pulses, an optical system for
splitting the optical beam, and an optical receiver with evaluation of the time interval between
the sent pulse and the reflected pulse.
The source is a semiconductor laser with a minimum power of 5-15 mW at 10 ns pulse
width. The optical system with lenses and semi-transmissive mirror serves to couple optical
power to the fibre and deliver reflected light back to the detector so that the ratio is 1:1; 50%
of optical power flowing through the optical splitter is lost for the measurement.
The relations between direct and reflected radiation in the fibre have already been
defined by a derived relation (9.21). To make the method clear, let us assume that the fibre
exhibits a serious failure or is interrupted; the power of light PR reflected from the failure at
a distance ℓ from the fibre beginning will be
R O ,2
R kP t P
where t is the time interval between the sent pulse and the reflected pulse, Po is the
power of transmitter pulse, R is the surface reflectance, k are losses due to the optical system
inclusive of beam splitting, 2α (ℓ) is the average of losses in forward and reverse direction.
For the accuracy of localizing the failure the reflectance of the surface under
examination, which is very different for different damages, is of great significance. It follows
from the Fresnel relations that the maximum reflection is for perpendicular light incidence,
and it holds
,
221
221
nn
nn
where n1 is the effective refractive index, and n2 is the refractive index of the
surroundings.
For the typical core refractive index n1 = 1.5 and n2 = 0 (the case of air), which is
a situation occurring in the ideal case, the result is 0.04, which corresponds to a back
reflection of 4% of energy. In practice, however, the fibre fracture surface may be rough,
cracked and surrounded with water (n2 = 1.33) so that a reflection of 0.1 to 1% of energy can
mostly be reckoned with.
During the return passage the pulse is directed by the divider to the photodetector,
where it is converted to electric pulse. The input and the output pulse are displayed
simultaneously on the screen and the time interval between them is subtracted. Distance to
failure is determined from the relation:
1 1
,2 2
t c t c
n n
where t is the time difference between the sent pulse and the returned pulse, Δt is the
widening of reflected pulse due to fibre dispersion, c is the velocity of light in vacuum, and n1
is the core refractive index.
Finally, to express the total dependence of detected power of backscatter P(t) on time
delay t or on the corresponding distance from the fibre end we calculate fibre attenuation
from two points. The total fibre attenuation with the boundaries 1 and 2 is established from
the relation
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2 1 2 1
2 12 1
1
.c 2
n P n P n P n Pa
t tn
The meter is usually equipped with an amplifier with logarithmic characteristic, which
enables displaying sections with constant optical power losses as straight lines, whose
gradient of line is a quality indicator of these losses. Any discontinuity (e.g. attenuation in
splices and connectors) shows as a sudden drop in the characteristic being measured.
An example of the waveform of received power of backscatter from optical route. In the
example (Fig. 49), the reflection of light on fibre input and on fibre interruption is evident.
From the decreases in (losses of) attenuation on splices, attenuation can be read directly in dB
if a calibrated scale is used. The line gradients give directly a picture of the size of attenuation
in the fibre.
Fig. 49: Waveform of received backscatter power shown on a display.
An illustration of the result on the display and the on the print-out of a fully automated
reflectometer is shown in Fig. 50.
Transmission media – laboratories 59
Fig. 50: Measurement protocol of OTDR reflectometer.
The reflectometer used was EXFO FTB300. A ballast fibre (L = 500 m) was connected
between points 2 and 3. Points 4, 5 and 6 are splices along the route, where cable transfer was
carried out. Mark 7 terminates the cable in RSU. Part of the meter is a table giving the
attenuation values at the route points given above. The measurement was conducted on the
1310 nm wavelength.
The speed of measuring by this method is evident; the print-out gives immediately the
values of splice attenuation, specific attenuation and operational attenuation of individual
lengths, and, last but not least, some important values of the measurement protocol such as
refractive index, length of measured fibres, time of measurement, etc.
The accuracy of the measurement itself is also noteworthy when compared with similar
measurements known for metallic lines. The high accuracy is due to the fact that pulses
modulated to the carrier frequency of optical radiation are used in the measurement. The pulse
frequency bandwidth is much smaller than the carrier frequency. The optical pulse is then
deformed to a lesser degree and this enable higher measurement accuracy. For example, for a
length of 70 km and a pulse width of 10 ns the accuracy of localizing the fibre fracture is
better than ± 10 m.
The above backscatter method of measurement is a precise method frequently used in
practice. Using this method the following measurements can be conducted:
measurement of optical power losses of the route (i.e. in the fibre, splices and
connectors),
measurement (localization) of failures and damage in fibres (currently the only method
for this kind of measurement),
measurement of the lengths of individual route sections.
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10 Measuring of attenuation by OTDR
10.1 Instruction
1) Familiarize with the principle of measuring method OTDR (Optical
Time Domain Reflectometery).
2) Familiarize with measuring device EXFO FTB-400.
3) Using the reflectometer measure and draw the curve of backscatter
for the two wavelengths.
4) Measure in auto mode, then set two values of the pulse width.
5) Check the fiber attenuation for the two wavelengths.
6) Check the length of fiber.
10.2 Introduction
Currently the most used device for mounting and operating in many measurement
parameters, and optical cable routes is optical reflectometer. By this device you can measure
the length of the fiber, its homogeneity, attenuation of splices, connectors and fiber optic
connectors also allows to locate the fault. An optical reflectometer uses a backscatter method.
An OTDR injects a series of optical pulses into the fiber under test. It also extracts,
from the same end of the fiber, light that is scattered (Rayleigh backscatter) or reflected back
from points along the fiber. (This is equivalent to the way that an electronic time-domain
reflectometer measures reflections caused by changes in the impedance of the cable under
test.) The strength of the return pulses is measured and integrated as a function of time, and is
plotted as a function of fiber length.
An OTDR may be used for estimating the fiber's length and overall attenuation,
including splice and mated-connector losses. It may also be used to locate faults, such as
breaks, and to measure optical return loss. To measure the attenuation of multiple fibers, it is
advisable to test from each end and then average the results, however this considerable extra
work is contrary to the common claim that testing can be performed from only one end of the
fiber.
In addition to required specialized optics and electronics, OTDRs have significant
computing ability and a graphical display, so they may provide significant test automation.
However, proper instrument operation and interpretation of an OTDR trace still requires
special technical training and experience.
OTDRs are commonly used to characterize the loss and length of fibers as they go from
initial manufacture, through to cabling, warehousing while wound on a drum, installation and
then splicing. The last application of installation testing is more challenging, since this can be
over extremely long distances, or multiple splices spaced at short distances, or fibers with
different optical characteristics joined together. OTDR test results are often carefully stored in
Transmission media – laboratories 61
case of later fiber failure or warranty claims. Fiber failures can be very expensive, both in
terms of the direct cost of repair, and consequential loss of service.
OTDRs are also commonly used for fault finding on installed systems. In this case,
reference to the installation OTDR trace is very useful, to determine where changes have
occurred. Use of an OTDR for fault finding may require an experienced operator who is able
to correctly judge the appropriate instrument settings to locate a problem accurately. This is
particularly so in cases involving long distance, closely spaced splices or connectors, or
PONs.
OTDRs are available with a variety of fiber types and wavelengths, to match common
applications. In general, OTDR testing at longer wavelengths, such as 1550 nm or 1625 nm,
can be used to identify fiber attenuation caused by fiber problems, as opposed to the more
common splice or connector losses.
Block diagram of OTDR can be seen in Fig. 51.
Fig. 51: Block diagram of OTDR.
The optical dynamic range of an OTDR is limited by a combination of optical pulse
output power, optical pulse width, input sensitivity, and signal integration time. Higher optical
pulse output power, and better input sensitivity, combine directly to improve measuring
range, and are usually fixed features of a particular instrument. However optical pulse width
and signal integration time are user adjustable, and require trade-offs which make them
application specific.
A longer laser pulse improves dynamic range and attenuation measurement resolution at
the expense of distance resolution. For example, using a long pulse length, it may possible to
measure attenuation over a distance of more than 100 km, however in this case an optical
event may appear to be over 1 km long. This scenario is useful for overall characterization of
a link, but would be of much less use when trying to locate faults. A short pulse length will
improve distance resolution of optical events, but will also reduce measuring range and
attenuation measurement resolution. The "apparent measurement length" of an optical event is
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referred to as the "dead zone". The theoretical interaction of pulse width and dead zone can be
summarized as follows:
Pulse length Event dead zone
1 ns 0.15 m (theoretically)
10 ns 1.5 m (theoretically)
100 ns 15 m
1 µs 150 m
10 µs 1.5 km
100 µ 15 km
The OTDR "dead zone" is a topic of much interest to users. Dead zone is classified in
two ways. Firstly, an "Event Dead Zone" is related to a reflective discrete optical event. In
this situation, the measured dead zone will depend on a combination of the pulse length (see
table), and the size of the reflection. Secondly, an "Attenuation Dead Zone" is related to a
non-reflective event. In this situation, the measured dead zone will depend on a combination
of the pulse length.
Backscatter curve for longitudinal homogeneous fiber is shown in Fig. 52. At the
beginning and end can be seen reflection of the Fresnel from the front input and output
fibers. This reflection is approximately three orders of magnitude larger than the amplitude
of the reflected signal.
Fig. 52: Ideal curve of backscatter for longitudinally homogeneous fiber.
Transmission media – laboratories 63
Measuring of the attenuation of the real optical fibers and optical lines are usually not
meet the ideal course of the backscatter curve. Real course of these curves are distorted in
various ways. These distortions can be caused by changes along the fiber attenuation,
inhomogeneity in the route, fluctuations of waveguide structure or incorrect measurements
mode.
Typical examples of possible failures in backscatter curve are shown in Figure 3.
Fig. 53: Typical examples of possible failures in backscatter curve.
Typical examples of possible failures in backscatter curve are shown in Fig. 53.
Reflection from the input fiber interface with a marked dead zone.
1) In this section is measured fiber homogeneous and backscatter curve has
a constant slope. In this section is the attenuation constant.
2) Local increase in attenuation. This increase may be caused by splice.
3) Sharp peak, which arises due to Fresnel reflection at the connector coupling of
two fibers or fiber defects.
4) Apparent amplification occurs when in the route is a fiber section with a larger
diameter of mode field.
5) Multiple reflections can occur an incorrect choice of the linear range.
6) Ripple of the curve is usually caused by a measuring device, fluctuations
of waveguide structure or polarization effects.
7) Changing the slope of the curve. The cause of this effect may be changing
attenuation along
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8) the fiber or continuous change of diameter mode field.
9) Reflection from the fiber end. This reflection is not always evident.
Transmission media – laboratories 65
11 Splicing of optical fibers and measuring of the
attenuation
11.1 Instructions
1) Familiarize with the fiber cleaver and hand-held fusion splicer.
2) Stripp the ends of the two attached fibers, clean by izopropylalkohol, cleave and then
splice it.
3) Restore the function of primary protection.
4) Measure the attenuation of carried splice.
11.2 Introduction
Splicing of the optical fibers is one of the most used methods of connections of optical
fibers.
In the laboratory is available splicer FITEL S122.
With its low profile and IP-52 rated super rugged body, the FITEL S122 series fusion
splicer offers speedy operation in every splicing field, FTTx, LAN, backbone or long-haul
installations.
Splicer S122 is used for making reliable connections of optic fibers with low
attenuation. It is equipped with software for all common single and multimode fiber with a
standard cladding diameter of 125 microns and coatings from 250 to 900 micrometers. Splicer
has a sytem PAS (Profile Alignment System) for extremely low attenuation of splice,
independent of the operator.
Depending on the splicing process (see specification Fig. 54) and cleaning of fibers
offers high-precision positioning to coat of optical fiber, optimization of each splicing process
through automatic control of splicing time AFC, evaluation of attenuation and fully
automatic splicing process by pressing a single key.
PAS system is used for positioning the core to the core of fibers and automatic control
of splicing time. This system is supplemented by a video image evaluation L-PAS. Using two
cameras and magnifying optics detects the position and quality of the of fibers ends. At
present it is the most used technology of optical fibers centering.
Splicer machines with technology PAS (Profile Alignment System) used for centering
the fibers an active mechanism, which centered on the core of optical fiber with a minimum
deviation. Centering of the fibers takes place in three axes (3-D technology) and the resulting
centering is controlled by video image in two axes, which captures the optical lenses and
evaluated in the microprocessor.
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Fig. 54: Details of splicer.
LCD (display see Fig. 55) shows the fiber simultaneously in two different views (plane
X and Y).
Fig. 55: LCD display with details.
Transmission media – laboratories 67
The control panel (see Fig. 56 and details in Tab. 8: Descriptions of buttons.) consists of a set
of buttons that have the following functions:
Fig. 56: Control buttens of equipment.
Tab. 8: Descriptions of buttons.
Button Name Main function
Start Start/Pase/Restart of splicing process.
Function 1 Select the function (s) shown (s) in the right bottom
corner of the LCD.
Function 2 Select the function (s) shown (s) in the left bottom
corner of the LCD.
Up Move up, increasing the value.
Down Move down, decreasing the value.
Left Move left, switch the view X/Y.
Right Move right, switch the view X/Y.
Heat Switch on/off heating.
Power Switch on/off.
Through the USB it is possible to send data (parameters of splices) from the splicer to
PC or printer.
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11.3 Splicing procedure
WARNING: The smallest possible values of attenuation can be achieved only when the
ends of fibers are carefully prepared for splicing.
1) Slide the protection (Fig. 57) of splice either to the left or to the right fiber.
Fig. 57: Example of using.
2) Stripp (Fig. 58) about 30 mm of fiber protection in each end of fiber.
Fig. 58: One of the equipments which is used.
3) Clean stripped (Fig. 59) fiber by izopropylalkohol and cleaning tissue.
Fig. 59: Equipment and manual for the stripped fiber.
Transmission media – laboratories 69
4) Cleave the bare fiber (Fig. 60) 10 mm of bare fiber will protrude from the protection
fiber.
Fig. 60: Equipment for the cleave the bare fiber.
Do not clean the bare fiber after it has been cleaved.
Do not touch the bare end of the fiber with any surface.
We recommend using safety glasses.
5) Inserting of fiber
Open the front cover.
Insert the fiber to the “V” groove (Fig. 61). Make sure that the end of the bare fiber
does not touch anything.
Make sure that the bare fiber is positioned correctly in the V-grooves. If not, remove
the fiber and set it again.
Repeat it for second fiber.
Close the front cover. READY screen will be displayed.
Fig. 61: Inserting of fiber.
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Do not insert the tip ends of the fibers through the V-groove.
!
Warning
If you will insert the bare fibers to the V-grooves,do not break
them about V-groove or another part of the splicer.
Broken fiber could get into your eyes!
6) Splicing
Make sure that READY is displayed on the screen.
Push for start of splicing cycle.
S122 performs the following functions automatically. For interruption splicer
during one of these functions, press On display will be shown „PAUSE To
restart the operation, press again
On LCD monitor will be shown right and left end of the fiber.
There will be cleaning discharge to clean the fibers end.
The fibers are configured with a gap of about 30 micron between the ends.
Axis shift is checked and the state of fiber cleaves. Electrodes will discharge.
Inspection of splice is performed.
It is performed estimate attenuation is displayed on the LCD monitor, as shown in
Fig. 62.
Fig. 62: LCD monitor with details of proces.
Transmission media – laboratories 71
11.4 Splicing proces on a S122C
For details see the following sequence Fig. 63.
Inserting of fibers Check Discharge Estimate attenuation
Fig. 63: Sequence of splicing proces.
When the splice is done we can heat the protection. Move the protection on stripped part
of fiber (through the splice) and put the protection to the heating part of splicer machine as
shown on Fig. 64. Push the button .
Fig. 64: Finalization of the splicing proces.
11.5 Measuring of attenuation
The following Fig. 65 shown the general scheme of measuring of attenuation.
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Fig. 65: General scheme of measuring of attenuation.
1) For measuring the attenuation of splice will be used measuring kit Optokon
LS(PM)420. Read the instructions for detailed measuring equipment and familiarize
with the measuring device.
!
Warning
Be sure to clean! In particular, the purity of the connectors.
2) For measuring will be used reference method. At first, do the splice. After that, switch
on the light source (LS) and power meter (PM). For measuring use wavelength of
1550 nm. Connect the connectors to the LS and PM. Set new reference on PM (REF -
CONFIRM). Switch off PM and LS. Do not disconnect connectors from PM and LS.
!
Warning
Never disconnect connectors from PM and LS during the
measurement.
3) Cut the fiber next to the first splice (about 20 cm next to the splice). Then make the
new splice.
4) Switch on the LS and PM. Switch the wavelength on LS to 1550 nm. PM shows the
value of attenuation of splice.
5) Compare the measured value of attenuation with the estimate value from splicer
machine. The value of attenuation should be about 0,05 dB.
Transmission media – laboratories 73
12 Optical wireless transmittion in laboratory
This chapter describes modification of cheap free technology point-to-point FSO
from project Ronja (Reasonable Optical Near Joint Access), which is used in optical networks
laboratory in department of Telecommunications, Technical University of Brno. The Ronja
is optoelectronics device, which uses narrow light beam as a transmission channel
in atmosphere. This beam is crated throw lens system. Purpose of device is wireless
connection of two separate computer networks with transfer speed of 10 Mbps. Maximum
distance to communicate is 900 meters and must be in line of sight. FSO Ronja is constructed
from three main devices, transmitter, receiver and interface. The transmitter contains LED
(light emitting diode) for transmitting data and the receiver contains PIN photodiode
with very short switching time as a detector. Ronja communicates in full-duplex (allows
communication in both directions simulatelously). The interface alters signal levels and
impedance for optical transmitting. It generates signal at 1 MHz, which is needed to foolproof
function the Ronja with interferences such as sunlight or another shining source, which should
influence connection.
12.1 Modification the Ronja in Laboratory
Several devices were added to the Ronja for purpose of acquaint students
with the functionality and capabilities of this FSO. Also was created own unique mechanical
construction.
12.2 Measuring device
This device is design for laboratory needs. It task is to measure RSSI voltage at both
receivers and send their values to PC via USB or RS232 port. Regulation of current
transmitting diode is from PC via this measuring device. Measuring device can turn off entire
Ronja device via keyboard in PC. There is special plug with relay inside and it is connected to
USB power supply, so if the PC is shutting down this plug disconnect power supply to entire
device and the Ronja. Measuring device controls microcontroller Atmega8 from Atmel.
Wiring diagram measuring device is at Fig. 66 on next page.
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Fig. 66: Wiring diagrammeasuring device to laboratory needs.
Transmission media – laboratories 75
12.3 Modification of the transmitter
The Ronja is designed to work at distance from 135 to 900 meters, but consoles are
in laboratory only few meters from each other. So it needs to reduce intensity of transmitter
light beam to simulate longer link. That‟s why there is added NPN transistor serially to
transmitting LED in the transmitter. The transistor is connected between cathode transmitting
LED and ground. The transistor reduces current in transmitting LED via increasing resistance
in base of the transistor, so light beam from LED is weaker. This regulation of current can be
makes electronically from PC via resistors R17, R18, R23 – R27 in the measuring device or
via potentiometer at the cover of the transmitter pipe.
12.4 Voltmeter at the receiver
At the cover of the receiver pipe is fit voltmeter to measures and displays actual value
of RSSI. There is used single-line LCD display with backlight. The voltmeter controls
microcontroller Attiny26 from Atmel. With this voltmeter is targeting both end of the Ronja
much more comfortable.
12.5 Measuring program in computer
Program Hyperterminal is used to communicate with the measuring device. This
program is part of each operating system from Microsoft. Hyperterminal job is to monitor and
record RSSI values from both receivers, which are send from the measuring device. Every
second Hyperterminal shows measured RSSI from both receivers. Data should log, so that it
is possible to create graph RSSI values in Excel from longer period of time.
Regulation of current one transmitting diode is represented via keys 1 to 7, where
number the most left has the most significant affect to limitation current and number the most
right has the least significant affect. Every number can have value 1 or 0, where 1 increase
light beam. Thought this program you should turn on and off power supply to the Ronja. If is
press the button “z”,
Ronja has power supply, button “v” cut off power supply. This has purpose to saving
energy, when is no measuring at the Ronja. Example of Hyperterminal window is at Fig. 67
on the next page.
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Fig. 67: Example of data in Hyperterminal window.
12.6 Mechanical construction
There is 100 mm lens in the front part of each pipe. Laser cutting metal plates
are supposed to fit transmitter and receiver to the center of pipe, where these devices move
inside pipes to reach focal distance of the lens.
The receiver setups in pipe only once, so construction of metal plates are simple. But
metal plates for the transmitter are much more complicated, because transmitter can move in
pipe via spinning the handle on the screw rod. This is cause change of diameter the light beam
from the transmitter. Under each pipe there is very fine targeting system, which is assemble
from two screw rods. Vertical screw rod has purpose to support pipe and vertical advance of
the pipe, while horizontal screw rod is for horizontal advance of the pipe.
For tight fix of the pipe on its position there are always two nuts to counter with each
other. Two pipes with targeting system are assembled to console, which is screwed to wall.
Transmission media – laboratories 77
Every iron part from this console is galvanized to the best protection against rust. Overall look
to assembled mechanical construction is photographed on Fig. 68.
Fig. 68: Overall look to assembled mechanical construction.
Modification the Ronja expanded possibilities for different kinds of measurements
at the Ronja and improved working with the entire device. The Ronja will very well serve
students in the lessons in optical network laboratory. Where they will try to work with
the Ronja, its behavior in the limit conditions and affect function of Ronja by various external
influences. Students will try complex problems of wireless optical transmission in practice.
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13 EDFA Measurement
13.1 Assignment
1) Become familiar with the amplifier EDFA CLA-P(B)-01F!
2) Measure the powers of wavelengths in the input of the optical amplifier!
3) Measure the output powers by various values of gain, output gain and the
current of pumping diode!
4) Evaluate the measured values with the values in the output of EDFA amplifier!
13.2 Theoretical Introduction
Optical amplifiers
Optical amplifiers are used with advantage in wavelength division multiplexing
systems. Unlike repeaters, they enable restoring luminous flux in the fibre without the
necessity of converting it to electric form. These amplifiers are universal elements that
amplify both analogous and digital signals of arbitrary transmission speed.
Optical fibre amplifiers EDFA (Erbium Doped Fibre Amplification)
A simplified block diagram of the EDFA amplifier is given in Chyba! Nenalezen zdroj
dkazů.Fig. 69. The amplifier is formed by the so-called laser pump and a special optical fibre,
which is doped with rare-earth elements (erbium, et al.). Due to the laser pump emission (of
980 nm or 1480 nm wavelength) coupled to a special fibre of several metres in length, atoms
of doped element are excited to a higher energy level. The energy obtained from the laser
pump radiation is thus temporarily stored in the atoms. This energy is released due to the
presence of the signal being transmitted, whose energy calls forth stimulated emission of
radiation, which is of the same wavelength and phase as the signal transmitted. This amplifies
the transmitted optical signal. Optical fibre amplifiers enable increasing the signal level by as
much as 50 dB (one channel, C – band). Via internal arrangement of the amplifier a wide
range of the amplified band can be obtained and thus the signal can be amplified in the C and
L bands simultaneously. Various possibilities of deployment in an optical transmission system
result from the principle of EDFA function. Basically, the amplifiers can be applied in four
manners:
Booster – which is located right after the optical transmitter; it serves to amplify its
signal to a maximum level that can be coupled to the fibre. It must be capable of
accommodating a relatively large input signal from optical transmitter.
In-line amplifier – which is located on the optical fibre route; it amplifies a small input
signal to a maximum output signal.
Pre-amplifier - serves to amplify very low signal levels to a level that is sufficient for a
correct functioning of optical amplifier at the end of transmission route. The
requirement put on the pre-amplifier is to have a minimum internal noise.
Transmission media – laboratories 79
Compensation of losses in optical networks (CATV) – in optical community antenna
television the reduction of signal level is primarily due to the requirement to divide the
optical signal into several fibres. Before being divided the signal is amplified by
means of EDFA such that the same signal level is obtained in output fibres as in the
original fibre.
These amplifiers are manufactured as single-channel EDFA amplifiers, WDM
amplifiers, and CATV amplifiers.
Fig. 69: Principle of optical EDFA amplifier.
Optical Raman amplifiers
The Raman type of amplifiers is used to amplify an optical signal. It is practically just
a laser source of radiation connected to the optical route. To amplify the optical signal the
Raman scattering on particles of the waveguide material is used. With this scattering there is,
among other things, a shift of energy from the lower wavelengths (wavelength of Raman
pump radiation) to higher wavelengths (wavelengths of the signal being transmitted) and
consequently an amplification of the signal. A simplified connection diagram is shown in Fig.
70.
Fig. 70: Optical Raman amplifier.
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The amplification of optical signal thus takes place directly in the transmission route
fibre. No special fibre is necessary here; any communication fibre can be used. Amplification
values obtained with this type of amplifier are not as high as with EDFA. The signal level can
be increased by ca. 15-20 dB.
The Raman amplifier is located at the end of optical transmission fibre and radiation
from the laser pump propagates against the signal being amplified. This amplifier can be used
to amplify an arbitrary wavelength, provided the appropriate wavelength of laser pump is
chosen (e.g. 1450 nm for the 1550 nm band). The EDFA amplifier and the Raman amplifier
can with advantage be combined. However, these amplifiers also amplify distortion and thus
in the case of long routes it is necessary to connect the classical repeater to restore the signal.
Example of four-wavelength multiplexer - is a frequently demanded multiplexer for
rapidly increasing the network capacity at a relatively low cost. It is based on the principle of
cascading interference filters with 8 nm spacing (see Fig. 71).
Fig. 71: WDM – wavelength division multiplexer.
Example of dense wavelength division multiplexer (DWDM) – leading manufacturers of
the DWDM device include Lucent Technologies, Alcatel, Nortel, NEC, and others. Currently
marketed DWDM have 16, 20, 40, 60 and up to 100 spectral channels.
The Wave Star OLS 806 device by Lucent Technologies will be given as an example. It
employs 16 wavelengths, spanning attenuation 33 dB, which corresponds to a distance of 120
km without amplification (non-zero dispersion fibre True wave is assumed). The device can
be used with advantage when creating ring topologies, as can be seen in Fig. 72Chyba!
Nenalezen zdroj odkazů..
Effects influencing the quality of multiplex transmissions
For a good quality transmission it is necessary to meet the respective limits, which are
verified by measuring and which include:
the average wavelength, which must meet the respective standards; accurate
measurement must be assured with respect to temperature changes, laser instability,
and back reflections,
the bandwidth must also meet the criteria of spectral characteristics,
Transmission media – laboratories 81
the insertion loss must provide the most favourable transmission conditions,
in DWDM installations, the cross-talk must satisfy the cross-talk quantities between
neighbouring wavelengths, as was earlier the case with metallic conductors,
Fig. 72: OLS 806 in “ring application”.
the back reflection may be different in individual channels and the values need to be
kept within tolerances, mainly with a view to system stability,
the peak power of individual channels must satisfy the respective tolerances,
the type of fibre used is a separate and very important consideration.
An example of the spectral characteristics of four-wavelength multiplexer is shown in .
In the case of new installation, when the deployment of DWDM system is assumed, the
technical parameters can be selected to meet the operator‟s requirement. This principle is
currently implemented in the construction of transport networks of many operators, using the
so-called “Telehouses”.
A substantially more difficult situation exists as regards the future exploitation of
already installed fibres for the needs of DWDM.
In the first place, earlier fibres (ITU G.652) are not ideal for DWDM. Relatively large
chromatic dispersion in the 1550 nm band limits the link range and compensation is necessary
for longer routes.
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Fig. 73: Spectrum of wavelength division multiplexer.
All the effects given above influence transmission and they must be taken into
consideration in the design of networks.
Application potentials of wavelength division multiplexers in academic computer
network of universities
One of the first applications is the deployment of two-wavelength multiplexer in the
experimental network of the Department of Telecommunications (DT). The system
demonstrates its functionality and enables the measurement of elements. The system was used
to demonstrate its application in the supervision for optical cables. Its connection is shown in
Fig. 74.
Fig. 74: Implementation and connection of WDM in DT network.
Transmission media – laboratories 83
Scheme of the workplace configuration:
Input
2% TAPcoupler
Insulator
Optical aplifier
Insulator0.1% TAPcoupler
Output
Back reflection Output monitor
Laser pump
Fig. 75: Scheme of the workplace configuration.
The workplace consists of optical source LS 420, two TAP Couplers, amplifier EDFA
CLA-P(B)-01F and optical power meter PM 420.
13.2.1 Optical source LS 420
Optical source LS 420 is conform to necessary technical requirements for
operational devices. Rechargeable battery secures long operational time with the minimal
lifetime period of 5 years. These sources offer the choice among six operational wavelengths
of 650, 850, 1310, 1490, 1550 and 1625 nm. It works with CW modulation, which enables to
create up to seven combinations of wavelengths. These sources are applicable in
measurements of optical fibres as well as testing fibre continuity.
13.2.2 Optical Power Meter PM 420
This device is designed for the measurement of absolute or relative optical power in
optical networks. Internal memory is available in this measuring equipment, which enables
saving as well as data uploading up to 512 measurements including number of fibre,
wavelength and other parameters. Internal SW enables memory download and generation of
test reports.
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13.2.3 SFT-TAP coupler
This splitter is designed for monitoring in optical networks by operation. It enables
bidirectional or unidirectional monitoring of optical fibre on whole span of CWDM
wavelengths. Splitting ratio may be chosen from 1:99 up to 10:90. It is independent of
protocol as well as bit rate.
SFT-TAP-AT
AT
B
Fig. 76: Block diagram of the splitter.
13.3 Amplifier EDFA CLA-P(B)-01F
This amplifier is of the family low-nose, high-powered amplifiers designed for the
realisation of the “key-solution” in optical networks. It consists of two EDFA amplifiers –
pre-amplifier Pre AMP and power amplifier Booster.
Characteristic features:
Low noise,
Broad wave span,
Low energy consumption,
Microcontroller enabling remote control,
Interface RS232, Ethernet, USB, optionally GSM/GPRS and WiFi,
Uni channel and multi channel,
LED signalling.
Transmission media – laboratories 85
The amplifier may be switched among following modes:
AGC – Automatic Gain Control,
APC – Automatic Power Control,
ACC - Automatic pumping Current Control.
The control is based on Linux. Remote control CLA proceeds via SSH or SNMP.
Critical reports are sent by e-mail. There are supervised all important parameters as input and
output power, the current of laser diodes and others.
The control is based on Linux. Remote control CLA proceeds via SSH or SNMP.
Critical reports are sent by e-mail. There are supervised all important parameters as input and
output power, the current of laser diodes and others.
13.4 Instructions RS232
The instruction consists of the name of instruction, one or more disjunctive marks,
which isolate chosen parameters and of terminating mark: ACC [1,2] value . Important
instructions are in the Tab. 9.
Tab. 9: Table of command for settings.
AM Apmlifier mode G, P, C, O
AGC Gain control 1, 2, value
APC Power control 1, 2, value
ACC Circuit of pump control 1, 2, value
Example: AGC 1 18. The gain for level 1 in the mode AGC OADM will be set on 18
dB.
Example of the instruction for report statement about adjustment of individual
parameters: AGC
Response: AGC 1: 30 dB
AGC 2: 17 dB
13.5 Procedure
Become familiar to the amplifier EDFA CLA-P(B)-01F. Connect it into CLI (Command
Line Interface). In case of direct connection of monitor and keyboard to the amplifier, there is
possible to survey and edit CLI directly. In case of direct connection of amplifier to the
computer via serial port, let connect to CLI by hyperterminal.
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For more details see in Tab. 10.
Tab. 10: Setting of Hyperterminal.
Parameter Setting Unit
Baud Rate 9 600 Baud
Data Bits 8 Bits
Parity None
Stop Bits 1 bit
Flow control None
Determine its actual setting in the modes AGC, APC and ACC by using instructions.
Connect optical source LS 420 to measured configuration. Attach power meter PM 420
to the monitoring output of the splitter SFT-TAP. Set the combination of wavelengths 1300,
1550 and 1650 nm. Read absolute and relative powers of optical signal on the display for
individual wavelengths. Add measured values into the table. Then disconnect the measuring
device from the input splitter.
Connect measuring device to the monitoring output of output splitter. Change modes of
amplifier among AGC, APC and ACC using instructions. Set the parameters of amplifier in
individual modes and read values of the optical power in the display. Fill in measured values
into the Tab. 11.
13.5.1 Working-out of protocol
Tab. 11: The table of measured values.
Measurement of input power of wavelengths
Wavelength Relative power Absolute power
nm dBm dBm
1 300
1 550
1 650
Measurement of output powers
AGC APC ACC
Adjusted
gain
Measured
gain
Adjusted
output
power
Measured
gain
Adjusted
current
Measured
gain
dB dBm mW dBm mA dBm
Transmission media – laboratories 87
13.6 Conclusion
After all measuremnt you need to do a protocol.
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14 Measurement of chromatic dispersion
14.1 Method of phase shift and differential phase shift
By the ITU-T G.650 recommendation, the method of phase shift is given as the
reference method for measuring the chromatic dispersion of optical fibres. A modulated
radiation source of several wavelengths is used for the measurement. At the receiver side the
instrument used for the detection of the test signal being received is an instrument for phase
measurement such as the vector-voltmeter. The output phase measured is compared with the
input phase of the signal and their difference is used to determine the change in the signal
phase after the passage through the optical cable route being measured. A disadvantage of this
method can be seen in the necessity to use a different fibre in the cable as the reference route
in Fig. 77Chyba! Nenalezen zdroj odkazů., via which information about the input phase is
transmitted from the transmitter to the receiver.
Fig. 77: Method of phase shift.
14.2 Method of delayed pulses in the time domain
This method consists in transmitting optical pulses in/on different wavelengths but with
a precisely determined pulse magnitude and spacing. A comparison of the spacing of input
pulses with that of the pulses received on the output is used to determine the delay due to
chromatic dispersion. The connection which is similar to that in the preceding methods but
without reference fibre is in Fig. 78.
Fig. 78: Method of delayed pulses.
Fig. 79 gives an example of the connection of generator of optical pulses with given
time spacing. A cascade of Bragg gratings serves as the monochromator. The pulse generator
modulates the radiation of a wide-spectrum source such as LED diode. From the coming
pulse the diode reflects components of selected wavelengths with certain time spacing back
into the route being measured. The base is a cascade of Bragg gratings, which is formed by
different gratings with sections of optical fibre between the gratings. Each of these gratings
reflects radiation of different wavelength. The result of this connection is that a sequence of
pulses of different wavelengths with given time spacing comes into the measured fibre of the
route. After the passage through the route the time spacing of pulses changes due to the effect
of chromatic dispersion. By comparing the spacing on the input with that on the output of the
route being measured the values of delay due to chromatic dispersion are established.
Transmission media – laboratories 89
Fig. 79: Method of delayed pulse, with a cascade of Bragg gratings.
An example of the resultant measurement of optical route using a chromatic dispersion
meter is given in Fig. 80.
Fig. 80: Waveform of chromatic dispersion.
14.3 Measurement of polarization mode dispersion (PMD)
The interferometric method of measuring PMD is based on the interference (wave
addition) of low-coherence (coherence – spectral purity) of optical radiation. The block
representation of the method is shown in Fig. 81. An interferometer is placed on the output of
the optical route being measured, which separates the radiation into two branches. A fix
mirror is in one branch and a movable mirror in the other branch. The movable mirror
changes the phase shift between received signals of the two branches and, with the aid of
interference, the delay due to PMD is shown on the detector.
Fig. 81: Measurement of PMD by interferometric method.
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A typical example of the plot obtained by this method is given in Fig. 82.
Fig. 82: Example of PMD plot of optical fibre, obtained by interferometric method.
In the bottom of the figure the interferogram shows the correlation functions of two
mutually perpendicular polarization planes. The pronounced peak is the autocorrelation
function of the measuring signal itself, which depends on the shape of its spectrum.
This method is sometimes referred to as TINTY (Traditional Interferometry Analysis);
the more recent method GINTY (General Interferometric Analysis) suppresses the effect of
autocorrelation peak. In this method the resultant signal, which contains optical radiation from
both branches of the interferometer, is again divided by polarization light into two mutually
perpendicularly polarized components, with each of them incident on a separate detector.
Interference occurs on the detectors and the two correlation components are expressed. By
subtracting the interferogram we obtain the mutual correlation, and by adding up we obtain
the pure autocorrelation. This method enables measuring also routes with EDFA amplifiers. It
is a quick method, it is not necessary to measure individual route sections separately.
14.3.1 Method of scanning the wavelength
The measurement of PMD by the method of scanning the wavelength is based on the
principle of measuring the optical power passing along the measured optical route in
dependence on the wavelength. The block representation is given in Fig. 83. The radiation
source can be a tunable laser or a wide-spectrum LED diode.
Fig. 83: Method of scanning the wavelength.
Transmission media – laboratories 91
Compared with the preceding method, this one is slower and the fibre is susceptible to
vibration.
14.3.2 Method of POTDR
This method of measuring combines the measurement of PMD with the method of
optical reflectometry. This means that it enables measuring the whole route and determining a
possible critical section with increased PMD value, which can be subsequently replaced and
thus the prescribed values of PMD are achieved (POTDR – Polarization Optical Time
Domain Reflectometry).
The POTDR method makes partial use of the classical OTDR method of backscatter
measurement. It operates on a similar principle but the POTDR method differs in that the
reflectogram is evaluated via polarization. The principle of the method: we try to transmit into
the route fibre a measuring signal in the form of a train of pulses and from the backscattered
radiation (effect of Rayleigh‟s backscatter) we read information about the PMD of individual
sites on the route fibre. The dependence of the PMD of route fibre can be expressed by the
relation
PMD ,h
where β is the double refraction in the fibre (ps·km-1
), i.e. the difference in the
propagation speed of the two polarization modes mentioned, l is the fibre length, and h gives
the coupling length at which there is a significant change in the axis (shape) of the double
refraction in the fibre, which leads to a marked exchange of energy between the polarization
modes. PMD increases with the magnitude of double refraction in the fibre, with the fibre
length and with the coupling length. With increasing length of the fibre and thus a smaller
energy exchange between the two modes, which propagate at different speeds, PMD will play
a greater role. For the longitudinal analysis of PMD we need to obtain from the backscattered
radiation from the fibre also information about its local double refraction and coupling length.
To establish this information we send short pulses of polarized optical radiation into the fibre.
This purpose is served by the DOP (Degree of Polarization) method, which establishes the
results from backscattered radiation. We monitor the degree of polarization. The schematic of
this connection is shown in Fig. 84. The radiation source is a DFB laser of a very narrow
spectrum, which is different from the current OTDR meters. It is used here in order to prevent
signal depolarization in the fibre because the signal might propagate via several wavelengths.
In this case, double refraction of the fibre would cause for different wavelengths different
changes in the state of polarization SOP and thus depolarize the signal. Such a (undesirable)
mechanism of depolarization must be suppressed by narrow-spectrum radiation source.
Polarized output radiation from the DFB laser is coupled to the fibre being measured. For
backscattered radiation from individual sites of the route fibre the DOP is analyzed using
a polarimeter and an OTDR detector.
Fig. 84: Measurement of PMD by the method of DOP analysis.
92 FEKT VUT v Brně
A strong double refraction in fibre β brings about a quick rotation of polarization state,
which leads to the depolarization of radiation within the measuring pulse, thus contributing to
a reduction of the degree of polarization. A weak double refraction of fibre β will result in
a high DOP measured, and vice versa. But the DOP will also depend on intermodal coupling
(coupling length h). With some simplification, the situation can thus be divided into three
groups:
1. fibres with weak double refraction (small β) – the DOP will be high (up to 1),
irrespective of intermodal coupling. In practice, these are optical fibres with
a small PMD value,
2. fibres with a strong double refraction and strong intermodal coupling (large β and
short coupling length h) – the DOP will be low due to the strong double refraction
(for a backscattered signal it will approximate the value 1/3) and will change
rapidly due to the strong intermodal coupling. In practice, these are optical fibres
with average PMD values,
3. fibres with a strong double refraction and weak intermodal coupling (large β and
long coupling length h) – here, in addition to β and h, it also depends on the
mutual position of SOP of polarization and on the shape of double refraction in
the fibre. The DOP can then fluctuate between low and high values but it will
change only slowly. In practice these are optical fibres with high PMD values.
It follows from the above that not only the DOP value itself but also the speed of DOP
change is important. The measuring instrument then performs an analysis of the results
measured. Because of the considerable speed of DOP change it is first necessary to determine
the average DOP value from several tens of samples. The measuring instrument then performs
measurement for two states of input polarization, which yields two measurement results: DOP
and DOPC (complementary), from which the DOPGEO parameter is calculated using the
relation
,DOPDOPDOP 2
C
2
GEO
which provides information about the real DOP value of radiation backscattered from
a given section of route fibre. The measuring instrument has a parameter hDOP for
monitoring the speed of DOP changes. This parameter is equal to the fibre length along which
the DOP changes markedly – the quicker the DOP changes, the smaller the hDOP parameter.
From an analysis of the DOP parameter of individual sections of optical fibre of route the
following can be concluded:
- on sections with high DOPGEO value the PMD value will be low because the there
is a small double refraction of fibre here,
- on sections with varying or low DOPGEO value and thus with a possible larger
double refraction of fibre, and on the assumption that the hDOP parameter is
small, then the PMD value will be low because there is a strong intermodal
coupling in the fibre,
- average, then the PMD value will be average,
- large, then the PMD value will be high because there is a weak intermodal
coupling in the fibre.
After the evaluation of the values measured, the polarization reflectogram (POTDR)
will display several measurement results and graphs, above all the POTDR reflectogram,
where individual cable sections and route sites can be followed. The longitudinal resolution
Transmission media – laboratories 93
power of this method is of the order of hundreds of metres; routes of tens of km in length can
be measured. In the POTDR reflectogram display, sites with low, increased and high PMD
values are presented in colour.
The second graphical representation that can be displayed is the waveform of the hDOP
function, which is yet another important curve of the overall evaluation of PMD. In addition
to the hDOP function, horizontal straight lines are also displayed, which mark the limit values
of PMD. These limit values can be set by the users themselves.
The third (and most important) graphical representation is the DOP curve. In one
graphical display several parameters can be seen in different colours – the DOP, DOPC and
DOPGEO curves. The DOPGEO curve has the greatest informative value regarding the actual
state of PMD.
The last possibility is the graphical representation of the curves (for both input states of
POTDR polarization) of normalized Stokes parameters S1 to S3.
On top of all this, information about splice and connector sites can, of course, also be
read from the final measurement protocol.
It is necessary to stress that the measurement of POTDR does not replace the total
absolute values of PMD delay, which are measured using one of the interferometric methods,
it only complements them.
94 FEKT VUT v Brně
15 References
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[3] Fiber optics handbook: fiber, devices, and systems for optical communications. Editor
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[8] IGA, Kenʾichi a Y KOKUBUN. Encyclopedic handbook of integrated optics. Boca
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[9] IIZUKA, Keigo. Engineering optics. 3rd ed. New York: Springer, c2008, xx, 525 p.
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[10] AGRAWAL, Govind P. Fiber-optic communication systems. 3rd ed. New York: Wiley-
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[11] OKAMOTO, Katsunari. Fundamentals of optical waveguides. 2nd ed. Burlington:
Academic Press, 2006, xvi, 561 s. ISBN 978-0-12-525096-2.