1 chapter 3 components couplers, isolators and circulators, multiplexers and filters, optical...
TRANSCRIPT
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Chapter 3 Components
Couplers, Isolators and Circulators, Multiplexers and
Filters, Optical Amplifiers, Transmitters, Detectors switches, Wavelength
converters.
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3.1 Couplers [ wavelength independent, wavelength
selective for 1.31/1.55 multiplexing]
α : coupling ratio3dB couple α= 1/2α = 0.95 (for monitoring)
1 α
1-α
3
Def: excess loss: the loss of the device above the fundamental loss introduced by the coupling ratio α Example: A 3dB coupler may have 0.2dB excess loss
For multiplexing 1310nm
1550nm
1310nm1550nm
For EDFA 1550nm 1550nm
980nmor 1480nm
980nmor 1480nm
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3.1.1 Principle of Operation
E: electrical fieldS-parametersFor lossless couplers
12
21
01 1
02 2
1 11 1
2 22 2
( ) ( )cos( ) sin( )(3.1)
( ) sin( ) cos( ) ( )ij
i
E f E fk i ke
E f i k k E f
b s s a
b s s a
1
:
: ,
, ...
the coupling length
k coupling coefficient depending on width
shape of waveguides n distance
a1→
a2→
→ b1
→ b2
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The power transfer function2
02
211
212
( )
: , :
( ) cos ( )(3.2)
( ) sin ( )
jij
ii
ET f
E
i input j output
T f kf
T f kf
11 12
2 2
3
1( ) ( )
21
sin ( ) cos ( )2
(2 1) 04
For a dB coupler
T f T f
kf kf
k n n
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3.1.2 Conservation of Energy (S-parameter)
11 1201 1
02 221 22
(3.3)i
i
s sE E
E Es s
11 12
21 22
0 01 02
1 2
0
2 20 0 1 2
:
( )
( )
ij
T
Ti i i
i
T
i i
The scattering matrix is
s sS s complex
s s
Denote E E E
E E E
E SE
The sum of input power is proportional to
E E E E
complex conjugate
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Similarly the sum of output power is proportional to
If it is lossless
This relation holds for arbitrary
Eq(3.4) can be extended to any number of ports
2 20 0 01 02
TE E E E
����������������������������
0 0 ( ) ( )
( )
T Ti i
T Ti i
Ti i
E E SE SE
E S S E
E E
��������������������������������������������������������
����������������������������
����������������������������
iE��������������
(3.4)
:
TS S I
I identity matrix
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For a 2 x 2 symmetrical coupler21 12 22 11
2 21 (3.5)
0 (3.6)
T
s s a s s b
S S I
a b
ab ba
( ) ( )
cos( ) sin( ) (3.7)
cos( ) , sin( )
0
cos( )sin( ) 0
cos( ) 0
2 1
2
a b
a b b a
i i
i i
a b
a b
a x b x
let a x e b x e
ab ba
x x e e
k
lossless combination is impossible
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3.2 Isolators and Circulators (nonreciprocal devices)
Isolators are for transmitter, circulators are for add and drop or others.
The insertion loss should be small ~ 1dBA circulator is similar to an isolator except it has
multiple ports.
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3.2.1 Principle of Operation of an Isolator
SOP= state of Polarization
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A spatial walk-off polarized splits the signal into
two orthogonally polarized components.
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3.3 Multiplexer and FiltersMultiplexers and filters are for WDM, add/drop. WXC,
13
Dynamic WXCs use optical switches and mux/demux.
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The desired characteristics of filters
1. Low insertion loss
2. Polarization-independent loss
3. Low temperature coefficient
4. Reasonable broad passbands
5. Sharp passband skirts
6. Low cost
a. integrated-optic (may be polarization dependent)
b. all-fiber devices
passband skirt
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3.3.1 GratingsAny device whose operation involves interference among
multiple optical signals originating from the same source but with different relative phase shifts. An exception is a device where the multiple optical signals are generated by repeated traversals of a single cavity (etalons).
F-P
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Principle of OperationThe pitch of the grating (distance between adjacent slits)=a
Assuming plane wave is incident at angle
: diffraction angle
The slits are small compared to λ,
phase changes across a slit is negligible
id
19
For construction interference at λ occurs at the image plane if
sin sin
sin sin
i d
i d
AB CD a a
a
sin sin
:
i da m
m the order of the grating
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The energy at a single λ is distributed over all the discrete angles that satisfy (3.9).
For WDM only light of a certain order m will be collected, the remaining energy is lost.
m=0 has most energy θi= θd
The wavelengths are not separated.blazed reflection grating maximize the light e
nergy at α
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3.3.2 Diffraction PatternRelax the constrain a <<λ, the phase change
across the slit is not negligible, consider a slit of length from
The relative phase shift of the diffracted light from y at an angle θ compared to that from y=0 is given by
,2 2w wy to w
(
2w
2w
y
sin( ) 2
yy
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The amplitude A(θ) at θ (Ref: Optics, page401)
Fourier Transform of rectangular slit.
For any diffracting aperture f(y)
2
2
2
2
0exp ( )
0exp 2 (sin )
sin( sin )0(3.10)
sin
w
w
w
w
AA i y dy
w
A yi dyw
wA
w
0 ( )exp 2 (sin ) (3.11)yA A f y i dy
2where A intensity distribution
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1= w
( ) 1
( )0
( )
( ) 0.5 2 2
0exp 2 (sin )2 22
0exp 2 (sin ) exp 2 (s22
If f y dy normalized
For a rectangular slit
y wf y
otherwise
For a pair of narrow slits infinite long with spacing d
d df y y y
A yd dA y y i dy
A di i
=
in ) 2
0 cos( sin )
d
dA
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3.3.3 Bragg Gratings (BGs)
BGs are widely used in WDM
BGs: any periodic perturbation in the
propagating medium. (periodic
variation of n)
(Fiber BGs are written by UV)
BGs can also be formed by acoustic waves.
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Principle of OperationConsider two waves with β0 and β1 propagating in opposit
e directions.
If the Bragg phase-matching condition is satisfied
when Λ= the period of the grating
Consider β1 wave propagating from left to right,
Then the energy from this wave is coupled onto a scattered wave traveling from right to left at the same wavelength provided.
0 0 0
0 0
0
0 0
2( ) 2
2 /
:
:
2
eff
eff
eff
let n
wavelength of the incident wave
n effective refractive index
n Bragg wavelength
0 1
2
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These reflections add in phase, when the path length in λ0 each period is equal to half the incident wavelength λ0
02effn Bragg condition
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Δλ: detuning from λ0
Δ is inversely proportional to the length of the grating
Apodized grating: the refractive index change is made small toward the edges of the grating
=> increasing the main lobe width
The index distribution over the length of BG is analogous to the grating aperture in sect3.3.2.
The side lobes arise due to the abrupt start and end of the grating, which result in a sinc(.) behavior for the side lobes.
Apodization is similar to pulse shaping to reduce the side lobes of signal spectrum.
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3.3.4 Fiber Gratings (FGs)A. Useful for filter, add/drop compensating dispersionB. Advantages: a. low loss (0.1dB) b. ease of coupling c. polarization insensitivity d. low temperature coefficient e. simple packaging f. extremely low costC. Made from photosensitive fiber (Ge-doped) UV intensity ↑ n↑] change of n ~ 10-4
D. Two kind of FGs a. short period (Bragg Grating Λ~ 0.5μm) b. long period (Λ~ 100+μm – 1000+μm)
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Fiber Bragg Gratings (FBG)
A. extremely low loss ~ 0.1dB
B. high wavelength accuracy (±0.05nm)
C. high crosstalk suppression (Fig 3.8) (40dB)
D. flat tops
E. typical temperature coefficient ~1.25x10-
2nm/℃ For passive temperature-compensated ~
0.07x10-2nm/℃
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Long-Period Fiber Grating (a few intermeters)
Useful for EDFA gain (equalization)
They may be cascaded to obtain the desired profile.
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Principle of OperationThe propagating mode in core couples onto the
modes in the cladding => induce loss
For a given λ
coupling occurs depending on Λ
β= propagation constant of the core mode
: propagation constant of the path order cladding mode
The phase matching condition
1Pc
2Pc
PcBecause is very small
long a few hundred m
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Let and be the refractive indices of the core and the path-order cladding modes
effn peffn
2
22
( )
eff
P Pc eff neff
Peff neff
n
n n
n n
, ,
.
Peff neffGiven n n obtain
It is a wavelength dependent loss element
core cladding
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3.3.5 Fabry-Perot Filters
This filter is called Fabry-Perot interferometer or etalon.
Principle of Operation The wavelengths for which the cavity length is an integral
multiple of half the wavelength in the cavity are called resonant wavelengths.
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A round trip through the cavity is an integral multiple of the wavelength.
The light waves add in phase.
Assume r1=r2 t1=t2
The reflectance R=r1r2
A: absorption loss of mirror
T=t1t2=transmission:
:
nOne way delay en reflective index
r1 r3t1
t2
iE
(1 )iiE e A R
3 (1 )iiE e R A R
5 2(1 )iiE e R A R
l
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2 2 40 (1 ) 1 ...i i i
iE E A R e Re R e
=E
E
2
2
1(1 )
1
(1 )
1
ii i
ii
i
A R eRe
A R e
Re
2
2
2
=+ -2
+ -2
- +4
2 2
02
2
2
2 2 2
2
2
2
2
2
2
(1 )( )
1
1
1 cos2 sin2
(1 )
(1 cos2 ) sin
(1 )cos2 1 2sin2
1 cos2
(1 )
1 4 sin
(1 )
(1 ) sin ( )
11
21
1
i
FP ii
E A R eT f
E Re
A R
R iR
A R
R R
A R
R R
A R
R R R
A R
R R
AR
R
2
2
2
2sin
2: ,
11
( ) (3.12)2
1 sin(2 )1
( ) sin2 0 / 2
FP
FP
nR
nnone way delay c
AR
T fR
fR
For maximam T f f f k
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A=0, R=0.75, 0.9 and 0.99TFP (f) is periodic function with period FSRWhere FSR: free spectral range = The spectral range between two successive passband = 1/2τ
1( ' ) , 1 '
2 2
kf f k FSR f f
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1: (3.12) 0
2sin2 ' 1, 3
11
sin2 '2
1 1 sin2 ' 2 ' , '
1' , 2 '
4
1 1(3.13)
2 12
Define finesse
F
FSRFWHM
R
Rproof Assume A
Rf for dB point
RR
fR
R f f f
Rf FWHM f
R
FSR RRF
FWHM RR
is the smallest value satisfied the condition
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Tunability
1. change cavity length
2. change refractive index n
Recall
The wave with frequency will be selected.
1. mechanical tuning
2. piezoelectric tuning
=> thermal instability, hysteresis
0 :2
kf k positive integer
0f
nc
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3.3.6 Multilayer Dielectric Thin-Film FiltersA thin-film resonant multicavity filter (TFMF) consist of
two or more cavitied.
Advantages: flat top, sharp skirt, low loss, insensitive to the polarization
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3.3.7 Mach-Zehnder Interferometers (MZI)
Usage: filter, MUX/DEMUX, modulator, switch
Problems:
a. wavelength drift caused by aging or temperature variation
b. not exact 50:50
c. not flat top passbands
Change temperature (or refractive index) of one arm=> tuning
46
Principle of Operation
01 1
02 2
2
01
02
( ) ( )cos( ) sin( )(3.1)
( ) sin( ) cos( ) ( )
( ) 0
( ) cos( )
( ) sin( )
:
ii
i
i
i
i
Recall
E f E fk i ke
E f i k k E f
let E f for DEMUX
E f e k
E f e i k phase lag due to i
let L length difference in lower arm
L
another phase lag
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At the upper output .
The signal all through the upper arm as reference.
The signal through the lower arm and the upper output has phase lag
At the lower output the phase difference
2 2L L
2 2L L
through low arm由第一個 3dB coupler產生 delayπ/2
through upper arm由第二個 coupler 到第二個 output產生 delayπ/2 所以互相 cancel
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(2 1)
(2 1) 2( 2)
, (2 1) (2 1)
( )
(2 )
,
If L k k is odd k n
n n in phase
The signals at the upper arm add in phase at upper arm
At the lower output the phase difference is n L n
out of phase no signal
If L n
At the upper arm output the
2
11
212
(2 ) (2 1)
(2 )
sin( ) 2(3.14)
( ) cos 2
1: cos( ) sin( )
phase difference is
n n out of phase no signal
At the lower arm output
L n signals add in phase
The transfer function of MZI is
LT f
T f L
hint k k
1 01
2 02
2
' '
' '
2
( ) ( )1 0
0( ) ( )
( ) 0 (3.14)
i
i
i
i L
i L
and
The input and output relation of the middle section is
E f E fe
eE f E f
let E f and Multiply three matrices
49
consider K MZI interconnected
The path length difference for the kth MZI is assumed to be 12kL
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MZI can be used as a 1x2 demultiplexer or multiplexer
λ1 λ2 chosen to be coincide with the peaks or troughs of the transfer function
If , and mi is odd, say mi=1 output 1 has signal, output 2 has no signal,
If and mi is even, output 1 has no signal.
λ1 λ2 λ1
λ2
MZI
2 2
2
2, :
2 2
sin sin2 2
eff
effi i i ii i
eff i eff
i
n
nm mlet L L m m integer
n n
mL
1
2 eff
i
Ln
m
2
2 eff
i
Ln
m
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3.3.8 Array wavelength Grating (AWG)
Usage: a. nx1 multiplexer b. 1xn demultiplexer c. crossconnect (wavelengths and FSR must be chosen)Advantages: low loss, flat passband, ease to realized on a
integrated-optic substrate (silicon), the waveguides are silica. Ge-doped silica, or SiO2-Ta2O5
Because the temperature coefficient = 0.01nm/ is large℃Temperature control may be needed.
目前除了用 Rowland circle 之外尚可用multimode interference (MMI) 做 coupler
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Principle of OperationLet number of inputs and outputs be n, and the numbe
rs of inputs and outputs of the couplers be nxm and mxn
ΔL=length difference between two adjacent waveguides. = difference in distance between input i and array waveguide k =difference in distance between array waveguide k and output j
inikdoutk jd
n
ik
m m
n
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The relative phase
(3.15)1 2 1
2 in outijk ik kjn d n k L n d
input outputthrough k
k= 1. 2. …m
= +
1 2 1 1 1
11 2 1
2
2 2(3.16)
in in inik i i
out out outkj j j
in out in outijk i j i j
in out in outi j i j
If we design that
d d k
d d k
Then n d n k L n d n k n k
n kd d n n L n
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Rowland circle constructiongrating circle
Rowland
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3.16)
1 2 1
1 2 1
1
,
,
22
(
( 1) '
'
j
in outi j j
jj
j
j
in outi j j
j
ini
If appears at input i and
n n L n p p is integer
kp kp
will add in phase at output j prob
will be present at output j
If n n L n p
will be also present at output j
let n
2 1
2
( 1)
'
'
outj
pc p cn L n p
f f
cFSR f f
n L
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3.3.9 Acoustic-Optic tunable Filter (AOTF)
polarization-dependent, polarization-independent.
59
Principle of OperationAs Fig 3.27 AOTF is constructed from a birefringent m
aterial and only supporting the lowest-order TE and TM modes.
If an acoustic wave is launched, the n varies to form gratings.
The Bragg condition is satisfied
TE mode is converted to TM mode.
For LiNbO3, |nTE-nTM|=0.07=Δn. at 1.55μm λ=ΛΔn (3.18)At 170MHz Λ=22μm, acoustic wavelength
1(3.17)TM TZn n
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The transfer function is
where Δλ=λ-λ0 λ0 satisfies (3.17) Δ=λ0
2/lΔn l : the length of acoustic-optic interaction FWHM bandwidth=0.8Δ
2 2
2
sin 1 (2 )2
( )1 (2 )
T
-10dB down is not enough => cross talk
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Disadvantages: high loss, large crosstalk, bulky
wide passband> 100GHz
dynamic crossconnect
response time ~ millisecond
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3.3.10 High Channel Count Multiplexer Architectures
A. Serial (only for small number of ports)
不同 channel 有不同 insertion loss
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B. Single stage (AWG)
最好的選擇
65
C. Multistage banding
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D. Multistage Interleaving
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3. 4 Optical Amplifiers
Advantages: transparent to bit rate, pulse format, large bandwidth, high gain
Disadvantages: noise accumulates
A. Erbium-doped fiber amplifiers (EDFA)
B. Raman amplifiers (RA)
C. Semiconductor optical amplifiers (SOA)
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3.4.1 Stimulated Emission (EDFA or SOA)
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Two energy levels E2>E1
hfc= E2-E1, h: Planck's constant= 6.63x10-34JS
(absorption)
E1→E2 excitation (by photons or population inversion)
E2→E1 emission photons
a. stimulated emission
b. spontaneous emission
If emission > absorption => amplification
N1: Population (number of atoms) at E1
N2: population at E2
If N2 > N1, population inversion occurs.
1 2
2 1
rate of E Er
rate of E E
70
3.4.2 Spontaneous Emission
If ASE is very large
=> Saturate the amplifier
E2
E1
noncoherent
hf=E2-E1
amplified spontaneous emission (ASE)(noise)
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3.4.3 EDAF
Erbium fiber = Er3+ doped silica fiber
Pumping wavelength = 980nm or 1480nmAdvantages(1) Availability of high power pump lasers(2) All fiber device, polarization independent, ease to
couple, reliable(3) Simple(4) Less crosstalk
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Principle of Operation
73
Stark splitting : an isolated ion of erbium is split into multiple energy levels.
Each stark splitting level is spread into a band.
Thermalization : the erbium ions are distributed in the various levels within the band.
Capable of amplifying several wavelengths simultaneously.
page 39, c-band from 1530~1565nm
2 1
32 3 2
21 2 1
2
1 sec
10 sec
, 980
1520 1570 50c
hfc E E
E E
E E m
atoms stay at E longer nm pump is usable
nm f nm BW nm
74
When 980nm pump is used
τ32≈ 1μsec << τ21
We have population inverse between E2 and E1
We can amplify 1530-1570nm signalsWhen 1480nmpump is used the absorption from
the bottom of E1 to the top of E2
1480nm pump is less efficient Less population inversionHigher noise figure980nm for low noise EDFAHigh power 1480nm pump is available => High output power and pump can be located
remotely
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77
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Gain Flatness
79
80
81
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Multistage Designs
The first stage: high gain, low noise The second stage: high output powerTwo-stage amplifier is more reliable (pump failure)The inserted loss element can be gain compensation,
add/drop or dispersion compensation,L-band EDFA needs high pumping power and produces
high ASE
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3.4.4 Raman Amplifiers (RA)
RA can provide gain about 100nm band (13THz) above the pumping wave λp<λs (Signal Wavelength)
A. RA is a distributed device and can provide gain in different bands
B. No special fibers are neededC. Required high pump power~1wD. Pump power fluctuations induce noise (propagating in same
direction), propagating in opposite directions will have lower noise
E. Crosstalk (modulated signals will deplete the pump power => fluctuation => noise) so, pumping opposite direction will lower the noise. (average out)
F. Another noise is due to Rayleigh scattering of the pumping signal
For example
1550~1600nmsignal
1460~1480nm pump
85
86
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3.4.5 Semiconductor Optical Amplifiers (SOAs)
Amplifier, Switches, wavelength converters
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
3. 5 Transmitters
A transmitter includes a driving circuit and a light source.
The light source can be laser or LED. For WDM systems, a laser needs to have the following important characteristics:
a. Reasonably high power 0~10dBm, low threshold current, high slop efficiency
b. Narrow spectral widthc. Wavelength stability (low aging effect)d. Small chirping (direct modulation)
104
105
106
Lasers Semiconductor lasers, fiber lasers, gas lasers, solid state lasers (Ruby lasers), free electron laser,
107
Principle of Operation (semiconductor laser)
Reference: John Gowar “Optical Communication Systems” PP262~323
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
Longitudinal ModesMultiple-longitudinal mode (MLM) lasers have large spectra
l widths~10nm (Fabry-Perot lasers) =>cause chromatic dispersion
Single—longitudinal mode (SLM) lasers have very narrow spectral widths
Side-mode suppression ratio is an Important parameter for SLM lasers. (~30dB)
126
Distributed-Feedback Lasers (DFB Lasers) Distributed Bragg reflector (DBR) Lasers
The temperature coefficient ~ 0.1nm/ at 1550nm.℃
127
External Cavity Lasers
Grating External Cavity Lasers
128
3.5.3 Tunable lasersTunable lasers are useful to reduce the inventory, (spare p
arts), to reconfigure the network, to be used for optical packet switched networks and for laboratory testing.
Tuning mechanismsa. Injecting current (change n) tuning range ~10~15 nm a
t 1550nmb. Temperature tuning 0.1nm/℃c. Mechanical tuning (wide range but bulky)Desirable propertiesa. Short tuning timeb. Wide tuning range (100nm)c. Stable over its lifetimed. Easily controllable and manufacturable
129
Two-and Three-Section DBR Lasers
Problemsa. Agingb. Temperature changesc. Current recalibrationd. Mode hopping
130
Vertical grating-assisted coupler filter (VGF) LasersThe coupling condition (3.17)
λ=ΛB(n1-n2)
ΛB: The period of the Bragg grating
n1 and n2 are refractive indices of two waveguides.
If n1 changes to n1+Δn11 1 2
1
1
1
1 2
' ( )B
B
B
n n n
n
n
n
n n
131
Sample Grating and Super-Structure Grating DBR lasers
132
Grating Coupled sampled Reflection lasers
133
3.5.4 Direct and External ModulationDirect modulation
Advantage: SimpleDisadvantage: induce chirping
Biasing above the threshold will reduce chirping but decrease the extinction ratio.
134
External Modulationa. Lithium niobate modulator, b. electro-absorption modula
tor
135
: coupling coefficient depending on width of the waveguide, refractive indices, distance of two waveguides
211
212
( ) cos ( )(3.2)
( ) sin ( )
T f k
T f k
k
136
211
212
( ) sin ( 2)(3.14)
( ) cos ( 2)
T f L
T f L
MZI can achieve high extinction ratio ~15 ~20dB with almost on chirping. Polarization control is needed.
137
3.6 Detectors
138
3.6.1 PhotodetectorsPhotons incident on a semiconductor are absorbed by
electrons in the valence band. These are excited into the conduction band and leave holes in the valence band. When a reversed bias voltage is applied, these electron –hole pairs produce photo current.
(3.19)
1.24( . )
cc g
c
g g
fhf eE
fm ev
eE E
139
140
4
(1 ) (3.20)
:
:
:
1 (3.21)
10 , 10 , 0.99
Labs in
in
Labs
in
P e P
P incident power
absorption coefficient
L Thickness of the semiconductor
The efficiency
Pe
P
Example L mcm
141
AW
AW
A= W1.24The quantum efficiency
(=
( / sec) /
/ ( / sec)
)/
P
in
inP
c
c
ph
in ph
ph
in ph
ph
in
The responsivity
IR
P
PI e hf
eR
hf
e
hc
I e electrons
P E photons
I eP E
I hc hcR
P e e
142
143
PIN Photodiodesa. A very lightly doped intrinsic
semiconductor between the p-type and n-type Layers can improve the efficiency. The depletion region extends across the intrinsic layer.
b. If the p-type or n-type layer is transparent the efficiency can be further improved.
144
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146
147
Avalanche Photodiodes (APD)
When the generated election in a very high electric field, it can generate more secondary electron-hole pairs. This process is called avalanche multiplication.
Gm: multiplicative gain
M: multiplication factor (Gm: M-1)
Large Gm will induce large noise.
If Gm→∞, avalanche breakdown occurs.
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3.6.2 Front-End Amplifiersa. High-impedance amplifier
b. Transimpedance amplifier
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3.7 Switches
Important parametersa. Number of portsb. Switching timec. The insertion lossd. The crosstalke. Polarization-dependent lossf. Latching (maintaining its switch state)g. Monitoring capabilityh. Reliability
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3.7.1 Large Optical SwitchedThe main considerationsa. Number of switch elements requiredb. Loss uniformityc. Number of crossoversd. Blocking characteristics blocking and nonblocking (strict sense, wide sens
e, rearrargeable)e. Synchronous or asynchronous
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Crossbar
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Spanke
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3.7.2 Optical Switch Technologies
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MEMS Switches
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Bubble-Based Waveguide Switch
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Liquid Crystal Switches
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A. Thermal-Optic Switches (MZI)
B. Semiconductor Optical Amplifier Switches
C. Large Electronic Switched
a) Single stage
b) Multistage
c) Line rate
d) Total capacity (line rate x number of ports)
e) Circuit switching V.S. packet switching
f) Cross bar V.S. shared memory
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3.8 Wavelength Converters
a. A device converters data from one incoming wavelength to another outgoing wavelength.
b. Used in WDM networks i. input wavelength is not suitable for the networks ii. Improving the wavelength utilization in WDM
networks iii. Converting to suitable outgoing wavelengthsc. Types i. fixed-input, fixed-output ii. Variable-input, fixed-output iii. Fixed-input, variable-output iv. Variable-input, variable-output
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d. Other important characteristics
i. convertion range
ii. Transparent to data rate or modulation format
iii. Loss (efficiency)
iv. Noise, crosstalk
e. Mechanism to achieve wavelength convertion
i. optoelectronic (commercial available)
ii. Optical gating
iii. Interferomatric
iv. Wave mixing
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3.8.1 Optoelectronic Approach (O/E, E/O)
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3.8.2 Optical GratingUsing the principle of cross-gain modulation
in a SOA. (For high input signal power, the carrier will be depleted => less gain for the probe wavelength)
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Disadvantages
i. small extinction ratio
ii. High input signal power to deplete the carriers (simultaneously changes n)
iii. Requiring to filter this high-powered signal
iv. Changing refractive index inducing pulse distortion
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3.8.3 Interferometric Techniques
1
1
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Principle of Operation (cross phase modulation CPM)
When λs presents, the carrier densities (or n) change to induce different phase changes of λp. At the port A, the intensity of λp will be modulated.
i. digital signal only
ii. Higher extinction ratio
iii. Providing reamplification and reshaping
iv. Low input power
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Stage1 samples the dataStage2 reshapes and retimes the data (inverse)Stage3 reamplifies
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3.8.4 Wave Mixing