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Transmitters, Receivers

Optical WirelessCommunications

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Optical Communication Systems

Communication systems with light as the carrierand optical fiberascommunication medium

Optical fiber is used to contain and guide light waves

Typically made of glass or plastic

Propagation of light in atmosphere is impractical

This is similar to cable guiding electromagnetic waves

Capacity comparison

Microwave at 10 GHz

Light at 100 Tera Hz (1014)

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Optical Fiber Construction

Corethin glass center of the fiber

where light travels.

Claddingouter optical material

surrounding the core Buffer Coatingplastic

coating that protects

the fiber.

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Types Of Optical Fiber

Single-mode step-index Fiber

Multimode step-index Fiber

Multimode graded-index Fiber

n1core

n2cladding

noair

n2cladding

n1core

Variable

n

noair

Light

ray

Index profile

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Optical Transmitter LED

Laser

Lamp

Optical ReceiverDetection Techniques:

Direct Detection

Coherent Detection

Photodetectors

p-i-n

Avalanche Photo Diode (APD)

Photo Multiplier Tube (PMT)

Modulation Techniques

Transmitters/Receivers

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LED

Semiconductor device

Medium modulation speed

Incoherent output light

Mainly used for short range FSO systems (shorter than 1 km)

Laser

Highly directional beam profile Used for long range FSO systems

High modulation speed

Coherent output light

Lamp

Lower efficiency compared to LED and laser Lower cost

Low modulation speed

Incoherent output light

Provides higher power

Optical Transmitters

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A semiconductor pn junction device that gives off spontaneous

optical radiation when subjected to electronic excitation

The electro-optic conversion process is fairly efficient, thus resulting

in very little heat compared to incandescent lights

Mainly used for short-range FSO systems (shorter than 1 km)

Ultraviolet communications

Indoor FSO systems

Optical Transmitters: LED

Illustration of the radiated

optical power against driving

current of an LED

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LED Types

Optical Transmitters: LED

Dome LED

Edge-Emitting LED

Planar LED

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Laser: light amplification by stimulated emitted radiation

Has highly directional beam profile

Is used for long range FSO systems

Has narrow spectral width compared to LED

Optical Transmitters: Laser

Laser output power

against drive current plot

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Laser Types

Optical Transmitters: Laser

Fabry-Perot Laser

Distributed Feedback Laser Vertical-cavity surface-

emitting Laser (VCSEL)

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Hecht: Understanding Fiber

Optics. (C) 2006 PearsonEducation, Upper Saddle River,

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Distributed Feedback Laser

Light scatters

from grating in

laser substrate

Limits

oscillation to

one wavelength

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Hecht: Understanding Fiber

Optics. (C) 2006 PearsonEducation, Upper Saddle River,

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Distributed Bragg reflection

Grating etched insubstrate

In plane of activelayer, but outsidelaser zone

Feedback limits

oscillation to onewavelength Light scattered

From here

into active layer

Semiconductor covers

grating layer

In unpumped region

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Can be used in FSO communications, not in fiber optics

Wideband and continuous spectrum Have very high power, but undirected

The electro-optic process is inefficient, and huge amount of

energy is dissipated as heat (causes high temperature in lamps) Has very low modulation bandwidth

Divided as follows

Carbon button lamp Halogen lamps

Globar

Nernst lamp

Optical Transmitters: Lamp

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Optical Receivers

The purpose of the receiver is:

To convert the optical signal to electrical domain Recover data

Direct-Detection Receiver:

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Coherent-Detection Receiver

For detecting weak signal, coherent detection scheme is applied

where the signal is mixed with a single-frequency strong localoscillator signal.

The mixing process converts the weak signal to an intermediate

frequency (IF) in the RF for improved detection and processing.

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Optical Receivers

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Photodetectors

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A square-law optoelectronic transducer that generates an electrical signal

proportional to the square of the instantaneous optical field incident on its

surface

The ratio of the number of electronhole (eh) pairs generated by a

photodetector to the incident photons in a given time is termed the quantum

efficiency,

Dark current: the current through the photodiode in the absence of light

Noise-equivalent power (NEP): the minimum input optical power to

generate photocurrent equal to the root mean square (RMS) noise current ina 1 Hz bandwidth

Responsivity: photocurrent generated per unit incident optical power

(W/A)

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Photodetectors

p-i-nphotodetector

Consists ofp- and n-type semiconductor materials separated by a very

lightly n-doped intrinsic region

In normal operating conditions, a sufficiently large reverse bias

voltage is applied across the device

The reverse bias ensures that the intrinsic region is depleted of any

charge carriers

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Photodetectors

Avalanche Photo-Diode (APD)

provides an inherent current gain through the process called repeated

electron

This culminates in increased sensitivity since the photocurrent is now

multiplied before encountering the thermal noise associated with the

receiver circuit

Multiplication (or gain) factor:

:the average value of the total output current

= : the primary unmultipliedphotocurrent

Typical gain values lie in the range 50300

Excess noise factor:

= + 2 1

1

: the ratio of the hole impact ionization

rate to that of electrons

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AVALANCHE Photodiodes.

An APD internally amplifies the photocurrentby an avalanche process when a large reverse-

bias voltage is applied across the active

region.

The gain of the APD can be changed by

changing the reverse-bias voltage.

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AVALANCHE Photodiode.