Optical ReceiversAbdul Rehman

Receiver Components

Most lightwave systems employ the digital format. Figurebelow shows a digital optical receiver

Its components are arranged into three groups, the frontend, the linear channel and data-recovery section

Front End

It consists of photodiode (modeled as current source)followed by a preamplifier

Photodiode converts the optical bit stream into anelectrical time-varying signal

The role of the preamplifier is to amplify the electricalsignal for further processing

∆f

The design of the front end requires trade-off betweenspeed and sensitivity

A high impedance front end with large load resistance RL

Increases input voltage

Reduces thermal noise

Reduces bandwidth

∆f 1

2πRLCT

can’t be considerably less than the bit rate

Equalizer is used to increase the bandwidth, thatattenuates low-frequency components of the signal morethan the high-frequency components, thus increasing theeffective bandwidth

ver sensitivity is not of concern, RL can simply be

decreased to increase the bandwidth thus

aking low-impedance front ends

Transimpedance front end provides high

sensitivity together with large bandwidth

Its dynamic range is also improved compared

with high-impedance front end

These are often used in optical receivers

because of their improved characteristics

as

The load resistance is

connected feedback

resistance around an

inverting amplifier

Thus the bandwidth is

enhanced by a factor of G

compared with high-

impedance front ends

Stability of feedback is

major design issue

Linear ChannelLinear channel in optical receivers consists of high-gain

amp and low pass filter.

An equalizer is sometimes added to just before the

amplifier to correct for the limited bandwidth of the

front end

It has AGC to limit the average output voltage to a

fixed level irrespective of the incident average optical

power at the receiver

The low pass filter shapes the voltage pulse and

reduces noise without intersymbol interference (ISI).

The low pass filter has the smallest bandwidth so the

The combination of preamplifier, main amplifier, and thefilter acts as a linear system hence given the name linearchannel

Equalizer Ip(t)Preamp Amp

Filter

Zin(ω) Gp(ω) GA(ω) HF(ω)

ZT(ω)

Vout(t)

% I

% %

The output voltage can be written as∞

Vout t zT t Ip t ∫zT t −t 'Ip t 'dt ' -------(i)−∞

Where Ip t is the photocurrent generated in response

to the incident optical power (Ip RPin )

In freq domain Vout (ω) = ZT (ω) ⋅ %p (ω) ---------(ii)

Vout (ω) = Vin ω) ⋅ G p (ω) ⋅ G A (ω) ⋅ H F (ω)

Where ZT is the total impedance at the frequency ωand tilde represents the Fourier transform

~ ~~

~~

~

ZT (ω ) = ~

Viin ω ) = Ziin ω ) ⋅ I p ω ) =I p ωYin ω

Where Yin ωis the input admittance

Vout (ω ) =I p (ω)

Yin (ω)⋅G p (ω ) ⋅ GA (ω ) ⋅ H F (ω )

Where G p (ω), GA (ω) and H F () are transfer functions

of the preamplifier, the main amplifier and the filter

Vout (ω ) G p (ω ) ⋅ GA (ω ) ⋅ H F (ω )

I p (ω ) Yin (ω )- - - - - - - -(iii)

~ ~

~ ~ ⋅~~

Vout 0ZT 0⋅I p 0

Vout (ω) ZT (ω) I p (ω)

Vout (0) ZT (0) I p (0)

Hout(ω) HT(ω) Hp(ω)

H out (ω) H T (ω) ⋅H p (ω) - - - - - - - - - -(iv)

Where H T (ω) is the total transfer function

of the linear channel

Data RecoveryThe data-recovery section of optical receivers consists

of a decision circuit and a clock-recovery circuit

The purpose of clock-recovery circuit is to isolate aspectral component at f=B from the received signal

This component provides information about the bit slot

TB1B to the decision circuit and helps to synchronize

the decision process

Clock recovery is more difficult in the case of NRZformat because the signal received lacks a spectralcomponent at f=B (In RZ format a spectral componentat f=B is present)

The decision circuit compares the output from the linear

channel to threshold level, at sampling times determined by

the clock-recovery circuit and decides whether the signal

corresponds to to bit 1 or bit 0

Eye diagram is formed by superposing 2-3 bit-long

electrical sequences in the bit stream on top of each other.

It is called because of its appearance

Noise and the timing jitter causes closing of the eye. The

best sampling time corresponds to maximum opening of

the eye. Eye diagram provides a visual way of monitoring

the receiver performance

⎬data

B1

B2

Input data

Clockrecovering

input f

Decisioncircuit

Clock, f=B

RZ format,

NRZ- format

Regenerate d ⎫

Re −timed ⎭

Bit slot T B

component at f=B presentSAW filterno component at f=B

Circuit Input

Ref DFFData out

Decision comparatorclock

Eye diagramSuperimposing in same time slot

Ideal Eye diagram for NRZ format

•

•

•

Degraded Eye diagram for NRZ format

Noise and time jitter close the eye

Best sampling time for maximum opening

BER related to eye closure