Optical receiver

DIRECT DETECTION RECEIVERS

Direct Detection Optical ReceiverAn optical receiver consists of a photodetector, an amplifier, and signal processing circuitry.It first converting the optical energy emerging from the end of a fiber into an electric signal, and then amplifying this signal to a large enough level so that it can be processed by signal processing circuits for reducing the noise and improving the output pulse shape.

Noise sources and disturbances in the optical pulse detection mechanism


The power signal-to-noise ratio at the output of an optical receiver is defined by

N

S Signal power from photocurrentPhotodetector noise power+amplifier noise power

Noise Equivalent Power (NEP)NEP is the minimum optical signal power that produces SNR = 1.

This is the optical power necessary to produce a photocurrent of the same magnitude as total noise current.

NEP determines the weakest optical signal that can be detected in the presence of noise.

Signal-to-Noise Ratio (SNR)

2

2

noise

p

i

I

N

S

For both signal power and noise power are released at the same load resistance,

average photocurrent

noise

p

i

I

root mean square value of the noise induced current


Quantum Noise•The detection of light by a photodiode is a discrete process - an electron-hole pair is generated from the absorption of a photon.•The photocurrent generated is dictated by the statistics of photon arrivals.• When the detector is illuminated by an optical signal P0, the average number of electron-hole pairs generated in a time is

hf

Prz em

0

•The actual number of electron-hole pairs z that are generated fluctuates from the average according to the Poisson distribution, where the probability that z electrons are generated in an interval is

!

exp)(

z

zzzP m

zm

quantum noise - it is not possible to predict exactly how many electron-hole pairs are generated by a known optical power incident on the detector.


Digital Signaling Quantum Noise• For an ideal receiver (Idark= 0, =1 and able to detect an individual photon), the probability of no electron-hole pairs (z = 0) being generated when an optical pulse of energy E falls on the photodetector in the time interval is

mzP exp10•This error probability represents the bit-error-rate of digital system, [ P(0/1)=10 -9, on the average, one error occurs for every billion pulses sent].

•The minimum optical power (or pulse energy) required to maintain a specific bit- error-rate performance in a digital system is known as the quantum limit.

Analog Transmission Quantum Noise•In analog optical receiver quantum limit manifests itself as a shot noise which has Poisson statistics. The shot noise current is on the photocurrent Ip is given by

ps qBIi 22 •Neglecting other sources of noise the SNR at the receiver is

qB

I

i

I

N

S p

s

p

22

2

•The minimum incident optical power necessary to achieve a specific S/N is hfB

P

qBhf

qP

2200

• In term of the absolute optical power requirements analog transmission compares unfavorably with digital signaling.

hfz

P mmin

hfB

N

SP

2min


Dark Current Noise• A small leakage current flows from the device terminals when there is no optical power incident on the photodiode.• This current contribute to the random fluctuations about the average particle flow of the photocurrent and manifests itself as shot noise.• The mean square value of dark current noise is dd qBIi 22 Thermal Noise• Electron motion due to temperature (external thermal energy) occurs in a random way.• The number of electrons flowing through a given circuit at any instance is a random variable.• The mean square value of thermal-noise current in a resistor R,

R

TBki Bt

42 kB = Boltzmann’s constantT = absolute temperature

Shot Noise•The detector average current Ip exhibits a random fluctuation about it mean value as a result of the statistical nature of the quantum detection process.•The number of electrons producing photocurrent will vary because of their random absorption and recombination.• Deviation of an instantaneous number of electrons from their average value is known as shot noise and its current mean square value is

B = post-detection bandwidthps qBIi 22


Noise in a P-I-N Photodiode

• Three sources of noise: Shot noise, Dark current noise, Shot noise due to background radiation

bdPTS IIIqBi 22• The total shot noise, Ib = background radiation induced current

• For photodiode without internal gain, thermal noise from the detector load resistor and from active elements in the amplifier tends to dominate.

Noise in an APD

• Due to avalanche multiplication gain in an APD, the amount of noise is higher than that in a P-I-N photodiode• An excess noise in the output photocurrent due to gain fluctuation

)(2 22 MFMqBIi ps

)(2 22 MFMqBIi dd

)(2 22 MFMqBIi bb

Shot noise

Dark current noise

Background noise


Receiver Noise

Noise sources within an amplifier can be represented by a series voltage noise source and a shunt current noise source .

The equivalent circuit for the front end of an optical fiber receiver, including the effective input capacitance Ca and resistance Ra.

2av 2

ai

The total noise associated with the amplifier is dfYviiB

aaamp 0

2222

where Y is the shunt admittance and f is frequency. may be reduced with low detector and amplifier capacitance.

2ampi


When the noise associated with the amplifier is referred to the load resistance RL the noise figure Fn of the amplifier may be obtained. This allows to be combined with the thermal noise from the load resistance to give

Then the SNR can be written as

L

nBdp

p

RTBFk

IIqB

I

N

S4

)(2

2

The SNR at the output of the P-i-N photodiode receiver is

2

2

42 amp

L

Bdp

p

iR

TBkIIqB

I

N

S

SNR of P-i-N Photodiode Receiver

2ampi

2ampi

2ti

L

nBampt R

TBFkii

422

The thermal noise contribution may be reduced by increasing the value of the load resistor RL, however this will decrease the post detection bandwidth

adL CCRB

2

1


The SNR at the output of the APD receiver is

2

2

2

22

42

42

MR

TBFkMIIqB

I

R

TBFkMIIqB

MI

N

S

L

nBxdp

p

L

nBxdp

p

SNR of APD ReceiverThe total shot noise current multiplied through impact ionization is given by

xdPSA MIIqBi 22 2 where

xMMF )( , x ~0.3 to 0.5 for Si APDs x ~ 0.7 to 1.0 for Ge or III-VAPDs

For low M the combined thermal and amplifier noise term dominates and giving an improved SNR.For large M the SNR decreases with increasing M at the rate of Mx.For the maximum SNR,

xMRTBFk

MIIqB

opLnB

xopdp 2

4

)(22

and

dpL

nBxop IIxqR

TFkM

42


Structures of the receiver front end

Three basic configurations for optical receiver front end are used : (i) Low impedance front end (ii) high impedance front end (iii) the transimpedance front end.

Low impedance front end

• Voltage amplifier with an effective input resistance Ra

• The total input resistance• The bandwidth is limited by , CT = total input capacitance• For an optimum B, RTL is small, but thermal noise will dominate within the receiver.

• Impractical for long-haul, wideband optical fiber communication system.

baTL RRR // TTLCRB 21


High Impedance (integrating) Front End

• A large detector bias resistor Rb in order to reduce the effect of thermal noise but degraded frequency response• Detector output is effectively integrated over a large time constant and must be restored by differentiation• It has problems of limited dynamic range - the ratio of maximum to minimum input signals


Transimpedance Front End

Thus when feedback is employed, the thermal noise can be reduced by increasing Rf and the bandwidth can be increased by increasing the gain of the amplifier.

This configuration can overcome the drawbacks of the low and high impedance front end configurations by virtue of a negative feedback amplifier with high input impedance.

Open loop gain TLTTLOL RCjGRH 1 and TTLCRB 21

Close loop gain GCRjRH TffCL 1 and Tf CRGB 2

Optical receiver

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