Optical Receivers Theory and Operation

Optical - ElectricModulation and Demodulation

Modulation

bit interval

• Conversion of digital electrical data into optical format

• On-off keying (OOK) is the most commonly used scheme– NRZ format most commonly used

– RZ format (requiring 2X NRZ bandwidth)

– Pulse format (not commercially deployed)

Demodulation: Receiver• Recovering the transmitted data

– Recovering the bit clock– Determining the bit value within each bit

interval

Optical Receivers

• Optical receivers convert optical signal (light) to electrical signal (current/voltage)– Hence referred ‘O/E Converter’

• Photodetector is the fundamental element of optical receiver, followed by amplifiers and signal conditioning circuitry

• There are several photodetector types:– Photodiodes, Phototransistors, Photon multipliers,

Photo-resistors etc.

Receiver Functional Block Diagram

Fiber-Optic Communications Technology-Mynbaev & Scheiner

Receiver Types+Bias

Is

RL 50 Amplifier

Output

+Bias

Is

Amplifier

Output

Ct

Rf+Bias

Is

RL

Amplifier

Output

EqualizerCt

Low Impedance

Low SensitivityEasily MadeWide Band

High Impedance

Requires Equalizer for high BWHigh SensitivityLow Dynamic RangeCareful Equalizer Placement Required

Transimpedance

High Dynamic RangeHigh SensitivityStability ProblemsDifficult to equalize

Equivalent Circuits of an Optical Receiver

High Impedance Design Transimpedance Design

Transimpedance with Automatic Gain Control

Fiber-Optic Communications Technology-Mynbaev & Scheiner

Receiver Noise Sources

•Photon NoiseAlso called shot noise or Quantum noise, described by poisson statistics

•Photoelectron NoiseRandomness of photodetection process leads to noise

•Gain Noiseeg. gain process in APDs or EDFAs is noisy

•Receiver Circuit noiseResistors and transistors in the the electrical amplifier contribute to circuit noise

Photodetector without gain Photodetector with gain (APD)

Noise

2

2Noise Power=4

4 4

nn

rms rms

VkTB i R

R

kTBi V kTRBR

2

m

spectral density= V /Hz

for FETs4kTK=

gwhere is the FET corner frequency and is the channel noise factor

c

c

Kf

f

f

Frequency

Nois

e P

ow

er

Frequency

Nois

e P

ow

er

Frequency

Nois

e P

ow

er 1/f noise

Fc

Johnson noise (Gaussian and white)

1/2 1/22rms noise current 2ni qIB

Shot noise (Gaussian and white)

“1/f” noise

Johnson (thermal) Noise

Noise in a resistor can be modeled as due to a noiseless resistor in parallel with a noise current source

2 2

The variance of the noise current source is given by:

4

Where is Boltzman's constant

T is the Temperature in Kelvins

B is the bandwidth in Hz (not bits/sec)

Bi

B

k TBi

R

k

s = »

Photodetection noise

The electric current in a photodetector circuit is composed of a superposition of the electrical pulses associated with each photoelectron

The variation of this current is called shot noise

If the photoelectrons are multiplied by a gain mechanism then variations in the gain mechanism give rise to an additional variation in the current pulses. This variation provides an additional source of noise, gain noise

Noise in photodetector

Noise in APD

Circuit Noise

Optical receiver schematic

Bandwidth of the front end:

CT: Total Capacitance = Cd+Ca

RT: Total Resistance = Rb // Ra

Try Example 6.7

1 2 T TB R C

Signal Path through an Optical Link

Noise sources and disturbancesat an optical receiver