Optical Receivers Theory and Operation

Optical - Electric Modulation and Demodulation

Modulation 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) bit interval

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 Low Impedance Low Sensitivity Easily Made Wide Band High Impedance Requires Equalizer for high BW High Sensitivity Low Dynamic Range Careful Equalizer Placement Required Transimpedance High Dynamic Range High Sensitivity Stability Problems Difficult 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 Noise Also called shot noise or Quantum noise, described by poisson statistics Photoelectron Noise Randomness of photodetection process leads to noise Gain Noise eg. gain process in APDs or EDFAs is noisy Receiver Circuit noise Resistors and transistors in the the electrical amplifier contribute to circuit noise Photodetector without gain Photodetector with gain (APD)

Noise Johnson noise (Gaussian and white) Frequency Noise Power Johnson noise (Gaussian and white) Frequency Noise Power Shot noise (Gaussian and white) “1/f” noise Frequency Noise Power 1/f noise Fc

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

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 Noise in photodetector 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 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 4

Signal Path through an Optical Link 1

Noise sources and disturbances at an optical receiver 2