1. Optical Receivers Theory and Operation

2. Optical - Electric Modulation and Demodulation

3. 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

4. Demodulation: Receiver
Recovering the transmitted data
Recovering the bit clock
Determining the bit value within each bit interval

5. 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.

6. Receiver Functional Block Diagram
Fiber-Optic Communications Technology-Mynbaev & Scheiner

7. 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

8. Equivalent Circuits of an Optical Receiver
High Impedance Design
Transimpedance Design
Transimpedance with Automatic Gain Control
Fiber-Optic Communications Technology-Mynbaev & Scheiner

9. 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)

10. 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

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

12. 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

13. Circuit Noise

14. Optical receiver schematic
Bandwidth of the front end:
CT: Total Capacitance = Cd+Ca
RT: Total Resistance = Rb // Ra
Try Example 6.7 4

15. Signal Path through an Optical Link1

16. Noise sources and disturbances at an optical receiver2