Fiber Optic Communication Lec 13 By Engr.Muhammad Ashraf Bhutta

Optical Receivers The purpose of the receiver is: i) to convert the optical signal to electrical ii) recover the data

Photo detectors The most critical component is the photodetector. It should have: high sensitivity fast response time low noise size compatible with fibers high reliability This means that semiconductor materials are exclusively used in light wave systems. In these, photons are absorbed to generate electron-hole (e-h) pairs producing a photo-current. A basic requirement is that the detector material bandgap energy (Eg) must be smaller than the photon energy (h ).

Photodiodes: Responsivity The detector responsivity R is defined as the photo- current/incident optical power: 

p-i-n diodes

Receiver components The digital receiver consist of three blocks front end (photodetector, transimpedance amplifier) linear channel (amplifier, low-pass filter) data recovery (clock recovery, decision circuit)

The front end consists of a photodiode and a preamplifier. It is a trade off between sensitivity (low thermal noise) and high bandwidth. A high-impedance front end suffers from low bandwidth. A low-impedance front end suffers from high thermal noise. Usually a transimpedance design is used:

Linear Channel The linear channel consists of: a high-gain (limiting) amplifier, which provides a constant output voltage irrespective of the average input power. (within certain limits) a low-pass filter with bandwidth chosen to minimize noise (proportional to filter bandwidth) while not introducing much inter- symbol-interference (ISI). The best situation is when the filter (and not other components) limits the overall bandwidth of the receiver.