Photodetectors What is photodetector (PD)? Photodetector properties Detector types Optical detection principles Absorption coefficient Quantum efficiency Responsivity PD structures Optical receiver and noises

What is photodetector? Photodetector is an important elements in OFC, which converts optical signal into electrical form. A PD should have the following characteristics: High sensitivity at the operating wavelength High fidelity Short response time to obtain a suitable bandwidth Noise should be minimum Stability of performance characteristics Small size Low cost

Photodetector types PD used in OFC

V-I characteristics of PD Photovoltaic mode Photoconductive mode Region 2 Region 1 V Increasing optical power Region 3

Photodetection principles + - p n hf >Eg Eg CB CB VB Band structure under reversed bias VB

Absorption coefficient Absorption coefficient is a measure of how good the material is for absorbing light of a certain wavelength The photo current Ip produce by incident light of optical power P0 d e : Electronic charge r : Fresnel reflection coefficient

Absorption coefficient of various materials

Quantum efficiency The quantum efficiency n is defined as the fraction of incident photons which are absorbed by the photodetector and generated electrons which are collected at the detector terminal n = Number of electrons collected/ Number of incident photons rp: Incident photon rate re: Corresponding electron rate

Output Ch. of a typical p-n photodiodes Reverse bias (V) Current A 10 20 30 40 200 400 600 800 High light level Low light level Dark current (no light)

p-i-n Photodiode hf E-Field p Depletion region i Absorption region n Load x

Avalanche photodiodes E-field hf n Gain region p i Absorption region p+ Load

Basic structure of an optical receiver Optical signal PD Preamplifier Post-amplifier Pre-detection filter Electrical signal

Sources of noise in an optical receiver Photodetector Electrical signal Optical signal Amplifier Photo- detection Avalanche gain Detector load bias Electronic gain Noise Quantum shot Dark current Surface leakage Noise Excess noise due to random gain mechanisms Noise Thermal Noise Thermal noise Device (active element) Surface leakage currents