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

2. 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

3. Photodetector types
PD used in OFC

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

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

6. 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

7. Absorption coefficient of various materials

8. 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

9. 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)

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

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

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

13. 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