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 PDI

V

Region 2

Region 1

Region 3

Increasing optical power

Photovoltaic mode

Photoconductive mode

Photodetection principles

Eg

hf >Eg

- +p n

CB

VB CB

VB

Band structure under reversed

bias

Absorption coefficient

Absorption coefficient is a measure of how good the material is for absorbing light of a certain wavelength

d

)exp(1)1(0 d

hf

rePI p

The photo current Ip produce by incident light of optical power P0

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

p

e

r

r

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

E-Field

Depletion region

Absorption region

hf

x

p

i

Loadn

Avalanche photodiodeshf

Load

np

p+

i

Gain region

Absorption region

E-field

Basic structure of an optical receiver

PD

Preamplifier

Post-amplifier

Pre-detection filter

Electrical signal

Optical signal

Sources of noise in an optical receiver

Photo-detection

Avalanche gain

Detector load bias

Electronic gain

Optical signal

Photodetector Amplifier

Electrical signal

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