NAME : SYED KAMRAN

HAIDER

PRESENTATION:OPTICAL DETECTORS

FOUNDATION UNIVERSITY RAWALPINDI CAMPUS

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OPTICAL COMMUNICATION SYSTEMS

Communication systems with light as the carrier and optical fiber as communication medium

Optical fiber is used to contain and guide light waves Typically made of glass or plastic Propagation of light in atmosphere is impractical

This is similar to cable guiding electromagnetic waves Capacity comparison

Microwave at 10 GHz Light at 100 Tera Hz (1014 )

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HISTORY

1880 Alexander G. Bell Photo phone, transmit sound waves over beam of light

1930: TV image through uncoated fiber cables

Few years later image through a single glass fiber 1951: Flexible fiberscope: Medical applications 1956: The term “fiber optics” used for the first time 1958: Paper on Laser & Maser

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HISTORY CONT’D

1960: Laser invented 1967: New Communications medium: cladded fiber 1960s: Extremely lossy fiber:

More than 1000 dB /km 1970: Corning Glass Work NY, Fiber with loss of less

than 2 dB/km 70s & 80s : High quality sources and detectors Late 80s : Loss as low as 0.16 dB/km 1990: Deployment of SONET systems

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OPTICAL FIBER ARCHITECTURE

Transmitter

InputSignal

Coder orConverter

LightSource

Source-to-FiberInterface

Fiber-to-lightInterface

LightDetector

Amplifier/ShaperDecoder

Output

Fiber-optic Cable

Receiver

TX, RX, and Fiber Link

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OPTICAL FIBER ARCHITECTURE – COMPONENTS

Light source: Amount of light emitted is

proportional to the drive current

Two common types: LED (Light Emitting

Diode) ILD (Injection Laser

Diode) Source–to-fiber-coupler

(similar to a lens): A mechanical interface to

couple the light emitted by the source into the optical fiber

InputSignal

Coder orConverter

LightSource

Source-to-FiberInterface

Fiber-to-lightInterface

LightDetector

Amplifier/ShaperDecoder

Output

Fiber-optic Cable

Receiver

Light detector: PIN (p-type-intrinsic-n-type) APD (avalanche photo diode) Both convert light energy into

current

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LIGHT DETECTORS

PIN Diodes photons are absorbed in the intrinsic layer sufficient energy is added to generate carriers

in the depletion layer for current to flow through the device

Avalanche Photodiodes (APD) photogenerated electrons are accelerated by

relatively large reverse voltage and collide with other atoms to produce more free electrons

avalanche multiplication effect makes APD more sensitive but also more noisy than PIN diodes

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BLOCK DIAGRAM OF FIBER OPTIC RECEIVER.

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OPTICAL DETECTORS. These are transducers that convert

optical signals into electrical signals.

Transducers are devices that convert input energy of one form into output energy of another.

An optical detector does so by generating an electrical current proportional to the intensity of the incident optical light.

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OPTICAL DETECTOR REQUIREMENTS. Compatible in size to low-pass optical

fibers for efficient coupling and packaging.

High sensitivity at the operating wavelength of the source.

Low noise contribution.

Maintain stable operation in changing environmental conditions.

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PHOTO DETECTION PRINCIPLES

(Hitachi Opto Data Book)

Device Layer Structure

Band Diagramshowing carriermovement in E-field

Light intensity as a function of distance below the surface

Carriers absorbed here must diffuse to the intrinsic layer before they recombine if they are to contribute to the photocurrent. Slow diffusion can lead to slow “tails” in the temporal response.

Bias voltage usually needed to fully deplete the intrinsic “I” region for high speed operation

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CURRENT-VOLTAGE CHARACTERISTIC FOR A PHOTODIODE

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CHARACTERISTICS OF PHOTODETECTORS

Number of Collected electrons1

Number of Photons *Entering* detector

/Number of Collected electrons1 1

Number of Photons *Incident* on detector /

Photo Current (Amps)

Wi

ph We p

o

e

i qR e

P h

R

1 1Incident Optical Power (Watts)

1 1ph o

ph Wp

o

Wp

oi RP

i qR e

P h

R ePqh

• Internal Quantum Efficiency

•External Quantum efficiency

• Responsivity

•Photocurrent

Incident Photon Flux (#/sec)

Fraction Transmitted into Detector

Fraction absorbed in detection region

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RESPONSIVITY

Mh

eR

Output current per unit incident light power; typically 0.5 A/W

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OPTICAL DETECTOR MATERIALS. Si,GaAs, GaAlAs – 850nm Ge, InP, InGaAs -1300nm and 1550nm.

Materials determine the responsivity of the detector which is the ratio of the output photocurrent to the incident optical power.

It’s a function of the wavelength and efficiency of the device.

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PIN PHOTODIODE. Semiconductor positive-negative

structure with an intrinsic region sandwiched between the other two regions.

Normally operated by applying a reverse-bias voltage.

Dark current can also be produced which is a leakage current that flows when a reverse bias is applied without incident light.

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PIN PHOTODIODES

Energy-band diagram

p-n junction

Electrical Circuit

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BASIC PIN PHOTODIODE STRUCTURE

Front Illuminated Photodiode

Rear Illuminated Photodiode

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PIN DIODE STRUCTURES

Diffused Type (Makiuchi et al. 1990)

Etched Mesa Structure(Wey et al. 1991)

Diffused Type(Dupis et al 1986)

Diffused structures tend to have lower dark current than mesa etched structures although they aremore difficult to integrate with electronic devices because an additional high temperature processing step is required.

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RESPONSE TIME FACTORS.

Thickness of the active area. -Related to the amount of time

required for the electrons generated to flow out of the detector active area.

Detector RC time constant. -Depends on the capacitance of the

photodiode and the resistance of the load.

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ADVANTAGE OF PIN PHOTODIODES.

The output electrical current is linearly proportional to the input optical power making it a highly linear device.

Low bias voltage(<4v). Low noise Low dark current High-speed response

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AVALANCHE PHOTODIODE.

•High resistivity p-doped layer increases electric field across absorbing region•High-energy electron-hole pairs ionize other sites to multiply the current•Leads to greater sensitivity

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AVALANCHE PHOTODIODES. An APD internally amplifies the photocurrent by an

avalanche process when a large reverse-bias voltage is applied across the active region.

The gain of the APD can be changed by changing the reverse-bias voltage.

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APD DETECTORS

Signal Current

s

qi M P

h

APD Structure and field distribution (Albrecht 1986)

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APDS CONTINUED

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DETECTOR EQUIVALENT CIRCUITS

Iph

Rd

Id Cd

PIN

Iph

Rd

Id Cd

APD

In

Iph=Photocurrent generated by detectorCd=Detector CapacitanceId=Dark CurrentIn=Multiplied noise current in APDRd=Bulk and contact resistance

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MSM DETECTORS

Semi insulating GaAs

•Simple to fabricate

•Quantum efficiency: MediumProblem: Shadowing of absorption region by contacts

•Capacitance: Low

•Bandwidth: HighCan be increased by thinning absorption layer and backing with a non absorbing material. Electrodes must be moved closer to reduce transit time.

•Compatible with standard electronic processesGaAs FETS and HEMTs InGaAs/InAlAs/InP HEMTs

To increase speeddecrease electrode spacingand absorption depth

Absorptionlayer

Non absorbing substrate

E Fieldpenetrates for ~ electrode spacinginto material

Simplest Version

Schottky barriergate metal

Light

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WAVEGUIDE PHOTODETECTORS

(Bowers IEEE 1987)

•Waveguide detectors are suited for very high bandwidth applications•Overcomes low absorption limitations•Eliminates carrier generation in field free regions•Decouples transit time from quantum efficiency•Low capacitance•More difficult optical coupling

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CARRIER TRANSIT TIME

Transit time is a function of depletion width and carrier drift velocity

td= w/vd

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DETECTOR CAPACITANCE

p-n junction

xp xn

For a uniformly doped junction

Where: =permitivity q=electron charge

Nd=Active dopant density

Vo=Applied voltage Vbi=Built in potential

A=Junction area

C A

Ww xp xn

C A

2

2qVo Vbi

Nd

1/ 2

W 2(Vo Vbi)

qNd

1/ 2

P N

Capacitance must be minimized for high sensitivity (low noise) and for high speed operation

Minimize by using the smallest light collecting area consistent with efficient collection of the incident light

Minimize by putting low doped “I” region between the P and N doped regions to increase W, the depletion width

W can be increased until field required to fully deplete causes excessive dark current, or carrier transit time begins to limit speed.

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BANDWIDTH LIMIT

C=0K A/w

where K is dielectric constant, A is area, w is depletion width, and 0 is the permittivity of free space (8.85 pF/m)

B = 1/2RC

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PIN BANDWIDTH AND EFFICIENCY TRADEOFF

Transit time

=W/vsat

vsat=saturation velocity=2x107 cm/s

R-C Limitation

Responsivity

Diffusion

=4 ns/µm (slow)

RC in

ARW

1 1 W

pRq

R eh

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DARK CURRENT

Surface Leakage

Bulk Leakage

Surface Leakage

Ohmic Conduction

Generation-recombinationvia surface states

Bulk Leakage

Diffusion

Generation-Recombination

Tunneling

Usually not a significant noise source at high bandwidths for PIN StructuresHigh dark current can indicate poor potential reliabilityIn APDs its multiplication can be significant

SIGNAL TO NOISE RATIO

ip= average signal photocurrent level

based on modulation index m where

2

222 pp

Imi

LBLDp

p

RTBkBqIBMFMIIq

Mi

N

S

/422 2

22

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OPTIMUM VALUE OF M

where F(M) = Mx and m=1

Dp

LBLxopt IIxq

RTkqIM

/422

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NOISE EQUIVALENT POWER (NEP)

Signal power where S/N=1Units are W/Hz1/2

L

xD RM

kTMeI

e

hNEP

2

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TYPICAL CHARACTERISTICS OF P-I-N AND AVALANCHE PHOTODIODES

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COMPARISONS

PIN gives higher bandwidth and bit rate APD gives higher sensitivity Si works only up to 1100 nm; InGaAs up to

1700, Ge up to 1800 InGaAs has higher for PIN, but Ge has

higher M for APD InGaAs has lower dark current