Chapter 6: Optical Detectors

Overview of Optical Detector An essential component of an optical fibre communication

system, dictates the overall system performance. Its function is to convert the received optical signal into an

electrical signal. It must satisfy a significant requirements for performance and

compatibility: High sensitivity at the operating wavelengths (1200-1600nm). High fidelity. Linearity is important for analog transmission. Short response time to obtain a suitable bandwidth. A minimum noise introduced by the detector. Stability of performance characteristics. Small size for efficient coupling to the fibre and easy

packaging with the following electronics. Low bias voltages. High reliability and low cost.

Overview of Optical Detector Device types:

Photodiodes Phototransistors Photoconductive detectors Photomultiplier tubes Phototube

Optical Detection Principles

A photon incident in the depletion region (hf> Eg) will excite an electron from the valence band into the conduction band, leaving an empty hole in the valence band – absorption.

Carrier pair generated near the junction are separated and swept (drift) under the influence of the electric field to produce a displacement by current in the external circuit in an excess of any reverse leakage current.

Reverse bias condition

• When external battery is connected to the p-n junction with positive terminal to the n-type and vice versa, the junction is said reverse biased.

• Depletion region increased, barrier potential increases and prevents any majority carriers flowing across the junction.

• A reverse bias widens the depletion region, but allows minority carriers to move freely with the applied field.

Optical generation of carriers in a PN junction.

The high electric field present in the depletion region causes the carrier to separate and be collected across the reversed biased junction.

The high electric field present in the depletion region causes the carriers to separate and be collected across the reverse biased junction

This gives rise to a current flow in an external circuit, with one electron flowing for every carrier pair generated. This current flow is known as photocurrent.

Absorption Assume only bandgap transitions,

the photocurrent IP produced by incident light of optical power P0 is:

• The upper wavelength cutoff λC is determined by the bandgap energy of the material:

Light penetration depth (µm)

Wavelength (µm)

Photodiode Material

Some of the important photodiode characteristic for several photodiode material:-

Example A photodiode is constructed of Ga As, which

has a band-gap energy of 1.43 eV at 300 K. Find the long-wavelength cutoff for the photodiode.

Given: 1 eV = 1.6 x 10-19 J/eV

Quantum efficiency

η ≤ 1η is a function of the photon wavelengthTo obtain a high η the width of the depletion layer,

One of the important characteristic of photodetectorThe quantum efficiency is the number of electron-hole carrier pairs generated per incident photon of energy hv.IP is the average photocurrent generated by a steady-state average optical power, P0 incident on the photodiode.

Responsivity The performance of photodiode is often

characterized by the responsivity, R. This is related to quantum efficiency by:-

Example

Long Wavelength Cut-off

Photodiodes Two types of photodiodes commonly used

PIN (p-type, intrinsic, n-type) diodes, and Avalanche photodiodes (APDs)

PIN Photodiode the thickness of the depletion region is controlled by i-layer, not by the reverse

voltage most of the incident photons absorbed in the thick i-layer - high η large electric field across the i-layer - efficient separation of the generated

electrons & holes The p and n layers are extremely thin compare to i-layer - diffusion current is

very small The increase in the i-width reduces the speed of a photodiode

The speed of response of the photodiode is limited by the time it takes to collect the carriers (drift time) the capacitance of the depletion layer (RC time constant of the detector circuit)

Photodiodes Avalanche photodiodes (APDs)

It is a photodiodes with internal gain An additional layer is added in which secondary electron-hole pairs are generated

through impact ionization. Internally multiplied the primary photocurrent before it enters the input circuitry of

the following amplifier. Commonly used structure: Reach-through APD (RAPD) The RAPD is composed of a high-resistivity p-type and p+ (heavily doped p-type )

p-n Photodiode

Typical p-n Photodiode Output Characteristic

p-i-n Photodiode

III-V P-i-N photodiodes - InGaAs/InP Epitaxial growth of several layers on a n-type InP

substrate. Incident light is absorbed in the low-doped n-type InGaAs

layer lattice matched In0.53Ga0.47As/InP system, C = 1.67m Drawback - optical absorption in the undepleted p+ region.

Comparisons of Common P-i-N Photodiodes

Dark CurrentResponsivity

Avalanche Photodiode (APD)

Impact Ionization

Impact Ionization• The average number of e-h pairs created by a carrier per

unit distance travelled is called the ionization rate/coefficient

• In the high field region of an APD, photogenerated electrons and holes can acquire sufficient energy to create new electron-hole pairs through impact ionization process.

• These secondary carriers gain enough energy to ionize other carriers, causing the avalanche process of creating new carriers.

Electron Electric Field e Electron Impact Hole Impact hv Ionization Ionization h Ec Hole Ev

Multiplication Factor

The measured value of M is expressed as an average quantity since the avalanche mechanism is a statistical process; not every carrier pair generated in the diode experiences the same multiplication.

Benefits and Drawbacks with the APD

PiN or APDCharacteristics of common P-i-N Photodiodes

Characteristics of common APDs