detectors for optical communicationsee.sharif.edu/~opticsicsys-alif/ch3_02.pdf · • transit time:...

Chapter 3: Optical Devices

Detectors for Optical Communications

Optical Communications: Circuits, Systems and Devices

Sep 2012 Sharif University of Technology 1

lecturer: Dr. Ali Fotowat Ahmady

Photo Detectors

• All detectors for optical communications use optical absorption in a depletion region to convert photons into electron-hole pairs, and then sense the number of pairs.

- Because of the electric field in the depletion region, the electron-hole pairs give rise to a photocurrent, Ip- One figure of merit is the responsivity, defined as the ratio of the

Chapter 3 Optical Devices Detectors


p- One figure of merit is the responsivity, defined as the ratio of the photocurrent to the optical power, Pin:

where ηQ = quantum efficiency and q = charge generated per photon

p Q Q

in

I q qR

P hvη η

ω= = =

helectron-hole pair generation rate

photon incident rateQη =

(units: A/W)

Detector Materials

• Bandgaps and emission wavelengths (at 300◦ K) of semiconductors used as detectors for optical communications



Characteristics of Light Detectors (1 of 2)

• Responsitivity: Is a measure of the conversion efficiency of a photodetector.

e = electron charge (1.6×10-19 coulomb)v = frequency of the light


[A/W]e GRhην

=


v = frequency of the lightη = quantum efficiencyG = internal gain (>1 for APD)

• Dark current: The leakage current that flows through a photodiode with no light input. Thermally generated.

Characteristics of Light Detectors (2 of 2)

• Transit time: The time it takes a light-induced carrier to travel across the depletion region of a semiconductor. This parameter determines the maximum bit rate possible with a particular photodiode.

• Light sensitivity: The minimum optical power a light detector can receive



and still produce a usable electrical output signal.

• Spectral response: The range of wavelength values that a given photodiode will respond.

Types of Optical Detectors

• p-n photodiodes- Electron-hole pairs are created in the depletion region of a p-n junction in proportion to the optical power- Electrons and holes are swept out by the electric field, leading to a current

• p-i-n photodiodes



• p-i-n photodiodes- Electric field is concentrated in a thin intrinsic (i) layer

• Avalanche photodiodes- Like p-i-n photodiodes, but have an additional layer in which an average of M secondary electron-hole pairs are generated through impact ionization for each primary pair

◦ Leads to a responsivityqR M ηω

=h

p-n Photodiodes

• Operated in reverse-biased regime for detection, instead of forward-biased regime for emission• Wide depletion region

- Advantage: High quantum efficiency- Problem: Diffusion of carriers created in the boundary p and n regions limits the detector bandwidth



regions limits the detector bandwidth- Problem: Transit time across the depletion region also limits the detector bandwidth- RC time constant:

τRC = (RL + Rs)CpRL = load resistance, Rs = internal series resistance, Cp = parasitic capacitance

Photoconductive vs. Photovoltaic Operation

• Photoconductive regime: Reverse-biased• Photovoltaic regime: Unbiased



Current-voltage characteristic curves for a silicon photodetector

p-i-n Photodiodes

• Basic idea: Eliminate diffusion of carriers created outside the depletion region by:

- Sandwiching a thin layer of a different semiconductor material (of intrinsic conductivity) between the outer p and n layers- Choosing the outer p and n layers to be transparent to light in the working wavelength range



working wavelength range• Typical sensitivities for a BER of 10−10 are:−26 dBm at a bit rate B = 2.5Gb/s, or −18 dBm at a bit rate B = 10Gb/s

InGaAs p-i-n DC Responsivity vs. Wavelength



p-i-n Sensitivity vs. Bit Rate



Load-Line Analysis of a p-i-n Circuit

• Photoconductive regime: Voltage across load resistor is proportional to optical power

- For optical powers above a certain critical value (40 μW in this example), the voltage across the load grows very slowly as a function of optical power



(a) simple PIN circuit. (b) Graphical analysis of the circuit

Noise in PIN Diodes

• Shot noise (Poisson process ~ Gaussian)

- R is responsivity of detector- Be is electrical BW of detector (typically between BR and ½ BR)

• Thermal noise (Gaussian process)


2 ( ) 2 s ei t eIB I RP= =


• Thermal noise (Gaussian process)

- Fn is the noise figure of front-end amplifier, typically 3-5 dB• Total noise power

2 4( ) Bt n eL

k Ti t F BR

=

( )2 2 2( ) ( )s tI i t i t∆ = +

Avalanche Photodiodes (1 of 2)

• Internal gain- Electron-hole pairs created by absorption of photons are accelerated to energies at which more pairs are created, and then the new pairs are accelerated and create more pairs, in an “avalanche”

◦ Overall gain is M pairs generated for each pair created optically



optically

where vd = reverse bias voltage, VBR = breakdown voltage, and n > 1

- Avalanche multiplication creates excess noise- Noise scales nonlinearly with M, while the signal scales linearly

( )1

1 nd BRM

v V≈

−

Avalanche Photodiodes (2 of 2)

- Therefore there’s an optimal value, Mopt◦ Typically 3 < Mopt < 9

- Much better signal-to-noise ratio than with external amplification

• Typical sensitivities for a BER of 10−10 are:−32 dBm at a bit rate B = 2.5Gb/s, or −22 dBm at a bit rate B = 10Gb/s



−32 dBm at a bit rate B = 2.5Gb/s, or −22 dBm at a bit rate B = 10Gb/s

Noise in APD

• Shot noise


( )

( )( )

2

1

( ) 2

( ) 1 2

s e m A m

m

A m m G

i t eRPB G F G

I G RP

F G Gβ βα α

=

=

= + − −


- Where FA is the excess noise factor - InGaAs detectors typically have β/α =0.7

( )( )( ) 1 2 mA m m GF G Gα α= + − −

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



An APD typically has 10dB better sensitivity than a PIN

Questions



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