Detectors for Optical Communications

Transcription

1 Optical Communications: Circuits, Systems and Devices Chapter 3: Optical Devices for Optical Communications lecturer: Dr. Ali Fotowat Ahmady Sep 2012 Sharif University of Technology 1

2 Photo 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 electronhole pairs give rise to a photocurrent, I p - One figure of merit is the responsivity, defined as the ratio of the photocurrent to the optical power, P in : Ip ηqq ηqq R = = = P hω hv η Q = in (units: A/W) electron-holepairgenerationrate photonincidentrate where η Q = quantum efficiency and q = charge generated per photon Sep 2012 Sharif University of Technology 2

3 Detector Materials Bandgaps and emission wavelengths (at 300 K) of semiconductors used as detectors for optical communications Sep 2012 Sharif University of Technology 3

4 Characteristics of Light (1 of 2) Responsitivity: Is a measure of the conversion efficiency of a photodetector. eη G R = [A/W] hν e = electron charge ( coulomb) v = frequency of the light η = quantum efficiency G = internal gain (>1 for APD) Dark current: The leakage current that flows through a photodiode with no light input. Thermally generated. Sep 2012 Sharif University of Technology 4

5 Chapter 3 Optical Devices Characteristics of Light (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. Sep 2012 Sharif University of Technology 5

6 Types of Optical 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 -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 q Leads to a responsivity R = M η hω Sep 2012 Sharif University of Technology 6

7 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 -Problem: Transit time across the depletion region also limits the detector bandwidth - RC time constant: τ RC = (R L + R s )C p R L = load resistance, R s = internal series resistance, C p = parasitic capacitance Sep 2012 Sharif University of Technology 7

8 Photoconductive vs. Photovoltaic Operation Photoconductive regime: Reverse-biased Photovoltaic regime: Unbiased Current-voltage characteristic curves for a silicon photodetector Sep 2012 Sharif University of Technology 8

9 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 Typical sensitivities for a BER of are: 26 dbm at a bit rate B = 2.5Gb/s, or 18 dbm at a bit rate B = 10Gb/s Sep 2012 Sharif University of Technology 9

10 InGaAs p-i-n DC Responsivity vs. Wavelength Sep 2012 Sharif University of Technology 10

11 p-i-n Sensitivity vs. Bit Rate Sep 2012 Sharif University of Technology 11

12 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 Sep 2012 Sharif University of Technology 12

13 Noise in PIN Diodes Shot noise (Poisson process ~ Gaussian) i 2 () t = 2 eib I = RP s e -R is responsivity of detector -B e is electrical BW of detector (typically between BR and ½ BR) Thermal noise (Gaussian process) 2 4kT B it () t = FB n e RL -F n is the noise figure of front-end amplifier, typically 3-5 db Total noise power ( ) = + I i () t i () t s t Sep 2012 Sharif University of Technology 13

14 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 1 M n 1 v V ( ) d where v d = reverse bias voltage, V BR = breakdown voltage, and n > 1 -Avalanche multiplication creates excess noise -Noise scales nonlinearly with M, while the signal scales linearly BR Sep 2012 Sharif University of Technology 14

15 Avalanche Photodiodes (2 of 2) -Therefore there s an optimal value, M opt Typically 3 < M opt < 9 -Much better signal-to-noise ratio than with external amplification Typical sensitivities for a BER of are: 32 dbm at a bit rate B = 2.5Gb/s, or 22 dbm at a bit rate B = 10Gb/s Sep 2012 Sharif University of Technology 15

16 Noise in APD Shot noise I ( ) i () t = 2 erpb G F G 2 s e m A m = G RP m 1 ( β )( ) α F ( G ) = β G A m α m G m -Where F A is the excess noise factor -InGaAs detectors typically have β/α =0.7 Sep 2012 Sharif University of Technology 16

17 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 Sep 2012 Sharif University of Technology 17

18 Questions Sep 2012 Sharif University of Technology 18