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EE 230: Optical Fiber Communication Lecture 12 Receivers From the movie Warriors of the Net Receiver Functional Block Diagram Fiber-Optic Communications Technology-Mynbaev & Scheiner Receiver Types +Bias +Bias +Bias Is Is Is Output RL 50 Output Output RL Amplifier Rf Ct Ct Amplifier Equalizer Amplifier Low Impedance High Impedance Transimpedance Low Sensitivity Easily Made Wide Band Requires Equalizer for high BW High Sensitivity Low Dynamic Range Careful Equalizer Placement Required High Dynamic Range High Sensitivity Stability Problems Difficult to equalize Equivalent Circuits of an Optical Receiver High Impedance Design Transimpedance Design Transimpedance with Automatic Gain Control Fiber-Optic Communications Technology-Mynbaev & Scheiner Receiver Noise Sources Photodetector without gain •Photon Noise Also called shot noise or Quantum noise, described by poisson statistics •Photoelectron Noise Randomness of photodetection process leads to noise •Gain Noise eg. gain process in APDs or EDFAs is noisy •Receiver Circuit noise Resistors and transistors in the the electrical amplifier contribute Photodetector with gain (APD) to circuit noise Johnson noise (Gaussian and white) Vn Noise Power=4kTB R in 2 R Frequency 4kTB R Noise Power Vrms 4kTRB Shot noise (Gaussian and white) rms noise current in2 1/ 2 2qIB “1/f” noise spectral density= K f V 2 /Hz 1/ 2 Frequency Noise Power i rms 2 Noise Power Noise 1/f noise Fc for FETs Frequency 4kT K= fc gm where fc is the FET corner frequency and is the channel noise factor Johnson (thermal) Noise Noise in a resistor can be modeled as due to a noiseless resistor in parallel with a noise current source The variance of the noise current source is given by: s i2 = i 2 » 4kBTB R Where kB is Boltzman's constant T is the Temperature in Kelvins B is the bandwidth in Hz (not bits/sec) Photodetection noise The electric current in a photodetector circuit is composed of a superposition of the electrical pulses associated with each photoelectron Noise in photodetector The variation of this current is called shot noise If the photoelectrons are multiplied by a gain mechanism then variations in the gain mechanism give rise to an additional variation in the current pulses. This variation provides an additional source of noise, gain noise Noise in APD Circuit Noise Signal to Noise Ratio Signal to noise Ratio (SNR) as a function of the average number of photo electrons per receiver resolution time for a photo diode receiver at two different values of the circuit noise Signal to noise Ratio (SNR) as a function of the average number of photoelectrons per receiver resolution time for a photo diode receiver and an APD receiver with mean gain G=100 and an excess noise factor F=2 At low photon fluxes the APD receiver has a better SNR. At high fluxes the photodiode receiver has lower noise Dependence of SNR on APD Gain Curves are parameterized by k, the ionization ratio between holes and electrons Plotted for an average detected photon flux of 1000 and constant circuit noise Receiver SNR vs Bandwidth Double logarithmic plot showing the receiver bandwidth dependence of the SNR for a number of different amplifier types Basic Feedback Configuration Ii Is A Vi + Is If Ri Ro Parallel Current Feedback Lowers Input Impedance is i f ii bVo V is b AVi i Ri Zin Vi Ri is 1 Rm b Parallel Voltage Sense: Voltage Measured and held Constant => Low Output Impedance Zo Vtest Ro Ro I test 1 b ARi 1 b Rm Stabilizes Transimpedance Gain Vo Aii Ri ii is i f is b Vo Ii ZtIi + Vo ARi is b Vo Zt Vo ARi Rm is 1 ARi b 1 Rm b Zi - Zo Transimpedance Amplifier Design i + Zi Output Voltage Proportional to Input current Zero Input Impedance Vi A Vi + Ri Ro Typical amplifier model With generalized input impedance And Thevenin equivalent output is + Vi - A Vi + Ri - Vo AVi ARi ii Calculation of Openloop transimpedance gain: Rm V ARi Rm is Ro Vo Transimpedance Amplifier Design Example Vcc1 Controls open loop gain of amplifier, Reduce to decrease “peaking” Vcc2 See Das et. al. Journal of Lightwave Technology Vol. 13, No. 9, Sept.. 1995 Rc Q2 Q1 Out Photodiode Most Common Topology Vbias Has good bandwidth and dynamic Range Rf For an analytic treatment of the design of maximally flat high sensitivity transimpedance amplifiers Transimpedance approximately equals Rf low values increase peaking and bandwidth “Off-the-shelf” Receiver Example Sensitivity i2 i2 Detector Re sistor 2qId I2B 1.8x1017 A2 4kT 2 I2B i Detector 1.9 x1012 A2 Rs NF i 2 i 2 Re sistor Amp1 1 4kT 2 10 10 I2B iDetector 7.5x1012 A2 Rs Re sistor Amp1 Amp 2 4kT 10 Rs NFTotal 10 2 I2B iDetector 7.6 x1012 A2 45.22dBm 20.14dBm 16.63dBm 16.59dBm Bit Error Rate BER is equal to number of errors divided by total number of pulses (ones and zeros). Total number of pulses is bit rate B times time interval. BER is thus not really a rate, but a unitless probability. Q Factor and BER Q Vth Voff off Von Vth on 1 Q BER 1 erf 2 2 BER vs. Q, continued When off = on and Voff=0 so that Vth=V/2, then Q=V/2. In this case, 1 V BER 1 erf 2 2 2 Sensitivity The minimum optical power that still gives a bit error rate of 10-9 or below (Smith and Personick 1982) Receiver Sensitivity Sensitivity= Average detected optical power for a given bit error rate P hv Q q i2 1/2 Probability of error vs. Q is to good approximation: For pin detectors i2 i2 amplifier 2qId I2B Q2 /2 P E 1 e 2 Q eg. for a SNR = Q = 6 Bit Error Rate= P(E)=10-9 Dynamic Range and Sensitivity Measurement Dynamic range is the Optical power difference in dB over which the BER remains within specified limits (Typically 10-9/sec) Input Optical Power Dynamic Range The low power limit is determined by the preamplifier sensitivity The high power limit is determined by the nonlinearity and gain compression High Rf Feedback Resistance Low Rf (High Impedance Preamplifier) (Transimpedance Preamplifier Patten Generator Transmitter Adjustable Attenuator Optional Clock Experimental Setup Optical Receiver Bit Error Rate Counter Eye Diagrams Transmitter “eye” mask determination Formation of eye diagram Eye diagram degradations Computer Simulation of a distorted eye diagram Fiber-Optic Communications Technology-Mynbaev & Scheiner Power Penalties • Extinction ratio • Intensity noise • Timing jitter Extinction ratio penalty Extinction ratio rex=P0/P1 1 rex 2 RP Q 1 rex on off 1 rex ex 10 log 1 rex Intensity noise penalty rI=inverse of SNR of transmitted light I R PrI I 10 log 1 r Q 2 I 2 Timing jitter penalty Parameter B=fraction of bit period over which apparent clock time varies 4 2 2 b 8 B 3 1 b / 2 J 10 log 2 2 2 1 b / 2 b Q / 2