PPT On Optical Recievers

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Optical receivers convert optical signal (light) to electrical signal.The photodetector is the fundamental element of an optical receiver, followed by amplifiers and signal conditioning circuitry.This PPT will give a brief idea and notes on Optical Fiber Receivers channels and modes.

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    Optical Receivers Theory and Operation
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    Optical Receivers Optical receivers convert optical signal (light) to electrical signal (current/voltage) — Hence referred 'O/E Converter' Photodetector is the fundamental element of optical receiver, followed by amplifiers and signal conditioning circuitry There are several photodetector types: — Photodiodes, Phototransistors, Photon multipliers, Photo-resistors etc.
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    Photodetector Requirements ' High sensitivity (responsivity) at the desired wavelength and low responsivity elsewhere -5 high wavelength selectivity Low noise and reasonable cost Fast response time -5 high bandwidth Insensitive to temperature variations Compatible physical dimensions Long operating life
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    Photodiodes ' Due to above requirements, only photodiodes are used as photo detectors in optical communication systems Positive-Intrinsic-Negative (pin) photodiode — No internal gain Avalanche Photo Diode (APD) — An internal gain of M due to self multiplication Photodiodes are sufficiently reverse biased during normal operation -5 no current flow, the intrinsic region is fully depleted of carriers
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    Avalanche Photodiode (APD) The internal gain of the APD is obtained by having a high electric field that energizes photo- generated electrons and holes These electrons and holes ionize bound electrons in the valence band upon colliding with them This mechanism is known as impact ionization The newly generated electrons and holes are also accelerated by the high electric field They gain enough energy to cause further impact ionization This phenomena is the avalanche effect
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    APD vs PIN APD has high gain due to self multiplying mechanism, used in high end systems The tradeoff is the 'excess noise' due to random nature of the self multiplying process. APD's are costly and need high reverse bias voltage (Ex: 40 V) APD's have the same excess noise at longer wavelengths, but they have an order of magnitude lower avalanche gain.
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    Basic pin photodiode circuit Bias voltage Hole Photodiode Electron RL Load resistor Output n Photon Incident photons trigger a photocurrent I in the external circuitry by pumping energy Photocurrent Incident Optical Power
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    pin energy-band diagram Photogenerated electron Band gap Eg Conduction band Photon hv>E Photogenerated hole Valence band 1+—Depletion region —H I .24 Cut off wavelength depends on the pm band gap energy
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    Responsivity Quantum Efficiency (n) = number of e-h pairs generated / number of incident photons Ip/q Ip mA/mW Avalanche PD's have an internal gain M M 1M : average value of the total multiplied current M = 1 for PIN diodes
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    Responsivity 300/0 When x c absorption will be low
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    Signal to Noise Ratio Signal power from photocurrent SNR = Detector Noise + Amplifier Noise For high SNR The Photodetector must have a large quantum efficiency (large responsivity or gain) to generate large signal current Detector and amplifier noise must be low SNR Can NOT be improved by amplification
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    Notation: Detector Current The direct current value is denoted by, Ip ; capitol main entry and capital suffix. The time varying (either randomly or periodically) current with a zero mean is denoted by, i small main entry and small suffix. Therefore, the total current Ip is the sum of the DC component Ip and the AC component i 1 i} (t)dt
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    Quantum (Shot Noise) Due optical power fluctuation because light is made up of discrete number of photons 02 - 2q1pBM 2F(M) F(M): APD Noise Figure I : Mean Detected Current B = Bandwidth
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    Dark/Leakage Current Noise There will be some (dark and leakage ) current without any incident light. This current generates two types of noise 2q1DBM2F(M) Bulk Dark Current Noise I Surface Leakage Current Noise (not multiplied by M) ID', Dark Current = 2q1LB IL: Leakage Current
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    Thermal Noise The photodetector load resistor RL contributes a mean-square thermal (Johnson) noise current = 4K BTB/R KB: Boltzmann's constant = 1.38054 X 10(-23) J/K T is the absolute Temperature Quantum and Thermal are the important noise mechanisms in all optical receivers RIN (Relative Intensity Noise) will also appear in analog links
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    Signal to Noise Ratio Detected current AC component (i ) + DC component (Ip) Signal Power = 1912 2 SNR - 2q(1p + + +4kBTB/R Typically not all the noise terms will have equal weight
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    SNR Dark current and surface leakage current noise are typically negligible, If thermal noise is also negligible SNR - For analog links, (RIN= Relative Intensity Noise) SNR = [2q(1p +
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    Response Time in pin photodiode Reverse bias voltage Optical POwer Depletion layer O Carrier drift n Hole diffusion Lo ad resistor O Electron diffusion Transit time, td and carrier drift velocity Vd are related by t VV / v d For a high speed Si PD, = 0.1 ns
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    Rise and fall times Time Photodiode has uneven rise and fall times depending on: 1. Absorption coefficient us(X) and 2. Junction Capacitance C
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    Junction Capacitance 80 8 r A eo = 8.8542 x 10(-12) F/m; free space permittivity er = the semiconductor dielectric constant A = the diffusion layer (photo sensitive) area width of the depletion layer Large area photo detectors have large junction capacitance hence small bandwidth (low speed) A concern in free space optical receivers
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    Signal Path through an Optical Link b 1 Electric input pulses LED or laser transmitter pin or Optical power pulses avalanche photodiode Decision circuit and pulse regenerator Electric current pulses containing photodetector noise 1 o 1 Optical fiber Amplifier and filter Signal- processing equipment Regenerated output voltage pulses Attenuated and distorted optical power pulses Voltage pulses and amplifier noise 4 The arrows denote I the time slot centers.
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    Noise sources and disturbances at an optical receiver Photon stream Photon detection quantum noise (Poisson fluctuation) Photodetector (gain M) Bulk dark current Surface leakage current Statistical gain fluctuation (for avalanche photodiodes) Bias resistor Thermal noise Amplifier Amplifier noise
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    Bit Error Rate (BER) BER is the ratio of erroneous bits to correct bit Estimate of BER often needed to estimate the performance of a communication link BER depends on the signal and noise power (Signal to Noise Ratio) BER requirement is different for different services and systems - Wireless -Y BER < 10(-6)•, Optical: BER < 10(-12) - Voice Low BER•, Data -Y High BER
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    Pulse spreading in digital links that leads to Inter Symbol Interference (ISI) Tb-.---...-.-H bnhp(t-nT) bn = Data bit (1,0) Tb = Bit period = Received pulse shape h
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    Gaussian Noise A Gaussian RV, with jnean value variance is denoted as N(tn, Probability density function (pdf*) is fx(r) Also known as normal distribution. 1 N (0, l): standard normal distribution. pdf and CDF are given respectively as: 1 Define function Q(x) as the tail integration of normal Gaussian: 1 e—crdx 27t From definition of Q-function, we can see that Fx(a;) = 1 Q(x) z; C—• CIT (3) (4)
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    Gaussian Noise For general Gaussian RV, N(m o 1 2TÜ 20 dt Letting z (t vn)/a, we have 1 Approximation: for a: > 3. 2 1 2rx -02/2
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    Error Probability - On-Off Signaling si(t) 8(t) = 81 (t) S2(t) -O Oft < T What would be the best threshold 10? The input signal energy of (t) is ,A2T and the energy of S2(t) is zero. The average energy per bit is E .42 T/ 2 El // 2, The error probability can be written as to 200 2N0 where E El /2 has been applied in the last equal sign, and E denotes the average bit energy. therefore. Pe Q
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    Logic 0 and 1 probability distributions level PO(v) PI(V) O level A&iüve noiee, y Asymmetric distributions Select v th to minimize Pe th p&11) vehge POT 10) th
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    Noise variances Variance Variance coff Time t Probability of error depends on v th and noise power Threshold level th (con off - I—erf 2
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    Fig. 7-7: BER (Pe) versus Q factor Q— 5.99781 — 10-9
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    BER vs SNR (equal standard deviations and boff
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    SNR vs, received power APD (5 MHz) APD (25 MHz) 30 pin (5 MHz) pin (25 MHz) Pr (dBm)

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