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Optical Fibre Communication Systems Lecture 4 - Detectors & Receivers Professor Z Ghassemlooy Northumbria Communications Laboratory Faculty of Engineering and Environment The University of Northumbria U.K. http://soe.unn.ac.uk/ocr Prof. Z Ghassemlooy 1 Contents Properties and Characteristics Types of Photodiodes PIN APD Receivers Noise Sources Performance SNR BER Prof. Z Ghassemlooy 2 Optical Transmission - Digital • The design of optical receiver is much more complicated than that of • optical transmitter because the receiver must first detect weak, distorted signals and then make decisions on what type of data was sent. • analogue receiver • But offers much higher quality than analogue receiver. Prof. Z Ghassemlooy 3 Optical Receiver – Block Diagram Optical signal (photons – hf) Photodetection Converting optical signal into an electrical signal To recover the information signal Amplification (Pre/post) Filtering Limiting the bandwidth, thus reducing the noise power Prof. Z Ghassemlooy Signal Processing Information signal 4 Photodetection - Definition It converts the optical energy into an electrical current that is then processed by electronics to recover the information. Detection Techniques • Thermal Effects • Wave Interaction Effects • Photon Effects Prof. Z Ghassemlooy 5 Photodiode - Characteristics An electronics device, whose vi-characteristics is sensitive to the intensity of an incident light wave. I Forward-biased “Photovoltic” operation Dark current V Po Reverse-biased “photoconductive” operation Short-circuit “photoconductive” operation Prof. Z Ghassemlooy • Small dark current due to: • leakage • thermal excitation • Quantum efficiency (electrons/photons) • Responsivity • Insensitive to temperature variation 6 Photodetector - Types The most commonly used photodetectors in optical communications are: – Positive-Intrinsic-Negative (PIN) • • • • • No internal gain Low bias voltage [10-50 V @ = 850 nm, 5-15 V @ = 1300 –1550 nm] Highly linear Low dark current Most widely used – Avalanche Photo-Detector (APD) • • • • • Internal gain (increased sensitivity) Best for high speed and highly sensitive receivers Strong temperature dependence High bias voltage[250 V @ = 850 nm, 20-30 V @ = 1300 –1550 nm] Costly Prof. Z Ghassemlooy 7 Photodiode (PIN) - Structure Photons Depletion region n electron I Io p hole Output • No carriers in the I region • No current flow p I hole Bias voltage The power level at a distance x into the material is: Where is the photon absorption coefficient n electron RL (load resistor) • Reverse-biased • Photons generated electron-hole pair • Photocurrent flow through the diode and in the external circuitry Prof. Z Ghassemlooy 8 Photodiode (PIN) - Structure Depletion region width The capacitance of the depletion layer Cj (F) is: Prof. Z Ghassemlooy 9 Photodetector - Reponsivity PIN: R = Io/Po Io = Photocurrent; G = APD gain; Note: rp = Po/hf APD: RAPD = G R A/W Po = Incident (detected) optical power = Quantum efficiency = average number of electron-hole pairs emitted re / average number of incident photons rp and re = Io/q = 99% ~ 1 l is the length of the photoactive region Io = qPo/hf Thus normally is very low, therefore = 0. So Prof. Z Ghassemlooy 10 Photodetector - Responsivity Silicon (Si) – Least expensive Germanium (Ge) – “Classic” detector Indium gallium arsenide (InGaAs) – Highest speed G Keiser , 2000 Prof. Z Ghassemlooy 11 Photodetector - Equivalent Circuit Contact leads Photodiode Rs Io Cj Rj Amplifier L RL Ramp Camp Output L = Large, (i.e o/c)Rs = Small, (i.e s/c) CT = Cj + Camp RT = Rj || RL || Ramp The transfer function is given by: Prof. Z Ghassemlooy 12 Photodetector - Equivalent Circuit 1 The detector behaves approximately like a first f B order RC low-pas filter with a bandwidth of: 2CT RT Prof. Z Ghassemlooy 13 Photodiode Pulse Responses Fast response time High bandwidth • At low bias levels rise and fall times are different. Since photo collection time becomes significant contributor to the rise time. G Keiser , 2000 Prof. Z Ghassemlooy 14 Photodiode Pulse Responses Small area photodiode Small area photodiode Large area photodiode w = depletion layer s = absorption coefficient Due to carrier generated in w Due to diffusion of carrier from the edge of w G Keiser , 2000 Prof. Z Ghassemlooy 15 Photodetetor – Typical Characteristics Si Parameters PIN Wavelength range Peak (nm) Ge APD 400-1100 900 830 PIN APD 800-1800 1550 1300 InGaAS PIN APD 900-1700 1300 1300 (1550) (1550) Responsivity (A/W) 0.350.55 50-120 0.5-0.65 2.5-25 0.5-0.7 - Quantum Efficiency (%) 65-90 77 50-55 55-75 60-70 60-70 Bias voltage (-V) 45-100 220 6-10 20-35 5 <30 Dark current (nA) 1-10 0.1-1 50-500 10-500 - 1-5 Rise time (ns) 0.5-1 0.1-2 0.1-0.5 0.5-0.8 0.06-0.5 0.1-0.5 Capacitance (pF) 1.2-3 1.3-2 2-5 2-5 0.5-2 0.1-0.5 Source: R. J. Hoss Prof. Z Ghassemlooy 16 Minimum Received Power • Is a measure of receiver sensitivity defined for a specific: • Signal-to-noise ratio (SNR), • Bit error Rate (BER), • Bandwidth (bit rate), at the receiver output. Detector Pr Amplifier Power loss Po MRP = Minimum Detected Power (MDP) – Coupling Loss Prof. Z Ghassemlooy 17 MRP Vs. Bandwidth -20 SNR (dB) 50 MRP (-dBm) -30 -40 30 -50 10 -60 =1300 0 -70 1 2 5 10 20 50 100 200 500 1000 Bandwidth (MHz) Prof. Z Ghassemlooy 18 Selection Criteria and Task Optical Optical Sensitivity for a given BER and SNR Operating wavelength Dynamic range Simplicity Reliability and stability Electrical Data rate Bit error rate (digital) Maximum Bandwidth (analogue) Signal-to-noise ratio (analogue) Task: •To extract the optical signal (low level) from various noise disturbances •To reconstruct the original information correctly Prof. Z Ghassemlooy 19 Receivers: Basics The most important and complex section of an optical fibre system It sensitivity is design dependent, particularly the first stage or front-end Main source of major noise sources: – Shot noise current – Thermal noise: Due to biasing/amplifier input impedance – Amplifier noise: • Current • Voltage – Transimpedance noise Prof. Z Ghassemlooy 20 Receiver - Bandwidth A range of frequencies that can be defined in terms of: • Spectral profile of a signal • Response of filter networks • Equivalent bandwidth: Defines the amount of noise in a system Types of Bandwidth • Ideal • Baseband • Passband • Intermediate-Channel • Transmission • Noise Prof. Z Ghassemlooy 21 Ideal, Low-pass and Band-pass Filters Band-pass filter Low-pass filter 0 dB -3 Higher order filter Ideal Frequency Bbp Prof. Z Ghassemlooy Blp 22 Noise Equivalent Bandwidth (NEB) B 0 NEB -3 dB Area under the response cure = Area under the noise curve. B3dB B Defines as the ideal bandwidth describing the point where: Filter response Prof. Z Ghassemlooy 23 Optical System P(t) m(t) Optical drive circuit Light source Fibre Photodiode ip(t) Amplifier P(t ) Pt (1 Mm(t )) Photocurrent i p (t ) R P (t ) R Pt (1 Mm(t )) Signal current Average photocurrent Photocurrent = + io(t) (DC current) Io Prof. Z Ghassemlooy 24 Optical Receiver - Model The received digital pulse stream incident at the photodetector is given by: P(t ) b h n n p (t nTb ) where Tb is bit period, bn is an amplitude parameter of the nth message digit and h p (t )is the received pulse shape which is positive for all t. Prof. Z Ghassemlooy 25 Optical Receiver - contd. For m(t) = sin t The mean square signal current is is io (t ) 2 2 is io (t )G 2 2 for PIN 2 for APD For a digital signal The mean square signal current is is io (t ) RP (t ) 2 2 for PIN is io (t )G RG P(t ) 2 2 2 2 Prof. Z Ghassemlooy for APD 26 Optical System - Noise Is a random process, which can’t be described as an explicit function of time In the time domain – Can be characterized in probabilistic terms as: Mean - correspond to the signal that we are interested to recover Variance (standard deviation) - represents the noise power at the detector’s output Can also be characterized in terms of the Root Mean Square (RMS) value Time average Prof. Z Ghassemlooy 27 Optical System - Noise • The electric current in a photodetector circuit is composed of a superposition of the electrical pulses associated with each photoelectron • The variation of this current is called shot noise Prof. Z Ghassemlooy 28 Optical System - Noise Sources At the receiver: Additive Signal dependent Modal noise Due to interaction of (constructive & destructive) multiple coherent modes, resulting in intensity modulation. Photodetector Noise Preamplifier (receiver) Noise Distortion due to Non-linearity Crosstalk and Reflection in the Couplers Prof. Z Ghassemlooy 29 Noise - Source Noise - contd. LED: Due to: – In-coherent intensity fluctuation – Beat frequencies between modes LD: Due to: – – – – Non-linearities Quantum noise: In the photon generation Mode hopping: Within the cavity Reflection from the fibre back into the cavity, which reduces coherence – Difficult to measure, to isolate and to quantify – Most problematic with multimode LD and multimode fibre Prof. Z Ghassemlooy 30 Noise Currents Let noise current be defined as: inoise(t) = i(t) - IDC IDC = Photocurrent Io (Amps) Noise current from random current pulses is termed as shot-noise. Shot-noise: • Quantum • Dark current Prof. Z Ghassemlooy 31 Quantum Shot Noise The photons arrive randomly in a packet form, with no two packets containing the same amount of photons. Random generation of electron-hole pair, thus current. Variation of the total current generated, about an average value. This variation is best known as QUANTUM SHOT NOISE. Prof. Z Ghassemlooy 32 Quantum Shot Noise The average number of electronholes pairs per bits is: Where the time period. The probability of detecting n photons in a time period is follows the Poisson Distribution: Incoherent light Y Semenova, DIT, Ireland Coherent light Prof. Z Ghassemlooy 33 Quantum Shot Noise The rate of electron-hole pairs generated by incident photons is: With an ideal receiver with no noise we have: Note that, the minimum pulse energy of the quantum limit is: Prof. Z Ghassemlooy 34 Shot Noise - PIN • The mean square quantum shot noise current on Io ish 2qI o B 2 (A 2 ) • The mean square dark current noise (also classified as shot noise) ids 2qI d B 2 (A 2 ) Where Id = surface leakage current, and B is the electrical bandwidth of the system Q is the electron charge. Total shot noise current ITs = Dark current + Photocurrent The total mean square shot noise iTs 2q( I o I d ) B 2 Prof. Z Ghassemlooy (A 2 ) 35 Noise Power Spectrum Power spectrum I2o ITs2 Shot noise 0 B Modulation bandwidth Prof. Z Ghassemlooy Frequency 36 Shot Noise - APD • The mean square photocurrent noise iTs 2q[( I o I d )G F ]B 2 2 where F = The noise figure = Gx for 0<x<1 G = The optical gain 2 (A ) Bias voltage hf Av RL Prof. Z Ghassemlooy Vo Vi 37 Noise Currents - contd. Thermal Noise ith 2 4 KTB RL RL = Total load seen at the input of the preamplifier K = Boltzmann’s constant = 1.38x10-23 J/K T = Temperature in degree Kelvin = Co + 273 Total Noise PIN iT ish ids ith APD iT ish ids ith 2 2 2 2 Prof. Z Ghassemlooy 2 2 2 2 38 Electrical Amplifier Noise Amplifier type - Voltage Noise - Current Noise BJT va 2 JEFT qI c 2 gm 2 va 2 2 B Total amplifier noise i A Prof. Z Ghassemlooy B ia 2qI g B ia 2qIb B 1 B gm 2 2 2 2 qI d B 2 2 [ i ( v a a / Z )] df 0 39 Receiver Signal-to-Noise Ratio (SNR) hf io • PIN SNR io SNR 2 iT iT Io 2 4 KTB 2 2qB( I o I d ) i A RL 2 • APD iA G2Io SNR 2qB[( I o I d )G Note: SNR cannot be improved be amplification Prof. Z Ghassemlooy 2 x 2 4 KTB ] F i2 A RL 40 SNR - Quantum Limit The mean square quantum shot noise current on Io ish 2qI o B 2 ( Io ) Io SNR)Q 2qIoB (A 2 ) RP oq / hf 2qB RP o / hf 2B nelectron re nelectron / s N B bit / s bit Shot noise Poisson Prof. Z Ghassemlooy 41 Type of Receivers - Low Impedance Voltage Amplifier +Bias - Simple Low sensitivity Limited dynamic range It is prone to overload and saturation Is Output RL 50 Av hf CT RL Amplifier Vo Vi 1 • RC limited bandwidth B 2CT RL RL = Rdetector || Ramp. Prof. Z Ghassemlooy Ramp= High 42 Type of Receivers - High Impedance Voltage Amplifier with Equaliser +Bias Is • High sensitivity • Low dynamic range Output RL Ct Amplifier Equalizer Equaliser Av hf Vo Vi CT RL • Rdetector is large to reduce the effect of thermal noise • Detector out put is integrated over a long time constant, and is restored by differentiation Prof. Z Ghassemlooy 43 Type of Receivers - Transimpedance Feedback Amplifier +Bias • The most widely used Rf Is • Wide bandwidth •High dynamic range • No equalisation • Greater dynamic range (same gain at all frequencies) • Slightly higher noise figure than HIVA Output Ct Amplifier RF Bandwidth Av hf B CT RL Vi Prof. Z Ghassemlooy Vo Av 2CT RF 44 Transimpedance Feedback Amplifier R V* F F V* A -A I* sh RL CT SNR I* Th I* A Vi Vi G 2 I o2 V 1 R {1 3 (2BR C ) }B 2qI G 2 F (G ) 4kT I * o A RT 2 * 2 A T T 2 * Where I . is the noise power spectral density, and RT = RL||RF Prof. Z Ghassemlooy 45 Optical Receiver - Analogue Employ an analogue preamplifier stage, followed by either an analogue output stage (depending on the type of receiver). Comms. Special. Inc. Prof. Z Ghassemlooy 46 Optical Receiver - Digital 1st stage is a current-to-voltage converter. 2nd stage is a voltage comparator, which produces a clean, fast rise-time digital output signal. The trigger level may be adjusted to produce a symmetrical digital signal. Prof. Z Ghassemlooy 47 Optical Transmission - ISI Optical pulse spread after traversing along optical fiber Thus leading to ISI, where some fraction of energy remaining in appropriate time slot, whereas the rest of energy is spread into adjacent time slots. Prof. Z Ghassemlooy 48 Receiver Performance Signal-to-Noise Ratio (SNR) Bit Error Rate (BER) Prof. Z Ghassemlooy 49 SNR In analogue transmissions the performance of the system is mainly determined by SNR at the output of the receiver. In case of amplitude modulation the transmitted optical power P(t) is in the form of: P(t ) Pt [1 Mm(t )] where M is modulation index, and m(t) is the analogue signal. The photocurrent at receiver can be expressed as: is (t ) RGPr [1 Mm(t )] Prof. Z Ghassemlooy 50 SNR The S/N can be written as is2 S (1 / 2)( RGPr ) 2 2 N 2q( RPr I d )G 2 F (G ) B (4k BTB / RT ) F iN (1 / 2)(GI P ) 2 2q ( I P I d )G 2 F (G ) B (4k BTB / RT ) F Note, F is the amplifier noise figure. For PIN: G = 1 So we have S (1 / 2)( I ) 2 2 2 2 ( 1 / 2 ) G R Pr P N (4k BTB / RT ) F (4k BTB / RT ) F And for large signal level Low input signal level S RPr N 4qB Prof. Z Ghassemlooy 51 SNR Vs Receiver Sensitivity Note: Io =RPo G Keiser , 2000 Po(dBm) Prof. Z Ghassemlooy 52 Bit Error Rate (BER) Probability of Error = probability that the output voltage is less than the threshold when a 1 is sent + probability that the output voltage is more than the threshold when a 0 has been sent bo n Variance 2on 1 vth v P1 (v) p( y | 1)dy probablity that the equalizer output vol tage is less than v, if 1 transmitt ed P0 (v) p ( y | 0)dy probablity that the equalizer output vol tage exceeds v, if 0 transmitt ed v Variance 2off Pe q1 P1 (vth ) q 0 P0 (vth ) q1 0 boff vth p( y | 1)dy q p( y | 1)dy 0 vth where q1 and q0 are the probabilities that the transmitter sends 0 and 1 respectively. Note, q0 = 1- q1. Prof. Z Ghassemlooy 53 Bit Error Rate (BER) BER = No. of error over a given time interval/Total no. of bits transmitted P1 (vth ) vth p( y | 1)dy P0 (vth ) p( y | 0)dy vth 1 2 on 1 2 off (v bon ) 2 exp dv 2 2 on vth (v boff ) 2 exp dv 2 2 off vth If we assume that the probabilities of 0 and 1 pulses are equally likely 1 Q BER Pe 1 erf 2 2 where Prof. Z Ghassemlooy vth boff Q off bon vth on 54 Bit Error Rate (BER) - contd. For • off = on = RMS noise • bon = V, and boff = 0 • Thus vth = V/2 and Q = V/2 1 V Pe 1 erf 2 2 2 In terms of power signal-to-noise ratio (S/N) Therefore: 1 S Pe 1 erf 0.345 2 N Prof. Z Ghassemlooy 55 BER Performance Minimum input power depends on acceptable bit error rate Many receivers designed for 1E-12 or better BER G Keiser , 2000 Prof. Z Ghassemlooy 56 Basic Receiver Design Bias Clock Recovery AGC -g Temperature Control Decision Circuit Monitors & Alarms Optimized for one particular – Sensitivity range – Wavelength 0110 Remote Control Can include circuits for telemetry Agilent Tech. – Bit rate Prof. Z Ghassemlooy 57 Optical Receivers - Commercial Devices 28 GHz Monolithic InGaAs PIN Photodetector 100 kHz- 40 Gb/s DC - 65 Gb/s InGaAs PIN Photodiodes 100 GHz Dual-Depletion InGaAs/InP Photodiode Prof. Z Ghassemlooy 58 Wide-Band Optical Receiver (40 Gb/s) • Operating current 75 mA • Bandwidth: 100 KHz to 35 GHz • Power dissipation: 400 mW • Responsivity: 0.6 A/W • Wavelength response: 800 - 1600 nm • Power gain: 8 dB Linearity response Sensitivity response Typical eye diagram Prof. Z Ghassemlooy 59 Wide-Band Optical Receiver (DC - 65 Gb/s) InGaAs PIN Photodiodes Reverse bias voltage: +3V Responsivity: 0.5 A/W at 1300 and 1550 nm wavelength. Opto-electronic Integrated Circuits (OEICs) which combine optical, microwave, and digital functions on the same chip Application: – – – – – Ethernet fiber local area networks Synchronized Optical Network SONET, ISDN, Telephony Digital CATV). Prof. Z Ghassemlooy 60 Regenerator (3R) Receiver followed by a transmitter – No add or drop of traffic – Designed for one bit rate & wavelength Signal regeneration – Reshaping & timing of data stream – Inserted every 30 to 80 km before optical amplifiers became commercially available – Today: reshaping necessary after about 600 km (at 2.5 Gb/s), often done by SONET/SDH add/drop multiplexers or digital crossconnects Fibre Fibre Prof. Z Ghassemlooy 61 Summary Photodiode characteristics Types of photodiode: PIN and APD Photodiode responsivity & equivalent circuit Minimum received power Optical receiver: – Types – Bandwidth Noise Signal-to-noise ratio Bit error rate Receiver design Regenerator Prof. Z Ghassemlooy 62 Next Lecturer Optical Devices Prof. Z Ghassemlooy 63