Optical Fibre Communication Systems

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

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) To recover the information signal Photodetection Amplification (Pre/post) Filtering Signal Processing Converting optical signal into an electrical signal Limiting the bandwidth, thus reducing the noise power Information signal Prof. Z Ghassemlooy 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. Dark current P o Reverse-biased photoconductive operation I Forward-biased Photovoltic operation V Short-circuit photoconductive operation Small dark current due to: leakage thermal excitation Quantum efficiency (electrons/photons) Responsivity Insensitive to temperature variation Prof. Z Ghassemlooy 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[50 V @ = 850 nm, 0-30 V @ = 1300 1550 nm] Costly Prof. Z Ghassemlooy 7

Photodiode (PIN) - Structure Depletion region Photons n electron I p hole No carriers in the I region No current flow Output The power level at a distance x into the material is: Where is the photon absorption coefficient I o n electron R L (load resistor) p hole Reverse-biased Photons generated electron-hole pair Photocurrent flow through the diode and in the external circuitry Prof. Z Ghassemlooy 8 I Bias voltage

Photodiode (PIN) - Structure Depletion region width The capacitance of the depletion layer Cj (F) is: Prof. Z Ghassemlooy 9

Photodetector - Reponsivity PIN: APD: R = I o /P o A/W R APD = G R I o = Photocurrent; P o = Incident (detected) optical power G = APD gain; = Quantum efficiency = average number of electron-hole pairs emitted r e / average number of incident photons r p Note: r p = P o /hf and re = I o /q = 99% ~ 1 l is the length of the photoactive region I o = qp o /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, 000 Prof. Z Ghassemlooy 11

Photodetector - Equivalent Circuit Photodiode R s Contact leads L Amplifier I o C j R j R L R amp C amp Output L = Large, (i.e o/c)r s = Small, (i.e s/c) C T = C j + C amp R T = R j R L R amp The transfer function is given by: Prof. Z Ghassemlooy 1

Photodetector - Equivalent Circuit The detector behaves approximately like a first order RC low-pas filter with a bandwidth of: f B 1 C R T T 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, 000 Prof. Z Ghassemlooy 14

Photodiode Pulse Responses Small area photodiode Large area photodiode Small area photodiode w = depletion layer s = absorption coefficient G Keiser, 000 Due to carrier generated in w Due to diffusion of carrier from the edge of w Prof. Z Ghassemlooy 15

Photodetetor Typical Characteristics Parameters Wavelength range Peak (nm) PIN Si APD 400-1100 900 830 PIN Ge APD 800-1800 1550 1300 PIN InGaAS APD 900-1700 1300 1300 (1550) (1550) Responsivity (A/W) 0.35-0.55 50-10 0.5-0.65.5-5 0.5-0.7 - Quantum Efficiency (%) 65-90 77 50-55 55-75 60-70 60-70 Bias voltage (-V) 45-100 0 6-10 0-35 5 <30 Dark current (na) 1-10 0.1-1 50-500 10-500 - 1-5 Rise time (ns) 0.5-1 0.1-0.1-0.5 0.5-0.8 0.06-0.5 0.1-0.5 Capacitance (pf) 1.-3 1.3- -5-5 0.5-0.1-0.5 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 P r Power loss P o Amplifier MRP = Minimum Detected Power (MDP) Coupling Loss Prof. Z Ghassemlooy 17

MRP (-dbm) MRP Vs. Bandwidth -0-30 SNR (db) 50-40 -50-60 30 10 0 =1300-70 1 5 10 0 50 100 00 500 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 0

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 1

Ideal, Low-pass and Band-pass Filters 0 db Band-pass filter Low-pass filter -3 Higher order filter Ideal B bp B lp Frequency Prof. Z Ghassemlooy

Noise Equivalent Bandwidth (NEB) B 0-3 db NEB Defines as the ideal bandwidth describing the point where: Area under the response cure = Area under the noise curve. B 3dB B Filter response Prof. Z Ghassemlooy 3

Optical System m(t) Optical drive circuit Light source Fibre P(t) Photodiode i p (t) Amplifier Photocurrent i p Photocurrent = P( t) Pt (1 Mm( t)) ( t) R P( t) R P(1 Mm( t)) Average photocurrent (DC current) I o t + Signal current i o (t) Prof. Z Ghassemlooy 4

Optical Receiver - Model The received digital pulse stream incident at the photodetector is given by: P( t) wheret and h p n b b n h p ( t nt ) is bit period, b b n is an amplitude parameter of the nth message ( t)is the received pulseshape which is positivefor all t. digit Prof. Z Ghassemlooy 5

Optical Receiver - contd. For m(t) = sin t The mean square signal current is i s i o ( t) for PIN i s i o ( t) G for APD For a digital signal The mean square signal current is i s i o ( t) RP( t) for PIN i s i o ( t) G RG P( t) for APD Prof. Z Ghassemlooy 6

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 7

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 8

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 9

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: i noise (t) = i(t) - I DC (Amps) I DC = Photocurrent I o 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 3

Shot Noise - PIN The mean square quantum shot noise current on I o i sh qi o B (A ) The mean square dark current noise (also classified as shot noise) i ds qi d B (A Where I d = surface leakage current, and B is the electrical bandwidth of the system Q is the electron charge. Total shot noise current I Ts = Dark current + Photocurrent ) The total mean square shot noise i Ts q( I o I d ) B (A ) Prof. Z Ghassemlooy 33

Noise Power Spectrum Power spectrum I o I Ts Shot noise 0 Modulation bandwidth B Frequency Prof. Z Ghassemlooy 34

Shot Noise - APD The mean square photocurrent noise i Ts q[( I o I d ) G F ] B (A ) where F = The noise figure = G x for 0<x<1 G = The optical gain hf Bias voltage V o A v R L V i Prof. Z Ghassemlooy 35

Noise Currents - contd. Thermal Noise i th 4KTB R L R L = Total load seen at the input of the preamplifier K = Boltzmann s constant = 1.38x10-3 J/K T = Temperature in degree Kelvin = C o + 73 Total Noise PIN i T i sh i ds i th APD i T i sh i ds i th Prof. Z Ghassemlooy 36

Electrical Amplifier Noise Amplifier type BJT JEFT - Voltage Noise v qi c a gm B v qi d a gm B - Current Noise i qi B i qi B a b a g Total amplifier noise i 1 A B B 0 [ i a ( v a / Z)] df Prof. Z Ghassemlooy 37

Receiver Signal-to-Noise Ratio (SNR) hf i o i T i A SNR i o i T PIN APD SNR SNR Note: SNR cannot be improved be amplification o I 4KTB qb( Io Id ) i R A x qb[( Io Id ) G ] F RL Prof. Z Ghassemlooy 38 G I o L 4KTB i A

SNR - Quantum Limit The mean square quantum shot noise current on I o i sh qi o B (A ) SNR) Q ( Io) Io qiob Poq / hf qb Po / hf B re n B bit electron / / s s n electron bit N Shot noise Poisson Prof. Z Ghassemlooy 39

Type of Receivers - Low Impedance Voltage Amplifier - Simple - Low sensitivity - Limited dynamic range - It is prone to overload and saturation I s +Bias RL 50 Amplifier Output hf A v V o C T R L V i RC limited bandwidth R L = R detector R amp. B 1 C T R L R amp = High Prof. Z Ghassemlooy 40

Type of Receivers - High Impedance Voltage Amplifier with Equaliser +Bias I s High sensitivity Low dynamic range RL Ct Amplifier Equalizer Output Equaliser hf A v V o V i C T R L R detector 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 41

Type of Receivers - Transimpedance Feedback Amplifier The most widely used Wide bandwidth High dynamic range No equalisation Greater dynamic range (same gain at all frequencies) Slightly higher noise figure than HIVA I s Ct +Bias Rf Amplifier Output R F hf C T Av R L V i V o Bandwidth B AT C R F v Prof. Z Ghassemlooy 4

Transimpedance Feedback Amplifier V * F R F V * A -A I * sh CT RL I * Th I * A V i V i SNR qi o G F( G) 4kT RT G I V R * A I { 1 ( BRTC) } B A o * T 1 3 Where I *. is the noise power spectral density, and R T = R L R F Prof. Z Ghassemlooy 43

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 44

Optical Receiver - Digital 1st stage is a current-to-voltage converter. nd 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 45

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 46

Receiver Performance Signal-to-Noise Ratio (SNR) Bit Error Rate (BER) Prof. Z Ghassemlooy 47

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: i s ( t) RMP [1 Mm( t)] r Prof. Z Ghassemlooy 48

SNR The S/N can be written as S N i i s N q( RP r I d (1/ )( RMGPr ) ) G F( G) B (4k B TB/ R T ) F q( I P I d (1/ )( GMIP ) ) G F( G) B (4k B TB/ R T ) F Note, F is the amplifier noise figure. For PIN: G = 1 So we have S (1/ )( I PM ) N (4k TB/ R ) F And for large signal level B T S N (1/ ) MG R Pr (4k TB/ R ) F B M RP 4qB Prof. Z Ghassemlooy r T Low input signal level 49

SNR Vs Receiver Sensitivity Note: I o =RP o G Keiser, 000 P o (dbm) Prof. Z Ghassemlooy 50

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 b o n Variance on 1 v th P ( v) 1 P ( v) 0 v v P p( y 1) dy p( y 0) dy e q probablity that theequalizer output voltage is less than v,if 1 transmitted probablity that theequalizer output voltage exceeds 1 P ( v 1 th ) q 0 P ( v 0 th ) v,if 0 transmitted Variance off b off 0 q v th 1 p( y 1) dy q 0 v th p( y 1) dy where q 1 and q 0 are the probabilities that the transmitter sends 0 and 1 respectively. Note, q 0 = 1- q 1. Prof. Z Ghassemlooy 51

Bit Error Rate (BER) BER = No. of error over a given time interval/total no. of bits transmitted P ( v 1 th P ( v 0 th ) ) v v th th p( y 1) dy p( y 0) dy 1 1 on off v th v exp th exp ( v b on on ( v b ) off off ) dv dv If we assume that the probabilities of 0 and 1 pulses are equally likely BER P 1 1 erf Q vth boff bon vth e where Q off on Prof. Z Ghassemlooy 5

Bit Error Rate (BER) - contd. For off = on = RMS noise b on = V, and b off = 0 Thus v th = V/ and Q = V/ Therefore: P 1 1 V e erf In terms of power signal-to-noise ratio (S/N) 1 P e 1 erf 0. 345 S N Prof. Z Ghassemlooy 53

BER Performance Minimum input power depends on acceptable bit error rate Many receivers designed for 1E-1 or better BER G Keiser, 000 Prof. Z Ghassemlooy 54

Basic Receiver Design Bias AGC Clock Recovery -g Decision Circuit 0110 Temperature Control Monitors & Alarms Remote Control Optimized for one particular Sensitivity range Wavelength Bit rate Can include circuits for telemetry Agilent Tech. Prof. Z Ghassemlooy 55

Optical Receivers - Commercial Devices 8 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 56

Wide-Band Optical Receiver (40 Gb/s) Bandwidth: 100 KHz to 35 GHz Responsivity: 0.6 A/W Wavelength response: 800-1600 nm Operating current 75 ma Power dissipation: 400 mw Power gain: 8 db Linearity response Sensitivity response Typical eye diagram Prof. Z Ghassemlooy 57

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 58

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.5 Gb/s), often done by SONET/SDH add/drop multiplexers or digital crossconnects Fibre Fibre Prof. Z Ghassemlooy 59

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 60

Next Lecturer Optical Devices Prof. Z Ghassemlooy 61