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ECE-VII- Optical Fiber Communication U5
Electronic and communication (ECE)
Visvesvaraya Technological University
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Optical Fiber Communication 10EC
UNIT - 5
OPTICAL RECEIVER
Introduction, Optical Receiver Operation, receiver sensitivity, quantum limit, eye diagrams, coherent detection, burst mode receiver operation, Analog receivers.
RECOMMENDED READINGS:
TEXT BOOKS:
Optical Fiber Communication – Gerd Keiser, 4th Ed., MGH, 2008.
Optical Fiber Communications – – John M. Senior, Pearson Education. 3rd Impression, 2007.
REFERENCE BOOK:
- Fiber optic communication – Joseph C Palais: 4th Edition, Pearson Education.
Optical Fiber Communication 10EC
5 Optical Receiver Design
An optical receiver system converts optical energy into electrical signal, amplify the signal and process it. Therefore the important blocks of optical receiver are :sdsfd - Photodetector / Front-end - Amplifier / Liner channel - Signal processing circuitry / Data recovery.
Noise generated in receiver must be controlled precisely as it decides the lowest signal level that can be detected and processed. Hence noise consideration is an important factor in receiver design. Another important performance criteria of optical receiver is average error probability.
Receiver Configuration
Configuration of typical optical receiver is shown in Fig. 5.1.
Photodetector parameters – - PIN or APD type
Optical Fiber Communication 10EC
is responsivity of photodiode
Neglecting dark current, the mean output current is given as –
... (5.1)
Then mean output current is amplified, filtered to give mean voltage at the output.
Preamplifier Types
The bandwidth, BER, noise and sensitivity of optical receiver are determined by preamplifier stage. Preamplifier circuit must be designed with the aim of optimizing these characteristics. Commonly used preamplifier in optical communication receiver are – 1. Low – impedance preamplifier (LZ) 2. High – impedance preamplifier (HZ) 3. Transimpedance preamplifier (TZ) 1. Low – impedance preamplifier (LZ) In low-impedance preamplifier, the photodiode is configured in low – impedance amplifier. The bias resister Rb is used to match the amplifier impedance. Rb along with the input capacitance of amplifier decides the bandwidth of amplifier. Low – impedance preamplifier can operate over a wide bandwidth but they have poor receiver sensitivity. Therefore the low – impedance amplifier are used where sensitivity is of not prime concern. 2. High – impedance preamplifier (HZ) In high – impedance preamplifier the objective is to minimize the noise from all sources. This can be achieved by – - Reducing input capacitance by selecting proper devices. - Selecting detectors with low dark currents. - Minimizing thermal noise of baising resistors. - Using high impedance amplifier with large Rb. The high impedance amplifier uses FET or a BJT. As the high impedance circuit has large RC time constant, the bandwidth is reduced. Fig. 5.1 shows equivalent circuit of high input impedance pre-amplifier.
Optical Fiber Communication 10EC
High-input impedance preamplifier are most sensitive and finds application in long – wavelength, long haul routes. The high sensitivity is due to the use of a high input resistance (typically > 1 MΩ), which results in exceptionally low thermal noise. The combination of high resistance and receiver input capacitance, results in very low BW, typically < 30 kHz, and this causes integration of the received signal. A differentiating, equalizing or compensation network at the receiver output corrects for this integration. 3. Transimpedance preamplifier (TZ) The drawbacks of ghigh input impedance are eliminated in transimpedance preamplifier. A negative feedback is introduced by a feedback resistor Rf to increase the bandwidth of open loop preamplifier with an equivalent thermal nose current if (t) shunting the input. An equivalent circuit of transimpedance preamplifier is shown in Fig. 5.1.
ea (t) = Equivalent series voltage noise source
ia(t) = Equivalent shunt current noise.
Rin = Ra ║Ca.
Rf = Feedback resistor.
if (t) = Equivalent thermal noise current.
Optical Fiber Communication 10EC
As the amplifier input resistance is very high, the input current noise spectral density S 1 is expressed as –
... (5.1)
Thermal noise associated with FET channel
The voltage noise spectral density is –
... (5.1)
where,
gm is transconductance.
Γ is channel noise factor.
Thermal noise characteristic equation is a very useful figure of merit for a receiver as it measures the noiseness of amplifier. The equation is reproduced here –
Substituting S1 and SE, the equalizer output is then
Where, C = Cd + Cgs + Cgd + Cs ... (5.1)
If bias resistor Rb is very large, so that the gate leakage current is very low. For this the detector output signal is integrated amplifier input resistence. It is to be compensated by differentiation in the equalizer. The integration – differentiation is known as high input impedance epreamplifier design technique. However, the integration of receive signal at the front end restricts the dynamic range of receiver. It may disrupt the biasing levels and receiver may fail. To correct it the line coded data or AGC may be employed such receivers can have dynamic ranges in excess of 20 dB. Of course, FET with high gm is selected. For high data rates GaAs MESFET are suitable while at lower frequencies silicon MOSFETs or JFET are preferred .
Optical Fiber Communication 10EC
High Impedance Bipolar Transistor Amplifier
High input impedance preamplifier using BJT is shown in Fig. 5.1.
Input resistance of BJT is given as –
... (5.1)
Where,
IBB is base bias current.
Spectral density of input noise current source because shot noise of base current is –
... (5.1)
Spectral height of noise voltage source is given as –
... (5.1)
Where, gm is transconductance.
The performance of receiver is expressed by thermal noise characteristic equation (W)
Optical Fiber Communication 10EC
Where,
A is frequency independent gain of amplifier.
Now the transfer function of feedback (transimpedance) amplifier is –
... (5.1)
This yields the BW of transimpedance amplifier.
... (5.1)
i. BW of transimpedance amplifier is A times that of high-impedance amplifier. Because of this equalization becomes easy.
Characteristic equation
The thermal noise characteristic equation (W) is reduced to –
... (5.1)
Where,
WHZ is noise characteristic of high-impedance amplifier (non-feedback amplifier).
Thus thermal nose of transimpedance amplifier is sum of ooutput noise of non-feedback amplifier and noise associated with Rf.
Benefits of transimpedance amplifier
- Wide dynamic range : As the BW of transimpedance preamplifier is high enough so that no integration takes place and dynamic range can be set by maximum voltage swing at preamplifier output.
- No equalization required : Since combination of Rin and Rf is very small hence the time constant of detector is small.
- Less susceptibe to external noise : The output resistance is small hence the amplifier is less susceptible to pick up noise, crosstalk, RFI and EMI.
- Easy control : Transimpedance amplifiers have easy control over its operation and is stable.
- Compensating network not required : Since integration of detected signal does not occur, compensating network is not required.
Optical Fiber Communication 10EC
High Speed Circuit
Now fiber optic technology is widely employed for long-distance communication, LAN and in telephone networks also because of improvement in overall performance, reliable operation and cost effectiveness. Fiber optic link offers wide bandwidth to support high speed analog and digital communication.
Because of advancement in technology minimized transmitters and receivers and available in integrated circuits package.
5 Receiver Noise
In a receiver system errors arises because of noises and disturbances in the signal detection system. Noise is an unwanted electric signal in signal processing. The noise sources can be internal or external to the system. Only the internal sources of noise are considered here. The nose is generated by spontaneous fluctuations of current and voltage (e. shot noise, thermal noise). When photons incident on the photodetector are random in nature, quantum noise (shot noise) is generated. This noise is significant for both PIN and APD receivers. Other sources of photodetector noise are from dark current and leakage current. These noise can be reduced considerably by choosing proper components. Thermal noise is generated from detector load resistances.
Intersymbol interference (ISI) also contributes to error which is causing from pulse spreading. Because of pulse spreading energy of a pulse spreads into neighbouring time slots, results in an interfering signal. Fig. 5.2 shows ISI.
Optical Fiber Communication 10EC
... (5.2)
Where, SI is the spectral density of amplifier input noise current source. iii) Thermal noise due to amplifier input noise voltage source ea (t) :
... (5.2)
Where,
SE is the spectral density of amplifier noise voltage source.
Be is noise equivalent BW of amplifier.
iv) Mean square shot noise :
... (5.2)
Where,
is mean square avalanche gain.
is photocurrent.
All constituents of mean square noise voltage are summarized here.
5 Receiver Sensitivity
To calculate optical receiver sensitivity, total noise in the receiver is calculated.
Optical Fiber Communication 10EC
Substituting these values and solving equation (5.2) gives
and
Substituting these values and solving equation (5.2) gives
... (5.3)
... (5.3)
Where, ... ( 5.3)
This equation is known as thermal noise characteristic of an optical receiver.
The optimum gain to achieve desired BER for receiver is given by –
... (5.3)
Assuming no ISI i. γ =
Where,
Q is parameter related so S/N ratio to achieve desired BER.
W is thermal noise characteristic of receiver. X is photodiode factor.
I2 is normalized BW.
Mean Square Input Noise Current
The mean square input noise current is gives as –
... (5.3)
i) Shot noise Current :
Optical Fiber Communication 10EC
Mean square quantum noise current
... Ans.
Mean spark dark current
= 2 (1 x 10-19) (20 x 10 6 ) (4 x 10-9)
= 0 x 10-19 Amp ... Ans.
Mean square thermal noise current
Where B is Boltzman constant = 1 x 10-23 J/K
T = (25o+273o) = 298 K
= 3 x 10-16 Amp ... Ans.
5 Analog Receivers
Fiber optic transmission also supports analog links i. voice channels. The performance of analog receiver is measured interms of S/N ratio (ratio of mean square signal current to mean square noise current). The current generated at optical receiver by analog optical signal is given as –
... (5.4)
Where,
Optical Fiber Communication 10EC
is responsivity.
M is photodetector gain.
Pr is average received power.
Ip is primary photocurrent
Mean – square signal current at photodetector, neglecting d. term is
... (5.4)
For a photodiode detector mean noise current is sum of i) Mean square quantum noise current. ii) Equivalent resistance thermal noise current. iii) Dark noise current. iv) Surface leakage noise current..
Where,
Ip is primary photocurrent.
ID is primary dark current.
IL is surface leakage current.
F (M) is photodiode noise factor.
B is effective noise BW.
Req is equivalent resistance of photodetector and amplifier.
Ft is noise figure of baseband amplifier.
Signal – to – noise ratio (S/N ratio) is given as –
... (5.4)
Optical Fiber Communication 10EC
BER for optical fiber communication system is ranging between 10-9 to 10-12. BER of receiver depends on S/N ratio. To compute the BER at receiver probability distribution of output signal is considered. Condiitonal PDF : P(y/x) is the probability that the output voltage is y when x was transmitted. The functions p (y/1) and p (y/0) are conditional PDF as shown in Fig. 5.5.
The probability distributions are given as –
It is the probability that output voltage is less than threshold when logic ‘1’ is sent.
It is the probability that output voltage exceeds threshold voltage when a logic ‘0’ is sent.
The error probability is expressed as
... (5.5)
Where,
a and b are probabilities that either 1 or 0 occurs.
... (5.5)
Where,
Optical Fiber Communication 10EC
V is the pulse amplitude.
σ is standard deviation (measure of width of probability distribution)
Quantum Limit
For an ideal photodetector having quantum efficiency η = 1 and has zero dark current (i. no output when light is absent) then the minimum received power for a specific bit – error rate is known as Quantum Limit. Let an optical pulse of energy E is incident on photoetector in time interval τ. Then the probability of emitting zero electrons during the interval is
... (5.5)
Where,
XXXX is average number of electron – hole pairs.
Example 5.5 : A digital fiber link operating at 850 nm requires a BER of 10-9. Calculate the quantum limit in terms of quantum efficiency.
Solutions : λ = 850 nm = 850 x 10-9 m
BER = 10-
Probability of error
No. of electron – hole paid generated (XXXX), quantum efficiency (η), photo energy (hv) and energy received (E) are related by,
ECE-VII- Optical Fiber Communication U5
Course: Electronic and communication (ECE)
University: Visvesvaraya Technological University
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