UNIT - 5 LECTURE-1 OPTICAL RECEIVER Introduction, Optical Receiver Operation, receiver sensitivity, quantum limit, eye diagrams, coherent detection, burst mode receiver operation, Analog receivers. RECOMMENDED READINGS: TEXT BOOKS: 1. Optical Fiber Communication Gerd Keiser, 4 th Ed., MGH, 2008. 2. Optical Fiber Communications John M. Senior, Pearson Education. 3 rd Impression, 2007. REFERENCE BOOK: 1. Fiber optic communication Joseph C Palais: 4 th Edition, Pearson Education. 5.1 Introduction and Operation of Optical Receiver: An optical receiver system converts optical energy into electrical signal, amplify the signal and process it. Therefore the important blocks of optical receiver are : - Photo detector / 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 criterion of optical receiver is average error probability. Receiver Configuration Configuration of typical optical receiver is shown in Fig. 5.1.2. Photo detector parameters - PIN or APD type - Gain M = 1 - Quantum efficiency η - Capacitance C d - Dias resistance R b - Thermal noise current i b (t) generated by R b. Amplifier parameters - Input impendence R a - Shunt input capacitance C o - Transconductance g m (Amp/volts) - Input noise current i a (t) because of thermal noise of R a - Input noise voltage source e a (t) Equalizer is frequency shapping filter used to mollify the effects of signal distortion and ISI. Expression for Mean Output Current from Photodiode Assumption : 1. All noise sources are Gaussian in statistics. 2. All nose sources are flat in spectrum. 3. All noise sources are uncorrelated (statistically independent). Binary digital pulse train incident on photodector is given by
(5.1.1) Where, P(t) is received optical power. T b is bit period. b n is amplitude parameter representing nth message bit. h p (t) is received pulse shape. At time t, the mean output current due to pulse train P(t) is (5.1.2) Where, M is gain of photodectector is responsivity of photodiode Neglecting dark current, the mean output current is given as Then mean output current is amplified, filtered to give mean voltage at the output. Preamplifier Types (5.1.3) 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 R b is used to match the amplifier impedance. R b 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 R b. The high impedance amplifier uses FET or a BJT. As the high impedance circuit has large RC time constant, the bandwidth is reduced. Fig. 5.1.3 shows equivalent circuit of high input impedance pre-amplifier. 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. Trans impedance preamplifier (TZ) The drawbacks of ghigh input impedance are eliminated in transimpedance preamplifier. A negative feedback is introduced by a feedback resistor R f to increase the bandwidth of open loop preamplifier with an equivalent thermal nose current i f (t) shunting the input. An equivalent circuit of transimpedance preamplifier is shown in Fig. 5.1.4. e a (t) = Equivalent series voltage noise source i a (t) = Equivalent shunt current noise. R in = R a C a. R f = Feedback resistor.
i f (t) = Equivalent thermal noise current. Although the resulting receiver is often not as sensitive as the integrating front end design, this type of preamplifier does not exhibit a high dynamic range and is usually cheaper to produce. High Impedance FET Amplifier High input impedance preamplifier using FET is shown in Fig. 5.1.5. Basic noise sources in the circuit are - Thermal noise associated with FET channel. - Thermal noise from load. - Thermal noise from feedback resistor. - Shot noise due to gate leakage current (I gate ) - FET 1/f noise.
As the amplifier input resistance is very high, the input current noise spectral density S 1 is expressed as (5.1.4) Thermal noise associated with FET channel The voltage noise spectral density is (5.1.5) where, g m 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 = C d + C gs + C gd + C s (5.1.6) If bias resistor R b 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 g m is selected. For high data rates GaAs MESFET are suitable while at lower frequencies silicon MOSFETs or JFET are preferred
High Impedance Bipolar Transistor Amplifier High input impedance preamplifier using BJT is shown in Fig. 5.1.6. Input resistance of BJT is given as (5.1.7) Where, I BB is base bias current. Spectral density of input noise current source because shot noise of base current is (5.1.8) Spectral height of noise voltage source is given as (5.1.9) Where, gm is transconductance. The performance of receiver is expressed by thermal noise characteristic equation (W)
Substituting R in, S I and S E in characteristic equation. (5.10.10) Where, If R b >> R in, then R R in, the expression reduces to (5.1.11) Transimpedance Amplifier An ideal transimpedance preamplifier provides an output voltage which is directly proportional to the input current and independent of course and load impedance. A transimpednace amplifier is a high-gain high-impedance amplifier with feedback resistor Rf Fig. 5.1.7 shows a simple CE/CC. Shunt feedback transimpedance receiver. Bandwidth (BW) To find BW, the transfer function of non-feedback amplifier and feedback amplifier is compared. The transfer function of non-feedback amplifier is (5.1.12)
Where, A is frequency independent gain of amplifier. Now the transfer function of feedback (transimpedance) amplifier is (5.1.13) This yields the BW of transimpedance amplifier. (5.1.14) i.e. 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 (5.1.5) Where, W HZ is noise characteristic of high-impedance amplifier (non-feedback amplifier). Thus thermal nose of transimpedance amplifier is sum of output noise of non-feedback amplifier and noise associated with R f. Benefits of Tran s impedance amplifier 1. Wide dynamic range: As the BW of Trans impedance preamplifier is high enough so that no integration takes place and dynamic range can be set by maximum voltage swing at preamplifier output. 2. No equalization required: Since combination of R in and R f is very small hence the time constant of detector is small. 3. Less susceptible to external noise: The output resistance is small hence the amplifier is less susceptible to pick up noise, crosstalk, RFI and EMI. 4. Easy control: Transimpedance amplifiers have easy control over its operation and is stable. 5. Compensating network not required: Since integration of detected signal does not occur, compensating network is not required.
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.