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Note for Optical Communication and Network - OCN By Sundram Gupta

  • Optical Communication and Network - OCN
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MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI - 621213 DEPARTMENT SUBJECT NAME SUBJECT CODE : ECE : OPTICAL COMMUNICATION & NETWORKS : EC 2402 UNIT – IV: FIBER OPTIC RECEIVER AND MEASUREMENT PART -A (2 Marks) 1. What are requirements of an optical receiver ?[AUC NOV 2006] • Light detector • Pre amplifier • Equalizer • Signal discriminator circuits 2. List out various error sources? [AUC MAY 2013/NOV 2012] • Quantum noise • Bulk dark current noise • Surface leakage current noise • Thermal noise • Amplifier noise 3. Why do we prefer trans-impedance pre amplifier rather than high impedance preamplifier? [AUC MAY 2007] • Since the high impedance produces large input RC time constant, the front end bandwidth is less than the signal bandwidth. This drawback is overcome in the trans-impedance amplifier. 4. Define threshold level. [AUC NOV 2009] • A decision circuit compares the signal in each time slot with a certain reference voltage known as threshold level. 5. Define quantum limit? [AUC MAY 2013] • It is possible to find the minimum received optical power required for a specific bit error rate performance in a digital system. This minimum received power level is known as quantum limit. EC 2402 OPTICAL COMMUNICATION & NETWORKS – IV/VII - V.SENTHAMIZH SELVAN ASST. PROF/ECE

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6. What are the methods used to measure the fiber refractive index profile? [AUC MAY 2012] • • • Interferometric method Near field method Refracted near field method 7. Define dark current. [AUC NOV 2012] • It is the current to flow through thr bias current of the device when no light is incident on photo diode. 8. What are the advantages of preamplifier [AUC NOV 2011] • Low noise level • High bandwidth • High dynamic range • High sensitivity • High gain 9. List out the advantages of outer diameter measurement. [AUC NOV 2009] • Speed is large • More accuracy • Faster diameter measurements 10. Define effective cutoff wavelength? [AUC April 2004, MAY2010] • It is defined as wavelength greater than the ratio between the total power to the launched higher order modes and fundamental mode power. 11. Define BER? [AUC MAY2012] • An approach is to divide the number of errors occurring over a certain time interval t by the number of pulses transmitted during this interval. This is called bit error rate or error rate. 12. What are the requirements of preamplifier. [AUC MAY 2008] • Preamplifier bandwidth must be greater than or equal to signal bandwidth. • It must reduce all source of noise • It must have high receiver sensitivity EC 2402 OPTICAL COMMUNICATION & NETWORKS – IV/VII - V.SENTHAMIZH SELVAN ASST. PROF/ECE

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13. Compare the performance of APD and PIN diode. [AUC NOV 2008] S.No Parameters 1 Sensitivity PIN Less sensitive (0- 12 dB) APD More sensitive (5-15 dB) 2 Biasing Low reverse biased voltage High reverse biased voltage (5 to 10 V) (20- 400 volts) 3 Wavelength region 300- 1100 nm 400 -1000 nm 4 Gain No Internal gain Internal gain PART (B) 1. Explain the fiber optic receiver operation? [AUC NOV 2010] The receiver must first detect weak, distorted signal and then make decisions on what type of data was sent based on amplified version of the distorted signal. To understand the function of the receiver, we first examine what happens to the signal as it is sent through the optical data link which is shown in the following figure. Fig: Signal path through an optical data link EC 2402 OPTICAL COMMUNICATION & NETWORKS – IV/VII - V.SENTHAMIZH SELVAN ASST. PROF/ECE

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A digital fiber transmission link is shown in the above figure. The transmitted signal is a two level binary data stream consisting of either a 0 or 1 in a time slot of duration T b . This time slot is referred to as bit period. Electrically there are many ways of sending a given digital message. One of the simplest techniques for sending binary data is Amplitude Shift Keying (ASK), wherein a voltage level is switched between two values, which are generally on and off. The resultant signal wave thus consists of a voltage pulse of amplitude V relative to zero voltage level when a binary 1 occurs and a zero voltage level space when a binary 0 occurs. When a 1 is sent, a voltage pulse of duration T b occurs, whereas for a 0 the voltage remains at its zero level. The function of the optical transmitter is to convert the electric signal to an optic signal. Here 1 is represented by a pulse of optical power (light) of duration T b , whereas a 0 is the absence of any light. The optical signal that gets coupled from the light source to the fiber becomes attenuated and distorted as it propagates along the fiber waveguide. Upon reaching the receiver either a pin or an avalanche photodiode converts the optical signal back to an electric format. The electric signal then gets amplified and filtered. A decision circuit compares the signal in each time slot with a certain reference voltage known as the threshold level. If the received signal level is greater than the threshold level, a 0 is assumed to be received. In some cases an optical amplifier is placed ahead of the photodiode to boost the optical signal level before photodetection. This is done so that the signal to noise ratio degradation caused by thermal noise in the receiver electronics can be suppressed. Compared to APD’s or optical heterodyne detectors, an optical preamplifier provides a large gain factor and a broader bandwidth. 2. Explain error sources of optical receiver. [AUC NOV 2010] Error Sources: Errors arise from various noise and disturbances associated with the signal detection system which is shown in the following figure. Fig: Noise sources and disturbances in the optical pulse detection mechanism. The term noise is used to describe unwanted components of an electric signal that tend to disturb the transmission and processing of the signal in a physical system. The noise sources can be either external or the system (for example atmospheric noise, equipment generated noise) or internal to the system. Let us consider the internal noise. This noise is caused by the EC 2402 OPTICAL COMMUNICATION & NETWORKS – IV/VII - V.SENTHAMIZH SELVAN ASST. PROF/ECE

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