Anna University Chennai 2005 B.E OPTO ELECTRONICS AND OPTICAL COMMUNICATIONS - Question Paper

(A) its low transmission losses.
(C) high optical carrier frequency.
(B) electrical isolation and immunity to
interference.
(D) good quality optical sources.
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PART I
ans any 3 ques.. every ques. carries 14 marks.
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Q.2
discuss the concept of electromagnetic modes in relation to a planar optical waveguide and discuss the modification needed in the theory to define optical propagation in a cylindrical fiber. (7)

define with the aid of suitable diagrams, the major strategies and structures utilized in the fabrication of single frequency injection lasers.
(7)
Q.3
define what is meant by the fusion splicing of optical fibers. explain the advantages and drawbacks of this jointing technique. (7)

Outline the 3 major categories of passive wave length division multiplexing coupler. define in detail 1 implementation of every category. discuss the relative merits and draw backs associated with every of the WDM devices you have defined.


(7)
Q.4
explain lens coupling of LED's to optical fibers and outline the different techniques employed. (8)

The minority carrier recombination lifetime for an LED is 5ns. when a constant d.c. drive current is applied to the device and the optical output power is 300 W . Determine the optical output power when the device is modulated with an rms drive current corresponding to the d.c. drive current at frequency of 100 MHz
(6)


Q.5
Outline the phenomena that can affect the stability of the transmission characteristics in optical fiber cables and define any technique by which these issues may be avoided. (6)

describe the noise equivalent power (NEP) for a photodetector. Starting with photocurrent, derive an expression for the NEP of a photo diode in which the dark current noise dominates. (8)
Q.6
explain the implications of the load resistance on both thermal noise and post detection bandwidth in optical fiber communication receivers.


(6)

Outline the major techniques employed to achieve nonsynchronous optical ASK and FSK heterodyne detection. Indicate the benefits of these schemes over the corresponding synchronous demodulation schemes.


(8)
_______________________________________________________________
PART I
ans any 3 ques.. every ques. carries 14 marks.
_______________________________________________________________
Q.7
define the degradation mechanisms in injection lasers. Comment on these with regard to the CW lifetime of the devices. (7)

A Distributed Feedback (DFB) laser has a 3dB linewidth of 50 MHz. It is connected to a short optical jumper cable such that . find the avg. reflectivity for every of the connectors so that the RIN is decreased beneath a level of -130 dB Hz -1 when the frequency f is also 50 MHz.


(7)


Q.8
What is meant by the excess avalanche noise factor ? provide 2 possible ways of expressing this factor in analytical terms. (7)

A germanium APD (with u = 1.0) operates at a wavelength of 1.35m where its responsitivity in 0.45 AW -1. The dark current is 200 nA at the operating temperature of 250 K and the device capacitance is 3PF. Determine the maximum possible SNR when the incident optical power is eight x 10-7W and the post detection bandwidth without equalization is 560 MHz.
(7)
Q.9
The subsequent specification are envisaged for a single mode fiber communication system employing cascaded travelling wave Semiconductor Laser Amplifier (SLA):
Operating wavelength 1.30 m
Power launched at transmitter two dBm
Fiber cable loss 0.40 dB Km -1
Fiber splice losses 0.02 dB Km -1
K (amplifiers) 6
Amplifier separation 50 Km
Received SNR 20dB
Assuming amplifier noise to be the limiting factor, estimate the maximum transmission rate allowed for the system when the total system length is 8000 km. (14)


Q.10
compute the absolute maximum repeater spacing that could be given to maintain a BER (Bit fault Rate) of 10 -9 within coherent optical fiber system operating at a wavelength of 1.55 m when the fiber and splice/connector losses avg. out at 0.2 dB/Km the optical power launched into the fiber link is 2.5 mW and the transmission rates are 50Mbits/sec and one Gbits/sec. For both bits rates consider the subsequent ideal receiver kinds :
(i) ASK heterodyne synchronous detection
(ii) PSK heterodyne detection (14)
Q.11
define the techniques used to provide both electrical and optical confinement in multimode injection lasers. Contrast these techniques when used in gain guided and index guided lasers.


(5)

define briefly Continuous Phase Shift Keying (CPSK) modulation with reference to coherent optical transmission.


(4)

Briefly explain the reasons for the use of block codes in digital optical fiber transmission. Indicate the advantages and drawbacks when a 5B6B code is
employed. (5)

Earning:   Approval pending.