Following Paper ID and Roll No. to be filled in your Answer Book

EEC7Ol

B.Tech.

(sEM. VID ODD SEMESTERTHEORY EXAMINATION 2}r2'r3OPTICAL COMMUNICATION

Time : 3 Hours Total Marks : 100

Note :- (1) All questions are compulsory.(2) All questions carry equal marks'

1. Attempt any FOUR parts of the following :- (5x4:20)(a) Sketch the biock diagram of optical fiber communication

system.

(b) List out various advantages of optical fiber communicationsystem over the conventional electrical communicationsYStem.

(c) Explain the concept ofacceptance angle in optical fiberwith the help of proper diagram. How it is related tothe numerical aperture of optical fiber ?

(d) With the suitable ray diagram, explain the propagationof skew rays in the optical waveguide and compare it'with Mendional raY.

(e) A graded index fiber has a core with a parabolicrefractive index profile which has a diameter of50 pm. The fiber has a numerical aperture of 0'2'Estimate the total number of guided modes propagatingin the fiber when it is operating at a wavelength of1 pm-

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(0 Mention any two materials system used in the fabricationof optical fibers. Explain tfre Vapor_phase depositiontechniques to produce silica rich glasses of highesttransparency.

2. Attempt any TWO parts of the following : l0x2=20(a) With the aid of suitable (iagram, briefly discuss the

following in the case optical fiber transmission :(i) Fiber bend losses;(ii) Dispersion shifted fibers.

(b) Describe the mechanism of intermodar dispersion in amultimode step index fiber. Show that the total broadeningof a light pulse AT. due to intermodal dispersion in amultimode step index fiber may be given by ;

n." _ L(NA)2drs ---

2nrc

where L is the length of the fiber, NA is the numericalaperture of the fiber, n, is the core refractive .indexand c is the velocity of light in vacuum.

(c) Explain what is meant by :(i) Modalbirefringence(iD The beat Iength in single mode fibers.The difference between the propagation constants forthe two orthogonal modes in a single mode fiber is250. It is illuminated with light of peak wavelength1.55 trrm from an injection laser source with a spectralline width of 0.g nm. Estimate the coherence lengthwithin the fiber.

EECTOT/DLT44I68

3. Attempt any TWO parts of the following : (10x2=20)(a) Explain the principle, construction and working of

Semiconductor Injection laser'

(b)Theradiativeandnon-radiativerecombinationlifetimesof the minority carri.ers in the active region of double

heterostructureLEDare60nsandl00nsrespectively.Dete-rninethetotalcarrierrecombinationlifetimeandthe power internally generated within the device whe-h

the peak emission wavelength is 0'87 pm at a drive

current of 40 mA-

(c) Explain the working principle of LED' How fhe quantumefficiency of a LED is defined ? List out vanousparameters which are needed to be optimi zed for getting

maximum output power from the I-ED'

Attempt any FOUR parts of the following : (5x4=20)

(a) Derive an expression for the coupling efficiency of asurface emitting LED into a step index fi'ber' assuming

the device to have a Lambertian output'

(b) A p-i-n photodiode on average generate one electron-hole pair per three incident photons at a wavelength of

0.8 pm. Assuming all the electrons are collected'calculate :

(i) the quantum efficiency of the device(ii) the maximum possible bandgap energy'

(c) Explain the principle, construction and working of APD'

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4.

, (d), Briefly discuss the possibre sources of noise in optical

fiber receivers. Describe the quantum noise ,, i",l,r.(e) Sketch the full equivalent circuit for a digital optical

fiber receiver. Briefly. explain its various parts.(0 Explain the detection process in p-rnphotodiode. Compare

this device with the p_i_n photodiode.5 Write short notes on any TWO of the following :

(10x2=20).

(a) The Oprical power Meter(b) orDR(c), Wavelengrh Division Multiplexing.

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