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BIRLA INSTITUTE OF TECHNOLOGY & SCIENCE, PILANI (RAJ)

I Semester 2009-10

EA C422 Fiber Optics & Optoelectronics

Comprehensive ExaminationDuration: 3Hrs Date: 04/12/2009 AN Max Marks: 40

____________________________________________________________________

Note: Attempt all questions. Parts of the same questions must be attempted

consecutively.

Q. 1: A typical fiber-optic communication link is shown in Fig.Q.1. The fibers betweenstations A and B and those between Cand D are normal GIfibers with a NA of 0.21, of

1.9, and exhibiting a total attenuation of 0.11 dB/km; the fiber between stations B and C

is DCF with a NA of 0.18, of 2.0, and exhibiting a total attenuation of 0.12 dB/km.There is perfect splicing of optical fibers at B and C; the link has two couplers (one each

at stations A and D), each giving a loss of 0.5 dB. A safety margin of 6 dB is desired.Fresnel reflection losses at B and Cmay be assumed negligible. Length AB = CD = 90

Km. AB and CD fibers are available in pieces of 30 Km each. The splice loss at each

splice may be taken to be 0.1 dB. Assume that the receivers and transmitters have risetimes of 10 ns and 12 ns. The multipath time dispersion in all the fibers is almost

negligible. The sources used in the transmitters have a spectral half-width of 20 nm. The

material dispersion parameter for the fibers between A and B and those between Cand D

is 70 ps km-1nm1, and that for the fiber between B and C is - 450 ps km1nm1.

(a) Calculate the length of DCF for achieving zero dispersion at D.

(b) Calculate the minimum optical power that must be launched by the transmitters on

the two sides for a duplex link (i.e., for two-way communication) if the receivers oneither side require a minimum of 0.5 W for signal recovery.

(c) Calculate the maximum bit rate at which the system can be operated using NRZ

format.

[8 M]

A B C D

Fig. Q.1

Q. 2: A typical CATV system uses an optical bus to distribute video signals to subscribers.The transmitter is coupling 0 dBm of optical power into the bus. Each receiver hassensitivity of 45 dBm. Each optical tap couples 3% of optical power to the subscriber

and has an insertion loss of 0.2 dB. On an average, the optical loss within the opticalfiber bus itself is 0.011 dB/m and the minimum distance between two subscribers is 40

m. How many subscribers can be added to the bus before the signal needs an in-lineamplification?

[4M]

Q. 3: (a) Calculate the change in the refractive index due to longitudinal electro-optic effect

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for a 3 mm long crystal of lithium niobate for an applied voltage of 50 V. If thewavelength of light propagating through the crystal is 550 nm, calculate the net phase

shift between the two polarization components after they emerge from the crystal. For

lithium niobate, no = 2.29, ne = 2.20 and the electro-optic coefficient, r33 = 3.0810-11

.

(b) For the crystal of part (a), calculate V .

[4M]Q. 4 A planar waveguide is formed from a 6 m thick core film of dielectric material of

refractive index 1.46 sandwiched between the cladding slabs of a similar material for

which the relative refractive index difference (w.r.t. core) is 0.02. Calculate the G-

factors for the modes supported by the waveguide when light of wavelength 1.165 m is

propagating through the waveguide.

[4M]Q. 5: An impulse is launched into an optical fiber. The power, P(t), in the received pulse

varies with time, t, according to the following relation:

P(t) = P0 exp (-t/ e); 0 < t <

Calculate : (a) The total energy in the pulse

(b) Mean pulse arrival time(c) Rms pulse width.

[4M]Q. 6: For a DH ILD, with a confinement factor of 0.9, the threshold gain coefficient, gth, to a

good approximation, may be given by the relation: gth = Jth, where is a constant

depending on the device configuration and Jth is the threshold current density for

stimulated emission. Consider a GaAs ILD, with a cavity of length 400m and width

90m. Assume that = 0.018 (cmA-1

), eff = 10 cm-1

, n = 3.7, and one of the facets has a

reflectivity of 100%. Calculate the threshold current density and the threshold current Ith,

assuming the current flow is confined to the cavity region.

[4M]

Q.7: Consider a DC single mode fiber. Assume that the core diameter is 10 m,

n1 = 1.47, = 0.3% and the inner (depressed) cladding diameter is 25m. Further assumethat the mode-field distribution is Gaussian and hence the power at a radius r is given by

P(r)=P0 exp (2 r2/w

2), whether w is the Gaussian mode field radius. Calculate the

fraction of power that may leak at the inner outer cladding interface for transmitting

= 1.55 m.[4M]

Q.8: A connector joins two compatible multimode s i. fibers with a lateral offset of 4 m, an

angular misalignment of core axes of 3 and 10 m end separation. If the core of eachfiber has a refractive index of 1.48, a relative refractive index difference of 2% and a

diameter of 100 m, calculate the total insertion loss at the connector.[4M]

Q. 9: (a) An APD has a quantum efficiency of 60% at 1.55 m. When illuminated with optical

power of 0.5 W at this wavelength, it produces an output photocurrent of 10 A, after

avalanche gain. Calculate the multiplication factor of the diode.

(b) A p-n photodiode has a quantum efficiency of 50% at = 0.90 m. Calculate the

rate of received photons if the mean photocurrent is 10-6

A.

[4M]