30-10-2012, 12:57 PM
IV B.Tech I Semester Regular Examinations - OPTICAL COMMUNICATION
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1. (a) List out the advantages and disadvantages of optical fiber communication.
[5+3]
(b) Distinguish between optical fiber communication system and conventional
communication system. [4+4]
2. Write short notes on
(a) Propagation modes in a single mode step index fiber.
(b) Graded index fiber structure with relevant expressions. [8+8]
3. (a) Explain material dispersion, waveguide dispersion & find an expression for ma-
terial & waveguide dispersions using electromagnetic field theory. [3+3+2+2]
(b) An LED operating at 850 nm has a spectral width of 45nm. What is the pulse
spreading in ns/ Km due to material dispersion. [6]
4. (a) With respect to LED what is internal quantum efficiency and derive the ex-
pression for the lifetime reduction caused by interfacial recombination. [3+5]
(b) If the radiative and non radiative recombination lifetimes of the minority car-
riers in the active region of an LED are 1ns and 100ns, respectively, find the
internal quantum efficiency and the bulk recombination lifetime in the absence
of self absorption and recombination at the heterojuntion. [4+4]
5. (a) With schematic representation explain the working principle of pin photo de-
tector and obtain the expression for cutoff wavelength. [2+6+2]
(b) An APD with multiplication factor of 20 operates at a wavelength of 1.5μm.
Calculate the quantum efficiency and the output photo current from the device
if its responsivity at this wavelength is 0.6A/Wand 1010 photons of wavelength
1.5μm are incident upon it per second. [3+3]
6. A high input impedance type of preamplifier is used in an optical receiver having
the bias resistance of 3M
estimate,
(a) Maximum Bandwidth that can be achieved without equalization if the total
capacitance is 5pf.
(b) The mean square thermal noise current generated by the impedance
RTL (parallel combination of two 3M
when the operating temperature is
270C for unity bandwidth).