EE409_Exam_May2009_Ans - Sample Answers for Optical...

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Sample Answers for Optical Communications (EE409) May 2009 Question 1 (a) Main parameters are the wavelength of the light, the radius of the optical fibre waveguide, and the refractive index difference between core and cladding. For a given radius and refractive index difference between core and cladding there will be a cut-off wavelength above which the fibre becomes single-mode. In multi-mode fibre the main dispersive mechanism is multi-mode dispersion caused by a different group delay for the different modes. In single-mode fibre the main dispersion mechanisms are material (due to the refractive index being a function of wavelength), and waveguide (due to a portion of the light propagating in the cladding) dispersion. (b) Main characteristics are attenuation, dispersion, nonlinearity. To overcome loss it is possible to use use optical amplifiers. To overcome dispersion in an optical communications system it is necessary to operate close to the dispersion zero wavelength or else employ dispersion compensation fibre. To overcome non- linearity it is normally sufficient to operate at low power levels, but it is also possible to operate in the soliton regime where dispersion and non-linearity can counteract each (c) External modulation means using an external modulator to modulate a constant optical output power from a laser with an electrical data signal. The external modulator is usually based on an interferometer which uses the electro-optic effect to vary the light passing through in relation to the electrical signal applied. With direct modutaion the electrical data signal is applied directly across the electrodes of the laser. A dc bias level is normally needed to overcome the threshold current of the laser. Main advantage of external modulation over direct current modulation is the reduced frequecy chirp on the optical data signal which means that the signal is less distorted by fibre dispersion.
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This note was uploaded on 03/23/2011 for the course EEE 174 taught by Professor Kasab during the Spring '11 term at ASU.

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EE409_Exam_May2009_Ans - Sample Answers for Optical...

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