EE409_Exam_May2008_solutions

EE409_Exam_May2008_solutions - ANSWERS FOR EE409 EXAM...

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EE409 Solutions 2007 Page 1 of 4 ANSWERS FOR EE409 EXAM SEMESTER 2 – 2007/2008 Question 1 (a) Main points : The terms on the right hand side of the equation for the photon density deal with the generation of photons by spontaneous and stimulated emission, and the loss of photons by recombination and emission from the laser facets. The terms on the right hand side of the equation for the carrier density deal with the loss of carriers by spontaneous and stimulated emission, and the gain of carrier by application of current to the laser device. [14 marks] Main points of the modulation response are that it is flat at low frequencies and then peaks around the relaxation oscillation frequency before rolling off. [6 marks] (b) 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 [13 marks]
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EE409 Solutions 2007 Page 2 of 4 Question 2 (a) The student should ideally plot how typical pulse spectral and temporal profiles are be influenced by non-linearity and dispersion in the fiber. With dispersion being zero the effect of the non-linearity is only to affect the spectrum of the pulse and to leave the temporal profile undistorted. Essentially the spectrum will broaden as the signal propagates due to self-phase-modulation with the spectral
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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_May2008_solutions - ANSWERS FOR EE409 EXAM...

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