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Unformatted text preview: Module Code: EE555 Page 1 of 11 EE555 Exam Solutions 2008/2009 Question 1 Write a brief note on each of the following topics (each part carries 5 marks): 1(a) Erbium Doped Fibre Amplifiers (EDFAs) EDFAs are optical amplifiers which use EDF as a gain medium. They use 980nm lasers as pumps through WDMs. They are used extensively to overcome loss in long haul optical transmission. They make WDM systems practical since they can amplifier multiply wavelength channels at once. Removing the need for O/E E/O conversion. They are sources of ASE. There important parameters are their gain/saturation output power/wavelength range and noise figure. Better performance than SOAs but cannot be integrated. [5 marks] 1(b) Direct and external modulation in optical transmitters Both are techniques for applying data to an optical carrier. Direct modulation involves modulating the drive current to the laser/LED itself. External modulation involves driving the laser/LED continuously (CW) and using an external optical modulator (e.g MZM, EAM etc.) modulate the light. Direct modulator has the advantage of being cheaper and more simple to implement. It has severally drawbacks including the large frequency chirp and jitter, the reduction in SMSR and the lower bandwidth (generally <10Gb/s) External modulation is more expensive and has more components and potentially polarisations issues but can give much greater bandwidth (>40Gb/s) with almost no chirp As a result External modulation is used in high speed long haul links (core network) while direct modulation is suitable for shorter lower rate systems (metro network). [5 marks] 1(c) FP Lasers A Fabry Perot laser is the most basic semiconductor laser configuration It uses the two cleaved facets of the devices as mirrors to form a fabry perot cavity. They generally employ double heterostructures and some form of lateral confinement (mesa, buried structure, ridge structure etc) to improve the gain/threshold characteristics. This can also result in [5 marks] Module Code: EE555 Page 2 of 11 a single transverse mode being generated In contrast to DFBs or DBRs, FP lasers do not have any bragg elements to select a single longitudinal mode . As a result they lase at a number of wavelengths across their whole gain band. This multi-mode output makes them no-ideal for use in most optical communications systems since their output is too susceptible to chromatic dispersion 1(d) Chromatic Dispersion Chromatic Dispersion is the effect of different optical wavelengths (colors) experiencing a different refractive indexes in a medium. Meaning they propagate through the medium at different speeds....
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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.
- Spring '11