EE555_Exam_Jan2008_Solutions - DUBLIN CITY UNIVERSITY...

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EE555 2007/2008 Page 1 of 11 DUBLIN CITY UNIVERSITY Optical Communications System Design EE555 Exam Solutions SEMESTER ONE EXAMINATIONS 2007/2008 Question 1 Write a brief note on each of the following topics (each part carries 5 marks): a) State the main differences between a LED to a LASER [2.7.4] A LED emits spontaneous radiation which is broad in its spectral nature and incoherent. A laser emits stimulated radiation, which has a narrower linewidth and is coherent (same phase, polarisation, wavelength, direction) Laser’s are more efficient due to stimulated emission dominating over spontaneous emission A laser is a threshold device, a LED isn’t A laser has a strong dependence of threshold current and hence output power, on temperature, whilst an LED has a weak temperature dependence Although the two devices can generate similar amounts of optical power, the better directed laser beam means that more light from a laser can be coupled into fibre. [5 marks] b) Molecular Beam Epitaxy (MBE) [3.2.2] Evaporation technique that is used in the manufacturing process for lasers System consists of deposition chamber that is maintained at very low pressure and one or more effusion cells that contain very pure base materials. Electron beam directed into effusion cell heats material to liquid state, resulting in atoms evaporating out of materials, exiting through shutter and depositing on the wafer. Shutter is computer controlled allowing precise control over the thickness of each layer Evaporated atoms interact with each other only when they reach the wafer Adv: ability to form multiple layers on wafter during one process step, very controlled process Disadv: low film growth resulting in low productivity and high expense [5 marks]
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EE555 2007/2008 Page 2 of 11 c) Operation of a Mach-Zehnder Interferometer as used for External Modulation [4.1.1] Uses electro-optic effect (refractive index of material is changed by the application of an applied electric field) in lithium niobate to alter the optical path length and cause either constructive or destructive interference at the output node of the device Structure of device consists of a block of lithium niobate from which two waveguides are cut with electrodes placed on either side of the waveguide (interaction length) Light enters the device, it is split equally among the two branches and propagates over the same distance until recombining at the output node. Electric field, which is proportional to the electrical data being transmitted applied to one arm of the modulator, altering the optical path length, resulting in either constructive or destructive interference at output node. Adv: signal nearly chirp free, low temporal jitter, large bandwidth
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EE555_Exam_Jan2008_Solutions - DUBLIN CITY UNIVERSITY...

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