Evaluation of the performance of the optical transmitters to be employed in mid-infrared optical communication systems

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Evaluation of the performance of the optical transmitters to be employed in mid-infrared optical communication systems


Author: Chan, Wing-yip
Title: Evaluation of the performance of the optical transmitters to be employed in mid-infrared optical communication systems
Degree: M.Sc.
Year: 1996
Subject: Laser communication systems
Optical communications
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Pages: v, 104 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1230699
URI: http://theses.lib.polyu.edu.hk/handle/200/3861
Abstract: The state-of-the-art in the various essential components suitable for the mid-infrared optical communication systems under development are identified in this dissertation. The ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) system of the heavy metal fluoride glasses is identified to be most suitable for the application as the fibre material in the mid-infrared range. The predicted minimum loss of the fibre is about 0.04 dB/km at a wavelength of 2.55um and there is a second minimum loss point at 2.35um with a loss value of 0.06 dB/km. Moreover, the fibre can be designed to be dispersion-shifted with a dispersion smaller than 1ps/(km*nm) between 1.8um and 2.8um. Three candidates, the InGaAs/InAsPSb semiconductor laser, the GaInAsSb/GaAlAsSb semiconductor laser and the thulium-doped fluoride fibre laser, are identified to have the potential to be employed as the optical source for the communication systems. After a detailed assessment, the thulium-doped fluoride fibre laser is concluded to be the most promising optical source for the application with the operating wavelength at 2.35um. To match the operating characteristics of both the fibre and the optical source recommended, InGaAs photodiodes are considered as the most suitable type of optical detector. In order to evaluate the performance of the most promising type of optical source identified, discussions in laser setup, pumping threshold condition calculation, laser efficiencies, output power, pump sources, tuning devices and external modulators are presented. For enhancing the output at 2.35um, the thulium-doped fluoride fibre laser should be able to maintain simultaneous oscillations at both transitions at 1.9um and 2.35um by using tuning devices. GaAlAs diode lasers emitting at 790nm are recommended to be employed as the pump source for the fibre lasers. The practical realization of the use of the fibre lasers requires the use of external modulators, e.g. the Ti-LiNbO3 type. Estimates for the power budget and rise-time budget are presented with all identified components taken into account as a completed system. The maximum allowable repeater spacing for a direct detection system is calculated to be 530 km under an assumed bit rate of 2.5 Gbit/s. Finally, conclusions are stated for the overall work and some suggestions on further works are mentioned.

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