Electrical and Electronic Engineering - Theses
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ItemFeedforward compensation of directly modulated semiconductor lasers for analogue fibre communications systems( 1991)Optical fibre communication has become very attractive in the field of telecommunication. Recently, subcarrier multiplexing has emerged as a method for providing a way for each customer to directly access voice, data, and video transmissions. Although digital transmission is becoming very popular in telecommunications, analogue transmission still has its attractive features due to its low cost and compatibility to recent customer electronics. Directly modulated semiconductor lasers are important optical sources in optical communication systems. In analogue fibre communications, nonlinear distortion and intensity noise in these directly modulated semiconductor laser are main obstacles. The thesis first presents experimental characterization of semiconductor lasers, including light-current characteristics, static longitudinal mode spectrum, and small-signal frequency response. Since the linearity of semiconductor lasers plays an important role in analogue fibre communications, distortion characterization of semiconductor lasers has been investigated in considerable detail. This thesis gives details of distortion characteristics, particularly in experimental work. The main work presented in this thesis is an optical feedforward compensation scheme. This feedforward system was successfully built for experimental investigation. Since nonlinear distortion in semiconductor lasers is a severe problem in analogue fibre-communications, the feedforward compensation has been employed to compensate the distortion. Reduction of harmonic distortion and intermodulation distortion was achieved by feedforward compensation. The thesis describes the optical feedforward system in considerable detail. A simultaneous reduction of intensity noise and distortion was achieved using the feedforward technique. This provided a great improvement in signal-to-noise ratio in analogue subcarrier multiplexed systems. The experimental results are presented in this thesis. Also, the theoretical calculation of noise limitations in the feedforward system is presented in this thesis.