Linearization of analogue optical transmitter by feedforward compensation

© 1993 Anthony Chiu-Chi KwanIn recent years, analogue optical systems have received a lot of attention. Although conventional optical systems mostly employ digital format because of the low power budget requirement and good immunity to noise and distortion, analogue systems prove to have significant advantage over digital systems in applications like video distribution and satellite communication systems. However, analogue transmission systems require transmitters of low noise and low distortion. A number of linearization schemes have been proposed to reduce the distortion introduced by the analogue transmitter. One of the most widely used linearization scheme for analogue transmitter is feedforward compensation. Previous research has shown that feedforward compensation can reduce laser intensity noise as well as distortion. The work described in this thesis is to investigate design optimization of feedforward compensation and to develop a prototype of this optimized system. Experimental testing shows that the feedforward prototype is capable of reducing distortion products by 15 dB over 2.7 GHz. This work also involves modelling the feedforward system to investigate the factors limiting the operation bandwidth of the system. The model developed is a useful design tool for feedforward systems because the distortion reduction performance of the feedforward system can be predicted by characterising the individual components that made up the linearization scheme. Theoretical analysis of an alternative implementation of the feedforward system is also performed. Unlike conventional feedforward systems this new implementation uses only one laser source. Although the distortion reduction performance is degraded, the advantage is that problems associated with the use of two optical sources of different wavelengths is eliminated