Adaptive electronic linearization of analog optical links

Linearization of optical links is imperative for obtaining the maximum dynamic range in an analog link. This dissertation presents an Electronic Linearization method of adaptively linearizing links that are limited by different sources of nonlinearities. Second order and third order distortions arising at the transmitter end in externally and directly modulated links have been suppressed by Predistortion Linearization to give Spurious Free Dynamic Range improvements of as much as 12 dB. A WDM technique for high sensitivity detection of distortions has been proposed and demonstrated for externally modulated links. Further on, a theoretical study of applying Predistortion to baseband signals for linearizing passband links with the signals upconverted to GHz frequencies has been carried out to show that single sideband modulation has distinct advantages over double sideband modulation. The dissertation then explores the nonlinearities produced at the receiver end due to photodetector saturation in intensity modulated links as well as from coherent detection of phase modulated links. Theoretical models and expressions for the dynamic range have been derived, followed by the experimental demonstration of Postdistortion Linearization of both links. The dynamic range was shown to achieve the RIN noise limit with improvements of more than 5 dB in 3rd order and 16 dB in 2nd order limited dynamic range, respectively for the intensity modulated links. Improvements of 2.4 dB were obtained for the phase modulated links. The principle of baseband linearization for passband signals has been applied to these coherently detected links. In an independent approach towards increasing the capacity of an optical link, an uncoordinated optical multiple access system using novel nonlinear trellis codes was also set up and demonstrated to give Bit Error Rates of less than 10-9 for a system with six users, with all interfering users treated as noise.