Advanced avalanche photodiode receivers in optical communications
This thesis is concerned with a detailed study of the performance of superlattice avalanche photodiodes (SAPDs) and the implications for high bit rate direct-detection optical fibre communication systems. In these advanced detectors the electron to hole ionisation rate ratio is artificially enhanced through selective heating of the electron distribution to reduce the excess noise associated with the randomness of the avalanche multiplication and to ensure high gain-bandwidth product. Thus SAPDs are suitable for long wavelength applications (1.3-1.6 pm) where most compound semiconductor materials otherwise have comparable electron and hole ionisation rates. A comprehensive discrete ionisation model is developed to assess the performance of SAPDs; emphasis being placed on the gain, excess noise factor, gain moment generating function (MGF), and gain-bandwidth product. The model is quite flexible and it is found that other device impairments such as dark current and the number of ionisations per stage caused by the injected carrier can be readily incorporated into the formulation. The performance of optical receivers employing SAPDs is examined using a Gaussian approximation (GA) and taking into account the influence of various device impairments. To assess the accuracy of GA a rigorous statistical analysis is developed using a MGF formulation. New signal designs for optical communications devised specifically for APD receivers are described. These signals achieve simultaneously both zero intersymbol interference and zero telegraph distortion with respect to a depressed optimum threshold and are thus well suited to untimed transmission. Importantly, they also offer improved tolerance to alignment jitter when they used in conventional fully retimed receivers.