Receiver Design for Digital Fiber Optic Communication Systems, I

The purpose of this paper is to provide insight into a systematic approach to designing the "linear channel" of a repeater for a digital fiber optic communication system. In particular, we are interested in how one properly chooses the biasing circuitry for the photodetector; and how the required power to achieve a desired error rate varies with the bit rate, the received optical pulse shape, and desired baseband output pulse shape. Throughout this paper, performance will be measured in terms of signal-to-noise ratios. Efforts to calculate exact error rates and bounds 843 (S46 THE BELL SYSTEM TECHNICAL JOURNAL, JULY-AUGUST 1973 to error rates are difficult to carry out, and, in the past, the results of such efforts have shown little deviation (for practical design purposes) from calculations of error rates using the signal-to-noise ratio (Gaussian approximation) approach. (See Refs. 2 through 5 and Appendix A.) An avalanche photodiode is the device of interest in fiber applications for converting optical power into current for amplification and equalization, ultimately to produce a baseband voltage for regeneration. In order to appreciate its performance in practical optical systems, we have to characterize the avalanche photodiode from three points of view: the physical viewpoint, the circuit viewpoint, and the statistical viewpoint. When we study the device from the physical viewpoint, we ask how does it operate, how do we develop circuit and statistical models of its operation, and what are the limitations of the models.