Equalize APD-based receiver for ultrafast optical communication system

摘要

Due to their high electrical bandwidth and good signal-to-noise ratio, optical receivers that utilize a combination of PIN photodetectors and erbium-doped fiber amplifiers (EDFAs) have emerged as an attractive technology for high-speed optical communication. However, the drawbacks of this technology are cost and bulkiness. Since avalanche photodiodes (APDs) are capable of amplifying the photocurrent internally, without the need for optical preamplification, they may offer a cost-effective and compact alternative to the PIN-EDFA combination. Unfortunately, this internal optoelectronic gain comes at the expense of uncertainty in the APD's gain, and more importantly, at the expense of reduced speed due to the notorious avalanche buildup time. The relatively slow response time of an APD, compared to a PIN photodiode, introduces significant inter-symbol interference (ISI) at high operational transmission rates. In this work, an equalization approach is undertaken to compensate for buildup-time-induced ISI by means of either the transversal equalizer (TE) or the decision-feedback equalizer (DFE). To design the equalizers, the APD-based receiver is viewed as a random linear channel whose impulse-response function is a stochastic process. The mean and the correlation matrix of the receiver's random impulse-response function are numerically determined by utilizing a recently developed analytical model. It is revealed that these equalizers can reduce the bit-error-rate (BER) remarkably at high transmission rates; this makes current InP APDs potentially suitable for near 40-Gbps digital operation.