Study of optical direct detection scheme using mach-Zehnder discriminator

Abstract

Direct detection optical receiver has become increasingly popular for its low cost and simplicity. In this work, the performance of a CPFSK direct detection optical receiver employing Mach-Zehnder interferometer as an optical filter is investigated. The recei;er sensitivity is expressed in terms of bit error rate(BER) and an analytical expression of BElt is developed including the effects of laser phase noise and receiver noise. Usually the frequency modulation(FM) response of a distributed feedback(DFB) laser is not uniform over the modulating frequency range. Due to this reason analysis is carried out to evaluate the BER performance of the receiver by taking into account the effect of non-uniform FM response of transmitting DFB laser in presence of laser phase noise and receiver noise. The possibiiity of performance improvement with the inclusion of an optical preamplifier in the receiver front-end is also investigated. The non-zero linewidth(LW) of laser causes a BER floor and performance enhancement cannot be achieved beyond the BER floor by increasing the signal strength .. However, the receiver performance can be improved by increasing the modulation index. With increasing bit rate, the shot noise increases but the floor remains at the same value of BER. In the presence of non-unifor:mity in the FM response of the semiconductor laser, the performance of the receiver is highly degraded and BER floor occurs at a much lower value of laser LW for long pseudo-random data pattern. It is also observed that with the addition of an optical preamplifier, the receiver performance can be improved within a certain range of amplifier gain. Beyond this range, the amplifier spontaneous emission(ASE) noise becomes dominant and further improvement cannot be achieved by increasing the amplifier gain.