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.
