Performance enhancement of optical QPSK systems with coherent reception for high speed links

Coherent optical QPSK systems have attracted the attention of the research community and the Optical industry on account of features such as low power requirement and high receiver sensitivity. This can bring about a reduction in the transmitter power levels as well as the link margin to maintain a required BER. However, in order to realize a coherent detection system, the local oscillator laser needs to be synchronized with the transmitter laser diode. This can be achieved by using an optical (Phase Lock Loop) PLL at the receiver for medium data rate (Mbps). But for high data rate applications (data rates in terms of Gbps), optical PLLs are not practically realizable. In this paper, we have analyzed the use of Viterbi and Viterbi Phase Estimation Algorithm for phase synchronization. We have carried out the BER calculations and determined the BER performance degradation when compared to ideal uncoded QPSK BER performance when synchronization is not maintained. The use of Viterbi and Viterbi phase estimation algorithm brings about an improvement in BER even in the absence of an optical synchronizing arrangement. This performance improvement is further enhanced with the use of suitable error control codes. A comparison has been made between various Error Control Codes such as Reed-Solomon (RS) codes, Concatenated codes comprising of a RS and convolutional code and Low Density Parity check (LDPC) codes. We have quantified the improvement achievable with the use of these codes.