Feasibility study on QPSK optical-heterodyne detection system

The feasibility of a QPSK (quadrature-phase-shift-keying) optical heterodyne detection system for a high-speed and long-span coherent transmission is investigated. The key factors, which are the modulation method and the spectral linewidth requirement for optical light sources, are discussed. Through theoretical analysis and the simulation experiment, it is confirmed that the required beat spectral linewidth for QPSK coherent detection is less than 7.5*10/sup -5/*bit rate. It is also shown that the required spectral linewidth for QPSK differentially coherent detection is less than 2.5*10/sup -5/*bit rate. These results show that the gigabits-per-second range QPSK optical heterodyne detection can be realized with conventional external cavity laser diodes.<
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