Carrier synchronization for homodyne and heterodyne detection of optical quadriphase-shift keying

Through analysis and simulation, the authors investigated the performance of four carrier-synchronization techniques suitable for both homodyne and heterodyne detection of optical quadriphase-shift keying: the discrete-time decision-directed loop, the analog decision-directed loop, the Costas quadriphase loop, and the fourth-power phase-locked loop. Accounting for shot noise, laser phase noise, and feedback delay, they optimize the loop natural frequency and specify laser-linewidth requirements. The performance discrepancy between the best and worst of these loops is found to be small; accounting for inherent loop delays only, the linewidth requirements range from ΔvT<2.5×10^-5 to ΔvT<5.2×10^-5, where Δv is the beat laser linewidth and T is the baud interval. Hence other considerations, such as ease of implementation, will govern the design choice for most practical systems. For the case when propagation delays in the feedback loop are significant, a simple and accurate method for estimating the laser-linewidth requirement and corresponding optimal natural frequency is presented