Performance optimization and laser linewidth requirement evaluation for a coherent optical PSK system with DDL

This paper presents the exact requirement laser linewidth evaluation for both heterodyne and homodyne receivers with a Decision-Driven Loop (DDL). Taking the laser phase noise and photodetector shot noise into account, the Probability Density Function (p.d.f.) of the phase error is determined. The optimal normalized loop bandwidth is determined by minimizing the Bit-Error Rate (BER). The power penalty dependence on laser linewidth is also determined. All calculations are based on the nonlinear Phase-Locked Loop (PLL) model. The laser phase noise and photodiode shot noise are described through loop parameter variations. The laser phase noise is presented specifically through frequency variations of both the transmitter laser and the local laser instead of phase fluctuations. Finally, the comparison between nonlinear phase loop model results determined here and linear phase loop model results obtained in [1, 2] is presented.