Nonlinear Phase-Shift Compensation for DQPSK Signals Using a Nonlinear Amplifying Loop Mirror

The application of a Nonlinear Amplifying Loop Mirror (NALM) as a nonlinear phase-shift compensator (NPSC) in phase-shift keyed transmission is investigated with emphasis on Differential Quadrature Phase-Shift Keying (DQPSK). The origin of the effective negative nonlinear phase shift in a NALM and the effects of the NALM parameters on the phase-shift compensation are discussed. Two possible application modes of the NALM as NPSC have been found: sole nonlinear phase-shift compensation up to 3 rad, and phase-shift compensation in a smaller range with simultaneous amplitude equalization. The points of operation for both modes and the limitations in their implementation are presented. Results show that the use of a NALM, optimized for phase-shift compensation, seems to be very promising, especially for the application as post-compensator in differential phase-shift-keyed transmission systems. To evaluate the performance of a NALM as post-compensator a simplified model of the NALM and a DQPSK transmission system was used. The BER with and without the NALM was calculated in dependence on the average nonlinear phase shift and on the input noise. The results show that with a NALM as NPSC the amount of nonlinear phase shift in a preceding transmission system can be approximately doubled for the same BER