Phase-preserving amplitude regeneration using a Nonlinear Amplifying Loop Mirror in a DPSK transmission system

With the implementation of differential phase-shift keying (DPSK) in optical transmission systems, phase-preserving amplitude regenerators become attractive to prevent the accumulation of nonlinear phase noise which is one of the major impairments in long-distance DSPK transmission systems [1]. We presented in [2] that a Nonlinear Amplifying Loop Mirror (NALM) can provide phase-preserving suppression of amplitude noise of a DPSK signal and, thus, suppress generation of nonlinear phase noise when a signal with deterministic amplitude fluctuations is transmitted. This work reports experimental results on the improved performance of a DPSK transmission system using a NALM when nonlinear phase noise generated by amplified spontaneous emission of optical amplifiers due to the fiber nonlinearity is the main limiting factor.