Sensitivity improvement for NRZ optical systems using NALM and narrow-band filter

Nonlinear amplifying loop mirror (NALM) has been shown experimentally to improve the system performance by restoring the extinction ratio (ER) in the input signals, and by reducing the input amplified spontaneous emission noise (ASE) for systems using return-to-zero (RZ) modulation format. Theoretical treatment for such improvement, from the system point of view, has yet been provided. Here, we present a theoretical analysis on optical systems using NALM with a narrow-band optical filter and under nonreturn-to-zero (NRZ) modulation format. We show these arrangement can eliminate BER floor, improve sensitivity, and achieve high system margins. The results can be easily extended to systems using other modulation formats.