Noise monitoring of optical signals using RF spectrum analysis and its application to phase-shift-keyed signals

The radio-frequency (RF) spectrum of an optical channel impaired by noncorrelated incoherent amplified spontaneous emission (ASE) is linked to the RF spectrum of the two sources and the RF spectrum of the ASE signal beat noise, which is proportional to the convolution of the spectral density of the two sources. This allows, essentially, dispersion-independent monitoring of noise on return-to-zero (RZ) phase-shift keyed signals using low bandwidth photodetection and electronics, which is experimentally demonstrated on a 10-Gb/s 50% RZ differential phase-shift-keyed signal.