Noise Monitoring of Optical Signals Using RF Spectrum Analysis and Its Application to Differential Phase-Shift-Keying

The RF spectrum of an optical channel impaired by non-correlated incoherent amplified spontaneous emission is shown to be the sum of 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. The relation between the RF power measured at a given frequency with the optical signal-to-noise ratio and the variance of the intensity of the pulses in the channel is used to perform disperion-independent noise monitoring of a 10 Gb/s 50% return-to-zero DPSK signal.