Signal-to-noise-ratio monitoring of optical data using narrowband RF analysis at the half-clock frequency

A technique for monitoring the signal-to-noise ratio (SNR) of a randomly modulated optical data signal is proposed and demonstrated. The signal is detected and then subjected to a narrowband RF demodulation at precisely half the data clock frequency. Due to symmetries present in the modulation spectrum at the half-clock frequency, the demodulated signal is confined to a single dimension in the recovered phase space. In contrast, the noise power is distributed isotropically within the phase space, and the SNR is measured by taking the ratio of the power recovered along the signal dimension to the power recovered along the orthogonal dimension. The technique is demonstrated for return-to-zero modulated data at 2.5 Gbits/sec and 10 Gbits/sec, and performs well for OSNR levels ranging from 13 dB to >30 dB. In addition, good performance is observed at chromatic dispersion levels of nearly 800 psec/nm for the case of 10 Gbit/sec transmission.