Phase noise in coherent analog AM-WIRNA optical links

One of the challenges facing coherent links is the degradation of performance in the presence of laser phase noise, and hence recent efforts have focused on AM-WIRNA (Amplitude Modulated-WIdeband filter-Rectifier-NArrowband filter) processing, due to its relative immunity to such noise. AM demodulation with a square law device significantly reduces the effects of phase noise, however phase-noise broadening of the photocurrent spectrum leads to distortion at the intermediate frequency (IF) filter which causes phase-to-intensity (PTI) noise. In this work, we present results from the first detailed analysis of the system performance of AM-WIRNA links in the presence of phase noise, using numerical methods to estimate the power spectral density (PSD) of the PTI noise. Comparison of the results of our analysis with the experimental data also presented here indicates that the numerical analysis accurately models the effects of PTI noise on link performance. We find that the PTI noise critically affects the performance of high SNR links. For high power, high SNR systems such as antenna remoting and AM-CATV, this limit will dominate for laser linewidths greater than a few MHz