A study of optical beat interference between Fabry-Perot lasers

Optical beat interference can be the dominant source of noise in subcarrier-multiplexed (SCM) passive-optical networks (PONs). We have studied how the relative-optical-beat-interference noise (ROBIN) between Fabry-Perot (FP) lasers varies using lasers with both equal, and unequal mode spacing. In our experiments, ROBIN ranged from -96 dB/Hz to -115 dB/Hz. Because the noise level depends on the distribution of optical power in the lasers' modes, using lasers with different mode spacing led to a 4-dB improvement over using lasers with equal mode spacing. The noise is also correlated with the linewidth of the interfering modes. Broadening these modes by driving the lasers below threshold lowered the noise by an additional 10 dB.