Operation of a passive optical network with subcarrier multiplexing in the presence of optical beat interference

When two lasers carrying subcarrier-multiplexed data operate with very closely spaced wavelengths, beating between the lasers can occur which increases the noise at the detector. This effect, optical beat interference (OBI), can greatly interfere with the operation of these systems. The authors have observed OBI with DFB lasers in a system carrying two 155.5-Mb/s data streams. With an optical modulation index of m=0.8, a bit-error rate floor of approximately 10^-2 was observed due to OBI, which is consistent with the noise measured by an RF power meter. When m is increased to 1.8, a significant reduction in OBI was observed, and it was possible to achieve transmission at the 10 ^-9 level with a penalty of only 1.8 dB due to the presence of OBI