Suppression of RF noise tones caused by Rayleigh backscatter in to an unisolated laser using audio-frequency external optical-phase modulation

When Fabry-Perot lasers are subject to Rayleigh backscatter, dramatic line-narrowing and frequency hopping up to 500 MHz, evidenced by radio-frequency (RF) tones, are observed. These RF tones will severely degrade the system performance of a subcarrier multiplexed transmission system, such as quadrature phase-shift keying. In this letter, it is demonstrated that audio-frequency external optical-phase modulation can suppress the noise tones and restore the laser linewidth. This scheme may offer an inexpensive alternative to optical isolators to recover the system performance and thus is very important for cost-effective customer access networks.