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We proposed and demonstrated a linewidth compression method of a laser based on stimulated Brillouin scattering (SBS) and a Rayleigh backscattering structure (RBS). The relationship between the output SBS laser linewidth and the input pump linewidth was studied theoretically and experimentally. It is shown that the narrower linewidth of the pump laser leads to the narrower bandwidth of the SBS gain and, finally, the bandwidth of the SBS will tend to its intrinsic value as the linewidth of a pump laser narrower than 10 kHz; then the linewidth of an SBS fiber ring laser would tend to 200 Hz. In order to further reduce its linewidth with low cost, RBS and a simple dual-cavity feedback structure were added and, finally, a ∼75  Hz narrow-linewidth laser with a side-mode suppression ratio of 70 dB was obtained.

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Single longitudinal mode (SLM) distributed feedback (DFB) lasers with a linewidth lower than a few kHz find applications in many coherent detection systems. In this paper, we proposed and experimentally demonstrated a novel method to compress the linewidth of a SLM DFB laser by utilizing a dual-cavity feedback structure (DCFS). The DCFS first provides optical self-injection feedback to compress the laser linewidth, and then the two feedback lengths are carefully optimized to achieve SLM output via the Vernier principle and the suppression of modes overlapping between two cavities. The linewidthes of 1 MHz and 200 kHz were successfully compressed to ~2.7 and 1.5 kHz with a side mode suppression ratio of 38 and 45 dB, respectively. The stability of the DCFS output power can be controlled within ~0.21%. Our method provides a simple, effective, low cost way to achieve DFB linewidth compression, which will greatly improve the performance of coherent detection systems using DFB laser as sources.

We demonstrate the evolution characteristics of laser linewidth in accordance to Rayleigh backscattering (RBS) signal accumulated from different lengths of the RBS fiber. An ultra-narrow-linewidth fiber laser operating in single longitudinal mode has been obtained. The output linewidth is approximately 210 Hz and the side-mode suppression ratio is as high as ∼60 dB provided the length of the RBS fiber is set to be 5 km. The theoretical prediction suggests linewidth of tens of Hertz could be achieved provided the experimental setup is further optimized and the temperature control device is introduced.