  A single longitudinal mode fiber laser with ultra-narrow linewidth based on
self-injection feedback by using the linewidth compress mechanism of Rayleigh
backscattering (RBS) are proposed and demonstrated. Since the linewidth of RBS
is narrower than that of the incident light in optical fibers and they have the
same centre wavelength, the RBS can act as a mechanism to compress the
linewidth of the incident light in fiber ring laser. In addition, more RBS
signal could be collected to help further compress the laser linewidth besides
the free spectral range is expanded when the self-injection feedback method is
used. Our experimental results show that the side-mode suppression ratio of our
laser is up to 75dB and the laser linewidth could be low to ~130Hz.
