Ultra-narrow spectral modes in generation of random distributed feedback Raman fiber laser

We directly observe ultra-narrow spectral modes in the generation of the random fiber laser by using a technique of optical heterodyning in real-time. These solitary spectral features appear slightly above the threshold of generation. We found that such modes could have a spectral width well below 3 MHz. These modes are spatially extended over the fiber laser length, and remain for tens of roundtrip times.