Experimental demonstration of mode structure in ultralong Raman fiber lasers

We present the first experimental demonstration of a resolvable mode structure with spacing c/2nL in the RF spectra of ultralong Raman fiber lasers. The longest ever demonstrated laser cavity (L = 84 km), RF peaks of similar to 100 Hz width and spacing similar to 1 kHz have been observed at low intracavity powers. The width of the peaks increases linearly with growing intracavity power and is almost independent of fiber length. (c) 2007 Optical Society of America.