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=84km), RF peaks of ∼100 Hz width and spacing ∼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. © 2007 Optical Society of America.