Spectrum narrowing of pumps with different statistical properties in fibers with normal dispersion

It was shown that the spectral width of a nominally continuous wave Raman fiber laser (RFL) output becomes narrower after propagation in normal dispersion (β2>0) fiber [1]. This feature, attributed to Inverse Four-Wave Mixing (IFWM), was especially pronounced when the RFL output had a double-peak spectral shape caused by efficient FWM inside the cavity [2]. This spectrum was transformed into a Lorentzian-like shape which propagated with remarkable stability over long (∼100 km) lengths of fiber.