Spectrum broadening in Raman fiber laser induced by cross-phase modulation

Raman fiber lasers (RFLs) are attractive laser sources providing almost any wavelength in the near infrared region. The RFL spectral performance is of great importance for their applications. At the same time, it is well known that the RFL radiation suffers from spectral broadening. Although a spectral broadening mechanism is not well justified yet, most of authors consider the four-wave mixing between Stokes wave longitudinal modes to be the main mechanism of the spectral broadening in the RFL. Here we show that it is not necessary the case. Comparing operations of the same RFL with co-and contra-propagating pumping we prove in a direct experiment that the modulation instability induced by cross-phase modulation of the broadband pump wave on the Stokes wave may significantly contribute to the RFL line. The ring Raman laser operates at 1550 nm. The laser is pumped at 1460 nm by another RFL emitting 6W. It is important for this experiment that the pump radiation exhibits rather broad spectral width of ~0.5 nm. The direction of the Stokes wave propagation in the ring cavity is determined by an optical circulator, so during the experiment we could easily invert this direction, other conditions being equal. The laser cavity comprises 200 m long Raman fiber with a Raman efficiency of 2.2/km/W and dispersion parameters of-29.6 and -18.5 ps/nm/km at 1460 and 1550 nm, respectively. The fiber Bragg grating (FBG) with a reflectivity of 20% is used to select the laser wavelength.