Grating-free Raman laser using highly nonlinear photonic crystal fiber

We demonstrate a Raman laser made from a grating-free highly-nonlinear photonic crystal fiber. The laser threshold power is lower than 600 mW and laser power characteristics recorded in experiments are accurately described from the usual simplest model dealing only with stationary evolutions of total optical powers [J. Opt. Soc. Am. 69, 803-807 (1979)]. In our theoretical treatment, reflectivity coefficients are fixed parameters, in strong contrast with procedures usually implemented to describe Raman fiber lasers made with fiber Bragg gratings. Experimental investigations of the spectral properties of our grating-free Raman fiber laser evidence that the shape of the Stokes power spectrum remains remarkably Gaussian whatever the incident pump power. Increasing the incident pump power induces a drift of the Stokes wavelength together with a broadening of the Stokes optical spectrum. Investigations on the role of light polarization on laser characteristics show that our grating-free Raman fiber laser behaves as a Raman laser made with a standard polarization maintaining fiber.