Control of spectral and statistical properties of Raman fiber lasers

The present paper numerically investigates in details the temporal and statistical properties of Raman fiber lasers (RFL) radiation generated in different types of cavities. It is shown that RFL can operate in two principally different operation regimes. In usual regime the laser generation exhibits strong intensity fluctuations of intensity probability density function (PDF) with maximum at zero intensities. The autocorrelation function is typical for quasi-CW PC radiation. The spectrum is broader than FBGs spectral profile, owing to intense turbulence-induced FWM interaction between numerous longitudinal modes. Contrary, in the second new regime fast intensity fluctuations are suppressed resulting in PDF with maximum at the mean intensity, like in conventional laser sources. The autocorrelation function confirms the almost CW nature of the laser radiation over 1-ns time scale. The optical spectrum is much narrower than in the previous case being even much narrower than FBGs spectral profiles despite the higher total power and several times higher spectral power density. The key role in the formation of the new type of the RFL generation should play a suppression of the spectral broadening which enables the generation of the spectrum being free from far spectral wings generated at one fiber pass. In this case, despite the fact that amplitude of each of numerous longitudinal modes fluctuates with Gaussian statistics, total intensity fluctuations could be suppressed because of correlations of different longitudinal modes over all spectrum.