Raman transformation properties of partially coherent laser pulses in phosphorus-doped silica fibre

The work reports for the first time on fibre-based Raman conversion with relatively large Stokes shift pumped by double-scale laser pulses having various degree of coherence. It was discovered that the degree of coherence of the pump pulses affects significantly the amount of the wavelength shift, intensity, and spectral width of frequency-downconverted radiation. At lower coherence within double-scale pulses, the magnitude of intra-pulse femtosecond field oscillation grows, leading to stronger nonlinear pulse interaction with the optical medium. This discovery suggests new approaches to nonlinear transformation of partially coherent laser pulses, typical of many mode-locked generation regimes of fibre lasers.