Unusual evolutions of pulses and spectra in an Yb-doped intra-cavity cascaded Raman fiber laser

Abstract

Based on nonlinear polarization rotation, a passively mode-locked Yb-doped intra-cavity cascaded Raman fiber laser was demonstrated, in which the unusual evolutions of spectra and pulses have been observed for the first time. The first-order stimulated Raman scattering (SRS) laser and the second-order cascaded SRS laser have been obtained successively under the different pump powers. Furthermore, due to the interaction of self-phase modulation, SRS, group velocity dispersion and other nonlinear effects, the spectra and pulses of the Raman fiber laser presented a unique anomalous evolution. With the increase in pump power, the decrease in the intensity of the first-order Stokes and the second-order Stokes waves and the narrowing of the pulses were observed for the first time. Finally, a flat broadband spectrum was achieved based on the unusual evolution of laser emission spectrum, which would be of great interest as a new method in generating the broadband continuum source.