Dynamics of passive Q-switching in SBS/Er fiber laser at low pump power

We observed and investigated self-starting quasi-periodic pulsation in Er-doped fiber laser at 30-100 mW pump power. Pulses with duration of 10 - 50 ns (FWHM) and peak power of 50-200 W are generated at a quite stable repetition rate in the range of 300-500 mus. In contrast with previous experiments the pump level in our experiment is significantly lower. At this low pump power we found no nonlinear effect except SBS influencing on the Laser dynamics. The experimental results were explained by a theoretical model based on cooperative dynamics of Rayleigh backscattering (RS) and Stimulated Brillouin Scattering (SBS). Using digital oscilloscope, we traced in details different stages of Q-switching pulse formation process: growth of the spontaneous radiation, lasing due to Rayleigh backscattering, appearance and growth of the first order SBS Stokes radiation and the second order Stokes radiation, lasing suppression due to saturation of the population inversion in Er-doped fiber by the SBS Stokes radiation. Good agreement between theory and experiment have been demonstrated.