Coherent Raman lasing in a short polarization-maintaining fiber with a random fiber Bragg grating array

We study Raman lasing in a 7 m polarization-maintaining fiber with an array of fiber Bragg gratings with random phases/amplitudes. After optimization of the grating number the threshold pump power decreases to 0.8 W, whereas generated Stokes power at 1092 nm grows linearly above the threshold, reaching ~3 W for both forward and backward waves. A comparison of experimental data with a developed model shows what such a random laser generates in a coherent regime. Near the threshold, single-longitudinal-mode generation with a ~50 kHz linewidth is obtained, whereas the number of generated modes grows with power exceeding ~100 at ~0.1 W. At higher power, a transition to an incoherent regime with different power increments is observed, while the linewidth grows nonlinearly reaching ~80 pm. Insertion of a 30 m fiber before gratings enables ~1.5 times higher backward Stokes power at a narrower linewidth. Additionally, tunable parametric generation around 1140 nm is observed.