Soliton compression and locking in a Brillouin fiber ring laser

Stimulated Brillouin scattering (SBS) in an optical fiber cavity resonantly couples an optical cw-pump and a backscattered Stokes wave with electrostrictively excited longitudinal GHz hypersound waves. Below a critical feedback the Stokes wave self-structurates into ns pulses, called Brillouin dissipative three-wave solitons; they are able in turn to electrostrictively excite lower frequency acoustic waves (f(n) = 5 MHz to 1 GHz), due to cladding Brillouin scattering (CBS), transversally propagating with respect to the fiber axis in the fiber's cladding. We derive a four-wave model which couples the transverse CBS dynamics to the nonlinear dissipative three-wave SBS dynamics. Above a critical coupling, the resonant CBS oscillations (f(n)/FSR = integer) are strong enough to break the pulses, while stable compressed or decompressed pulses are still obtained in finite frequency ranges between resonant frequencies: the small damped low frequency CBS vibrations contribute to lock further longitudinal cavity modes which compose the compressed SBS soliton. A stability map is computed. Compression up to a factor 0.4 with respect to the bare Brillouin soliton is numerically obtained, yielding ns pulses of peak intensity up to 14 times the cw launched pump, in fair quantitative agreement with experiments done in long fiber ring cavities.