Single-mode narrow-linewidth fiber ring laser with SBS-assisted parity-time symmetry for mode selection

A single-longitudinal-mode narrow-linewidth fiber ring laser with stimulated Brillouin scattering (SBS) assisted parity-time (PT) symmetry for mode selection in a single fiber loop is proposed and experimentally demonstrated. When an optical pump is launched into the fiber loop along one direction, an SBS gain for the Stokes light along the opposite direction is produced. For two light waves at the Stokes frequency propagating along the two opposite directions, one will have a net gain and the other will have a net loss. By incorporating a fiber Bragg grating (FBG) with partial reflection in the loop, mutual coupling between the two counterpropagating Stokes light waves is achieved. The SBS gain can be controlled by tuning the angle between the polarization directions of the pump and the Stokes light waves through a polarization controller (PC). Once the gain and loss coefficients between the two counterpropagating light waves are controlled to be identical in magnitude, and that the gain coefficient is greater than the coupling coefficient caused by the FBG, PT symmetry breaking is achieved, making the mainmode to sidemode ratio highly enhanced, single mode lasing is thus achieved. The approach is evaluated experimentally. For a fiber ring laser with a cavity length of 8.02 km, single-mode lasing with a narrow 3-dB linewidth of 368 Hz and a sidemode suppression ratio of around 33 dB is demonstrated. The wavelength tunable range from 1550.02 to 1550.18 nm is also demonstrated.