High-order-mode Brillouin random fiber laser based on distributed Rayleigh scattering in all-few mode fiber half-open cavity

A high-order-mode (HOM) Brillouin random fiber laser (BRFL) based on distributed Rayleigh scattering in an all-few-mode fiber random cavity is proposed and experimentally demonstrated. Compared to traditional BRFLs oscillating in the single-mode-fiber cavity, the few-mode-fiber random cavity allows the high-order spatial mode Stokes beams to be activated and amplified via intra-mode stimulated Brillouin scattering for sufficient gain. Besides random feedback of distributed Rayleigh scattering along few-mode fibers, a few-mode fiber Bragg grating (FM-FBG) is employed to provide the one-end transverse mode-dependent reflection, which facilitates a high-purity oscillating random laser mode switchable between LP01 and LP11 mode by readily tunning the working wavelength of the Brillouin pump. Consequently, the all few-mode fiber-based BRFL emits a 600-Hz narrow-linewidth LP11 mode with a high mode purity of similar to 91.3 %, which paves the way for spatial multiplexing coherent communication, high-resolution imaging, highly sensitive sensing, etc.