Noise-squeezed forward Brillouin lasers in multimode fiber microresonators

Stimulated Brillouin scattering (SBS) in low-power and compact microresonators has created a new field in cavity nonlinear photonics due to the marriage between acoustic and optical signal processing. Considering the fundamental differences between backward SBS and forward SBS processes, it is challenging to observe the coexistence of both processes in the same microresonator, as well as the photon noise suppression for the forward stimulated Brillouin laser (FSBL). In this paper, we demonstrate the first 20-dB-noise-squeezed FSBL generation excited by the coexisting backward SBL (BSBL) in an ultrahigh-quality-factor Fabry-Perot (FP) microresonator based on multimode fiber (MMF). Multiple FSBLs and BSBLs are cascaded by multiple intermodal SBS processes in the multimode microresonator, where the cascaded process between backward SBS and forward SBS process (pump-BSBL-FSBL) provides a route towards additional noise squeezing, rendering the FSBL phase noise to be -120 dBc/Hz at 1 MHz offset frequency. Furthermore, we demonstrate the first Brillouin-Kerr soliton from a high-order BSBL, which also coexists with FSBLs. Our experimental results show the potential of MMF FP microresonator as an ideal testbed for high-dimensional nonlinear cavity dynamics and laser source with ultrahigh coherence.