Excitation of noise-squeezed forward Brillouin lasers from intracavity backward Brillouin lasers in a multimode fiber microresonator

Abstract 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). Here, we demonstrate the first 20-dB-noise-squeezed FSBL generation excited by the coexisting backward SBL (BSBL) in an ultrahigh-quality-factor Fabry–Pérot (FP) microresonator based on multimode fiber (MMF). Multiple FSBLs and BSBLs are cascaded through multiple intermodal SBS processes in the multimode microresonator, where the cascaded process between backward SBS and forward SBS (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 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, benefitting critical applications such as acoustic and optical signal generation, processing, and sensing.