Sub-kilohertz intrinsic linewidth stimulated Brillouin laser in integrated lithium niobate microresonators

Ultra-narrow linewidth Brillouin microlasers with low-threshold power have been achieved in silica, silicon and silicon nitride platforms, but not in lithium niobate on insulator (LNOI), a pivotal material for integrated photonics. This challenge arises from the need to simultaneously confine optical and acoustic fields tightly while enhancing optomechanical coupling. Here we demonstrate an integrated Brillouin microlaser with 118-Hz intrinsic linewidth and 3.15-mW threshold power in a dispersion engineered and suspended LNOI microdisk resonator of 116 m diameter. Benefited from the ultrahigh Q factor and sub-millimeter-scale microresonator, large Brillouin gain is attained via the backward intermodal stimulated Brillouin scattering between the dual-resonant optical whispering gallery modes (WGMs) and a 10-GHz mechanical WGM, while satisfying the requirements of both energy and momentum conservations. The strong optomechanical coupling rate up to 12.1 kHz is readily for a record narrow linewidth and a lowest stimulated Brillouin laser threshold value within sub-millimeter-scale integrated microresonators reported so far. This advancement in integrated ultra-narrow linewidth Brillouin lasers paves the way for applications ranging from coherent information processing to precision metrology within the realm of high density photonic integration.