Narrow-linewidth microcavity Brillouin laser based on pump-locked high-Q silica microsphere resonator

Microcavity-based Brillouin lasers are promising high-performance light sources for integrating photonics and optoelectronics. One method to lock the pump light frequency is to utilize a complex system with optoelectronic feedback, which requires a high-cost narrow-linewidth pump laser and limits the application of microlasers in integrated optoelectronic systems. Another method reported recently is all-optical feedback to achieve the locking of microcavity laser. We propose to utilize Rayleigh scattering of microcavities to lock the frequency of the pump laser to the resonant frequency of the Brillouin laser microcavity with the all-optical method. While compressing the linewidth of the pump laser, it can greatly improve the long-term stability of the optically pumped microcavity Brillouin laser. In the experiment, the linewidth of the semiconductor pump laser is compressed from the MHz level to the kHz level. The microcavity Brillouin laser achieves an ultra-narrow intrinsic linewidth of 100 Hz, with an ultra-low frequency noise of 35 Hz2/Hz. The constructed microlaser obtains a locking time up to 1 h, which does not require any temperature control or vibration isolation of the laser system. This work demonstrated an optically pump-locked microcavity Brillouin laser, which provides a stable and reliable low-cost experimental platform for ultra-narrow-linewidth lasers, precision laser sensors, microwave-photonic signal synthesizer, and optomechanical systems.