Millihertz-linewidth self-injection–locked semiconductor laser based on a hollow FP cavity

An ultra-narrow-linewidth laser is a core device in fields such as optical atomic clocks, quantum communications, and microwave photonic oscillators. This paper reports an ultra-narrow-linewidth self-injection locked semiconductor laser, which is realized through optical feedback from a high-Q (258 million) Fabry-Perot (FP) cavity constructed with three mirrors, generating an output power of 12 mW. Employing a delay self-heterodyne method based on a signal source analyzer, the phase noise of the laser is -129 dBc/Hz at 100 kHz offset frequency, with an intrinsic linewidth of 3 mHz. This is the shortest, to our knowledge, intrinsic linewidth obtained in a self-injection-locked laser. Additionally, the estimated integral linewidth is 11.8 Hz, validating its extremely low noise level and outstanding narrow-linewidth performance. This study provides strong technical support for the extra-laboratory applications of narrow-linewidth semiconductor lasers, which is of great significance to the development of various coherent optical systems.