Ultra-narrow linewidth light generation based on an optoelectronic oscillator

Narrow-linewidth light sources are essential for both fundamental research and various technological applications, yet they are challenging to generate directly due to instabilities in active laser cavities and the misalignment between closely spaced resonator modes and broad gain bandwidth. In this study, we demonstrate the direct generation of low-noise light from an optoelectronic oscillator (OEO). By minimizing the delay in the optoelectronic link and employing a high-quality optical resonator, an OEO can simultaneously generate both a low-phase-noise optical oscillation that is resilient to phase fluctuations of its pump laser, and a radio frequency (RF) oscillation that captures these fluctuations. We leverage these RF-domain fluctuations to implement a high-performance feedback loop, which stabilizes the pump laser and significantly enhances the performance of optical oscillation. This approach achieves an outstanding phase noise level of -100 dBc/Hz at a 1 kHz offset and an integrated linewidth as narrow as 0.23 Hz. The integration of optoelectronic oscillation and feedback loop provides significant broadband noise suppression compared to conventional schemes for generating narrow-linewidth light, paving the way for developments in fields such as coherent optical communications, atomic spectroscopy, metrology, and quantum optics.