Ultra-narrow linewidth lasers have found widespread application in various fields such as precision gyroscopes, optical sensing, coherent optical communication, atomic clocks, and ultra-stable microwave generators. Recently, researchers have used stimulated Brillouin scattering (SBS) in ultra-high-quality-factor (Q-factor) fiber resonators as an effective method for linewidth compression. By increasing the Q-factor and Stokes light power in a long fiber cavity, the linewidth of Brillouin lasers can be compressed, resulting in a narrower output beam. However, achieving stable single-longitudinal-mode operation with a long fiber cavity can be challenging due to limitations of the free spectral range (FSR), relative to the gain bandwidth of SBS. The Vernier effect in a compound cavity can address this challenge and achieve single-longitudinal-mode operation with narrow linewidth, though this approach requires precise control of cavity lengths and can be complex. In the present study, we described a novel approach that combines the Pound-Drever-Hall (PDH) locking technique with the stimulated Brillouin effect in an ultra-high-Q-factor fiber resonator and achieved a significantly reduced linewidth for Brillouin lasers, reaching the sub-mHz level. We hope that this research will promote the development of narrow linewidth Brillouin lasers and drive progress in several key fields, including optical communication, optical spectrum analysis, and optical sensing.

We combine the Pound-Drever-Hall (PDH) locking technique with the stimulated Brillouin effect in an ultra-high-Q-factor fiber resonator to compress the linewidth of Brillouin lasers. Initially, we increase the coupled Q-factor to over 10¹⁰ by lengthening the fiber resonator and simultaneously reducing its free spectral range. In addition, thermal and mechanical isolation treatments are applied to the fiber resonator with foam and aluminum boxes to mitigate the influence of external environmental factors. The high-precision PDH locking technique is then used to generate a single-longitudinal-mode Brillouin laser with side-mode suppression exceeding 70 dB, even when the FSR of the fiber resonator is significantly smaller than the Brillouin gain bandwidth. Furthermore, we employ three methods to rigorously test the linewidth of the fiber Brillouin laser: the reference laser heterodyne method, the delayed self-heterodyne method, and the common cavity laser heterodyne method. These methods allow us to achieve a fundamental linewidth for the Brillouin laser at the sub-mHz (millihertz) level.

As shown in Fig. 5, the findings indicate that as the Brillouin laser power increases, there is a corresponding increase in side-mode power. Specifically, when the Stokes power is raised to 13 dBm, the Brillouin laser achieves a side-mode suppression ratio of 70 dB. However, further increasing the Brillouin laser power may potentially reduce noise performance. The analysis of the three linewidth measurement methods is shown in Fig. 6. The common cavity laser heterodyne method effectively suppresses common-mode, mechanical, and vibration noise. At a Brillouin laser output power of 9 dBm, the fundamental linewidth is calculated from the frequency noise in the white noise region as 31 μHz, which closely matches the theoretical expectation of 30 μHz. Results from the delayed self-heterodyne method, shown in Fig. 6(b), reveal a fundamental linewidth of 0.9 mHz at a Brillouin laser power of 13 dBm. However, experimental results are significantly influenced by variables such as the delay fiber length and external environmental conditions, which constrain measurement sensitivity and cause a notable deviation from theoretical values. In addition, comparing the phase noise of the Brillouin laser with that of the pump laser under PDH-locked conditions indicates that the Brillouin laser significantly mitigates the frequency noise of the pump light in this specified range.

In this study, we present a significant advancement in laser technology by successfully combining the PDH locking technique with SBS in an ultra-high-Q-factor fiber resonator. The result is a single-longitudinal-mode Brillouin laser with a narrow linewidth and substantial potential across various applications. Our meticulous approach involves three distinct measurement schemes—the reference laser heterodyne method, the delayed self-heterodyne method, and the common cavity laser heterodyne method—which allow for precise assessment of the SBS laser linewidth and comprehensive validation of our results. Using the common cavity laser heterodyne method, we achieve a remarkable fundamental linewidth of 31 μHz, closely matching the theoretical calculation of 30 μHz, affirming the robustness and accuracy of our methodology. This achievement marks a milestone in laser research, demonstrating our capability to attain sub-mHz linewidth for Brillouin lasers. It also highlights the crucial role of Stokes light in mitigating phase noise from the pump laser, enhancing the overall stability and performance of the laser system. The implications of this research are profound, potentially advancing narrow linewidth Brillouin lasers and fostering progress in optical communication, optical spectrum analysis, and optical sensing. As we continue to refine and expand upon these findings, we anticipate further significant advancements in laser technology and its diverse applications.