Modulation-free laser frequency locking using Fano resonance in a crystalline whispering-gallery-mode cavity

<正>A low-thermal-noise, small-sized, monolithic crystalline whispering-gallery-mode cavity can achieve a compact laser frequency locking system. In this study, we propose generating a Fano resonance spectrum within the crystalline cavity to achieve frequency locking without the need for traditional modulation techniques, aiming to further simplify the locking system. By coupling a prism with the crystalline cavity, we generate a Fano transmission spectrum to serve as the error signal for laser frequency locking. Experimental results show that our method achieves a level of noise suppression comparable to the classical Pound-Drever-Hall technique, reducing laser frequency noise to near the thermal noise limit of the crystalline cavity. It enables us to suppress the laser frequency noise to below 1 Hz2∕Hz in the offset frequency range of 103–105Hz and achieve a minimum noise of0.2 Hz2∕Hz. We also analyzed various unique optical noises in the Fano locking technique and found that the primary factor limiting laser frequency noise in this work is still the inherent thermal noise of the crystalline cavity. Our results indicate that the proposed Fano locking technique has significant potential to simplify laser locking systems, enhance stability, and reduce overall power consumption and cost.