Modulation-free laser frequency stabilization via balanced detection of common-path Fano resonance in all-PM fiber cavity

Laser frequency stabilization is indispensable for extensive applications involving precision measurements, and the best performance has been achieved with the Pound–Drever–Hall technique. However, it relies on a bulky ultrastable Fabry–Pérot cavity with demanding environment disturbance isolation techniques, along with electro-optical modulation. Here, we present a modulation-free approach for laser frequency locking to an all-polarization-maintaining (PM) fiber cavity, and the error signal is obtained from balanced detection of common-path Fano resonance. The asymmetric Fano resonance arises from the common-path interference of the two intrinsic polarization modes in the all-PM fiber cavity, and balanced detection is used to eliminate the non-zero bias of the Fano resonance. Moreover, we also adopt birefringence compensation to alleviate the influence of temperature fluctuations on the phase bias, which is crucial for a highly stable Fano resonance. After locking to the zero-biased stable Fano resonance, we achieve a laser frequency drift less than 1 MHz within 400 s, and the laser frequency noise is lowered to 3 × 103 Hz2/Hz at 1 kHz and 1 × 104 Hz2/Hz at 200 Hz. We believe that this is a versatile, compact, and simple laser frequency stabilization approach and can thus be widely used in diverse scenarios for laser interferometry and precision measurements.