Dual Laser-Cavity Frequency Stabilization

Lasers can be stabilized by locking their frequency to an optical cavity resonance. This is typically done with the Pound-Drever-Hall (PDH) scheme where frequency-modulated light is incident on the cavity, and demodulation of the reflected intensity produces an error signal proportional to the laser-cavity detuning. However, any drifts in the length of the cavity lead to an unavoidable drift in the laser frequency, which means that ultra-low expansion cavities under vacuum are used to suppress these drifts. In this work, we present a modified PDH scheme based on the idea of dual frequency modulation (DFM). Typical DFM methods perform modulation at two different frequencies - one to perform a standard PDH lock, and the other to create an error signal in the transmitted light that tracks the cavity free spectral range (FSR). We show that it is possible to recover both error signals even when the light is modulated at a single frequency equal to half the FSR. This allows for stability transfer between optical and microwave domains without the use of a frequency comb.