A digital servo for ultra-stable laser frequency stabilization

We present a fully digital servo optimized for ultra-stable laser frequency stabilization. Experiments such as optical clock experiments can achieve high laser frequency stability, imposing high bandwidth, high precision, and low noise requirements on servo systems. The laser system utilizes the Pound-Drever-Hall method, employing an ultra-stable cavity to generate an error signal for servo input. The input is separated into two independent channels, with one channel featuring high feedback bandwidth and the other channel featuring high gain in the low-frequency domain. The process is fully digitized using field-programmable gate arrays with custom-made infinite impulse response filters and proportional-integral-derivative algorithms. Thanks to the low latency of 120.5 ns and low input noise of 3.22 × 10-12 V2/Hz@1 Hz, our digital servo can easily lock an external-cavity diode laser to a typical ultra-low expansion ultra-stable cavity. The laser system has a fractional frequency stability of 10-16@1s, with the servo electrical noise contributing only 5.54 × 10-18@1s.