A multi-loop auto-locking system with fully digital electronics for ultra-stable laser.

To achieve high performance ultra-stable laser (USL), multiple locking loops are required to operate simultaneously, including such as Pound-Drever-Hall (PDH), fiber noise cancellation (FNC), and light power stabilization (LPS). In this paper, a highly integrated multi-loop auto-locking system designed for USLs is demonstrated using fully digital electronics. The auto-locking process is implemented using a field programmable gate array with high-speed and high precision analog-to-digital and digital-to-analog converters. The auto-locking system can automatically stabilize the laser carrier frequency or offset sideband frequency to its reference cavity and is capable of successfully relocking within 10 s if interrupted. The fully digital open-loop bandwidth of the fast feedback loop reached 1.3 MHz. The frequency instability of the USL's frequency in carrier- and offset-locking modes was measured to be 1.5 × 10-15 at 1 s and 2.5 × 10-15 at 1 s, respectively. The contribution of digital electronics to the instability in carrier-locking mode was 2.9 × 10-16 at 1 s, while the noise suppression effect of the digital servo inside the FNC and LPS loops reached 75 and 40 dB at 1 Hz, respectively. The results show that this highly flexible and high-reliability digital auto-locking system can fully satisfy the requirements of USL applications.