FPGA-based Low-Latency Digital Servo for Optical Physics Experiments

We propose a general-purpose dual-channel field-programmable gate array (FPGA) based digital servo with a minimum latency around 200 ns. This servo implements a proportional, dual-integration and derivative (PIID) controller along with internal numerical controlled oscillators (NCO), a phase detector (PD) utilizing a fast in-phase and quadrature (I/Q) detection algorithm (80 ns), NCO modulator, network analyzer, auto-locker, ramp generator, and low pass filters. A web interface and/or Python commands over TCP/IP allows full control and monitoring. The servo has been successfully used to lock Pound-Drever-Hall cavities, phase-locked loops and optical frequency combs. A simple experiment phase-locking two dielectric resonator oscillators (DRO) has been set up to demonstrate a 1 MHz closed-loop bandwidth.