The performance and limitations of FPGA-based digital servos for atomic, molecular, and optical physics experiments

TL;DR

This paper evaluates FPGA-based digital servos for laser control in AMO physics, highlighting their performance, advantages, limitations, and improvements over analog servos in high-bandwidth feedback applications.

Contribution

It provides a comprehensive benchmark of FPGA-based digital servos, comparing them to analog counterparts and demonstrating enhanced control features and bandwidth capabilities.

Findings

Digital servos achieve 2.5 MHz bandwidth.

Digital servos outperform analog in transfer function flexibility.

Limitations include signal latency and resonance suppression challenges.

Abstract

In this work we address the advantages, limitations, and technical subtleties of employing FPGA-based digital servos for high-bandwidth feedback control of lasers in atomic, molecular, and optical (AMO) physics experiments. Specifically, we provide the results of benchmark performance tests in experimental setups including noise, bandwidth, and dynamic range for two digital servos built with low and mid-range priced FPGA development platforms. The digital servo results are compared to results obtained from a commercially available state-of-the-art analog servo using the same plant for control (intensity stabilization). The digital servos have feedback bandwidths of 2.5 MHz, limited by the total signal latency, and we demonstrate improvements beyond the transfer function offered by the analog servo including a three pole filter and a two pole filter with phase compensation to suppress resonances. We also discuss limitations of our FPGA-servo implementation and general considerations when designing and using digital servos.