Laser Frequency Stabilization Using a Prescaler and a High-Resolution Frequency to Voltage Converter

Lasers operating at the hundreds of terahertz range are used to cool, trap, and manipulate atoms in quantum simulation experiments. Drifts and fluctuations of the laser frequency can diminish the overall performance of such systems. Although precise control solutions exist, the nonlinear and fast frequency variations of lasers are hard to monitor precisely in real-time. In this paper, we propose a laser frequency stabilization system, which involves a frequency prescaler, a low-latency Frequency to Voltage Converter (FVC) implemented in FPGA, and a PID. Our FVC implementation counts the zero crossings in the input signal, adapts a processing window size automatically, and scales the detected frequency to a
$2\ \mathrm{V}_{\text{pp}}$
DAC output in a configurable range. The prescaler enables the detection of frequency signals higher than the Nyquist rate, i.e. 62.5 MHz for our ADC. We verified the proposed laser stabilization system by integrating it successfully in an experiment generating Bose-Einstein condensates. The system can measure the instant change of the frequency between two lasers varying between 10 MHz and 1.25GHz, and is used to generate the error signal for stabilizing the respective frequencies. In our experiment, our FPGA implementation detects the frequency variations down to 72.5 kHz resolution (1.45 MHz before prescaling) with a 432 ns digital latency and an adaptive extension to 336 ns window length. Our straightforward approach can be easily adapted to experiments requiring high-resolution laser frequency detection, and further allow monitoring the output of generic frequency sources in real-time.