Errors in PDH offset locking due to spurious spectral features

TL;DR

This paper identifies and models a significant source of error in PDH laser frequency stabilization caused by unintended interactions between residual sidebands and higher-order modes, which can be mitigated with spectral purity improvements.

Contribution

It reveals a previously underestimated error source in PDH offset locking, provides experimental validation, and proposes spectral purity as a mitigation strategy.

Findings

Frequency deviations up to 50% of cavity linewidth observed.

Agreement between experimental data and a simple theoretical model.

Spectrally-pure frequency offsets reduce deviations by an order of magnitude.

Abstract

The Pound-Drever-Hall (PDH) technique is widely used to stabilize the frequency of lasers. Here we report on a routinely underestimated source of error in PDH offset-locking: a shift in the lock point due to the unintended interaction between residual optical sidebands and higher-order spatial modes in misaligned Fabry-Perot cavities. Significant frequency deviations-up to 50% of the cavity linewidth-can arise when the optical offset is obtained from a sinusoidally driven EOM. We measure this deviation experimentally, find agreement with a simple model, and show how a spectrally-pure frequency offset can reduce the deviation by an order of magnitude. Our findings draw attention to a systematic effect of importance to precision optical spectroscopy, optical clocks, and quantum information science.