Pdh Offset Locking Errors, up to 50% of Linewidth, Arise from Spurious Spectral Features

The Pound-Drever-Hall technique forms the bedrock of laser stabilisation, yet a subtle source of error routinely undermines its precision, as Roame Hildebrand, Wance Wang, and Connor Goham from the University of Maryland, along with Alessandro Restelli and Joseph Britton, now demonstrate. Their work reveals that unintended interactions between residual sidebands and higher-order spatial modes within misaligned optical cavities cause a shift in the laser’s lock point, potentially introducing significant frequency deviations, up to half the cavity linewidth. The team experimentally measured this deviation and validated a simple theoretical model, showing that using a spectrally pure frequency offset can dramatically reduce the error. These findings are crucial for advancing precision spectroscopy, atomic clocks, and the burgeoning field of quantum information science, highlighting a systematic effect previously overlooked in these sensitive applications.

PDH Locking, Frequency Deviations, and Metrology

This research thoroughly examines frequency deviations in Pound-Drever-Hall (PDH) locked lasers, identifying a frequently overlooked source of error. The study focuses on how slight misalignments between the laser and the optical cavity introduce unwanted frequencies that can cause the laser to drift from its intended frequency, a critical issue for precision measurements and advanced technologies. Researchers investigated the impact of these deviations on applications requiring highly stable laser frequencies, such as precision metrology and quantum control, aiming to understand and minimize these errors. The team compared two modulation techniques used in PDH locking, dual-sideband (DSB) and serrodyne modulation.

They discovered that serrodyne modulation effectively suppresses unwanted sidebands, leading to significantly lower frequency deviations compared to DSB modulation. Through detailed theoretical modelling and experimental measurements, scientists established a strong correlation between misalignment, modulation scheme, and the resulting frequency errors. Experiments confirmed that DSB modulation can cause substantial frequency shifts, while serrodyne modulation reduces these shifts to a much smaller level. This research offers a comprehensive analysis of a previously understudied error source in PDH locking, paving the way for more accurate and reliable precision measurements and advanced technologies.

Misalignment Effects on Laser Frequency Stability

This study meticulously investigates frequency deviations in Pound-Drever-Hall (PDH) offset-locked lasers, pinpointing a routinely underestimated error source: the interaction between residual optical sidebands and higher-order spatial modes within misaligned Fabry-Pérot cavities. Researchers engineered an experimental setup employing the PDH technique to stabilize laser frequency, deliberately introducing realistic misalignments to a Fabry-Pérot cavity to simulate practical conditions. The team precisely measured frequency deviations arising from these misalignments, utilizing a spectrally-resolved detection system to characterize the laser’s output and identify spurious modes. The core of the work involves a detailed model quantifying the impact of misalignment on cavity modes, specifically examining how tip-tilt and transverse displacement excite higher-order Hermite-Gaussian and Laguerre-Gaussian modes.

Scientists calculated the frequency shifts induced by these modes, demonstrating that deviations can occur when the offset is derived from a sinusoidally driven electro-optic modulator. To validate the model, experiments were conducted with a cavity of length 100mm and mirrors with radii of curvature 500mm and infinity. The team then explored mitigation strategies, focusing on serrodyne modulation to create a spectrally-pure frequency offset, demonstrating a reduction in deviation by an order of magnitude. This work establishes a crucial understanding of systematic errors in precision spectroscopy, optical clocks, and quantum information science, providing a pathway to enhance the stability and accuracy of these technologies. By identifying and quantifying the impact of misalignment, the research offers valuable insights for optimizing experimental setups and improving the performance of advanced laser-based applications.

Misalignment Drives Laser Frequency Errors

Scientists have identified a previously underestimated source of error in laser frequency stabilization techniques, specifically within Pound-Drever-Hall (PDH) locking systems. This work reveals that unintended interactions between residual sidebands and higher-order spatial modes within misaligned optical cavities can cause significant deviations in the laser’s lock point, potentially impacting precision measurements. Experiments demonstrate that these frequency deviations can reach as high as 50% of the cavity linewidth, a substantial error that had not been fully accounted for. The team meticulously measured these deviations using a setup incorporating a single-mode laser diode locked to an optical cavity, employing a double-sideband (DSB) frequency offset generated by an electro-optic modulator.

Cavity parameters were precisely determined. By sweeping the offset frequency and analyzing the resulting phase excursions, researchers were able to quantify the spurious frequency shifts induced by the unwanted interactions. Detailed analysis revealed that the magnitude of these shifts scales inversely with the cavity alignment contrast, allowing for predictable rescaling across different experimental setups. Theoretical predictions closely matched the experimental data, confirming the underlying mechanism driving the observed deviations. These findings are crucial for improving the accuracy of precision spectroscopy, atomic clocks, and information science applications reliant on highly stable laser frequencies.

Misalignment Drives PDH Stabilization Errors

This research systematically investigates a previously underestimated source of error in Pound-Drever-Hall (PDH) laser frequency stabilization, unwanted frequency shifts caused by interactions between residual sidebands and misaligned optical cavities. The team demonstrates that these shifts, potentially reaching half the cavity linewidth, arise when standard dual-sideband modulation is employed. Through both experimental measurement and modelling, they establish a clear link between cavity misalignment and the magnitude of these frequency deviations. The study further highlights the benefits of employing spectrally-pure optical offsets, specifically serrodyne modulation, which significantly reduces these unwanted shifts by an order of magnitude. By minimizing spurious signals, serrodyne modulation offers a pathway to more robust and accurate laser frequency control. This demonstrates the effectiveness of using advanced modulation techniques to mitigate the effects of misalignment.