Accounting for the detection response brings cavity ring-down spectroscopy to sub-permille accuracy

Abstract

For nearly four decades, cavity ring-down spectroscopy (CRDS) has been among the simplest, most widely used absorption techniques, requiring no calibration because it measures decay times, not intensities. Yet as accuracy requirements have tightened, CRDS has proven strikingly inconsistent: reported errors range from several percent to the permille level - the signature of an uncontrolled systematic effect so far worked around rather than resolved. This has confined applications demanding the highest accuracy - from testing ab initio theory to atmospheric sensing and quantum-based metrology - to a handful of laboratories relying on advanced calibration or architectures, at the cost of simplicity. Here we show the main source was hiding in plain sight: the detection chain's transient response. It distorts every ring-down, biasing the retrieved decay times differently from instrument to instrument - a bias that routine truncation of the early decay masks rather than removes. We measure this response with a calibration-free beat-note method and build it into the ring-down model. Validation uses a well-characterized carbon monoxide transition and an independent frequency-based reference. Across four detection configurations, line-intensity biases of up to 14 % shrink 41- to 87-fold, the results converge, reaching up to sub-permille agreement with the reference. This holds down to 4 us ring-down times, extending accurate CRDS to stronger absorption. Independent of the absorbing species and requiring no change to the CRDS setup, the approach turns a technique valued for its simplicity into one equally valued for its accuracy. Benchmark-quality measurement thus comes within reach of any CRDS laboratory.

16 pages, 8 figures