Heterodyne dispersive cavity ring-down spectroscopy exploiting eigenmode frequencies for high-fidelity measurements
arxiv(2024)
摘要
Measuring low light absorption with combined uncertainty < 1 permille is
crucial in a wide range of applications. Popular cavity ring-down spectroscopy
can provide ultra-high precision, below 0.01 permille, but its accuracy is
strongly dependent on the measurement capabilities of the detection system and
typically is about 10 permille. Here, we exploit the optical frequency
information carried by the ring-down cavity electromagnetic field, not explored
in conventional CRDS, for high-fidelity spectroscopy. Instead of measuring only
the decaying light intensity, we perform heterodyne detection of ring-downs
followed by Fourier analysis to provide exact frequencies of optical cavity
modes and a dispersive spectrum of a gas sample from them. This approach is
insensitive to inaccuracies in light intensity measurements and eliminates the
problem of detector band nonlinearity, the main cause of measurement error in
traditional CRDS. Using the CO and HD line intensities as examples, we
demonstrate the sub-permille accuracy of our method, confirmed by the best ab
initio results, and the long-term repeatability of our dispersion measurements
at 10^(-4) level. Such results have not been achieved in optical spectroscopy
before. The high accuracy of the presented method indicates its potential in
atmospheric studies, isotope ratio metrology, thermometry, and the
establishment of primary gas standards.
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