Dispersive heterodyne cavity ring-down spectroscopy exploiting eigenmode frequencies for high-fidelity measurements

Abstract

Measuring low light absorption with combined uncertainty <1 per mil (%) is crucial for many applications. Popular cavity ring-down spectroscopy can provide ultrahigh precision, below 0.01%, but its accuracy is often worse than 5% due to inaccuracies in light intensity measurements. To eliminate this problem, we exploit optical frequency information carried by the ring-down cavity electromagnetic field. 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 high-fidelity dispersive spectrum of a gas sample from them. We demonstrate the sub–per-mil accuracy of our method, confirmed by the best ab initio results for CO line intensity and for HD line shape, and the long-term repeatability of our dispersion measurements at 10−4 level. We see potential for our approach in atmospheric remote sensing, isotope ratio metrology, thermometry, and establishment of primary gas standards.