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

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 level. We see potential for our approach in atmospheric remote sensing, isotope ratio metrology, thermometry, and establishment of primary gas standards.