COMB-LOCKED CAVITY RING-DOWN SPECTROSCOPY FOR MOLECULAR TRANSITION FREQUENCY MEASUREMENTS BELOW 10−12 RELATIVE UNCERTAINTY

Abstract

The accurate determination of molecular transition frequencies can provide stringent tests and constraints on fundamental physics questions [1-3]. Here, we present recent work on a comb-locked cavity ring-down spectroscopy system which probes Doppler-broadened spectra in the linear absorption regime. These measurements have relative uncertainties in transition frequency below 10$^{-12}$, which are among the lowest values reported for optical molecular transition frequencies.

We use a probe laser which is phase-locked to a commercial optical frequency comb and subsequently coupled to a high-finesse optical cavity based on that of [4]. We observe stationary measurement statistics for measurements of more than 2000 spectra and find that measurements which are replicated in multiple experiments over several months are normally distributed. Single-spectrum signal-to-noise-ratios can exceed 50,000:1, with resulting line center uncertainties below 5 kHz. Because this method is based on linear absorption, it is an attractive alternative for the measurement of molecular transitions that cannot be probed by saturation-based Doppler-free spectroscopy. Furthermore, we demonstrate results having smaller systematic errors than those provided by more complicated nonlinear spectroscopy methods.

Finally, we report measurements for 44 $^{12}$C$^{16}$O$_{2}$ transition frequencies near 1.6 $\mu$m, obtaining combined uncertainties from approximately 200 Hz to 1 kHz. These results, which are included in a global fit yielding upper-state spectroscopic constants with reduced uncertainties, are expected to benefit spectroscopic retrieval algorithms for space-based measurements of atmospheric carbon dioxide[5].

[1] F. M. J. Cozijn et. al. Phys. Rev. Lett. 120, 153002 (2018).

[2] H. Fleurbaey et. al. Phys. Rev. Lett. 120, 183001 (2018).

[3] J. Baron et. al. Science 343, 269-272 (2014).

[4] H. Lin et. al. J. Quant. Spectros. and Rad. Trans. 161, 11-20 (2015)

[5] S. R. Kawa et. al. Tellus B: Chemical and Physical Meteorology 62, 759-769 (2010)