Abstract
A method of measuring double resonant two-photon signal and background from a single cavity ring-down decay is introduced. This is achieved by modulating the double resonance loss via one of the light sources exciting the transition. The noise performance of the method is characterized theoretically and experimentally. The addition of a new parameter to the fitting function introduces a minor noise increase due to parameter correlation. However, the concurrent recording of the background can extend the stable measurement time. Alternatively, the method allows a faster measurement speed, while still recording the background, which is often advantageous in double resonance measurements. Finally, the method is insensitive to changes in the cavity decay rate at short timescales and can lead to improved performance if they have significant contribution to the final noise level compared to the detector noise.
Highlights
Continuous-wave cavity ring-down spectroscopy (CRDS) is a powerful tool for trace gas sensing and measuring weak optical attenuation [1]
We compared the noise behavior of the decay rate extracted from measured CRDS decay traces using either the classic single exponential fits, or the Step-Modulated decay cavity Ring-down Spectroscopy (SMRS) fits
For the SMRS, the two ring-down rates extracted from a single decay should be the same and the noise of their difference reflects on the uncertainty of the modulated fits
Summary
Continuous-wave cavity ring-down spectroscopy (CRDS) is a powerful tool for trace gas sensing and measuring weak optical attenuation [1]. In the case of linear absorption, this light decay is exponential in time and characterized by a decay rate constant which is equal to the fractional optical power loss per round trip times the number of round trips per second (which equals the free spectral range of the cavity). This loss rate, often referred to as the ring-down rate, has a contribution from the finite reflectivity of the mirrors that make up the cavity as well as absorption, scattering, or reflection from any sample inside the cavity. Double resonance measurements provide enhanced selectivity towards the species and eigenstates involved in the transitions, which is of assistance in spectral assignment [6]
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