Broadband calibration-free cavity-enhanced complex refractive index spectroscopy using a frequency comb

TL;DR

This paper introduces a broadband, calibration-free cavity-enhanced spectroscopy method using a frequency comb to measure the complex refractive index of molecular bands with high precision and without prior calibration.

Contribution

The authors develop and demonstrate a novel broadband CE-CRIS technique that simultaneously measures absorption and dispersion spectra without calibration, using a frequency comb and cavity mode analysis.

Findings

Successfully measured CO2 spectra in the 1525-1620 nm range

Achieved good agreement with theoretical models

Demonstrated simultaneous measurement of absorption and dispersion

Abstract

We present broadband cavity-enhanced complex refractive index spectroscopy (CE-CRIS), a technique for calibration-free determination of the complex refractive index of entire molecular bands via direct measurement of transmission modes of a Fabry-Perot cavity filled with the sample. The measurement of the cavity transmission spectrum is done using an optical frequency comb and a mechanical Fourier transform spectrometer with sub-nominal resolution. Molecular absorption and dispersion spectra (corresponding to the imaginary and real parts of the refractive index) are obtained from the cavity mode broadening and shift retrieved from fits of Lorentzian profiles to the individual cavity modes. This method is calibration-free because the mode broadening and shift are independent of the cavity parameters such as the length and mirror reflectivity. In this first demonstration of broadband CE-CRIS we measure simultaneously the absorption and dispersion spectra of three combination bands of CO2 in the range between 1525 nm and 1620 nm and achieve good agreement with theoretical models. This opens up for precision spectroscopy of the complex refractive index of several molecular bands simultaneously.