Optical Frequency Comb Fourier Transform Spectroscopy with Sub-Nominal Resolution - Principles and Implementation

TL;DR

This paper presents a detailed theoretical and experimental framework for optical frequency comb Fourier transform spectroscopy that achieves sub-nominal resolution, enabling highly accurate, ILS-free molecular spectra measurement and analysis of line shapes.

Contribution

It introduces a comprehensive method for sub-nominal resolution spectroscopy using optical frequency combs, including theoretical description, experimental procedures, and validation with CO2 spectra.

Findings

Achieved ILS-free molecular spectra measurement.

Quantified collisional narrowing effects in CO2.

Demonstrated high-precision line center retrieval.

Abstract

Broadband precision spectroscopy is indispensable for providing high fidelity molecular parameters for spectroscopic databases. We have recently shown that mechanical Fourier transform spectrometers based on optical frequency combs can measure broadband high-resolution molecular spectra undistorted by the instrumental line shape (ILS) and with a highly precise frequency scale provided by the comb. The accurate measurement of the power of the comb modes interacting with the molecular sample was achieved by acquiring single-burst interferograms with nominal resolution precisely matched to the comb mode spacing. Here we give a full theoretical description of this sub-nominal resolution method and describe in detail the experimental and numerical steps needed to retrieve ILS-free molecular spectra, i.e. with ILS-induced distortion below the noise level. We investigate the accuracy of the transition line centers retrieved by fitting to the absorption lines measured using this method. We verify the performance by measuring an ILS-free cavity-enhanced low-pressure spectrum of the 3{\nu}1+{\nu}3 band of CO2. We observe and quantify collisional narrowing of absorption line shape, for the first time with a comb-based spectroscopic technique. Thus retrieval of line shape parameters with accuracy not limited by the Voigt profile is now possible for entire absorption bands acquired simultaneously.