Signal line shapes of Fourier transform cavity-enhanced frequency modulation spectroscopy with optical frequency combs

TL;DR

This paper analyzes the signal line shapes in Fourier transform cavity-enhanced frequency modulation spectroscopy with optical frequency combs, comparing models and verifying with experiments on CO2 spectra.

Contribution

It provides a comprehensive model and experimental validation of NICE-OFCS signal line shapes, especially regarding the modulation frequency to line width ratio.

Findings

The simplified analytical model accurately predicts line shapes at low f_m/Γ ratios.

Experimental spectra of CO2 match the theoretical models.

The study enhances understanding of signal dependence on modulation parameters.

Abstract

We present a thorough analysis of the signal line shapes of Fourier transform-based noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS). We discuss the signal dependence on the ratio of the modulation frequency, f${_m}$, to the molecular line width, {\Gamma}. We compare a full model of the signals and a simplified absorption-like analytical model that has high accuracy for low f${_m}$/{\Gamma} ratios and is much faster to compute. We verify the theory experimentally by measuring and fitting NICE-OFCS spectra of CO${_2}$ at 1575 nm using a system based on an Er:fiber femtosecond laser and a cavity with a finesse of ~11000.