# Sensitive and broadband measurement of dispersion in a cavity using a Fourier transform spectrometer with kHz resolution

## TL;DR

This paper demonstrates a broadband, high-resolution method to measure optical cavity dispersion using a Fourier transform spectrometer and frequency combs, enabling precise characterization of cavity modes and molecular refractivity.

## Contribution

It introduces a novel approach combining a Fourier transform spectrometer with a frequency comb for accurate, broadband dispersion measurements of optical cavities and gases.

## Key findings

- Measured 16,000 cavity modes over 16 THz bandwidth.
- Retrieved cavity mirror dispersion with 0.1 fs^2 precision.
- Determined CO2 refractivity with 5 x 10^-12 precision.

## Abstract

Optical cavities provide high sensitivity to dispersion since their resonance frequencies depend on the index of refraction. We present a direct, broadband, and accurate measurement of the modes of a high finesse cavity using an optical frequency comb and a mechanical Fourier transform spectrometer with a kHz-level resolution. We characterize 16000 cavity modes spanning 16 THz of bandwidth in terms of center frequency, linewidth, and amplitude. We retrieve the group delay dispersion of the cavity mirror coatings and pure N${_2}$ with 0.1 fs${^2}$ precision and 1 fs${^2}$ accuracy, as well as the refractivity of the 3{\nu}1+{\nu}3 absorption band of CO${_2}$ with 5 x 10${^{-12}}$ precision. This opens up for broadband refractive index metrology and calibration-free spectroscopy of entire molecular bands.

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Source: https://tomesphere.com/paper/1705.04729