# Large optical depth frequency modulation spectroscopy

## TL;DR

This paper introduces a modified frequency modulation spectroscopy technique that recovers signals in optically thick media by using larger modulation indices, enabling better measurement of atomic ensembles.

## Contribution

The authors demonstrate that increasing the modulation index allows signal recovery in high optical depth conditions, focusing on natural linewidth dominance despite inhomogeneous broadening.

## Key findings

- Signal can be recovered at large optical depths using higher modulation indices.
- The technique emphasizes the natural linewidth over inhomogeneous broadening.
- Application to cesium vapor shows effective spectroscopic measurements.

## Abstract

Band-resolved frequency modulation spectroscopy is a common method to measure weak signals of radiative ensembles. When the optical depth of the medium is large, the signal drops exponentially and the technique becomes ineffective. In this situation, we show that a signal can be recovered when a larger modulation index is applied. Noticeably, this signal can be dominated by the natural linewidth of the resonance, regardless of the presence of inhomogeneous line broadening. We implement this technique on a cesium vapor, and then explore its main spectroscopic features. This work opens the road towards measurement of cooperative emission effects in bulk atomic ensemble.