# Mid-infrared continuous-filtering Vernier spectroscopy using a doubly resonant optical parametric oscillator

## TL;DR

This paper introduces a mid-infrared continuous-filtering Vernier spectrometer based on a femtosecond doubly resonant optical parametric oscillator, capable of rapid, high-resolution molecular spectroscopy with high sensitivity.

## Contribution

The work demonstrates a novel spectrometer design that achieves fast, high-resolution, and sensitive molecular detection in the mid-infrared range using a femtosecond optical parametric oscillator and Vernier filtering.

## Key findings

- Spectral acquisition of 250 nm bandwidth in 25 ms at 8 GHz resolution.
- Minimum detectable CH4 concentration of 360 ppt, improving to 90 ppt with averaging.
- Stable, continuous spectra acquisition with active locking and calibration.

## Abstract

We present a continuous-filtering Vernier spectrometer operating in the 3.15-3.4 ${\mu}$m range, based on a femtosecond doubly resonant optical parametric oscillator, a cavity with a finesse of 340, a grating mounted on a galvo scanner and two photodiodes. The spectrometer allows acquisition of one spectrum spanning 250 nm of bandwidth in 25 ms with 8 GHz resolution, sufficient for resolving molecular lines at atmospheric pressure. An active lock ensures good frequency and intensity stability of the consecutive spectra and enables continuous signal acquisition and efficient averaging. The relative frequency scale is calibrated using a Fabry-Perot etalon or, alternatively, the galvo scanner position signal. We measure spectra of pure CH${_4}$ as well as dry and laboratory air and extract CH${_4}$ and H${_2}$O concentrations by multiline fitting of model spectra. The figure of merit of the spectrometer is 1.7x10${^-}$${^9}$ cm${^-}$${^1}$ Hz${^-}$${^1}$${^/}$${^2}$ per spectral element and the minimum detectable concentration of CH${_4}$ is 360 ppt Hz${^-}$${^1}$${^/}$${^2}$, averaging down to 90 ppt after 16 s.

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Source: https://tomesphere.com/paper/1702.00396