Multipass cell based on confocal mirrors for sensitive broadband laser spectroscopy in the near infrared

Abstract

We report on broadband absorption spectroscopy in the near IR using a multipass cell design based on highly reflecting mirrors in a confocal arrangement having the particular aim of achieving long optical paths. We demonstrate a path length of 314 m in a cell consisting of two sets of highly reflecting mirrors with identical focal length, spaced 0.5 m apart. The multipass cell covers this path length in a relatively small volume of 1.25 l with the light beam sampling the whole volume. In a first application, the absorption spectra of the greenhouse gases CO(2), CH(4), and CO were measured. In these measurements we used a femtosecond fiber laser with a broadband spectral range spanning the near IR from 1.5 to 1.7 μm. The absorption spectra show a high signal-to-noise ratio, from which we derive a sensitivity limit of 6 ppmv for methane observed in a mixture with air.

Selectivity in trace gas sensing: recent developments, challenges, and future perspectives

“…, quantum heterostructure detector, 173–176 avalanche photodiode (APD), 177,178 new types of gas cells. 179–184 Therefore, the sensitivity and selectivity depend on the quality of the sources and detectors indirectly. Thus, selectivity can be improved by utilizing the same spectroscopic technique but using a new laser source operating at a unique wavelength where a target gas would only absorb in the presence of other constituent gases.…”

Mid-Infrared Tunable Laser-Based Broadband Fingerprint Absorption Spectroscopy for Trace Gas Sensing: A Review

“…Recently, new types of gas cells have been developed to achieve long optical path in a smaller volume, including modified MPCs [111][112][113][114][115][116], circular multi-reflection (CMR) cells [117][118][119][120][121][122][123][124], and HWG [32,63,69,[125][126][127][128]. Guo and Sun reviewed the progress in modified MPCs and CMR cells and compared them in terms of optical pathlength (OPL), volume, and path-to-volume ratio (PVR) [116].…”

Analysis of Gas Mixtures with Broadband Dual Frequency Comb Spectroscopy and Unsupervised Learning Neural Network

“…The mid‐infrared (MIR) wavelength range exhibits many vibrational transitions of chemically important functional groups presenting the “fingerprint region” of the spectrum and enabling detection either remotely or locally via multipass cells (MPC). [ 2 ] Due to the large absorption cross sections in this spectral range, a potentially high detection sensitivity in direct absorption spectroscopy measurements can be reached, allowing the technology to progress further into trace gas detection. Various spectroscopic applications of optical frequency combs have been developed such as cavity enhanced Fourier transform spectroscopy, [ 3 ] MIR up‐conversion spectroscopy, [ 4 ] MIR virtually imaged phase array spectroscopy, [ 5 ] and dual comb spectroscopy (DCS), [ 6 ] which is based on superimposing two mutually coherent frequency combs.…”