Compact optical multipass matrix system design based on slicer mirrors

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

Selectivity is one of the most crucial figures of merit in trace gas sensing, and thus a comprehensive assessment is necessary to have a clear picture of sensitivity, selectivity, and their interrelations in terms of quantitative and qualitative views.

Modified ray transfer matrix method for accurate non-sequential ray tracing between arbitrary reflective mirrors

The ray transfer matrix method is widely used for analyzing beam-transmission properties and designing multi-pass systems. In this paper, a modified ray transfer matrix method is proposed for tracing an accurate non-sequential ray in three-dimensional space based on a vector reflection theory. The modified ray transfer matrix method can be utilized to analyze a ray transmitting between arbitrary surfaces, which is not confined to rotational symmetrical structures. There is no need to project rays onto two perpendicular planes, nor to introduce a paraxial approximation in our calculation. Since the length and angle of every ray can be calculated accurately, almost no deviation is accumulated in multi-pass system. The modified ray transfer matrix method can be applied in optical design, especially in the design of multi-pass cavities and multi-pass cells.

Silicon Multi-Pass Gas Cell for Chip-Scale Gas Analysis by Absorption Spectroscopy

Semiconductor and micro-electromechanical system (MEMS) technologies have been already proved as strong solutions for producing miniaturized optical spectrometers, light sources and photodetectors. However, the implementation of optical absorption spectroscopy for in-situ gas analysis requires further integration of a gas cell using the same technologies towards full integration of a complete gas analysis system-on-chip. Here, we propose design guidelines and experimental validation of a gas cell fabricated using MEMS technology. The architecture is based on a circular multi-pass gas cell in a miniaturized form. Simulation results based on the proposed modeling scheme helps in determining the optimum dimensions of the gas cell, given the constraints of micro-fabrication. The carbon dioxide spectral signature is successfully measured using the proposed integrated multi-pass gas cell coupled with a MEMS-based spectrometer.

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

The vast majority of gaseous chemical substances exhibit fundamental rovibrational absorption bands in the mid-infrared spectral region (2.5–25 μm), and the absorption of light by these fundamental bands provides a nearly universal means for their detection. A main feature of optical techniques is the non-intrusive in situ detection of trace gases. We reviewed primarily mid-infrared tunable laser-based broadband absorption spectroscopy for trace gas detection, focusing on 2008–2018. The scope of this paper is to discuss recent developments of system configuration, tunable lasers, detectors, broadband spectroscopic techniques, and their applications for sensitive, selective, and quantitative trace gas detection.

Simultaneous measurement of gas absorption spectra and optical path lengths in a multipass cell by FMCW interferometry

We present a novel method based on optical frequency-modulated continuous-wave interferometry that can realize simultaneous measurement of gas absorption spectra and optical path lengths (OPLs) in the widely used multipass cells (MPCs). This method involves a Fourier transform (FT) and an inverse FT, by which the gas absorption spectrum and the OPL are retrieved in frequency and time domains, respectively. Various OPLs are achieved by retrieving gas absorption spectra associated with different reflection positions in the MPC. As a demonstration, absorption spectra of acetylene around 1520 nm are measured using a commercial White type MPC, achieving a noise equivalent absorbance of 0.01 and a spatial resolution of 290 μm over 22.5 m OPL. Our proposed method shows advantages for MPC-based gas sensing applications in the significant relaxation of OPL calibration demands and flexible extension of measurement dynamic range.