Chirped Laser Dispersion Spectroscopy for Remote Open-Path Trace-Gas Sensing

Abstract

In this paper we present a prototype instrument for remote open-path detection of nitrous oxide. The sensor is based on a 4.53 μm quantum cascade laser and uses the chirped laser dispersion spectroscopy (CLaDS) technique for molecular concentration measurements. To the best of our knowledge this is the first demonstration of open-path laser-based trace-gas detection using a molecular dispersion measurement. The prototype sensor achieves a detection limit down to the single-ppbv level and exhibits excellent stability and robustness. The instrument characterization, field deployment performance, and the advantages of applying dispersion sensing to sensitive trace-gas detection in a remote open-path configuration are presented.

Advances in Mid-Infrared Spectroscopy for Chemical Analysis

“…Spectroscopy in the mid-infrared (MIR) spectral region extending from 4,000 cm −1 to 400 cm −1 (2.5-25 μm) provides quantitative chemical, structural, and compositional information on constituent molecules in the gas, liquid, and solid phases, thus offering a wide field of applications ranging from industrial production monitoring (1)(2)(3), materials science (4), medicine (5,6), and biotechnology (7,8) to environmental analysis (9)(10)(11)(12)(13). The inherent selectivity of MIR spectroscopy provides access to fingerprint spectra of organic and inorganic constituents and is of particular advantage when probing complex samples in the presence of interfering components, which are frequently encountered in real-world applications.…”

Suppressing the influence of optical fringes in dispersion spectroscopy

“…We assumed the presence of a 20-cm-long optical etalon (FSR =750 MHz in air). These are realistic assumptions: few hundreds ppbv is a typical ambient N 2 O concentration, a 10m http://www.photonics.pl/PLP © 2013 Photonics Society of Poland optical path can be easily obtained using a multi-pass cell and a similar etalon was present in previous CLaDS instruments due to a poor AR coating on the acoustooptical modulator [3,17] (similar fringes can also be created when a multi-pass cell is used [18]). Figure 5 shows amplitude simulation of a CLaDS dispersion signal as a function of frequency spacing Ω for both N 2 O and fringe spectrum.…”

“…[13]). CLaDS provides several advantages over other spectroscopic techniques including: immunity of the CLaDS signal amplitude to the fluctuations of light intensity [3], or linear dependence of the CLaDS signal amplitude on molecular concentration [15]. Despite many new measurement capabilities CLaDS (similarly to other methods based on coherent laser sources) suffers from optical fringes that affect the phase of light and create periodic fringe structures also in the dispersion spectrum.…”

“…Laser spectroscopy is a powerful tool in various fields including fundamental research [1], environmental monitoring [2][3], industrial process control [4][5], or medical diagnostics [6]. With sensitivity enhancement schemes based on optical paths increase using e.g.…”

Advanced Fabrication of Single-Mode and Multi-Wavelength MIR-QCLs

“…Some applications, such as open-path spectroscopy, benefit specifically from high-power single-mode operation with a proper beam shape [55,56]. Although DFB QCLs achieve high output power values, their maximum output power is limited due to the appearance of parasitic modes when increasing the length or width of the device.…”