Novel Helmholtz-based photoacoustic sensor for trace gas detection at ppm level using GaInAsSb/GaAlAsSb DFB lasers

A new and compact photoacoustic sensor for trace gas detection in the 2-2.5 μm atmospheric window is reported. Both the development of antimonide-based DFB lasers with singlemode emission in this spectral range and a novel design of photoacoustic cell adapted to the characteristics of these lasers are discussed. The laser fabrication was made in two steps. The structure was firstly grown by molecular beam epitaxy then a metallic DFB grating was processed. The photoacoustic cell is based on a Helmholtz resonator that was designed in order to fully benefit from the highly divergent emission of the antimonide laser. An optimized modulation scheme based on wavelength modulation of the laser source combined with second harmonic detection has been implemented for efficient suppression of wall noise. Using a 2211 nm laser, sub-ppm detection limit has been demonstrated for ammonia. © 2005 Elsevier B.V. All right reserved.

Gas traces measurement by photoacoustic spectroscopy using Helmholtz resonator-based sensors

Mario Mattiello

Photoacoustic spectroscopy is a well-established gas traces optical detection technique, which consists in the generation of an acoustic wave in the investigated gas compound excited by a modulated laser beam, and in the detection of this sound wave with a sensitive microphone. The sensitivity of this technique can be greatly enhanced through the use of acoustic resonators. A wide range of resonant configurations has been developed and reported in the literature in the last decades. Among these, Helmholtz resonators are known for their simplicity of implementation, though with a reduced efficiency. The main goal of this work is to demonstrate that Helmholtz resonators can be successfully applied to photoacoustic spectroscopy, delivering sensitivities in the same order of magnitude as other more common configurations, as well as offering a powerful tool to perform differential measurements. Two Helmholtz-based sensors are presented within this thesis. The first sensor has been developed for ammonia sensing, taking benefit from the properties of antimonide-based lasers; the final design has been the result of a carefully optimised compromise between high sensitivity, noise reduction, technical constraints, compact size and straightforward use. After several implemented improvements, an ultimate sub-ppm concentration detection limit has been achieved. The second sensor exploits the intrinsic phase shift existing between the two volumes of a Helmholtz resonator to perform differential measurements. Each of the two volumes of the resonator is filled with a different concentration of the same gas, whereas the exciting laser beam is split in two arms that separately illuminate the volumes, the resulting photoacoustic signal being proportional to the difference between the respective concentrations of the probed gas in the volumes. A particular detection scheme has been implemented to guarantee a linear measurement. The achieved sensitivity is not as high as obtained with the first sensor; nonetheless, the developed sensor offers the possibility to perform measurements that would otherwise require two different sensing devices, resulting in a clear gain in terms of cost and of detection complexity.

EPFL2008

Application of antimonide diode lasers in photoacoustic spectroscopy

Mario Mattiello, Luc Thévenaz

First investigations of photoacoustic (PA) spectroscopy (PAS) of methane using an antimonide semiconductor laser are reported. The laser fabrication is made in two steps. The structure is firstly grown by molecular beam epitaxy, then a metallic distributed-feedback (DFB) grating is processed. The laser operates at 2371.6 nm in continuous wave and at room temperature. It demonstrates single-mode emission with typical tuning coefficients of 0.04 nm mA-1 and 0.2 nmK-1. PA detection of methane was performed by coupling this laser into a radial PA cell. A detection limit of 20 ppm has been achieved in a preliminary configuration that was not optimised for the laser characteristics. © 2004 Elsevier B.V. All rights reserved.

Elsevier2004

Gas traces measurement by photoacoustic spectroscopy using Helmholtz resonator-based sensors

Mario Mattiello

EPFL2008