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

Abstract

A new and compact photoacoustic sensor for trace gas detection in the 2-2.5 m 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.

Signal-to-Noise Analysis of the Improved Open Photoacoustic Helmholtz Cell

“…The principle of the acousto-electric analogies is to create a model of a cell in which each of its acoustic parts is replaced by its electric counterpart, i.e., the cavity corresponds to capacitance, and the ducts are replaced by transmission lines converted into corresponding T-section impedances [20,26]. In the literature, different definitions for calculation of values of the electric counterparts can be found [20,21,24,28,29]; however, the best results are obtained when the loss-improved method is applied [17]. The model used in the analysis of the improved cell was described in considerable detail in previous papers [7,26].…”

Model of the Photoacoustic Helmholtz Resonator with Conical-Ended Duct

“…A major drawback of Helmholtz resonators used in photoacoustic applications is concerned with their relatively low quality factors. Practically, achievable Q-factor values range from a few to about 25 [10,11]. Taking into consideration that noticeable losses (resulting in a lower Q-factor) can occur at the duct-cavity boundaries, one of the attempts to develop a Helmholtz resonator with a higher quality factor led to a cell design with cone profiles at the ends of the duct (Fig.…”

“…2) [12]. Previously existing models of photoacoustic Helmholtz resonators allowed for modeling cells with ducts of constant diameters only [11,[13][14][15]. Such models were usually based on acousto-electrical analogies with either lumped elements (Fig.…”

“…In the model with lumped elements, the duct is represented by a series connection of resistance R and inductance L, while in the transmission line model, the duct is considered as a transmission line. Furthermore, the mentioned components (capacitors, inductors, resistors, transmission line) may be defined in many different ways [11,[13][14][15][16][17][18][19][20][21][22][23]. In order to limit the number of model variants, some preliminary experimental verification of the models was performed [24], leading to the conclusion that the best results are obtained with the transmission line model and the model with lumped elements presented by Kastle and Sigrist [15].…”

Near-infrared resonant photoacoustic gas measurement using simultaneous dual-frequency excitation

“…Sigrist ( ) Laser Spectroscopy and Sensing Laboratory, Institute of Quantum Electronics, ETH Zurich, Schafmattstr. 16, 8093 Zurich, Switzerland e-mail: sigrist@iqe.phys.ethz.ch Fax: +41- for photoacoustic cells have been proposed: acoustically resonant [4][5][6][7][8][9][10], multipass [11,12], intracavity [13,14], parallel [15], "windowless" [16], differential [17] and quartz enhanced [18]. In most known photoacoustic sensing schemes, the absolute values of the microphone signal are measured and used to derive the concentration of the gas of interest ( [3] and references therein).…”