Photoacoustic Gas Sensor Research Articles

Trace gas sensor based on quartz tuning fork enhanced laser photoacoustic spectroscopy

A compact photoacoustic gas sensor based on a quartz tuning fork and fiber-coupled distributed feedback (DFB) diode laser for detection of trace gas at atmospheric pressure has been developed. The sensor performance was evaluated by detection of water vapor in ambient air at normal atmospheric pressure. A normalized noise equivalent absorption coefficient of 1.68×10−8 cm−1 W/Hz1/2 was achieved. Influence of different acoustic microresonators and sample pressure on the sensor performance, and the characterization of the sensor response time were investigated. Approaches to improve the current sensor performance are discussed.

Sensitive and Selective Photoacoustic Gas Sensor Suitable for High-Volume Manufacturing

Sensitive and selective gas measurements are crucial for a large variety of applications. This paper describes the manufacturing and characterization of a photoacoustic gas sensor system. The system is based on a pressure sensor element with a sensitivity of 10 muV/V/Pa. To demonstrate and evaluate the concept, 12 prototypes for measuring CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> have been manufactured and characterized. Detection limits ranging from 92 ppm to below 6 ppm CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> were obtained with a path length of 10 cm, depending on the measurement time and photoacoustic cell design. Measurements showed no cross-sensitivity towards CO, CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , or humidity in any of the sensors. The temperature drift of the uncompensated raw signal of two sensor designs was below 117 ppm CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> in the range from 25degC to 50degC.

CMOS-compatible fabrication of thermopiles with high sensitivity in the 3–5 μm atmospheric window

A CMOS-compatible process is proposed for the fabrication of thermopiles with high sensitivity in the 3–5 μm window, suited for use in non-dispersive infrared (NDIR) or photo acoustic (PA) gas sensors. Since CMOS passivation layers present low infrared absorption at those wavelengths, an interferometric absorbing layer based on thin metals with a dielectric spacer is adopted and embedded in the fabrication process. Different CMOS-compatible thermoelement designs are investigated and compared in terms of performance, considering Al/n-polysilicon, Al/p-polysilicon and p-/n-polysilicon solutions. Electrical tests on the fabricated devices indicate a noise equivalent power of about 0.76 nW for 1 mm 2 wide p-/n-polysilicon thermopiles, which makes such devices applicable in NDIR or PA-based sensing of relevant gases like CO, CO 2, N 2O or CH 4 having absorption lines in the 3–5 μm wavelength range.

Gas detection with quantum cascade lasers: An adapted photoacoustic sensor based on Helmholtz resonance

A photoacoustic gas sensor exploiting a quantum cascade laser as a radiation source is demonstrated. A detection limit of ∼1 ppm with 1 ms response time is found using a Peltier-cooled Fabry–Pérot InGaAs-based quantum cascade laser emitting at 9.4 μm, and a commercial microphone as a detector. The photoacoustic cell consists of a Helmholtz resonator preceeded by a low-pass acoustic filter. This geometry is well adapted to the shape of the laser beam and allows for an effective filtering of ambient acoustical noise. The relative simplicity of the system is particularly attractive for applications where sensitivity, robustness, and ease of fabrication are all fundamental requirements.

Development of a Photoacoustic Trace Gas Sensor Based on Fiber-Optically Coupled NIR Laser Diodes

A novel photoacoustic sensor system for on-line monitoring of benzene, toluene, and xylene concentrations is described. The radiation emitted by near-infrared (NIR) laser diodes is coupled into a fiber and guided to the resonant cell. The resonator is operated at its lowest azimuthal resonance while the radiation enters the cell at two different locations. The fiber-optical approach allows for the use of various laser diodes with different wavelengths. This approach enables detection of many analytes in parallel and correction for spectral interferences with other substances. Measurements at the first overtone of the CH stretching vibrations (1.67 μm) are discussed. Detection limits obtained for benzene, toluene, and xylene are 70, 100, and 160 μg/L, respectively. In order to compensate for the spectral interferences of water vapor, the concentration of the latter was determined by using the 1.31 μm NIR laser diode. Water concentrations as low as 0.5 mg/L could be determined. Results of studies performed with two analytes at two different wavelengths simultaneously demonstrate the feasibility of the technique for analysis of mixtures.

Development of a photoacoustic gas sensor for in situ and on-line measurement of gaseous water and toluene†

Developments towards a portable sensor system for the detection of benzene, toluene and xylene for screening are described. The monitoring system is based on the resonant photoacoustic technology. As light sources, NIR laser diodes at wavelengths of overtone vibrations of the alkyl CH bonds are used. The resonator is of cylindrical shape with diameter 2R = 5 cm and total length L = 10 cm and is operated in its first azimuthal mode. The acoustic wave is detected by an electret microphone. Test measurements of water and toluene vapor are presented. For the detection of water vapor a 1.31 µm NIR laser diode was used. Water concentrations down to 0.5 mg l–1 in air could be measured. For the detection of toluene vapor a wavelength of 910 nm exciting the third overtone vibration of the alkyl CH bonds was used. The sensitivity of 3.2 mg l–1 in air may be increased by several orders of magnitude as soon as suitable laser diodes with wavelengths fitting the absorption lines of the first overtone become available.