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

Abstract

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.

Resonant photoacoustic simultaneous detection of methane and ethylene by means. of a 1.63-μm diode laser

“…Gas detectors based on fixed-wavelength diode lasers are already commercially available [9] but they are not suitable for the simultaneous detection of more than one molecule. The development of multi-component gas detectors has attracted, in recent years, strong research and technological efforts [10][11][12][13][14][15][16][17][18]. Detectors based on 1.7-2.1-µm diode lasers are in principle more efficient because many molecules show relatively strong absorption bands in this region, but such lasers are still in development and not easily available.…”

Development of MEMS photoacoustic spectroscopy

“…With some exceptions [6][7] PAS cells are often fabricated by traditional machining methods for use in the lab with emphasis on improving detection. While the optical excitation is typically through free-space paths, use of fiber optics has been reported [8], but only in the near-IR region (3000-12,500 cm -1 ), well outside the highly valuable "fingerprint" region (600-1200 cm -1 ). The fingerprint region is important for exact chemical identification for this is where small differences in molecular composition and structure result in significant spectral differences.…”

Breadboard sized photo-acoustic spectroscopy system using an FPGA based lock-in amplifier

“…Much work has been done over the last several years to reduce the size of the photo-acoustic cell to the MEMS scale [5][6][7][8] . Other work has been done in collaboration with Daylight Solutions to reduce the size of the tunable laser and its driver, see Figure 2.…”