Janos Sneider

Pulsed-laser excitation of acoustic modes in open high-Q photoacoustic resonators for trace gas monitoring: results for C 2 H 4

The pulsed excitation of acoustic resonances was studied with a continuously monitoring photoacoustic detector system. Acoustic waves were generated in C(2)H(4)/N(2) gas mixtures by light absorption of the pulses from a transversely excited atmospheric CO(2) laser. The photoacoustic part consisted of high-Q cylindrical resonators (Q factor 820 for the first radial mode in N(2)) and two adjoining variable acoustic filter systems. The time-resolved signal was Fourier transformed to a frequency spectrum of high resolution. For the first radial mode a Lorentzian profile was fitted to the measured data. The outside noise suppression and the signal-to-noise ratio were investigated in a normal laboratory environment in the flow-through mode. The acoustic and electric filter system combined with the averaging of the photoacoustic signal in the time domain suppressed the outside noise by a factor of 4500 (73 dB). The detection limit for trace gas analysis of ethylene in pure N(2) was 2.0 parts in 10(9) by volume (ppbV) (minimal absorption coefficient α(min) = 6.1 × 10(-8) cm(-1), pulse energy 20 mJ, 1-bar N(2)), and in environmental air, in which the absorption of other gas components produces a high background signal, we can detect C(2)H(4) to ~180 ppbV. In addition, an alternative experimental technique, in which the maximum signal of the second azimuthal mode was monitored, was tested. To synchronize the sampling rate at the resonance frequency, a resonance tracking system was applied. The detection limit for ethylene measurements was α(min) = 9.1 × 10(-8) cm(-1) for this system.

INTRACAVITY PHOTOACOUSTIC GAS DETECTION WITH AN EXTERNAL CAVITY DIODE LASER

The first use of an external cavity diode-laser light source in combination with a photoacoustic detector for high-sensitivity gas detection is described. This combined system is applicable for detecting gases with absorption coefficients as low as 5 x 10−8 cm−1 by operating the photoacoustic cell in an intracavity mode. Measurements were made on the 1.13 μm absorption lines of water vapour. For quantitative measurements, it was found to be necessary to introduce a reference cell into the system.

A high-sensitivity, near-infrared tunable-diode-laser-based photoacoustic water-vapour-detection system for automated operation

A photoacoustic sensor system for automatic detection of low concentrations of water vapour is described in this paper. A Littman-configuration external-cavity diode laser operating at 1125 nm was used as a light source in combination with a high-sensitivity measuring photoacoustic cell, a reference photoacoustic cell and PC-controlled electronics. The system was calibrated with synthetic gas samples and a detection limit of 13 µmol per mol of water vapour was determined. Adsorption/desorption phenomena at the walls of the measuring photoacoustic cell were found to be an important limiting factor for the sensitivity of the system.

Photoacoustic gas detection based on external cavity diode laser light sources

An external cavity diode laser is constructed and shown to have stable, narrow-linewidth operation around 840 nm. Based on this laser and a photoacoustic detector, a gas monitoring system capable of measuring absorption coefficients as low as 10 28 cm 21 is developed. The continuous (mode-hop-free) tuning range of the laser is found to exceed 100 GHz, which makes possible the spectrally resolved detec- tion of water vapor absorption lines. The photoacoustic cell with the ex- ternal cavity diode laser as a light source offers great potential for the development of a portable, automated gas detection system.

On the Possibility of Combining External Cavity Diode Laser with Photoacoustic Detector for High Sensitivity Gas Monitoring

Abstract An external cavity diode laser is constructed and demonstrated to possess stable, narrow linewidth operation and wide-range tunability, thus being an ideal light source for spectrally resolved detection of absorption lines of small molecules in the gas phase. Moreover, by using this laser in conjunction with a photoacoustic cell, detection limit for optical absorption coefficient about 3.5 × 10−8 cm−1 was achieved. The performance of the combined system was tested by measuring overtone and combination lines of water vapour, methane and acetylene around 850nm. Because of its cheapness, simplicity and stability, this combined system is a promising candidate for high sensitivity gas detection applications.

Methane detection with single laser photoacoustic Raman spectroscopy

Methane detection with a new arrangement of photoacoustic Raman spectroscopy has been carried out with self seeding of the Stokes beam generation. It employs only one fixed frequency pulsed laser source, the light of which is focused into a Raman cell, filled with the same gas to be detected. Both fundamental and Stokes shifted beams are sent through a carefully designed photoacoustic cell, resulting in a detection limit of 100ppm methane in N2.

Gas-detection instrument based on external-cavity diode lasers and photoacoustic detectors

Among the numerous detection methods applicable in spectroscopic gas detection systems photoacoustics is probably the simplest, most cost effective, yet a highly sensitive one. By using a properly designed photoacoustic cell, equipped with a commercial microphone and electronics for phase sensitive detection, absorbed electromagnetic radiation as low as 10 nW can be easily detected. The choice of a suitable light source for PA gas detection, however, is crucial in order to preserve the simplicity and practical advantages of the combined gas detection system. In the photoacoustic system presented here, therefore, external cavity diode lasers have been adopted. Contrary to ordinary diode lasers, external cavity diode lasers are ideal sources for spectroscopic studies as they have a narrow linewidth and ar continuously tunable in a relative wide wavelength range. External cavity diode lasers operating in different wavelength range have been developed and concentration determination of water vapor gaseous mixtures are presented. Sensitivity of the system presented here could reach the ppb. level under optimal conditions.

Development and application of external-cavity diode laser systems for photoacoustic gas detection

Two external cavity diode laser systems operating at 860 nm and at 1125 nm have been developed and assembled into photoacoustic gas detection systems. Both laser systems have been found to be suitable for water vapor detection applications. Comparison of the two systems has been made regarding their stability, mode structure, tunability etc. Their performance has been compared with other type of diode lasers as well.

Possible application areas of a diode-laser-based photoacoustic gas detection method

Due to its simplicity, high sensitivity, reliability and low price diode laser based photoacoustic gas detection can be expected to gain more and more widespread applications in the close future. Examples of the possible application areas such as environmental monitoring, agriculture, medical science chemical process control etc. are discussed here.

Optimization of diode-laser-based photoacoustic laser systems for high-sensitivity detection of water vapor, methane, and carbon dioxide

Developments towards a portable sensor system for in-situ detection of low concentration of water vapor are described. The system consists of a Littman type external cavity diode laser light source operating around 1125 nm, an acoustically optimized high sensitivity measuring photoacoustic ell, a reference photoacoustic cell, home made electronics and a personal computer for measurement and system control. For high sensitivity water vapor detection a fully computerized measurement protocol has been developed. Test measurements demonstrated that the system is able to detect water vapor concentration below the 10 ppm level.