András Miklós

Application of acoustic resonators in photoacoustic trace gas analysis and metrology

The application of different types of acoustic resonators such as pipes, cylinders, and spheres in photoacoustics is considered. This includes a discussion of the fundamental properties of these resonant cavities. Modulated and pulsed laser excitation of acoustic modes is discussed. The theoretical and practical aspects of high-Q and low-Q resonators and their integration into complete photoacoustic detection systems for trace gas monitoring and metrology are covered in detail. The characteristics of the available laser sources and the performance of the photoacoustic resonators, such as signal amplification, are discussed. Setup properties and noise features are considered in detail. This review is intended to give newcomers the information needed to design and construct state-of-the-art photoacoustic detectors for specific purposes such as trace gas analysis, spectroscopy, and metrology.

Detection of ammonia by photoacoustic spectroscopy with semiconductor lasers

Sensitive photoacoustic detection of ammonia with near-infrared diode lasers (1.53 microns) and a novel differential acoustic resonator is described; a sensitivity of 0.2 parts per million volume (signal-to-noise ratio = 1) is attained. To eliminate adsorption-desorption processes of the polar NH3 molecules, a relatively high gas flow of 300 SCCM was used for the ammonia-nitrogen mixture. The results are compared with recent ammonia measurements with a NIR diode and absorption spectroscopy used for detection and photoacoustic experiments performed with an infrared quantum-cascade laser. The performance of the much simpler and more compact setup introduced here was comparable with these previous state-of-the-art measurements.

Optoacoustic trace-gas monitoring with near-infrared diode lasers

An inexpensive resonant optoacoustic monitoring system using near-infrared laser diodes was developed. It was demonstrated that wavelength modulation at the resonance frequency of the cell provides a superior signal-to-noise ratio compared with amplitude modulation and eliminates background drifts and fluctuations. The system was tested out on ammonia. Its sensitivity is 8 parts in 10(9) (S/N = 1) at atmospheric pressure, which corresponds to a minimum detectable absorption coefficient of approximately 3.5 × 10(-11) cm(-1) W(-1). The pressure dependence of the optoacoustic resonance was also investigated. The monitor can be used as a continuous flow-through system up to a flow rate of approximately 3.5 L/min.

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.

Windowless resonant acoustic chamber for laser-photoacoustic applications

A windowless resonant photoacoustic chamber is described for environmental and agricultural applications. The windowless design is a solution to the lack of perfect windows in the infrared region. The design was made insensitive to external noise with the help of acoustic filters. The chamber was designed to have a high quality factor of about 400. Experimental results show good agreement with the theoretical calculations.

Effects of adsorption–desorption processes on the response time and accuracy of photoacoustic detection of ammonia

A distributed-feedback (DFB) diode laser radiating at 1.53 mum was used for photoacoustic detection of ammonia molecules in the gas phase under flow conditions. The influence of the adsorption-desorption processes that occur at the cell and tube walls on the measured gas concentration was studied. Dramatic differences in the adsorption behavior of a metal and a polypropylene cell are demonstrated. Simulations of the gas flow and adsorption-desorption processes yield the conditions that must be fulfilled for accurate concentration measurements in trace-gas analysis of polar molecules.

Quantitative signal analysis in pulsed resonant photoacoustics

The pulsed excitation of acoustic resonances was studied by means of a high-Q photoacoustic resonator with different types of microphone. The signal strength of the first radial mode was calculated by the basic theory as well as by a modeling program, which takes into account the acoustic impedances of the resonator, the acoustic filter system, and the influence of the microphone coupling on the photoacoustic cavity. When the calculated signal strength is used, the high-Q system can be calibrated for trace-gas analysis without a certified gas mixture. The theoretical results were compared with measurements and show good agreement for different microphone configurations. From the measured pressure signal (in pascals per joule), the absorption coefficient of ethylene was calculated; it agreed within 10% with literature values. In addition, a Helmholtz configuration with a highly sensitive 1-in. (2.54-cm) microphone was realized. Although the Q factor was reduced, the sensitivity could be increased by the Helmholtz resonator in the case of pulsed experiments. A maximum sensitivity of the coupled system of 341 mV/Pa was achieved.

Ammonia detection by using quantum-cascade laser photoacoustic spectroscopy

A pulsed quantum-cascade distributed-feedback laser, temperature tunable from -41 degrees C to +31.6 degrees C, and a resonant differential photoacoustic detector are used to measure trace-gas concentrations to as low as 66 parts per 10(9) by volume (ppbv) ammonia at a low laser power of 2 mW. Good agreement between the experimental spectrum and the simulated HITRAN spectrum of NH3 is found in the spectral range between 1046 and 1052 cm(-1). A detection limit of 30 ppbv ammonia at a signal-to-noise ratio of 1 was obtained with the quantum-cascade laser (QCL) photoacoustic (PA) setup. Concentration changes of approximately 50 ppbv were detectable with this compact and versatile QCL-based PA detection system. The performance of the PA detector, characterized by the product of the incident laser power and the minimum detectable absorption coefficient, was 4.7 x 10-9 W cm(-1).

Design and characterization of a windowless resonant photoacoustic chamber equipped with resonance locking circuitry

A novel design of a windowless resonant photoacoustic chamber with open acoustic filters and an electronic resonance locking circuitry is presented. The acoustic behavior of the cell and preliminary measurements on a certified gas mixture with a CO2 laser demonstrate the feasibility for trace gas monitoring.

Photoacoustic measurement of methane concentrations with a compact pulsed optical parametric oscillator

A pulsed periodically poled lithium niobate optical parametric oscillator operating in a cavity with a grazing-incidence grating configuration was used for sensitive and precise measurement of trace quantities of methane in nitrogen by photoacoustic spectroscopy with a novel differential photoacoustic detector. A sensitivity of 1.2 parts in 10(9) by volume of methane was obtained in direct calibration measurements (not extrapolated). With this apparatus, in situ measurement of the methane concentration in ambient air under atmospheric conditions was demonstrated.