WELCOME TO PHOTOACOUSTIC PAGES

Introduction

The Department of Optics and Quantum Electronics , at the University of Szeged has more than ten years of experience in laser development and its application in material testing, biology and chemistry. Since the beginning emphasis was always put on the development of practically applicable systems, and therefore one of our aims was to develop systems which are compact, automatic and operable under industrial conditions. Photoacoustic gas detection became one of our key subjects about 10 years ago, when we realized its potential for practical applications, due to its inherent simplicity, robustness, combined with its high sensitivity. However to develop practically applicable systems, we had to find a light source which not only keeps the above mentioned advantages of the photoacoustic method, but besides, it guarantees selective identification of the measured gas component. Currently our work concentrates on systems which combines the photoacoustic method with diode laser light sources. These systems are capable of long term unattended automatic operation over a long time period (several years), and the typically achievable ppm. (or even sub-ppm.) level sensitivity makes them ideal for measurement of minority (impurity) components in industrial gases (e.g. in natural gas) as an on-line instrument.

General overview of the photoacoustic method

History
The photoacoustic effect was discovered by A.G. Bell in 1881. The effect was later almost completely forgotten for more than half a century. Its revival started soon after the invention of laser light sources. In the seventies and eighties of the last century the method was really booming mostly by applying it in combination with gas lasers (CO and CO2 lasers) in the mid-infrared. These systems demonstrated ultra-high sensitivity down to the sub-ppb. concentration range and multi-component measurement capability. However the applied laser sources were bulky, complicated and needed expertise in operation. Therefore in the nineties these systems lost interests. In parallel research has been started to replace these lasers with alternative light sources. One idea was to use broad band light sources in combination with some wavelength selective elements. However these systems proved to have insufficient selectivity. Alternatively light sources such as optical parametric oscillator (OPO), quantum cascade lasers as well as room temperature diode lasers.
The photoacoustic effect
The photoacoustic effect is based on the absorption of modulated laser light in a photoacoustic cell, which creates an acoustic wave at the modulation frequency when the laser is tuned to an absorption line of at least one of the gas components. This photoacoustic signal generation happens through a periodic non-radiative relaxation of the molecular excitation resulting in a periodic heating of the irradiated gas volume, followed by a periodic thermally induced expansion and contraction of the gas volume, i.e. sound generation. The amplitude of the acoustic wave, which is sampled with a sensitive microphone, is proportional to the concentration of the absorbing component(s).
Why photoacoustics?
Diode laser based photoacoustic gas detection offers a combination of advantages with its simple construction and automatic operation, minimum detectable concentration in the low-ppm or even sub-ppm range (depending on the measured analyte), linear response over more than four orders of magnitude, high selectivity (i.e. insensitivity to the presence of other components), response time below one minute and accuracy in the few percentage range. Besides, our photoacoustic systems have high reliability in critical applications and they have to be calibrated only on a yearly (or even less frequent) time basis, due to the inherent stability of the system's components as well as the operation of the numerous in-built self testing and correction procedures.

Specialty of the diode laser based photoacoustic system

General system description
The main parts of our diode laser based photoacoustic systems are a diode laser light source, a photoacoustic cell, a reference unit, a control unit and a signal-processing unit. The laser light is modulated at a few kHz and then passed through the PA cell. This cell is constructed so that it amplifies the acoustic signal generated by the light absorption at the light modulation frequency. The reference unit is used for determining various spectroscopic and optical properties of the laser light. In a laboratory version the reference unit can contain various optical systems (e.g. wavelength meter, spectral analyzer, power meter etc.), but in a practical system these parts are replaced by a reference photoacoustic cell, which contains the measured component in a high concentration. With the help of the control unit the diode laser wavelength (through the diode laser temperature and current) and the laser modulation frequency are actively controlled and the sample temperature and pressure is measured, in order to ensure entirely reliable measurements. The signal processing unit amplifies, averages and measures the signal of the microphone attached to the PA cell, and finally it calculates from the measured photoacoustic signal the concentration of the measured component.
Detectable components and minimum detectable concentrations
In the near-infrared wavelength range (0.7-2.5 micron), where room temperature diode laser operates several gas components have relatively strong absorption. The table below lists some components which can be detected with a diode laser based photoacoustic gas detection system. The listed minimum detectable concentration corresponds to a photoacoustic system, which contains a diode laser light source with a 10 mW light power.

Gas Component	Optimum Wavelength Range (um)	Minimum Detectable Concentration (ppm.)
H2O	1.37	0.2
CH4	1.65	0.5
NH3	1.53	0.6
H2S	1.57	1

In addition a high power diode laser based photoacoustic system can be used for detection of particulate matter (soot) with a high (~200 ng m3) sensitivity. (See: L. Krämer, Z. Bozóki and R. Niessner: Characterisation of a mobile photoacoustic sensor for atmospheric black carbon monitoring. Analytical Sciences. 17. s563-566 (2001).)
Selectivity
Unlike many of other gas detection methods (e.g. solid-state sensors) photoacoustic gas detection can give true selectivity, thus satisfying the strong need both in scientific and in industrial application that the applied instrument really measures the selected component and the presence and variation of other constituent does not disturb the reliability the measurement. In our photoacoustic system the narrow linewidth of the applied diode laser wavelength, the special (wavelength) modulation scheme and the application of a multi-component analysis method ensures the necessary high selectivity and gives total confidence in the measured concentration values.
Reliability
We have several years of experience of operating our photoacoustic systems under tough industrial conditions. Our implemented systems proved to have high reliability, which stems from the fact that their different parts (i.e. the light source, the detector cells, the electronics and the measurement software) are carefully optimized and matched together. We design these parts, produce them and build them together. Besides all of our systems are carefully optimized and characterized from the point of view of acoustic and electric noise suppression, optical alignment. The completed system goes through a thorough calibration process. Into the system numerous self checking procedure is built. Besides, the performance of the system does not influenced by the variation of the physical (e.g. temperature and pressure) and the chemical (e.g. gas composition) parameters of the measured gas and the environment as, we carefully characterized the influence of these parameters on the photoacoustic signal and these effects are properly taken into account when the system determines the concentration of the measured component from the measured photoacoustic signal. The system operation does not require the use of any gas cylinders (except during calibration).
Calibration
One special advantage of our photoacoustic systems is the simplicity of their calibration. Calibration can be performed on-site almost completely automatically, without the use of special gas mixtures, or additional complicated instruments. One example is our water vapor measuring system, which can be calibrated simply with a gas cylinder containing 100% methane. Our experience shows that our systems have to be calibrated not more frequently than a year.

Our experience

In the last ten years our research was financed by the European Union, the Hungarian Government as well as various other sources both domestic and abroad (e.g. Germany and USA). We have several industrial R&D cooperation, and also cooperation with other research institutes. Every year new undergraduate and graduate students are coming to work in our laboratory and several Ph.D. thesis has been written during the years.