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Novel miniaturized photoacoustic cell integrated with compact mid-infrared widely-tunable laser for gas-detection applications

Final Report Summary - MICEPAS MIR-EC-QCL (Novel miniaturized photoacoustic cell integrated with compact mid-infrared widely-tunable laser for gas-detection applications)

Compact laser-spectroscopy gas detectors capable of analyzing the traces of chemical compounds in gas samples of small volume are in strong demand of applications, which need reliable substance-selective high-sensitivity analysis of gas media. Such gas detectors can be used as leak detectors for localizing explosive, toxic or narcotic substances in public accommodations. The detectors may be applicable in Life Sciences for analysis of weak gas flows emitted by small-scale biological samples: small animals and plants, their organs, tissue pieces or microscopic objects down to individual cells. The demand can be met by making of a gas detector based on a widely-tunable mid-infrared semiconductor laser and a novel miniaturized resonant photoacoustic cell. The design of this cell is optimized so as to reduce essentially the cell sizes and, simultaneously, to provide the high-performance cell operation at a selected acoustic resonance. For the properly optimized resonant cell, the window background (a signal arisen in the cell due to absorption and reflection of light beam by the cell windows) is absent at any modulation regime. Such a background-free cell is applicable to detection of any infraredactive compound whatever the spectral features of the compound. This cell design can be easily adapted to any laser-beam diameter and modulation frequency. Despite the essentially reduced sizes, the miniaturized cell is comparable to the well-developed macro-scale cells in the performance.
The project goal is to integrate this novel miniaturized cell with a mid-infrared external-cavity (EC) quantum cascade laser (QCL) into a compact photoacoustic gas detector and demonstrate the detector capabilities in experiments. Several detector prototypes, which answer to diverse applications, are created and tested. All principal components (laser chip, laser-cavity design, photoacoustic cell, etc) of the prototypes are fitted to each other for providing the best gas-detection performance.

During the project implementation, some significant results are achieved:
1. A new improved design of internal cavity for the miniaturized window-background-free photoacoustic cell is developed.
2. A prototype of compact photoacoustic gas detector intended to analyze absorption lines of gases is developed. Inserting the photoacoustic cell into the laser cavity results in a substantial increases of the gas-detection sensitivity because the intracavity power is 100÷300 times higher than the output laser power.
3. Our study demonstrates that the developed EC-QCL-based intracavity photoacoustic gas detector can be used as a high-sensitivity hygrometer or an analyzer of isotopic H2O(16), H2O(18) and H2O(17) species in water vapor at natural isotope abundances.

In general, according to our study, the combination of EC QCL and miniaturized photoacoustic cell has a great potential in gas-detection applications. The developed detector prototype shows a high gas-detection performance and a good capability of continuous wavelength tuning. The prototype will be exploited by the researchers of the host institution as a starting point for new improved versions of non-expensive photoacoustic gas detectors adapted to diverse practical applications. Such detectors are in strong demand for in situ multi-component analysis of atmospheric air in environmental pollution monitoring and exhaust gas monitoring, industrial process control and leak detection. The detectors must find an expanding application in biology-related areas for sensitive detection of gases being involved in the metabolic reactions for living systems. In the development of these new improved gas detectors, the host researchers (in cooperation with the group led by the applicant) will use a deeper knowledge and new practical experience accumulated during the project implementation. Results obtained in the project (e.g. practical realization of an intracavity EC-QCL-based photoacoustic gas detector) are used as a basis of new research proposal prepared recently and submitted to the DFG foundation.