Ultra-sensitive mid-IR gas sensor system

Within this project efforts will be made to further push the detection limits of already highly sensitive trace gas detection by mid-infrared laser based photoacoustic spectroscopy (PAS). The photoacoustic technique is based on the absorption of modulated light by molecules, which leads to a heating of the molecules. The heating in turn leads to a thermal expansion and thus to a pressure change in the media, which can be measured by sensitive microphones. A fundamental fact for generation of the photoacoustic signal is that it is directly proportional to the incident laser power. A modern variation of conventional PAS spectroscopy, called quartz-enhanced photoacoustic spectroscopy (QEPAS) uses a quartz tuning fork (QTF) to detect acoustic waves. The QTF is a piezo-electric element which converts its deformation by pressure changes into separation of electrical charges that can be measured as a voltage. The use of quantum cascade lasers (QCL) assures both selectivity and sensitivity by targeting single strong absorption lines of the analyst under investigation. It can be compared to the measurement of a characteristic molecular fingerprint. QCLs can be designed to emit light in the region from 3 to 25 m which corresponds to the mid-IR range. However, their power is typically limited to a few 10 to 100 mWs. An amplification of the laser power and thus an increase of the sensors sensitivity can be accomplished by coupling the beam into an optical resonator. Combining such a resonator with the QTF will lead to unprecedented level of measurement sensitivities. A further significant advantage of the resulting sensor systems is that they only require minute amounts of sample. As a result they will have rapid response times and a small overall size. It is expected that such highly sensitive, small and portable gas sensors will find numerous applications when the concentration of trace gases needs to be monitored accurately. Measurement sensitivities in the sub ppt range can be realized by this novel approach. 1 ppt refers to detecting 1 molecule in the presence of 1 000 000 000 000 other molecules. Applications are seen in biomedical diagnostic, environmental analysis and also for industrial process control.

Within the framework of the FWF project PIR 40-N34 a novel laser-based method for trace gas detection has been developed at the TU Wien. This method is characterized by its high sensitivity, as well as a compact design. The developed technique is based on the high-resolution measurement of the variation of the refractive index of a gas within an optical cavity. The target molecules are selectively heated by a dedicated laser. Thereby the temperature of the gas increases which in turn causes a change of its refractive index. The varying refractive index can be measured with high precision within a system consisting of two semi-transparent mirrors, i.e. an optical cavity. To this, a second laser beam is transmitted through the cavity, which only allows light of a certain wavelength to pass through optimally. If traces of the gas of interest are present and are heated up, the optical refractive index of the gas is altered, as well as the wavelength fitting between the two mirrors. This characteristic can be detected with high sensitivity by the measurement system. An improved performance of this sensing scheme was achieved by the novel implementation of an effective noise suppression. For this purpose the second laser beam is additionally split into two parts, which are both transmitted through the same cavity. Their intensities are simultaneously compared, which allows to effectively cancel out internal and external excess noise. Thereby this scheme greatly enhances the sensitivity as well as the ruggedness of the sensor. The metrological qualities of the developed prototype were demonstrated by trace gas detection of sulphur dioxide. Concentrations down to the single- digit part per billion range could be detected. Further refinements are expected to significantly lower the achievable detection limit. One of the key advantages of the new measurement technology is its feasibility of miniaturization. For classic absorption spectroscopy, the laser attenuate the laser beam. However, if instead the changes of the optical refractive index are measured, it is possible to perform measurements in an extremely confined space within a few mm. Thus many possible applications for the new measurement technology arise: these include environmental monitoring e.g. for detecting hazardous exhaust gases, medical applications such respiratory air analysis, as well as process gas monitoring in industry. Publication: Balanced-detection interferometric cavity-assisted photothermal spectroscopy, J. P. Waclawek et al., Optics Express, Vol. 27, Issue 9, pp. 12183-12195 (2019)