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WP 4 - Optimization of FP-PTI for gas sensing

Fig. 1. FP-PTI setup [1].

The goal of Workpackage 4 is to optimize the FP-PTI setup for spectroscopic field applications. This includes the identification of most suitable target wavelengths and the improvement of sensitivity, response time and to evaluate and mitigate possible interferences. After calibration for single and multiple analytes in the laboratory, the PTI sensor is incorporated into a portable analyser.

In the beginning of the project, the best suited wavelengths are selected using the HITRAN [2] database, considering other potentially interfering molecules as well. Theoretical investigations of the spectroscopic situation include variations in temperature and pressure.

In the next step the sensitivity and ruggedness of the PTI sensor, which has already been realized in a proof-of-concept experiment by PAG, will be further improved. This will be achieved in two ways: through the increase of the transcducer signal by optzimizing the properties of the used interferometer e.g. by an higher cavity finess and through a reduction of the sensor noise by identification and characterization of noise sources like laser or ambient noise. Such improvements, expecially concerning the reduction of ambient noise sources, will consequently also increase the ruggedness of the PTI sensor.

In addition, also selective excitation of the analyte is implemented by selecting the appropriate ICL wavelengths and driving the laser in continous-wave (CW) mode and applying a 2f-wavelength modulation scheme through current modulation. Choosing a modulation frequency of e.g. 5 kHz is consequently resulting in a modulation of the sample refractive index of 10 kHz.
To especially increase the suitability of the sensor for multi-species detection, a robust and compact coupling scheme for three excitation lasers will be elaborated throughout the project. A parallel analysis of the measurement signals can be realized by utilizing a special multiplexing technique. This includes a specific laser driving and analysis scheme.

Further, the available gas mixing station at PAG is used for invesitigating the concentration dependence of the ATMO-SENSE gas sensor by first using CO2, followed by N2O and O3, and finally all three species in parallel. To evalutate the long term stability of the system, Allan-Werle variance stuies will be performed.

[1] J. P. Waclawek, V. C. Bauer, H. Moser, and B. Lendl, "2f-wavelength modulation
     Fabry-Pérot photothermal interferometry",
Opt. Express 24(25), 28958, 2016.
[2] Hitran database (https://www.cfa.harvard.edu/hitran).