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Versatile Infrared Laser source for Low-cost Analysis of Gas Emissions

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Recent demonstrations of tunable infrared laser sources based on novel nonlinear optical materials have largely renewed the interest for vibrational molecular spectroscopy. Specific absorption features in the midinfrared (MIR) range of the spectrum are indeed recognized as a powerful and often unique way to provide high sensitivity detection and identification of a large array of molecules. This is particularly relevant in the gas phase in order to avoid preconditioning steps associated with other detection methods (wet chemistry, gas chromatography, mass spectroscopy). Yet, many promising results have remained confined to laboratories for lack of suitable MIR sources, leaving complex Fourier-Transform spectrometers as the only alternative. To promote direct MIR spectroscopy as a competitive solution for gas analysis, the main technical and scientific objective of the VILLAGE project is the development of a cost-effective widely tunable MIR laser source of high spectral purity. This source combines a 2 micron Thulium (Tm)-doped fibre laser device including a tunable Bragg grating stage and a nonlinear frequency converting crystal (Orientation-Patterned Gallium Arsenide, OP-GaAs) implemented either in a Difference Frequency Generation (DFG) setup or in a high spectral purity optical parametric oscillator (OPO) cavity. Such a design has the potential for unprecedented performance in terms of both primary specifications and suitability to target multi-gas analysis of main pollutants generated by and emitted from industrial processes and more specifically of the gases believed to contribute to global warming. In agreement with the work plan, the first twelve months of the project have enabled the VILLAGE Consortium to specify and fabricate all the subparts needed to implement a first version of MIR tunable source in order to provide useful feedback to the design of the targeted spectrometer. The corresponding technical achievements thus included: - The fabrication by ORC of prototype Tm-doped fibres with high thulium concentration and the demonstration of narrow-linewidth Tm-doped DFB fibre lasers at 1935 nm and 1943 nm with output power > 0.3 W, further scaled to about 1 W. This has been the highest output power so far reported for a Tm-doped DFB laser operating in this wavelength regime. - The design, fabrication and delivery by TRT of an OP-GaAs sample exceeding initial expectations in terms of propagation losses and geometrical characteristics, suited to DFG around 8 micron and the microscopic characterizations by UVA of various samples to validate a growth model. - The implementation by HHUD of a preliminary DFG experiment around 3 micron and of the planned DFG source around 8 micron. Building on the knowledge and sub-parts obtained during the first year, an upgraded version of this DFG-based MIR source has been implemented, leading to an exceptional tunability and to the first spectrometric experiment of the project, demonstrating methane detection. In parallel with such a successful technical achievement, the second year of the project also enabled to model numerous OPO configurations and more precisely assess the main technical options for the final implementation. This led the VILLAGE partners to choose to pursue the OPO development work in HHUD laboratories and to select a DFG design as the preferred solution for the final spectrometer prototype assembled by NEO.

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