Tunability improvement of a doubly resonant OPO for fast and high resolution gas spectroscopy

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Abstract
High resolution gas spectroscopy in the mid-infrared in a transportable device is a big challenge allowing to address numerous applications: air quality or industrial process monitoring, defense and security, medical diagnostics... Together with high tunability in the mid-infrared, spectral purity, narrow bandwidth, compactness and robustness are needed. Nested Cavity doubly resonant OPO (NesCOPO) fulfill all those requirements. This architecture is already commercialized (in the X-FLR8 portable gas analyzer from Blue Industry and Science) and allows to reach low threshold compatible with the use of compact micro-chip nanosecond YAG laser. A wide spectral range can be obtain (2 - 10 μm). In the most mature version NesCOPO takes benefit of down-conversion of a laser radiation at 1.064 μm in a PPLN bulk crystal and give rise to two secondary radiation around 1.5 μm and between 3.2 and 4.25 μm. This last radiation is used to probe rovibrational absorption lines of species of interest using absorption or transmission spectroscopy. Speed in the selection of the emitted wavelength can be an important requirement especially when security is involved. We use engineering of the crystal using fan-out configuration. Evolution of the bandwidth and phase shift between the three waves after reflection onto the end cavity mirror has to be managed to maintain high conversion efficiency. Experiment show more flexible behavior than expected with theory. This lead to fine wavelength control on the overall emission spectrum (over 1 μm) without using crystal temperature tuning that slow down tuning speed.

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