Compact, broadly tunable, mid-IR source for the spectroscopic investigation of molecular reference lines in the 27- to 33-THz range

We report on the improvement of a tunable, high resolution, diode laser-based, difference-frequency spectrometer using an AgGaS₂ nonlinear crystal. We use a type-II cut crystal as a part of the improvement compared with a type-I cut, which was used in our preliminary setup. The two tunable laser diodes are operating near λ₃ = 778 nm (pump) and λ₂ = 842 nm (signal) with a sub-100-kHz linewidth. The high resolution spectrometer is being developed as an alternative to CO₂ laser spectrometers in the 9- to 11-μm range. Using a dual-arm cavity to enhance the two radiation powers, and with 35 mW in front of the 778-nm arm and 100 mW in front of the 842-nm arm, about 70 nW of the tunable 10-μm radiation are generated. This power level is enough to investigate the linear absorption spectroscopy of SF₆. Doppler-limited spectra over 2 GHz are recorded, showing the wide continuous tunability of the spectrometer.