A Fully Automated Diode Laser Based Difference Frequency Spectrometer Tunable from 12-16 μm

The generation of tunable narrow-band light in the mid-infrared region is of considerable interest in pollution monitoring, remote sensing and trace gas detection. Nonlinear difference frequency mixing (DFM) is one of several techniques which allow access to the important 3-20 μm region, where many molecules have fundamental absorptions. Recent advances in single longitudinal mode high-power diode-laser technology, coupled with new developments based on periodic poling of non-linear crystals and improved growth techniques for birefringently phasematched crystals (such as GaSe), has made DFM a promising approach for a compact broadly tunable, narrowband infrared source. A judicious selection of pump laser wavelengths and non-linear crystals will allow most of the mid-infrared spectral region to be accessible by DFM. Our approach involves DFM of two high power single longitudinal mode diode lasers in the non-linear crystal, GaSe. We combine the broad tuning-range of the diode lasers with the extended critical phasematching range of GaSe to produce a widely tunable system with which we have implemented computer controlled diode frequency tuning, automatic crystal angle positioning, and automatic alignment of the pump beams into the crystal.