Compact cw laser source for high-resolution spectroscopy between 6.8 and 12.5 μm

Tunable monochromatic laser sources in the mid-infrared (MIR) spectral range are particularly important for high-sensitivity and high-resolution molecular spectroscopy. The optimum combination of frequency stability and spectral resolution can be provided only by cw laser radiation. Recently considerable progress has been achieved in developing such devices emitting above 3 μm that are based on difference-frequency generation (DFG) in AgGaS2, but the tunability of previously demonstrated all-solid-state DFG schemes was limited to a narrow region near 5 μm because of the requirement of 90° phase matching [1]. We report here substantial improvement of the tunability range using for the first time to our knowledge critical phase matching in AgGaS2 in the cw regime. The freedom with respect to the spectral position of the two pumping laser diodes allows the choice of relatively powerful pumping diodes which compensates for the reduced interaction length. This, in combination with type-II phase-matching, provides maximum output powers on the μW level and tunability between 6.8 and 12.5 μm with a single crystal as short as 1 cm.