High-repetition-rate, tunable and coherent mid-infrared source based on difference frequency generation from a dual-wavelength 2 µm laser and GaSe crystal

Although high-repetition-rate dual-wavelength 2 mu m KTP optical parametric oscillators (OPOs) have been realized for quite a long time, we have treated one in generating coherent mid-infrared waves in this paper for the first time. The dual-wavelength 2 mu m laser is generated from a KTP OPO intracavity, pumped by an acousto-optically Q-switched side-pumped Nd:YAG laser. Treating an 8mm long GaSe crystal for difference frequency generation (DFG), a maximum average output power of 360 mu W is obtained at a pulse- repetition rate of 10 kHz at the wavelength of 9.6 mu m for type-I phase matching DFG. The peak power amounts to 3.3 W and the conversion efficiency is 0.12%. The continuously tunable and coherent radiation in the mid-infrared range of 9-16 mu m has been achieved. Moreover, type-II phase matching DFG with an output wavelength continuously tunable in the range of 9-13.7 mu m has also been achieved. The output mid-infrared power scaling is still feasible with a higher pump power and a nonlinear DFG crystal with a higher damage threshold.