A high energy 3.75 μm KTA optical parametric oscillator at a critical angle

A high energy 3.75 μm nano-second KTiOAsO4 (KTA) optical parametric oscillator (OPO) at a critical angle is demonstrated in this letter. The OPO is pumped by a 1.064 μm laser and the KTA crystal is cut at 41.2° in the XZ plane. The phase matching angle is at the edge of the phase match curve and the edge of the absorption region for mid-infrared lasers. The cavity is a signal resonant oscillator. The spatial distribution of the pump light is a quasi-plane wave and the pulse width is 10 ns. A 218 mJ signal at 1.484 μm and a 60 mJ idler at 3.75 μm are achieved with a pump energy of 795 mJ, corresponding to a conversion efficiency of 35%. The idler pulse width is 7 ns. The divergence angle in the walk-off plane is 4 mrad and it is 14 mrad in the other plane. The results prove that the OPO phase matching of KTA exists at 3.75 μm and the KTA OPO can generate a high energy idler at 3.75 μm.