Efficient difference frequency generation (DFG) employing injection-seeded optical parametric oscillator (OPO)

Efficient high energy OPO´s employing LiNbO₃ or KTP crystals pumped by conventional Nd:YAG laser have been demonstrated in the range of 1-5 μm. To extend the source beyond 5 μm range an additional nonlinear step may be employed to generate difference-frequency of the two waves (signal and idler) obtained from an OPO. The difference frequency mixing is most easily done in nonlinear crystals such as AgGaSe₂ and AgGaS₂ which phase match over 3-18 μm and 1-12 μm range respectively. OPO output beam divergence and spectral linewidth should be very narrow for efficient difference-frequency mixing in a nonlinear crystal. An injection-seeded OPO with unstable resonator can have such properties