Rb:PPKTP optical parametric oscillator with intracavity difference-frequency generation in temperature-tuned OPGaAs

Recently we investigated cascaded schemes for down-conversion to the mid-IR spectral range above 5 μm by implementing the second stage based on a non-oxide nonlinear crystal inside the cavity of a ns optical parametric oscillator (OPO) based on Rb:PPKTP, pumped by a Q-switched Nd:YAG laser [1-3]. Intracavity difference-frequency generation (DFG) [2, 3] turned out to be more efficient compared to intracavity pumped OPO [1], however wavelength tuning is restricted and a broadband dichroic half-wave plate is needed for either type-I or type-II phase-matching in the studied crystals of AgGaSe₂ and BaGa₄Se₇. Low-symmetry cubic semiconductors, such as orientation-patterned GaAs offer greater variety of phase-matching configurations in which intracavity polarization rotation becomes redundant. With a similar spectral coverage of up to ~18 μm and 2 to 3 times higher effective nonlinearity, OPGaAs exhibits 50 to 100 times higher thermal conductivity, better damage resistivity, and offers the unique possibility of convenient temperature tuning in both stages in non-critical configurations. These advantages point at possible extensions to much higher repetition rates (1-100 kHz). On one hand such low energy regimes will utilize perfectly the limited aperture of the presently available OPGaAs samples but on the other hand will enable the generation of unprecedented average powers in the mid-IR spectral range. In a first demonstration of such intracavity DFG we implement OPGaAs in the same Rb:PPKTP OPO operated at 100 Hz [1-3], using tighter focusing.