Walk-off-compensated type-I and type-II SHG using twin-crystal AgGaSe2 and KTiOPO4 devices

Abstract
The walkoff-compensated (WOC) twin-crystal device enables to reduce the aperture effects in critically phase-matched parametric generation. In that device, instead of using a monolithic bulk crystal of length 21c, two identically-cut crystals of length lc are disposed in tandem such that the relative sign of the walkoff angle (rho) in the first and second items are opposite, while keeping unchanged the relative sign of their non-linear effective susceptibilities through a proper orientation of their optic axes. Applied to KTiOPO4 (KTP) for type-II (oeo) second-harmonic generation (SHG) of a color center laser, the 2 X lc twin- crystal device leads to a conversion efficiency 3.5 times higher than that of a 21c single bulk crystal. The same WOC technique has been applied successfully in the type-I (ooe) SHG of a CO2 laser using an AgGaSe2 (AGSE) device. The extension of the WOC method to a periodical structure composed of N twin-device stacks is considered. We report the first demonstration of the SHG efficiency enhancement of such a structure, which is made of four KTP plate optically contacted in the WOC configuration.

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