Beam walk-off in uniaxial and biaxial crystals occurs when the phase normal of the propagating electromagnetic wave deviates from the direction of the Poynting vector. This beam walk-off limits frequency conversion efficiency and restricts the OPO tuning range. The beam walk-off angle in nonlinear single crystals can be alleviated by bonding similar nonlinear crystals rotated by 180° with respect to each other. An even number of twisted twins of single crystals is formed that is stress-free and has negligible loss at the AFB~R (Adhesive-Free Bond) interfaces. Since no adhesive is employed and the bonding force consists primarily of Van der Waals attractive forces, there is no adverse effect or absorption at the bond interface. The theory of walk-off angles as a function of orientation for uniaxial and biaxial crystals is derived. Correcting beam walk-off by producing an AFB~R composite configuration results in more efficient frequency conversion and thereby allows the generation of higher power output of frequency converted radiation for a given input power. Beam correction is demonstrated experimentally for zinc germanium phosphide (ZGP) as representative of a uniaxial nonlinear crystal, and on biaxial KTP crystals. AFB~R composites of ZGP with inactive ends of gallium phosphide have been produced in an effort to further improve damage resistance of a ZGP optical parametric oscillator for frequency conversion into the mid-IR range.
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