Twin-crystal walk-off-compensated type-II second-harmonic generation: Single-pass and cavity-enhanced experiments in KTiOPO₄

A twin-crystal device consisting of a pair of two identically cut KTP crystals of length I = 6 mm mounted with their optical axes symmetrically crossed is used to circumvent conversion-efficiency limitations that are due to the aperture effect in type-II (oeo) critically phase-matched second-harmonic generation (SHG) at A = 1.3 and A = 2.532, um. A single-pass increase of as great as 3.2-3.5 times the conversion efficiency of a single crystal is obtained with this angle-tuned device at these wavelengths. This enhancement results from an increased effective coherence length of the interaction compared with that of a single bulk crystal of length 21. We show that cavity-enhanced critically phase-matched type-II SHG can be easily performed with the twin device, allowing for walk-off as well as phase compensation for the fundamental-resonating ordinary and extraordinary waves at A = 1.30, um. Potential implementations of this device in other single-cavityenhanced parametric interactions, such as doubly resonant optical parametric oscillation and degenerate sumor difference-frequency mixing, are discussed.