TWIN-CRYSTAL WALK-OFF-COMPENSATED TYPE-II 2ND-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 l = 6 mm mounted with their optical axes symmetrica lly crossed is used to circumvent conversion-efficiency limitations th at are due to the aperture effect in type-II (oeo) critically phase-ma tched second-harmonic generation (SHG) at lambda = 1.3 and lambda = 2. 532 mu m. A single-pass increase of as great as 3.2-3.5 times the conv ersion efficiency of a single crystal is obtained with this angle-tune d device at these wavelengths. This enhancement results from an increa sed effective coherence length of the interaction compared with that o f a single bulk crystal of length 2l. We show that cavity-enhanced cri tically phase-matched type-II SHG can be easily performed with the twi n device, allowing for walk-off as well as phase compensation for the fundamental-resonating ordinary and extraordinary waves at lambda = 1. 30 mu m. Potential implementations of this device in other single-cavi ty-enhanced parametric interactions, such as doubly resonant optical p arametric oscillation and degenerate sum- or difference-frequency mixi ng, are discussed.