Second-harmonic generation with monolithic walk-off-compensating periodic structures II Experiments

The theory of periodic walk-off compensation in monolithic structures is tested with type II second-harmonic generation experiments in the YZ plane of KTiOPO4 by use of 2N optically contacted, walk-off-compensating (OCWOC) structures with numbers of plates 2N=10 and 2N=4. The results confirm the theoretical prediction that such structures behave as harmonic birefringence filters whose N-dependent transfer functions select ranges of wavelengths for maximum conversion at normal incidence and extinguish others within the tuning bandwidth curve of nominal birefringence phase matching. The residual plate orientation mismatches that alter the periodicity of the phase-mismatch gratings were found to be responsible both for the reduced second-harmonic enhancement compared with that of a reference bulk crystal of the same total length and for broadening of the tuning bandwidth. The shapes of the tuning curves depend critically on the pump wavelength, displaying a variety of modulated patterns that were previously attributed to plate orientation mismatches. The tuning filter response was found to change with periodicity defects. An enhancement factor of 15 (scalable to 22) compared with a reference bulk crystal was measured with a 10-OCWOC structure of length LC=10 mm.