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Crystals can convert wavelengths emitted by lasers using a second-order process. They allow light to be generated with smaller or higher wavelengths than those of the input beam. The issue is to select the nonlinear crystal having the best conversion efficiency at the targeted wavelengths, which requires a high non-linearity and suited transparency range as well as phase-matching conditions.

Type II second-harmonic generation was performed in a 15.8-mm-long KTiOPO_4 (KTP) micrometric ridge waveguide with an average transversal section of 38  μm^2. Theoretical predictions are compared with experiments. Strong agreements are obtained for both phase-matching wavelengths and second-harmonic intensity. This work opens wide perspectives for integrated parametric optics.

We derive an analytic solution for the spectral and radial distribution of a sum-frequency generated wave, in a cylindrical whispering gallery mode (WGM) resonator with an arbitrarily modulated nonlinear coefficient along the angular direction. The cylindrical geometry dictates different wave vectors, spatial distribution, and spectrum of the generated wave with respect to a linear cavity with an identical round-trip length. The geometrical correction term to the phase-mismatch parameter is derived by exact analytic solution; it is found to be different by a factor of 2 with respect to the slowly varying envelope approximation result for WGMs, which has been previously suggested in the literature. This change occurs because the properties of the generated wave in the nonlinear process depend on the spatial distribution of the pump and signal, in addition to the geometrical boundary conditions. We also propose a scheme for the spectral shaping of optical pulses in a WGM resonator, based on a method taken from computer-generated holograms. This method, apart from having an advantage in the form of a considerable reduction of the necessary pump power due to the intracavity enhancement by the resonator, also should allow for arbitrarily shaping of the pulse envelope as well as the cancellation of group velocity dispersion per round trip.

We directly measured phase-matching directions of second harmonic, sum, and difference frequency generations in the Langatate La_3Ga_5.5Ta_0.5O_14 (LGT) uniaxial crystal. The simultaneous fit of the data enabled us to refine the Sellmeier equations of the ordinary and extraordinary principal refractive indices over the entire transparency range of the crystal, and to calculate the phase-matching curves and efficiencies of LGT for infrared optical parametric generation.

A 5 mm thick and 38 mm long 5% MgO-doped periodically poled lithium niobate (MgO:PPLN) cylinder with a grating period of 28 μm has been engineered as a partial cylinder with an angular aperture of 45°. An optical parametric oscillator based on this crystal and pumped at 1.064 μm is reported. The output energy is comparable to that generated in the same kind of sample cut as a slab, but the wavelength tunability from 1.41 to 4.3 μm is much broader and continuous.