Christophe Bonnin

Relative sign and absolute magnitude of d (2) nonlinear coefficients of KTP from second-harmonic-generation measurements

We determine the absolute magnitude of the dij(2) coefficients of KTP, using type-I and type-II phase-matching second-harmonic-generation efficiency measurements: |d15 (0.532 μm)| = 1.4 ± 0.07 pm/V, |d24 (0.532 μm)| = 2.65 ± 0.13 pm/V, and |d33 (0.532 μm)| = 10.7 ± 0.5 pm/V. We establish that these coefficients have the same sign. We compare the obtained magnitudes with the main previously published results.

Absolute measurement of quadratic nonlinearities from phase-matched second-harmonic generation in a single KTP crystal cut as a sphere

We determine within an accuracy of ∼10% the absolute magnitude of the quadratic effective coefficients of types I and II phase-matched second-harmonic generation from conversion efficiency measurements in a single nonlinear crystal cut as a sphere. The agreement is good with measurements performed in thin parallelepipedal samples. The material studied is KTiOPO4, for which improved Sellmeier equations are given.

Phase-matching measurements and Sellmeier equations over the complete transparency range of KTiOAsO 4 , RbTiOAsO 4 , and CsTiOAsO 4

The sphere method is used for the direct measurement of sum- and difference-frequency generation phase matched in the principal planes of KTiOAsO4, RbTiOAsO4, and CsTiOAsO4, including the tuning curves of 1.064-µm-pumped OPO’s emitting between 3 and 5 µm. The nonlinear least-squares fitting of our experimental data leads to refined dispersion equations of the principal refractive indices; the resulting dual-oscillator form equations are valid over the complete transparency range of the three studied materials.

Compact 21-W 2-µm intracavity optical parametric oscillator

We report on an intracavity optical parametric oscillator (OPO) placed within a compact diode-pumped Nd:YALO laser cavity. This OPO utilizes a pair of KTP crystals, which are diffusion bonded together in a walk-off-compensated configuration. We have generated up to 21.4 W of 2-mum radiation, operating in a few-kilohertz range.

Walk-off-compensated type-I and type-II SHG using twin-crystal AgGaSe2 and KTiOPO4 devices

The walkoff-compensated (WOC) twin-crystal device enables to reduce the aperture effects in critically phase-matched parametric generation. In that device, instead of using a monolithic bulk crystal of length 21c, two identically-cut crystals of length lc are disposed in tandem such that the relative sign of the walkoff angle (rho) in the first and second items are opposite, while keeping unchanged the relative sign of their non-linear effective susceptibilities through a proper orientation of their optic axes. Applied to KTiOPO4 (KTP) for type-II (oeo) second-harmonic generation (SHG) of a color center laser, the 2 X lc twin- crystal device leads to a conversion efficiency 3.5 times higher than that of a 21c single bulk crystal. The same WOC technique has been applied successfully in the type-I (ooe) SHG of a CO2 laser using an AgGaSe2 (AGSE) device. The extension of the WOC method to a periodical structure composed of N twin-device stacks is considered. We report the first demonstration of the SHG efficiency enhancement of such a structure, which is made of four KTP plate optically contacted in the WOC configuration.

Second-harmonic generation with monolithic walk-off-compensating periodic structures. I. Theory

Second-harmonic generation in a periodic structure made from N pairs of optically contacted, birefringence phase-matched, walk-off-compensating bulk plates is theoretically investigated. In the undepleted-pump approximation, analytical (heuristic) expressions for conversion efficiency versus N are derived for both type I and type II phase matching. An explicit split-step beam propagation scheme that solves exactly the coupled paraxial-wave equations is used to check the validity of the heuristic results. For type II, stronger conversion enhancement than for bulk crystal is predicted in the low-depletion regime, whereas for type I such structures avoid harmonic beam ellipticity. The periodic structures are found to behave as nonlinear harmonic birefringent filters because of the presence of periodic wave-vector mismatch grating ±Δk that results from walk-off compensation. The effect of periodicity imperfections, such as residual plate orientation mismatches, was found to be responsible for broadening of the tuning bandwidth in walk-off-compensating devices.

Study of RTP Crystal Used as Electro-Optic Modulator

The purpose of this work is to highlight behaviour of RTP crystal in Electro-Optic configuration. We will review main features of interest such as extinction ratio, voltage applied, lifetime and damage threshold.