Type-II frequency doubling at λ = 130 µm and λ = 253 µm in flux-grown potassium titanyl phosphate

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Abstract
Continuous-wave type-II (oeo) second-harmonic-generation (SHG) characterization of the linear and nonlinear properties of KTiOPO4 in the mid-IR range with a InGaAsP diode laser at λ = 1.30 µm and a tunable KCl:Li color-center laser at λ = 2.53 µm are reported. With accurate phase-matching-angle measurements we have tested the accuracy of the numerous published Sellmeier data. We also discuss a birefringence-assisted noncollinear phase-matching technique as an efficient means to increase the coherence length of type-II walk-off-limited SHG. Through measurements of the angular and wavelength acceptance bandwidths and experimental analysis of the far-field second-harmonic transverse pattern, we provide an experimental test of the relevancy of the modified Boyd-Kleinman theory of the walk-off-limited type-II parametric interaction. Measured values of the d24 nonlinear susceptibility at λ = 0.65 µm (d24 = 2.45 pm/V) and λ = 1.265 µm (d24 = 2.30 pm/V) with an accuracy of 8.5% have been deduced from our conversion-efficiency measurements performed in the undepleted cw pump regime. The extrapolated value at λ = 0.532 µm (d24 = 2.58 pm/V) is found to be 30% smaller than those reported thus far from phase-matched pulsed measurements.