PHOTOREFRACTIVE DAMAGE IN KTP USED AS 2ND-HARMONIC GENERATOR

A common application of KTP is as a second-harmonic generator. However, scaling up second-harmonic efficiencies of KTP is limited by generation of "dark tracks." It is possible to circumvent the problem of dark tracking by raising the crystal temperature. The heating extends the life time of the nonlinear medium and permits an increase of the second harmonic efficiency. At 170-degrees-C an astigmatic beam distortion of the harmonic beam was observed. The astigmatism was strongly dependent on the beam intensities. The direction of the astigmatic distortion was parallel to the z crystallographic direction. The development of the beam distortion resulted from optically induced nonuniformity of the n(z) refractive index of KTP. It represented photorefractive damage to KTP. Analysis of the astigmatic distortion versus second-harmonic power indicates that the photorefractive damage resulted from multiphoton absorption involving fundamental and harmonic photons.