The efficiency of second‐harmonic generation at 425 nm has been measured in periodically segmented waveguides in undoped and 120 ppm Sc3+‐doped c‐axis‐oriented KTiOPO4 (KTP) plates. Conversion efficiencies up to 200% and 325%/W cm2 are obtained for the two types of crystals. These efficiencies are found to have decreased by up to a factor of 2 after the generation of 2–4 mW of cw blue output power for more than 5 min, while the transmission of the pump beam is decreased by about 20%. These effects are semipermanent in undoped KTP, and are therefore ascribed to optical damage. A full recovery within a few minutes is, however, found in the Sc3+‐doped KTP waveguides. The ionic conductivity along the polar axis of 120 ppm Sc3+‐doped KTP has been measured to be about equal to that of undoped KTP, so that the difference in the optical damage characteristics of the waveguides is not caused by a difference in the conductivity of the substrate KTP. The conductivity of 230 ppm doped crystals is found to be about a factor of 30 lower than that of undoped KTP, and has a different dependence on current frequency. In these crystals domain inversion in the exchanged waveguide segments was not obtained.
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