External cavity type second harmonic generator using KTP crystal was fabricated. KTP crystal was prepared in the direction of type II phase matching with the maximum value of $d_{eff}$(effective second order nonlinear optic coefficient). The crystal surfaces were polished with the flatness of λ/10 and coated with the anti-reflection coating for the fundamental and second harmonic waves. Nd:YLF cw laser with the wavelength 1.047 ㎛ and the repetition rate 100 MHz was used as a light source.
The output power for the KTP resonator was observed to be about 1.4 mW, which is much less than the theoretical value, 60 mW, when the input power was 300 mW. The observed value is about 30 times greater than the single pass output power. The observed low output power could be explained by the fact that the two waves with the perpendicular polarizations do not satisfy the resonant conditions at the same time due to the different refractive indices for the two polarizations.
