SHG PHASE-MATCHING CONDITIONS FOR UNDOPED AND DOPED LITHIUM-NIOBATE

We measured the refractive indices of undoped and Zn- or Mg-doped lith ium niobate in the wavelength range 400 to 1200 nm. The results are de scribed by a generalized Sellmeier equation which takes into account t he defect structure of the material. From the generalized Sellmeier eq uation a simple relation between the phase matching temperature for no ncritical type I second harmonic generation (SHG) and the internal cri tical phase matching angle is derived. Good correspondence with experi mental data over a wide composition and fundamental wavelength range i s observed.