Second-harmonic generation in a waveguide with domain-inverted regions like periodic lens sequence on z-face KTiOPO4 crystal

A domain-inverted waveguide with refractive-index distribution of a periodic lens sequence instead of a conventional square shape was fabricated in a z-cut KTiOPO4 (KTP) crystal. In the periodic lens sequence waveguide, the propagated laser beam through the waveguide was collimated and focused into a spot with a minimum amount of dispersion. The reason for the phenomenon of this periodic lens sequence waveguide structure can be explained from the viewpoint of ray matrix approximation.