Method of processing a substrate made of a ferroelectric single crystalline material

A method of processing a substrate made of a ferroelectric single crystalline material, comprising the steps of forming a desired proton-exchanged layer in the substrate by proton-exchanging portions of the substrate, and selectively removing the proton-exchanged layer to form a convex ridge structure, wherein the desired proton-exchange layer is formed by using an acid containing a lithium salt as a proton-exchanging source, the surface of the substrate from which the convex ridge structure is formed is an X-cut surface or a Z-cut surface, as a main surface, of the ferroelectric single crystalline material used as the substrate, and the convex ridge structure has shoulder portions with their height equal to or larger than a radius of curvature of each of curved portions.

A substrate made of a ferroelectric single crystalline material, said substrate having a concave ditch structure, wherein a surface of the substrate from which the concave ditch structure is formed is an X-cut surface or a Z-cut surface, as a main surface, of the ferroelectric single crystalline material used as the substrate, and the concave ditch structure has a recessed portion with its depth equal to or larger than its half opening width.

An embedded-type optical waveguide element comprising a substrate made of a ferroelectric single crystalline material and having a concave ditch structure, and an optical waveguide formed in the concave ditch structure, wherein a surface of the substrate from which the concave ditch structure is formed is an X-cut surface or a Z-cut surface, as a main surface, of the ferroelectric single crystalline material used as the substrate, and the concave ditch structure has a recessed portion with its depth equal to or larger than its half opening width, and the optical waveguide was made of another ferroelectric single crystalline material having a larger refractive index than that of the substrate by a liquid phase epitaxial method.

A substrate made of a ferroelectric single crystalline material, said substrate having a convex ridge structure having curved portions at base portions of said ridge structure, wherein a surface of the substrate from which the convex ridge structure is formed is an X-cut surface or a Z-cut surface, as a main surface, of the ferroelectric single crystalline material used as the substrate, and the convex ridge structure has shoulder portions with their height equal to or larger than a radius of curvature of curved portions of said ridge structure.