Patent ID: 7184639

Claim:
A slab waveguide comprising a two-dimensional crystal grating having columnar members having a refractive index different from the refractive index of a slab and two-dimensionally and periodically arranged along a surface of the slab, wherein the refractive index of a slab refractive index portion other than said columnar members in the slab, the number, the shape and the refractive index of said columnar members in the slab are selected so that when a beam of light entering the slab waveguide and traveling periodically, said columnar members and said slab refractive index portion other than said columnar members expands to a maximum extent, the size of the beam in the slab thickness direction does not exceed the slab thickness, wherein the refractive index of said slab refractive index portion in a direction perpendicular to the slab surface is maximized at a predetermined point other than end portions in the slab refractive index portion, and is decreased with the increase in distance from the predetermined point, and wherein the refractive index of said slab refractive index portion in the direction perpendicular to the slab surface is distributed symmetrically about the predetermined point.