Optical waveguide and thermally-assisted magnetic recording head therewith

An optical waveguide of the invention includes a core that is a waveguide through which light propagates; and a cladding that surrounds the core. The core has a plate shape and includes a wide core base part onto which the light is incident, a taper part that is connected to the core base part and of which a width is gradually tapered along a propagation direction, and a narrow front end core part that is connected to the taper part and that extends along the propagation direction. A grating is provided on one of planar surfaces of the wide core base part, the grating is formed by engraving a number of concave grooves having a rectangular cross section on the planar surface along a width direction thereof, the grating is formed to be optically coupled with laser light that is incident perpendicularly onto the grating formation surface, a frequency (grating pitch: pitch of the concave grooves) of the grating is smaller than a wavelength (defined as a wavelength in the cladding) of the perpendicularly incident laser light, and a groove depth H of the grating is formed with respect to a thickness H of the core base part so that a relationship H=(0.33 to 0.67)×H is satisfied.