Patent ID: 7835252

Claim:
An optical head apparatus comprising: a light source emitting a linearly-polarized light; an objective lens focusing a light source light emitted from the light source on an optical recording medium; a light detector receiving a return light generated by reflecting the light source light by the optical recording medium; and a variable diffraction device comprising: a first polarized light diffraction grating; and a second polarized light diffraction grating, the variable diffraction device: diffracting an incident light of the linearly-polarized light by one of the first polarized light diffraction grating and the second polarized light diffraction grating; and transmitting the incident light without diffraction by the other thereof, the first polarized light diffraction grating having: first stripes comprising an anisotropic transparent material; and second stripes comprising an isotropic transparent material, the first stripes and the second stripes being alternately arranged in a first direction and in a first pitch to fill each space; the second polarized light diffraction grating having: third stripes comprising an anisotropic transparent material; and fourth stripes comprising an isotropic transparent material, the third stripes and the fourth stripes being alternately arranged in a second direction and in a second pitch to fill each space; the first polarized light diffraction grating and the second polarized light diffraction grating being different from each other in at least one of their respective pitches and directions, the variable diffraction device being capable of switching: either the first polarized light diffraction grating or the second polarized light diffraction grating diffracting the incident light; and the other thereof transmitting the incident light without diffraction by changing refractive indices sensed by the first polarized light diffraction grating and the second polarized light diffraction grating with respect to the incident light by an externally applied electric signal, wherein, provided that a polarizing direction diffracted by the first polarized light diffraction grating is defined as a first polarizing direction and a polarizing direction that is orthogonal to the first polarizing direction and not diffracted by the first polarized light diffraction grating is defined as a second polarizing direction, a ratio obtained from dividing a diffraction efficiency in the second polarizing direction by a diffraction efficiency in the first polarizing direction, of the first polarized light diffraction grating, is 0 to 0.5, and a ratio obtained from dividing a diffraction efficiency in the first polarizing direction by a diffraction efficiency in the second polarizing direction, of the second polarized light diffraction grating, is 0 to 0.5.