Patent ID: 8064314

Claim:
An optical head, comprising: a first light source for emitting light having a wavelength λ 1 ; a second light source for emitting light having a wavelength λ 2 ; a third light source for emitting light having a wavelength λ 3 ; an objective lens for converging the light having the wavelength λ 1 onto a first optical disc at a numerical aperture NA 1 , converging the light having the wavelength λ 2 onto a second optical disc at a numerical aperture NA 2 , and converging the light having the wavelength λ 3 onto a third optical disc at a numerical aperture NA 3 ; and at least one light detector for detecting reflected light respectively from the first, second, and third optical discs, wherein the objective lens is formed of at least a diffraction element and a refracting lens and has numerical apertures in a relation expressed as: NA 1 >NA 2 >NA 3 , wherein the diffraction element is divided into an inner peripheral region corresponding to the numerical aperture NA 3 , an intermediate peripheral region corresponding to the numerical aperture NA 3 to the numerical aperture NA 2 , and an outer peripheral region corresponding to the numerical aperture NA 2 to the numerical aperture NA 1 , and is configured to differ diffraction orders of diffracted light that is generated most strongly in each of the inner peripheral region, the intermediate peripheral region, and the outer peripheral region, and wherein the diffraction element is formed of a diffraction grating having: a grating shape in the inner peripheral region that generates third-order diffracted light most strongly as the light having the wavelength λ 1 and converged onto the first optical disc, generates second-order diffracted light most strongly as the light having the wavelength λ 2 and converged onto the second optical disc, and generates second-order diffracted light most strongly as the light having the wavelength λ 3 and converged onto the third optical disc; a grating shape in the intermediate peripheral region that generates sixth-order diffracted light most strongly as the light having the wavelength λ 1 and converged onto the first optical disc, generates fourth-order diffracted light most strongly as the light having the wavelength λ 2 and converged onto the second optical disc, and generates third-order diffracted light most strongly for the light having the wavelength λ 3 ; and a grating shape in the outer peripheral region that generates m-th order diffracted light most strongly as the light having the wavelength λ 1 and converged onto the first optical disc.