Patent ID: 7656775

Claim:
An optical head comprising: a light source; a transforming element for transforming a light beam emitted from the light source into a parallel beam; an objective lens for concentrating the parallel beam upon an optical-information recording medium that includes three or more information recording layers and corresponding light transmission layers, such that each corresponding light transmission layer has a respective thickness measured from a surface upon which the light beam is incident up to the corresponding information recording layer of the three or more information recording layers, and such that each respective thickness of each corresponding light transmission layer is different; and an aberration correcting element correcting a spherical aberration generated according to a thickness of a light transmission layer up to an information recording layer, the aberration correcting element being provided between the light source and the objective lens, wherein: the objective lens is set so that an absolute value of a fifth-order spherical aberration, which is generated when a focal point of the objective lens upon which the parallel beam is incident coincides with the information recording layer of the optical-information recording medium on an incidence-nearest side, is equal to an absolute value of a fifth-order spherical aberration that is generated when the focal point coincides with the information recording layer on a deepest side; and when: (i) a thickness of a light transmission layer up to the j-th (j is an integer of 1≦j≦m and m is a number of information recording layers) information recording layer from the side upon which the light beam is incident is d j ; (ii) a refractive index of the j-th light transmission layer is n j ; (iii) a design refractive index of the objective lens is N; and (iv) a thickness of a light transmission layer is A, the light transmission layer having the thickness of A being a light transmission layer at which a spherical aberration, generated by the objective lens on which a parallel beam is incident, is offset in a state where the focal point of the objective lens is coincident between the information recording layer that is closest to the incidence-side surface and the information recording layer on the deepest side, the objective lens satisfies the following formula (1) N 2 - 1 N 3 · A ≈ ( ∑ j = 1 m ⁢ ⁢ n j 2 - 1 n j 3 · d j + n 1 2 - 1 n 1 3 · d 1 ) / 2. ( 1 )