Objective lens for optical pick-up
AssigneeAsahi Kogaku Kogyo Kabushiki Kaisha
InventorKoichi Maruyama
Life of this patent
Estimating the term…
Abstract
An objective lens includes a refractive lens having a positive refractive power, and a diffractive grating having a plurality of concentric ring-shaped steps that are formed on at least one lens surface of the refractive lens. The objective lens is a biconvex plastic lens of which first and second surfaces are aspherical. A diffractive grating is formed on the first surface of the objective lens. The diffractive grating is similar to a Fresnel lens, it is formed as many concentric rings each of which has a wedge sectional shape. The boundary between the adjacent rings is a step to give a predetermined optical path difference. The diffractive grating has wavelength dependence such that spherical aberration varies in the undercorrected direction as wavelength of incident light increases.
Claims7
An objective lens for an optical head, comprising:
- a refractive lens having a positive refractive power;
- and
- a diffractive grating having a plurality of concentric ring-shaped steps that are formed on at least one lens surface of said refractive lens,
- wherein said diffractive grating is configured such that spherical aberration changes in the undercorrected direction as wavelength of incident light increases.
Dependencies are inferred from the claim text.