Objective lens for optical pick-up

摘要

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.