| 摘要 |
<p>An objective lens includes a refractive lens having a positive refractive power, and a diffractive lens structure 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 having first and second aspherical surfaces. A diffractive lens structure is formed on the first surface of the objective lens. The diffractive lens structure 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 formed as a step giving a predetermined optical path difference. The diffractive lens structure has wavelength dependence such that at least two light beams having different wavelength with an identical diffractive order form appropriate wavefronts for at least two kinds of optical discs respectively provided with cover layers having different thickness.</p> |
