Patent ID: 7755998

Claim:
An objective lens receiving light beams with different wavelengths λn (n≧3)for at least three kinds of optical recording media and having a positive power to focus each light beam on an information recording surface of a transparent substrate of each optical recording medium only by refraction, wherein, if distances between points Pn (n>3) where incident light beams or extension lines of incident light beams with wavelengths λn (n≧3)to the objective lens crosses an optical axis and a point Q where a lens surface of the objective lens located farther from each optical recording medium than another lens surface crosses the optical axis is expressed by Sn (n>3), and a sign of the distance Sn is defined as positive if a position of the point Pn is located in a different side from the optical recording medium with respect to the point Q, and defined as negative if the position of the point Pn is located in the same side as the optical recording medium with respect to the point Q, an incident light beam satisfying following expressions enters the objective lens: λ 1 <λ 3 and (1 /S 1 )<(1 /S 3 ) λ 2 <λ 3 (λ 2 >λ 1 ) and (1 /S 2 )<(1 /S 3 ) each light beam is focused on the information recording surface with RMS wavefront aberration of 0.035 λRMS or below, at least one lens surface is radially sectioned into a plurality of zones, each aspherical surface shape of the plurality of zones is formed so that spherical aberration due to a difference in transparent substrate thickness and chromatic aberration due to a difference in refractive index of the objective lens, which result from a difference between light beam wavelengths, cancel each other, and an optical path length of a light beam passing through each zone of the objective lens is different from an optical path length passing through another zone by substantially 2 mλ (m is an integral number) when the light beam is one of a first light beam, a second light beam, and a third light beam, and by substantially mλ (m is an integral number) for a rest of the light beams.