| 摘要 |
PURPOSE:To compensate well the spherical aberration and the coma aberration, by combining plural distributed refractive index lenses different in refractive index distribution from one another and using one diverging distributed refractive index lens at least. CONSTITUTION:In one distributed refractive index lens, a value n2 which gives a coefficient sigmai to cancel an aberration due to the refraction on the end face of the lens and the face of an optical disc exists certainly with respect to one optional aberration. Generally, other aberrations are not canceled simultaneously at this time. The number of degrees of freedom for aberration compensation of the refractive index distribution is 1. That is, when plural distributed refractive index lenses are combined and high-degree terms of respective refractive index distributions of these lenses are selected properly, two or more aberrations are compensated simultaneously. Thus, a diverging distributed index lens Ln and a converging distributed refractive index lens Lp are brought closely into contact with each other to the lens system, thereby compensating well not only the spherical aberration but also the coma aberration. |
