Optical information recording/reproducing element

Claim 2

An optical information recording/reproducing element for focusing and projecting a parallel or substantially parallel light beam onto an optical information recording medium, wherein the element comprises a transparent columnar body having a convex surface as an incident surface of the light beam, and a flat surface as an output surface thereof, said columnar body having a refractive index distribution according to which a refractive index gradually decreases from a central axis toward a periphery of the columnar body, said refractive index distribution being given by: N.sup.2 (r)=n.sub.0.sup.2 [1-(gr).sup.2 +h.sub.4 (gr).sup.4 +h.sub.6 (gr).sup.6 +. . . ] where n 0 is a refractive index on the central axis, n(r) is a refractive index at a point of a distance r from the central axis, and g, h 4 and h 6 are distribution constants, and said element satisfies the following conditions: -150G+52 mm≦r.sub.0 ≦-100G+40.5 mm r.sub.0 ≦2.5 mm G≦0.45 where r 0 is an effective element radius and G is a product of n 0 , g and r 0 .

Claim 3

An element according to claim 1, wherein an absolute value of a deficiency amount of a sine condition for an incident light beam at least at a portion of the incident surface of the element is not more than 0.2%.

Claim 4

An optical information recording/reproducing element for focusing and projecting a parallel or substantially parallel light beam onto an optical information recording medium, wherein the element comprises a transparent columnar body having a convex surface as an incident surface of the light beam, and a flat surface as an output surface thereof, said columnar body having a refractive index distribution according to which a refractive index gradually decreases from a central axis toward a periphery of the columnar body, said refractive index distribution being given by: N.sup.2 (r)=n.sub.0.sup.2 [1-(gr).sup.2 +h.sub.4 (gr).sup.4 +h.sub.6 (gr).sup.6 +. . . ] where n 0 is a refractive index on the central axis, n(r) is a refractive index at a point of a distance r from the central axis, and g, h 4 and h 6 are distribution constants, and said element satisfies the following conditions: -3.87G+2.85 mm≦r.sub.0 ≦-4G+3.41 mm r.sub.0 ≦2.5 mm G≦0.45 where r 0 is an effective element radius and G is a product of n 0 , g and r 0 .

Claim 5

An optical information recording/reproducing element for focusing and projecting a parallel or substantially parallel light beam onto an optical information recording medium, wherein the element comprises a transparent columnar body having a convex surface as an incident surface of the light beam, and a flat surface as an output surface thereof, said columnar body having a refractive index distribution according to which a refractive index gradually decreases from a central axis toward a periphery of the columnar body, said refractive index distribution being given by: N.sup.2 (r)=n.sub.0.sup.2 [1-(gr).sup.2 +h.sub.4 (gr).sup.4 +h.sub.6 (gr).sup.6 +. . . ] where n 0 is a refractive index on the central axis, n(r) is a refractive index at a point of a distance r from the central axis, and g, h 4 and h 6 are distribution constants, and said element satisfies the following conditions: -3.87G+2.85 mm≦r.sub.0 ≦-4G+3.41 mm r.sub.0 ≦2.5 mm G≦0.45 where r 0 is an effective element radius and G is a product of n 0 , g and r 0 .

Claim 6

An element according to claim 3, wherein an absolute value of a deficiency amount of a sine condition for an incident light beam at least at a portion of the incident surface of the element is not more than 0.2%.