Objective lens, optical pickup device having the same, and recording and/or reproducing apparatus for optical recording medium, equipped with the optical pickup device

1. An objective lens for converging used light on a desired position of an optical recording medium which information is recorded in and reproduced from, the objective lens consisting of:

a single lens element having at least one aspheric surface, wherein the following conditional expressions (1) to (3) are satisfied

0.70<NA<0.98  (1)

1.10 ≦d/f≦ 1.28  (2′)

0.48<X<0.55  (3)

where NA denotes a numerical aperture of the objective lens on an optical recording medium side,

d denotes a thickness of the objective lens on an optical axis in mm,

f denotes a focal length of the objective lens in mm,

X is equal to (X1−X2)·(n−1)/(NA·f)

n denotes a refractive index of the objective lens at a wavelength of the used light,

X1 denotes a distance, in mm and in an optical axis direction, between (a) a first tangential plane that is perpendicular to the optical axis and is tangent to a vertex of a light source side surface of the objective lens and (b) a most outside position within an effective diameter of the light source side surface, which is a position on the light source side surface on which a marginal ray having the NA is incident, and X1 is defined so that a direction toward the optical recording medium from the first tangential plane serving as a reference point is a positive direction and that a direction toward the light source from the first tangential plane is a negative direction, and

X2 is a distance, in mm and in the optical axis direction, between (a) a second tangential plane that is perpendicular to the optical axis and is tangent to a vertex of an optical recording medium side surface and (b) a most outside position within an effective diameter of the optical recording medium side surface, which is a position on the optical recording medium side surface on the marginal ray having the NA is incident, and X2 is defined so that a direction toward the optical recording medium from the second tangential plane serving as a reference point is a positive direction and a direction toward the light source from the second tangential plane is a negative direction.

2. The objective lens according to claim 1 , wherein the following conditional expression (4) is satisfied

1.0≦ΦA≦5.0  (4)

where ΦA denotes the effective diameter of the light source side surface of the objective lens.

3. The objective lens according to claim 1 , wherein the following conditional expression (5) is satisfied

0.25<y<0.65  (5)

where Y is equal to R1/(n·f), and

R1 denotes a radius of curvature of the light source side surface of the objective lens near the optical axis.

4. The objective lens according to claim 1 , wherein the wavelength of the used light is 405.0±5.0 nm.

5. The objective lens according to claim 1 , wherein

the wavelength of the used light is 405.0±5.0 nm,

the numerical aperture NA is 0.85, and

a thickness of a protection layer of the optical recording medium is 0.1 mm.

6. The objective lens according to claim 1 , wherein

the wavelength of the used light is 405.0±5.0 nm,

the numerical aperture NA is 0.85,

aberration is minimized at a position being distant t1 mm from a surface of the optical recording medium to an inside of the optical recording medium, and

the following conditional expression (6) is satisfied

0.075≦t1≦0.1  (6).

7. An optical pickup device comprising:

the objective lens according to claim 1 ; and

an actuator that performs a focusing operation of the objective lens and a tracking operation of the objective lens.

8. A recording and/or reproducing apparatus for an optical recording medium, the apparatus comprising:

the optical pickup device according to claim 7 .