Objective lens, optical head and optical disc device

1. An objective lens to be incorporated into an optical head, wherein:

the objective lens includes a first lens and a second lens substantially in close contact with each other such that optical axes thereof match each other, and acts as a convex lens as a whole;

the first lens and the second lens each include a central portion including the optical axis and a peripheral portion located in a periphery of the central portion;

the central portion of the first lens acts as a convex lens;

the central portion of the second lens acts as a concave lens; and

where the refractive index of the first lens at d line (wavelength: 587.56 nm) is nd1, the Abbe constant of the first lens represented using the refractive indices at d line, F line (wavelength: 486.13 nm) and C line (wavelength: 656.27 nm) is νd1, the refractive index of the second lens at d line is nd2, and the Abbe constant of the second lens represented using the refractive indices at d line, F line and C line is νd2, the objective lens fulfills

nd1>nd2 and νd1>νd2; and

the objective lens has three lens faces crossing the optical axis of:

a first lens face which is a lens face of the first lens which is not in contact with the second lens,

a second lens face which is a border between the first lens and the second lens substantially in close contact with each other, and

a third lens face which is a lens face of the second lens which is not in contact with the first lens; and

among effective areas of at least one of the first lens face, the second lens face and the third lens face, a central effective area including the optical axis and a peripheral effective area located in a periphery of the central effective area are formed in different aspheric shapes;

wherein in each of the first lens face, the second lens face and the third lens face, the central effective area and the peripheral effective area are formed in different aspheric shapes, and light from a light source of the optical head is incident on the peripheral effective area.

2. The objective lens of claim 1 , wherein:

the refractive index nd1 fulfills 1.60<nd1<1.85;

the refractive index nd2 fulfills 1.45<nd2<1.60;

the Abbe constant νd1 fulfills 40<νd1<60; and

the Abbe constant νd2 fulfills 20<νd2<40.

3. The objective lens of claim 1 , wherein the first lens is formed of a glass material, and the second lens is formed of a resin material.

4. The objective lens of claim 1 , wherein the first lens and the second lens are formed of glass materials, and the first lens and the second lens are formed of glass materials having different glass transition temperatures.

5. The objective lens of claim 1 , wherein where an angle between a tangent plane contacting an arbitrary point on a plane within an effective diameter of the second lens face and a reference plane which is a plane perpendicular to the optical axis is e, the value of e varies at a border between the central effective area and the peripheral effective area of the second lens face.

6. An optical head, comprising:

a plurality of light sources for emitting light of different wavelengths; and

an objective lens of claim 1 for converging light emitted from one of the plurality of light sources, wherein,

the objective lens converges light from a first light source among the plurality of light sources by the central portion and the peripheral portion thereof; and

the objective lens converges light from a second light source among the plurality of light sources substantially by only the central portion thereof.

7. The optical head of claim 6 , further comprising a conversion section for converting the light into substantially parallel light, diverging light or converging light in accordance with the wavelength of the light and causing the converted light to be incident on the objective lens,

wherein the first light source emits light of a shorter wavelength than the second light source.

8. The optical head of claim 6 , further comprising a third light source for emitting light of a longer wavelength than the first light source and the second light source, wherein:

the first light source emits light of a first wavelength λ 1 , the second light source emits light of a second wavelength λ 2 , and the third light source emits light of a third wavelength λ 3 ; and

the wavelength λ 1 , the wavelength λ 2 and the wavelength λ 3 fulfill:

350 nm<λ 1 <450 nm,

600 nm<λ 2 <700 nm, and

700 nm<λ 3 <850 nm,

the optical head further comprising an optical element acting as a phase step for a part of the light of the wavelength λ 1 ,

wherein the phase step is constructed such that a difference in an optical path length caused when the light of the wavelength λ 1 passes the phase step is substantially 10 times the wavelength λ 1 , and thus corrects the spherical aberration caused at a position of a focal point of the light by the change in the wavelength of the light of the wavelength λ 1 .

9. The optical head of claim 8 , wherein among the spherical aberrations caused along with the change in the wavelength of the light of the wavelength of λ 1 ,

a conversion section mainly corrects a third-order spherical aberration; and

the phase step mainly corrects a five- or higher-order spherical aberration.

10. An optical drive, comprising:

an optical head of claim 6 ;

a motor for driving an optical recording medium to rotate; and

a control section for controlling the optical head and the motor.

11. The objective lens of claim 1 , wherein the optical head comprises:

a first light source for emitting light of a first wavelength λ 1 , a second light source for emitting light of a second wavelength λ 2 and a third light source for emitting light of a third wavelength λ 3 ,

the wavelength λ 1 , the wavelength λ 2 and the wavelength λ 3 fulfill:

350 nm<λ 1 <450 nm,

600 nm<λ 2 <700 nm, and

700 nm<λ 3 <850 nm;

wherein, the central portion defined by the central effective area converges the light of the first, the second and the third wavelength λ 1 , λ 2 and λ 3 respectively and the peripheral portion defined by the peripheral effective area only converges the light of the wavelength λ 1 .

12. An objective lens to be incorporated into an optical head, wherein:

the objective lens includes a first lens and a second lens substantially in close contact with each other such that optical axes thereof match each other, and acts as a convex lens as a whole;

the first lens and the second lens each include a central portion including the optical axis and a peripheral portion located in a periphery of the central portion;

the central portion of the first lens acts as a convex lens;

the central portion of the second lens acts as a concave lens; and

at least one lens face of the objective lens crossing the optical axis includes a central effective area including the optical axis and a peripheral effective area not including the optical axis, and the central effective area and the peripheral effective area are formed in aspheric shapes represented by different aspheric coefficients; and

the first lens and the second lens define:

a first lens face which is a lens face of the first lens which is not in contact with the second lens,

a second lens face which is a border between the first lens and the second lens substantially in close contact with each other, and

a third lens face which is a lens face of the second lens which is not in contact with the first lens; and

among effective areas of at least one of the first lens face, the second lens face and the third lens face, a central effective area including the optical axis and a peripheral effective area located in a periphery of the central effective area are formed in different aspheric shapes;

wherein in each of the first lens face, the second lens face and the third lens face, the central effective area and the peripheral effective area are formed in different aspheric shapes, and light from a light source of the optical head is incident on the peripheral effective area.

13. The objective lens of claim 12 , wherein the optical head comprises:

a first light source for emitting light of a first wavelength λ 1 , a second light source for emitting light of a second wavelength λ 2 and a third light source for emitting light of a third wavelength λ 3 ,

the wavelength λ 1 , the wavelength λ 2 and the wavelength λ 3 fulfill:

350 nm<λ 1 <450 nm,

600 nm<λ 2 <700 nm, and

700 nm<λ 3 <850 nm;

wherein, the central portion defined by the central effective area converges the light of the first, the second and the third wavelength λ 1 , λ 2 and λ 3 respectively and the peripheral portion defined by the peripheral effective area only converges the light of the wavelength λ 1 .