Optical pickup apparatus for recording or reproducing information for an optical information recording medium and objective lens for optical pickup apparatus
An objective lens of an optical pickup apparatus converges a divergent light flux onto an information recording surface. The following conditional formula is satisfied: | δSA 1 /δU|·|δU|+|δSA 2 /δT|·|δT |≦0.07 λrms where λ represents a wavelength of a light source, δSA 1 /δU represents a change of a spherical aberration for an object-to-image distance change δU (|δU|≦0.5 mm) and δSA 2 /δT represents a change of spherical aberration for a temperature change δT (|δT|≦30° C.), the object-to-image distance is a distance between the light source (a light emitting point) and the information recording surface.
1. An objective lens for use in an optical pickup apparatus, comprising:
a first light source for emitting a light flux having a wavelength λ1;
a second light source for emitting a light flux having a wavelength λ2, where (λ1≦λ2);
a light converging optical system;
a first optical information recording medium including a transparent base board having a thickness t 1 , the optical pickup apparatus recording or reproducing information for the first optical information recording medium by using the first light source and the light converging optical system;
a second optical information recording medium including a transparent base board having a thickness t 2 , where (t 1 <t 2 ), the optical pickup apparatus recording or reproducing information for the second optical information recording medium by using the second light source and the light converging optical system; and
at least two types of optically functional surfaces arranged in a direction perpendicular to an optical axis, the optically functional surfaces having a different optical action from others,
wherein a light flux passing through at least an outermost optically functional surface among the optically functional surfaces is used for recording or reproducing information only when recording or reproducing information for the first optical information recording medium,
wherein the objective lens is a pasted lens in which there are at least two types of optical elements each made of an optical material different from the other,
wherein a value of refractive index change dn/dT for a temperature change of the optical material used for the optical elements having a power component stronger than that of other satisfies the following conditional formula under the conditions of the wavelengths of the first light source and the second light source and a room temperature environment
| dn/dT|≦ 10.0×10 −6 (/° C.),
wherein both of the outermost optically functional surface and the optically functional surface closest to the optical axis comprise a diffractive structure, and
wherein a diffraction order n ot of a diffracted light ray having a highest intensity generated at the diffractive structure on the outermost optically functional surface and a diffraction order n in of a diffracted light ray having a highest intensity generated at the diffractive structure on the optically functional surface closest to the optical axis satisfy the following conditional formula when recording or reproducing information for the first optical information recording medium
| n ot |>| n in |.
2. The objective lens of claim 1 , wherein at least one of optical elements other than those having stronger power components among the plural optical elements is made of plastic material.
3. The objective lens of claim 1 , wherein image forming magnification m 1 of the objective lens for recording or reproducing information for the first optical information recording medium satisfies the following conditional formula
−¼≦ m 1≦⅛.
4. The objective lens of claim 1 , wherein an aperture-stop in the recording or reproducing of information for the first optical information recording medium is the same as that in the recording or reproducing of information for the second optical information recording medium.
5. The objective lens of claim 1 , wherein a necessary numerical aperture NA 1 in the recording or reproducing of information for the first optical information recording medium satisfies the following conditional formula
NA1≧0.60.
6. The objective lens of claim 1 , wherein the wavelength λ1 of the first light source is not more than 670 nm.
7. The objective lens of claim 1 , wherein at least one of the optical materials is an optical glass and has a dispersion value vd greater than 50.
8. The objective lens of claim 1 , wherein a spherical aberration generated in recording or reproducing information for the second optical information recording medium is discontinuous at a boundary section of the optically functional surface, and a difference in the spherical aberration at the discontinuity is in a range from 10 μm to 30 μm.
9. An optical pickup apparatus, comprising:
a first light source for emitting a light flux having a wavelength λ1;
a second light source for emitting a light flux having a wavelength λ2, where (λ1<λ2);
a light converging optical system including an objective lens;
a first optical information recording medium including a transparent base board having a thickness t 1 , the optical pickup apparatus recording or reproducing information for the first optical information recording medium by using the first light source and the light converging optical system; and
a second optical information recording medium including a transparent base board having a thickness t 2 , where (t 1 <t 2 ), the optical pickup apparatus recording or reproducing information for the second optical information recording medium by using the second light source and the light converging optical system,
wherein the objective lens includes at least two types of optically functional surfaces arranged in a direction perpendicular to an optical axis, the optically functional surfaces having a different optical action from others,
wherein a light flux passing through at least an outermost optically functional surface among the optically functional surfaces is used for recording or reproducing information only when recording or reproducing information for the first optical information recording medium,
wherein the objective lens is a pasted lens in which there are at least two types of optical elements each made of an optical material different from the other,
wherein a value of refractive index change dn/dT for a temperature change of the optical material used for the optical elements having a power component stronger than that of other satisfies the following conditional formula under the conditions of the wavelengths of the first light source and the second light source and a room temperature environment
| dn/dT|≦ 10.0×10 −6 (/° C.),
wherein both of the outermost optically functional surface and the optically functional surface closest to the optical axis comprise a diffractive structure, and
wherein a diffraction order n ot of a diffracted light ray having a highest intensity generated at the diffractive structure on the outermost optically functional surface and a diffraction order n in of a diffracted light ray having a highest intensity generated at the diffractive structure on the optically functional surface closest to the optical axis satisfy the following conditional formula when recording or reproducing information for the first optical information recording medium
| n ot |>| n in |.
10. The optical pickup apparatus of claim 9 , wherein at least one of optical elements other than the optical element having stronger power component among the plural optical elements is made of a plastic material.
11. The optical pickup apparatus of claim 9 , wherein image forming magnification m 1 of the objective lens for recording or reproducing information for the first optical information recording medium satisfies the following conditional formula
−¼≦ m 1≦⅛.
12. The optical pickup apparatus of claim 9 , wherein an aperture-stop in the recording or reproducing of information for the first optical information recording medium is the same as that in the recording or reproducing of information for the second optical information recording medium.
13. The optical pickup apparatus of claim 9 , wherein a necessary numerical aperture NA 1 in the recording or reproducing of information for the first optical information recording medium satisfies the following conditional formula
NA1≧0.60.
14. The optical pickup apparatus of claim 9 , wherein the wavelength λ 1 of the first light source is not more than 670 nm.
15. The optical pickup apparatus of claim 9 , wherein at least one of the optical materials is an optical glass and has a dispersion value vd greater than 50.
16. The optical pickup apparatus of claim 9 , wherein a spherical aberration generated in recording or reproducing information for the second optical information recording medium is discontinuous at a boundary section of the optically functional surface, and a difference in the spherical aberration at the discontinuity is in a range from 10 μm to 30 μm.