Objective lens and optical information recording/reproducing apparatus
View Patent ↗An optical information recording/reproducing apparatus for an optical disc using a light beam having a wavelength of λ including: an objective lens that converges the light beam onto a recording surface of the optical disc and satisfies a condition: 0.40 < d · ( n - 1 ) f 2 < 0.70 ( 3 ) where f represents a focal length (unit: mm) of the objective lens with respect to the wavelength λ, d represents a thickness (unit: mm) of the objective lens along an optical axis of the objective lens, and n represents a refractive index with respect to the wavelength λ; a photoreceptor to receive the beam reflected from the optical disc; a detector to detect quality of a signal, and an objective lens driving unit to tilt the objective lens in a direction to improve the quality of the signal based on a result of detection by the detector.
1. An optical information recording/reproducing apparatus for recording information to and/or reproducing information from an optical disc based on a predetermined standard by using a light beam having a wavelength of λ (unit: nm),
the optical information recording/reproducing apparatus comprising:
an objective lens that converges the light beam onto a recording surface of the optical disc and satisfies a condition:
0.40
<
d
·
(
n
-
1
)
f
2
<
0.70
(
3
)
where f represents a focal length (unit: mm) of the objective lens with respect to the wavelength λ, d represents a thickness (unit: mm) of the objective lens along an optical axis of the objective lens, and n represents a refractive index with respect to the wavelength λ;
a photoreceptor that receives the light beam reflected from the recording surface of the optical disc;
a detector that detects quality of a signal outputted by the photoreceptor based on the light beam reflected from the recording surface of the optical disc; and
an objective lens driving unit configured to tilt the objective lens in a direction to improve the quality of the signal based on a result of detection by the detector,
wherein:
at least one of surfaces of the objective lens is formed to be a phase shift surface having a phase shift structure including a plurality of refractive surface zones which are concentrically formed and are divided by minute steps, each of the steps being formed at a boundary between adjacent ones of the plurality of annular zones to give a predetermined optical path length difference to the light beam incident thereon;
when (Σφ 0.00-1.00 ) represents a sum of heights of all the steps formed in an effective beam diameter on the phase shift surface, and (Σφ 0.95-1.00 ) represents a sum of heights of the steps formed in a range of 95% to 100% of an effective beam radius with respect to the optical axis, the objective lens satisfies a condition:
0.2
<
(
∑
ϕ
0.95
-
1.00
)
(
∑
ϕ
0.00
-
1.00
)
<
0.6
.
(
4
)
2. The optical information recording/reproducing apparatus according to claim 1 ,
wherein the steps of the phase shift structure are defined by a following optical path difference function φ(h):
φ( h )=( P 2 ×h 2 +P 4 ×h 4 +P 6 ×h 6 +P 8 ×h 8 +P 10 ×h 10 +P 12 ×h 12 ) mλ
where P 2 , P 4 , P 6 . . . represent coefficients of the 2 nd order, 4 th order, 6 th order, h represents a height from the optical axis, and m represents a diffraction order at which diffraction efficiency is maximized.
3. The optical information recording/reproducing apparatus according to claim 1 ,
wherein the objective lens is configured such that an offence against a sine condition takes a local minimum value within a range larger than or equal to 20% and smaller than 80% of the effective beam radius with respect to the optical axis, the offence against the sine condition takes a maximum value within a range of 80% to 100% of the effective beam radius, and the maximum value of the offence against the sine condition falls within a range of 0.001 to 0.01.
4. The optical information recording/reproducing apparatus according to claim 1 , wherein the optical disc has a protective layer thickness of 0.05 to 0.15 (unit: mm).
5. The optical information recording/reproducing apparatus according to claim 1 , wherein the objective lens is configured such that an offence against a sine condition and a spherical aberration become equal to each other at a 100% point of the effective beam radius when a protective layer thickness of the optical disc lies within a range of 0.105 to 0.15 (unit: mm).
6. The optical information recording/reproducing apparatus according to claim 1 , wherein a numerical aperture NA of the objective lens defined on an optical disc side lies within a range of 0.8 to 0.87 when the light beam having the wavelength λ is used.
7. The optical information recording/reproducing apparatus according to claim 1 , wherein the objective lens is made of resin.
8. The optical information recording/reproducing apparatus according to claim 1 ,
wherein the refractive index n of the objective lens satisfies a condition:
1.4<n<1.7 (5).
9. An objective lens for an optical information recording/reproducing apparatus for recording information to and/or reproducing information from an optical disc based on a predetermined standard, the optical disc having a protective layer thickness of 0.05 to 0.15 (unit: mm),
the objective lens satisfying a condition:
0.40
<
d
·
(
n
-
1
)
f
2
<
0.70
(
3
)
where f represents a focal length (unit: mm) of the objective lens with respect to a light beam having a wavelength λ used for recording information to and/or reproducing information from the optical disc, d represents a thickness (unit: mm) of the objective lens along an optical axis of the objective lens, and n represents a refractive index with respect to the wavelength λ,
at least one of surfaces of the objective lens being formed to be a phase shift surface having a phase shift structure including a plurality of refractive surface zones which are concentrically formed and are divided by minute steps, each of the steps being formed at a boundary between adjacent ones of the plurality of annular zones to give a predetermined optical path length difference to the light beam incident thereon;
when (Σφ 0.00-1.00 ) represents a sum of heights of all the steps formed in an effective beam diameter on the phase shift surface, and (Σφ 0.95-1.00 ) represents a sum of heights of the steps formed in a range of 95% to 100% of an effective beam radius with respect to the optical axis, the objective lens satisfying a condition:
0.2
<
(
∑
ϕ
0.95
-
1.00
)
(
∑
ϕ
0.00
-
1.00
)
<
0.6
.
(
4
)
10. The objective lens according to claim 9 , wherein the optical disc has a protective layer thickness of 0.05 to 0.15 (unit: mm).
11. The objective lens according to claim 9 ,
wherein the steps of the phase shift structure are defined by a following optical path difference function φ(h):
φ( h )=( P 2 ×h 2 +P 4 ×h 4 +P 6 ×h 6 +P 8 ×h 8 +P 10 ×h 10 +P 12 ×h 12 ) mλ
where P 2 , P 4 , P 6 . . . represent coefficients of the 2 nd order, 4 th order, 6 th order, h represents a height from the optical axis, and m represents a diffraction order at which diffraction efficiency is maximized.
12. The objective lens according to claim 9 ,
wherein a numerical aperture NA of the objective lens defined on an optical disc side lies within a range of 0.8 to 0.87 when the light beam having the wavelength λ, is used.
13. The objective lens according to claim 9 , wherein the objective lens is made of resin.
14. The objective lens according to claim 9 , wherein the refractive index n of the objective lens satisfies a condition:
1.4<n<1.7 (5).