High NA objective lens for optical pick-up
View Patent ↗A single-element objective lens is employed in an optical pick-up. The objective lens directs an incident light beam to a data recording surface of an optical disc through a cover layer to form a beam spot thereon. A numerical aperture of the objective lens is 0.7 or more. The objective lens is configured such that a characteristic of aberration which is generated due to decentering of surfaces of the objective lens is substantially comparable to a characteristic of aberration due to an inclination of an optical axis of the objective lens and an inclination of a central axis of the light beam incident on the data recording surface of the optical disc with respect to the optical disc.
1. A single-element objective lens for an optical pick-up, said objective lens directing an incident light beam to a data recording surface of an optical disc through a cover layer to form a beam spot, a numerical aperture of said objective lens being 0.7 or more,
said objective lens being configured such that a characteristic of aberration due to decentering of surfaces of said objective lens is substantially comparable to a characteristic of aberration due to an inclination of an optical axis of said objective lens and a central axis of the light beam incident on the data recording surface of the optical disc with respect to the optical disc, and such that there exists a coefficient α with which condition:
-
0.01
<
X
1
+
X
2
×
(
-
1
+
n
0
2
+
(
n
0
2
-
1
)
X
2
)
-
h
(
n
0
-
1
)
c
+
α
(
tNA
(
-
1
n
d
+
1
-
NA
2
n
d
2
-
NA
2
)
)
<
0.01
is satisfied,
wherein, X represents a differentiation, with respect to h, of a function g(h), which is a SAG amount of the first surface of the objective lens at a height h from the optical axis thereof, c represents the differentiation of X with respect to h, n 0 represents a refractive index of the objective lens, n d represents a refractive index of the cover layer, t represents a thickness along the optical axis of the cover layer, NA represents a function NA(h) which is defined as NA(h)=h/f, f being a focal length of the objective lens, h being any value within a range 0<h≦h max , wherein h max is a maximum effective radius of a light beam on the first surface.
2. The objective lens according to claim 1 , which is configured such that coma is compensated for by inclining an optical axis of said objective lens and the central axis of the light beam incident on the data recording surface of the optical disc with respect to the optical disc.
3. The objective lens according to claim 1 , which is configured such that there exists a coefficient α with which condition:
-
0.005
<
X
1
+
X
2
×
(
-
1
+
n
0
2
+
(
n
0
2
-
1
)
X
2
)
-
h
(
n
0
-
1
)
c
+
α
(
tNA
(
-
1
n
d
+
1
-
NA
2
n
d
2
-
NA
2
)
)
<
0.005
is satisfied.
4. The objective lens according to claim 1 , which is configured to satisfy a condition:
0.08
<
X
1
+
X
2
×
(
-
1
+
n
0
2
+
(
n
0
2
-
1
)
X
2
)
-
h
(
n
0
-
1
)
c
<
0.30
,
wherein h being equal to h max .
5. The objective lens according to claim 1 , wherein the coefficient α being greater than five and less than twelve.
6. An optical pick-up, comprising:
a laser source that emits a laser beam;
a single-element objective lens, said objective lens directing the laser beam emitted by said laser source to a data recording surface of an optical disc through a cover layer to form a beam spot, a numerical aperture of said objective lens being 0.7 or more; and
an inclination adjusting mechanism which adjusts an inclination of an optical axis of said objective lens and a central axis of the laser beam incident on the cover layer with respect to the optical disc,
wherein said objective lens is configured such that a characteristic of aberration due to decentering of surfaces of said objective lens is substantially comparable to a characteristic of aberration due to an inclination of an optical axis of said objective lens and a central axis of the light beam incident on the data recording surface of the optical disc with respect to the optical disc, and such that there exists a coefficient α with which condition:
-
0.01
<
X
1
+
X
2
×
(
-
1
+
n
0
2
+
(
n
0
2
-
1
)
X
2
)
-
h
(
n
0
-
1
)
c
+
α
(
tNA
(
-
1
n
d
+
1
-
NA
2
n
d
2
-
NA
2
)
)
<
0.01
is satisfied,
wherein, X represents a differentiation, with respect to h, of a function g(h), which is a SAG amount of the first surface of the objective lens at a height h from the optical axis thereof, c represents the differentiation of X with respect to h, n 0 represents a refractive index of the objective lens, n d represents a refractive index of the cover layer, t represents a thickness along the optical axis of the cover layer, NA represents a function NA(h) which is defined as NA(h)=h/f, f being a focal length of the objective lens, h being any value within a range 0<h≦h max , wherein h max is a maximum effective radius of a light beam on the first surface.
7. The optical system according to claim 6 , wherein said objective lens is configured such that coma is compensated for by inclining an optical axis of said objective lens and the central axis of the light beam incident on the data recording surface of the optical disc with respect to the optical disc.
8. The optical system according to claim 6 , wherein said objective lens is configured such that there exists a coefficient α with which condition:
-
0.005
<
X
1
+
X
2
×
(
-
1
+
n
0
2
+
(
n
0
2
-
1
)
X
2
)
-
h
(
n
0
-
1
)
c
+
α
(
tNA
(
-
1
n
d
+
1
-
NA
2
n
d
2
-
NA
2
)
)
<
0.005
is satisfied.
9. The optical system according to claim 6 , wherein said objective lens is configured to satisfy a condition:
0.08
<
X
1
+
X
2
×
(
-
1
+
n
0
2
+
(
n
0
2
-
1
)
X
2
)
-
h
(
n
0
-
1
)
c
<
0.30
,
wherein h being equal to h max .
10. The optical system according to claim 6 , wherein the coefficient α being greater than five and less than twelve.