Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
1999-05-28
2001-02-20
Mack, Ricky (Department: 2873)
Optical: systems and elements
Diffraction
From grating
C359S565000, C359S718000, C359S719000
Reexamination Certificate
active
06191889
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a high NA (numerical aperture) objective lens installed in an optical pick-up of an optical disc apparatus for a digital versatile disc (DVD), a magneto-optical disc (MO) or the like. Particularly, the present invention relates to the objective lens that has a diffractive grating formed on a surface of a refractive lens.
An optical pick-up of an optical disc apparatus employs, in general, a single-piece plastic objective lens having both aspherical surfaces. Plastic lenses have advantages since the weight is relatively light, and the lenses can be formed relatively easily by an injection molding process.
However, the plastic lens has some disadvantages as follows. The major disadvantage is significant performance change depending on the temperature. It is because, the plastic lens has higher sensitivity to temperature in terms of the refractive index, and larger heat expansion property than a glass lens. For instance, when temperature rises, the refractive index of the plastic lens decreases, then spherical aberration changes to be overcorrected, and consequently, wave front aberration increases. When the wave front aberration exceeds the predetermined permissible level, the objective lens cannot form a sufficiently small beam spot on an optical disc. The sensitivity, which is a ratio of the refractive index variation with respect to a temperature change, is approximately −10×10
−5
/° C.
FIG. 27
is a graph showing a change of wave front aberration (unit:&lgr;, wavelength) with respect to the NA for a plastic lens (focal length: 3.0 mm at wavelength 650 nm), when the temperature rises by 40 degrees and the refractive index is changed by −400×10
−5
. As shown in the graph, the change of the wave front aberration when the temperature is changed is substantially proportional to the fourth power of NA.
The NA of an objective lens for a CD apparatus is approximately 0.45 and the permissible level of the wave front aberration is approximately 0.04&lgr;. This allows temperature variation in a range of 90 degrees. For example, the range is 30±45° C. Since the temperature range allowing the permissible level of the wave front aberration is sufficiently large, the deterioration of the wave front aberration due to temperature variation is virtually out of question in the CD apparatus.
On the other hand, the NA of an objective lens for the DVD apparatus is about 0.60, that for the MO disc apparatus is about 0.65 and the permissible level of the wave front aberration is approximately 0.03&lgr;. In this case, when the temperature changes by 40 or 50 degrees, the wave front aberration exceeds the permissible level, which may cause a problem in reading and/or writing information.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an objective lens for an optical pick-up, which is capable of reducing the change of the wave front aberration due to temperature change to widen the permissible range of the temperature change even when the objective lens is applied to the DVD apparatus or the MO disc apparatus.
For the above object, according to the present invention, there is provided an improved objective lens for an optical pick-up, which includes a refractive lens having a positive refractive power, and a diffractive grating having a plurality of concentric ring-shaped steps that are formed on at least one lens surface of the refractive lens. The diffractive grating has wavelength dependence such that spherical aberration varies in an undercorrected direction as wavelength of incident light increases.
As described above, spherical aberration of a positive refractive lens varies in the overcorrected direction as temperature rises. A semiconductor laser, which is generally used as a light source of an optical pick-up, has temperature dependence such that wavelength of the emitted laser increases as temperature rises. When the temperature rises, the refractive lens changes the spherical aberration in the overcorrected direction, and the diffractive grating changes the spherical aberration in the undercorrected direction because the wavelength of the light emitted from the semiconductor laser increases. Thus the changes of the spherical aberrations caused by the refractive lens and the diffractive grating can be counterbalanced to each other.
The refractive lens is desirable to be a plastic lens to make an inexpensive and lightweight objective lens and to easily transfer a grating pattern of a mold to the lens. Since a high NA lens requires strict aberration correction, a double aspherical biconvex lens is desirable to make a high NA lens.
An additional optical path length added by a diffractive grating is expressed by the following optical path difference function &PHgr;(h):
&PHgr;(
h
)=(
P
2
h
2
+P
4
h
4
+P
6
h
6
+ . . . )×&lgr;
where P
2
, P
4
and P
6
are diffractive coefficients of second, fourth and sixth orders, h is a height from the optical axis and &lgr; is wavelength of incident light.
The objective lens according to the present invention may satisfy the following condition (1);
−75.0<
P
4
×(
h
max
)
4
/(
f×NA
4
)<−25.0 (1)
where h
max
is the maximum height from the optical axis in the effective diameter, NA is a numerical aperture, and f is a total focal length of the refractive lens and the diffractive grating.
Further, it is desirable for the refractive lens and the diffractive grating to have total longitudinal chromatic aberration such that the backfocus increases as the wavelength of the incident light increases. In such a case, it is advisable that the following condition (2) is satisfied;
−1<&Dgr;
CA/&Dgr;SA<
0 (2)
where &Dgr;CA is a movement of a paraxial focal point with wavelength shift and &Dgr;SA is a variation of spherical aberration for marginal rays with wavelength shift. In special cases, the objective lens satisfies the following condition (2′);
−0.7<&Dgr;
CA/&Dgr;SA<−
0.5. (
2′)
The diffractive grating of the objective lens according to the present invention may have a positive paraxial power, and it is desirable to satisfy the following condition (3);
40.0<
f
D
/f<
140.0 (3)
where f
D
is the focal length of the diffractive grating that is defined as the following equation;
f
D
=1/(−
P
2
×2&lgr;).
REFERENCES:
patent: 4815059 (1989-03-01), Nakayama et al.
patent: 5629799 (1997-05-01), Maruyama et al.
patent: 5633852 (1997-05-01), Maruyama et al.
patent: 5796520 (1998-08-01), Maruyama
patent: 5838496 (1998-11-01), Maruyama et al.
patent: 5883744 (1999-03-01), Maruyama
patent: 5914822 (1999-06-01), Maruyama
patent: 0840144 (1998-05-01), None
patent: 11-337818 (1999-10-01), None
Asahi Kogaku Kogyo Kabushiki Kaisha
Greenblum & Bernstein P.L.C.
Mack Ricky
LandOfFree
Objective lens for optical pick-up does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Objective lens for optical pick-up, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Objective lens for optical pick-up will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2604609