Optical: systems and elements – Lens – Including a nonspherical surface
Reexamination Certificate
2002-04-19
2003-07-22
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
Including a nonspherical surface
C369S112030
Reexamination Certificate
active
06597519
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an objective lens which is used for an optical pick-up apparatus, optical pick-up apparatus and optical information recording reproducing apparatus, and particularly to, by using an objective lens which is an single lens at the definite magnification, an objective lens of the optical pick-up apparatus by which recording and/or reproducing of the optical information recording medium can be conducted, optical pick-up apparatus, and optical information recording reproducing apparatus.
To the optical information recording medium such as a DVD, an optical pick-up apparatus by which the information can be recorded at high density, or reproduced, is developed, and used in various purpose of uses. In such the optical pick-up apparatus, there are various requirements such as the compactness of structure, or the reliability at the time of the environmental temperature change.
In this connection, in the optical pick-up apparatus, as the objective lens by which the information recording light is light converged onto the information recording surface of the optical information recording medium, a material in which the plastic material is a raw material, is used for many cases, because it is advantageous for the mass production. However, in the temperature change of the refractive index, it is well known that the plastic material is about 2 digits larger than the glass material.
Herein, when the environmental temperature of the optical pick-up apparatus having the objective lens formed of the plastic material is increased, and the refractive index of the objective lens is changed, it is well known that the spherical aberration is deteriorated, as the light converging optical system. For example, in the present DVD optical pick-up apparatus, an objective lens of a plastic lens material of the numeral aperture NA is 0.60, and the image formation magnification m=0 is widely spread, however, even when the objective lens is designed so that it becomes no aberration at the focal distance is 3 mm, and the temperature change of the refractive index of the plastic material is dn/dT=−0.00012 (/° C.), and the refractive index of the plastic material at the wavelength of the light source &lgr;=650 nm, is n=1.53, when the temperature of the optical pick-up apparatus is increased by 30° C. from the room temperature, the refractive index of the objective lens is 1.5264, thereby, the residual aberration of 0.033 &lgr;rms is generated in the calculation.
Generally, it is said to be necessary that it is suppressed lower than the diffraction limit of the optics (Marèchal's criterion 0.07 arms) in the whole optical system, and because, other than the objective lens, there are various factors to generate the aberration, it is said to be initially important that the aberration generated by the environmental temperature change is suppressed as small as possible.
The spherical aberration deterioration &dgr;SA/&dgr;T when the refractive index is changed by the environmental temperature change, is expressed by the following expression,
&dgr;
SA/&dgr;T∝f
(1−
m
)
4
NA
4
·(
dn/dT
)/&lgr;. (1)
Where, the focal distance at the infinity object of the objective lens is f, the numerical aperture on the optical information recording medium side is NA, the wavelength of the light source is &lgr;, the changed amount of the refractive index of the objective lens at the time of the increase of temperature 1° C. is dn/dT, and the image formation magnification is m.
In the optical pick-up apparatus to conduct the recording or reproducing of the information onto the high density optical information recording medium such as the recent DVD, there is a tendency that the numerical aperture NA is further increased. Further, in order to secure the compactness of the optical pick-up apparatus, a trial that it is used at the definite magnification and the definite degree is increased, is conducted. On the other hand, according to the expression (1), the more the numerical aperture NA of the objective lens is increased, or for the very objective lens used in the specification in which the definite degree of the optical system is the more increased, it can be understood that the spherical aberration deterioration becomes conspicuous. For example, as described above, at f=3 mm, m=0, when the numerical aperture is increased from 0.60 to 0.85, the aberration is deteriorated in proportional to 4-th power of NA, and the approximation value of the spherical aberration at the time of 30° C. temperature rise from the room temperature (for example, 25° C.) is 0.12 &lgr;rms, and it is presumed that it largely exceeds Marèchal's criterion. Further, when the light source such as the blue laser developed recently is used, the further high densification of the information can be expected, however, the light source wavelength &lgr; is more reduced to the short wavelength thereby, it can be said that the problem of the spherical aberration deterioration to the temperature change is increased to the more severe direction.
To such the problem, from the expression (1) in the same manner, it can be seen that, when the focal distance f is reduced, the spherical aberration deterioration to the temperature change can be suppressed a certain degree. For example, even when the numerical aperture is increased from NA 0.60 to NA 0.85, when the focal distance is decreased to ¼, the same temperature characteristic as before the numerical aperture increase can be obtained.
However, when the focal distance f is reduced, the image height characteristic becomes disadvantageous. This is because, when the obtaining of the same image height is tried, the incident angle onto the objective lens is increased. The more the incident angle is increased, the more the astigmatism or coma is deteriorated. Accordingly, in the image height characteristic, it is preferable that the focal distance is larger.
SUMMARY OF THE INVENTION
In view of the above problems, the present invention is attained, and the object of the present invention is, even in the optical pick-up apparatus in which the numerical aperture NA is increased, or the light source wavelength is reduced, to provide an objective lens which is an objective lens of a single lens having the good temperature characteristic and by which the whole of the optical system which is well-balanced with the image height characteristic can be made compact, optical pick-up apparatus by using it, and optical information recording reproducing apparatus.
The above object of the invention can be attained by the following structures.
An objective lens for an optical pick-up apparatus described in (1), which has the light source of the wavelength &lgr;, and objective lens to conduct the recording and/or reproducing of the information by image forming a luminous flux from the light source onto the optical information recording medium, the objective lens for an optical pick-up apparatus is characterized in that: the objective lens is a single lens, and the focal distance f at the infinite object of the objective lens, numerical aperture NA on the optical information recording medium side, wavelength &lgr; of the light source, changed amount dn/dT of the objective lens refractive index at the time of 1° C. increase of the temperature, and image formation magnification m respectively satisfy the following expressions,
0.1
<f<
1 (1)
0.50
<NA<
0.90 (2)
350 nm<&lgr;<850 nm (3)
{fraction (1/10)}<|
m|<
⅓ (4)
0.0<((1
−m
)
NA
)
4
×f×|dn/dT|/&lgr;<
0.2 (5)
and when the aberration of the objective lens when the temperature of whole the optical pick-up apparatus is increased from the room temperature (for example, 25° C.) by 30° C., is &dgr;Wtemp, and the aberration at the image height of the objective lens Y=0.02 mm is &dgr;Wheight, it is that
0.0&lgr;rms<&dgr;Wtemp&pl
Collins Darryl J.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Konica Corporation
Sugarman Scott J.
LandOfFree
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