Optical: systems and elements – Lens – With reflecting element
Reexamination Certificate
2000-05-15
2001-07-03
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
With reflecting element
C369S112040
Reexamination Certificate
active
06256154
ABSTRACT:
TECHNICAL FIELD
This invention relates to a catadioptric lens usable with advantage for exploiting evanescent light. This invention also relates to an optical head and an optical recording and/or reproducing apparatus employing the catadioptric lens.
BACKGROUND ART
As a technique for realising high recording density of an optical recording medium, such a technique has been devised which exploits evanescent light to enable recording and/or reproduction at an extremely small recording pit not larger than the diffraction threshold. In recording and/or reproducing an optical disc using the evanescent light, the incident light beam to a lens is converged on an end face of the lens such that its major portion undergoes total reflection on the lens end face. If the distance between the lens end face and the optical recording medium is made sufficiently narrow, a portion of the evanescent light is coupled with the optical recording medium so as to be taken out to outside the lens to enable recording and/or reproduction exploiting the evanescent light.
The distance for which the evanescent light can be coupled in this manner in air is on the order of 100 nm if the numerical aperture NA of the lens is 1.5. Therefore, if recording and/or reproduction of an optical recording medium is to be performed using the evanescent light, the distance between the lens end face and the optical recording medium needs to be maintained at a level not larger than approximately 100 nm. This can be realized using the flying head technique used e.g., in a magnetic disc.
That is, if, with the use of the flying head technique used in a magnetic disc, the distance between the lens end face and the optical recording medium is maintained at approximately 100 nm or less, a portion of the evanescent light is coupled with the optical recording medium to enable recording and/or reproduction at an extremely small recording pit not larger than the limit of diffraction.
The technique of exploiting a catadioptric lens for recording and/or reproduction exploiting this evanescent light has been proposed by C. W. Lee of Samson Electronics Inc. at an optical data storage meeting in USA in May 1998.
However, analysis of lens data of the catadioptric lens proposed by C. W. Lee reveals that correction of the coma aberration is not optimum, such that use of this catadioptric lens raises the following problem:
That is, if the evanescent light is to be used, the incident light beam needs to be converged with a small spot diameter on the lens end face. However, since the coma aberration can be corrected only incompletely with the catadioptric lens, such that, if the angle of incidence of the light beam exceeds ±1°, due to an error in the mounting angle of the catadioptric lens, the coma aberration is produced outstandingly, such that it becomes impossible to converge the incident light beam on the lens end face with a small spot diameter.
Moreover, in a near-field optical system, exploiting the evanescent light, it may be estimated that the effect of the coma aberration presents itself more significantly than if a light beam is converged in a far-field optical system with a small spot diameter. Therefore, if assumed that the catadioptric lens is used in the near-field optical system, it may be premeditated that, when mounting the catadioptric lens, the error in the mounting angle thereof needs to be appreciably smaller than ±1°.
That is, since the correction of the coma aberration is incomplete in the catadioptric lens, extremely high precision is required in mounting the catadioptric lens. In particular, it is extremely difficult to realize the mounting at a precision which permits use of the catadioptric lens in the near-field optical system. If this could be achieved, the manufacturing cost is necessarily prohibitive.
Disclosure of the Invention
An object of the present invention, proposed with the above-described status of the art in mind, is to provide a catadioptric lens in which the coma aberration has been corrected nearly completely for use in exploiting the evanescent light. Another object is to provide an optical head and an optical recording and/or reproducing apparatus exploiting this catadioptric lens.
A catadioptric lens according to the present invention is such a lens in which a first surface is a concave refractive surface, a second surface is a plane mirror, and a third surface is a concave non-spherical mirror, arranged co-axially with the concave refractive surface of the first surface, and in which a parallel incident light beam is converged through an aperture provided in the plane mirror of the second surface on a plane of total reflection of the fourth surface.
Let r
1
and r
3
be the radius of curvature of the first surface and the center radius of curvature of the third surface, and let d be an optical length from the first surface to an apex of the third surface. Also, let f
a
(N) and f
b
(N) be coefficients represented as functions of the refractive index N of a lens medium, and also let a condition satisfying the sine condition be as shown in the following equation:
d/r
3
=f
a
(
N
)
r
1
/r
3
×f
b
(
N
) (1).
Moreover, let the value of the radius of curvature r
1
of the first surface and the value of the optical length d from the first surface to the apex of the third surface, when the above equation (1) is satisfied, be R and D, respectively.
Then, the radius of curvature r
1
, of the first surface is R±1% and the optical length d from the first surface to the apex of the third surface is D±0.5%.
In the catadioptric lens of the present invention, a lens may be arranged upstream of the first surface. By providing the lens upstream of the first surface, the diffused light may be turned into collimated light which then is allowed to fall on the first surface. The lens arranged upstream of the catadioptric lens is preferably a meniscus convex lens.
The catadioptric lens according to the present invention may be configured so that each of the first to third surfaces is constituted by a first lens medium and the fourth surface is constituted by a second lens medium and so that the second lens medium is bonded to the plane containing the second surface constituted by the lens medium. A magnetic field generating coil may be buried in the second lens medium.
The catadioptric lens according to the present invention may be configured so that third surface is a routine non-spherical surface represented by the equation
X=
(
Y
2
/R
)/[1+{1−(1 +
K
)(
Y/R
)
2
}
½
]+AY
4
+BY
6
+CY
8
+DY
10
(2)
where, as to the third surface, X is the depth from the apex of the surface, Y is the height from the optical axis, R is the center radius of curvature, K is the conical coefficient, A is the non-spherical coefficient of the term Y
4
, B is the non-spherical coefficient of the term Y
6
, C is the non-spherical coefficient of the term Y
4
, and D is the non-spherical coefficient of the term Y
10
.
In the catadioptric lens of the present invention, the refractive index of the lens medium is preferably not less than 1.4 in the working wavelength range. By employing a lens medium having a larger refractive index, it is possible to reduce the diameter of the light spot formed on the fourth surface.
With the catadioptric lens according to the present invention, the coma aberration, which is the asymmetric aberration, is corrected nearly completely, such that the off-axis aberration is only the astigmatic aberration which is the symmetric aberration. Thus, with the catadioptric lens of the present invention, an ideal light spot can be formed on the end face of the lens within an operating range within which the effect of the astigmatic aberration is tolerated, that is, within the range of ±3° of the light beam incident angle.
An optical head according to the present invention is such an optical head provided with the above-described catadioptric lens of the present invention as an objective lens
Kubota Shigeo
Watanabe Kenjiro
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Sony Corporation
Sugarman Scott J.
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