Optical: systems and elements – Lens – Including a nonspherical surface
Reexamination Certificate
2002-05-02
2003-07-08
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
Including a nonspherical surface
C359S199200, C369S112230
Reexamination Certificate
active
06590717
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an optical system for use with an optical disk, which focuses a luminous flux from a light source on an information recording surface of an optical disk such as a digital video disk, a digital audio disk, or an optical memory disk for use in a computer. The present invention also relates to an optical head device that is provided with the optical system for use with an optical disk, and to an optical drive device that is provided with the optical system for use with an optical disk.
BACKGROUND ART
Generally, in optical head devices for use with optical disks, many of the lenses used as objective lenses for recording information or reproducing recorded information by focusing light so as to form a point image at the diffraction limit on an information recording surface of an optical disk are single lenses having aspherical surfaces.
The following will describe a conventional optical head device, while referring to the drawings.
FIG. 5
is a view illustrating a schematic configuration of a conventional optical head device. As shown in
FIG. 5
, an optical path of a luminous flux emitted from a semiconductor laser
151
is redirected by a beam splitter
152
, and the luminous flux is converted into a substantially collimated light by a collimator lens
153
. The optical path of the laser beam is redirected further by a turning mirror
154
, and the laser beam is focused on an information recording surface
157
of an optical disk
156
by an objective lens
155
. Here, the objective lens
155
is actuated by an actuator
160
. The laser beam focused on the information recording surface
157
of the optical disk
156
is diffracted by recesses and projections formed on the information recording surface
157
. The laser beam reflected and diffracted by the information recording surface
157
of the optical disk
156
passes through the objective lens
155
, and its optical path is redirected by the turning mirror
154
. Then, the laser beam passes the collimator lens
153
, the beam splitter
152
, and a cylindrical lens
158
, thereby being focused on a photodetector
159
. Changes in the light quantity due to modulation at the information recording surface
157
of the optical disk
156
are detected according to electric signals from the photodetector
159
, and thus, the reading of data on the information recording surface
157
is carried out.
Here, the objective lens
155
has a wave aberration due to manufacturing errors caused during the manufacture. To consider the wave aberration, it can be separated into aberration components, which are a third-order spherical aberration, a third-order coma, a third-order astigmatism, and a high-order aberration.
Among these aberration components, the third-order coma does not occur according to design as long as lens surfaces are rotationally symmetric surfaces. However, actually, a third-order coma occurs due to, as predominant factors, the decentering (deviation in a direction perpendicular to the optical axis) between a first surface
161
of the objective lens
155
on a collimated luminous flux side and a second surface
162
thereof on a light-focusing side, as well as a tilt (relative to a surface perpendicular to the optical axis) of the first surface
161
of the objective lens
155
, or of the second surface
162
thereof, or of the both. The decentering and the tilt are produced in the manufacturing process.
Recently, higher densification of recording has been achieved, as seen in digital versatile disks (DVDs), DVD-RAMs, etc. To achieve higher densification, to form as small a light spot as possible on an optical disk is significant, and it is known that the spot diameter can be decreased by increasing a numerical aperture (NA) of a lens and by shortening a wavelength of light. The numerical aperture of a lens has increased gradually for higher densification. The NA and the wavelength for DVDs are 0.6 and 660 nm, respectively. At the present time, one disk (single-sided) is required to have a memory capacity of approximately 25 GB to record high-definition video data for 2 hours, assuming a movie and the like. This means a recording density approximately five times that of a DVD, and therefore, it is necessary to reduce the spot area to one fifth. In other words, the spot diameter has to be 0.45 time. For shortening the wavelength, laser beam sources with wavelengths in the vicinity of 405 nm has become available in the market. As it is known that the spot diameter is proportional to the NA and inversely proportional to the wavelength, a lens is required to have a NA of not less than 0.82.
Conventionally, lenses are designed, considering manufacturing errors that could occur when a lens is manufactured or assembly errors that could occur when a lens is assembled in an optical head, so that the aberration should not increase even in the case where the first and second surfaces of the lens have the decentering, as well as so that the aberration should not increase even in the case where light enters from outside the axis. One of the reasons why lenses with greater NAs, for instance, 0.82, are demanded significantly but have not yet been brought into the actual use is as follows: merely to decrease the aberration on the axis in lens designing is easy, but in the case where a lens is designed with the fabrication tolerance and the assembly tolerance taken into consideration, it is very difficult to design a single lens having sufficient tolerances.
Therefore, as a method for increasing the numerical aperture of the objective lens, a technique has been applied such that two or three lenses are combined to configure an objective lens, so as to increase the manufacturing tolerances for each lens.
A problem remaining in the objective lens is that a third-order coma generated due to a tilt of a disk increasing as the numerical aperture of the objective lens increases, and it also increases as the thickness of a transparent substrate of the disk (transparent substrate formed on the objective lens side of an information recording surface of an optical disk; also referred to as “optical disk substrate”) increases.
The thickness of the disk preferably is as small as possible from the above-described viewpoint. However, in the case where the disk is thin, the luminous flux from the objective lens has a smaller area on the surface of the transparent substrate of the disk, thereby resulting in that the recording/reproduction performance deteriorates due to dust and scratches on the surface of the transparent substrate. As a result of studies by the inventors of the present invention, it has been discovered that the luminous flux desirably has a diameter of approximately 130 &mgr;m on the disk, in order not to impair the recording/reproduction performance even in the case where dust and scratches in a size of several tens of micrometers each are present. To satisfy this requirement, in the case where the NA is assumed to be 0.82 and the refractive index of the transparent substrate of the disk is assumed to be 1.5, the transparent substrate desirably is 0.1 mm thick, which is found by calculation. Besides, for manufacturing, the thickness of the transparent substrate of approximately 0.1 mm is appropriate at the present stage, since the productivity decreases in mass production in the case of too thin disk transparent substrates. In light of the foregoing, it is adequate that the disk has a thickness of 0.1 mm.
At present, there is a possibility that the disk surface could tilt approximately 0.7 degree relative to the optical axis due to warpage of the disk or the like. When tilting of this degree occurs, a third-order coma at a level of 79 m&lgr; is generated in the case of an optical system having a numerical aperture of 0.85, a focal distance of 1.8 mm, and a transparent substrate thickness of 0.1 mm. To make a spot sufficiently small on the information recording surface and reproducible by a DVD system, it is required to suppress the third-order coma to a level of 70 m&lgr; or below. Ther
Sasano Tomohiko
Tanaka Yasuhiro
Yamagata Michihiro
Merchant & Gould P.C.
Sugarman Scott J.
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