Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
2003-01-21
2004-11-16
Boutsikaris, Leo (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S565000, C359S571000, C359S574000, C369S112030, C369S112080
Reexamination Certificate
active
06819491
ABSTRACT:
TECHNICAL FIELD
This invention relates to an objective lens unit for an optical pickup used for a disc driving device illuminating the laser light on an optical recording medium for recording and/or reproducing the information, an optical pickup employing this objective lens unit and to a disc driving device employing this objective lens unit. Specifically, this invention relates to an objective lens unit capable of converging the laser light close to the diffraction limit on a recording surface of the optical recording medium, an optical pickup employing this objective lens unit, and to a disc driving device employing this optical pickup.
BACKGROUND ART
As a recording medium for information signals, a non-contact optical recording medium, having a high recording density, and adapted for illuminating the laser light on the recording surface for recording and/or reproducing the information, is finding widespread application. Of the various types of the optical recording medium, a disc-shaped optical recording medium, referred to below as an optical disc, is used most extensively because of ease in retrieving the information recorded thereon.
An optical disc has concentric recording tracks or a spirally-shaped recording track. The separation between neighboring recording tracks or neighboring turns of the recording track, or a track pitch, is approximately 1.6 &mgr;m in the case of, for example, a Compact Disc (CD). In a DVD (Digital Video Disc/Digital Versatile Disc), developed more recently, this track pitch is of a narrower value of 0.74 &mgr;m, which means appreciably improved recording density of the information signals. For radiating the laser light on an optical disc in which the track pitch is reduced to improve the information recording density for recording and/or reproducing information signals, such as DVD, it is necessary to form a beam spot, smaller in size than for an optical disc with a larger track pitch, on its recording surface.
The diameter of the beam spot converged by the objective lens unit is proportionate to the to the design diameter of the laser light and inversely proportionate to the numerical aperture NA of the laser light. Thus, for diminishing the diameter of the beam spot illuminated on the optical disc, it is necessary to increase the numerical aperture of the objective lens unit and to reduce the wavelength of the laser light.
On the other hand, since a high energy laser light is required for recording the information by a phase change system, or the like. For reducing the laser noise by the reflected laser light, the driving power is varied by a method of superposing the high frequency on the voltage or the driving current of the laser light emitting device to vary the laser light wavelength in a short period. Thus, in an optical pickup adapted for illuminating the coherent laser light on the optical disc, chromatic aberration ascribable to wavelength variation on the order of several nm is produced to enlarge the beam spot on the optical disc.
Meanwhile, the optical pickup is provided with an objective lens unit
201
for converging the laser light
200
on the signal recording surface of the optical disc. This routine objective lens unit
201
is formed by a single lens molded of glass and has an aspherical surface
201
a
having high light converging power, as shown in FIG.
1
.
Referring to the charts of
FIGS. 2A
,
2
B and
2
C, showing the spherical aberration astigmatic aberration and distortion aberration, respectively, it may be seen that the chromatic aberration on the order of ±0.6 &mgr;m
m is produced against wavelength variation of ±2 nm, even with the use of the aspherical surface
201
a.
In the aberration charts of
FIGS. 2A
to
2
C, solid lines, dotted lines and chain-dotted lines depict the values for 405 nm, 403 nm and 407 nm, respectively, whereas, in
FIG. 2B
showing the astigmatic aberration, thick and thin lines depict a sagittal image surface and a tangential image surface, respectively.
For recording the information on an optical disc, in which the track pitch of the recording track is narrowed for increasing the recording density, it is desirable to converge the laser light close to the diffraction limit by the objective lens unit to form a beam spot of a smaller diameter. With the single lens type objective lens unit
201
, produced as conventionally by casting glass with the aspherical surface
201
a
, it is difficult to converge the laser light close to the diffraction limit because of the chromatic aberration distortion aberration.
DISCLOSURE OF THE INVENTION
In view of the above depicted status of the art, it is an object of the present invention to provide an objective lens unit that may be used with advantage for an optical pickup used for recording the information on an optical recording medium, in which the track pitch is narrowed to raise the recording density, and for reproducing the information recorded on the optical recording medium. It is a more specific object of the present invention to provide a small-sized objective lens unit that is capable of effectively correcting the chromatic aberration and converging the light beam close to the diffraction limit.
It is another object of the present invention to provide an optical pickup employing an objective lens unit that is capable of effectively correcting the chromatic aberration and converging the light beam close to the diffraction limit, and an optical disc employing this optical pickup.
For accomplishing the above object, the present invention provides an objective lens unit for an optical pickup including a resin layer having, sequentially from an object side, a first surface as an aspherical surface and a second surface as an aspherical surface, at least one of the first surface and the second surface including a diffractive surface, and a lens of glass having the second surface as an aspherical surface and a third surface as an aspherical surface. The lens unit is corrected for the chromatic aberration on an image surface on the optical axis with respect to the light of a reference wavelength not larger than 420 nm within several nm of the reference wavelength, with the numerical aperture of the lens unit being not less than 0.8. With this objective lens unit, the light can be converged on the image surface close to the limit of diffraction.
According to the present invention, the base surface defining the objective lens unit necessarily has a value of the aspherical coefficient, so that the diffraction type lens can be formed to an optically well-balanced stabilized state.
With the objective lens unit of the present invention, the aspherical surface of the first surface and the aspherical surface of the second surface are of the same base curvature and the same aspherical coefficient, so that the resin layer is of the constant film thickness to increase stability against changes in temperature. With the objective lens unit of the present invention, the resin layer, the first surface or the second surface of which is the diffractive surface, is of the transmission phase type, with the first surface or the second surface being of the blazed shape, so that the numerical aperture of the lens unit may be increased without diminishing the radius of curvature of the base surface of the lens unit. Since the first or second surface, as the diffractive surface, is of the blazed shape, it is possible to eliminate diffraction efficiency deteriorating factors, such as non-machinable portions, or transmittance deteriorating factors, such as a serrated shape, during the forming of the diffraction surface.
With the objective lens unit of the present invention, an aperture is formed in the outer rim of the first surface so as to allow usage of only the lens surface portion exhibiting desirable optical properties and so as not to allow usage of the lens rim to suppress production errors to stabilize the performance in assembling the lens unit.
With the objective lens unit of the present invention, in which the design degree of orders is not less
Hineno Satoshi
Takahashi Toyokazu
Boutsikaris Leo
Kananen Ronald P.
Rader & Fishman & Grauer, PLLC
Sony Corporation
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