Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2000-03-20
2003-07-15
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S112240
Reexamination Certificate
active
06594204
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a lens holder for supporting an objective lens loaded on an optical head used for recording and/or reproducing an information recording medium, such as an optical disc. This invention also relates to a method for manufacturing such a lens holder, a metal die used in manufacturing a lens holder and an objective lens device constituted by the objective lens and the lens holder.
2. Description of the Related Art
The information recording mediums, such as a replay-only optical disc, a phase change disc, a magneto-optical disc or an optical card, are finding extensive use for storing the image information, speech information or data for computer programs. Thus, the demand for raising the recording density and the recording capacity of these information recording mediums is increasing year by year.
For increasing the recording density of the information recording medium, it is effective to increase the numerical aperture NA of the objective lens or to shorten the light emission wavelength of the light source. For example, in an optical head for a Compact Disc (CD), which is a digital optical disc or recording mainly music signals, the numerical aperture NA of the objective lens and the light emission wavelength of the light source are 0.45 and 780 nm, respectively, whereas, in a so-called optical head for the Digital Versatile Disc (DVD), numerical aperture NA of the objective lens and the light emission wavelength of the light source are 0.6 and 650 nm, respectively. With this DVD, the recording density is improved over that with the CD to render it possible to record picture signals.
The objective lens, used for recording and/or reproducing the information for the routine CD or DVD, is prevalently a single non-spherical lens molded from glass or plastics. This single non-spherical lens, carried by a lens holder, makes up an objective lens device, and is loaded on the optical head. For example, the single non-spherical lens is bonded to and mounted on a reference surface of the lens holder. The lens holder, carrying this single non-spherical lens, is loaded with pre-set accuracy on the optical head.
Recently, a higher recording density and a larger recording capacity of the information recording medium are desired, such that a larger numerical aperture NA of the objective lens and the wavelength of the light radiated from picture signals shorter than e.g., 650 nm are required.
It is however impossible to produce the single non-spherical lens with the numerical aperture NA not smaller than 0.75 because of difficulties in metal die machining and in controlling the eccentricity at the time of lens molding. That is, in machining a metal die for molding the single non-spherical lens with the numerical aperture NA not less than 0.75 the tilt angle of the lens surface with respect to the optical axis in the vicinity of the lens is less than 40°, so that machining becomes difficult in consideration of the size of the distal end of the cutting edge, such as a diamond byte. Also, if the curvature of the lens surface is increased, the sag (depth along the optical axis from the apex of the lens surface to the outer rim of the lens) is increased to render metal die machining difficult. For this reason, it is retained to be difficult to constitute an objective lens with a numerical aperture NA not less than 0.75 as a single lens.
Recently, as a technique of realizing an objective lens with the numerical aperture NA of not less than 0.75, a double set objective lens has come to be used. With this double set objective lens, the objective lens is constituted by plural lenses to diminish the refractive power of each lens. This renders it possible to increase the radius of curvature of the non-spherical lens surface to manufacture an objective lens with the numerical aperture NA not less than 0.75.
However, with the double set objective lens, with a large numerical aperture NA, an extremely high precision is required as the relative lens position accuracy when assembling the plural lenses into one set. For example, micron order accuracy is required of the eccentricity and spacing between the lenses, whilst the minute order accuracy is required of the lens tilt. By making three-dimensional position adjustment, it is sufficiently possible to assemble the lens to meet this precision requirement. However, this three-dimensional position adjustment is in need of an expensive jig and an advanced position adjustment technique and hence does not lend itself to a mass production process.
As means for positioning the lenses to assemble them to a sole lens unit, it may be envisaged to assemble the respective lenses in the lens holder whose reference portion having a reference surface for mounting the lenses thereon has been formed to high precision.
The reference portion formed in the lens holder is set to a shape capable of positioning the respective lenses as to the lens offset, tilt and the lens-to-lens separation. The lens offset, tilt and separation are referred to below as three elements. By forming the reference portion to high precision with respect to these three elements, the objective lens can be assembled to high precision without requiring the position adjustment of the respective lenses. That is, the objective lens can be assembled to high precision solely by sufficiently optimizing the shape designing of the lens holder.
The shape of the lens holder is hereinafter explained inclusive of the presumed manufacturing method.
FIGS. 15 and 16
show the structure of an objective lens device
201
having a first lens
202
and a second lens
203
of the double set objective lens set assembled into a lens holder
204
.
The first lens
202
is a lens into which falls the laser light radiated from a light source, not shown. The first lens
202
has its mid portion facing the second lens
203
, referred to below as the radiating surface, is formed as a non-spherical lens surface
202
a
, on an outer rim of which is formed a planar portion perpendicular to the optical axis. On the other hand, the surface to which falls the light radiated from the light source, and which is the opposite surface of the first lens
202
, has its mid portion formed to a non-spherical lens surface
202
b
. On the outer rim of the lens surface
202
b
is formed a planar portion perpendicular to the optical axis. The above-mentioned opposite surface is referred to below as the incident surface.
The second lens
203
is a lens of the double set objective lens set which opposes to a digital optical disc, such as a phase change optical disc or a magneto-optical disc. The surface of the second lens
203
facing the disc, not shown, referred to below as the facing surface, is formed to a planar shape, whilst the mid portion of the surface facing the first lens
202
as the opposite surface, referred to below as the incident surface, is formed as a non-spherical lens surface
203
b
. On the outer rim of the lens surface
203
b
is formed a planar section perpendicular to the optical axis.
The side of the objective lens device
201
on which falls the light radiated from the light source is termed an object point side, whilst the side of the objective lens device
201
lying along the disc arraying direction, that is the side on which an image point is formed by the objective lens device
201
by the light radiated from the light source, is termed an image point side. Thus, with the first and second lenses
202
,
203
, non-spherical lens surfaces are formed on the object side.
The lens holder
204
is formed substantially to a toroidal shape. On the inner peripheral side of the object point is formed a first mounting portion
204
a
carrying the first lens
202
, whereas, on the inner periphery on the image point side, there is formed a second mounting portion
204
b
carrying the second lens
203
.
The first mounting portion
204
a
is formed step-wise as one with the inner rim of the image point side aperture, and is formed as one from a first axial refere
Maeda Fumisada
Osato Kiyoshi
Yamamoto Kenji
Sony Corporation
Tran Thang V.
Vuong Bach
LandOfFree
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