Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1998-09-24
2002-07-02
Korzuch, William (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S044230, C369S013330, C369S094000
Reexamination Certificate
active
06414931
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an objective lens driving device for driving an objective lens for condensing a light beam on a signal recording surface of an optical recording medium, and an optical head having such lens driving device.
2. Description of the Related Art
Among recording mediums for information signals, there is an optical recording medium for recording/reproducing the information signals using an optical head. This recording medium may be enumerated by an optical disc, a magneto-optical disc or a phase-change optical disc. For increasing the information recording density of the optical recording medium, it is effective to increase the numerical aperture NA of the objective lens of the optical head. By increasing the numerical aperture NA of the objective lens, the spot size of the light beam condensed on the signal recording surface of the optical recording medium can be decreased to improve the information recording density.
Meanwhile, an objective lens of the conventional optical head, used for collecting the light from a light source on a signal recording surface, includes a sole lens. That is, the objective lens used in the conventional optical head is a so-called single-lens type lens.
If, in this optical head, the numerical aperture NA of the objective lens is to be increased, the single-lens type lens needs to have an extremely large refractive power. If the refractive power of the single-lens type lens is to be increased, the radius of curvature of the reflective surface needs to be extremely small, while refractive surfaces need to be positioned extremely accurately. Thus, if the objective lens of the optical head is a single-lens type lens, the numerical aperture NA cannot be raised beyond approximately 0.6.
In an optical recording medium, there is usually provided on the signal recording layer a layer adapted for protecting the signal recording surface (termed herein a cover glass). The light beam from the optical head falls on the signal recording surface via this cover glass. If the cover glass thickness deviates from its prescribed value, spherical aberration is produced. If the spherical aberration of a higher order is disregarded, this spherical aberration is proportionate to the numerical aperture NA raised to the fourth power. Therefore, if the numerical aperture NA of the objective lens is increased, the tolerance value of the thickness error of the cover glass is decreased significantly. That is, if the numerical aperture NA of the objective lens is increased, there is raised an extremely severe demand on the thickness tolerance of the cover glass to render it difficult to manufacture the optical recording medium.
As an optical head for solving the problem inherent in the conventional optical head, there has been proposed in U.S. Pat. No. 5,712,842 or U.S. Pat. No. 5,764,613 an optical head in which an objective lens is constituted by two lenses
101
,
102
, as shown in FIG.
1
.
In this optical head, the objective lens is constituted by a first lens
101
on which falls the light beam from the light source and a second lens
102
on which falls the light beam converged by the first lens
101
. The light beam from the light source is condensed by the first lens
101
and by the second lens
102
in this order before being illuminated on the optical recording medium. When the light beam is condensed by the first and second lenses
101
,
102
so as to be then illuminated on the optical recording medium, the first and second lenses
101
,
102
are moved by a biaxial actuator in a direction perpendicular to the optical axis in order o effectuate tracking servo. In addition, the first and second lenses
101
,
102
are moved by the biaxial actuator along the optical axis in order to effectuate focussing servo.
In this optical head, if the refractive power of each of the first and second lenses
101
,
102
is small, the two-lens system, consisting of the combination of the two lenses
101
,
102
, can have a large value of the numerical aperture NA. This realizes the numerical aperture exceeding 0.6, which it has been difficult to achieve with the optical head having a single-lens type lens as an objective lens.
Also, in the optical head employing the two-lens system, employing the biaxial actuator to effectuate tracking servo and focussing servo as described above, it is also possible to have a voice coil motor mounted on the second lens
102
, instead of using the biaxial actuator, and to cause the second lens
102
to be moved by this voice coil motor along the optical axis to suppress the spherical aberration.
That is, not only the two-lens type lens system, made up of the first and second lenses
101
,
102
, is moved by the biaxial actuator, but also the second lens
102
is moved along the optical axis in the manner of cancelling the spherical aberration brought about by changes in film thickness of the cover glass of the optical recording medium by the voice coil motor mounted on the second lens
102
, in order to reduce the spherical aberration.
That is, if, with the optical head having the two-lens type lens system as an objective lens, the voice coil motor is mounted on the second lens
102
to render it possible to adjust the distance between the first lens
101
and the second lens
102
, it is possible to overcome the problem that the spherical aberration is increased with increased numerical aperture of the objective lens.
If, for cancelling the spherical aberration, the second lens
102
is adapted to be movable with respect to the first lens
101
, it is necessary to increase the attenuation factor of the voice coil motor adapted for causing movement of the second lens
102
facing the optical recording medium. The attenuation factor of the voice coil motor can be increased by, for example, charging the viscous fluid. However, since the viscosity of the viscous fluid in general has temperature dependency, the viscous fluid is changed in viscosity by fluctuations in the external air temperature if the viscous fluid is charged to increase the attenuation factor of the voice coil motor, with the result that the focussing servo is rendered unstable.
For maintaining stability of focussing servo, the attenuation factor of the voice coil motor mounted on the second lens
102
needs to be increased. However, if the attenuation factor of the voice coil motor is increased, there is produced hysteresis in displacement characteristics with respect to the input current to render it impossible to realize high-speed stable operation.
Also, with the two-lens type lens system shown in
FIG. 1
, the second lens
102
facing the optical recording medium can be displaced, so that, if the second objective lens
102
collides against the optical recording medium, the distance between the first lens
101
and the second lens
102
is varied significantly, with the result that it is difficult to maintain an appropriate distance between the two lenses. Moreover, if the second lens
102
collides against the optical recording medium, there is imposed a large load on the member movably supporting the second lens
102
, such that there is the risk of destruction of the supporting member.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a lens driving device for driving an objective lens designed to converge a light beam on an optical recording medium in which the numerical aperture NA of the objective lens can be increased to suppress the spherical aberration and in which focussing servo can be effectuated speedily and stably.
It is another object of the present invention to provide an optical head having such lens driving device.
In one aspect, the present invention provides a lens driving device for driving a lens adapted for collecting the light from a light source on a signal recording surface of an optical recording medium, first driving means having a movable portion which is adapted for being moved in a direction along the optical axis of the light from the lig
Ichimura Isao
Maeda Fumisada
Ohsato Kiyoshi
Watanabe Toshio
Yamamoto Kenji
Chu Kim-Kwok
Kananen, Esq. Ronald P.
Korzuch William
Rader & Fishman & Grauer, PLLC
Sony Corporation
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