Optical head device, optical information apparatus, and...

Details Optical head device, optical information apparatus, and... Optical head device, optical information apparatus, and...

1998-04-16

2001-09-18

Hindi, Nabil (Department: 2753)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S044410, C369S112150

Reexamination Certificate

active

06292441

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical head device for recording information on an information medium or reproducing or erasing information which has been recorded on an information medium; an optical information apparatus including an optical head device; and a method for detecting a tracking error.
2. Description of the Related Art
In recent years, improvements have been made in optical disks so that various optical disks have been developed, e.g., read/write optical disks, read-only (ROM) optical disks. Against such backgrounds, optical head devices have been proposed which are capable of reproducing information carried on various kinds of optical disks. Such an optical head device is disclosed in, for example, Japanese Laid-Open Publication No. 8-022624.
Hereinafter, a conventional optical head device
600
will be described with reference to FIG.
18
.
FIG. 18
illustrates a conventional optical head device
600
.
FIG. 18
shows x, y, and z axes, which are employed for clarifying spatial directions in the figure. Unless otherwise specified, it is assumed that each of the x, y, and z axes represents the same direction throughout the present specification and the figures. In the case where one or more mirror faces are present in an optical system, the x, y, and z axes will represent axes of coordinates in a mirror image created on such a mirror face throughout the specification and the figures.
The optical head device
600
includes a LD-PD unit
160
, a collimate lens
102
, a polarization anisotropic hologram
180
(hereinafter referred to as “PA holo-gram”), a ¼ wavelength plate
115
, and an object lens
103
.
The LD-PD unit
160
includes a light source
2
for emitting a light beam L
0
, a light detector
191
for receiving a light beam L
1
reflected from an information medium
105
, and a light detector
192
for receiving a light beam L
2
reflected from the information medium
105
. The light source
2
and the light detectors
191
and
192
take specific relative positions with respect to one another in the LD-PD unit
160
.
The optical head device
600
further includes a support member
106
for supporting the PA hologram
180
, ¼ wavelength plate
115
, and the object lens
103
, as well as a driving section
112
for driving the support member
106
along the z-axis direction.
The information medium
105
is positioned so that the tangential direction thereof substantially coincides with the y-axis direction.
Hereinafter, the operation of the optical head device
600
will be described.
The light source
2
emits the linear polarization light beam L
0
, which is polarized along the x-axis direction. The light beam L
0
emitted from the light source
2
is converted into collimated light through the collimate lens
102
. Thereafter, the light beam L
0
is transmitted through the PA hologram
180
without being refracted by the PA hologram
180
. The polarization state of the light beam L
0
is changed from linear polarization into circular polarization through the ¼ wavelength plate
115
. The light beam L
0
is focused on the information medium
105
through the object lens
103
.
The light beam reflected from the information medium
105
travels in the opposite direction from the direction of travel of the light beam L
0
emitted from the light source
2
, so as to travel back through the ¼ wavelength plate
115
. The polarization state of this light beam is changed from circular polarization into linear polarization through the ¼ wavelength plate
115
. The polarization direction of the linear polarization light is a direction (i.e., the y-axis direction) which is perpendicular to the polarization direction of the light beam L
0
emitted from the light source
2
. The light beam which has been transmitted through the ¼ wavelength plate
115
is split by the PA hologram
180
into +1st diffracted light L
1
and −1st diffracted light L
2
. The +1st diffracted light L
1
enters the light detector
191
. The −1st diffracted light L
2
enters the light detector
192
. The light detectors
191
and
192
detect servo signals and/or a signal representing the information recorded on the information medium
105
.
Hereinafter, the details of the signal detection process will be described with reference to
FIGS. 19 and 20
.
FIG. 19
schematically represents the pattern of the PA hologram
180
. The PA hologram
180
is divided into four regions (regions A, B, C, and D) by a line which intersects the center of the PA hologram
180
and extends in parallel to the x axis and a line which intersects the center of the PA hologram
180
and extends in parallel to the y axis. Herein, the center of the PA hologram
180
coincides with its optic axis.
Each of regions A, B, C, and D is subdivided into a plurality of strip regions by a plurality of lines extending in parallel to the y axis. For example, regions Ab and regions Af alternate in region A; regions Bb and regions Bf alternate in region B; regions Cb and regions Cf alternate in region C; and regions Db and regions Df alternate in region D.
In
FIG. 19
, regions which are indicated by the same region name have a hologram pattern generated from the same function; regions which are indicated by different region names have hologram patterns generated from different functions. As a result, regions indicated by different region names diffract light beams in different directions or wave fronts.
FIG. 20
illustrates a light-sensitive surface
191
a
of the light detector
191
and a light-sensitive surface
192
a
of the light detector
192
provided in the LD-PD unit
160
.
The light-sensitive surfaces
191
a
and
192
a
are disposed in a symmetrical manner with respect to an emission point (i.e., a point from which the light beam L
0
is emitted) or a point equivalent to the emission point. In
FIG. 20
, point P represents the emission point or a point equivalent to the emission point.
The light-sensitive surface
191
a
is divided into three regions (regions FE
1
, FE
2
, and FE
3
) by two lines L
1
X
1
and L
1
X
2
extending in parallel to the x axis.
The light-sensitive surface
192
a
is divided into four regions (regions TEa, TEb, TEc, and TEd) by a line L
2
X
1
extending in parallel to the x axis and a line L
2
Y
1
extending in parallel to the y axis.
Due to the diffraction action of the PA hologram
180
, the light beam reflected from the information medium
105
is converted into the +1st diffracted light L
1
and the −1st diffracted light L
2
. The +1st diffracted light L
1
enters the light-sensitive surface
191
a,
whereas the −1st diffracted light L
2
enters the light-sensitive surface
192
a.
Light spots are created on the light-sensitive surface
191
a
corresponding to the light beams entering the respective regions of the PA hologram
180
. The light spots L
1
Ab, L
1
Af, L
1
Bb, L
1
Bf, L
1
Cb, L
1
Cf, L
1
Db, and L
1
Df are formed by light beams entering regions Ab, Af, Bb, Bf, Cb, Cf, Db, and Df, respectively, of the PA hologram
180
.
Light spots are created on the light-sensitive surface
192
a
corresponding to the light beams entering the respective regions of the PA hologram
180
. The light spots L
2
Ab, L
2
Af, L
2
Bb, L
2
Bf, L
2
Cb, L
2
Cf, L
2
Db, and L
2
Df are formed by light beams entering regions Ab, Af, Bb, Bf, Cb, Cf, Db, and Df, respectively, of the PA holo-ram
180
.
The PA hologram
180
is designed so as to satisfy the following conditions (1) to (4) in a state (defined as the “focused state”) where the size of the light spot converged on the recording face of information medium
105
becomes minimum:
(1) a light beam entering region Ab (
FIG. 19
) is converged at a point behind the light-sensitive surface
191
a,
whereas a light beam entering region Af (
FIG. 19
) is converged at a point in front of the light-sensitive surface
191
a
;
(2) a light beam entering region Bb (
FIG. 19
) is converged at a point behind the light-sensitiv

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