Optical head having a phase-shifting diffraction grating

Details Optical head having a phase-shifting diffraction grating Optical head having a phase-shifting diffraction grating

1998-10-23

2001-08-28

Neyzari, Ali (Department: 2752)

Dynamic information storage or retrieval

Specific detail of information handling portion of system

Radiation beam modification of or by storage medium

C369S044370, C369S112150

Reexamination Certificate

active

06282164

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical head apparatus and in particular, to an optical head apparatus causing no offset in a track error signal even if an objective lens is shifted and capable of detecting a land/groove position.
2. Description of the Related Art
In a conventional optical head apparatus, the push-pull method is known as one of the track error signal detecting methods. The push-pull method is realized by a simple configuration of an optical system and an electric circuit but an offset is caused in the error detecting signal if an objective lens is shifted.
To cope with this, there is known a method to use a diffraction grating to generate three beams of 0-th order light, plus and minus 1
st
-order diffracted lights, so that the offset of the track error signal caused by the objective lens shift is cancelled by a difference between the 0-th order light and the plus and minus 1
st
-order diffracted lights. An optical recording medium has a land and a groove. In this method, the 0-th order light is applied to the land (or the groove) and the plus and minus 1
st
-order diffracted lights are applied to the adjacent grooves (or lands). However, in an optical recording medium having a track pitch different from a design, it is impossible to apply the three focal spots as mentioned above and accordingly, it is impossible to cancel the track error signal offset upon an objective lens shift.
Japanese Patent Publication (Unexamined) No. 9-81942 discloses a method to use a diffraction grating consisting of two regions one of which has a phase delayed by &pgr; from the phase of the other, so as to generate three beams of 0-th order light and plus and minus 1
st
-order diffracted lights so that differences between the 0-th order light and the plus and minus 1
st
-order lights are used to cancel a track error signal offset at an objective lens shift. In this method, the 0-th order light and the plus and minus 1
st
-order diffracted lights is applied to a single land (or groove). Consequently, even in an optical recording medium having a track pitch different from a predetermined design, the arrangement of the three focal spots is not changed, enabling to cancel the offset of the track error signal caused by the objective lens shift.
FIG. 17
shows a configuration of a conventional optical head apparatus using the aforementioned method.
A light emitted from a semiconductor laser
51
is made into parallel lights by a collimator lens
52
and divided by a diffraction grating
53
d
into 0-th order light and plus and minus diffracted lights. Approximately half of these lights are passed through a beam splitter
54
and focused by an objective lens
55
on a disc
56
. The three lights reflected from the disc
56
are introduced via the objective lens
55
into the beam splitter
54
, where about half of the lights is reflected to be received via a composite lens
57
by a photo detector
58
d
. The composite lens
57
consists of a convex lens and a cylindrical lens. The photo detector
58
d
is arranged in an intermediate position between two focal lines of the composite lens
57
.
FIG. 18
is a plan view of the diffraction grating
53
d
. The diffraction grating
53
d
is divided into a region
78
a
and a region
78
b
. The line of this division is a straight line in a tangential direction (parallel to the track) passing through the optical axis of the incident light
59
. The phase difference between the region
78
a
and the region
78
b
is &pgr;. Accordingly, there is a phase difference of &pgr; between the plus and minus 1
st
-order diffracted lights from the region
78
a
and the plus and minus 1
st
-order diffracted lights from the region
78
b.
FIG. 19
shows an arrangement of the focal spots on the disc
56
. The 0-th order light, the plus 1
st
-order diffracted light, and the minus 1
st
-order diffracted light respectively correspond to focal spots
79
a
,
79
b
, and
79
c
, which are arranged on a single track
61
(land or groove). The focal spots
79
b
and
79
c
have two peaks having an identical intensity in a radial direction (vertical direction to the track).
FIG. 20
shows light receiving blocks of the photo detector
58
d
and a light spot arrangement on the photo detector
58
d
. A light spot
80
a
corresponds to the 0-th order light which is received by the light receiving block divided into four light receiving sections
81
a
to
81
d
by two straight lines of tangential direction passing through the optical axis and the radial direction. A light spot
80
b
corresponds to the plus 1st-order diffracted light, which is received by a light receiving block divided into a light receiving sections
81
e
and
81
f
by a tangential line passing through the optical axis. A light spot
80
c
corresponds to the minus 1
st
-order diffracted light, which is received by a light receiving block divided into light receiving sections
81
g
and
81
h
by a tangential line passing through the optical axis. The focal spots
79
a
,
79
b
, and
79
c
are arranged in the tangential direction on the disc
56
, but the light spots
80
a
,
80
b
, and
80
c
on the photo detector
58
d
are arranged in the radial direction by the function of the composite lens
57
.
If it is assumed that outputs of the light receiving sections
81
a
to
81
h
are V
81
a
to V
81
h
, the focus error signal can be obtained from the calculation (V
81
a
+V
81
d
)−(V
81
b
+V
81
c
) according to the astigmatism. The track error signal can be obtained by the differential push-pull method as follows: {(V
81
a
+V
81
b
)−(V
81
c
+V
81
d
)}−K{(
81
e
+V
81
g
)−(V
81
f
+V
81
h
)} (wherein K is a constant). Moreover, the reproduction signal can be obtained from the calculation of V
81
a
+V
81
b
+V
81
c
+V
81
d.
FIG. 21
shows a phase change of the 0-th order light, the plus and minus 1
st
-order diffracted lights from the disc
56
caused by a position difference between the focal spot
79
a
on the disc
56
and the track
61
. The focal spot
79
a
is formed by a beam
66
d.
FIG. 21A
, case (1), the light beam
66
d
is applied to a groove
67
a
. Here, if the 0-th order light is assumed to have phase 0, the plus and minus 1
st
-order diffracted lights have a phase of −&pgr;/2. In
FIG. 21A
, case (2), the light beam
66
d
is applied to a boundary of the groove
67
a
and the land
67
b
. Here, with respect to case (1), the plus 1
st
-order diffracted light has a phase delayed by &pgr;/2, and the minus 1
st
-order diffracted light has a phase advancing by &pgr;/2. Accordingly, if the 0-th order light has a phase 0, the plus 1
st
-order diffracted light has a phase of plus and minus &pgr;, and a minus 1
st
-order diffracted light has a phase 0. In
FIG. 21A
, case (3), the light beam
66
d
is applied to the land
67
b
. Here, with respect to case (2), the plus 1
st
-order diffracted light has a phase delayed by &pgr;/2, and the minus 1
st
-order diffracted light has a phase advancing by d &pgr;/2. Accordingly, if the 0-th order light has phase 0, the plus and minus 1
st
-order diffracted lights have a phase of &pgr;/2. In
FIG. 21A
, case (4), the light beam
66
d
is applied to a boundary between the land
67
b
and the groove
67
a
. Here, with respect to case (3), the plus
1
st
-order diffracted light has a phase delayed by &pgr;/2, and the minus 1
st
-order diffracted light has a phase advancing by &pgr;/2. Accordingly, if the 0-th order light has phase 0, the plus 1
st
-order diffracted light has phase 0 and the minus 1
st
-order diffracted light has phase plus and minus &pgr;.
FIG. 21B
shows a region
82
a
containing both of the 0-th order light and the plus 1
st
-order diffracted light and a region
82
b
containing both of the 0-th order light and the minus 1
st
-order diffracted light. These regions
82
a
and
82
b
have light intensities as follows. In
FIG. 21A
, case (1), the phase difference between the 0-th order light and th

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