Tracking error detection device

Details Tracking error detection device Tracking error detection device

2001-08-06

2004-05-25

Tran, Thang V. (Department: 2653)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S044280, C369S044290

Reexamination Certificate

active

06741533

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a tracking error detecting apparatus which detects a tracking error of an optical spot that is obtained by radiating light on an optical recording medium.
BACKGROUND ART
As a method of obtaining a tracking control signal from an optical disk typified by a CD or a DVD in which information is recorded in the form of concavo-convex pits, a phase difference method has been employed in recent years.
As disclosed in Japanese Published Patent Application No. Hei.10-162381, the phase difference method is one which obtains a tracking error signal utilizing that when an optical spot deviates from the center of an information pit while the optical spot radiated on an information recording surface of an optical disk passes traversing on the information pit, a mapped image of the information pit on a photo detector (diffraction pattern) varies. That is, when the photo detector is divided in a track length direction for the information pit mapped image to see an output signal level according to respective accepted light quantities, the way of variation differs according to the direction and amount of the deviation of the optical spot from the center of the information pit. Therefore, by seeing the phase difference of the binarized signal which is obtained by binarizing the output of the photo detector with a prescribed level, a tracking error signal indicating the direction and amount of the deviation of the optical spot can be obtained.
A conventional method for detecting a tracking error will be described with reference to
FIGS. 4
to
14
.
FIG. 4
is a schematic diagram illustrating a main configuration of an optical pickup part
100
in
FIG. 4
, an astigmatic method is employed for a detection of a focus error signal.
A luminous flux radiated from a light source
1
, such as a semiconductor laser is converted into a parallel beam at a collimator lens
3
, goes through a half mirror
6
, is converged by an objective lens
4
, and is radiated on an information recording surface
51
on an optical recording medium (such as an optical disk)
5
as a small optical spot. A reflected light of the optical spot goes through the objective lens
4
, has its optical path inflected in the right hand direction of the figure by the half mirror
6
, and reaches a photo detector
2
through a convex lens
61
and a cylindrical lens
62
to be a convergent light having two focuses characteristic in the astigmatic method. Information on the optical recording medium
5
is recorded by an information pit line having unevenness.
Next, a description will be given of a method of obtaining a tracking error signal which indicates a positional deviation of the optical spot in a vertical direction against the pit line (track) in the information recording surface, by utilizing a diffraction pattern of light generated when the optical spot passes traversing on the pit.
An intensity distribution pattern (far field pattern) of the reflected light quantity of the optical spot changes according to the position of an information pit on which the optical spot passes traversing,
FIGS.
5
(
a
)-(
c
),
6
(
a
)-(
c
), and
7
(
a
)-(
c
) are diagrams exemplifying changes of the far field pattern of the reflected light quantity when the optical spot passes traversing on the pit. (a) of each figure is a diagram illustrating the physical relationship between an optical spot
12
and an information pit
13
(the center of the information pit
13
is described by a dotted line), and the optical spot
12
proceeds on the information pit
13
in a direction of arrows. (b) of each figure shows the transition of the intensity distribution pattern (far field pattern) of the reflected light quantity on the photo detector
2
, and the three patterns shown in (b) of each figure respectively represent patterns when the optical spot
12
is at three positions shown in (a). (c) of each figure shows two signals obtained from the photo detector
2
. Further, the photo detector
2
has photo acceptance units
2
a
-
2
d
, respective twos being arranged vertically and horizontally, and the two signals obtained in (
c
) of each figure are ones which are obtained as a result of adding signals, that are obtained from the four photo acceptance units
2
a
-
2
d
, for the photo acceptance units in a diagonal direction, respectively (i.e.,
2
a
+
2
d
and
2
b
+
2
c
).
For example, as shown in FIG.
5
(
a
), when the optical spot
12
passes traversing on the left of the center of the information pit
13
in the direction of movement, the pattern changes to rotate clockwise as shown in FIG.
5
(
b
), resulting in two signals out of phase as in FIG.
5
(
c
).
As shown in FIG.
6
(
a
), when the optical spot
12
passes traversing on the center of the information pit
13
, that is, the center of a track, the pattern changes symmetrically as in FIG.
6
(
b
), resulting in two signals in phase as in FIG.
6
(
c
).
As shown in FIG.
7
(
a
), when the optical spot
12
passes traversing on the right of the center of the information pit
13
in the direction of movement, the pattern changes in a counterclockwise direction as shown in FIG.
7
(
b
), resulting in two signals out of phase as in FIG.
7
(
c
).
As described above, it is proved that the transition of the field pattern changes when the optical spot deviates from the center of the information pit. The phase difference method is the one that utilizes the changes of the far filed pattern so as to detect a tracking error signal. That is, the method comprises comparing phases of two adding signals obtained from the photo detector
2
, and detecting the degree of phase advancement or delay, thereby recognizing a positional deviation between the optical spot
12
and the information pit
13
.
A conventional tracking error detecting apparatus will be described with reference to FIGS.
8
and
9
(
a
)-(
h
).
FIG. 8
is a block diagram illustrating an example of a tracking error detecting apparatus which detects a phase difference to detect a tracking error signal, and FIGS.
9
(
a
)-(
h
) diagrams of illustrating waveforms of signals denoted by (
a
)-(
h
) in FIG.
8
. Further, FIGS.
9
(
a
)-(
h
) diagrams of waveforms in a case where according to a passage of time, the optical spot
12
passes traversing on the information pit
13
, crossing from the left side to the right side in the direction of movement, that is, changing from the state in FIGS.
5
(
a
)-(
c
) to that in FIGS.
7
(
a
)-(
c
).
The photo detector
2
has the photo acceptance units
2
a
,
2
b
,
2
c
, and
2
d
, respective twos being arranged vertically and horizontally, and detects optical signals to project into respective units as a photoelectric current. The detected photoelectric current is converted into voltage signals by current/voltage conversion circuits
7
a
,
7
b
,
7
c
, and
7
d
, respectively.
Next, adder
8
a
and
8
b
adds signals which are obtained from two pairs of units in a diagonal direction of the photo detector
2
, for respective pairs. That is, an adder
8
a
adds outputs of the current/voltage conversion circuits
7
a
and
7
c
, and an adder
8
b
adds outputs of the current/voltage conversion circuit
7
b
and
7
d
. Two adding signals (a) and (b) become waveforms shown in FIGS.
9
(
a
) and
9
(
b
), respectively.
The adding signals (a) and (b) pass through binary circuits
9
a
and
9
b
so that binary signals (c) and (d) are obtained, respectively.
A phase difference detector circuit
10
detects a phase difference of rise or fall of the binary signals (c) and (d). In the circuit configuration shown in
FIG. 8
, a phase difference of fall is detected employing D-type flip flops (D-FF)
101
a
and
101
b
. The D-FFs
101
a and
101
b
have input terminals D, clock input terminals T, reset input terminals R, and output terminals Q and Q-, and when an input of the reset input terminal R is at logic level, an output of the output terminal Q is unconditionally at level, and when an input of the reset input terminal R is at logic level, a signal the

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