Optical device

Details Optical device Optical device

1998-10-13

2002-07-16

Hindi, Nabil (Department: 2753)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

C369S112280, C369S121000, C369S053280, C369S044120

Reexamination Certificate

active

06421307

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical device for use in receiving and detecting a returned light reflected from an irradiated portion by irradiating a light from a light-emitting portion, for example, on the irradiated portion of an optical recording medium such as an optical disk, a phase-change type optical disk and so on, and particularly to an optical device for use in detecting a tracking error signal relative to an optical disk having a pit depth&lgr;/4n or a recording portion equivalent thereto.
2. Description of the Related Art
In optical devices such as an optical pickup of an optical disk drive of a so-called compact disc (CD) player and a magnetooptical disk drive, respective optical assemblies such as a grating, a beam splitter and so on are individually fabricated so that an overall arrangement of a device becomes complicated and large. Moreover, when optical assemblies are fabricated on a base in a hybrid fashion, optical assemblies should be disposed with a strict alignment accuracy.
FIG. 1
is a structural diagram showing an example of a conventional optical pickup
81
that is exclusively used for reproducing a compact disc (CD). This optical pickup
81
comprises a semiconductor laser
82
, a diffraction grating
83
, a beam splitter plate
84
, an objective lens
85
and a light-receiving element
86
composed of a photo-diode. A laser light L from the semiconductor laser
82
is reflected on the beam splitter plate
84
, converged by the objective lens
85
and thereby irradiated on an optical disk
90
. A returned light reflected on the optical disk
90
is traveled through the objective lens
85
and the beam splitter plate
84
and received and detected by the light-receiving element
86
.
However, such optical pickup
81
has not only many assemblies and become very large in size but also many assemblies thereof should be disposed with a high accuracy so that its productivity is low accordingly.
As a tracking servo method in an optical device such as an optical pickup or the like, there are generally used a push-pull method, a 3-beam method, a heterodyne method and the like.
Of these methods, according to the conventional push-pull method, when a beam spot of incident light on a disc is displaced from a track or a pit, a difference of intensity occurs in +first-order light and −first-order light, whereby a far field pattern (FFP) becomes asymmetric. Then, two photo-detectors, for example, detect signals corresponding to the asymmetric far field pattern, and a computing device computes these signals to detect a displacement of a beam spot (see FIG.
2
).
FIGS. 2A and 2B
are each a schematic structural diagram showing a tracking servo using a push-pull method.
As shown in
FIG. 2B
, when a light is irradiated on concavities and convexities of pits formed on the surface of a disk
52
, the concavities and the convexities diffract the light to provide a 0-order diffracted light (main beam B) and ±first-order diffracted lights (sub-beams B′).
In
FIG. 2B
, reference numerals S
0
, S
1
denote irradiated spots of the 0-order diffracted light and the ±first-order diffracted lights, respectively. The irradiated spot S
0
becomes circular because of an aperture of an objective lens.
In this case, as shown in
FIG. 2A
, two split photo-diodes PD
R,
PD
L
are disposed as a light-receiving unit. Lights received by these photo-diodes PDR, PDL are computed by a suitable device such as a differential amplifier or the like, not shown, like (PD
L
−PD
R
), for example, to thereby obtain a tracking error signal TE as a tracking signal.
When the track and the center axis of incident beam are shifted from each other, there is caused a difference in diffracted information between ±first-order diffracted lights, so that TE=(PD
L
−PD
R
) does not become zero but indicates a positive or negative value in response to the shifted direction. Thus, it is possible to detect the direction and the amount in which the center axis of the incident beam is shifted from the track.
Although the tracking servo system using the push-pull method may be realized by the two split photo-diodes and may be made inexpensive, there arises a problem that, when the lens is shifted, the returned light from the disk is vertically shifted on the light-receiving surface with respect to the split line of the light-receiving element, thereby resulting in a large offset being generated in the signal.
As shown in
FIG. 3A
, when a lens
51
is shifted in the lateral direction, spots of lights received by the photo-diodes PDL, PDR are shifted as shown by dashed line concurrently therewith. Thus, even when the tracking is properly made, the tracking error signal TE=0 is not satisfied.
Also, as shown in
FIG. 3B
, when the lens
51
is skewed relative to the disk
52
, spots of received lights are also shifted as shown by dashed lines. Thus, even when the tracking is properly made, the tracking error signal TE=0 is not satisfied.
FIG. 4
shows measured results of influences exerted upon the tracking error signal by the lens shift in the case of the conventional push-pull signal as described above. Incidentally, the vertical axis represents measured results in the form of a relative value. Influences were computed by using a disk such that a groove pitch was 1.60 &mgr;m, a groove depth (depth) was wavelength/8 and a duty (duty:groove ratio) was 65%. Also, a wavelength was 0.78 &mgr;m.
A study of
FIG. 4
revealed that, according to the conventional push-pull signal, when the lens is shifted, the whole of the tracking error signal also is shifted concurrently therewith.
According to the push-pull method, when a wavelength of reproducing laser is &lgr; and a refractive index of the transparent base of the disk
52
is n, if a depth of pits on the disk
52
is &lgr;/4n, then a signal due to interference between the 0-order diffracted light and ±first-order diffracted lights becomes zero. As a consequence, it becomes impossible to detect the tracking error signal from a principle standpoint. Accordingly, the push-pull method may not be applied to the standardized disk
52
in which the pit depth is &lgr;/4n.
For example, a DVD (Dedital Versatile Disk)-ROM and a DVD-Video have the pit depth of &lgr;/4n according to the standards, so that the push-pull method may not be applied to such disks.
According to the three-beam method, the diffraction grating separates a light to provide a main beam and two sub-beams on both sides of the main beam.
FIG. 5
shows positions of spots formed on the disk surface according to the three-beam method. As illustrated, reflected beams of two sub-beams are respectively detected by irradiating a spot S
0
based on the main beam and spots S
1
, S
2
based on the sub-beams on both sides of the main beam on the grooves or pits of the disk
52
and a difference signal is calculated, thereby effecting a tracking servo similar to that of the push-pull method.
When the spot S
0
of the main beam is shifted from the center of the track, the reflected lights based on the spots S
1
, S
2
of the sub-beams become asymmetrical so that the tracking error signal provided by the difference signal is fluctuated from zero. Since the fluctuated amount of this tracking error signal changes in accordance with the amount in which the spot S
0
of the main beam is shifted from the center of the track, there may be effected the tracking servo.
Incidentally, the reflected light of the main beam is used to detect a disk recording signal.
Although the three-beam method may cope with the above-mentioned lens shift, the three-beam method has the drawback such that the light should travel through the diffraction grating such as a grating or the like, causing the number of assemblies to increase, an amount of light of the main beam decreases, causing a power consumption to increase, an adjustment is complicated, requiring much manufacturing cost and so on.
Further, as a method whi

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