BACKGROUND OF THE INVENTION
Field of the invention
The present invention relates to optical pickups, and more
particularly to an optical pickup and method of adjusting same used to read information
from an optical disc (hereinafter merely referred to as "disc"), such as CD and
Description of the prior art
There is disclosed one example of a conventional optical
pickup of this kind in Japanese Patent Publication No. H5-9851. This conventional
art has a diffraction element 3 to cause diffraction in a main beam reflected from
a disc 6 and split it into two. These diffracted beams are converged onto regions
7a, 7b and 7c of a light receiving element 7 so that the difference (Sa-Sb) in output
signal of between the regions 7a and 7b provides a focusing error signal while the
sum (Sa + Sb + Sc) of the output signals from the areas 7a, 7b and 7c gives a pit
In such an optical pickup, the adjustment in positional
relationship between the diffraction element 3 and the light receiving element 7
requires actual rotation of the disc 6 to perform focusing servo using a servo circuit.
The diffraction element 3 is displaced such that the jitter value observed by a
jitter meter is brought to a best.
In the conventional optical pickup, however, there has
been a problem that the adjustment is troublesome and consumes a long time, because
it requires actual rotation of a disc 6 in order to implement focussing servo. Also,
there has been another problem that the provision of the servo circuit results in
mounting-up of cost. Moreover, there has been still another problem that the optical
pickup is difficult to stabilize its characteristics, because the displacement of
the diffraction element is based on a jitter value as a reference that is susceptible
SUMMARY OF THE INVENTION
Therefore, it is a primary object of the present invention
to provide a novel optical pickup.
In the present invention, observation is made on a relationship
between the lens position in a direction perpendicular to a disc surface and respective
output values of the first and second portions. If the diffraction element is displaced
such that an intersection of a first waveform and a second waveform, i.e., the point
to which focus servo is effected, is brought into overlap with a center point of
a maximum value range of a third waveform representative of the lens position and
an output value of a third portion, i.e., the point at which the spot size assumes
a minimum, then it is possible to prevent against focusing offset and bring best
the jitter value.
Also, the third portion may be divided, for example, as
a portion 3a and a portion 3b, in order to grasp as waveforms 3a and 3b a relationship
between the change in lens position and respective outputs of the portions 3a and
3b. The diffraction element m,ay be displaced such that the intersection of the
first and second waveforms is brought into overlap with the intersection of the
waveform 3a and the waveform 3b, i.e., such that the spot size assumes a minimum.
Due to this, the focusing offset can be prevented with the jitter value taken best.
In also this case, the number of output signals increases due to the division of
the third portion into the portions 3a and 3b. However, if an optical pickup of
the invention to be referred later is utilized, two signals can be selectively taken
out through one terminal by a switch means. This can prevent the number of terminals
A conventional optical pickup according to the preamble
of claim 1 is known from JP-A-08111026.
An optical pickup according to the invention comprises:
a first diffraction element for causing a laser beam from a laser device to be split
into one main beam and two sub-beams; a lens for converging the main beam and the
sub-beams on a disc; a second diffraction element divided into two of first and
second regions to respectively cause diffraction in the reflection beams from the
disc; a photo-detector having four light emitting elements to converge thereon reflection
beams of the main beam respectively diffracted by the first region and the second
region, and two light receiving elements for converging thereon reflection beams
of the sub-beams; and a switching means for switching over between taking out an
output signal associated with any of the four light receiving elements and taking
out a tracking error signal by subtracting between output signals of the two light
According to the present invention, the positional relationship
between the diffraction element and the photo-detector is easy to adjust. Also,
there is no neccesity of separately providing a servo circuit or the like, thus
reducing cost. Meanwhile, Because the jitter value can be determined best as a result
that the spot size is brought into a minimum at a point where focusing servo is
effected, the optical pickup is stabilized in characteristic. Moreover, the number
of terminals is prevented from increasing, thereby preventing against increase in
The above described objects and other objects, features,
aspects and advantages of the present invention will become more apparent from the
following detailed description of the present invention when taken in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
- Figure 1 is an illustrative view showing one embodiment of the present invention;
- Figure 2 is an illustrative view showing a photo-detector used in the Figure
- Figure 3 is an illustrative view showing a manner to adjust a positional relationship
between the photo-detector and the hologram unit;
- Figure 4 is an illustrative view showing how to assemble the optical pickup;
- Figure 5 is a graph showing how to adjust a position of the hologram unit;
- Figure 6 is a graph showing how to determine a point Q;
- Figure 7 is a graph showing another method to adjust a position of the hologram
- Figure 8 is an illustrative view showing another photo-detector;
- Figure 9 is a circuit diagram showing the photo-detector of Figure 8; and
- Figure 10 is a graph showing how to adjust a position of a hologram unit when
using the photo-detector of Figure 11.
An optical pickup 10 of the present embodiment, as shown
in Figure 1, is to read information out of an optical disc (hereinafter merely "disc")
14, such as CD and DVD, being rotated by a disc motor 12, and includes a semiconductor
laser device 16 as a light source to emit a predetermined wavelength of laser light.
A hologram unit 18 as a diffraction element and an objective lens 20 are arranged
between the semiconductor laser device 16 and the disc 14. A photo-detector 22 is
provided obliquely below the hologram unit 18. Note that these semiconductor laser
device 16, hologram unit 18 and photo-detector 22 are formed in one body as a unit
24 as shown in Figure 4.
The hologram unit 18 includes a substrate 26 made of a
quartz glass or the like. The substrate 26 has a grating pattern 28 formed at an
underside surface in order to create three beams, and a hologram pattern 30 on a
top surface to cause polarization of light. The hologram pattern 30 is divided with
a first region 30a and a second region 30b by a divisional line A0 extending in
a diametrical direction of the disc 14, as shown in Figure 3. The regions 30a and
30b are provided with respective gratings different in period. The hologram unit
18 is held by a support member 32 (Figure 1) which is to be displaced in directions
X, Y and &thgr; by a servo motor.
The objective lens 20 is fixed to an actuator coil 34 constituting
a focusing actuator so that it is vertically displaced by energizing the actuator
The photo-detector 22 is divided into five portions, i.e.,
light receiving elements 22a, 22b, 22c, 22e and 24f. The light receiving element
22a and the light receiving element 22b are separated by a divisional line B0 is
given almost in a same direction as but slightly deviated from a direction of diffraction
by the hologram pattern 30 in order to prevent against focusing offset.
Meanwhile, a computer 36 is connected to the support member
32, the actuator coil 34 and the light receiving elements 22a-22f of the photo-detector
22. The computer 36 is connected with an APC (Automatic Power Control) 38 to control
the output of the semiconductor laser device 16.
During assembling such an optical pickup 10, an adjusting
method is employed to conduct adjustment on the positional relationship between
the hologram unit 18 and the photo-detector 22 as shown in Figure 4, in order to
prevent against focusing offset and bring best the jitter value.
In step ①, first prepared is a stem 24a having a
semiconductor laser device 16 and a photo-detector 22 incorporated therein in one
body. A UV resin 40 is applied to a top surface of the stem 24a. In step ②,
the semiconductor laser device 16 is caused to emit light, to thereby determine
its light emission center with using a CCD camera 42. On the other hand, in step
③ a hologram unit 18 is recognized by the CCD camera 42 to determine its
device center. In succeeding step ④, the hologram unit 18 is mounted on the
top surface of the stem 24a. In step ⑤, this hologram unit 18 is fixed at
a predetermined position as shown in Figure 1.
Then, a half mirror (or total reflection mirror) 46, in
place of a disc 14, is placed on the turntable 44 to be rotated by the disc motor
12, and the semiconductor laser device 16 is caused to emit light. Thereupon the
laser beam from the semiconductor laser device 16 is diffracted by the grating pattern
28, and split into 0-order diffraction light (main beam) and two of 1-order diffraction
light (sub-beams). The three beams thus split by the grating pattern 28 travels
through the hologram pattern 30, and then converged on the half mirror 46 through
the objective lens 20. The reflection light of the main beam from the half mirror
46 travels through the objective lens 20, being diffracted by the hologram pattern
30. Thereupon the light diffracted by the region 30a shown in Figure 3 is converged
onto the light receiving elements 22a and 22b of the photo-detector 22, while the
light diffracted by the region 30b is onto the light receiving element 22c.
Then the actuator coil 34 is energized to move the objective
lens 20 in vertical directions. Such a graph as shown in Figure 5 is grasped by
the computer 36 as to a relationship between a vertical position of the objective
lens 20, i.e., a position perpendicular to the record surface of the disc 14 (voltage
on the actuator coil 34) and output values from the light receiving elements22a,
22b and 22c of the photo-detector 22. Note that spots 1- 9 are shown in Figure 3
corresponding to lens positions 1- 9 in Figure 5.
In the graph of Figure 5, a point P represents an intersection
of between an output waveform A given from the light receiving element 22a and an
output waveform B from the light receiving element 22b, i.e., the point toward which
focusing servo is effected. On the other hand, a point Q represents a center point
in a maximum value range (the range the output assumes its maximum) of an output
waveform C given from the light receiving element 22c. For example, the point Q
can be determined as an intersection of the output waveform C with a vertical line
L2 as shown in Figure 6, wherein a horizontal line L1 is drawn through a point lower
by 0.3 V than a peak value of the output waveform C and a horizontal line L2 is
then drawn to equally divide into two an area S (the area hatched in Figure 6) above
the horizontal line L1. This point Q represents a point at which the size of a laser
spot assumes its minimum. Accordingly, if the intersection P and the point Q are
in overlap with each other, focus servo is effected at that point with a minimum
spot size. As a result, the jitter value becomes best. In order to achieve this,
a not-shown servo motor is controlled by the computer 36 to move the support member
32 so that the hologram unit 18 is aligned in position to bring the intersection
P overlap with the point Q. Note that Figure 3 shows a state wherein there exists
a deviation in angle &thgr; between the hologram unit 18 and the photo-detector
22. When the hologram unit 18 is rotated by an angle &thgr; in an arrow direction
from the state of Figure 3, the point P and the point Q become overlapping with
each other. The hologram unit 18 is optimally determined of position by repeating
several times such positional adjustment of the hologram unit 18 as described above.
In step ⑥, light radiation is made to a contact
area between the stem 24a and the hologram unit 18, thus fixing the hologram unit
It is possible to obtain a tracking error signal by subtracting
between output signals respectively given from the light receiving element 22e and
the light receiving element 22f during actual rotation of the disc 14.
This embodiment can lower cost because of no necessity
of separately providing a servo circuit and the like to enable positional adjustment.
Furthermore, the jitter value can be obtained best as a result of minimizing the
spot size at a point to which focus servo is effected. It is therefore possible
for the optical pickup 10 to stabilize the characteristic.
Incidentally, in the above embodiment the hologram unit
18 was aligned in position so as to bring the intersection P of the output waveforms
A and B into overlapping with the point Q. Alternatively, the hologram unit 18 may
be aligned in position in a manner overlapping a zero cross point (the point to
which focus servo is effected) P1 of an S waveform (A - B) with a point Q as shown
in Figure 7, so that the difference in output value between the light receiving
elements 22a and 22b can be grasped as an S waveform (A - B) by the computer 36.
Also, the above-described embodiment employed the photo-detector
22 having five divided portions (light receiving elements). In place of the photo-detector
22, a photo-detector 48 as shown in Figures 8 and 9 may be used which is divided
into six portions (light receiving elements). As will be well understood from Figure
9, the photo-detector 48 includes six light receiving elements 48a, 48b, 48cl, 48c2,
48e, 48f. The light receiving elements 48a - 48f are connected with corresponding
amplifiers 50a - 50f. The amplifiers 50a and 50b have output signals respectively
given onto terminals 52a and 52b. The amplifiers 50c1 and 50c2 have output signals
which are added together by an adder 54 and then given onto a terminal 52c. The
output signals of the amplifiers 50c1 and 50c2 are respectively given onto terminals
52d and 52e, when the switches 56a and 56b are in a first state. The amplifiers
50e and 50f have output signals which are respectively given onto terminals 52d
and 52e, when switches 56a and 56b are in a second state. The switches 56a and 56b
are operated based on an output signal of a voltage comparator circuit 58 that is
dependent upon a reference voltage applied to a terminal 52f.
In an optical pickup 10 using the photo-detector 48, when
the hologram unit 18 is aligned in position, the objective lens is vertically displaced
with the switches 56a and 56b rendered in the first state. As shown in Figure 10,
the hologram unit 18 is adjusted in position such that an intersection P1 (the point
to which focus servo is effected) of between an output wave A of the light receiving
element 48a taken out through the terminal 52a and an output waveform B of the light
receiving element 48b taken through the terminal 52b is brought into overlap with
an intersection P2 (the point at which the laser spot size assumes a minimum) of
between an output waveform C1 of the light receiving element 48c1- through the terminal
52d and an output waveform c2 of the light receiving element 48c2 - through the
terminal 52e. On the other hand, when the disc 14 is being rotated, the switches
56a and 56b are put in the second state. Subtraction is made between an output signals
of the light receiving element 50e taken through the terminal 52d and an output
signal of the light receiving element 50f taken through the terminal 52e to thereby
obtain a tracking error signal. The output signals through the terminals 52a, 52b
and 52c are added together to thereby obtain a pit signal.
If employing such a photo-detector 48, the hologram unit
18 becomes easy to adjust of position depending on increased number of signals.
Furthermore, because two or more signals can be selectively obtained by switching
the switches 56a and 56b, it is possible to prevent against increase in the number
of terminals and in package size.
Also, a photo-detector 60 as shown in Figure 11 may be
used in place of the above-described photo-detector 22 (Figure 2) or photo-detector
48 (Figure 8, Figure 9). This photo-detector 60 includes six light receiving elements
60a, 60b, 60c1, 60c2, 60e, 60f. The light receiving elements 60a - 60f are connected
with corresponding amplifiers 62a - 62f. The amplifiers 62a, 62b, 62c1 and 62c2
have output signals which are added together by an adder 64 and then taken as a
pit signal Rf through a terminal 66a. The amplifiers 62a and 62b have output signals
which are subjected to subtraction by a subtracter 68 and when taken through a terminal
66b. Also, the amplifiers 60e and 60f have output signals which are respectively
taken through terminals 66c and 66d. The amplifiers 62c1 and 62c2 have output signals
which are subjected to subtraction by a subtracter 72 when the switch 70e in the
first state, while the amplifiers 62e and 62f have output signals which are subjected
to subtraction by a subtracter 74 and taken through a terminal 66e when the switch
70 is in the second state. The switch 70 is operated based on an output signal of
a voltage comparator circuit 76 that is dependent upon a reference voltage applied
to a terminal 66f.
In an optical pickup 10 using the photo-detector 60, when
the hologram unit 18 is aligned in position, the objective lens 20 may be vertically
moved with the switch 70 placed in the first state. As shown in Figure 12, the hologram
unit 18 may be adjusted in position such that a zero cross point (the point to which
focussing servo is effected) P1 in an S waveform (A - B) taken through the terminal
66b is overlapped with zero cross point (the point at which the laser spot size
assumes a minimum) P2 in an S waveform C1 - C2) taken through the terminal 66e.
On the other hand, when the disc 46 is being rotated, the switch 70 is put in the
second state to obtain a tracking error signal TE through the terminal 66e and a
pit signal RF through the terminal 66a.
In the case where the photo-detector 60 is used, the hologram
unit 18 can be easily adjusted in position without increasing the number of terminals
in a manner similar to the case using the photo-detector 48.
Incidentally, the above described photo-detectors 48 (Figure
8 and Figure 9) and 60 (Figure 11) are one examples of a photo-detector having six
light receiving element. The adders 54 and 64 and the subtracters 68, 72 and 74
may be arbitrarily combined to take other signals from the terminals.
Although the present invention has been described and illustrated
in detail, it is clearly understood that the same is by way of illustration and
example only and is not to be taken by way of limitation, the scope of the present
invention being limited only by the terms of the claims.