Optical pickup with position adjusting means

An information reading device is used as an optical pickup in a compact disk player or the like for playing back an optical disk. The optical pickup has an adjusting fulcrum about which a base supported on a base mount can be turned with respect to the base mount, a first adjusting screw for angularly adjusting the position of the base with respect to the base mount in a tangential direction of the disk, and a second adjusting screw for angularly adjusting the position of the base with respect to the base mount in a radial direction of the disk. The adjusting fulcrum and the first and second adjusting screws are positioned in a particular positional relationship to prevent any crosstalk from being produced when the base is adjusted in the tangential or radial direction after the base has been adjusted in the radial or tangential direction. When the first and second adjusting screws are turned, the position of the base with respect to the base mount is adjusted about the principal point on the base mount side of an objective lens supported on the base. Therefore, incident and exit angles of the objective lens are equalized to each other through the adjustments.

BACKGROUND OF THE INVENTION 
The present invention relates to an information reading device or optical 
pickup for optically reading recorded information from an information 
storage medium such as a compact disk (CD), a laser vision disk (LVD), or 
the like, and more particularly to a skew adjusting mechanism for use in 
such an optical pickup. 
Recorded information stored in optical information storage mediums such as 
CDs, LBDs, or the like is reproduced by CD players, LVD players, or the 
like. The CD players and the LVD players have an optical pickup for 
optically reading recorded information from a disk. The optical pickup has 
a laser diode for emitting a laser beam to read recorded information. The 
laser beam generated by the laser diode is focused into a beam spot on the 
information recording surface of the disk by an objective lens. The laser 
beam thus applied as a beam spot to the information recording surface of 
the disk is reflected thereby as a light beam bearing information 
represented by pits on the disk. The reflected light beam goes through the 
objective lens to a photodetector, which converts the light beam into an 
electric RF signal for the reproduction of the recorded information. 
The optical pickup is required to apply the laser beam as a sufficiently 
small beam spot to the information recording surface of the disk. To meet 
this requirement, the optical pickup has to be mounted in the player in a 
proper position or a attitude so that the optical axis of the optical 
pickup extends perpendicularly to the information recording surface of the 
disk and the laser beam will not be defocused on the disk due to 
aberrations of the objective lens. When the player is assembled, 
therefore, the position or inclination of the optical pickup or objective 
lens is adjusted through a process known as skew adjustment in order to 
minimize the aberrations of the objective lens and any deviation of the 
optical axis of the optical pickup. Such skew adjustment is made by a skew 
adjusting mechanism on a base of the optical pickup. 
One conventional skew adjusting mechanism has three adjusting screws which 
supports an objective lens supporting device on the base of the optical 
pickup. The operator adjusts the height of the points where the objective 
lens supporting device is supported by the adjusting screws by turning the 
adjusting screws, thereby setting the objective lens for a desired focal 
length and inclination. 
The prior skew adjusting mechanism however requires a complex, tedious, and 
time-consuming adjusting procedure since fine adjustments have to be 
repeated using the three adjusting screws until proper objective lens 
position and inclination are achieved. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an optical pickup 
having a skew adjusting mechanism for allowing easy skew adjustments and 
improving image height characteristics. 
According to the present invention, there is provided an optical pickup for 
reading information recorded along tracks on an information storage 
medium, comprising an optical element for reading the recorded information 
as an optical signal, a base supporting the optical element, a base mount 
supporting the base, an adjusting fulcrum supported on the base and 
serving as a fulcrum about which the base can be turned when the position 
of the base with respect to the base mount is adjusted, the adjusting 
fulcrum being positioned on either side of a tangential line extending 
parallel to a line tangential to one of the tracks of the information 
storage medium and passing through an optical center of the optical 
element, the adjusting fulcrum being disposed near a center line extending 
radially of the tracks and passing through the optical center of the 
optical element, and a plurality of adjusting screws supported on the base 
for angularly adjusting the position of the base with respect to the base 
mount, the adjusting screws including a first adjusting screw which is 
disposed near the center line for angularly adjusting the position of the 
base with respect to the base mount about a principal point on the base 
mount side of the optical element primarily in a radial direction of the 
tracks, and a second adjusting screw which is disposed near the tangential 
line remotely from the optical center for angularly adjusting the position 
of the base with respect to the base mount about the principal point only 
in a tangential direction of the tracks, after the base has been adjusted 
by the first adjusting screw. 
According to the present invention, there is also provided an optical 
pickup for reading information recorded along tracks on an information 
storage medium, comprising an optical element for reading the recorded 
information as an optical signal, a base supporting the optical element, a 
base mount supporting the base, an adjusting fulcrum supported on the base 
and serving as a fulcrum about which the base can be turned when the 
position of the base with respect to the base mount is adjusted, the 
adjusting fulcrum being positioned on either side of a center line 
extending radially of the tracks and passing through an optical center of 
the optical element, the adjusting fulcrum being disposed near a 
tangential line extending parallel to a line tangential to one of the 
tracks of the information storage medium and passing through the optical 
center of the optical element, and a plurality of adjusting screws 
supported on the base for angularly adjusting the position of the base 
with respect to the base mount, the adjusting screws including a first 
adjusting screw which is disposed near the tangential line for angularly 
adjusting the position of the base with respect to the base mount about a 
principal point on the base mount side of the optical element primarily in 
a tangential direction of the tracks, and a second adjusting screw which 
is disposed near the center line remotely from the optical center for 
angularly adjusting the position of the base with respect to the base 
mount about the principal point only in a radial direction of the tracks, 
after the base has been adjusted by the first adjusting screw. 
With the above arrangements, the adjusting fulcrum about which the base can 
be turned, the first adjusting screw for adjusting the base in the 
tangential direction, and the second adjusting screw for adjusting the 
base in the radial direction, are held in a certain positional 
relationship. First, the first adjusting screw is turned, and then the 
second adjusting screw is turned to angularly adjust the position of the 
base with respect to the base mount about the principal point on the base 
mount side of the optical element. In this manner, any crosstalk is 
prevented from being produced when the base is adjusted in the tangential 
or radial direction after the base has been adjusted in the radial or 
tangential direction. Skew adjustments can be effected simply in a short 
period of time, and image height characteristics are improved. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description when 
taken in conjunction with the accompanying drawings in which preferred 
embodiments of the present invention are shown by way of illustrative 
example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before describing embodiments of the present invention, a general optical 
pickup will first be described with reference to FIGS. 1, 2A and 2B for a 
better understanding of the present invention. 
As shown in FIG. 1, an information reading device or optical pickup is 
positioned in confronting relation to a disk 4 as an information storage 
medium which stores recorded information as spiral pits along tracks 41. 
The optical pickup comprises an objective lens 1 for applying a light beam 
emitted from a light source (not shown) as a focused beam spot to the disk 
4, a base 2 supporting the objective lens 1 and capable of adjusting the 
skew of the objective lens 1, and a base mount 3 on which the base 2 is 
supported. 
The base 2 has an adjusting fulcrum 21 and two adjusting screws 24, 25. The 
adjusting fulcrum 21 is positioned at one end of the base 2 on a lefthand 
side (as shown in FIG. 1) of a central line 42 which extends radially of 
the disk 4 and passes through the optical center 11 of the objective lens 
1. The adjusting fulcrum 21 is also disposed on a tangential line 43 
extending through the optical center 11 of the objective lens 1 
tangentially to a track of the disk 4. When the base 2 is adjusted in its 
position with respect to the base mount 3, the base 2 is turned about the 
adjusting fulcrum 21. The adjusting screws 24, 25 are positioned at the 
other end of the base 2 on a righthand side (as shown in FIG. 1) of the 
center line 42 and symmetrical with respect to the tangential line 43. The 
adjusting screws 24, 25 serve to adjust the position of the base 2 in a 
radial direction, indicated by the screw R, of the disk 4 and in a 
tangential direction, indicated by the arrow T, of the disk 4. 
Skew adjustment of the optical pickup shown in FIG. 1 will be described 
below with reference to FIGS. 2A and 2B. 
The adjusting screws 24, 25 are turned to adjust the distance of the base 2 
from the base mount 3. Such an adjustment is effected alternately in the 
radial direction R and the tangential direction T until the angle 
.theta..sub.R1 of the objective lens 1 with respect to an optical axis 51 
of the optical pickup in the radial direction R becomes 90.degree. and the 
angle .theta..sub.T1 of the objective lens 1 with respect to the optical 
axis 51 in the tangential direction T becomes also 90.degree. . 
In the above skew adjusting procedure, after the angle .theta..sub.T1 in 
the tangential direction T has been adjusted to 90.degree. through a 
tangential skew adjusting step, the angle .theta..sub.R1 in the radial 
direction R is adjusted to 90.degree. through a radial skew adjusting 
step. Since a crosstalk is produced at each adjusting step, the tangential 
and radial skew adjusting steps have to be repeated. Therefore, many skew 
adjusting steps and a long period of time are required before both the 
angles .theta..sub.T1, .theta..sub.R1 in the tangential and radial 
directions T, R become 90.degree. . 
It would be possible to move the base 2 angularly on the base mount 3 for 
skew adjustments. However, if the center about which the base 2 is 
angularly moved is located at the focal point of the objective lens 1 or 
the vertex of the spherical or aspherical surface of the objective lens 1, 
then image height characteristics would not be appreciably improved when 
adjusted. 
An optical pickup according to a preferred embodiment of the present 
invention will now be described below with reference to FIGS. 3, 4, 5A, 
and 5B. 
The optical pickup according to the present invention comprises an 
objective lens 1, a base 2, and a base mount 3 which are disposed between 
a disk 4 and a light source (not shown), the optical pickup having an 
optical axis 51. The base 2 is supported at a skew supporting portion. 
The base 2 has an adjusting fulcrum 21 and first and second adjusting 
screws 22, 23 for skew adjustments. The adjusting fulcrum 21 is positioned 
on an upper side (as shown in FIG. 4) of a tangential line 43 which 
extends through the optical center 11 of the objective lens 1 parallel to 
a tangential direction of a track 41 of the disk 4. The adjusting fulcrum 
21 is disposed near a center line 42 extending radially of the disk 4 
through the optical center 11 of the objective lens 1. The adjusting 
fulcrum 21 is used to adjust the position of the base 2 with respect to 
the base mount 3. The first adjusting screw 22 is positioned on a lower 
side (as shown in FIG. 4) of the tangential line 43 remotely from the 
adjusting fulcrum 21 and near the center line 42. The first adjusting 
screw 22 serves to angularly adjust the position of the base 2 with 
respect to the base mount 3 about a principal point H.sub.1 on the base 
mount side of the objective lens 1 in a direction A (see FIG. 3) with 
respect to the radial direction of the track 41. The second adjusting 
screw 23 is disposed new the tangential line 43 remotely from the optical 
center 11 of the objective lens 1. The second adjusting screw 23 serves to 
angularly adjust the position of the base 2 with respect to the base mount 
3 about the principal point H.sub.1 on the base mount side of the 
objective lens 1 in a direction A (see FIG. 3) with respect to the 
tangential direction of the track 41, after an adjustment is made by the 
first adjusting screw 22. 
The adjusting fulcrum 21 and the first and second adjusting screws 22, 23 
are located in respective positions which are predetermined by experiments 
and simulation processes. Typical experimental positional examples and a 
process of determining the positions of the adjusting fulcrum 21 and the 
first and second adjusting screws 22, 23 will now be described with 
reference to FIGS. 6A through 6E and 7A through 7E. In FIGS. 7A through 
7E, a circular mark (.largecircle.) indicates skew angles in case that the 
first adjusting screw 22 is rotated while a square mark (.quadrature.) 
indicates skew angles in case that the second adjusting screw 23 is 
rotated. 
FIG. 6A shows an example in which the adjusting fulcrum 21 and the first 
adjusting screw 22 are located on the center line 42 and the second 
adjusting screw 23 is disposed on the tangential line 43. FIG. 7A shows 
adjustments achieved for skew angles in the tangential and radial 
directions T, R with the example shown in FIG. 6A. It can be seen from 
FIG. 7A that when the first and second adjusting screws 22, 23 are turned, 
the skew angles on the coordinate axes in the tangential and radial 
directions T, R can successively be adjusted, with no crosstalk produced. 
Since, however, the adjusting fulcrum 21 and the adjusting screws 22, 23 
are on the center line 42 and the tangential line 43, their positions are 
fixed and impose some limitations on the designing and manufacturing 
processes. 
FIG. 6B shows an example in which the adjusting fulcrum 21 and the first 
adjusting screw 22 are located on the center line 42 and the second 
adjusting screw 23 is disposed slightly off the tangential line 43. FIG. 
7B shows adjustments achieved for skew angles in the tangential and radial 
directions T, R with the example shown in FIG. 6B. It can be seen from 
FIG. 7B that when the first adjusting screw 22 is turned, a large 
crosstalk is produced, and when the second adjusting screw 23 is turned, a 
crosstalk is somewhat produced. It will be understood that when the second 
adjusting screw 23 is displaced from the tangential line 43, a large 
crosstalk is produced with respect to the skew angle in the radial 
direction R. 
FIG. 6C shows an example in which the adjusting fulcrum 21 and the first 
adjusting screw 22 are displaced from the center line 42 parallel thereto 
and the second adjusting screw 23 is disposed on the tangential line 43. 
FIG. 7C shows adjustments achieved for skew angles in the tangential and 
radial directions T, R with the example shown in FIG. 6C. It can be seen 
from FIG. 7C that when the adjusting fulcrum 21 and the first adjusting 
screw 22 are displaced from the center line 42 parallel thereto, a large 
crosstalk is produced with respect to the skew angle in the radial 
direction R, and only a slight crosstalk is produced with respect to the 
skew angle in the tangential direction T. 
FIG. 6D shows an example which is similar to the example of FIG. 6C except 
that the second adjusting screw 23 is displaced largely from the 
tangential line 43. FIG. 7D shows adjustments achieved for skew angles in 
the tangential and radial directions T, R with the example shown in FIG. 
6D. It can be seen from FIG. 7D that even when the second adjusting screw 
23 is displaced from the position shown in FIG. 6C, the magnitude of the 
crosstalk with respect to the tangential direction T remains substantially 
unchanged. 
FIG. 6E shows an example in which a line interconnecting the adjusting 
fulcrum 21 and the first adjusting screw 22 is inclined with respect to 
the center line 42, with the second adjusting screw 23 displaced largely 
from the tangential line 43. FIG. 7E shows adjustments achieved for skew 
angles in the tangential and radial directions T, R with the example shown 
in FIG. 6E. It can be seen from FIG. 7E that even when the line 
interconnecting the adjusting fulcrum 21 and the first adjusting screw 22 
is not parallel to the center line 42, no crosstalk is produced with 
respect to the skew angle in the tangential direction T. However, it is 
preferable that the line interconnecting the adjusting fulcrum 21 and the 
first adjusting screw 22 be inclined by a small angle with respect to the 
center line 42. If the line interconnecting the adjusting fulcrum 21 and 
the first adjusting screw 22 is inclined by a large angle with respect to 
the center line 42, then it is necessary to increase the distance between 
the adjusting fulcrum 21 and the first adjusting screw 22. From the 
experimental examples shown in FIGS. 5A through 6E can be derived the 
following conditions for the positioning of the adjusting fulcrum 21 and 
the first and second adjusting screws 22, 23 (see FIG. 8): 
(1) The line interconnecting the adjusting fulcrum 21 and the first 
adjusting screw 22 should be as parallel as possible to the center line 
42, and the distance t between the above line and the center line 42 
should be as small as possible. 
(2) If the above requirement (1) is not met, then the distance l.sub.4 
between the adjusting fulcrum 21 and the center line 42 should be as small 
as possible. At this time, the adjusting fulcrum 21 and the first 
adjusting screw 22 are disposed one on each side of the tangential line 
43, and should preferably be spaced from the tangential line 43 by large 
distances l.sub.1, l.sub.2. 
(3) The second adjusting screw 23 should be located on one side of the 
center line 42 where the adjusting fulcrum 21 and the first adjusting 
screw 22 are not positioned. The distance l.sub.2 between the second 
adjusting screw 23 and the tangential line 43 should be small, and the 
distance l.sub.6 between the second adjusting screw 23 and the center line 
42 should be large. 
An adjusting procedure for the example shown in FIG. 6E will be described 
below with reference to FIG. 7E. 
It is assumed that skew angles are at a coordinate point P.sub.1 before 
adjustments are made. First, the first adjusting screw 22 is turned to 
vary the skew angles from the coordinate point P.sub.1 to a coordinate 
point P.sub.2 in a direction indicated by the broken line arrow until the 
skew angle in the radial direction R becomes 0.degree. (at this time, the 
angle .theta..sub.R with respect to the optical axis 51 is 90.degree. , 
see FIG. 5A). As the first adjusting screw 2 is thus turned, the skew 
angle in the tangential direction T also varies to -1.degree. at the 
coordinate point P.sub.2. Then, the second adjusting screw 23 is turned to 
vary the skew angle in the tangential direction from the coordinate point 
P.sub.2 to the coordinate origin O in a direction indicated by the broken 
line arrow until the skew angle in the tangential direction T becomes 
0.degree. (at this time, the angle .theta..sub.T with respect to the 
optical axis 51 is 90.degree. , see FIG. 5B). When the second adjusting 
screw 23 is turned, no crosstalk is produced in the radial direction R. 
Therefore, the skew angles of the objective lens 1 can be adjusted to zero 
in two adjusting steps. 
A skew adjusting procedure with respect to the optical pickup shown in 
FIGS. 3, 4, 5A, and 5B will now be described below. 
For adjustments in the radial direction R, the first adjusting screw 22 is 
turned to vertically move the base 2 with respect to the base mount 3 in 
the direction indicated by the arrow B in FIG. 5A. This vertical movement 
causes the base 2 to turn on the base mount 3 about the principal point 
H.sub.1 on the base side of the objective lens 1 in the direction A (FIG. 
3) along an arcuate path having a radius R (R=a). Upon such turning 
movement of the base 2 at the radius R, the position of the base 2 varies 
between the fixed disk 4 and the light source 5, skewing the objective 
lens 1 mounted on the base 2 through an angle .DELTA..theta..sub.R with 
respect to the optical axis 51. 
Adjustments in the tangential direction T are performed after the 
adjustments in the radial direction R are made. For adjustments in the 
tangential direction T, the second adjusting screw 23 is turned to 
vertically move the base 2 with respect to the base mount 3 in the 
direction indicated by the arrow C in FIG. 5B. This vertical movement 
causes the base 2 to turn on the base mount 3 about the principal point 
H.sub.1 in the direction A (FIG. 3) along the arcuate path having the 
radius R. The objective lens 1 is now skewed through an angle 
.DELTA..theta..sub.T with respect to the optical axis 51. 
Since the adjusting fulcrum 21 and the first and second adjusting screws 
22, 23 are located in the particular preferred positions determined from 
the experimental examples, the adjustments in the tangential direction T 
can be made without producing any crosstalk with respect to the 
adjustments in the radial direction R. 
The relationship between the above skew adjustments and image height 
characteristics will be described in comparison with conventional optical 
pickups with reference to FIGS. 9a through 9C. FIG. 9A shows skew 
adjustments about the principal point H.sub.1 on the base mount side, FIG. 
9B skew adjustments about a principal point H.sub.2 on the image side, and 
FIG. 9C skew adjustments about an image-side focal point P of an objective 
lens 1. 
When skew adjustments are made about the principal point H.sub.1 on the 
base mount side of the objective lens 1 as shown in FIG. 9A, since the 
principal point H.sub.1 on the base mount side and the principal point 
H.sub.2 on the image side are in conjugate relationship to each other, an 
incident angle i.sub.1 and an exit angle r.sub.1 are equal to each other, 
and an image height d.sub.1 is expressed as follows: 
EQU d.sub.1 =b.multidot.r.sub.1 (1) 
where b is the distance to the image point. 
When skew adjustments are made about the principal point H.sub.2 on the 
image side of the objective lens 1 as shown in FIG. 9B, an incident angle 
i.sub.2 and an exit angle r.sub.2 differ from each other, and an image 
height d.sub.2 is given by: 
EQU d.sub.2 =b.multidot.r.sub.2 (2) 
When skew adjustments are made about the focal point P of the objective 
lens 1 (focused point on the disk 4) as shown in FIG. 9C, an image height 
d.sub.3 is expressed as follows: 
EQU d.sub.3 =b.multidot.r.sub.3 =b.times.(i.sub.1 +.phi.) (3) 
where .phi. is the angle of the principal ray from the light source 5 with 
respect to the line extending from the light source perpendicularly to the 
disk 4. 
In FIG. 9C, from the relationship between images at the principal point 
H.sub.1 on the base mount side is established the following equation: 
EQU (b+l).multidot.sin i.sub.1 .perspectiveto.a.multidot.sin .phi. 
EQU (b+l).multidot.i.sub.1 .perspectiveto.a.multidot..phi. (4) 
Substituting the equation (4) for the equation (3), the following equation 
is obtained: 
##EQU1## 
Comparison between the image height d.sub.1 according to the above 
embodiment and the image height d.sub.2, d.sub.3 in the other arrangements 
indicates that the image height d.sub.1 is the smallest, and serves as the 
most suitable position as the center for the skew adjustments. More 
specifically, since the angle r.sub.2 is larger than the angle r.sub.1, 
the image height d.sub.1 is smaller than the image height d.sub.2. The 
image height d.sub.1 smaller than the image height d.sub.3 by 
(b.multidot.i.sub.1 (b+l)/a). 
The image heights d.sub.1, d.sub.2, d.sub.3 will be described below with 
reference to specific numerical values. It is assumed that the 
magnification of the objective lens 1 is about 5.5 times, the distance a 
to the object point (light source 5) is -4.56 [mm], the distance b to the 
image point (on the disk 4) is 25.09 [mm], and the thickness l of the 
objective lens 1 is 0.64 [mm]. If a light beam is applied from the light 
source 5 at an incident angle i.sub.1 =0.5C (=0.00873 [rad]), then the 
image heights are d.sub.1 =0.0398 [mm], d.sub.2 =0.0408 [mm], and d.sub.3 
=0.0481 [mm]. Therefore, the image height d.sub.1 according to the 
embodiment of the present invention is smaller than the other image 
heights d.sub.2, d.sub.3. 
The relationship between the image heights and wavefront aberrations is 
shown in the image height/wavefront aberration diagram of FIG. 10. 
In the above embodiment, the adjusting fulcrum 21 is located on the side of 
the base 2 which is closer to the center of the disk 4 and the first and 
second adjusting screws 22, 23 are located on the side of the base 2 which 
is farther from the center of the disk 4. However, the adjusting fulcrum 
and the first and second adjusting screws may be positioned otherwise, 
i.e., on the opposite sides farther from and closer to, respectively, the 
center of the disk 4. 
An optical pickup according to another embodiment of the present invention 
will be described below with reference to FIG. 11. 
The optical pickup shown in FIG. 11 has a base 2 which supports an 
adjusting fulcrum 21 and a first and a second adjusting screws 22, 23, 
just like the optical pickup according to the preceding embodiment, but 
differs therefrom as to its orientation with respect to the disk 4. 
More specifically, the adjusting fulcrum 21 is disposed on the lefthand 
side of the center line 42 near the tangential line 43, and serves as a 
fulcrum about which the base 2 is turned when the position of the base 2 
is adjusted. The first adjusting screw 22 is disposed on the righthand 
side of the center line 42 near the tangential line 43, and is used to 
adjust the skew angles primarily in the tangential direction T. The second 
adjusting angle 23 is disposed near the center line 42 remotely from the 
optical center 11, and is used to adjust the skew angles in the radial 
direction R after the adjustments have been made using the first adjusting 
screw 22. 
The adjustments are made using the second adjusting screw 23 after the 
adjustments have been made using the first adjusting screw 22, as 
described above. Therefore, the skew angles can be adjusted without any 
crosstalk produced when the final adjustments are made using the second 
adjusting screw 23. 
The invention may be embodies in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description and all changes 
which come within the meaning and range of equivalency of the claims are 
therefore intended to be embraced therein.