Optical pickup for reading or recording information on a recording surface

An optical pickup reads or records information on a recording surface of a first optical record medium or a second optical record medium, each of which has a protection film different in thickness from each other. The optical pickup includes: a light beam source for emitting a light beam; a dividing unit for dividing the light beam into a non-diffracted light, which is a portion of the light beam other than diffracted lights, and a first order diffracted light; and a converging unit for converging the non-diffracted light and the first order diffracted light. Further, an optical path length between the light beam source and the converging unit is determined so that the non-diffracted light is converged onto the recording surface of the first optical record medium and a spherical aberration of the non-diffracted light converged onto the recording surface of the first optical record medium is minimized. And the dividing unit is constructed so that the first order diffracted light is converged onto the recording surface of the second optical record medium and a spherical aberration of the first order diffracted light converged onto the recording surface of the second optical record medium is minimized.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical pickup for reading or recording 
information on a recording surface of a first optical record medium such 
as a compact disk or a second optical record medium such as a digital 
video disk, each of which has a protection film different in thickness 
from each other. 
2. Description of the Related Art 
There is known an optical system for recording and reproducing digital 
information including video data on an optical record medium. The optical 
system records and reproduces the digital information recorded onto the 
optical record disk in the higher density compared with conventional 
compact disk. 
In order to reproduce the optical record disk, in which the digital video 
information is recorded in the high density, an optical pickup system, the 
resolution of which is so high as to reproduce the information recorded in 
the high density, is used. 
There are two methods of increasing the resolution as follows: 
(1) According to first method of increasing the resolution, the wave length 
of a light beam emitted by a light beam source is decreased. A 
semi-conductor laser, in which the wave length of a light beam is so short 
as to achieve the high resolution, has already been used. 
(2) According to second method of increasing the resolution, the numerical 
aperture of an objective lens is increased. In case of increasing the 
resolution by increasing the numerical aperture, a detection signal 
obtained from the pickup system is influenced by the effect of aberration. 
Particularly, if the recording surface of the optical record disk is not 
vertical to the optical axis of the objective lens due to the curve or the 
tilt of the optical record disk, the detection signal is considerably 
influenced by the effect of the aberration and it is difficult to increase 
the numerical number. 
In order to overcome this difficulty, new disk standard is proposed. 
According to the new disk standard, an optical disk, which includes a 
protection film having thickness (0.6 mm as an example) less than that 
(1.2 mm) of the conventional optical disk, is proposed. Namely, since the 
effect of aberration due to the curve or the tilt of the optical record 
disk is decreased when the thickness of the optical disk is decreased, the 
effect of the aberration is decreased when the thickness of the optical 
disk is half of that of the conventional optical disk. 
Therefore, when the thickness of the optical disk is half of that of the 
conventional optical disk, a small light spot is converged on the 
recording surface of the optical record disk by using an objective lens 
having an numerical number greater than that of the conventional disk and 
then information recorded on the recording surface of the optical disk in 
the high density can be reproduced by using the small light spot. 
However, if a compatible player, which can reproduce the conventional 
optical disk and the optical disk based on the new optical disk standard, 
is constructed, the optical pickup of the compatible player should read 
information recorded on the conventional optical disk and the optical disk 
based on new optical disk standard. 
However, if the objective lens of the optical pickup provided in the 
compatible player is arranged so that a light spot of a light beam 
converged by the objective lens on the optical disk having a protection 
film thickness less than that of the conventional optical disk is 
minimized, the light spot of the light beam converged on the conventional 
optical disk becomes broader and it is impossible to reproduce the 
information recorded in the conventional optical disk by using the light 
spot. 
In order to overcome the problems, there is an optical pickup, which uses a 
bifocal lens, which consists of a hologram. 
According to the optical pickup, which uses a bifocal lens, a laser beam 
emitted from the laser diode is reflected by a half-mirror and converted 
into a parallel light beam by a collimator lens. The parallel light beam 
is divided by an optical device, which consists of a hologram, into a 
non-diffracted light, which is a portion of the light beam other than 
diffracted lights and a first order diffracted light. The non-diffracted 
light is converged onto a recording surface of the conventional optical 
disk and the first order diffracted light is converged onto a recording 
surface of the optical disk based on the new optical disk standard by an 
objective lens, which is integrally formed with the optical device. 
However, this optical pickup, which uses a bifocal lens, is disadvantageous 
in that it is complex in structure and is constructed on a large scale, 
since an objective lens is integrally formed with the heavy optical device 
and the strong driving force of an actuator for performing the focus 
control and the tracking control is needed. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to substantially 
eliminate defects and drawings encountered in the prior art and to provide 
an optical pickup, which is simpler in structure and downsized, for 
reading or recording information on a recording surface of each of a 
plurality of optical record mediums, each of which has a protection film 
different in thickness each other. 
According to one aspect of the present invention, there is provided an 
optical pickup for reading or recording information on a recording surface 
of a first optical record medium or a second optical record medium, each 
of which has a protection film different in thickness from each other 
including: a light beam source for emitting a light beam; a dividing unit 
for dividing the light beam into a non-diffracted light, which is a 
portion of the light beam other than diffracted lights, and a first order 
diffracted light; and a converging unit for converging the non-diffracted 
light and the first order diffracted light, whereby an optical path length 
between the light beam source and the converging unit is determined so 
that the non-diffracted light is converged onto the recording surface of 
the first optical record medium and a spherical aberration of the 
non-diffracted light converged onto the recording surface of the first 
optical record medium is minimized; and the dividing unit is constructed 
so that the first order diffracted light is converged onto the recording 
surface of the second optical record medium and a spherical aberration of 
the first order diffracted light converged onto the recording surface of 
the second optical record medium is minimized. 
According to the construction of the present invention, a light beam 
emitted from the light beam source is divided by a dividing unit into a 
non-diffracted light, which is a portion of the light beam other than 
diffracted lights, and a first order diffracted light. By a converging 
unit, the non-diffracted light is converged on a recording surface of a 
first optical record medium and the first order diffracted light is 
converged on a recording surface of a second optical record medium. 
Further, an optical path length between the light beam source and the 
converging unit is determined so that the non-diffracted light is 
converged onto the recording surface of the first optical record medium 
and a spherical aberration of the non-diffracted light converged onto the 
recording surface of the first optical record medium is minimized and the 
dividing unit is constructed so that the first order diffracted light is 
converged onto the recording surface of the second optical record medium 
and a spherical aberration of the first order diffracted light converged 
onto the recording surface of the second optical record medium is 
minimized. 
In accordance with the present invention, the dividing unit may include a 
collimator lens, which consists of a hologram, and an optical path length 
between the light beam source and the dividing unit may be determined so 
that the first order diffracted light is parallel to an optical axis of 
the converging unit. 
According to another aspect of the present invention, there is provided an 
optical pickup for reading or recording information on a recording surface 
of each of more than two optical record mediums, each of which has a 
protection film different in thickness each other including: a light beam 
source for emitting a light beam; a dividing unit for dividing the light 
beam into a non-diffracted light, which is a portion of the light beam 
other than diffracted lights, and a plurality of diffracted lights; and a 
converging unit for converging the non-diffracted light and the plurality 
of diffracted lights, whereby an optical path length between the light 
beam source and the converging unit is determined so that the 
non-diffracted light is converged onto the recording surface of one of the 
more than two optical record mediums and a spherical aberration of the 
non-diffracted light converged onto the recording surface is minimized; 
and the dividing unit is constructed so that each of the plurality of the 
diffracted lights is converged onto the recording surface of the more than 
two optical record mediums other than the one of the more than two optical 
record mediums and a spherical aberration of each of the plurality of the 
diffracted lights converged onto the recording surface is minimized. 
According to the construction of the present invention, a light beam 
emitted from a light beam source is divided by a dividing unit into a 
non-diffracted light, which is a portion of the light beam other than 
diffracted lights, and a plurality of diffracted lights. The 
non-diffracted light and the plurality of diffracted lights are converged 
by a converging unit. Further, an optical path length between the light 
beam source and the converging unit is determined so that the 
non-diffracted light is converged onto the recording surface of one of the 
more than two optical record mediums and a spherical aberration of the 
non-diffracted light converged onto the recording surface is minimized; 
and the dividing unit is constructed so that each of the plurality of the 
diffracted lights is converged onto the recording surface of the more than 
two optical record mediums other than the one of the more than two optical 
record mediums and a spherical aberration of each of the plurality of the 
diffracted lights converged onto the recording surface is minimized. 
According to the present invention, the dividing unit may include a 
plurality of collimator lenses, each of which consists of a hologram. An 
optical path length between the light beam source and the dividing unit 
may be determined so that one of said plurality of diffracted lights, 
which is divided by the dividing unit, is parallel to an optical axis of 
the converging unit. Further, the dividing unit may divide the light beam 
into (2n+1)th order diffracted lights (n is an integral number greater 
than or equal to zero), which are used as the plurality of diffracted 
lights. 
The nature, utility, and further features of this invention will be more 
clearly apparent from the following detailed description with respect to 
preferred embodiments of the invention when read in conjunction with the 
accompanying drawings briefly described below.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment according to the present invention will now be 
described below with reference to the accompanying drawings. 
First Embodiment 
FIG. 1 is a diagram showing a basic structure of an optical pickup 
according to the first embodiment of the present invention. 
As shown in FIG. 1, the optical pickup is provided with: a laser diode 11 
as a light source; a half-mirror 12 for reflecting a laser beam emitted 
from the laser diode 11 toward an optical disk 15 (disk 15A or 15B) and 
for transmitting the laser beam reflected by the optical disk 15; a 
collimator lens 13, which consists of a hologram, for dividing the laser 
beam reflected by the half-mirror 12 into a non-diffracted light L.sub.0, 
which is a portion of the light beam other than diffracted lights and a 
first order diffracted light L.sub.1 ; an objective lens 14. for receiving 
the light L.sub.0 and the light L.sub.1, which is parallel to an optical 
axis of the objective lens 14, and converging the light L.sub.0 onto a 
recording surface of the optical disk 15B and the first order diffracted 
light L.sub.1 onto a recording surface of the optical disk 15A; and a 
photodetector 16 for receiving the reflected lights from the disk 15 
through the objective lens 14, the collimator lens 13 and the half-mirror 
12 and converting it to a detection signal. 
Operation of the optical pickup according to the first embodiment of the 
present invention will be described below. 
A laser beam, which consists of diffused lights emitted from the laser 
diode 11, is reflected toward a collimator lens 13 by a half-mirror 12. 
The collimator lens 13 divides the laser beam emitted from the laser diode 
11 into a non-diffracted light L.sub.0, which is a portion of the light 
beam other than diffracted lights and a first order diffracted light 
L.sub.1. An objective lens 14 receives the light L.sub.0 and converges the 
light L.sub.0 onto a recording surface of the optical disk 15B. For 
example, the optical disk 15B, which comprises a protection film having 
thickness greater than that of a protection film of the optical disk 15A, 
corresponds to a Compact Disk having thickness of 1.2 mm. 
An optical path length from the objective lens 14 to the laser diode 11 is 
determined so that the spherical aberration of the light L.sub.0 converged 
onto the recording surface of the optical disk 15B is minimized. 
Further, the collimator lens 13 is designed so that the first order 
diffracted light L.sub.1 received by the objective lens 14 is parallel to 
an optical axis of the objective lens 14. The objective lens 14 receives 
the first order diffracted light L.sub.1, which is parallel to an optical 
axis of the objective lens 14, and the first order diffracted light 
L.sub.1 is converged onto a recording surface of the optical disk 15A so 
that the spherical aberration of the light L.sub.1 converged onto the 
recording surface of the optical disk 15A is minimized. For example, the 
optical disk 15A, which comprises a protection film having thickness less 
than that of a protection film of the optical disk 15B, corresponds to a 
Digital Video Disk having thickness of 0.6 mm. In this embodiment, a 
numerical aperture of the objective lens 14 is approximately 0.6. 
The lights reflected by the disk 15 is transmitted through the objective 
lens 14, the collimator lens 13 and the half-mirror 12 and received by a 
photodetector 16, which converts the reflected lights to a detection 
signal. 
Therefore, in case of reproducing or recording into the optical disk 15A, 
which comprises a protection film having thickness less than that of a 
protection film of the optical disk 15B, the first order diffracted light 
L.sub.1 is used and the first order diffracted light L.sub.1 is converged 
onto a recording surface of the optical disk 15A. In case of reproducing 
or recording into the optical disk 15B, the light L.sub.0 is used and the 
light L.sub.0 is converged onto a recording surface of the optical disk 
15B. 
Here, an optical path length between the laser diode 11 and the collimator 
lens 13 is determined so that the first order diffracted light L.sub.1 is 
parallel to the optical axis of the objective lens 14. Since the first 
order diffracted light L.sub.1 is parallel to the optical axis of the 
objective lens 14, the spherical aberration at the recording surface of 
the optical disk 15A is not increased if the position of the optical disk 
15 is moved toward the direction of the optical axis of the objective lens 
14. Therefore, since a small light spot of the first order diffracted 
light L.sub.1 is shaped on the recording surface of the optical disk 15A 
by the objective lens 14, an optical disk, in which data is recorded in 
high density, can be easily reproduced. 
According to the first embodiment, the focus control and the tracking 
control is performed by only driving an objective lens without driving a 
collimator lens, which consists of a hologram being heavy. Therefore, 
since the driving force of an actuator for performing the focus control 
and the tracking control is decreased, the actuator is downsized and then 
the optical pickup is also downsized. 
The above mentioned first embodiment suppose that there are no difference 
in recording density or the size of pits for recording data between the 
optical disks 15A and 15B. The only difference between the optical disks 
15A and 15B is the thickness of the protection film. 
Second Embodiment 
The above mentioned first embodiment relates to the optical pickup, which 
can record into or reproduce the optical disks 15A and 15B, each of which 
comprises a protection film different in thickness from each other. 
According to the second embodiment of the present invention, more than two 
optical disks, each of which comprises a protection film different in 
thickness from each other, can be reproduced by using the first order 
diffracted light L.sub.1 and (2n+1)th order diffracted lights (n is an 
integral number greater than or equal to zero). 
In this case, the (2n+1)th order diffracted lights are used, since the 
quantity of the diffracted light is easily controlled by using the 
(2n+1)th diffracted lights. 
Third Embodiment 
With respect to the above mentioned first and second embodiments, the 
pickup system is constructed by using one collimator lens which consists 
of a hologram. If the pickup system is constructed by using a plurality of 
collimator lenses, each of which consists of a hologram, for dividing 
laser beam into a non-diffracted light which is a portion of the light 
beam other than diffracted lights and one or more than one diffracted 
lights, one of which is parallel to an optical axis of the collimator 
lenses, a plurality of diffracted lights are reproduced by the plurality 
of collimator lenses. Therefore, a plurality of optical disks, each of 
which has a protection film different in thickness from each other, can be 
reproduced by using the plurality of diffracted lights. 
FIG. 2 is a diagram showing a basic structure of an optical pickup 
according to the third embodiment of the present invention. FIG. 3 is a 
diagram showing a basic structure of a collimator lens 13C, which consists 
of a first collimator lens 13A and a second collimator lens 13B. 
As shown in FIG. 2 and FIG. 3, the optical pickup is provided with: a laser 
diode 11 as a light source; a half-mirror 12 for reflecting a laser beam 
emitted from the laser diode 11 toward an optical disk 15 and for 
transmitting the laser beam reflected by the optical disk 15; a first 
collimator lens 13A, which consists of a hologram, for dividing the laser 
beam reflected by the half-mirror 12 into a non-diffracted light L.sub.0, 
which is a portion of the light beam other than diffracted lights and a 
first order diffracted light L.sub.01 ; a second collimator lens 13B, 
which consists of a hologram, for dividing the non-diffracted light 
L.sub.0 into the non-diffracted light L.sub.0 and a first order diffracted 
light L.sub.1, which is parallel to an optical axis of the collimator lens 
13B, and the first order diffracted light L.sub.01, into a second order 
diffracted light L.sub.2 and a third order diffracted light L.sub.3 ; an 
objective lens 14 for converging the light L.sub.0 onto a recording 
surface of an optical disk 15D, the first order diffracted light L.sub.1, 
onto a recording surface of an optical disk 15C, the second order 
diffracted light L.sub.2 onto a recording surface of an optical disk 15B, 
and the third order diffracted light L.sub.3 onto a recording surface of 
an optical disk 15A; and a photodetector 16 for receiving the reflected 
lights from the disk 15 through the objective lens 14, the collimator lens 
13 and the half-mirror 12 and converting it to a detection signal. 
Operation of the optical pickup according to the third embodiment of the 
present invention will be described below. 
A laser beam emitted from the laser diode 11 is reflected toward a 
collimator lens 13A by a half-mirror 12. The collimator lens 13A divides 
the laser beam emitted from the laser diode 11 into a light L.sub.0, which 
is a portion of the light beam other than diffracted lights and a first 
order diffracted light L.sub.01. The collimator lens 13B divides the light 
L.sub.0 into the light L.sub.0 and a first order diffracted light L.sub.0 
and the first order diffracted light L.sub.01 into the second order 
diffracted light L.sub.2 and the third order diffracted light L.sub.3. An 
objective lens 14 receives the light L.sub.0 and converges it onto a 
recording surface of an optical disk 15D; receives the light L.sub.1, 
which is parallel to an optical axis of the objective lens 14, and 
converges it onto a recording surface of an optical disk 15C; receives the 
light L.sub.2 and converges it onto a recording surface of an optical disk 
15B; and receives the light L.sub.3 and converges it onto a recording 
surface of an optical disk 15A. 
An optical path length from the objective lens 14 to the laser diode 11 is 
determined so that the spot of the light L.sub.0 converged onto a 
recording surface of the optical disk 15D is minimized. An optical path 
length between the laser diode 11 and the collimator lens 13A is 
determined so that one of said plurality of diffracted lights is parallel 
to the optical axis of the objective lens 14. 
The lights reflected by the disk 15 are transmitted through the objective 
lens 14, the collimator lenses 13A, 13B and the half-mirror 12 and 
received by a photodetector 16, which converts the reflected lights to a 
detection signal. 
Therefore, in case of reproducing or recording into the optical disk 15A, 
which comprises a protection film having thickness d1, the third order 
diffracted light L.sub.3 is used. In case of reproducing or recording into 
the optical disk 15B, which comprises a protection film having thickness 
d2, the second order diffracted light L.sub.2 is used. In case of 
reproducing or recording into the optical disk 15C, which comprises a 
protection film having thickness d3, the first order diffracted light 
L.sub.1 is used. In case of reproducing or recording into the optical disk 
15D, the light L.sub.0 is used. 
Although a focus control and a tracking control are not explained in the 
above description, a conventional focus control system and a conventional 
tracking system are used to construct the above optical pickup of the 
present invention. 
According to the present invention, the focus control and the tracking 
control is performed by only driving an objective lens without driving a 
collimator lens, which consists of a hologram being heavy. Therefore, 
since the driving force of an actuator for performing the focus control 
and the tracking control is decreased, the actuator is downsized and then 
the optical pickup is also downsized. 
According to one aspect of the present invention, the non-diffracted light 
is converged onto the recording surface of the first optical record medium 
and a spherical aberration of the non-diffracted light converged onto the 
recording surface of the first optical record medium is minimized and the 
first order diffracted light is converged onto the recording surface of 
the second optical record medium and a spherical aberration of the first 
order diffracted light converged onto the recording surface of the second 
optical record medium is minimized. Therefore, since the smallest light 
spot of the non-diffracted light is converged onto the recording surface 
of the first optical record medium and the smallest light spot of the 
first order diffracted light is converged onto the recording surface of 
the second optical record medium, the first and second record mediums, 
each of which has a protection film different in thickness from each 
other, in which data is recorded in high density, can be easily 
reproduced. 
According to another aspect of the present invention, the non-diffracted 
light is converged onto the recording surface of one of more than two 
optical record mediums and a spherical aberration of the non-diffracted 
light converged onto the recording surface is minimized and each of a 
plurality of the diffracted lights is converged onto the recording surface 
of said more than two optical record mediums other than said one of the 
more than two optical record mediums and a spherical aberration of each of 
said plurality of the diffracted lights converged onto the recording 
surface is minimized. Therefore, since the smallest light spot of the 
non-diffracted light is converged onto the recording surface of the one of 
more than two optical record mediums and the smallest light spot of each 
of said plurality of the diffracted lights converged onto the recording 
surface of another optical record medium of said more than two optical 
record mediums other than said one of the more than two optical record 
mediums, more than two optical record mediums, each of which has a 
protection film different in thickness each other, in which data is 
recorded in high density, can be easily reproduced. 
In the above description, although the optical pickup, which records into 
or reproduces the optical disk, is explained as an example, the present 
invention is applied to an optical card, which can be recorded into or 
reproduced by the optical pickup of the present invention. 
The present invention may be embodied in other preferred forms by using the 
conventional optical system without departing from the spirit or essential 
characteristics thereof. In the other embodiments, if the collimator lens, 
which consists of a hologram, is used to divide a laser beam into a 
non-diffracted light, which is a portion of the light beam other than 
diffracted lights, and a diffracted light and is designed so that the 
non-diffracted light is converged onto a recording surface of an optical 
disk and the diffracted light is converged onto a recording surface of 
another optical disk, the thickness of which is different from that of the 
optical disk, an optical pickup, which can reproduce two optical disks, 
each of which has a protection film different in thickness from each 
other, can be constructed. 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.