Optical pickup apparatus for use with various types of optical disks

An optical pickup apparatus comprises a first semiconductor laser, a first beam splitter which partially reflects or transmits a laser beam generated by the first semiconductor laser in a given direction, a second semiconductor laser which emits a laser beam having a different wave length from the laser beam of the first semiconductor laser, a second beam splitter which partially reflects or transmits the laser beam generated by the second semiconductor laser in a given direction, a single photodetector which receives laser beams from the first and second semiconductor lasers when the laser beams are reflected by a recording medium, and a first reflective portion which is formed on the first beam splitter. The first reflective portion partially reflects a given polarized light beam from the first semiconductor laser and transmits a polarized light beam from the second semiconductor laser beam, which differs from the given polarized light beam. A second reflective portion is formed on the second beam splitter. The second reflective portion partially reflects a given polarized light beam from the second semiconductor laser and transmits a polarized light beam from the first semiconductor laser beam, which differs from the given polarized light beam.

BACKGROUND OF THE INVENTION 
a) Field of the Invention 
The present invention relates to an optical pickup apparatus used in an 
optical disk device, in particular, it relates to an optical pickup 
apparatus which can be commonly used for recording media having protective 
layers with a different thickness. 
b) Description of the Related Art 
Conventional compact discs ("CDs" hereafter) have been widely used as 
optical discs on which information signals are optically recorded. 
Recently, with progress in technology to densify optical discs, digital 
video discs ("DVD" hereafter), in which animated images with a length of 
several hours can be written in and read from an optical disc with a 
diameter same as a CD, has come into actual use. Currently, various kinds 
of optical pickup apparatuses are suggested for reading both CDs and DVDs. 
Although CDs and DVDs share basic principles, they differ in the thickness 
such that CDs have a thickness of 1.2 mm while DVDs have a thickness of 
0.6 mm, which is a half of the thickness of CDs. Therefore, in order to 
realize an optical pickup apparatus for reading both CDs and DVDs, it is 
required to be structured such that spherical aberrations due to 
differences in the disc thickness are canceled. 
Examples to realize an optical pickup apparatus for reading both CDs and 
DVDs are an objective lens switching method and a compensation device 
method. Both of these methods utilize a common light source. In the 
objective lens switching method, two objective lenses facing optical discs 
are used for CDs and DVDs, which are switched according to which type of 
disc is read; in the compensation device method, spherical aberrations due 
to the differences in the disc thickness are canceled by a compensation 
device. 
Both CDs and DVDs are optical discs which share basic principles. However, 
in an optical pickup device for CDs, a semiconductor laser with a 
wavelength of 780 nm as a light source is used, while a semiconductor 
laser with a short wavelength of 650 to 630 nm is used as a light source 
in an optical pickup device for DVDs. Hence, in order to enable reading of 
both CDs and DVDs using a common light source as described above, a 
semiconductor laser with a short wavelength of 650 to 630 nm shall be used 
as a light source. Also, there is no negative effect, such as damaging a 
reflective film of CDs, caused by reading CDs with such a short wavelength 
laser. As a result, if a semiconductor laser with a short wavelength of 
650 to 630 nm is applied as a light source, the semiconductor laser can be 
commonly used for CDs and DVDs by switching objective lenses or by using a 
compensation device. 
Nonetheless, CDs have been developed in various modes; for example, CD-Rs 
are capable of addition and writing. The reflective film of the CD-R is 
designed such that maximum performance can be obtained by using a laser 
for CDs with a wavelength of 780 nm, therefore, the reflective film is 
said to be highly dependent of wavelength. As a result, in the case of 
using a semiconductor laser with a short wavelength of 650 to 630 nm in 
order to be applicable for both CDs and DVDs, as described above, the 
reflective film of the CD-R cannot reflect the short wavelength laser 
beam, therefore, information signals recorded on the CD-R cannot be 
reproduced. In addition, when such a short wavelength laser beam is 
irradiated on the reflective film of the CD-R, the reflective film is 
heated by absorbing the short wavelength laser beam such that it could be 
damaged. 
In order to realize an optical pickup apparatus which can be commonly used 
for CDs and DVDs and which can read CD-Rs, in general, it is possible to 
use a plurality of kinds of light sources corresponding to kinds of 
recording media, as well as to connect detecting devices corresponding to 
each of the light sources for selecting a light source and a detecting 
device corresponding to the kind of used recording media. However, an 
increase in the number of parts causes increased costs in addition to 
inconvenience that the detecting device has to be switched corresponding 
to the kind of the used recording medium. 
OBJECT AND SUMMARY OF THE INVENTION 
In consideration of the above issues, the primary object of the present 
invention is to provide an optical pickup apparatus in which: 
for reading CDs and DVDs, a common optical system and a single photo 
detector are used; 
signals on CD-Rs can be read without damaging the reflective film; 
a loss of luminous energy is small; 
dependency of the reflective film to angles is low such that stable 
performance is obtained; and 
stable reading can be performed on a recording medium with a large 
birefringence. 
In accordance with the invention, an optical pickup apparatus comprises a 
first semiconductor laser, a first beam splitter which partially reflects 
or transmits a laser beam generated by the first semiconductor laser in a 
given direction, a second semiconductor laser which emits a laser beam 
having a different wave length from the laser beam of the first 
semiconductor laser, a second beam splitter which partially reflects or 
transmits the laser beam generated by the second semiconductor laser in a 
given direction, a single photodetector which receives laser beams from 
the first and second semiconductor lasers when the laser beams are 
reflected by a recording medium, and a first reflective portion which is 
formed on the first beam splitter. The first reflective portion partially 
reflects a given polarized light beam from the first semiconductor laser 
and transmits a polarized light beam from the second semiconductor laser 
beam, which differs from the given polarized light beam. A second 
reflective portion is formed on the second beam splitter. The second 
reflective portion partially reflects a given polarized light beam from 
the second semiconductor laser and transmits a polarized light beam from 
the first semiconductor laser beam, which differs from the given polarized 
light beam.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the optical pickup apparatus of the present invention are 
described herein with reference to the drawings. 
In FIGS. 1 and 2, optical pickup apparatus 1 comprises base 3 which is 
contactingly attached along two guide shafts 2a, 2b attached in parallel 
with the frame (not illustrated) of the apparatus. The optical system 
described herein is constructed on base 3. 
As shown in FIG. 3, the optical system of optical pickup apparatus 1 
comprises first semiconductor laser 4 which emits first laser beam L1 and 
second semiconductor laser 5 which emits second laser beam L2; wherein 
first and second laser beams L1, L2 emitted from semiconductor lasers 4, 5 
are guided to a common optical path, thereby performing reading/writing of 
any data storage media 25 including CDs, CD-Rs, and DVDs. 
The common optical path is constructed with first beam splitter 21, second 
beam splitter 22, collimator lens 7, mirror 8, objective lens 9, sensor 
lens 10 and photo detector 11. 
First semiconductor laser 4 and second semiconductor laser 5 are in 
conjugate with single photo detector 11 and collimator lens 7. 
First semiconductor laser emits laser beam L1 of 780 nm wavelength for 
reading CDs. Second semiconductor laser 5 emits laser beam L2 of 650 to 
630 nm wavelength which is shorter than the laser beam of first 
semiconductor laser 4. First semiconductor laser 4 and second 
semiconductor laser emit laser beams L1, L2 in parallel to each other. 
Laser beam L1 enters first slope 23 such as a first reflective portion of 
first beam splitter 21 comprising a prism; laser beam L2 enters second 
slope 24 such as a second reflective portion of second beam splitter 22 
comprising a prism. First slope 23 and second slope 24 are arranged in 
parallel to each other and also are slanted 45 degree from the axial 
center of laser beams L1, L2. In addition, multi-layer films are formed on 
each of slopes 23 and 24 to reflect laser beams L1 and L2 for guiding to 
the same direction (upper left in FIG. 1). Laser beam L1 is S-polarized 
with respect to first and second slopes 23 and 24; laser beam L2 is 
P-polarized with respect to second slopes 23 and 24. 
Multi-layer films are formed on first slope 23 and second slope 24 to 
adjust optical transmissivity and reflectivity on these slopes to an 
appropriate value. The multi-layer films formed on first slope 23 
functions as a half mirror to S-polarized laser beam L1 partially 
reflecting the laser beam while it transmits P-polarized laser beam L2. 
The multi-layer film formed on second slope 24 functions as a half mirror 
formed to P-polarized laser beam L2 partially reflecting the laser beam 
while it transmits S-polarized laser beam L1. 
FIG. 7 shows an example of a design for spectroscopic characteristics of 
first slope 23 and second slope 24. In FIG. 7, "T" indicates a 
transmissivity, "R" indicates reflectivity, and "-" indicates an arbitrary 
value. In this design example, first slope 23 has more than 90% of 
transmissivity to laser beam L2, that is a P-polarized beam of 650 nm 
wavelength, so that most of the P-polarized beam of 650 nm wavelength is 
transmitted; on the other hand, it has 50% of transmissivity and 
reflectivity to laser beam L1, that is S-polarized laser beam of 780 nm 
wavelength, so that it functions as a half mirror to laser beam L1. Second 
slope 24 functions as a half mirror to the P-polarized beam of 650 nm 
wavelength (laser beam L2) with 50% transmissivity and reflectivity while 
it transmits most of the S-polarized beam of 780 wavelength (laser beam 
L1) with more than 90% transmissivity. Other characteristics are 
arbitrary. 
FIG. 5 shows the spectroscopic characteristics of first slope 23 as 
mentioned above. FIG. 6 shows the spectroscopic characteristics of second 
slope 24 as mentioned above. In FIGS. 5 and 6, Tp indicates transmittivity 
with respect to the P-polarized laser beam; Ts indicates transmittivity 
with respect to the S-polarized laser beam. First and second slopes 23, 24 
are structured with multi-layer film coated as mentioned above. The more 
layers, the more dependency to angles the film has, thereby providing 
different transmissivity for different incidence angles. As shown in FIG. 
4, spectroscopic characteristics of first and second slopes are different 
for laser beams a, b, c where; a is a laser beam which enters the slopes 
at 45 degree angle and is reflected; b is a laser beam which enters the 
slopes at a larger angle and is reflected; and c is a laser beam which 
enters the slope at a smaller angle and is reflected. Therefore, in FIGS. 
5 and 6, spectroscopic characteristics are shown according to each of the 
laser beams a, b and c. 
In the example of the spectroscopic characteristics shown in FIG. 5, first 
slope 23 has a good transmissivity to the P-polarized beam of 650 
wavelength (laser beam L2); also, it has both transmissivity and 
reflectivity at about 50% to the S-polarized beam of 780 wavelength (laser 
beam L1) to satisfy the characteristics required for first slope 23 of the 
design example shown in FIG. 7. In the example of the spectroscopic 
characteristics of second slope 24 shown in FIG. 6, on the other hand, 
transmissivity to the P-polarized beam of 650 wavelength is about 50%, 
that is reflectivity is also about 50%; also, transmissivity to the 
S-polarized beam of 780 wavelength is high so that the characteristics 
required for second slope 24 of the design example shown in FIG. 7 are 
satisfied. 
Furthermore, it is obvious from FIGS. 5 and 6 that first and second slopes 
23, 24 have low dependency to angles at critical points on the 
characteristics curves. This is due to the fact that it is relatively easy 
to satisfy the above required characteristics such that a small number of 
layers for the multi-layer films is needed to satisfy the required 
characteristics. The fact that first and second slopes have low dependency 
to angles is advantageous in terms of minimizing the size of the optical 
pickup apparatus. In other words, if dependency to angles are low in the 
spectroscopic characteristics of first and second slopes 23, 24, as shown 
in FIG. 4, positioning the slopes at emitting portions of laser beams does 
not have much effect so that it is not necessary to position the slopes in 
parallel light beams, that is, an infinity optical system. Hence, there is 
not much impact even when a beam is emitted directly from the 
semiconductor laser into the slope for reflection without placing a 
collimator lens between the semiconductor laser and the slope. Therefore, 
it is advantageous that the number of parts decreases since the collimator 
lens is unnecessary as well as that the size of the optical pickup 
apparatus can be reduced. 
First slope 23 of first beam splitter 21 and second slope 24 of second beam 
splitter 22 could logically be constructed as a standard half mirror. 
However, if first and second slopes 23, 24 are constructed as a standard 
half mirror, luminous energy would decrease to about 1/8th during the time 
when laser beams are emitted from semiconductor lasers 4, 5; this would be 
problematic in actual practice. In contrast, according to the mode of the 
above embodiment, luminous energy does not decrease while laser beams L1, 
L2 are emitted from semiconductor lasers 4, 5 and reach photo detector 11; 
the amount of luminous energy reaching photo detector 11 is twice the 
amount in the case of using the above standard half mirror. As a result, 
errors in reading signals recorded on recording media 25 can be reduced. 
Also, a conventional polarizing type using a 1/4 plate and a polarizing 
beam splitter is effective in terms of laser beam L1 emitted from first 
semiconductor laser 4 and laser beam L2 emitted from second semiconductor 
laser 5 to effectively enter photo detector 11. In other words, a linearly 
polarized laser beam emitted from the semiconductor laser is reflected by 
the polarized beam splitter and converted to a circularly polarized beam 
by the 1/4 plate. Then, the laser beam, which returns as a circularly 
polarized beam after being reflected by a disc, is transmitted through the 
1/4 plate again such that it is converted to a linearly polarized beam 
which is perpendicular to the original linearly polarized beam. This 
polarizing type provides a high efficiency in use of the laser beams; 
however, it does not correspond well to birefringence of recording media 
such that reading cannot be performed if there is birefringence. On the 
contrary, this embodiment is a non-polarizing type by eliminating the 1/4 
plate; therefore, birefringence on recording media does not affect the 
process as much, and errors in reading signals recorded on recording media 
25 can be reduced. 
Recording media 25 comprises a reflective film which is protected by a 
transparent protection film with an even thickness. The above mentioned 
recording tracks are formed on the reflective film. The reflected beam by 
the reflective film of recording media returns going through in order, 
objective lens 9, mirror 8 and collimator lens 7; then it is transmitted 
through first slope 23 of first beam splitter 21 and second slope 24 of 
second beam splitter 22. After passing through sensor lens 10, the beam is 
detected by photo detector 11. Wave length 780 nm of laser beam L1 emitted 
from first semiconductor laser 4 and wavelength of 650 to 630 nm emitted 
from second semiconductor laser 5 are reflected on recording media with a 
different thickness; then they are detected by single photo detector 11. 
In other words, first and second semiconductor lasers 4 and 5 are placed 
optically conjugate with above single photo detector, respectively. As a 
result, laser beam L1 or laser beam L2, reflected by recording media 25, 
is converged onto the photo detecting surface of photo detector 11. 
As is widely known, photo detector 11 comprises quarterly split devices and 
the like. When the laser beam converged onto these devices can be deviated 
on the split devices, tracking errors and focusing errors are detected. 
Objective lens 9 is driven in the tracking direction or the focusing 
direction according to the tracking error detecting signals or focusing 
error detecting signals such that tracking control and focusing control 
are performed. It is also known that signals recorded on recording media 
25 are detected by the split devices. 
The following briefly explains the operation of the above embodiment. To 
regenerate CDs or CD-Rs, S-polarized laser beam L1 is emitted at a 
wavelength of 780 nm from first semiconductor laser 4 to the reflective 
surface of the beam splitter. 50% of laser beam L1 is reflected on first 
slope 23, then converged on the recording tracks of the CD or CD-R after 
passing through collimator lens 7, mirror 8 and objective lens 9. Once 
laser beam L1 is reflected on the CD or CD-R, it goes back passing through 
in order, objective lens 9, mirror 8, and collimator lens 7. Then, 50% of 
the laser beam is transmitted through first slope 23; most of the 
transmitted beam is transmitted through second slope 24. Thereafter, it 
enters photo detector 11 via sensor lens 10. By a detecting output of 
photo detector 11, regeneration of the CD or CD-R is carried out, as well 
as detection of tracking and focusing. 
To regenerate DVDs, P-polarized laser beam L2 is emitted at a wavelength of 
650 nm from second semiconductor laser 5 to the reflective surface of the 
beam splitter. 50% of laser beam L2 is reflected at second slope 24. Most 
of the reflected beam is transmitted through first slope 23, then 
converged on the recording tracks of the DVD after passing through 
collimator lens 7, mirror 8 and objective lens 9. Laser beam L2 reflected 
on the DVD goes back passing through in order, objective lens 9, mirror 8, 
and collimator lens 7. 50% of the beam is transmitted through first slope 
23, then, most of the transmitted beam is transmitted through second slope 
24. Thereafter, it enters photo detector 11 via sensor lens 10. By a 
detecting output of photo detector 11, regeneration of the DVD is carried 
out, as well as detection of tracking and focusing. 
According to the above explained embodiment, laser beam L1 from first 
semiconductor laser 4 is to be S-polarized in relation with the reflective 
surface of the beam splitter; laser beam L2 from second semiconductor 
laser 5 is to be P-polarized. Splitters 21, 22 comprise first slope 23 
which partially reflects laser beam L1 from first semiconductor laser 4 
and which transmits laser beam L2 from second semiconductor laser 5; and 
second slope 24 which partially reflects laser beam L2 from second 
semiconductor laser 5 and which transmits laser beam L1 from first 
semiconductor laser. Therefore, the lost amount of laser beams L1, L2, 
which are emitted from first and second semiconductor lasers 4, 5, is 
small such that by allowing laser beams L1, L2 to effectively enter the 
photo detector, reading can be reliably performed. Also, it is possible to 
regenerate DVDs and CDs by using a common optical system comprising beam 
splitters 21, 22, collimator lens 7, objective lens 9 and photo detector 
11. As a result, an optical pickup apparatus for two kinds of wavelengths 
with a simple structure can be provided; at the same time, semiconductor 
lasers 4, 5 can be selectively used to emit a laser beam of a wavelength 
appropriate for regeneration of each medium. 
In addition, in the case of regenerating CD-Rs, first semiconductor laser 4 
is used to emit a laser beam of a long wavelength appropriate for the 
purpose; therefore, CD-Rs can be regenerated without damaging the 
reflective film. 
Transmissivity of first and second slopes 23, 24 is determined by 
selectively using semiconductor lasers 4, 5 for two kinds of wavelengths 
and by selectively using polarized beams from the semiconductor lasers to 
the reflective surface of the beam splitters between a S-polarized beam 
and P-polarized beam. As a result, the characteristics required for the 
reflective film formed on each of slopes 23, 24 can be obtained by 
relatively easy coating. Hence, the reflective film with low dependency to 
angles and stable performance can be obtained. Also, it is not necessary 
to place a collimator lens between the semiconductor lasers and beam 
splitters; a single collimator lens can be used commonly for laser beams 
L1, L2 with different wavelengths. Therefore, the number of parts can be 
reduced such that it is advantageous in terms of the costs. 
Also, the non-polarizing type, in which a 1/4 plate or a polarizing beam 
splitter is not used, is employed; therefore, a recording medium with 
large birefringence can be regenerated in a stable manner. 
The following describes another embodiment of the optical pickup system of 
the present invention. 
In the above embodiment, first and second slopes are formed as prisms 21, 
22; however, it is possible to form first slope 23 as a prism and second 
slope, that is the slope on the side of photo detector 11, as a horizontal 
plate, that is a plate mirror. 
Also, in the above embodiment, first semiconductor laser 4 and first slope 
23 are placed closer to the side of objective lens 9 than second 
semiconductor laser 5 and second slope 24. However, the reversed 
positioning of these is possible; that is, second semiconductor laser 5 
and second slope 24 can be placed closer to the side of objective lens 9 
than first semiconductor laser 4 and first slope 23. In this case, first 
semiconductor lasers 4 and second semiconductor laser 5 still shall be 
placed conjugate with collimator lens 7 and photo detector 11. 
A diffraction grating can be placed between first semiconductor laser 4 and 
first slope 23 or between second semiconductor laser 5 and second slope 
24. Or, diffraction gratings can be placed both between first 
semiconductor laser 4 and first slope 23 and between second semiconductor 
laser 5 and second slope 24. By placing the diffraction grating, each of 
the laser beams can be separated into zero order light, plus-one order 
light and minus-one order light such that tracking control by three beams 
can be performed. 
As previously described, the above embodiment employs the non-polarizing 
type such that dependency of the slopes to angles is low; therefore, the 
slopes can be placed in the diffuse optical path without serious negative 
effects. Hence, the collimator lens can be completely omitted to make the 
optical system definite. As a result, the number of parts can be further 
reduced such that in turn the costs are decreased. 
The combination of first and second semiconductor lasers 4, 5 can be 
changed. In other words, one semiconductor laser has a P-polarized beam of 
780 wavelength, and another has a S-polarized beam of 650 to 630 nm. Also, 
the wavelengths are not limited to 780 nm and 650 to 630 nm; combinations 
of other wavelengths can be applied. In addition, the wavelengths of first 
and second semiconductor lasers can be switched with the positioning of 
the rest of the optical system remains the same while the mode of the 
embodiment shown in the figures. In this case, the spectroscopic 
characteristics and polarization dependency of beam splitters 21, 22 shall 
be adjusted according to the semiconductor lasers corresponding to these 
beam splitters 21, 22. 
According to the present invention, the lost amount of laser beams emitted 
from first and second semiconductor lasers is small such that by allowing 
the laser beams to effectively enter the photo detector, reading can be 
reliably performed due to the facts that the wavelength of the first 
semiconductor laser differs from the wavelength of the second 
semiconductor laser such that the laser beam from the first semiconductor 
laser is a given polarized beam and the laser beam from the second 
semiconductor laser is a polarized beam different from the above given 
polarized beam; and a first reflective portion which partially reflects 
the laser beam from the first semiconductor laser and which transmits the 
laser beam from the second semiconductor laser; and a second reflective 
portion which partially reflects the laser beam from the second 
semiconductor laser and which transmits the laser beam from the first 
semiconductor laser. 
Also, except for first and second semiconductor lasers, a common optical 
system comprising the first and second beam splitters, collimator lens, 
objective lens and photo detector is employed; therefore, regeneration of 
DVDs and CDs is possible. In addition, an optical pickup apparatus for two 
kinds of wavelengths with a simple structure can be provided; at the same 
time, semiconductor lasers can be selectively used to emit a laser beam of 
a wavelength appropriate for regeneration of each medium. 
In the case of regenerating CD-Rs, it is established such that one of the 
semiconductor lasers is used for emitting a laser beam of a long 
wavelength appropriate for CD-Rs; therefore, CD-Rs can be regenerated 
without damaging the reflective film. 
Transmissivity of the first and second reflective portion is determined by 
selectively using the two semiconductor lasers for two kinds of 
wavelengths and by selectively using polarized beams from the 
semiconductor lasers to the reflective surface of the beam splitters 
between a S-polarized beam and P-polarized beam. As a result, the 
characteristics required for the reflective film formed on each of the 
slopes can be obtained by relatively easy coating. Hence, the reflective 
film with low dependency to angles and stable performance can be obtained. 
Also, it is not necessary to place a collimator lens between the 
semiconductor lasers and the beam splitters; a single collimator lens can 
be used commonly for the laser beams with different wavelengths. 
Therefore, the number of parts can be reduced such that it is advantageous 
in terms of the costs. 
Also, the non-polarizing type, in which a 1/4 plate or a polarizing beam 
splitter is not used, is employed; therefore, a recording medium with 
large birefringence can be regenerated in a stable manner. 
While the foregoing description and drawings represent the preferred 
embodiments of the present invention, it will be obvious to those skilled 
in the art that various changes and modifications may be made therein 
without departing from the true spirit and scope of the present invention.