Title: Optical pickup having two wavelength laser and simple structure
Abstract: In an optical pickup used in a DVD player, a photo detector has a predetermined photo sensing area pattern for detecting a returning laser beam from an optical disc. The returning laser beam is originated from a first laser beam or a second laser beam. The first and the second laser beams are alternatively emitted from a two wavelength laser having first and second light sources. Either the first laser beam or the second laser beam travels to the optical disc through a grating, a polarizing beam splitter, a collimating lens, a rising mirror, and an object lens and is reflected by the optical disc. The reflected laser beam reflected from the optical disc travels to the photo detector as the returning laser beam through the object lens, the rising mirror, the collimating lens, and the polarizing beam splitter. The predetermined photo sensing area pattern enables the photo detector to detect the returning laser beam regardless of the origin of the returning laser beam.
Patent Number: 6,925,045 Issued on 08/02/2005 to Sugawara, et al.
| Inventors:
|
Sugawara; Masayoshi (Yamagata, JP);
Sanpei; Hiroshi (Yamagata, JP)
|
| Assignee:
|
Mitsumi Electric Co., Ltd. (Tokyo, JP)
|
| Appl. No.:
|
835205 |
| Filed:
|
April 13, 2001 |
Foreign Application Priority Data
| Apr 13, 2000[JP] | 2000-111988 |
| Current U.S. Class: |
369/112.01; 369/112.05; 369/112.21 |
| Intern'l Class: |
G11B 007/00 |
| Field of Search: |
369/4437,112.05,441.1,441.4,442.3,442.6,442.7,444.1,112.01,112.02,112.03,112.04,112.06,112.1,112.15,112.16,112.21,112.29
|
References Cited [Referenced By]
U.S. Patent Documents
| 5986998 | Nov., 1999 | Park.
| |
| 6043911 | Mar., 2000 | Yang.
| |
| 6084843 | Jul., 2000 | Abe et al.
| |
| 6091691 | Jul., 2000 | Yoo et al.
| |
| 6236633 | May., 2001 | Chang et al.
| |
| 6358764 | Mar., 2002 | Nemoto.
| |
| 6480456 | Nov., 2002 | Kawamura et al.
| |
| 6507009 | Jan., 2003 | Ohnishi et al.
| |
| 6646975 | Nov., 2003 | Uchizaki et al.
| |
| Foreign Patent Documents |
| 11-144284 | May., 1999 | JP.
| |
| 11-149652 | Jun., 1999 | JP.
| |
Primary Examiner: Tran; Thang V.
Attorney, Agent or Firm: Frishauf, Holtz, Goodman, & Chick, P.C.
Claims
1. An optical pickup for applying a reading laser beam to an optical disc and
for detecting a returning laser beam reflected from said optical disc, said optical
pickup comprising:
a two wavelength laser having first and second light sources to emit first and
second laser beams, respectively, for alternatively applying one of said first
laser beam and said second laser beam to said optical disc as said reading laser
beam, said first and said second laser beams having optical axes parallel to a
first direction and having different wavelengths,
a polarizing beam splitter disposed adjacent to said two wavelength laser in
the first direction for one of partially passing and partially reflecting said
reading laser beam from said two wavelength laser to lead said reading laser beam
to said optical disc, and for one of partially reflecting and partially passing
said returning laser beam formed by reflecting said reading laser beam with said
optical disc to lead said returning laser beam in a second direction different
from said first direction,
a photo detector disposed adjacent to said polarizing beam splitter in the second
direction and having a predetermined photo sensing area pattern for detecting said
returning laser beam traveling in the second direction from said polarizing beam
splitter regardless of which one of the first laser beam and the second laser beam
forms the returning laser, and
a grating disposed between said two wavelength laser and said polarizing beam
splitter for dividing said reading laser beam into three divided laser beams,
wherein said photo detector comprises three photodiodes which respectively correspond
to said three divided laser beams, and which form said photo sensing area pattern,
and
wherein a middle one of said photodiodes comprises first and second photo sensing
areas each of which serves as four divisional photodiodes, and each of the first
and the second photo sensing areas receives a middle one of said three divided
laser beams originating from a respective one of said first laser beam and said
second laser beam.
2. The optical pickup as claimed in claim 1, wherein said first sensing area
includes a portion in common with said second sensing area.
Description
BACKGROUND OF THE INVENTION
This invention relates to an optical pickup for use in an optical disc driver
such as a DVD player and, in particular, to an optical pickup which has a two wavelength
laser for emitting two laser beams with different wavelengths.
As is well known in the art, a DVD (Digital Versatile or video Disc) player can
plays not only a DVD but also a CD (Compact Disc). To permit playing both of the
DVD and the CD, the DVD player has a special optical pickup with a special structure.
The special optical pickup has two laser diodes to alternatively emit a first reading
laser beam for the DVD or a second reading laser beam for the CD. The first reading
laser beam is different from the second reading in wavelength. That is, the first
reading beam has a shorter wavelength of about 650 nm while the second reading
laser beam has a longer wavelength of about 780 nm. The special optical pickup
is called two-wavelength correspondence type optical pickup.
A conventional two-wavelength correspondence type optical pickup comprises the
first and the second laser diodes as mentioned above. The first and the second
laser diodes are apart from each other and produce the first and the second reading
laser beams, respectively. The first laser diode is called a DVD-LD while the second
diode is called a CD-LD.
The optical pickup further comprises an optical system to lead the first laser
beam from the first laser diode to an optical disc loaded in an optical disc player
to which the optical pickup is attacked. The optical system also leads the second
laser beam from the second laser diode to the optical disc. In addition, the optical
system leads a reflected light beam reflected from the optical disc to a photo detector.
The conventional optical pickup needs two gratings and two polarizing beam splitters
to correspond to two of the laser diodes in the optical system, because the laser
diodes are relatively distant from each other. Accordingly, the conventional optical
pickup has a problem that it has a large number of components and a complicated structure.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide an optical pickup having
a simple structure.
It is another object of this invention to provide an optical pickup which can
be easily assembled.
It is still another object of this invention to provide an optical pickup which
is inexpensive.
It is further still another object of this invention to provide an optical pickup
which is reliable.
Other objects of this invention will become clear as the description proceeds.
On describing the gist of an aspect of this invention, it is possible to be understood
that an optical pickup is for applying a reading laser beam to an optical disc
and for detecting a returning laser beam reflected from the optical disc.
According to one aspect of this invention, the optical pickup comprises
a two wavelength laser having first and second light sources to emit first and
second laser beams, respectively, in a first direction for alternatively applying
the first laser beam or the second laser beam to the optical disc as the reading
laser beam. The first and the second laser beams have optical axes parallel to
a first direction and are different from each other in wavelength. A polarizing
beam splitter is disposed on a side of the first direction against the two wavelength
laser and partially passes or reflects the reading laser beam from the two wavelength
laser to lead the reading laser beam to the optical disc. The polarizing beam splitter
also partially reflects or passes the returning laser beam which is formed by reflecting
the reading laser beam with the optical disc to lead the returning laser beam in
a second direction different from the first direction. A photo detector is disposed
on a side of the second direction against the polarizing beam splitter and has
a predetermined photo sensing area pattern. The photo detector detects the returning
laser beam traveling in the second direction from the polarizing beam splitter
regardless of whether the returning laser is originated from the first laser beam
or the second laser beam.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a conventional optical pickup;
FIG. 2 is a schematic view of a photo sensing area pattern of a photo detector
used in the conventional optical pickup of FIG. 1;
FIG. 3 is a schematic view of an optical pickup according to a preferred embodiment
of this invention; and
FIG. 4 is a schematic view of a photo sensing area pattern of a photo detector
used in the optical pickup of FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT:
Referring to FIGS. 1 and 2, description will be at first directed to a
conventional optical pickup for a better understanding of this invention.
FIG. 1 is a system construction diagram of an optical system of the conventional
optical pickup used in an optical disc driver such as a DVD driver.
In FIG. 1, the optical pickup comprises first and second laser diodes (LD
1)
11 and (LD
2)
12, first and second gratings (GRT
1)
13
and (GRT
2)
14, first and second polarizing beam splitters (PBS
1)
15 and (PBS
2)
16, a collimating lens (CL)
17, a rising
mirror (or a 45 degree mirror) (FM)
18, an object lens (OL)
19, and
a photo detector (PD)
20.
The first laser diode
11 emits a first reading laser beam (L
1)
having a first optical axis and a wavelength of about 650 nm for playing a DVD
(Digital Versatile or video Disc) and is called a DVD-LD. The second laser diode
12 emits a second reading laser beam (L
2) having a second optical
axis and a wavelength of about 780 nm for playing a CD (Compact Disc) and is called
a CD-LD. The first laser diode
11 and the second laser diode
12 are
arranged so as to leave a predetermined interval between them and so that the first
optical axis and the second optical axis are parallel to each other and extend
in a first direction (or a vertical direction of FIG.
1).
The first grating
13 divides the first laser beam L
1 from the first
laser diode
11 into three first divided laser beams (i.e. a first central
beam and two first side beams located on both sides of said first central beam).
Similarly, the second grating
14 divides the second laser beam L
2
from the second laser diode
12 into three second divide laser beams (i.e.
a second central beam and two second side beams located on both sides of said second
central beam).
The first polarizing beam splitter
15 reflects fractions of the first
divided laser beams in a direction of the second polarizing beam splitter
16.
Moreover, the first polarizing beam splitter
15 passing reflected laser
beams (reflected from an optical disc
21) transmitted from the second polarizing
beam splitter
16 therein as mentioned later. Here, the first polarizing
beam splitter
15 substantially has a reflectance of 50% and a transmissivity
of 50% for light having a wavelength of about 650 nm and a transmissivity of 100%
for light having a wavelength of about 780 nm.
The second polarizing beam splitter
16 reflects fractions of the second
divided laser beams in a direction of the collimating lens
17. In addition,
the second polarizing beam splitter
16 passing the first divided laser beams
and the reflected beams between the first polarizing beam splitter
15 and
the collimating lens
17 as mentioned below. Here, the second polarizing
beam splitter
16 substantially has a transmissivity of 100% for the light
having the wavelength of about 650 nm and a reflectance of 50% and a transmissivity
of 50% for the light having the wavelength of about 780 nm.
The first and the second laser diodes
11 and
12, the first and
the second gratings
13 and
14, and the first and the second polarizing
beam splitters
15 and
16 are arranged so that optical axes of the
first divided laser beams coincide with that of the second divided laser beams
between the second polarizing beam splitters
15 and the collimating lens
17.
The collimating lens
17 collimates the first and the second divided laser
beams from the second polarizing beam splitter
16 to form parallel laser
beams. The rising mirror
18 reflects the parallel laser beams from the collimating
lens
17 at a right angle to lead the parallel laser beams to the object
lens
19. The object lens
19 condenses the parallel laser beams from
the rising mirror
18 to form condensed laser beams. The condensed laser
beams are applied to a recording layer of the optical disc.
The optical disc
21 reflects the condensed laser beams according to pits
formed in the recording layer thereof and forms the reflected beams. The reflected
beams from the optical disc
21 travel to the photo detector
20 through
the object lens
19, the rising mirror
18, the collimating lens
17,
the second polarizing beam splitter
16 and the first polarizing beam splitter
15. The reflected beams passed through the first polarizing beam splitter
15 have a second direction (or a horizontal direction of FIG. 1) which is
perpendicular to the first direction.
The photo detector
20 has enough photosensitivity to detect the reflected
beams even if total luminous energy of the reflected beams is equal to or smaller
than 25% of that of the first laser beam L
1 or the second laser beam L
2.
Next, the description will be made about an operation of the optical pickup
of FIG. 1. A case where the optical disc
21 is a DVD is first, then a case
where the optical disc
21 is a CD.
When the optical disc
21 is the DVD, the first laser diode
11
is in an operation state while the second laser diode
12 is not in an operation
state. Accordingly, the first laser diode
11 alone emits the first laser
beam L
1.
The first laser beam L
1 emitted from the first laser diode
11 travels
to the first grating
13. The first grating
13 divides the first laser
beam L
1 into the first divided laser beams.
The first divided laser beams travel to the first polarizing beam splitter
15.
The first polarizing beam splitter
15 reflects the fractions of the first
divided laser beams. That is, each of the first divided laser beams is partly reflected
by the first polarizing beam splitter
15 and partly passes through the first
polarizing beam splitter
15. Because the first polarizing beam splitter
15 has the reflectance of 50% and the transmissivity of 50% for the light
having the wavelength of about 650 nm as mentioned above, the luminous power of
the first divided laser beams reflected from the first polarizing beam splitter
15 falls 50%.
The first divided laser beams reflected from the first polarizing beam splitter
15 travel to the collimating lens
17 through the second polarizing
beam splitter
16. Inasmuch as the second polarizing beam splitter
16
has the transmissivity of 100% for the light having the wavelength of about 650
nm as mentioned above, all the first divided laser beams which reach to second
polarizing beam splitter
16 pass through the second polarizing beam splitter
16.
The collimating lens
17 collimates the first divided laser beams passing
through the second polarizing beam splitter
16. The collimated first divided
laser beams collimated by the collimating lens
17 are reflected by the rising
mirror
18 and condensed by the object lens
19 to be applied to the
recording layer of the optical disc
21.
The recording layer of the optical disc
21 reflects the condensed laser
beams condensed by the object lens
19 according to the pits and forms first
reflected laser beams. The first reflected laser beams travel to the photo detector
20 through the object lens
19, the rising mirror
18, the collimating
lens
17, the second polarizing beam splitter
16, and the first polarizing
beam splitter
15. Because the first polarizing beam splitter
15 has
the reflectance of 50% and the transmissivity of 50% for the light having the wavelength
of about 650 nm, all the luminous power of the first reflected laser beams falls
50%. That is, the luminous power of the first reflected laser beams is equal to
25% of that of the first laser beam L
1.
Next, the description is directed to the case where the optical disc is the CD.
When the optical disc
21 is the CD, the second laser diode
12
is in the operation state while the first laser diode
11 is not in an operation
state. Accordingly, the second laser diode
12 alone emits the second laser
beam L
2.
The second laser beam L
2 emitted from the second laser diode
12
travels to the second grating
14. The second grating
14 divides the
second laser beam L
2 into the second divided laser beams.
The second divided laser beams travel to the second polarizing beam splitter
16. The second polarizing beam splitter
16 reflects the fractions
of the second divided laser beams. That is, each of the second divided laser beams
is partly reflected by the second polarizing beam splitter
16 and partly
passes through the second polarizing beam splitter
16. Because the second
polarizing beam splitter
16 has the reflectance of 50% and the transmissivity
of 50% for the light having the wavelength of about 780 nm as mentioned above,
the luminous power of the second divided laser beams reflected from the second
polarizing beam splitter
16 falls 50%.
The second divided laser beams reflected from the second polarizing beam splitter
16 are applied to the optical disc
21 through the collimating lens
17, the rising mirror
18, and the object lens
19 like the
first divided laser beams passing through the second polarizing beam splitter
16.
The optical disc
21 reflects the second divided laser beams and forms
second reflected laser beams. The second reflected laser beams travel to the photo
detector
20 like the first reflected laser beams through the object lens
19, the rising mirror
18, the collimating lens
17, the second
polarizing beam splitter
16, and the first polarizing beam splitter
15.
Because the second polarizing beam splitter
16 has the reflectance of 50%
and the transmissivity of 50% for the light having the wavelength of about 780
nm, the luminous power of the second reflected laser beams passing through the
second polarizing beam splitter
16 falls 50%. That is, the luminous power
of the second reflected laser beams is equal to 25% of that of the second laser
beam L
2. The first polarizing beam splitter
15 passes the second
reflected laser beams through therein without reflecting them.
In the optical disc player, focusing control and tracking control of the optical
pickup are indispensable for playing the optical disc. To carry the focusing control
and the tracking control, it is necessary to obtain a focus error signal and a
tracking error signal from the reflected laser beams.
The first (or the second) reflected laser beams includes three laser beams because
the first (or second) laser beam L
1 (or L
2) is divided into three
divided laser beams by the first (or second) grating
13 (or
14).
The central one corresponding to the first (or second) center beam is used for
producing both of a reading signal and the focus error signal. The remaining two
corresponding to the first (or second) side beams are used for producing the tracking
error signal.
As illustrated in FIG. 2, the photo detector
20 comprises a center photodiode
25 and side photodiodes
26 and
27 arranged on both sides of
the center photodiode
25 at predetermined intervals.
The center photodiode
25 has four sensing areas each of which serves as
one photodiode and is called a fourfold photodiode. The center photodiode receives
the center beam and produces first through fourth detecting signals in response
to the center beam.
The side photodiodes
26 and
27 receive the side beams and produce
fifth and sixth detecting signals in response to the side beams, respectively.
When the focusing control is made so that the condensed laser beams are exactly
condensed on the recording layer of the optical disc
21, a beam spot
251
formed on the center photodiode
25 has a circle in shape as shown in FIG.
2. In other words, when a distance between the object lens
19 and
the optical disc
21 is equal to a predetermined value, the beam spot
251
on the center photodiode
25 has a perfect circle in shape. On the other
hand, when the condensed laser beams are not condensed on the recording layer of
the optical disc
21, the beam spot formed on the center photodiode
25
has an ellipse in shape. In other words, the optical system makes the beam spot
251 thin when the distance between the object lens
19 and the optical
disc
21 increases or decreases from the predetermined value.
When the tracking control is made so that the condensed laser beams exactly
trace a recording track of the recording layer, beam spots
261 and
271
are entirely on the side photodiodes
26 and
27, respectively, as
shown in FIG.
2. On the other hand, when the condensed laser beams do not
exactly trace a recording track of the recording layer, the beam spot
261
(or
271) is partially out of the photodiode
26 (or
27), though
the beam spot
271 (or
261) is entirely on the photodiode
27
(or
26).
The optical disc driver provides a signal processing circuit (not shown) including
a first processing unit and a second processing unit. The first processing unit
produces the reading signal and the focus error signal on the basis of the first
through the fourth detecting signals supplied from the center photodiode
25.
The second processing unit produces the tracking signal on the basis of the fifth
and the sixth detecting signals supplied from the side photodiodes
26 and
27.
As mentioned above, the conventional optical pickup has the first laser diode
for the DVD and the second laser diode for the CD. The first laser diode is relatively
distant from the second laser diode. Accordingly, the optical pickup has a problem
that it has complicated constitution because it must have two gratings and two
polarizing beam splitters to correspond to the laser diodes.
Japanese Unexamined Patent Publication No. 11-144284 discloses a one-chip
laser diode which has two laser diodes. The one-chip laser diode is used in an
optical pickup of a DVD player to reduce cost and components of the optical pickup.
However, the optical pickup has two photo detectors which are separated from each
other. One of the photo detectors is integrated into the one-chip laser diode while
the other is independent of the one-chip laser diode. Nothing is made about a photo
detector for detecting two laser beam sets corresponding to the two laser diodes
in the above mentioned document.
Another one-chip laser diode is disclosed in Japanese Unexamined Patent Publication
No. 11-149652. The one-chip laser is integrated into an optical pickup unit together
with a photo detector for detecting two laser beam sets corresponding to two laser
diodes included in the one-chip laser. However, the one-chip laser has an impractical
structure that a distance between emitting points of the two laser diodes is equal
to a mere 10 μm. In addition, the optical pickup including the one-chip laser
diode does not have gratings for dividing laser beams supplied from the laser diodes.
Still another one-chip laser diode is disclosed in the above mentioned document
(No. 11-149652). The one-chip laser diode has a practical structure that a distance
between emitting points of two laser diodes is equal to a 200-500 μm. In
this case, laser beams emitted by the two laser diodes travel on different optical
paths different from each other. Accordingly, the laser beams can not be detected
by a conventional photo detector. However, nothing is made about a concrete means
for enabling a photo detector to detect the laser beams travelling on the different
optical paths.
Additionally, the above mentioned document (No. 11-149652) discloses
a hologram element for perpendicularly applying the laser beams to the optical
disc. However, the hologram element makes one of the laser beams coincide with
the other laser beam.
Referring to FIGS. 3 and 4, the description will proceed to an optical
pickup according to a preferred embodiment of this invention.
FIG. 3 is a system constitution diagram of the optical pickup which is as a
two wavelength corresponding optical pickup.
In FIG. 3, the two wavelength corresponding optical pickup comprises a two wavelength
laser
31, a grating (GRT)
32, a polarizing beam splitter (PBS)
33,
a collimating lens (CL)
34, a rising mirror (FM)
35, an object lens
(OL)
36, and a photo detector (PD)
37.
The two wavelength laser
31 includes a semiconductor chip on which first
and second laser diodes (not shown) are integrated. The first laser diode produces
a first laser beam L
1 which has a first optical axis and a wavelength of
about 650 nm and which is used for playing a DVD. The second laser diode produces
a second laser beam L
2 which has a second axis and a wavelength of about
780 nm and which is used for playing a CD. A distance between the first and the
second laser diodes (or emitting points) is relatively smaller. The distance is,
for example, equal to about 100 nm and can be made with precision of 1 μm.
The grating
32 divides the first and the second laser beams into first
and second three divided laser beams, respectively.
The polarizing beam splitter
33 has a reflectance of 50% and a transmissivity
of 50% against both of wavelengths of 650 nm and 780 nm. Accordingly, the polarizing
beam splitter
33 reflects the first and the second divided laser beams at
a rate of 50 percent towards the collimating lens
34. Moreover, the polarizing
beam splitter
33 at a rate of 50 percent passes return laser beams mentioned
below through therein toward the photo detector
37 regardless of whether
the returning laser beams are originated from the first laser beams or the second
laser beams.
The collimating lens
34 collimates the laser beams reflected from the
polarizing beam splitter
33 to form parallel beams and lead the parallel
beams to the rising mirror
35.
The rising mirror
35 reflects the parallel beams from the collimating
lens
34 and changes a traveling direction of the parallel beams at an angle
of 90 degrees to lead the parallel beams to the object lens
36.
The object lens
36 condenses the parallel beams from the rising mirror
35 on the recording layer of the optical disc
38.
The optical disc
38 reflects the condensed laser beams condensed by the
object lens
36 according to the pits of the recording layer. The laser beams
reflected from the optical disc
38 returns to the polarizing beam splitter
33 through the object lens
36, the rising mirror
35 and the
collimating lens
34 as the return laser beams. The polarizing beam splitter
33 passes the return laser beams at a rate of 50 percent through therein
to the photo detector
37.
As illustrated in FIG. 4, the photo detector
37 has a photo sensing area
pattern formed by three photodiodes
41,
42 and
43. The photodiodes
41,
42 and
43 detect the return laser beams and produce electrical
signals in response to optical strength of the return laser beams. The photodiode
41 receives the center laser beam of the return laser beams. The photodiodes
42 and
43 receive the side laser beams of the return laser beams.
The photodiode
41 has first and second photo sensing areas
411
and
412, which include a common area
413 common to both of the first
and the second photo sensing areas
411 and
412. The first and the
second photo sensing areas
411 and
412 selectively operate. The first
photo sensing area
411 operates when the first laser diode emits the first
laser beam LI while the second photo sensing area
412 operates when the
second laser diode emits the second laser beam L
2. That is, the first photo
sensing area
411 is used for detecting the center laser beam originated
from the first laser beam L
1 while the second photo sensing area
412
is used for detecting the center laser beam originated from the second laser beam
L
2. Switching between the first and the second photo sensing areas
411
and
412 is, for example, performed by means of a changeover switch (not
shown). Each of the first and the second photo sensing areas
411 and
412
serves as a fourfold photodiode.
The photodiode
42 has a single photo sensing area. Though the photodiode
42 may have two photo sensing areas like the photodiode
41, the two
photo sensing areas are unnecessary for the photodiode
42 because the photodiodes
42 and
43 merely detect the quantity of the side laser beam. The
photodiode
43 is similar to the photodiode
42.
Next, an operation of the optical pickup is described soon with referring to
FIGS. 3 and 4.
The two wavelength laser
31 alternatively emits the first laser beam L
1
or the second laser beam L
2. That is, when the optical disc
38 is
the DVD, the first laser diode is selectively driven. On the other hand the second
laser diode is selectively driven when the optical disc
38 is the CD. The
first laser diode and the second laser diode are arranged so as to face to the
same direction and to be about 100 μm apart from each other. Accordingly,
the first laser beam L
1 and the second laser beam L
2 have optical
axes which are parallel to each other and which are distant from each other.
When either of the laser beam L
1 or L
2 emitted from the two wavelength
laser diode
31 reaches the grating
32, the grating
32 divides
the laser beam L
1 or L
2 into three of the divided laser beams. The
divided laser beams travel to the polarizing beam splitter
33.
When the divided laser beams reach the polarizing beam splitter
33, the
polarizing beam splitter
33 passes fractions of the divided laser beams
through therein and reflects the remains of the divided laser beams. The divided
laser beams passing through the polarizing beam splitter
33 are absorbed
and are not used in the optical pickup. The divided laser beams reflected by the
polarizing beam splitter
33 travel to the collimating lens
34.
The collimating lens
34 collimates the divided laser beams from the polarizing
beam splitter
33 to produce the collimated laser beams. The collimated laser
beams travel to the rising mirror
35.
The rising mirror
35 reflects the collimated laser beams to lead the collimated
laser beams to the object lens
36.
The object lens
36 condenses the collimated laser beams on the recording
layer of the optical disc
38. That is, the condensed laser beams condensed
by the object lens
36 are applied to the optical disc
38.
The optical disc
38 reflects the condensed laser beams according the pits
of the recording layer. The returning laser beams reflected by the optical disc
38 returns to the polarizing beam splitter
33 through the object
lens
36, the rising mirror
35, and collimating lens
34.
The polarizing beam splitter
33 reflects fractions of the returning laser
beams and passes the remains of the returning laser beams through therein to lead
the returning laser beams passed through the polarizing beam splitter
33
to the photo detector
37.
The photo detector
37 detects the return laser beams by the use of the
photodiodes
41,
42, and
43 when the return laser beams are
originated from the first laser beam L
1. On the other hand, the photo detector
37 detects the return laser beams by the use of the photodiodes
41,
42, and
43 when the return laser beams are originated from the second
laser beam L
2. In each case, the photo detector
37 produces the electric
signals in response to the strength of the return laser beams.
Thus, the optical pickup can deal with both of the DVD and the CD.
As mentioned above, the optical pickup can detect the return laser beams by the
use of the single photo detector
37 regardless of the origin of the return
laser beams. The optical pickup has a simple construction because it has the single
laser
31, the single grating
32 and the single polarizing beam splitter
33. The simple construction improves reliability of the optical pickup and
reduces both of manufacturing steps and cost.
While this invention has thus far been described in conjunction with the preferred
embodiment thereof, it will readily be possible for those skilled in the art to
put this invention into practice in various other manners. For example, the collimating
lens
34, the rising mirror
35, and the object lens
36 may
be disposed so that the divided laser beams passing through the polarizing beam
splitter
33 are led to the optical disc
38. In this case, e photo
detector
37 detects the return laser beams reflected by the polarizing beam
splitter
33.
Moreover, a cylindrical lens may be disposed between the polarizing beam
splitter
33 and the photo detector
37 to expand the return laser
beams in a direction and to improve a detecting precision.
Furthermore, it is unnecessary that the optical axes of the first and
the second laser beams L
1 and L
2 are parallel to each other. The
optical axes may, for instance, intersect to each other. In this case, if the optical
axes intersect to each other again at an incident surface of the collimating lens
34, the photo detector
37 can detect the return laser beams like
the optical pickup illustrated in FIG.
3.
*
