JPH09265656A - Optical pickup - Google Patents

Abstract

PROBLEM TO BE SOLVED: To compatibly reproduce a high density optical disk having a small disk thickness and a low density optical disk having a large disk thickness by a single optical pickup with high luminous efficacy of laser light by providing a hologram plate and two photodetectors in prescribed positions. SOLUTION: This optical pickup possesses a laser light source 1, an objective lens 6, the two photodetectors 11 and 12 for detecting reflected light from the optical disk, a beam splitter 7 and the hologram plate 2 for generating sub-light source provided between the laser light source 1 and the beam splitter 7. Then, the hologram plate 2 is provided in a position other than an optical path of the reflected light from the optical disk as from the objective lens 6 to the photodetectors 11 and 12, while the photodetectors 11 and 12 in positions where the lengths of the optical path from the objective lens 6 are different from each other respectively. Consequently, since the reflected light from the optical disk is not passed through the hologram plate 2, attenuance of light due to the hologram is reduced, and the luminous efficacy for the optical pickup is enhanced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup for reading information from an optical disc, and more particularly to an optical pickup for reproducing a plurality of types of optical discs having different recording densities and disc thicknesses.

[0002]

2. Description of the Related Art A low-density optical disc that has already been widely used, such as a compact disc (CD) for music, and a high-density optical disc that is being standardized newly, such as a DVD (digital video disc), The disc thickness and the recording density of the disc are different. Various proposals have been made to make it possible to reproduce such different types of optical discs with the same optical pickup.
For example, Japanese Patent Application Laid-Open No. 7-98431 proposes a two-focus type optical pickup using a hologram plate. FIG. 5 is a diagram showing a first conventional optical pickup.

In the optical pickup 50 shown in FIG. 5,
A light beam 21 emitted from a laser light source 1 such as a semiconductor laser is converted into substantially parallel light by a collimator lens 4, passes through a polarization beam splitter (PBS) 3 and a quarter wavelength plate 5, and enters a hologram plate 2 and an objective lens 6. Then, the light is focused on the points 9a and 9b on the optical disk 25. Since the light beam reflected by the optical disk 25 follows the original optical path in reverse,
The transmitted light 23 of the hologram plate 2 again passes through the hologram plate 2, the 0th-order diffracted light of the hologram plate 2 is again diffracted as the 0th-order diffracted light by the hologram plate 2, and the 1st-order diffracted light 24 of the hologram plate 2 is the hologram plate 2. Then, the light is diffracted as the first-order diffracted light again, and both are incident on the polarization beam splitter 3 through the same optical path as the outward path. The light flux reflected by the PBS 3 is condensed by the converging lens 26 and the cylindrical lens 8 and enters the photodetector 27.

The servo signal (focus error signal and tracking error signal) and information signal (RF signal) for performing focus control and tracking control are detected by the photodetector 27.
Can be obtained by calculating the output of each photodetector that constitutes The objective lens 6 needs to be moved at a high speed by the driving means 22 to correct the eccentricity and surface wobbling of the optical disk, but the hologram plate 2 is a flat optical element, so it is lightweight (several tens of milligrams or less), and the hologram Driving means 22 using a combination of the plate 2 and the objective lens 6
It can also be integrally driven by. Further, by integrally molding the hologram plate 2 directly on the objective lens 6, it is possible to achieve further weight reduction and cost reduction.

In the following description, the optical path of the light beam emitted from the laser light source 1 to the optical disk 25 will be referred to as the forward path, and the optical path of the light beam reflected by the optical disk 25 to the photodetector 27 will be referred to as the return path. In the first conventional example, between the polarization beam splitter 3 and the optical disc 25, the light beam follows the same optical path in the forward path and the backward path. That is, the transmitted light 23 is a light beam that has passed through the hologram plate 2 again on the return path, and the first-order diffracted light 24 that has been diffracted when passing through the hologram plate 2 on the forward path is the first-order diffracted light that has been diffracted on the hologram plate 2 again on the return path. Enters the polarization beam splitter 3 through the same optical path as the outward path.

The light beam reflected by the polarization beam splitter 3 is condensed by a converging lens 26 and a cylindrical lens 8 and enters a photodetector 27. 2 focus points 9
The condensing point of the light beams reflected from a and 9b on the side of the photodetector 27 has a conjugate image relationship with the emitting point of the laser light source 1 and coincides. Therefore, the converging lens 26, the cylindrical lens 8 and the photodetector 27 can be commonly used when obtaining the servo signal or RF signal of the astigmatism system, and the optical pickup is small in size, light in weight and low in cost with a small number of parts. However, it is said that the recording / reproducing of the optical disks 25 having different substrate thicknesses can be performed by one optical pickup.

In the above-described method using the hologram plate 2, the light flux at the central portion of the parallel light (this light quantity is 1) in the outward optical system is the 0th order light with a light quantity ratio of about 0.4 to 0.4. And primary light and split. A hologram is formed on the center side of the hologram plate 2, and a parallel light beam passing through the outside of the hologram and a 0th-order light beam of the parallel light passing through the hologram are incident on a high NA objective lens, for example, 0. 6 mm
A high density signal reading light spot 9b is formed on the thick high density optical disk 25. The first-order diffracted light separated by the hologram plate 2 becomes a diffused light beam and enters the objective lens 6. As a result, the light source position becomes a substantially finite position.

The objective plate 6 is formed by the hologram plate 2.
, The objective lens 6 is used as an objective lens having a substantially low NA, and a low-density signal reading light spot 9a is formed on the low-density optical disc 25 having a thickness of 1.2 mm. The information pits of the density optical disk 25 are read. The light quantity ratio of the light spots for the high density optical disk and the low density optical disk is about 6 to 3. The light flux emitted from the laser light source with a half-value angle of 1, for example, a radiation angle of horizontal 9 ° / vertical 30 °, has a relatively short focal length.
And the coupling efficiency (use efficiency of laser light) is 30%.
It is a high value. About 60% of the light amount is a light spot for a high density optical disc, and about 27% is a light spot for a low density optical disc.

The reflected light from the optical disk 25 is again divided into a 0th-order light beam and a 1st-order light beam by the hologram plate 2 in the returning optical path.
It is divided so that the light amount ratio is 6 to 0.4. Since the image forming position on the photodetector 27 is different between the 0th-order light and the 1st-order light, they are not simultaneously focused on the small-area photodetector, and therefore a large-area photodetector is used. Assuming that the amount of light emitted from the laser beam is 100%, the amount of light entering the photodetector 27 is the optical disc 2
If 5 is a high density optical disc, the optical disc 25
Assuming that the reflectance at 100% is 100% and there is no loss in the optical path of the return path, it is reduced to the next ratio. Outgoing path efficiency of hologram plate × Incoming path efficiency of hologram plate × Collimator coupling efficiency = 60% * O. 6 * 0.3 = 1
0.8%

In the case of the low density optical disk 25, the efficiency of the hologram plate in the forward path × the efficiency of the hologram plate in the backward path × the collimator coupling efficiency = 26.7% * 0.4 * 0.3 =
It becomes 3.2%. In this way, the reflected light from the optical disk 25 is divided again by the hologram plate 2, so that the light utilization efficiency becomes low. In particular, the optical disc 25 is provided with two recording layers separated by about 50 μm in the direction perpendicular to the disc surface.
In the case of a layered optical disc, the reflectance of the optical disc is about 30% or less, so that the optical power of the laser light source 1 is 4. to reproduce the optical disc with a practical C / N ratio.
It should be as large as 5 mW to 5 mW. Currently, the self-excited oscillation semiconductor laser having a wavelength of 650 nm has only a maximum output of about 5 mW, and the operation is at the full rating.

It is difficult to dissipate heat in the optical pickup in which the laser light source is arranged.
There is a problem that the life of the laser light source is shortened. When the optical disc 25 is a low density optical disc, at the time of reproduction,
The forward path of the laser light is a primary light beam, and the return path is a primary light beam of 4
The light enters the split photodetector 27 and the servo signal is formed. However, the hologram of the hologram plate 2 is formed with a non-uniform pitch also for the purpose of aberration correction, and in addition to the primary light flux necessary for forming the reproduction light spot of the low density optical disc, it is of a higher order. A luminous flux component is generated. Especially 0
When the next outgoing light flux passes through the hologram plate 2 and becomes a secondary light flux on the return path, the secondary light may be incident on the photodetector 27 set for reproduction.

At this time, the secondary light becomes unnecessary light (stray light), and if this is equally incident on each photodetector constituting the four-division photodetector 27, there is no problem. Since the offset is superimposed, it becomes a problem in detecting the servo signal. Further, as a second conventional example of the bifocal type optical pickup, there is Japanese Patent Laid-Open No. 7-654.
There is proposed a device in which an optical path length difference is optically formed, as disclosed in Japanese Patent Publication No. 07-2007. FIG. 6 is a diagram showing a second conventional optical pickup. In the optical pickup 60 shown in FIG. 6, the light flux 3 emitted from the laser light source 1
The light beam 1 is reflected by the half mirror 32, enters the prism 34, and is directly reflected by the polarization separation film 35, and the reflection surface 36 a, 36 of the prism 36 that passes through the polarization separation film 35.
The light beam 39 reflected by b is separated.

That is, by making the optical path length of one light flux longer than the optical path length of the other light flux, two light sources having substantially different light source positions are formed, and the light spots 9a and 9b corresponding to the respective light sources are formed. It is formed on the optical disc 25. One light flux 39 is made into a parallel light flux (infinite system) by the condenser lens 41, is incident on the objective lens 6, forms a light spot 9b, and reproduces a high density optical disc. The other light flux 2a passes through the condenser lens 41 and is equivalently diverged from a light source at a finite position, enters the objective lens 6, forms a light spot 9a, and reproduces a low density optical disc.

Since this method uses the property of polarization, there is no loss of light in the light beam separating means, but since the half mirror 32 is used when separating the emitted light from the light source and the reflected light from the optical disk 25. , Light utilization efficiency is reduced in this part. Further, in consideration of reproduction of the two-layer optical disc, since the reflectance thereof is as low as about 30%, the optical power of the laser light source is also increased, and the problem of the life of the laser light source occurs. At present, it is difficult to obtain a laser light source that has a sufficient light output and is excellent in noise characteristics that enables the above-described configuration, and it is difficult to realize a desired optical pickup. is there.

Further, as a method of detecting the tracking error signal, the one-beam type tracking error signal detection method can use a laser light source having a relatively small output as compared with the three-beam method, but on the other hand, the variation in the depth of the information pits. On the other hand, it has a drawback that the allowable range is narrow. Therefore, it is desirable to adopt the 3-beam method, but in the 3-beam method, since the luminous flux is divided, the light amount becomes small, and there is a problem that a desired optical pickup cannot be realized.

[0016]

The optical pickup of the present invention has been made in view of the above problems, and its main purpose is to provide a high density optical disc with a small disc thickness and a low density optical disc with a large disc thickness. Is to enable compatible reproduction with a single optical pickup, and another object is to provide the compatible reproduction capable of high reproduction efficiency. Further, it is an object of the present invention to provide an optical pickup capable of performing tracking control of a three-beam method in the compatible optical pickup, and an optical pickup capable of stably reproducing a two-layer optical disc. Another object is to provide an interchangeable and reproducible optical pickup with a small number of assembling steps and a high assembling accuracy.

[0017]

An optical pickup according to the present invention detects at least a laser light source, an objective lens for condensing light emitted from the laser light source onto an optical disc, and reflected light from the optical disc. Provided between the laser light source and the beam splitter, and a beam splitter for guiding the light emitted from the laser light source to the objective lens and guiding the reflected light from the optical disk to the photodetector. An optical pickup having a hologram plate for generating a sub-light source, wherein the hologram plate is provided in a region other than the optical path of the reflected light from the objective lens to the photodetector, and the photodetector is provided from the objective lens. The optical pickups are provided at two positions having different optical path lengths. Further, the optical pickup of the present invention is the same as the optical pickup, in which the two photodetectors are provided on the same plane.

In the optical pickup of the present invention, the light beam emitted from the laser light source is diffracted by the hologram plate installed between the laser light source and the beam splitter, and as a result, two light sources having substantially different light source positions are present. It will be. Therefore, the light beam emitted from the laser light source passes through the beam splitter and is then condensed by the objective lens to form a light spot at two points having different optical path lengths. By arranging the high density optical disc or the low density optical disc at the positions corresponding to the two light spots, it is possible to reproduce any of the optical discs with one laser light source.

Here, only the outward path from the laser light source to the optical disk passes through the hologram plate,
Light reflected from the optical disc does not pass through the hologram. Therefore, the light attenuation due to the hologram is small. The reflected light from the optical disc passes through the polarization beam splitter and is applied to the two photodetectors. Here, the two photodetectors are arranged on the optical axis of the substantially backward path, but are arranged at positions having different optical path lengths.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION The optical pickup of the present invention uses a single laser light source, but there are substantially two light sources. One of these light sources reproduces the information pits of a high-density optical disc with a small disc thickness by using an objective lens designed in an infinite system, and the other light source that makes the position of the light emission source substantially finite. , A conjugate image is formed to reproduce information pits of a low density optical disc having a large disc thickness. When the objective lens designed as an infinite system is used as it is, a collimator lens is arranged between the beam splitter and the objective lens. Further, the beam splitter, in the polarization optical system,
A polarization beam splitter is adopted, and a quarter wavelength plate is inserted between the collimator lens and the objective lens.

The light beam reflected by the optical disc passes through the first beam splitter, is split by the second beam splitter, and is incident on two photodetectors. The second beam splitter uses a semitransparent glass plate in the central portion and a transparent glass plate in the outer peripheral portion, but a transparent glass plate in which only the central portion is a total reflection mirror may be used. Furthermore, in order to arrange the two photodetectors on the same plane,
A prism is provided between the first beam splitter and the photodetector to make the optical path lengths of the light beams different from each other. Further, a converging cylindrical lens is provided between the first beam splitter and at least one photodetector.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The principle of conjugate correction used in the present invention will be described with reference to FIG. In FIG. 3, (1) uses a laser light source, a collimator lens, and an objective lens designed according to the specifications of an infinite system for a high-density optical disc having a disc thickness of 0.6 mm. An optical system for reproduction is shown. (2) shows an optical system for reproducing a low-density optical disc having a disc thickness of 1.2 mm by using the objective lens shown in (1), in which the position of the laser light source is moved from an infinite point to a finite distance, This shows that the information is reproduced by focusing on the information pit surface of the optical disk. (3)
In the optical system shown in (2), it shows how the aberration of the light spot changes depending on the position of the laser light source. Here, TH indicates the distance between the objective lens and the laser light source.

In FIG. 3, when the disc thickness of the optical disc is 0.6 mm, the objective lens is designed in advance so that the focused light incident on the disc substrate becomes a spherical wave and forms a light spot having a minimum diameter on the reading surface. It is designed by distorting the wavefront of focused light in air from a spherical wave. As shown in (3) of FIG. 3, in the case of an optical disc having a disc thickness of 1.2 t, even if the light source position is set to a finite point by the objective lens, the light spot is Aberration is minimized. Therefore, in order to be able to read a high-density optical disk of 0.6t and a low-density optical disk of 1.2t with a single objective lens, two light sources with different light source positions are prepared and the optical disk substrate thickness is changed. You can see that it is enough to switch. For this purpose, a hologram is placed at the center of the divergent light from the laser light source to form a 0th-order and 1st-order light flux, and thereby a plurality of light sources are substantially obtained.

A first embodiment of the optical pickup of the present invention will be described below with reference to FIG. FIG. 1 is a diagram showing a first embodiment of the optical pickup of the present invention. In FIG. 1, the same components as those of the optical pickup shown in FIG. 5 are designated by the same reference numerals, and the description thereof will be omitted. In the optical pickup 10 shown in FIG. 1, a hologram plate (HO) is provided between a laser light source (LD) 1 and a polarization beam splitter (PBS) 3.
E) 2 is provided, the light beam emitted from the laser light source 1 passes through the hologram plate, enters the PBS 3 and is reflected, and then passes through the collimator lens 4, the quarter-wave plate 5 and the objective lens 6 and onto the optical disc. To form light spots 9a and 9b.

The reflected light from the optical disk enters the PBS 3 through the optical path opposite to the outward path, and the transmitted light enters the beam splitter (BS) 7. A part of this BS7 is reflected and enters the first photodetector 11, and the transmitted light is condensed by the cylindrical lens 8 and enters the second photodetector 12. The photodetector 11 is a three-beam photodetector including photodetectors E1, F1, A1, A2, and A3 as shown in an enlarged view in FIG. Further, the photodetector 12 is, for example, a multi-division photodetector composed of a plurality of photodetectors as shown in an enlarged view in FIG. 1, and photodetectors E, F, and G are provided on the outer peripheral side thereof. , H are arranged, and photodetectors A, B, C, D are arranged on the inner peripheral side thereof. The return path of the optical system of the optical pickup means that the reflected light from the optical disk is the PBS
The hologram plate 2 is installed outside the optical path of the return path of the laser light, although it refers to the optical path from passing through to the photodetector.

The hologram plate 2 forms a hologram for generating a sub-light source on the center portion (inner peripheral side) of one surface of the hologram plate 2 and forms a grating (grating) on the opposite surface to form a low density optical disc. It is also possible to form a light spot for detecting the servo signal by the three-beam method during reproduction. Due to the hologram plate 2, the laser light source is substantially arranged at the infinite point and the finite point S, and the luminous flux from the infinite point and the diffused luminous flux from the finite point S are NA.
The light enters the objective lens 6 designed to have an infinite system with a substrate thickness of 0.6 and a thickness of 0.6 t.

That is, a light beam diverging from a red laser light source with a wavelength of 650 nm at a radiation angle of half angle of 9 ° / 30 ° is reflected by the PBS 3 and then converted into parallel light by the collimator lens 4. On the other hand, the hologram provided in the central portion of the hologram plate 2 forms a sub light source for a low density optical disc. Here, the position of the sub light source is set by the collimator lens 4 and the hologram plate 2 so as to be approximately 60 mm from the objective lens 6.

For this purpose, for example, when the focal length of the collimator lens 4 is 22 mm, the focal length of the objective lens 6 is 3.3 mm, and the distance between the objective lens 6 and the collimator lens 4 is 18 mm, the laser light source 1 is , The distance from the sub light source formed by the hologram plate 2 for generating the sub light source is 8 m
m. The optical system of the optical pickup 10 is a polarization optical system, and the outer peripheral side of the light beam emitted from the laser light source 1 does not pass through the hologram of the hologram plate 2 but is reflected by the polarization beam splitter (PBS) 3 and this reflected light is collimated. The light is collimated by the lens 4, circularly polarized by the λ / 4 plate, and condensed by the objective lens 6 to form a light spot 9b for reproducing a high density optical disc on the optical disc.

The light beam of the sub-light source generated by the hologram plate 2 is not collimated by the collimator lens 4 but is condensed by the objective lens 6 to form a light spot 9a for reproducing a low density optical disc on the optical disc. . Here, the light quantity ratio of each light beam divided by the hologram plate 2 is 0: 0 order light: 1st order light: Loss light: 4: 4: 2. The coupling efficiency of the collimator lens is about 21.9%. At this time, the amount of light for the low-density optical disc is about 3.7%, and in the high-density optical disc, the value obtained by subtracting 3.7% from the coupling coefficient of 21.9% of the collimator lens 4 is the utilization efficiency of 18.2. %.

As described above, the reflected light from the light spot 9b for the high density optical disk does not pass through the hologram plate 2, and therefore there is no light amount loss due to diffraction of the hologram plate as in the conventional example, and sufficient reflection is achieved. The amount of light is incident on the photodetector 12, and good signal quality can be obtained even with a two-layer optical disc. In addition, the light amount for the low density optical disc is 3.7%.
Even if the outgoing light of is separated into three beams, the reflected light is
A practical amount of light is incident on the photodetector 11 without passing through the hologram 2. Further, since the reflected light does not pass through the hologram 2, the 0th-order diffracted light on the outward path becomes a 2nd-order diffracted light and enters the sensor as in the conventional example, and is not an unnecessary stray light. No need for stray light measures.

As described above, the low-density optical disc has a small amount of light and low light utilization efficiency, but the high-density optical disc has a sufficiently large utilization efficiency. Further, the reflected light from the optical disk traces the reverse path and is transmitted through the PBS 3, and the reflected light from the low density optical disk is reflected by the BS 7 and is detected by the photodetector 1.
The reflected light from the high-density optical disc is transmitted through the beam splitter 7 and is focused on the photodetector 12 via the cylindrical lens 8. The cylindrical lens 8 is for detecting a focus error signal by the astigmatism method.

The beam splitter 7 is a transparent glass plate provided with a reflection mirror having a reflectance of about 25% at the center thereof. The photodetector 11 detects the tracking error signal of the low density optical disc by the three-beam method and the focus error signal by the light spot size method, respectively. Each error signal is formed by calculating the output of each photodetector that constitutes the photodetector 11. The BS 7 transmits 75% of the luminous flux, and the transmitted light is incident on the second photodetector 12 via the cylindrical lens 8. The reflected light from the low-density optical disk forms an image on the PBS 3 side from the cylindrical lens 8.

From the output of the photodetector 12, the information signal (RF signal) is obtained when reproducing the low density optical disc, and the RF signal and the servo signal are obtained when reproducing the high density optical disc. The RF signal is obtained by adding the eight outputs of the photodetector 12 because the reflected light spot diameter is large in the reproduction of the low-density optical disc, and the four photodetectors on the inner peripheral side in the reproduction of the high-density optical disc. Only the output of is added and obtained.

At the central portion of the light flux that has passed through the beam splitter 7, the light intensity is reduced because a part of the light flux is reflected by the BS 7, but the pattern of the light flux is preserved, and the tracking error signal is thus detected. It does not hinder the detection by the phase difference detection method. For high density optical discs,
The focus error signal is detected by the astigmatism method from the outputs of the photodetectors A to D on the inner peripheral side of the photodetector 12. The RF signal is formed by the sum of the outputs of the photodetectors A to D.

Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 2 is a diagram showing a second embodiment of the optical pickup of the present invention. The components having the same functions as those in FIG. 2 are designated by the same reference numerals and the description thereof will be omitted. The main difference between FIG. 1 and FIG. 2 is that the cylindrical lens 8 is arranged between the polarization beam splitter 3 and the beam splitter 7 in FIG. In the optical pickup 20 shown in FIG. 2, both the reflected light from the low density optical disc and the reflected light from the high density optical disc pass through the cylindrical lens 8 and enter the beam splitter 7. By doing so, the astigmatism method can be used for both the low-density optical disk and the high-density optical disk in detecting the servo signal.

That is, a part of the light flux passing through the cylindrical lens 8 has a reflectance of 2 at the center of the transparent glass plate.
Beam splitter 7 configured with a mirror of about 5%
Is reflected in the direction orthogonal to the optical axis and enters the photodetector 11.
The photodetector 11 is a photodetector divided into four parts A1 to D1,
It consists of three photodetectors E1 and F1, and the astigmatism method and 3
The servo signal can be detected by the beam method. Also,
The light flux transmitted through the beam splitter 7 is a photodetector 12
Incident on. Each output of the photodetector 12 is calculated to obtain an RF signal for a low density optical disc.

In the second embodiment, the reflectance of the central portion of the beam splitter 7 is set to about 25%, but the reflectance is set to about 100%, and all the signals for the low density optical disk are detected by the light detection. When the light is received by the device 11, the photodetector 11 can have a relatively small area, so that a sufficient signal band can be obtained even for reproduction of a low-density optical disc of 2 × speed or 4 × speed. In the case of a high density optical disc, the photodetector 11
The signals obtained from the respective photodetectors of 1 and the signals obtained from the respective photodetectors of the photodetector 12 are combined to form an RF signal and a tracking error signal (phase difference signal).

FIG. 4 is a diagram showing a third embodiment of the present invention, in which the components having the same functions as those in FIG. 1 are designated by the same reference numerals and the description thereof is omitted. The main difference between FIG. 1 and FIG. 4 is that the prism 12 is arranged and that the two photodetectors 11 and 12 are arranged.
Is a point on the same plane. In the optical pickup 30 shown in FIG. 4, the prism 12 having three reflecting surfaces is
The optical path length is made different between the reflected light from the low density optical disc and the reflected light from the high density optical disc, and the reflected light from both optical discs is imaged on the same plane. By arranging the two photodetectors 11 and 12 on the same plane in this way, the two photodetectors can be combined into one photodetector, or the number of adjustment steps of the photodetector can be reduced. , The adjustment accuracy can be improved.

[0039]

According to the optical pickup of the present invention, the reflected light flux reflected by the optical disk is prevented from passing through the hologram plate, that is, only the light flux of the forward path passes through the hologram plate, so that the laser light is doubled. It is possible to solve the problem that the light utilization rate is extremely reduced due to the separation. This makes it possible to provide an optical pickup with high light utilization efficiency.
Further, for this reason, the tracking control of the optical pickup can be performed by the three-beam method, and the allowable range of the variation of the information pit depth of the optical disk becomes large. Furthermore, a two-layer optical disc can use a laser light source having a relatively small output. In the optical pickup according to the second aspect of the present invention, the number of steps for adjusting the optical pickup can be reduced and the adjustment accuracy can be improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of an optical pickup of the present invention.

FIG. 2 is a diagram showing a second embodiment of the optical pickup of the present invention.

FIG. 3 is a diagram showing a principle of conjugate correction.

FIG. 4 is a diagram showing a third embodiment of the optical pickup of the present invention.

FIG. 5 is a diagram showing a first conventional optical pickup.

FIG. 6 is a diagram showing a second conventional example optical pickup.

[Explanation of symbols]

 1 Laser Light Source 2 Hologram Plate 3 Beam Splitter (Polarizing Beam Splitter) 4 Collimator Lens 5 1/4 Wave Plate 6 Objective Lens 7 Beam Splitter 8 Cylindrical Lens 11, 12 Photo Detector

Claims (2)

[Claims] 1. A laser light source, an objective lens for converging light emitted from the laser light source on an optical disc, a photodetector for detecting reflected light from the optical disc, and an emission from the laser light source. Light having a beam splitter for guiding the reflected light to the objective lens to the reflected light from the optical disk to the photodetector, and a hologram plate for generating a sub light source provided between the laser light source and the beam splitter. A pickup, wherein the hologram plate is provided in an area other than the optical path of the reflected light from the objective lens to the photodetector, and the photodetector is
An optical pickup, wherein the optical pickup is provided at two positions having different optical path lengths from the objective lens. 2. The optical pickup according to claim 1, wherein
An optical pickup in which the two photodetectors are provided on the same plane.