JPH0520711A - Optical pickup - Google Patents

Abstract

PURPOSE:To attain the proper recording and reproduction of the information by preventing the mixture of a sub-beam signal into a photodetector which detects a main beam. CONSTITUTION:A hologram optical element 6 leads the reflected light received from a non-irradiation body to a photodetector as the + or -1-degree diffracted light after diffracting the reflected light. Then the element 6 is divided into three areas, i.e., a 1st area 8a where a main beam passes and the 2nd and 3rd areas 8b and 8c where two sub-beams pass respectively. At the same time, the interference fringes are formed in both areas 8b and 8c so that these areas have the different lens functions to the area 8a. Thus a sub-beam spot is never mixed into a photodetector which detects the main beam. As a result, the noises of the information signals are reduced and no offset is caused to a focusing error signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup used in an information recording / reproducing apparatus for recording and / or reproducing information on an optical information recording medium such as an optical disk.

[0002]

2. Description of the Related Art An optical pickup irradiates a focused beam spot on an optical disk or the like while detecting the focus of the beam spot to be irradiated in order to properly record and reproduce information on the optical disk or the like. Is required. As a focus detection device for this purpose, a hologram element is arranged between a light source and a converging optical system, light emitted from the light source is made incident on the hologram element, and the 0th-order light is projected onto an irradiated object through the converging optical system. Then, the reflected light is made incident on the hologram element again through the converging optical system, and the + 1st order diffracted light or ± 1
There is a system in which the next-order diffracted light is received by a photodetector and the focus state of the converging optical system with respect to the irradiated object is detected based on the output thereof.

However, in the optical pickup provided with the focus detecting device as described above, when the light source and the photodetector are brought close to each other for the purpose of downsizing, the diffraction angle in the hologram element is small, so that the forward path (light source) The first-order diffracted light or the first-order diffracted light generated in the optical path from the object to the object to be irradiated) is reflected by the object to be irradiated (the following optical path is a return path) and enters the photodetector as stray light. Therefore, there is a problem that the photodetector cannot detect the focus state with high accuracy. On the other hand, if an aperture limit (diaphragm) is provided in the hologram element in order to solve this problem, a new component called a diaphragm is required, and an assembly work for accurately aligning the hologram pattern and the diaphragm is required. Occurs, which leads to an increase in cost.

Therefore, the present applicant has proposed an optical pickup as shown in FIGS. 8 and 9 in Japanese Patent Application No. 3-21492. In this optical pickup, photodetection sections 52 and 53 are formed on both sides of the optical path of the light emitted from a laser diode 51 mounted on a semiconductor substrate 50, and the surface from the laser diode 51 to the substrate 50 surface is formed. The light emitted in a direction parallel to is reflected by the rising mirror 54. After that, through the hologram element 55 and the objective lens 56 in which the grating pattern is formed on one surface and the diaphragm pattern area for light detection and the hologram area for hologram are formed on the other surface, the recording medium 57 which is the irradiated object is passed. A light beam is projected. Then, the reflected light from the recording medium 57 is again reflected by the objective lens 56.
The first and second order diffracted light beams are made incident on the hologram element 55 through the above, and the powers in the opposite directions are generated. In this way, the focus state of the beam spot with respect to the object to be illuminated can always be detected with high accuracy without increasing the cost.

[0005]

However, in the above-mentioned conventional example proposed by the present applicant, the ± first-order lights generated by the hologram element 55 are different in power from each other. Therefore, for example, assuming that the negative power (the action of the concave lens) is generated for the + 1st-order light and the positive power (the action of the convex lens) is generated for the −1st-order light, the light detection unit 52 from the illuminated object is detected. , 53, the image forming magnification increases with + 1st order light,
It becomes smaller with primary light. By the way, the light detector 52 is
3 photodetectors 52 to receive the + 1st order diffracted light
The photodetector 52b having a, 52b and 52c and receiving the central main beam has three-divided light receiving regions 52d, 52e and 52f having dividing lines in the diffraction direction of the hologram element 55. The same applies to the light detection unit 53.

Therefore, when the photodetectors 52 and 53 are irradiated with the light beams and are received by the respective photodetectors, the spot 58a of the main beam on the photodetector of the + 1st order light and the sub-beams are detected. As compared with the distance between the spots 58b and 58c, the distance between the main beam spot 59a and the sub-beam spots 59b and 59c on the -1st-order light photodetector becomes narrower. As described above, when the interval between the beam spots on the photodetector for the -1st order light is narrow, the spots 59b and 59c of the sub beam are generated.
Would enter the photodetectors 53d, 53e, 53f for detecting the main beam, and the sub-beam signal would become noise and enter the information signal (RF signal). Further, the focus error signal (FoE signal) is detected by using only the main beam, but there is a problem that the sub beam signal also enters there and the FoE signal offset occurs.

As a method for solving such a problem,
By increasing the imaging magnification of the converging optical system or increasing the inclination of the optical axes of the main beam and the sub beam (that is, widening the spot distance on the recording medium), the main beam and the sub beam on the photodetector are increased. -A possible method is to increase the spot distance from the spot. However, the method of increasing the imaging magnification of the converging optical system increases the distance between the object and image points, which leads to the enlargement of the optical system and the enlargement of the optical pickup, and the NA on the light source side decreases to reduce the coupling efficiency. However, there is a problem in that On the other hand, the method of widening the spot interval on the recording medium has a problem that the mounting accuracy in assembling the optical pickup becomes strict.

The present invention has been proposed to solve the above-mentioned problems, and is intended for a photodetector for detecting a main beam when a photodetector receives 3 beams and detects an RF signal or the like. Make sure that the sub-beam signal does not enter and R
It is an object of the present invention to provide an optical pickup capable of recording and reproducing proper information by reducing the noise of the F signal and preventing the offset of the FoE signal.

[0009]

In order to achieve the above object, the present invention provides a converging optical system for projecting light emitted from a light source onto an object to be irradiated, and an optical path between the converging optical system and the light source. The grating is provided to separate the light emitted from the light source into a main beam for reading information and two sub-beams for tracking error detection, and an optical path between the grating and the converging optical system. The hologram optical element is provided with a hologram optical element that diffracts the reflected light from the irradiated object and guides the ± 1st-order diffracted light to the photodetector, and a photodetector that receives the main beam and the sub-beam respectively. A first region including at least a region through which the main beam passes, a second region through which only one sub beam passes, and a third region including a region through which only the other sub beam passes. At least three areas Together are separated in the second region to the first region, the third region has an optical pickup to form an interference pattern to have different lens action. Further, in the above optical pickup, the first area of the hologram element is an interference fringe having a predetermined lens power, and the second area and the third area have a lens power close to zero.
And the interference fringes.

[0010]

As described above, the hologram pattern area is divided into the area through which the main beam passes and the area through which only the sub beam passes, and at the same time, with respect to the lens action of the area through which the main beam passes. The lens action of the region through which only the sub-beam passes is configured to be different so that the beam spot of the sub-beam does not enter the photodetector for detecting the main beam. Therefore, the noise of the information signal is reduced and the focus error signal is not offset.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. 1 to 4 show a first embodiment of the present invention, FIG. 1 is a schematic view of an optical pickup, and FIG. 2 is a plan view showing the arrangement state of a laser diode and a photodetector. 3, FIG. 3 shows the grating surface and the hologram surface of the hologram element, and FIG. 4 is a perspective view showing the relationship between the hologram element and the light detecting portion. On a semiconductor substrate 1, a laser diode 2 and photodetection sections 3 and 4 provided on both sides of an optical path of light emitted from the laser diode 2 are arranged (see FIG. 1, FIG. 2). A startup mirror 5 is provided in the vicinity of the laser diode 2 of the semiconductor substrate 1, and light emitted from the laser diode 2 in a direction parallel to the surface of the semiconductor substrate 1 is directed to the semiconductor substrate 1 surface. The light is made incident on the hologram element 6 provided in the direction perpendicular to (FIG. 4).

A grating surface 6a is formed on the surface of the hologram element 6 facing the semiconductor substrate 1, and a hologram surface 6b is formed on the side opposite to the grating surface 6a. Further, on the grating surface 6a of the hologram element 6, a grating 7 for forming 3 beams is formed at a substantially central position thereof (FIG. 3A). Therefore, the light incident on the grating surface 6a is divided into three beams, one main beam and two sub-beams, by the grating 7. Although the grating surface 6a and the hologram surface 6b are integrally formed in one hologram element 6 in this embodiment, they may be provided separately. Hologram pattern areas 8a, 8b and 8c divided into three parts are formed on the hologram surface 6b so that diffracted light can be generated at substantially the central position (FIG. 3B) and the grating 7 (FIG. 3B).
The light divided into 3 beams in A) is transmitted through the hologram pattern areas 8a, 8b and 8c as zero-order light.
The transmitted light that has passed through the hologram element 6 enters the objective lens 9 provided in the traveling direction of the transmitted light, and is converged and projected on the irradiated object 10 such as an optical disk (FIG. 1).

The reflected light reflected by the illuminated object 10 passes through the objective lens 9 again, and then enters the hologram element 6. The hologram element 6 produces powers in opposite directions ±
The 1st-order diffracted light is generated, and the ± 1st-order diffracted lights are received by the photodetectors 3 and 4 provided on the semiconductor substrate 1 (FIG. 1). According to FIGS. 2, 3 and 4,
The state in which the reflected light from the irradiated object 10 passes through the hologram element 6 and is received by the photodetectors 3 and 4 will be described in detail. First, the main beam of the reflected light from the recording medium 10 is incident on the hologram surface 6b and passes through the main beam passing region 11 to be diffracted in the hologram pattern region 8a (FIGS. 1 and 2). 3B).

As shown in FIG. 2, the photo-detecting section 4 has three photo-detectors 4a, which receive the + 1st-order diffracted light of 3 beams.
The photodetector 4b for receiving the main beam at the center is constituted by three divided light receiving regions 4d, 4e, 4f having a dividing line in the diffraction direction of the hologram element 6 in addition to being constituted by 4b, 4c. Similarly, the photo-detecting section 3 is constituted by three photo-detectors 3a, 3b and 3c so as to receive each -1st-order diffracted light of 3 beams, and a photo-detector 3b for receiving the central main beam. Is composed of three divided light receiving regions 3d, 3e and 3f having a dividing line in the diffraction direction of the hologram element 6. As described above, the hologram pattern area 8a (see FIG.
The + 1st order diffracted light diffracted in B) is the photodetector of the photodetector unit 4.
The light is guided to 4d, 4e, 4f and received, and the minus first-order diffracted light is guided to the photodetectors 3d, 3e, 3f of the photodetector 3 and received (FIG. 2). The hologram pattern area 8a is configured to give a negative power to the + 1st order diffracted light and a positive power to the -1st order diffracted light.

Here, a focus error signal (FoE signal)
A method of detecting the will be described. Main beam +1
The outputs of the light receiving regions 4d, 4e, and 4f of the photodetector 4b in the photodetector 4b that receives the diffracted light of the next order are B1, B2, and B3, and the light detector 3b of the photodetector 3 that receives the -1st order diffracted light receives the light. region
When the outputs of 3d, 3e and 3f are B4, B5 and B6, they are detected by FoE = (B1 + B3 + B5)-(B2 + B4 + B6).

Next, the light incident on the hologram pattern area 8a (FIG. 3B) in the sub beam passes through the sub beam passage areas 12 and 13 in the same manner as the main beam, so that the hologram pattern is formed. The + 1st order diffracted light is guided to the photodetectors 4a and 4c of the photodetector 4, and the -1st order diffracted light is guided to the photodetectors 3a and 3c of the photodetector 3 (FIG. 2). In this case, as described above, the hologram pattern area 8a is configured to give a negative power to the + 1st order diffracted light and a positive power to the -1st order diffracted light, so that the light is detected from the irradiated object 10. The imaging magnification on the containers 4a to 4b and 3a to 3b becomes large for the + 1st-order diffracted light and becomes small for the -1st-order diffracted light. Therefore, the beam spots on the sub-beam photodetectors 3 and 4 are beam spots 14a on the + 1st order diffracted light side, as shown in FIG.
14b moves away from the beam spot 14c of the main beam. Also, the beam spots 15a and 15b on the -1st order diffracted light side
Approaches the beam spot 15c of the main beam.

As described above, the hologram pattern area 8a is constructed so as to give a negative power (the action of a concave lens) to the + 1st order diffracted light and a positive power (the action of a convex lens) to the -1st order diffracted light. Therefore, the beam detectors 14a and 14b receive the beam detectors 14a and 14b with respect to the beam spots 14a and 14b.
The beam spots 15a and 15b received by 3a and 3c are inverted (FIG. 4). In this perspective view, only the beam spots 14c, 15c of the main beam and the beam spot 14a of a part of the sub beam are shown, and the beam spots of other sub beams are shown. Are not shown. Therefore, the beam spots 15a and 15b of the sub-beams on the photo-detectors 3a and 3c approaching the beam spot 15c of the main beam on the photo-detector 3b are inverted and only the missing parts are formed. It will be in a separated state and will not enter the photodetector 3b (FIG. 2).

Next, the sub-beams that have entered the hologram pattern regions 8b and 8c (FIG. 3B) pass through the sub-beam passage regions 12 and 13 to form the hologram pattern regions 8b and 8c. The + 1st-order diffracted light is guided to the photodetectors 4a and 4c of the photodetector 4, and the -1st-order diffracted light is guided to the photodetectors 3a and 3c of the photodetector 3 (FIG. 2). The hologram pattern areas 8b and 8c are formed as a diffraction grating having a smaller diffraction angle and no power than the hologram pattern area 8a through which the main beam passes. That is, the hologram pattern area 8a for the main beam is formed to have a predetermined lens power, and the hologram pattern area for the sub beam is formed.
8b and 8c do not have lens power. Therefore, the + 1st order diffracted light incident on the hologram pattern regions 8b and 8c, -1
Photodetectors 3a, 3c, 4a, with the same imaging magnification as the second-order diffracted light
Image on 4c. Therefore, the ± first-order diffracted light of the light beam incident on the hologram pattern area 8b becomes beam spots 14d and 15d on the photodetectors 4a and 3a. Further, the ± first-order diffracted lights of the light beam incident on the hologram pattern area 8c become beam spots 14e and 15e on the photodetectors 4c and 3c (FIG. 2). The beam spots 14d, 15d, 14e, and 15e are imaged on the photodetectors 4a, 3a, 4c, and 3c, so that they are dot-shaped beam spots.

Here, a method for detecting the tracking error signal (TrE signal) will be described. 2 subbies
Photodetector 4 in the photodetector 4 for receiving the + 1st order light
a, 4c receive the beam spots 14a, 14d, 14b, 14e, and the photodetectors 3a, 3c in the photodetector 3 for receiving the -1st order diffracted light are beam spots 15a, 15d, 15b, 15e
To receive. Outputs of photodetectors 4a, 4c, 3a, 3c respectively
Given that C1, C2, C3, and C4, TrE is detected by the 3 beam method by TrE = (C1 + C3)-(C2 + C4).

FIGS. 5 to 7 show a second embodiment of the present invention, in which parts corresponding to those of the first embodiment are designated by the same reference numerals. On a semiconductor substrate 1, a laser diode 2 and photodetection sections 3 and 4 provided on both sides of an optical path of light emitted from the laser diode 2 are arranged (see FIG. 5, FIG. 6).
A start-up mirror 5 is provided near the laser diode 2 of the semiconductor substrate 1, and light emitted from the laser diode 2 in a direction parallel to the surface of the semiconductor substrate 1 is emitted from the semiconductor substrate 1.
Hologram element 6 provided in the direction perpendicular to the plane
(Fig. 5).

A grating surface 6a is formed on the surface of the hologram element 6 facing the semiconductor substrate 1, and a hologram surface 6b is formed on the opposite side of the grating surface 6a. On the grating surface 6a of the hologram element 6, a grating 7 for forming 3 beams is formed at a substantially central position (FIG. 7A). Therefore, the light incident on the grating surface 6a is divided into three beams, one main beam and two sub-beams, by the grating 7. Hologram pattern areas 8a, 8b, 8c divided into three parts are formed on the hologram surface 6b so that diffracted light can be generated at substantially the central position (FIG. 7B), and grating 7 (FIG. 7A). The light divided into 3 beams is transmitted through the hologram pattern areas 8a, 8b and 8c as zero-order light. The transmitted light that has passed through the hologram element 6 enters the objective lens 9 provided in the traveling direction of the transmitted light, and is converged and projected on the irradiated object 10 such as an optical disk (FIG. 5).

The reflected light reflected by the illuminated object 10 passes through the objective lens 9 again, and then enters the hologram element 6. The hologram element 6 produces powers in opposite directions ±
The 1st-order diffracted light is generated, and the ± 1st-order diffracted lights are received by the photodetectors 3 and 4 provided on the semiconductor substrate 1 (FIG. 5). The + 1st-order diffracted light diffracted by the hologram pattern area 8a (FIG. 7B) as described above is guided to the photodetectors 4d, 4e, 4f of the photodetector 4 and received, and the -1st-order diffracted light is the light. The light is guided to the photodetectors 3d, 3e, 3f of the detection unit 3 and received (FIG. 6). Hologram pattern area 8a
Is configured to give a negative power to the + 1st-order diffracted light and a positive power to the -1st-order diffracted light. Here, a method of detecting a focus error signal (FoE signal) will be described. Photodetector 4 for receiving the + 1st order diffracted light of the main beam
The outputs of the light receiving regions 4d, 4e, 4f of the photodetector 4b at D1,
Let D2 and D3 be the outputs of the light receiving regions 3d, 3e and 3f of the photodetector 3b in the photodetector 3 that receives the −1st order diffracted light, and output D4, D5 and D6.
Then, FoE = (D1 + D3 + D5)-(D2 + D4 + D6) is detected.

Next, the light incident on the hologram pattern area 8a (FIG. 7B) in the sub-beam passes through the sub-beam passing areas 12 and 13 in the same manner as the main beam, so that the hologram pattern is formed. The + 1st-order diffracted light diffracted in the region 8a is guided to the photodetectors 4a and 4c of the photodetector 4, and the -1st-order diffracted light is guided to the photodetectors 3a and 3c of the photodetector 3 (FIG. 6). In this case, as described above, the hologram pattern area 8a is configured to give a negative power to the + 1st order diffracted light and a positive power to the -1st order diffracted light, so that the light is detected from the irradiated object 10. The imaging magnification on the containers 4a to 4b and 3a to 3b becomes large for the + 1st-order diffracted light and becomes small for the -1st-order diffracted light. Therefore, the beam spots on the sub-beam photodetectors 3 and 4 are beam spots 14a on the + 1st order diffracted light side, as shown in FIG.
14b moves away from the beam spot 14c of the main beam. Also, the beam spots 15a and 15b on the -1st order diffracted light side
Approaches the beam spot 15c of the main beam. Next, in the sub beam, hologram pattern areas 8b and 8c (see FIG.
The light incident on B) is diffracted by the hologram pattern areas 8b and 8c by passing through the sub-beam passage areas 12 and 13, and the + 1st order diffracted light is directed to the photodetectors 4a and 4c of the photodetection unit 4. The −1st order diffracted light is guided to the photodetectors 3a and 3c of the photodetector 3 (FIG. 6).

At this time, since the hologram patterns 8b and 8c are diffraction gratings having a larger diffraction angle and smaller power than the hologram pattern 8a (+ 1st order is positive and -1st order is negative power), the + 1st order diffracted light, There is almost no difference in the imaging magnification of the -1st-order diffracted light (imaging magnification of the converging optical system). Therefore, the distance between the beam spots of the main beam and the sub-beams hardly changes for the ± first-order diffracted light. Therefore, the ± 1st order diffracted light of the light incident on the area of the hologram 8b is detected by the photodetector.
Beam spots 14d and 15d are formed on 4a and 3a, and ± 1st-order diffracted light of the light incident on the area of hologram 8c is detected by photodetector 4c and
Spots 14e and 15e on 3c. Here, a method of detecting the tracking error signal (TrE signal) will be described. Photodetector 4 for receiving + 1st order light of two subbeams
The photodetectors 4a and 4c in FIG.
, 14b, 14e, and the photodetectors 3a, 3c in the photodetector 3 for receiving the -1st order diffracted light are the beam spot 15a.
, 15d, 15b, 15e are received. Photodetectors 4a, 4c, 3
If the outputs of a and 3c are E1, E2, E3, and E4, respectively, TrE
Is detected by the 3 beam method by TrE = (E1 + E3)-(E2 + E4).

The present invention is not limited to the above embodiment, but various modifications and variations are possible. For example, in the above embodiments, the case where various signal detections are performed by the beam size method is shown, but the present embodiment can be applied to the case where signal detections are performed by the astigmatism method. That is, in the present embodiment, the hologram pattern area 8a (FIG. 3) for the main beam is constructed so as to generate the + 1st order diffracted light and the -1st order diffracted light, but in the case of the astigmatism method, the main beam is generated. The lens function of the hologram pattern area 8a for use may be configured to have a lens power in the direction orthogonal to the dividing line of the photodetector so that signal detection can be performed. In addition, Fo
The method of detecting various signals such as the E signal is almost the same as the beam size method.

As described above, according to the present invention, the hologram pattern region through which the reflected light from the illuminated object passes is separated into the region through which the main beam passes and the region through which only the sub beam passes. In addition, the ± 1st order diffracted light has powers in opposite directions in the region where the main beam passes,
Since the region through which only the sub-beam passes does not have power, the beam spot of the sub-beam as the -1st order diffracted light is a photodetector that receives the beam spot of the main beam. Therefore, the beam spot of the sub beam does not enter the photodetector for detecting the main beam. Therefore, the noise of the information signal is reduced, and the focus error signal is not offset, so that proper information recording and reproduction can be performed.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an optical pickup according to a first embodiment of the present invention.

2 is a plan view of the laser diode and photodetector of FIG. 1. FIG.

3A and 3B are a plan view and a bottom view showing the configuration of the hologram element of FIG.

FIG. 4 is a perspective view showing a relationship between a hologram element and a light detection unit.

FIG. 5 is a schematic diagram of an optical pickup according to a second embodiment of the present invention.

FIG. 6 is a plan view of the laser diode and photodetector of FIG.

7A and 7B are a plan view and a bottom view showing the configuration of the hologram element of FIG.

FIG. 8 is a schematic diagram of an optical pickup according to a conventional example.

9A and 9B are a plan view and a sectional view showing the arrangement of the laser diode and the photodetector of FIG.

[Explanation of symbols]

1 Semiconductor substrate 2 Laser diode 3 Photodetector 4 Light detector 5 launch mirror 6 Hologram element 6a Grading surface 6b hologram surface 8a Hologram pattern area 8b Hologram pattern area (for sub beam) 8c Hologram pattern area (for sub beam) 11 Main beam passage area 12 Sub beam passage area 13 Sub beam passage area 14a Beam spot (sub beam) 14c Beam spot (sub beam) 15c Beam spot (main beam)

Claims (2)

[Claims] 1. A converging optical system for projecting light emitted from a light source onto an object to be illuminated, and an optical path between the converging optical system and the light source. The light emitted from the light source is a main beam for reading information.
Is arranged in the optical path between the grating and the focusing optical system, which separates the beam and the two tracking error detection sub-beams, and diffracts the reflected light from the illuminated object. A first optical system comprising a hologram optical element for guiding the first-order diffracted light to a photodetector and a photodetector for receiving a main beam and a subbeam respectively, and including at least a region through which the main beam passes in the hologram optical element. The first region is divided into at least three regions, a region, a second region through which only one sub-beam passes, and a third region including a region through which the other sub-beam only passes through.
The optical pickup is characterized in that interference fringes are formed so that the second region and the third region have different lens functions with respect to the region. 2. A hologram element in which a first region is an interference fringe having a predetermined lens power, and a second region and a third region are interference fringes having a lens power close to zero. The optical pickup according to item 1.