JP2003223728A - Optical pickup and hologram element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To securely detect a focus error signal using a special objective lens. <P>SOLUTION: When respective returned optical beams from a first and a second optical disks are diffracted by a hologram element through the special objective lens so as to be incident on one and another photodetectors in the form of spots, the power of a convex lens and the power of a concave lens of a hologram surface of the hologram element is made larger than that in the case of the first optical disk only and in the case of the second optical disk only. The positions of one and the other photodetectors are further shifted to the side of the hologram element than that in the case of the first optical disk only and in the case of the second optical disk only, in a state that the interval between the convergence point of a +1st order diffracted optical beam and the convergence point of a -1st order diffracted optical beam which are respectively diffracted by the hologram surface of the hologram element is enlarged, for respective returned optical beams from the first and the second optical disks. When the grating curve shape of the hologram surface of the hologram element is calculated using a polynomial, the coefficient of the term in the polynomial which works for correcting aberration is optimized. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention selectively irradiates two types of laser beams having different wavelengths emitted from one special objective lens onto two types of optical discs having different disc substrate thicknesses. Each return light of the above is diffracted by the hologram element, and the ± first-order diffracted light obtained by the hologram element is received by one and the other photodetector in a spot shape, and SSD (Spot Size Detectio
The present invention relates to an optical pickup and a hologram element configured to favorably detect a focus error signal with outputs from one and the other photodetectors by the method n).

[0002]

2. Description of the Related Art Generally, an optical disc records information signals such as video information, audio information, computer data, etc. on a disc-shaped disc substrate in a spiral or concentric circular pattern with high density, and It is often used because a desired track can be accessed at high speed when reproducing a recorded track. At this time, the optical disc can be roughly classified into a read-only type and a recordable / reproducible type.

Here, the read-only type optical disc is
A track is formed in a spiral or concentric pattern on a disk-shaped disk substrate by injection molding using a transparent resin material, and a reflective film such as aluminum is formed on the uneven pit array. To form a signal surface. On the other hand, a recordable / reproducible type optical disc is formed by injection molding using a transparent resin material on a disc-shaped disc substrate in which tracks are preliminarily formed in spiral or concentric circles with uneven grooves and lands. A signal film is formed by sequentially depositing a recording film and a reflective film on the land.

The read-only type optical disc is
An optical pickup, which is movably provided in the optical disc apparatus in the radial direction of the optical disc, emits a reproducing laser beam emitted through an objective lens onto a signal surface, and a return light reflected from the signal surface is detected by a photodetector. Is detected and played back. On the other hand, a recordable / reproducible type optical disc records a signal surface with a recording laser beam emitted through an objective lens from an optical pickup provided movably in a radial direction of the optical disc in an optical disc device, and thereafter, The recorded signal surface is reproduced with a laser beam for reproduction in the same manner as above.

Among the above optical discs, the first optical disc such as a DVD (Digital Versatile Disc) having an increased recording density for information signals is a reproduction-only disc for reproducing digitized and compressed video or audio, or computer data. Playback-only DVD-ROM (DVD-Read
Only Memory), a recordable and reproducible DVD-R (DVD-Recordable) capable of recording an information signal only once, and a recordable and reproducible DVD-RW (DVD-R) capable of recording an information signal a plurality of times.
e recordable), DVD-RAM (DVD-Random Access Me
mory or DVD-Writable), and these optical discs are treated as DVDs because the structures of two disc substrates bonded together are similar.

On the other hand, a second optical disc such as a CD (Compact Disc) having a recording density lower than that of the first optical disc is a reproduction-only CD in which music information is pre-recorded or a reproduction-only CD in which computer data is pre-recorded. -ROM (CD-Read Only
Memory) and recordable and reproducible CD-R (CD-Recordable) that can record information signals only once. These optical discs are treated as CDs because the structure of a single disc substrate is similar. Has been.

FIGS. 14 (a) and 14 (b) are diagrams schematically showing the case where the first optical disc (DVD) and the second optical disc (CD) are reproduced by the optical disc device, respectively.

First, a first optical disc 10 such as a DVD shown in FIG. 14A has a transparent disc substrate 11 made of a transparent resin material and a reinforcing disc substrate 1 made of a resin material.
4 and 120 mm diameter, 15m center hole diameter
m, the disk substrate has a thickness of 0.6 mm and is formed in a disk shape, and both disk substrates 11 and 14 are bonded to each other to have a total thickness of 1.2 mm. At this time, since the disk substrate thickness of the transparent disk substrate 11 from the laser beam incident surface 11a side to the signal surface 12 is 0.6 mm, which is thin, the first optical disk (DVD) 10 will be described below.
Is referred to as a first optical disc having a thin disc substrate.
Further, in the first optical disk (DVD) 10, the signal surface 12 is formed on the transparent disk substrate 11 with a narrow track pitch to have a high density, and the adhesive layer 13 is further formed on the signal surface 12.
The reinforcing disk substrate 14 is attached via the.

Then, the above-mentioned first optical disc (DV
D) When the 10 is reproduced by the optical pickup in the optical disk device, a short wavelength laser beam emitted from a semiconductor laser (not shown) having a wavelength of 650 nm is narrowed down by the objective lens OBL1 having a numerical aperture (NA) of 0.6. A laser beam L1 is obtained by irradiating the signal surface 12 with the laser beam L1 from the laser beam incident surface 11a side of the transparent disk substrate 11 and reproducing by the return light reflected by the signal surface 12.

Next, in the second optical disk (CD) 20 shown in FIG. 14 (b), a transparent disk substrate 21 made of a transparent resin material has a diameter of 120 mm, a central hole diameter of 15 mm, and a disk substrate thickness of 1.2 mm. It is formed in a disk shape.
At this time, the thickness of the disk substrate from the laser beam incident surface 21a side of the transparent disk substrate 21 to the signal surface 22 is 1.
Since the thickness is 2 mm, the second optical disc (CD) 20 will be referred to as a second optical disc having a thick disc substrate in the following description. In addition, in the second optical disc (CD) 20,
A signal surface 22 is formed on the transparent disk substrate 21 with a track pitch wider than that of the first optical disk (DVD) 10, and a protective film 23 is formed on the signal surface 22.

Then, the above-mentioned second optical disc (CD)
20 is an optical pickup (not shown) in the optical disk device
In the case of reproduction by the laser beam L2, a laser beam having a long wavelength, for example, a wavelength of 780 nm emitted from a semiconductor laser (not shown) is focused by an objective lens OBL2 having a numerical aperture (NA) of 0.45 to obtain a laser beam L2. L
2 is a laser beam incident surface 21 of the transparent disk substrate 21
The signal surface 22 is irradiated with light from the a side, and is reproduced by the return light reflected by the signal surface 22.

Here, the above-mentioned first optical disk (DV
D) 10 and the second optical disc (CD) 20 are recorded and / or reproduced in the same optical disc device, see
Japanese Patent Laid-Open No. 000-56216 discloses an optical system for an optical pickup and an optical pickup using one special objective lens for two laser beams having different wavelengths.

FIGS. 15 (a) and 15 (b) are views for explaining the special objective lens, and FIG. 15 (a) is an optical path diagram for reproducing a first optical disc (DVD) having a thin disc substrate, (B) is a diagram showing an optical path diagram when reproducing a second optical disc (CD) having a thick disc substrate.

The special objective lens SOBL shown in FIGS. 15 (a) and 15 (b) is disclosed in the above-mentioned Japanese Patent Laid-Open No. 2000-56216, and here, the special objective is referred to with reference to the same. The lens SOBL will be briefly described.

The optical pickup disclosed in Japanese Patent Laid-Open No. 2000-56216 mentioned above has a first wavelength of about 610 nm to 670 nm for reproducing the first optical disc (DVD) 10 having a thin disc substrate. A first semiconductor laser (not shown) that emits a laser beam and a second semiconductor that emits a second laser beam having a wavelength of about 740 nm to 870 nm for reproducing the second optical disc (CD) 20 having a thick disc substrate. It is assumed that a laser (not shown) is built in.

As shown in FIGS. 15 (a) and 15 (b), in the special objective lens SOBL, the refracting surface S1 on the laser light source side and the refracting surface S2 on the first and second optical discs 10 and 20 side are both aspherical. Is a convex lens having a positive refractive power. Further, a plurality of refracting surfaces S1 of the special objective lens SOBL on the laser light source side are concentric with the optical axis K (here, three).
The first ring zone surface Sd1 to the third ring zone surface Sd3 are formed from the inner circumference toward the outer circumference. At this time, the first ring surface Sd1
And a second ring surface Sd2 provided between the third ring surface Sd3 and
Is formed on a ring-shaped groove surface with a step at the boundary portion.

In this special objective lens SOBL, the first light flux passing through the first annular surface Sd1 including the optical axis K is the first light flux.
The second light flux, which is used for reproducing the information recorded on the optical disc 10 and for reproducing the information recorded on the second optical disc 20 and passes through the second ring zone surface Sd2 outside the first ring zone surface Sd1, is mainly 2 Used for reproducing information recorded on the optical disk 20, the third light flux passing through the third annular surface Sd3 outside the second annular surface Sd2 is mainly used for reproducing information recorded on the first optical disk 10. It is shaped to be used.

Then, as shown in FIG. 15A, when the signal surface 12 formed on the transparent disk substrate 11 of the first optical disk (DVD) 10 is reproduced, the thickness of the transparent disk substrate 11 becomes 0. 0.6 mm, special objective lens S
The first laser beam L1 having a wavelength of 610 nm to 670 nm narrowed down by OBL is made incident from the transparent disk substrate 11 side.

At this time, the first luminous flux and the third luminous flux (the luminous flux indicated by the diagonal lines) passing through the first annular zone surface Sdl and the third annular zone surface Sd3 are imaged at substantially the same first image forming position. The wavefront aberration (wavefront aberration excluding the second light flux passing through the second ring surface Sd2) is 0.07 λ1 rms or less, preferably 0.05 λ1 rms or less. Here, λ1 is the wavelength of the first laser light emitted from the first semiconductor laser. Further, at this time, the second light flux (light flux indicated by the broken line) passing through the second ring surface Sd2 is imaged at a second image formation position different from the first image formation position. If the second objective position is 0 (zero) and the special objective lens SOBL side is negative and the opposite side is positive, the second imaging position is −40 μm or more and −4 μm or less from the first imaging position, Preferably-
The distance is set to 27 μm or more and −4 μm or less (the second image forming position is brought closer to the objective lens than the first image forming position). As a result, the reproduction of the first optical disc 10 is performed mainly with the first light flux and the third light flux.

On the other hand, as shown in FIG.
When reproducing the signal surface 22 formed on the transparent disk substrate 21 of the optical disk (CD) 20, the thickness of the transparent disk substrate 21 is 1.2 mm, and the special objective lens SOB is used.
The second laser beam L2 having a wavelength of 740 nm to 870 nm narrowed by L is made incident from the transparent disk substrate 21 side.

At this time, in the case of a predetermined light beam (parallel light beam) incident on the special objective lens SOBL, a light beam passing near the optical axis of the first light beam (indicated by an oblique line rising to the right) is the optical axis K. The second light flux () is formed between the intersecting position and the position where the light ray passing through the end portion of the first annular zone surface Sd1 (on the side of the second annular zone surface Sd2) in the direction orthogonal to the optical axis K intersects the optical axis K. A light ray with a downward-sloping diagonal line intersects with the optical axis K (forms an image).
Like Therefore, the first light flux and the second light flux are condensed near the information recording surface of the second optical disc 20, and the reproduction of the second optical disc 20 is performed. At this time, the third light flux (partially shown by a broken line) is generated as flare, but
The nucleus formed by the light flux and the second light flux makes it possible to reproduce the second optical disc 20.

As described above, it is possible to reproduce the first and second optical disks 10 and 20 having different transparent disk substrate thicknesses by one special objective lens SOBL.

Further, the special objective lens SOBL has the first and second
Focus control is performed on the second optical disks 10 and 20 so that the first and second laser beams L1 and L2 are focused on the signal surfaces 12 and 22 formed on the transparent disk substrates 11 and 21, and Tracking control is performed so as to follow 222 tracks.

[0024]

By the way, in the conventional optical pickup adopting the above-mentioned special objective lens SOBL, the first and second laser beams L1 and L2 having different wavelengths focused by one special objective lens SOBL are recorded on the disk. Two types of first and second optical disks 10, 2 having different substrate thicknesses
0 to selectively irradiate the first and second laser beams L
1 and L2 are focused on the signal surfaces 12 and 22 formed on the transparent disk substrates 11 and 21, respectively.
Although the information signal recorded on the SSD can be reproduced, at this time, the SSD is used as a focus control for the special objective lens SOBL.
(Spot Size Detection) method is drawing attention.

The above-mentioned SSD method is obtained by diffracting the return light from the optical disc through the objective lens by the hologram element and receiving the ± first-order diffracted light obtained by the hologram element by the two photodetectors. Good focus control is possible by calculating the difference between the two spot diameters and detecting the focus error signal.

Therefore, the first and second laser beams emitted from the first and second semiconductor lasers having different wavelengths are respectively focused by the above-mentioned one special objective lens.
A second laser beam is selectively applied to the corresponding first and second optical disks, and a hologram element and a plurality of hologram elements are provided so that the SSD method described above can be applied to focus control of this special objective lens. The optical pickup is configured with the photodetector and the detection of the focus error signal is performed by the SSD method using this optical pickup. When the ± 1st-order diffracted light rays are disturbed, the two optical detectors are disturbed. The spot could not be received well, and the focus error signal could not be detected accordingly. As a result of investigating the cause of this, the longitudinal aberration curve of the ± first-order diffracted light obtained by diffracting the return light from the optical disc by the hologram element through the special objective lens is well separated at the position of the photodetector. For not ±
It was found that the first-order diffracted light could not be condensed at one point and spread.

[0027]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and the first invention emits a short wavelength first laser beam corresponding to a first optical disc having a thin disc substrate. The first semiconductor laser to be emitted, the second semiconductor laser to emit a second laser beam having a long wavelength corresponding to the second optical disc having a thick disc substrate, and the first or second semiconductor laser to be selectively emitted respectively. First
Alternatively, a hologram element having a hologram surface for allowing the second laser light to pass therethrough and diffracting the return light from the first or second optical disk, and numerical apertures corresponding to the first and second laser lights ( NA) is obtained, and a first or second laser beam obtained by narrowing the first or second laser light that has passed through the hologram element is formed on the first or second optical disc. A special objective lens for selectively irradiating the optical disc, and for detecting the ± first-order diffracted light obtained by diffracting the return light from the first or second optical disc on the hologram surface of the hologram element, Divide into one and the other and make this set
A plurality of photodetectors provided in combination, and among the ± 1st-order diffracted light obtained by reproducing the first or second optical disc, the + 1st-order diffracted light is generated by the convex lens power of the hologram surface of the hologram element. The spot light is received by the other photodetector, and the minus first-order diffracted light is received by the other photodetector in the spot by the concave lens power of the hologram surface of the hologram element.
In the optical pickup configured to detect the focus error signal by the output from each of the one and the other photodetectors by the method (1), the convex lens power and the concave lens power of the hologram surface of the hologram element are dedicated to the first optical disc. And a converging point of the + 1st-order diffracted light diffracted by the hologram surface of the hologram element corresponding to each return light from the first or second optical disc, which is larger than that for the second optical disc. -1
In the state where the distance between the condensing point of the second-order diffracted light is widened, the position of each of the one and the other photodetectors is closer to the hologram element side than when the first optical disc is dedicated and when the second optical disc is dedicated. In addition to the above, when the lattice curve shape of the hologram surface of the hologram element is calculated using a polynomial, by optimizing the coefficient of the term that acts on aberration correction in this polynomial, the optical axis of the special objective lens The longitudinal aberration curve of the + 1st order diffracted light showing the amount of deviation along
An optical pickup is characterized in that the longitudinal aberration curve of the second-order diffracted light is separated with the positions of the one and the other photodetectors as boundaries.

A second aspect of the present invention selectively reproduces a first optical disc having a thin disc substrate and a second optical disc having a thick disc substrate, and further, the first or second optical disc.
The ± 1st-order diffracted light obtained by diffracting each return light from the optical disk through a special objective lens having a plurality of ring-shaped surfaces and then diffracting the hologram surface produces an SSD (Spot Size Detectio).
In the hologram element used for detecting the focus error signal by the n) method, the convex lens power and the concave lens power of the hologram surface of the hologram element are made larger than those for the first optical disk and those for the second optical disk. At the same time, when the lattice curve shape of the hologram surface is calculated using a polynomial, the coefficient of the term that acts on aberration correction in this polynomial is optimized.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of an optical pickup and a hologram element according to the present invention will be described in detail below with reference to FIGS. 1 to 13.

FIG. 1 is a perspective view for explaining an optical pickup and a hologram element according to the present invention, FIG. 2 is a conceptual view for explaining the first to third photodetectors shown in FIG. 1, and FIG.
FIG. 3 is a conceptual diagram for explaining the special objective lens shown in FIG. 1.

As shown in FIG. 1, the optical pickup 30 according to the present invention is a first optical disc (DVD) having a thin disc substrate described above with reference to FIGS. 14 (a) and 14 (b).
10 and a second optical disc (C
D) 20 and 20 can be selectively recorded and / or reproduced by using one special objective lens 38.

In the optical pickup 30 described above, the mount base 32 is fixed on the silicon substrate 31, and the first semiconductor laser 33 for the first optical disk and the second semiconductor for the second optical disk are mounted on the mount base 32. The laser 34 and the laser 34 are installed close to each other. At this time, the first
The semiconductor laser 33 is installed with its center aligned with the optical axis K of the optical pickup 30, while the second semiconductor laser 34 has its center aligned with the optical axis K of the optical pickup 30 by 10 mm.
It is installed at a distance of 0 μm to 130 μm.

The above-mentioned first semiconductor laser 33 has a short wavelength, for example, a wavelength of 650n when the first optical disc (DVD) 10 having a thin disc substrate is mounted.
The second semiconductor laser 34 emits a first laser beam of m, while the second semiconductor laser 34 has a long and short wavelength corresponding to the second optical disc (CD) 20 having a large disc substrate thickness, for example, a wavelength of 780 nm. 2 Laser light is emitted.

A standing mirror surface 35a is formed on the silicon substrate 31 so as to face the laser light emitting sides of the first and second semiconductor lasers 33 and 34 and be inclined by 45 ° with respect to the silicon substrate 31. The raising mirror 35 is fixed.

Further, first to third photodetectors 31a to 31c are formed on the silicon substrate 31. On this occasion,
The first photodetector 31a and the second photodetector 31b are substantially symmetrical with respect to the optical axis K of the optical pickup 30, and the raising mirror 3 is used.
5 is formed separately on one side surface side and the other side surface side, and the third photodetector 31c is formed on the other side surface side of the rising mirror 35 and slightly apart from the second photodetector 31b to the outside. ing.

Then, these first to third photodetectors 31a
Denoted at 31c are focus error signals detected by the respective return lights from the first and second optical disks 10 and 20 using the SSD (Spot Size Detection) method. When detecting a signal, a set of the first and second photodetectors 31a and 31b formed on the silicon substrate 31 is used. On the other hand, when detecting a focus error signal from the second optical disc (CD) 20, , A set of first and third photodetectors 31a and 31c formed on the silicon substrate 31 is used.

Here, as shown in an enlarged manner in FIG. 2, the first to third photodetectors 31a to 31a formed on the silicon substrate 31.
31c has a light-receiving surface divided into three parts, and one and the other photodetectors (31a, 31b), (31) of each set.
a, 31c) where the output values of the respective spots received on the respective light receiving surfaces are V1 to V6, the focus error signal by the SSD method is: focus error signal = {(V1 + V3-V
2)-(V4 + V6-V5)}, which is obtained by just calculating, for example, V1 +
Each light-receiving surface is 3 so that V3 = V2, V4 + V6 = V5.
It is divided.

Returning to FIG. 1, a hologram element 36 is provided above the raising mirror 35 in accordance with the optical axis K of the optical pickup 30. This hologram element 36
Is a member that constitutes an essential part of the present invention, and has a hologram surface 36a1 formed on the upper surface 36a side and a lower surface 36b.
Grating is applied to the side.

Here, the upper surface 36a of the hologram element 36
As will be described later, the hologram surface 36a1 formed on the side has a larger convex lens power and concave lens power than those for the first optical disk and for the second optical disk. The coefficient of the term that acts on the aberration correction in this polynomial is optimized when the calculation is performed using these, which will be described in detail later.

Further, the first and second semiconductor lasers 3 are formed by the grating provided on the lower surface 36b side of the hologram element 36.
The first or second selectively emitted from each of
The laser light is diffracted and is incident on the upper collimator lens 37.

Next, the collimator lens 37 provided above the hologram element 36 is for collimating the first or second laser light passing through the hologram element 36 and making it enter the special objective lens 38 above. is there.

Further, the special objective lens 38 provided above the collimator lens 37 is previously shown in FIGS. 15 (a) and 15 (b).
Is formed into a shape substantially similar to that described in the related art, and as shown in FIG.
The refracting surface S1 on the side of the semiconductor lasers 33, 34 and the refracting surface S2 on the side of the first and second optical disks 10, 20 are both aspherical convex lenses having a positive refracting power. Further, the refracting surface S1 of the special objective lens 38 has the optical pickup 30
The first ring zone surface Sd1 to the third ring zone surface Sd3 are formed concentrically with the optical axis K from the inner circumference toward the outer circumference. And
A numerical aperture (NA) of 0.6 corresponding to the first laser beam is obtained from the first ring surface Sd1 to the third ring surface Sd3, and
A numerical aperture (NA) = 0.45 corresponding to the second laser light is obtained by the first and second ring zones Sd1 and Sd2. At this time, the first ring surface Sd1 and the third ring surface S
The second annular zone surface Sd2 provided between the d3 and d3 is formed as a ring-shaped concave groove surface with a step at the boundary portion. Also,
The third ring zone surface Sd3 is formed without aberration.

In the present invention, the first to third ring zones S
d1 to Sd3 are used for recording and / or reproducing information on the first optical disc (DVD) 10, and the first ring surface Sd
1 and the second ring-shaped surface Sd2 are the second optical disc (CD) 20.
Is used for recording and / or reproducing information.

The optical pickup 30 constructed as described above
In the first optical disc (D
When recording and / or reproducing the VD) 10, the first semiconductor laser 33 is driven to emit the first laser light having a wavelength of 650 nm. At this time, when recording, the power of the first laser beam is increased and emitted, while during reproduction, the first laser beam is emitted.
The power of the laser light is weakened and emitted.

The first laser light emitted from the first semiconductor laser 33 is reflected upward by the mirror surface 35a of the rising mirror 35, and then passed through the hologram element 36 and the collimator lens 37 in this order. The first optical disc (DVD) 10 is irradiated with the first laser beam L1 focused by the special objective lens 38.

After that, the signal surface 12 formed on the transparent disk substrate 11 of the first optical disk (DVD) 10 (FIG. 14).
The return light reflected by the reflection film of (a)} returns to the special objective lens 38 and the collimator lens 37 in order, is diffracted by the hologram surface 36a1 of the hologram element 36, and becomes the + 1st-order diffracted light D1 and the −1st-order diffracted light D2. The first photodetector 3 in which the + 1st-order diffracted light D1 is separated and formed on the silicon substrate 31.
The first-order diffracted light D2 is received in a spot shape on the first substrate 1a, and the −1st-order diffracted light D2 is received in a spot shape on the second photodetector 31b formed on the silicon substrate 31.
The spots on a and 31b are received substantially symmetrically with respect to the optical axis K of the optical pickup 30.

On the other hand, when recording and / or reproducing the second optical disc (CD) 20 having a thick disc substrate, the second semiconductor laser 34 is driven to drive the second optical disc having the wavelength of 780 nm.
It emits laser light. Again, the second time when recording
The power of the laser light is increased and emitted, while the power of the second laser light is decreased and emitted during reproduction.

The second laser light emitted from the second semiconductor laser 34 is reflected upward by the mirror surface 35a of the rising mirror 35, and then passed through the hologram element 36 and the collimator lens 37 in this order. The second optical disk (CD) 20 is irradiated with the second laser beam L2 focused by the special objective lens 38.

After this, the signal surface 22 formed on the transparent disk substrate 21 of the second optical disk (CD) 20 (FIG. 14).
The return light reflected by the reflection film of (b)} returns to the special objective lens 38 and the collimator lens 37 in order, and is diffracted by the hologram surface 36a1 of the hologram element 36 to become + 1st-order diffracted light E1 and −1st-order diffracted light E2. The first photodetector 3 in which the + 1st order diffracted light E1 is separated and formed on the silicon substrate 31.
The first-order diffracted light E2 is received in a spot shape on the first substrate 3a, and the −1st-order diffracted light E2 is received in a spot shape on the third photodetector 31c formed on the silicon substrate 31.
The spots on a and 31c are received asymmetrically with respect to the optical axis K of the optical pickup 30. The reason for this is the second
The position where the semiconductor laser 34 is installed is slightly apart from the optical axis K of the optical pickup 30, and the wavelength of the second laser light emitted from the second semiconductor laser 34 is the first emitted from the first semiconductor laser 33. This is because the angle of diffraction by the hologram element 36 is different because it is different from the wavelength of the laser light.

Here, the case where the focus error signal is detected by using the SSD method, which is an essential part of the present invention, will be described in detail.

FIGS. 4A and 4B are optical path diagrams when the first and second optical disks are reproduced by one special objective lens, and FIG. 5 is a state where they are optically designed by using an objective lens of NA 0.6. 3A and 3B are diagrams for explaining a diffraction characteristic when the first optical disc is reproduced, (a) shows a longitudinal aberration curve of ± 1st-order diffracted light, and (b) shows first and second ± 1st-order diffracted light. The figure showing the light receiving state to the photodetector.
5A is a diagram for explaining a diffraction characteristic when a second optical disc is reproduced in an optical design state using the objective lens of FIG. 5, (a) shows a longitudinal aberration curve of ± first-order diffracted light, FIG.
(B) is a diagram showing the light receiving state of the ± 1st order diffracted light to the first and third photodetectors, and FIG. 7 shows the diffraction characteristics when the first optical disc and the second optical disc are reproduced by using the special objective lens. It is a figure for explaining, (a) shows a longitudinal aberration curve of each ± 1st order diffracted light, and (b) shows each (1st, 2nd), (1st, 3rd) light of ± 1st order diffracted light. FIG. 8 is a diagram showing a light receiving state to a detector, FIG. 8 is a conceptual diagram for explaining a case where the convex lens power and the concave lens power of the hologram surface of the hologram element are increased in the optical pickup according to the present invention, and FIG. 9 is the present invention. In the optical pickup according to the present invention, when a special objective lens is used and the lens power of the hologram element is increased to reproduce the first optical disc and the second optical disc, a diagram showing longitudinal aberration curves of each ± first-order diffracted light, Figure 10
In the optical pickup according to the present invention, a special objective lens is used, and after the lens power of the hologram element is increased, the positions of the first to third photodetectors are moved to change the first optical disc and the second optical disc. When reproduced, each ±
FIG. 11 is a diagram showing the longitudinal aberration curve of the first-order diffracted light, and FIG. 11 shows the first-order diffracted light after the high-order coefficient (= aberration term coefficient) of the hologram curve polynomial of the hologram element is manipulated from the state of FIG. 10 in the optical pickup according to the present invention. FIG. 12 is a diagram showing the longitudinal aberration curves of the ± 1st-order diffracted lights when the first optical disc and the second optical disc are reproduced, and FIG. 12 is a diffraction characteristic when the first optical disc (DVD) is reproduced in the optical pickup according to the present invention. FIG. 13A is a diagram showing a longitudinal aberration curve of ± first-order diffracted light, and FIG. 13B is a diagram showing a light-receiving state of ± first-order diffracted light to the first and second photodetectors. FIG. 4 is a diagram showing diffraction characteristics when a second optical disc (CD) is reproduced in the optical pickup according to the present invention, (a) shows a longitudinal aberration curve of ± first-order diffracted light, and (b) shows ± 1. First, third of diffracted light
It is the figure which showed the light receiving state to the photodetector.

As shown in FIG. 4A, when the first optical disc (DVD) 10 having a thin disc substrate is reproduced, the return light from the first optical disc (DVD) 10 is a special objective lens as described above. 38, collimator lens 3
7 in that order, the hologram surface 36 of the hologram element 36
Diffracted at a1.

Here, on the hologram surface 36a1 of the hologram element 36, the lens power exerted on the + 1st order diffracted light D1 has the function of a convex lens, while the lens power exerted on the −1st order diffracted light D2 has the function of a concave lens. There is. Therefore, the + 1st-order diffracted light D1 diffracted by the convex lens power on the hologram surface 36a1 of the hologram element 36 is condensed at the point position D1a before reaching the first photodetector 31a formed on the silicon substrate 31, and thereafter. , Are received in spots on the first photodetector 31a. On the other hand, the −1st-order diffracted light D2 diffracted by the concave lens power on the hologram surface 36a1 of the hologram element 36 reaches the second photodetector 31b formed on the silicon substrate 31 and is received as a spot, and then the second The light is virtually collected at a point position D2a behind the photodetector 31b.

Further, as shown in FIG. 4B, when the second optical disk (CD) 20 having a thick disk substrate is reproduced, the second optical disk (CD) 10 is reproduced similarly to the first optical disk (DVD) 10 described above. ) 20 returns from the special objective lens 38 and the collimator lens 37 in order and is diffracted by the hologram surface 36a1 of the hologram element 36. Then, the hologram surface 36 of the hologram element 36
The + 1st order diffracted light E1 diffracted by the convex lens power a1 is the first photodetector 31 formed on the silicon substrate 31.
It is focused at the point position E1a before reaching a, and then
The light is received in spots on the first photodetector 31a.
On the other hand, the −1st order diffracted light E2 diffracted by the concave lens power on the hologram surface 36a1 of the hologram element 36 is
The third photodetector 31c formed on the silicon substrate 31 reaches the third photodetector 31c and is received in a spot shape.
It is virtually condensed at a point position E2a behind c.

By the way, in the optical system of the optical pickup 30 shown in FIG. 4A, instead of the special objective lens 38, an objective lens OBL1 {NA0.6 dedicated to the first optical disc (DVD) 10 is used. The wavelength of 650 nm narrowed down by this objective lens OBL1 using FIG.
The first laser beam L1 of the first optical disc (DVD)
Objective lens OB with NA 0.6 when irradiating on 10
Hologram element 36 and silicon substrate 3 according to L1
1. First and second photodetectors 31a ′, 31b ′ formed in 1
When the position in the optical axis direction of (FIG. 5) is geometrically designed, the detection of the focus error signal from the first optical disk (DVD) 10 by the SSD method becomes an ideal state. Hologram element 3 obtained in this ideal state
The diffraction characteristics of No. 6 are as shown in FIGS.
5A and 5B, the ± first-order diffracted lights D1 ′ and D2 ′, the photodetectors 31a ′, and
31b ', ± 1st-order diffracted light focusing point positions D1a', D2
A description will be given below with reference to a '.

Furthermore, in the following description, in general,
When the vibration direction of a wave is the same as the traveling direction, it is called a longitudinal wave, and when the vibration direction is perpendicular to the traveling direction, it is called a transverse wave.Even with a lens, the direction along the optical axis is vertical and the direction perpendicular to the optical axis is transverse. become. Therefore, the longitudinal aberration represents the amount of deviation along the optical axis of the lens when measuring the amount of aberration, and here represents the amount of deviation along the optical axis of the objective lens or the special objective lens.

That is, as shown in FIG. 5A, when the return light from the first optical disk (DVD) 10 is diffracted by the hologram element 36, the objective lens OBL having NA 0.6.
The longitudinal aberration curve of the + 1st-order diffracted light D1 ′ indicating the amount of deviation along the optical axis of 1 (FIG. 14A) is the + 1st-order diffracted light D1 ′.
On this optical axis, a straight line having no inclination is obtained at a point position D1a 'separated from the position of the first photodetector 31a' formed on the silicon substrate 31 on the first optical disc 10 side by a distance d1. On the other hand, when the return light from the first optical disc (DVD) 10 is diffracted by the hologram element 36, the longitudinal aberration curve of the −first-order diffracted light D2 ′ showing the amount of deviation along the optical axis of the objective lens OBL1 with NA 0.6. Is the -1st order diffracted light D2 '.
A straight line having no inclination is obtained on the optical axis of the point position D2a ', which is a distance d2 away from the position of the second photodetector 31b' formed on the silicon substrate 31 on the side opposite to the first optical disc 10 side. At this time, the longitudinal aberration curve of the + 1st-order diffracted light D1 'and the longitudinal aberration curve of the -1st-order diffracted light D2' are
The first and second photodetectors 31a 'and 31b' formed in No. 1 are separated into left and right with the position as a boundary. This allows
As shown in FIG. 5B, the first optical disc (DVD)
The return light from 10 was diffracted by the hologram element 36 ±
The 1st-order diffracted light D1'D2 'is located at point positions D1a', D, respectively.
It is in an ideal state where light is condensed at each one point of 2a '.

Further, similarly to the above, in the optical system of the optical pickup 30 shown in FIG. 4B, the special objective lens 38 is replaced with a dedicated NA 0.45 for the second optical disc (CD) 20. Objective lens OBL2 {Fig. 14
(B)} is used to irradiate the second optical disc (CD) 20 with the second laser beam L2 having a wavelength of 780 nm narrowed down by the objective lens OBL2, the hologram element is aligned with the objective lens OBL2 with NA 0.45. 36 and 31 of the first and third photodetectors formed on the silicon substrate 31.
When the positions of a ′ and 31c ′ (FIG. 6) in the optical axis direction are geometrically designed, the second optical disc (C
D) The detection of the focus error signal from 20 becomes an ideal state. The diffraction characteristics of the hologram element 36 obtained in this ideal state are as shown in FIGS. 6 (a) and 6 (b). In addition, in FIGS. 6A and 6B, FIG.
Corresponding to the ± 1st order diffracted light E1 ', E2', photodetector 3
1a ', 31c', the focal point position E1 of the ± 1st-order diffracted light
A'and E2a 'will be described below.

That is, as shown in FIG. 6A, the return light from the second optical disk (CD) 20 is reflected by the hologram element 3
Objective lens OBL with NA 0.45 when diffracted by 6
The longitudinal aberration curve of the + 1st-order diffracted light E1 ′ indicating the amount of deviation along the optical axis of 2 {FIG. 14 (b)} is the + 1st-order diffracted light E1 ′.
A straight line having no inclination is obtained on the optical axis of the point E1a ', which is a distance e1 away from the position of the first photodetector 31a' formed on the silicon substrate 31 toward the second optical disc 20 side. On the other hand, when the return light from the second optical disc (CD) 20 is diffracted by the hologram element 36, the longitudinal aberration curve of the −first-order diffracted light E2 ′ indicating the deviation along the optical axis of the objective lens OBL2 having NA 0.45. Is the -1st order diffracted light E2 '
A straight line having no inclination is obtained on the optical axis of the point position E2a 'at a distance e2 from the position of the third photodetector 31c' formed on the silicon substrate 31 to the side opposite to the second optical disc 20 side. At this time, the longitudinal aberration curve of the + 1st order diffracted light E1 ′ and the longitudinal aberration curve of the −1st order diffracted light E2 ′ are
The first and third photodetectors 31a ′ and 31c ′ formed in 1 are separated into left and right sides. This allows
As shown in FIG. 6B, the second optical disc (CD) 2
The return light from 0 was diffracted by the hologram element 36 ± 1
The second-order diffracted lights E1 'and E2' are located at point positions E1a 'and E, respectively.
It is in an ideal state where light is condensed at each one point of 2a '.

On the other hand, the present invention overcomes the differences in the disk substrate thickness of the first and second optical disks 10 and 20, the wavelengths of the first and second laser beams L1 and L2, and the numerical aperture (NA) of the objective lens. Although one special objective lens 38 is used for the first and second optical disks 10 and 20,
When the hologram element 36 is used especially when reproducing the first and second optical disks 10 and 20, the hologram element 3
± 1st-order diffracted lights D1 and D having a wavelength of 650 nm diffracted by D6
2 and the respective rays of the ± 1st-order diffracted lights E1 and E2 having a wavelength of 780 nm are disturbed to some extent.
The focus error signal of the optical disks 10 and 20 could not be detected.

The reason for this will be described with reference to FIGS. 7A and 7B. The special objective lens 38 is used to selectively reproduce the first optical disk (DVD) 10 and the second optical disk (CD) 20. In this case, the first and second optical disks 10, 20
It was found that the ± first-order diffracted lights (D1, D2) and (E1, E2) obtained by diffracting the respective return lights from the respective holograms by the hologram element 36 are largely deviated from the ideal state described above.

That is, as shown in FIG. 7A, when the return light from the first optical disk (DVD) 10 is diffracted by the hologram surface 36a1 of the hologram element 36, the special objective is produced by the convex lens power of the hologram surface 36a1. A longitudinal aberration curve of the + 1st-order diffracted light D1 indicating the shift amount of the lens 38 along the optical axis and a −longitudinal diffracted light D2 of the special objective lens 38 indicating the shift amount along the optical axis due to the concave lens power of the hologram surface 36a1. The aberration curves are not separated into left and right with the positions of the first and second photodetectors 31a and 31b as boundaries, and are largely deviated from the ideal state, that is, ± first-order diffracted lights D1 and D2. As shown by the solid line, the longitudinal aberration curve of appears in the right side of the positions of the first and second photodetectors 31a and 31b, and is inclined toward the left side.
It straddles the positions of the first and second photodetectors 31a and 31b.

In the same manner as above, the second optical disc (C
D) When the return light from 20 is diffracted by the hologram surface 36a1 of the hologram element 36, the hologram surface 36a1
A longitudinal aberration curve of the + 1st-order diffracted light E1 showing the amount of deviation along the optical axis of the special objective lens 38 due to the convex lens power of
The vertical aberration curve of the −1st order diffracted light E2, which indicates the amount of deviation along the optical axis of the special objective lens 38 due to the concave lens power of the hologram surface 36a1, is the first and third photodetectors 31a, 3
It is not separated into left and right with the position of 1c as a boundary and is largely deviated from the ideal state, that is, the longitudinal aberration curves of the ± first-order diffracted lights E1 and E2 are the first and second as shown by the dotted lines.
Since it appears to the left of the positions of the third photodetectors 31a and 31c and is inclined toward the right, it straddles the positions of the positions of the first and third photodetectors 31a and 31c.

Further, the longitudinal aberration curve of the + 1st order diffracted light D1 by the first optical disc (DVD) 10 and the second optical disc (C
D) The longitudinal aberration curve of the −1st-order diffracted light E2 due to 20 intersects with each other, so that a crossover occurs between the two.

Therefore, as shown in FIG. 7B, when reproducing the first and second optical disks 10 and 20, especially when the special objective lens 38 and the hologram element 36 are used, the hologram element 36 diffracts light. Wavelength 650n
± 1st-order diffracted lights D1 and D2 of m and ± 1 of wavelength 780 nm
Each of the rays of the next-order diffracted lights E1 and E2 has point positions D1a and D2a.
And the point positions E1a and E2a cannot be focused well,
Along with this, the first and second photodetectors 31a and 31b and the first and third photodetectors 31a and 31c cannot receive light properly, so the SSD method is used to detect the first and second optical discs 10 and 20.
In this state, the focus error signal of can not be detected well.

Therefore, in the present invention, when one special objective lens 38 is adopted, the first and second optical disks 10 and 2 are used.
When each return light from 0 is diffracted by the hologram surface 36a1 of the hologram element 36, ± 1st order diffracted lights D1 and D2 having a wavelength of 650 nm and ± 1st order diffracted light E having a wavelength of 780 nm
Paying attention to the longitudinal aberration curve with 1, 1 and E2, ± 1st order diffracted light D1,
The longitudinal aberration curve of D2 is the first and second photodetectors 31a and 31b.
Is completely separated to the left and right with the position of as a boundary, and ± 1
The following measures have been taken so that the longitudinal aberration curves of the next-order diffracted lights E1 and E2 are completely separated left and right with the positions of the first and third photodetectors 31a and 31c as boundaries.

First, the convex lens power and the concave lens power of the hologram surface 36a1 of the hologram element 36 are made larger than those for the first optical disk and for the second optical disk. As described above, the hologram element 36 has the function of a convex lens for the + 1st order diffracted light D1 (or E1) and the function of a concave lens for the −1st order diffracted light D2 (or E2). , By increasing the lens powers of the convex lens function and the concave lens function, the ± first-order diffracted lights D1, D2 and (± first-order diffracted light E
1, E2 ... (not shown) increase the distance between the condensing points.

Specifically, as shown in FIG. 8, for example, when the return light from the first optical disc (DVD) 10 is diffracted by the hologram surface 36a1 of the hologram element 36, as shown in FIG. Objective lens OB with NA 0.6
When the optical design is performed with L1 {FIG. 14 (a)}, the focal point of the + 1st-order diffracted light D1 'is at the point position D1a', and the focal point of the -1st-order diffracted light D2 'is at the point position D2a'. ing.
Then, in this case, the condensing point interval between the point positions D1a 'and D2a' is X1.

Here, when the convex lens power and the concave lens power of the hologram surface 36a1 of the hologram element 36 are increased, the converging point of the + 1st order diffracted light D1 is at the point position D1a '.
From the point position D2a 'to the hologram element 3 at the point position D1a moved in the direction (left side in the drawing) closer to the hologram element 36.
The point position D2a is moved in a direction away from 6 (right side in the figure). In this case, the point position D1a and the point position D2a
The distance between the condensing points between and is X2. This focus point interval X2
Is wider than the above-mentioned condensing point interval X1.

At this time, by repeating the simulation by all optical calculations, expansion of the convex lens power and the concave lens power of the hologram element 36 is suppressed to the minimum. Although not shown here, the return light from the second optical disc (CD) 20 is diffracted by the hologram surface 36a1 of the hologram element 36 in the same manner as described above ± 1.
Also for the next-order diffracted lights E1 and E2, the light is condensed with the convex lens power and the concave lens power of the hologram surface 36a1 of the hologram element 36 being larger than those for the first and second optical disks. The focus of the folded light E1 moves to the left, and the focus of the −1st order diffracted light E2 moves to the right.

Next, as shown in FIG. 9, the special objective lens 38 is used, and as described above, the hologram element 36 is used.
When the first optical disc (DVD) 10 is reproduced by increasing the convex lens power and the concave lens power of the hologram surface 36a1 of the above, the focus point intervals of the ± first-order diffracted lights D1 and D2 are widened, and the + 1st-order diffracted light D1 is collected. The light point moves to the left side, the condensing point of the -1st order diffracted light D2 moves to the right side, and a part (longitudinal end portion in the drawing) of the longitudinal aberration curves of the ± 1st order diffracted lights D1 and D2 is the first and second. The photodetectors 31a and 31b are separated into the right and left with the position of the boundary as a boundary.

Further, in the same manner as described above, the hologram element 36
When the convex lens power and the concave lens power of the hologram surface 36a1 are increased to reproduce the second optical disc (CD) 20, the ± 1st-order diffracted light E1, E2 has a wider condensing point interval and the + 1st-order diffracted light E1 is collected. Although the light spot moves to the left and the focal point of the −1st order diffracted light E2 moves to the right, the longitudinal aberration curves of the ± 1st order diffracted lights E1 and E2 are
It is in a state where the third photodetectors 31a and 31c are not separated into the left and right sides with the positions of the boundaries as boundaries.

Furthermore, the longitudinal aberration curve of the + 1st order diffracted light D1 by the first optical disc (DVD) 10 and the second optical disc (C
D) The longitudinal aberration curve of the −1st-order diffracted light E2 due to D does not intersect with each other, so that there is no crossover between them.

Next, as shown in FIG. 10, after using the special objective lens 38 and increasing the convex lens power and the concave lens power of the hologram surface 36a1 of the hologram element 36, the first to third photodetectors 31a ... The position of 31c is moved from the same position as the position shown in FIG. The position in this case is the same position as in the case of exclusive use for the first optical disk and the case of exclusive use of the second optical disk, while the position for this is a hologram rather than the position for exclusive use of the first optical disk and exclusive case for the second optical disk. Element 3
The position is slightly shifted to the 6 side (the first and second optical discs 10 and 20 side). Then, the first to third photodetectors 31a
The first optical disc (DV
D) 10 and the second optical disc (CD) 20 are selectively reproduced, the ± first-order diffracted light D as in the case of FIG.
Part of the longitudinal aberration curves of 1 and D2 (lower end in the figure) are separated into left and right with the positions of the first and second photodetectors 31a and 31b as boundaries, and further, here, ± first-order diffracted lights E1 and E2. Part of the longitudinal aberration curve (lower end in the figure) of the first and third photodetectors 3
With the positions of 1a and 31c as boundaries, they are separated into left and right.

Next, as shown in FIG. 11, after operating the high-order coefficient (= aberration term coefficient) of the hologram curve polynomial of the hologram element 36 from the state of FIG. 10, the first optical disk 10 and the second optical disk 20 are changed. When reproduced, the longitudinal aberration curves of the ± first-order diffracted lights D1 and D2 show the first and second photodetectors 31.
It is completely separated to the left and right with the positions of a and 31b as boundaries.
In addition, the longitudinal aberration curves of the ± first-order diffracted lights E1 and E2 are also completely separated right and left with the positions of the first and third photodetectors 31a and 31c as boundaries.

Specifically, the hologram element 36 described above is used.
The polynomial expressing the lattice curve shape of the hologram surface 36a1 formed in a concave-convex shape is functionalized based on the x-axis and the y-axis which are orthogonal to each other on the hologram surface 36a1 of the hologram element 36, and f (x, y ) = Ax + Bx ² + Cx
y + Dy ² + Ex ³ + Fx ² y + Gxy ² + Hy ³ + I
When calculating the lattice curve shape of the hologram surface 36a1 of the hologram element 36 by using x ⁴ + ... ...... = nλ (where n is a natural number and λ is the wavelength of the laser light), the term Ax represents the lattice shape. Acting on bending, the term Bx ² acts on convex lens power and concave lens power, and Cxy + Dy ² + Ex ³ +
Each higher order term of Fx ² y + Gxy ² + Hy ³ + Ix ⁴ + ... Becomes an aberration term that acts on aberration correction. In this embodiment, the higher order coefficient of the hologram curve polynomial of the hologram element 36 (= aberration term coefficient). ) Is optimized.

As described above, the convex lens power and the concave lens power of the hologram element 10 are increased, and
As a result of moving the positions of the third detectors 31a to 31c and optimizing the higher-order term coefficient (= aberration term coefficient) of the hologram curve polynomial of the hologram element 36, the first and the first objective lenses 38 are used. When the two optical discs 10 and 20 are selectively reproduced, the diffraction characteristics shown in FIGS. 12A and 12B are obtained for the first optical disc (DVD) 10, and the second optical disc (CD) 20 is obtained. For Figure 13
The diffraction characteristics shown in (a) and (b) were obtained.

The ± first-order diffracted light D shown in FIG.
In the longitudinal aberration curves of D1 and D2, the vertical axis shows the scale equally divided in the state where NA = 0.6 is set, and in the longitudinal aberration curves of the ± first-order diffracted lights E1 and E2 shown in FIG. ,
The vertical axis shows the scale equally divided with NA = 0.45. Further, the longitudinal aberration curves of the ± first-order diffracted lights D1 and D2 shown in FIG. 12A and the ± shown in FIG.
In the longitudinal aberration curves of the first-order diffracted lights E1 and E2, the parts that protrude at a distance substantially parallel to the horizontal axis are the special objective lens 38.
Each edge of the concave groove surface of the second ring zone surface Sd2 (FIGS. 1 and 3) of FIG.

As is clear from FIGS. 12A and 12B, when the first optical disc (DVD) 10 is reproduced by using one special objective lens 38, the return light from the first optical disc 10 is returned. Indicates the amount of deviation along the optical axis of the special objective lens 38 when diffracted by the hologram element 38+
The longitudinal aberration curve of the first-order diffracted light D1 and the longitudinal aberration curve of the -1st-order diffracted light D2 are completely separated into left and right with the positions of the first and second photodetectors 31a and 31b formed on the silicon substrate 31 as boundaries. As a result, ± 1st order diffracted light D1, D
2 is in an ideal state in which light is condensed at each one of the point positions D1a and D2a, so that the first optical disc (DV
D) The focus error signal from 10 can be reliably detected by the first and second photodetectors 31a and 31b.

As is apparent from FIGS. 13A and 13B, when the second optical disc (CD) 20 is reproduced by using one special objective lens 38, the second optical disc 20 is reproduced.
When the return light from is diffracted by the hologram element 38,
The longitudinal aberration curve of the + 1st-order diffracted light E1 and the longitudinal aberration curve of the −1st-order diffracted light E2, which show the amount of deviation of the special objective lens 38 along the optical axis, are the first and third photodetectors 31a formed on the silicon substrate 31, Since the light is completely separated to the left and right with the position of 31c as the boundary, the light collection performance is slightly worse than that of the first optical disc 10, but the ± first-order diffracted light E
1 and E2 are in a state of substantially condensing at each one of the point positions E1a and E2a, the second optical disk (C
D) The focus error signal from 20 can be reliably detected by the first and third photodetectors 31a and 31c.

The hologram element 38 formed as described above can be handled as one of the components of the optical pickup.

[0082]

In the optical pickup and the hologram element according to the present invention described in detail above, according to the optical pickup of claim 1, the return light from the first and second optical disks is passed through the special objective lens to the hologram element. The ± 1st-order diffracted light obtained by diffracting on the hologram surface is received by one and the other photodetectors provided corresponding to the first and second optical discs in a spot shape, and SSD (Spot Size Detectio
When detecting the focus error signal by the n) method, especially when the convex lens power and the concave lens power of the hologram surface of the hologram element are dedicated to the first optical disc and the second optical disc.
Larger than in the case of exclusive use for the optical disc, the focal point of the + 1st-order diffracted light and the focal point of the -1st-order diffracted light which are diffracted on the hologram surface of the hologram element in response to the respective return lights from the first and second optical discs. With the gap between the points widened,
When the positions of the one and the other photodetectors are shifted toward the hologram element side more than when the first optical disk is dedicated and the second optical disk is dedicated, and when calculating the lattice curve shape of the hologram surface of the hologram element using a polynomial By optimizing the coefficient of the term that acts on the aberration correction in this polynomial, the longitudinal aberration curve of the + 1st-order diffracted light and the longitudinal aberration curve of the -1st-order diffracted light showing the amount of deviation along the optical axis of the special objective lens are obtained. To
Since the positions of the one and the other photodetectors are separated at the boundary, even if a special objective lens is used, the respective returning lights from the first and second optical discs are diffracted by the hologram element to obtain ± 1st-order diffracted lights. Since the light is focused substantially at one point, each focus error signal from the first and second optical disks can be reliably detected by each set of photodetectors.

According to the hologram element of the second aspect, the first optical disc having a thin disc substrate and the second optical disc having a thick disc substrate are selectively reproduced, and the first or second optical disc is used. Each of the return lights of 1) is passed through a special objective lens having a plurality of ring-shaped surfaces and then diffracted by the hologram surface to obtain SS
When the focus error signal is detected by the D (Spot Size Detection) method, the convex lens power and the concave lens power of the hologram surface of the hologram element are made larger than those for the first optical disk and for the second optical disk, and the hologram surface is When the lattice curve shape of is calculated using a polynomial, the coefficient of the term that acts on aberration correction in this polynomial is optimized. Therefore, when this hologram element is used for an optical pickup, Each focus error signal can be reliably detected.

[Brief description of drawings]

FIG. 1 is a perspective view for explaining an optical pickup and a hologram element according to the present invention.

FIG. 2 is a conceptual diagram for explaining the first to third photodetectors shown in FIG.

FIG. 3 is a conceptual diagram for explaining the special objective lens shown in FIG.

FIG. 4A and FIG. 4B are optical path diagrams when reproducing the first and second optical discs with a single special objective lens, respectively.

FIG. 5 is a diagram for explaining a diffraction characteristic when a first optical disc is reproduced in an optical design state using an objective lens of NA 0.6, and (a) is a longitudinal aberration curve of ± first-order diffracted light. And (b) is a diagram showing a light receiving state of ± first-order diffracted light to the first and second photodetectors.

FIG. 6 is a diagram for explaining a diffraction characteristic when a second optical disc is reproduced in an optical design state using an objective lens of NA 0.45, and (a) is a longitudinal aberration curve of ± first-order diffracted light. And (b) is a diagram showing a light receiving state of ± first-order diffracted light to the first and third photodetectors.

FIG. 7 shows a first optical disc and a second optical disc using a special objective lens.
It is a figure for demonstrating the diffraction characteristic at the time of reproducing an optical disk, (a) shows the longitudinal-aberration curve of each ± 1st-order diffracted light, (b) shows each (1st, 2nd) of ± 1st-order diffracted light. , (1st, 3rd) It is the figure which showed the light reception state to a photodetector.

FIG. 8 is a conceptual diagram for explaining a case where the convex lens power and the concave lens power on the hologram surface of the hologram element are increased in the optical pickup according to the present invention.

FIG. 9 is a longitudinal aberration of each ± first-order diffracted light when an optical pickup according to the present invention uses a special objective lens and reproduces the first optical disc and the second optical disc by increasing the lens power of the hologram element. It is the figure which showed the curve.

FIG. 10 is a diagram showing an optical pickup according to the present invention, in which a special objective lens is used, and after the lens power of a hologram element is increased, the positions of the first to third photodetectors are moved to move the first optical disc and the first optical disc. It is a figure showing a longitudinal aberration curve of each ± 1st-order diffracted light when two optical disks are reproduced.

FIG. 11 shows an optical pickup according to the present invention.
The longitudinal aberration curves of the ± 1st-order diffracted lights are shown when the first optical disc and the second optical disc are reproduced after the high-order coefficient (= aberration term coefficient) of the hologram curve polynomial of the hologram element is manipulated from the state of 0. It is a figure.

FIG. 12 shows a first example of the optical pickup according to the present invention.
It is a figure showing a diffraction characteristic when reproducing an optical disc (DVD), (a) shows a longitudinal aberration curve of ± 1st-order diffracted light,
FIG. 6B is a diagram showing a light receiving state of ± first-order diffracted light to the first and second photodetectors.

FIG. 13 shows a second optical pickup according to the present invention.
It is a figure showing a diffraction characteristic when reproducing an optical disc (CD), (a) shows a longitudinal aberration curve of ± 1st-order diffracted light,
FIG. 6B is a diagram showing a light receiving state of ± first-order diffracted light to the first and third photodetectors.

14A and 14B are first optical discs (DV).
FIG. 6D is a diagram schematically showing a case where the second optical disc (CD) is reproduced by the optical disc device.

15 (a) and 15 (b) are diagrams for explaining a special objective lens, and FIG. 15 (a) is a first diagram showing a thin disc substrate.
The optical path diagram when reproducing an optical disc (DVD) is shown,
(B) shows a second optical disc (C
It is the figure which showed the optical path diagram at the time of reproducing D).

[Explanation of symbols]

10 ... 1st optical disk (DVD), 20 ... 2nd optical disk (CD), 30 ... Optical pickup, 31 ... Silicon substrate, 31a-31c ... 1st photodetector-
Third photodetector, 32 ... Mount base, 33 ... First semiconductor laser, 34 ... Second semiconductor laser, 35 ... Stand-up mirror, 36 ... Hologram element, 36a ... Top surface, 36a1 ... Hologram surface, 36b ... Bottom surface, 37 ... Collimator lens, 38 ... Special objective lens, Sd1-Sd3 ... First ring zone surface-
Third ring zone surface, X2 ... Interval between condensing points of ± 1st order diffracted light.

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 5D118 AA13 AA26 BA01 CD02 CG03                       CG07 CG26 DA20                 5D119 AA28 AA41 BA01 EC45 EC47                       FA08 JA14 JA44                 5D789 AA28 AA41 BA01 EC45 EC47                       FA08 JA14 JA44

Claims (2)

[Claims] 1. A first semiconductor laser for emitting a first laser beam of a short wavelength corresponding to a first optical disc having a thin disc substrate, and a second wavelength of a long wavelength corresponding to a second optical disc having a thick disc substrate. A second semiconductor laser that emits a laser beam and a first or second laser beam that is selectively emitted from the first or second semiconductor laser, respectively, and returns light from the first or second optical disc. And a hologram element having a hologram surface for diffracting light, and a plurality of orbicular zones formed so as to obtain respective numerical apertures (NA) corresponding to the first and second laser beams, and passed through the hologram element. A special objective lens for selectively irradiating the first or second optical disc with a first or second laser beam obtained by focusing the first or second laser beam; In order to detect the ± first-order diffracted light obtained by diffracting the return light from the optical disc on the hologram surface of the hologram element, each optical disc is divided into one set and the other set, and two sets are provided. A photodetector, and the ±± obtained by reproducing the first or second optical disc.
Of the first-order diffracted light, the + 1st-order diffracted light is received by the one photodetector in a spot shape by the convex lens power on the hologram surface of the hologram element, and the -1st-order diffracted light is received by the concave lens power on the hologram surface of the hologram element. The other photodetector is made to receive light in a spot shape, and SS
In an optical pickup configured to detect a focus error signal by the output from each of the one and the other photodetectors by the D (Spot Size Detection) method, the convex lens power and the concave lens power of the hologram surface of the hologram element are set to the first lens power. The + 1st-order diffracted light diffracted by the hologram surface of the hologram element corresponding to each return light from the first or second optical disc is made larger than that for the optical disc and for the second optical disc. In the state where the distance between the condensing point and the condensing point of the -1st-order diffracted light is widened, the positions of the photodetectors on the one side and the other side of When shifting to the hologram element side than the case and calculating the lattice curve shape of the hologram surface of the hologram element using a polynomial By optimizing the coefficient of the term that acts on aberration correction in this polynomial,
The + indicating the amount of deviation along the optical axis of the special objective lens
An optical pickup, wherein the longitudinal aberration curve of the first-order diffracted light and the longitudinal aberration curve of the -1st-order diffracted light are separated with the positions of the one and the other photodetectors as boundaries. 2. A first optical disc having a thin disc substrate and a second optical disc having a thick disc substrate are selectively reproduced, and each return light from the first or second optical disc is formed into a plurality of ring zones. A hologram element used for detecting a focus error signal by an SSD (Spot Size Detection) method with ± 1st-order diffracted light obtained by diffracting on a hologram surface after passing through a special objective lens having a surface The convex lens power and the concave lens power of the hologram surface of the element are made larger than those for the first optical disk and those for the second optical disk, and when calculating the lattice curve shape of the hologram surface using a polynomial 2. A hologram element characterized by optimizing the coefficient of a term that acts on aberration correction.