JP2006031824A - Optical pickup device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a high-performance aberration correction element in an optical pickup device capable of selectively recording or reproducing Blu-ray (R) Disk and DVD. <P>SOLUTION: When recording or reproduction is selectively executed in a first optical recording medium 1 or a second optical recording medium 2 different in substrate thickness by using a laser beam L1 and a second laser beam L2 different in wavelength, especially, when a diffraction type optical element 30 for correcting a spherical aberration generated due to the difference in substrate thickness between the first and second optical recording media 1 and 2, and an objective lens 31 designed corresponding to the first optical recording medium 1 are housed in a lens holder 29, a diffraction pattern 30a formed in the inner peripheral area of a diffraction type optical element 30 is designed to satisfy sine wave conditions with respect to the second laser beam L2, and a space k is set between the diffraction type optical element 30 and the objective lens 31 so as to reduce image height characteristics tilted by a predetermined amount from the optical axis of the objective lens 31 to the second laser beam L2. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  In the present invention, when the first and second optical recording media having different substrate thicknesses are selectively recorded using the first and second laser beams having different wavelengths, the numerical aperture (NA) is 0.75. An optical pickup comprising at least one objective lens as described above and a diffractive optical element for correcting spherical aberration caused by a difference in substrate thickness between the first and second optical recording media when the objective lens is used. It relates to the device.

  In general, an optical recording medium such as a disk-shaped optical disk or a card-shaped optical card has a high density on a track in which information signals such as video information, audio information, and computer data are spirally or concentrically formed on a transparent substrate. When a recorded track is recorded and a recorded track is reproduced, a desired track can be accessed at high speed.

  For example, CDs (Compact Discs) and DVDs (Digital Versatile Discs) are already on the market as optical discs of this type of optical recording media. A Blu-ray Disc capable of recording or reproducing information signals at a higher density than CD and DVD has been actively developed.

  First, the above-mentioned CD is irradiated with a laser beam obtained by narrowing a laser beam having a wavelength of around 780 nm with an objective lens having a numerical aperture (NA) of about 0.45, and is separated from the laser beam incident surface by about 1.2 mm. An information signal is recorded or reproduced on a signal surface at a certain position.

  The above-described DVD is irradiated with a laser beam obtained by narrowing a laser beam having a wavelength of around 650 nm with an objective lens having a numerical aperture (NA) of about 0.6, and is separated by about 0.6 mm from the laser beam incident surface. An information signal is recorded or reproduced on a signal surface at a certain position. At this time, the recording capacity of the DVD is 6 to 8 times higher than that of the CD, and when the diameter of the disk substrate is 12 cm, it is about 4.7 GB (gigabyte) on one side.

  Also, the above-described Blu-ray Disc is irradiated with a laser beam obtained by narrowing a laser beam having a wavelength of 450 nm or less with an objective lens having a numerical aperture (NA) of 0.75 or more, and is approximately 0 from the laser beam incident surface. Development is progressing so that an information signal can be recorded or reproduced on a signal surface at a position 1 mm apart. At this time, the recording capacity of the Blu-ray Disc is around 25 GB (gigabyte) on one side when the diameter of the disk substrate is 12 cm.

By the way, with a composite objective lens composed of a diffraction hologram and an objective lens, a Blu-ray Disc having a substrate thickness of about 0.1 mm corresponding to a blue light beam and a substrate thickness of about 0.6 mm corresponding to a red light beam are used. There is an optical head device capable of recording and reproducing compatible with a DVD (for example, see Patent Document 1).
JP 2004-71134 A.

  20A to 20C are cross-sectional views respectively showing specific examples of a composite objective lens composed of a diffraction hologram and an objective lens in a conventional optical head device.

  Compound objective lenses 100A to 100C in the conventional optical head device shown in FIGS. 20A to 20C are disclosed in the above-mentioned Patent Document 1 (Japanese Patent Laid-Open No. 2004-71134), and here Then, it demonstrates simply with reference to patent document 1. FIG.

  As shown in FIGS. 20A to 20C, the composite objective lenses 100A to 100C in the conventional optical head device have a substrate thickness of about 0.1 mm (0.06 mm to 0.11 mm). The disc 1 and the DVD 2 having a substrate thickness of about 0.6 mm (0.54 mm to 0.65 mm) can be selectively recorded and reproduced.

  In the above-described compound objective lens 100A (or 100B or 100C), the hologram 102A (or 102B or 102C) is attached as a diffractive optical element to the lower part in the lens holder 101, and the upper part in the lens holder 101 is attached. An objective lens 103 is attached as a refractive lens.

  At this time, the hologram 102A (or 102B or 102C) has a plurality of concave and convex (or stepped or sawtooth) diffraction gratings formed in a ring shape, and a blue light beam for recording and reproducing the Blu-ray Disc 1 104 transmits a large amount of light without being diffracted, and on the other hand, the red light beam 105 for recording / reproducing the DVD 2 is diffracted in the inner peripheral region, so that the substrate thickness of the Blu-ray Disc 1 and the DVD 2 can be increased. The spherical aberration that occurs due to the difference is corrected.

  By the way, in the composite objective lens 100A (or 100B or 100C) described above, if there is an error in the relative position between the diffraction hologram 102A (or 102B or 102C) and the objective lens 103, the designed wavefront is incident on the objective lens 103. In the composite objective lens 100A (or 100B or 100C) described above, the diffractive hologram 102A (or 102B or 102C) and the objective lens 103 are deteriorated. Are integrally fixed in the lens holder 101 to improve the relative position error between them, but the diffraction hologram 102A (or 102B or 102C) and the objective lens 103 are attached to the lens. At any interval in the holder 101 Specific disclosure without for Luke, not even suggested.

  Therefore, when the diffractive optical element and the objective lens are designed for the first optical recording medium (for example, Blu-ray Disc) and the second optical recording medium (for example, DVD), There is a demand for an optical pickup device that can satisfactorily correct spherical aberration caused by a difference in substrate thickness between the first and second optical recording media by optimally setting an interval between the objective lens and the objective lens.

The present invention has been made in view of the above problems. The invention according to claim 1 is a first optical recording medium, a recording density lower than that of the first optical recording medium, and lower than that of the first optical recording medium. An optical pickup device for selectively recording or reproducing a second optical recording medium having a large substrate thickness and a combined optical recording medium in which the signal surfaces of the first and second optical recording media are combined and laminated integrally In
A first laser light source that emits a first laser beam having a wavelength of 450 nm or less corresponding to the first optical recording medium;
A second laser light source that emits a second laser beam having a wavelength longer than that of the first laser beam corresponding to the second optical recording medium;
The parallel light of the first and second laser lights is selectively incident and the first laser light is transmitted as it is, but the diffraction pattern portion formed in the inner peripheral region with respect to the second laser light. A diffractive optical element for correcting spherical aberration caused by a difference in substrate thickness of the first and second optical recording media by emitting diffracted diffracted light;
For the first optical recording medium, the numerical aperture (NA) is set to 0.75 or more, and at least one of the first and second surfaces facing each other is formed as an aspheric surface, and the first laser Provided above the diffractive optical element while satisfying a sine condition with respect to the light, and selectively collect the first and second laser beams on each signal surface of the first and second optical recording media. And at least an objective lens that emits light,
When the diffractive optical element is stored in a lower part of the lens holder and the objective lens is stored in an upper part of the lens holder, the diffraction pattern of the diffractive optical element with respect to the second laser light The diffractive optical element between the objective lens and the objective lens so that an image height characteristic to the second laser light tilted by a predetermined amount from the optical axis of the objective lens is reduced. The optical pickup device is characterized in that an interval is set.

According to a second aspect of the present invention, in the optical pickup device according to the first aspect,
An optical pickup device characterized in that a value of wavefront aberration when recording or reproducing the second optical recording medium with the second laser light tilted by a predetermined amount from the optical axis of the objective lens is equal to or less than a Marechal criterion. .

  According to the optical pickup device of the present invention, the first optical recording medium (for example, Blu-ray Disc) and the second optical recording medium (for example, Blu-ray Disc) having different substrate thicknesses using the first laser light and the second laser light having different wavelengths. For example, a diffractive optical element for correcting spherical aberration caused by a difference in substrate thickness between the first and second optical recording media, and the first optical recording medium. When the objective lens designed for (for example, Blu-ray Disc) is housed in the lens holder, the diffraction pattern portion formed in the inner peripheral region of the diffractive optical element with respect to the second laser light does not satisfy the sine condition The distance between the diffractive optical element and the objective lens is set so that the image height characteristic to the second laser beam tilted by a predetermined amount from the optical axis of the objective lens becomes small. As a result, the first and second optical recording media can be recorded or reproduced satisfactorily without generating spherical aberration. In addition, since the first and second laser beams are incident on the diffractive optical element in the state of parallel light, the optical axes of the first and second laser beams are slightly shifted from the optical axis of the objective lens. However, the deterioration of the spherical aberration is reduced and the optical axis can be easily adjusted when the optical pickup device is assembled.

  An embodiment of an optical pickup device according to the present invention will be described below in detail with reference to FIGS.

  FIG. 1 is a diagram showing the overall configuration of an optical pickup device according to the present invention.

  As shown in FIG. 1, an optical pickup device 20 according to the present invention provided movably in an optical disk drive device 10 has an information signal transmitted by a first laser beam L1 having a wavelength λ1 of 450 nm or less. An information signal is generated by a first optical recording medium (for example, Blu-ray Disc) 1 that records or reproduces ultra-high density on 1b and a second laser beam L2 having a wavelength λ2 longer than the wavelength λ1 of the first laser beam L1 and around 650 nm. Either the second optical recording medium (for example, DVD) 2 for recording or reproducing at a high density on the signal surface 2b whose substrate thickness is thicker than the signal surface 1b described above, and any of the first and second laser beams L1 and L2 are incident. A combination type optical recording medium in which the laser beam incident surfaces to be used are shared and the signal surfaces 1b and 2b of the first and second optical recording media 1 and 2 are combined and laminated integrally is selectively applied. It was developed to function.

  Although not shown here, the combined optical recording medium in which the signal surfaces 1b and 2b of the first and second optical recording media 1 and 2 are combined has a total disk substrate thickness of approximately 1.2 mm. Although formed, in the following description, each of the Blu-ray Disc 1 and DVD 2 will be described in detail, and in the case of a combination type optical recording medium, the description thereof will be omitted because it is an application thereof.

  In the following description, a case where the first and second optical recording media 1 and 2 are applied to disk-shaped optical disks will be described. However, the present invention is not limited to this, and a card-shaped optical recording medium may be used. .

  The first and second optical recording media 1 and 2 are selectively mounted on the turntable 12 fixed to the shaft of the spindle motor 11 that is rotatably provided in the optical disk drive 10. ing.

  Here, the above-described Blu-ray Disc 1 serving as the first optical recording medium has a disc substrate thickness t1 between the laser beam incident surface 1a and the signal surface 1b of approximately 0.05 mm to 0 based on the next-generation optical disc standard. The reinforcing plate 1c is bonded to the reinforcing plate 1c so that the total thickness is increased. The total thickness is approximately 1.2 mm, for example. In the following description, the first optical recording medium is described as “Blu-ray Disc 1”.

  Also, in the DVD 2 as the second optical recording medium described above, the disc substrate thickness t2 between the laser beam incident surface 2a and the signal surface 2b is thicker than the Blu-ray Disc 1 and is approximately 0.6 mm based on the DVD standard. The total thickness is set to about 1.2 mm by bonding the reinforcing plate 2c thereon. In the following description, the second optical recording medium is described as DVD2.

  At this time, in the embodiment, it is assumed that the disc substrate thicknesses t1 and t2 of the Blu-ray Disc 1 and the DVD 2 are set to 0.1 mm and 0.6 mm, for example.

  An optical pickup device 20 according to the present invention is provided below the laser beam incident surface 1a of the Blu-ray Disc 1 or the laser beam incident surface 2a of the DVD 2 so as to be movable in the radial direction of the Blu-ray Disc 1 or DVD2. Yes.

  In the optical pickup device 20 described above, a first laser light source (hereinafter referred to as a blue semiconductor laser) 22 that emits a first laser light L1 having a wavelength of 450 nm or less corresponding to the Blu-ray Disc 1 in the pickup housing 21; A second laser light source (hereinafter referred to as a red semiconductor laser) 23 that emits a second laser light L2 having a wavelength of around 650 nm is provided corresponding to DVD2.

  At this time, in the embodiment, the reference wavelength λ1 of the first laser light L1 emitted from the blue semiconductor laser 22 is set to 405 nm, for example, while the reference wavelength λ2 of the second laser light L2 emitted from the red semiconductor laser 23 is set. Is set to 660 nm, for example.

  First, the blue semiconductor laser 22 side will be described in correspondence with the Blu-ray Disc 1. The first laser light L1 having a wavelength λ1 = 405 nm emitted from the blue semiconductor laser 22 is a divergent light of linearly polarized light (p-polarized light). The divergent light is converted into parallel light by the collimator lens 24, and the parallel light of the first laser light L1 is reflected by the polarization-selective dielectric multilayer film 25a (p-polarized light: reflected, s-polarized light: transmitted) of the polarizing beam splitter 25 to be 90 °. Then, the first laser beam L1 passes through the phase plate 26 and becomes circularly polarized light. At this time, the phase plate 26 for the first laser beam gives a phase difference of (λ1) / 4 when the first laser beam L1 having the wavelength λ1 = 405 nm is transmitted.

  Further, the first laser light L 1 that has passed through the phase plate 26 passes through the dichroic film 27 a of the dichroic prism 27. At this time, the dichroic film 27a of the dichroic prism 27 transmits the first laser light L1 having the wavelength λ1 = 405 nm emitted from the blue semiconductor laser 22 and the first light having the wavelength λ2 = 660 nm emitted from the red semiconductor laser 23. Two films are formed so as to reflect the laser beam L2.

  Further, the first laser light L1 transmitted through the dichroic prism 27 is turned 90 ° by the rising plane mirror 28, and then the parallel light of the first laser light L1 is converted into a lower part in the lens holder 29. After entering the diffractive optical element 30 in one example or the diffractive optical element 30 ′ in another example and transmitting the 0th-order light as it is without being diffracted by the diffractive optical element 30 (or 30 ′). Further, the first laser beam obtained by making it incident on the objective lens 31 housed in the upper part of the lens holder 29 and narrowing down the first laser light L1 by the objective lens 31 is the laser beam incident surface 1a of the Blu-ray Disc 1. And is condensed on the signal surface 1b.

  At this time, one example of the diffractive optical element 30 or another example of the diffractive optical element 30 ′ housed in the lower part of the lens holder 29 is different in the shape of the diffraction pattern portion formed in the inner peripheral region as will be described later. In this embodiment, one of the diffractive optical element 30 of one example and the diffractive optical element 30 'of another example is used.

  Details of the action of the diffractive optical element 30 (or 30 ') on the first laser light L1 will be described later.

  The objective lens 31 described above has a numerical aperture of 0.75 or more for Blu-ray Disc, and at least one of the first and second surfaces 31a and 31b facing each other is formed as an aspheric surface. In this embodiment, the first surface is a single lens having a numerical aperture (NA) of 0.85 and faces the diffractive optical element 30 (or 30 ′) side as will be described later. The first surface 31b and the second surface 31b facing the optical discs 1 and 2 are both formed as aspherical surfaces, and are optimized infinite conjugate with respect to the first laser light L1 having the wavelength λ1 = 405 nm. The distance between the objective lens 31 having the smallest spherical aberration with respect to the first laser beam L1 and the laser beam incident surface 1a of the Blu-ray Disc 1, that is, the working distance is about 0.5 mm.

  In addition, the diffractive optical element 30 (or 30 ′) housed in the lower part of the lens holder 29 and the objective lens 31 housed in the upper part of the lens holder 29 are aligned with the optical axis in the lens holder 29. By integrating, the occurrence of coma is suppressed, and furthermore, the interval k between the diffractive optical element 30 (or 30 ′) and the objective lens 31 is set to an appropriate value as will be described later. The wavefront aberration (image height characteristic) when the light beam of the second laser beam L2 slightly tilted with respect to the optical axis of the objective lens 31 enters the diffractive optical element 30 (or 30 ′) is optimized. The diffractive optical element 30 (or 30 ′) and the objective lens 31, which are the main parts of the embodiment, will be described in detail later.

  The focus coil 32 and the tracking coil 33 are integrally attached to the outer periphery of the lens holder 29, and the lens holder 29 is attached via a plurality of suspension wires (not shown) fixed to the outer periphery of the lens holder 29. The Blu-ray Disc 1 or DVD 2 is supported so as to be swingable in the focus direction and the tracking direction.

  Then, by the focus coil 32, the tracking coil 33, and a permanent magnet (not shown), the diffractive optical element 30 (or 30 ') and the objective lens 31 are integrated with the lens holder 29 and the focus direction of the Blu-ray Disc 1 is set. The tracking direction is controlled. In the case of the DVD 2 described later, the diffractive optical element 30 (or 30 ') and the objective lens 31 are integrated with the lens holder 29 to be controlled in the focus direction and tracking direction of the DVD 2.

  Thereafter, reproduction, recording, or erasure of the Blu-ray Disc 1 on the signal surface 1b is performed by the first laser beam condensed by the objective lens 31.

  Thereafter, the first reflected light returned by the first laser light L1 reflected by the signal surface 1b of the Blu-ray Disc 1 is circularly polarized in the direction opposite to the forward path and is incident on the objective lens 31 again. 31 becomes parallel light, passes through the diffractive optical element 30 (or 30 ′), turns 90 ° by the plane mirror 28, passes through the dichroic film 27a of the dichroic prism 27, passes through the phase plate 26, and travels forward. Is linearly polarized light (s-polarized light) having orthogonal polarization directions. At this time, since the first reflected light transmitted through the phase plate 26 is linearly polarized light (s-polarized light) whose polarization direction is orthogonal to the forward path, the first reflected light is transmitted through the polarization-selective dielectric multilayer film 25a of the polarization beam splitter 25 and is detected. The light converges at 34 and is condensed on the first photodetector 35. A tracking error signal, a focus error signal, and a main data signal when the signal surface 1b of the Blu-ray Disc 1 is reproduced by the first photodetector 35 are detected.

  Next, the red semiconductor laser 23 side corresponding to DVD2 will be described. The second laser light L2 having a wavelength λ2 = 660 nm emitted from the red semiconductor laser 23 is a divergent light of linearly polarized light (p-polarized light). Passes through the hologram element 37 in the DVD integrated device 36 and becomes parallel light by the collimator lens 39, and this parallel light passes through the phase plate 40 for the second laser light and becomes circularly polarized light. At this time, the phase plate 40 for the second laser beam gives a phase difference of (λ2) / 4 when the second laser beam L2 having the wavelength λ2 = 660 nm is transmitted.

  The DVD integrated device 36 includes the red semiconductor laser 23, the hologram element 37 installed above the red semiconductor laser 23, and the second photodetector 38 installed on the right side of the red semiconductor laser 23. These are integrated on a semiconductor substrate (not shown).

  Further, the second laser light L2 transmitted through the phase plate 40 is reflected by the dichroic film 27a of the dichroic prism 27 to turn the 90 ° light direction, and further, the rising plane mirror 28 turns the 90 ° light direction, Thereafter, parallel light of the second laser beam L2 is incident on the diffractive optical element 30 (or 30 ′), and the numerical aperture (to the objective lens 31) in the outer peripheral region of the diffractive optical element 30 (or 30 ′) ( NA) is limited so as to be equivalent to 0.65, but after correcting the spherical aberration with the primary light diffracted in the inner peripheral region, the diffracted primary light is incident on the objective lens 31, and this first A second laser beam obtained by narrowing the two laser beams L2 with the objective lens 31 is incident from the laser beam incident surface 2a of the DVD 2 and focused on the signal surface 2b.

  Details of the action of the diffractive optical element 30 (or 30 ') on the second laser light L2 will be described later.

  Thereafter, reproduction, recording, or erasure of the DVD 2 on the signal surface 2b is performed by the second laser beam condensed by the objective lens 31.

  Thereafter, the second reflected light returned by the second laser light L2 reflected by the signal surface 2b of the DVD 2 is circularly polarized in the direction opposite to the forward path and re-enters the objective lens 31, and the second light of the primary light. The reflected light becomes convergent light by the objective lens 31, further becomes parallel light by the diffractive optical element 30 (or 30 ′), turns 90 ° light direction by the plane mirror 28, and is reflected by the dichroic film 27 a of the dichroic prism 27. Then, after turning through the 90 ° ray direction, after passing through the phase plate 40, it becomes linearly polarized light (s-polarized light) opposite to the forward path, becomes converging light by the collimator lens 39, and is diffracted by the hologram element 37, and the second photodetector. 38 is condensed. A tracking error signal, a focus error signal, and a main data signal when the signal surface 2b of the DVD 2 is reproduced by the second photodetector 38 are detected.

  At this time, the diffractive optical element 30 (or 30 ′) corrects the spherical aberration generated by the condensing optical system disposed on the optical axis between the red semiconductor laser 23 and the signal surface 2b of the DVD 2. When the spherical aberration is minimized, the distance between the objective lens 31 and the laser beam incident surface 2a of the DVD 2, that is, the working distance is about 0.27 mm.

  As described above, although it is a non-polarizing optical system on the DVD 2 side, it is linearly polarized light orthogonal to the forward path, so that there is almost no influence of the second reflected light on the red semiconductor laser 23.

  Here, an example diffractive optical element 30 or another example diffractive optical element 30 ′, which is a main part of the embodiment, and an objective lens 31 will be described in order with reference to FIGS. 2 to 6.

2A to 2C are diagrams for explaining the example diffractive optical element shown in FIG. 1, wherein FIG. 2A is a top view, FIG. 2B is a longitudinal sectional view, and FIG. Enlarged view of the circumferential uneven diffraction pattern part,
FIGS. 3A to 3C are diagrams for explaining another example of the diffractive optical element shown in FIG. 1, wherein FIG. 3A is a top view, FIG. 3B is a longitudinal sectional view, and FIG. Enlarged view of the inner periphery side stepped diffraction pattern portion and the outer periphery side uneven diffraction pattern portion,
FIG. 4 is an enlarged view showing a case where a Blu-ray Disc, DVD is recorded or reproduced using an objective lens optimized for infinite conjugation for Blu-ray Disc.
FIG. 5 is a diagram showing spherical aberration and sine conditions when the first laser beam is incident on the objective lens shown in FIG.
FIG. 6 is a diagram for explaining a sine condition in a general lens.

  First, as shown in FIGS. 2A and 2B, the diffractive optical element 30 as an example in the embodiment uses BK7 (borosilicate crown glass) having a thickness of 0.925 mm and having optical transparency. The outer shape is a square of 5 mm square, and the inner peripheral side irregularity is formed in the inner peripheral region with a diameter of 2.96 mm centered on the center “O” on the upper surface 30 a side facing the objective lens 31 (FIG. 1). The diffraction pattern portion 30a1 is formed in a concavo-convex ring shape, and the outer peripheral side flat portion 30a2 is on the upper surface within a diameter φ2.96 mm to 3.8 mm in the outer peripheral region adjacent to the outer periphery of the inner peripheral concavo-convex diffraction pattern portion 30a1. It is flat at the same height as 30a. Note that the inner-side uneven diffraction grating pattern 30a1 of the diffractive optical element 30 may be on the lower surface 30b side (semiconductor laser side) instead of on the upper surface 30a side (objective lens side).

At this time, as shown in an enlarged view in FIG. 2C, the unevenness of the inner-side uneven diffraction pattern portion 30a1 formed in the inner peripheral region of the upper surface 30a of the diffractive optical element 30 is considered as an n-step staircase structure. In this case, the number of steps n is equivalent to one having two steps and the step number n-1 is set to one. The height dimension of the projections and depressions is the reference wavelength λ1 = 405 nm of the first laser beam L1 as the design wavelength. Yes. The depth d1 of the recess in the inner-side uneven diffraction pattern portion 30a1 having one step number n-1 is the first laser beam L1 having the wavelength λ1 = 405 nm emitted from the blue semiconductor laser 22 (FIG. 1). If the following equation 1 is used so that no diffractive action occurs with respect to the 0th-order light, the 0th-order light of the first laser light L1 is transmitted without being diffracted at all. The depth d1 of the recess in the diffractive pattern portion 30a1 is a phase difference 2π (an optical path difference equivalent to the wavelength λ1 of the first laser beam L1).

If k1 is the periodic coefficient (natural number) of the first laser beam L1 with respect to the 0th-order light in the above equation 1, the periodic coefficient k1 with respect to the 0th-order beam of the first laser beam L1 is k1 = 1 and the wavelength λ1 = When the refractive index N1 of the diffractive optical element 30 with respect to the first laser beam L1 of 405 nm is N1 = 1.5302, the concave portion in the inner circumferential uneven diffraction pattern portion 30a1 with respect to the 0th order light of the first laser beam L1 The depth d1 is expressed by the following equation 1 and d1 = 0.405 / (1.5302-1) μm = 0.664 μm. Thereby, the depth d1 = 0.664 μm of the recess in the inner-side uneven diffraction pattern portion 30a1 is a value that can obtain an optical path difference equivalent to the wavelength λ1 of the first laser light L1, and therefore the inner-side unevenness The depth d1 of the recess in the diffractive pattern portion 30a1 is set so as to have an optical path difference equal to the wavelength λ1 of the first laser light L1.

On the other hand, the radial dimension of the unevenness of the inner-side uneven diffraction pattern portion 30a1 formed in the inner peripheral region of the upper surface 30a of the diffractive optical element 30 has the reference wavelength λ2 = 660 nm of the second laser light L2 as the design wavelength. . In this case, the inner circumferential concave-convex diffraction pattern portion 30a1 of the diffractive optical element 30 emits parallel light of the second laser beam L2 having a wavelength λ2 = 660 nm emitted from the red semiconductor laser 23 (FIG. 1) as will be described later. When the primary light diffracted and emitted is irradiated onto the signal surface 2b of the DVD 2 (FIG. 1) via the objective lens 31 (FIG. 1), the spot of the second laser light L2 on the signal surface 2b is The numerical aperture (NA) is equivalent to 0.65, and the inner circumferential side irregularities are obtained by the phase difference function Φ (x) shown in the following formula 2 so that the spherical aberration of the primary light of the second laser beam L2 is minimized. The phase difference at the radial distance x from the center “O” of the diffractive pattern portion 30a1 is obtained, and the uneven shape in the radial direction is determined by binarizing the phase difference.

Table 1 below shows an example of the phase difference function coefficients A _{2 to} A _{8 in} the phase difference function Φ (x) with respect to the inner circumferential concave-convex pattern portion 30a1 of the diffractive optical element 30 in the above formula 2.

The values of the phase difference function coefficients A _{2 to} A ₈ shown in Table 1 are described later when the distance k (FIG. 1) between the diffractive optical element 30 and the objective lens 31 is set to 2 mm. This is optimized so that the image height characteristic is minimized with respect to the second laser light L2.

At this time, the phase difference function coefficient A ₂ term in the phase difference function Φ (x) is to correct spherical aberration by finite correction, and the value of this phase difference function coefficient A ₂ term is set to A ₂ > 0. Then, the second laser beam L2 is diffracted by the inner circumferential concave-convex diffraction pattern portion 30a1 of the diffractive optical element 30, and is emitted from the substantially parallel light to the objective lens 31 side as diffused light, while the phase difference function coefficient A ₂ term Is set to A ₂ ≦ 0, the second laser beam L2 is diffracted by the inner-side uneven diffraction pattern portion 30a1 of the diffractive optical element 30 and emitted from the substantially parallel light to the objective lens 31 side as convergent light. Therefore, in this embodiment, the value of the phase difference function coefficient A ₂ term is set to A ₂ > 0 in order to design the inner-side uneven diffraction pattern portion 30a1 unsatisfactory with respect to the second laser light L2, as will be described later. Is set. Further, the term of the phase difference function coefficient A ₄ or less in the phase difference function Φ (x) is for correcting spherical aberration by wavefront correction.

  Returning to FIG. 2 (b), on the lower surface 30b of the diffractive optical element 30, the light-transmitting flat portion 30b1 has a diameter centered on the center “O” so as to be opposed to the above-described inner-side uneven diffraction pattern portion 30a1. The second laser beam L2 is formed in a circular shape in the inner peripheral area within φ2.96 mm, and is adjacent to the outer periphery of the light-transmitting flat portion 30b1 in the outer peripheral region having a diameter of φ2.96 mm and a diameter of φ3.8 mm. On the other hand, a second laser light aperture limiting portion 30b2 for limiting the numerical aperture to the objective lens 31 to be equivalent to 0.65 is formed in a ring shape using a dichroic film.

  At this time, among the diffractive optical elements 30, the numerical aperture to the objective lens 31 is set to be equivalent to 0.85 for the first laser light L1 that passes through the outer peripheral region having a diameter of 3.8 mm or less. However, as will be described later, the diameter of the objective lens 31 is φ3.74 mm, and it is considered that the center deviation between the diffractive optical element 30 and the objective lens 31 has a tolerance of ± 3 μm at the maximum.

  Note that the second laser beam opening restricting portion 30b2 does not have to form the dichroic film or the concave-convex diffraction pattern portion, and the outer peripheral flat portion 30a2 is flat with respect to the second laser light L2. Is transmitted as it is, the aberration at the outer flat portion 30a2 of the diffractive optical element 30 is large with respect to the second laser light L2, and the wavefronts of the inner and outer peripheral regions change discontinuously, and the wavefront is continuous. Therefore, the peripheral light does not contribute to spot formation on the signal surface 2b of the DVD 2. In the following description, it is assumed that a dichroic film is formed on the second laser light aperture limiting portion 30b2.

  Next, as shown in FIGS. 3A and 3B, another example of the diffractive optical element 30 ′ in the embodiment is made of BK7 (borosilicate crown glass) having a light transmittance of 0.925 mm in thickness. The upper surface facing the objective lens 31 (FIG. 1) is the same as the diffractive optical element 30 of the example described above with reference to FIG. 2 in that the outer shape is formed into a 5 mm square. On the 30a side, an inner peripheral stepwise diffraction pattern portion 30a3 is formed in a step ring shape in an inner peripheral region having a diameter φ of 2.96 mm centered on the center “O”, and this inner peripheral stepwise diffraction pattern portion. In contrast to the diffractive optical element 30 as an example, the outer peripheral side uneven diffraction pattern portion 30a4 is formed in an uneven ring shape at an equal pitch within a diameter φ2.96 mm to φ3.8 mm in an outer peripheral region adjacent to the outer periphery of 30a3. Different That. Incidentally, the inner peripheral side stepped diffraction grating pattern 30a3 and the outer peripheral side uneven diffraction pattern portion 30a4 of the diffractive optical element 30 ′ are not on the upper surface 30a side (objective lens side) but on the lower surface 30b side (semiconductor laser side). Also good.

  At this time, as shown in an enlarged view in FIG. 3C, the inner circumferential stepped diffraction pattern portion 30a3 formed in the inner circumferential region of the upper surface 30a of the diffractive optical element 30 ′ is microscopic (microscopic). As shown in FIG. 5, since the number of steps n of the staircase structure is five, the number of steps n-1 is four. On the other hand, when viewed macroscopically (macroscopically), it becomes a multistage blaze (sawtooth) shape. The height dimension of the staircase is designed so that the reference wavelength λ1 = 405 nm of the first laser beam L1 is the design wavelength. Then, in determining the depth of the entire stepped concave portion in the inner circumferential stepwise diffraction pattern portion 30a3 having a five-step structure and having four steps, based on the number of steps n of the inner circumferential stepwise diffraction pattern portion 30a3. In general, it can be expressed by n-value and expressed by the following formula 3, but the number of stages n may be an integer of 3 or more.

That is, in the stepped diffraction pattern portion on the inner circumference side where the number of steps of the staircase structure is n and the number of steps is n−1, the entire stepped recess in the stepped diffraction pattern portion for the 0th order light of the first laser beam L1. D (n-1) can be obtained from the following equation (3).

  When the depth of the entire step-like concave portion in the inner-side staircase diffraction pattern portion 30a3 having a five-step structure and four steps is expressed as d (5-1) = d4, a stepped shape per step The depth of the concave portion is (d4) / 4, and the depth (d4) / 4 of the stepped concave portion per step is an optical path difference equivalent to twice the wavelength λ1 of the first laser light L1 (substantially with a phase difference of 2π). 2).

  On the other hand, the radial dimension of the step of the inner stepped diffraction pattern portion 30a3 formed in the inner peripheral region of the upper surface 30a of the diffractive optical element 30 ′ is the reference wavelength λ2 = 660 nm of the second laser beam L2 as the design wavelength. Yes. In this case, the radial direction of the staircase of the inner circumferential stepwise diffraction pattern portion 30a3 is also applied from the center “O” of the inner circumferential stepwise diffraction pattern portion 30a3 by applying Equation 2 and Table 1 described above. A phase difference at a distance x in the radial direction is obtained, and a step shape in the radial direction is determined by converting the phase difference into five values.

  Further, the depth of the irregularities of the outer circumferential concave / convex diffraction pattern portion 30a4 formed in the outer circumferential region of the upper surface 30a of the diffractive optical element 30 'is set to the design wavelength of the reference wavelength λ1 = 405 nm of the first laser beam L1. The depth of the concave portion of the outer circumferential concave-convex diffraction pattern portion 30a4 is approximately twice the wavelength λ1 of the first laser beam L1 from the lowest position in the inner circumferential step-like diffraction pattern portion 30a3 having a five-step structure. The optical path difference is set to a value that can be obtained, and the height of the convex portion of the outer circumferential concave-convex diffraction pattern portion 30a4 is the position of the first laser light L1 from the lowest position in the stepped diffraction pattern portion 30a3 having a five-step structure. The optical path difference corresponding to approximately 6 times the wavelength λ1 is set to a value that can be obtained. At this time, the average phase of the outer circumferential concave / convex diffraction pattern portion 30a4 is substantially equivalent to four times the wavelength λ1 of the first laser light L1, and the entire chromatic aberration can be reduced most. It should be noted that the same effect can be obtained by using a stepped shape so that the entire outer peripheral concave-convex diffraction pattern portion 30a4 has a substantially average phase of the inner peripheral stepped diffraction grating pattern 30a3.

  Returning to FIG. 3 (b), on the lower surface 30b of the diffractive optical element 30 ′, the light-transmitting flat portion 30b1 faces the above-mentioned inner peripheral stepped diffraction pattern portion 30a3 and the center “O” is the center. A second laser beam is formed in a circular shape in an inner peripheral region having a diameter of less than 2.96 mm, and in the outer peripheral region having a diameter of not less than 2.96 mm and a diameter of not more than 3.8 mm adjacent to the outer periphery of the light-transmitting flat portion 30b1. A second laser light aperture limiting portion 30b2 that limits the numerical aperture to the objective lens 31 to be equal to 0.65 with respect to L2 is formed in a ring shape using a dichroic film. Even in the case of the diffractive optical element 30 ′, the second laser beam aperture restricting portion 30 b 2 may not be formed.

  Next, as shown in FIG. 4, the objective lens 31 which is a main part of the embodiment is optimally designed with an infinite conjugate for Blu-ray Disc, and as a glass material, for example, NBF1 (optical glass made by HOYA). The first surface 31a facing the diffractive optical element 30 (or 30 ') is formed as an aspherical surface, and the second surface 31b facing the Blu-ray Disc 1 or DVD2 is also formed as an aspherical surface. ing.

  At this time, the working distance WD1 between the second surface 31b of the objective lens 31 and the laser beam incident surface 1a of the Blu-ray Disc 1 is about 0.5 mm, and the second surface 31b of the objective lens 31 and the DVD 2 The working distance WD2 with respect to the laser beam incident surface 2a is about 0.27 mm.

  When NBF1 (HOYA optical glass) is used for the glass material of the objective lens 31, the refractive index N3 for the first laser light L1 emitted from the blue semiconductor laser 22 (FIG. 1) having the wavelength λ1 = 405 nm is 1. In addition, the refractive index N4 for the second laser light L2 emitted from the red semiconductor laser 23 (FIG. 1) and having the wavelength λ2 = 660 nm is 1.735532.

Table 2 below shows the specifications of the objective lens 31 that is optimally designed infinite conjugate so as to record or reproduce the Blu-ray Disc 1 with the first laser beam L1 having the wavelength λ1 = 405 nm.

  From Table 2, the same wavelength as the reference wavelength λ1 = 405 nm of the first laser beam L1 emitted from the blue semiconductor laser 22 (FIG. 1) is set as the design wavelength, and the objective lens 31 has a numerical aperture (NA) of 0. .85 is used.

Next, when the first surface 31a and the second surface 31b of the objective lens 31 are formed to be aspherical surfaces, the aspherical surfaces are represented by using the following polynomial expression 4.

Table 3 below shows an example of the aspheric coefficients B _{4 to} B ₁₂ for forming the first surface 31a of the objective lens 31 into an aspheric surface when the above-described polynomial of Equation ₄ is used.

Table 4 below shows an example of the aspheric coefficients B _{4 to} B ₁₀ for forming the second surface 31b of the objective lens 31 into an aspheric surface when the above-described polynomial of Equation 4 is used.

Further, when the diffractive optical element 30 of the example shown in FIG. 2 or the diffractive optical element 30 ′ of the other example shown in FIG. 3 and the objective lens 31 shown in FIG. The optical surface forming members for Blu-ray Disc 1 and DVD 2 are shown in Table 5 below.

  From Table 5, the radius of curvature at the apex of the first surface 31a of the objective lens 31 is 1.812171 mm, and the radius of curvature at the apex of the second surface 31b is −6.507584 mm. The lens thickness between the two surfaces 31a and 31b is 3.104 mm, the working distance of the objective lens 31 to the Blu-ray Disc 1 is 0.5 mm, and the working distance of the objective lens 31 to the DVD 2 is 0.269 mm. is there.

  The objective lens 31 is designed so as to satisfy a sine condition with respect to the first laser light L1 corresponding to the Blu-ray Disc 1, and in FIG. 5, spherical aberration of the objective lens 31 to the first laser light L1. And the sine condition. In FIG. 5, the vertical axis indicates the beam height normalized with respect to the numerical aperture (NA) = 0.85 to the objective lens 31 of the first laser beam L1, and the horizontal axis indicates the amount of deviation (mm). ing.

  By the way, the state satisfying the above sine condition means that, as shown in FIG. 6, a light beam having a height h from the optical axis enters the lens parallel to the optical axis and exits from the lens. When the emission angle is α, h / sin α satisfies a certain value, and when the sine condition is satisfied, the lateral magnification of each ray within the effective diameter can be regarded as constant.

  When the objective lens 31 described above satisfies the sine condition with respect to the Blu-ray Disc 1, it is designed using the 0th-order light of the first laser light L1, so that the diffractive optical element shown in FIG. 30 does not depend on the shape of the inner peripheral side uneven pattern portion 30a1 or the shape of the inner peripheral stepped pattern portion 30a3 of the diffractive optical element 30 ′ shown in FIG.

  Next, the diffractive optical element 30 of the example shown in FIG. 2 or the diffractive optical element 30 ′ of the other example shown in FIG. 3 and the objective lens 31 shown in FIG. The case where the Blu-ray Disc 1 and the DVD 2 are recorded or reproduced will be described in order with reference to FIGS.

FIG. 7 is a diagram schematically showing a case where a Blu-ray Disc is recorded or reproduced by the diffractive optical element of the example shown in FIG. 2 and the objective lens shown in FIG.
FIG. 8 is a diagram schematically showing a case where a Blu-ray Disc is recorded or reproduced by the diffractive optical element of another example shown in FIG. 3 and the objective lens shown in FIG.
FIG. 9 is a diagram schematically showing a case where a DVD is recorded or reproduced by the diffractive optical element of the example shown in FIG. 2 and the objective lens shown in FIG.
FIG. 10 is a diagram schematically showing a case where a DVD is recorded or reproduced by the diffractive optical element of another example shown in FIG. 3 and the objective lens shown in FIG.

  First, as shown in FIG. 7, when the Blu-ray Disc 1 is recorded or reproduced by the diffractive optical element 30 and the objective lens 31 stored in the lens holder 29, the second surface 31b of the objective lens 31 is used. The first laser beam having a wavelength λ1 = 405 nm emitted from the blue semiconductor laser 22 (FIG. 1) in a state where the working distance WD1 is set to about 0.5 mm between the laser beam incident surface 1a of the Blu-ray Disc 1 When L1 is collimated by the collimator lens 24 (FIG. 1) and this collimated light is incident from the lower surface 30 side of the diffractive optical element 30, the parallel light of the first laser beam L1 is circularly formed in the inner peripheral region of the lower surface 30. And a second laser light aperture formed in a ring shape using a dichroic film outside the light transmissive flat portion 30b1. After passing through the limiting portion 30b2 as it is, the parallel light of the first laser beam L1 is not diffracted by the inner-side uneven diffraction pattern portion 30a1 formed in the inner peripheral region of the upper surface 30a of the diffractive optical element 30. The 0th-order light is transmitted as it is, and the outer peripheral flat portion 30a2 formed in the outer peripheral region of the upper surface 30a is transmitted as it is, and is incident on the first surface 31a of the objective lens 31 as parallel light.

  Then, the first laser beam focused by the first and second surfaces 31a and 31b of the objective lens 31 is made incident from the laser beam incident surface 1a of the Blu-ray Disc 1 on the signal surface 1b having a disc substrate thickness of 0.1 mm. It is focused on.

  In this case, since diffraction does not occur in the inner concavo-convex diffraction pattern portion 30a1 formed on the upper surface 30a of the diffractive optical element 30 with respect to the first laser light L1 having the wavelength λ1 = 405 nm, the diffractive optical element 30 There is no light loss other than reflection and absorption at the surface (there is a diffraction loss due to a shape error, details will be described later), and as described above, the depth d1 of the recess in the inner-side uneven diffraction pattern portion 30a1 is 0. When the thickness is 764 μm, the diffraction efficiency of the 0th order light is 100%. At present, since the output of the blue semiconductor laser 22 (FIG. 1) having the wavelength λ1 = 405 nm is low, it is essential that each optical component of the optical pickup device 20 (FIG. 1) of the embodiment has a small light amount loss. Yes.

  On the other hand, as shown in FIG. 8, when the Blu-ray Disc 1 is recorded or reproduced by the diffractive optical element 30 ′ of another example housed in the lens holder 29 and the objective lens 31, the diffractive optical element 30 ′ is recorded. The parallel light of the first laser beam L1 incident from the lower surface 30b is diffracted by the inner-side stepwise diffraction pattern portion 30a3 and the outer-side uneven diffraction pattern portion 30a4 formed in the inner peripheral region and the outer peripheral region of the upper surface 30a, respectively. The light is transmitted as it is, and is incident on the first surface 31a of the objective lens 31 as parallel light.

  Next, as shown in FIG. 9, when the DVD 2 is recorded or reproduced by the diffractive optical element 30 and the objective lens 31 of an example housed in the lens holder 29, the second surface 31b of the objective lens 31 and the DVD 2 are recorded. The second laser light L2 having a wavelength λ2 = 660 nm emitted from the red semiconductor laser 23 (FIG. 1) in a state where the working distance WD2 is set to 0.269 mm with respect to the laser beam incident surface 2a of the collimator lens 39 ( 1), when the parallel light is incident from the lower surface 30b side of the diffractive optical element 30, the parallel light of the second laser light L2 is formed in a ring shape using a dichroic film in the outer peripheral region of the lower surface 30b. Although the aperture is restricted by the filmed second laser light aperture restricting portion 30b2 so that the numerical aperture (NA) to the objective lens 31 is equivalent to 0.65, the diffraction type After the light transmitting flat portion 30b1 formed in a circular shape is transmitted to the inner peripheral region of the lower surface 30b of the optical element 30, the parallel light of the second laser light L2 is further transmitted to the inner periphery of the upper surface 30a of the diffractive optical element 30. The spherical aberration is corrected by the primary light diffracted by the inner-side uneven diffraction pattern portion 30a1 formed in the region, and the primary light obtained by diffracting is incident on the first surface 31a of the objective lens 31. .

  Then, the second laser beam focused by the first and second surfaces 31a and 31b of the objective lens 31 is made incident from the laser beam incident surface 2a of the DVD 2 and condensed on the signal surface 2b having a disc substrate thickness of 0.6 mm. is doing.

In this case, since the objective lens 31 is designed for Blu-ray Disc, the spherical aberration becomes larger with respect to the second laser light L2 having the wavelength λ2 = 660 nm emitted from the red semiconductor laser 23 (FIG. 1). Finite correction (phase difference function coefficient A ₂ term) and wavefront correction (phase difference function coefficient A ₄ ) with respect to the second laser light L2 at the inner circumferential uneven diffraction pattern portion 30a1 formed on the upper surface 30a of the diffractive optical element 30. Since the spherical aberration is corrected according to the following items, recording or reproduction on the DVD 2 is not hindered.

  On the other hand, as shown in FIG. 10, when the Blu-ray Disc 1 is recorded or reproduced by the diffractive optical element 30 ′ of another example housed in the lens holder 29 and the objective lens 31, the diffractive optical element 30 ′ is recorded. The parallel light of the second laser beam L2 incident from the light transmissive flat portion 30b1 formed in the inner peripheral region of the lower surface 30b of the upper surface 30a is diffracted by the inner peripheral stepwise diffraction pattern portion 30a3 formed in the inner peripheral region of the upper surface 30a. The primary light obtained by correcting the spherical aberration with the primary light and diffracting it is incident on the first surface 31 a of the objective lens 31.

  From the above, in the optical pickup device 20 (FIG. 1) of the embodiment, the diffractive optical element 30 of the example or the first laser light L1 for Blu-ray Disc and the second laser light L2 for DVD in the state of parallel light. Since the light is incident on the diffractive optical element 30 ′ of another example, coma is deteriorated even when the optical axes of the first and second laser beams L1 and L2 are slightly shifted from the optical axis of the objective lens 31. The optical axis can be easily adjusted when the optical pickup device 20 is assembled.

  Next, the diffractive optical element 30 of the example shown in FIG. 2 or the diffractive optical element 30 ′ of the other example shown in FIG. 3 and the objective lens 31 shown in FIG. 4 are placed in the lens holder 29 (FIG. 1). The distance k (FIG. 1) between the diffractive optical element 30 (or 30 ′) and the objective lens 31 will be described with reference to FIGS.

FIG. 11 is a diagram showing wavefront aberration when the diffractive optical element and the objective lens are decentered with respect to the working distance WD2 between the objective lens and the DVD.
FIG. 12 is a diagram showing axial chromatic aberration due to the wavelength error of the second laser beam to the diffractive optical element and the objective lens with respect to the working distance WD2 between the objective lens and the DVD,
FIG. 13 is a diagram showing image height characteristics (wavefront aberration) when a light beam of the second laser beam tilted from the optical axis is incident on the diffractive optical element and the objective lens with respect to the working distance WD2 between the objective lens and the DVD. ,
FIG. 14 is a diagram showing the minimum pitch of the diffractive optical element with respect to the working distance WD2 between the objective lens and the DVD,
FIG. 15 is an aberration diagram when the distance k between the diffractive optical element and the objective lens is set to 2 mm, and the diffractive optical element is designed to satisfy the sine condition with respect to the second laser beam.
FIG. 16 is an aberration diagram when the distance k between the diffractive optical element and the objective lens is set to 2 mm, and the diffractive optical element is designed to satisfy the sine condition with respect to the second laser beam.
FIG. 17 is an aberration diagram when the distance k between the diffractive optical element and the objective lens is set to 4 mm, and the diffractive optical element is designed to satisfy the sine condition with respect to the second laser beam.
FIG. 18 is an aberration diagram when the distance k between the diffractive optical element and the objective lens is set to 4 mm, and the diffractive optical element is designed to satisfy the sine condition with respect to the second laser beam.
FIG. 19 is an aberration diagram when the interval k between the diffractive optical element and the objective lens is set to 1 mm, and the diffractive optical element is designed to satisfy the sine condition with respect to the second laser light.

  Here, the diffractive optical element 30 of the example shown in FIG. 2 or the diffractive optical element 30 ′ of the other example shown in FIG. 3 and the objective lens 31 shown in FIG. 4 are placed in the lens holder 29 (FIG. 1). In this case, the objective lens 31 is designed to satisfy the sine condition with respect to the first laser light L1 corresponding to the Blu-ray Disc 1 as described above. 2, the inner-side uneven diffraction pattern portion 30a1 of the diffractive optical element 30 shown in FIG. 2 or the inner-side stepwise diffraction pattern portion 30a3 of the diffractive optical element 30 ′ shown in FIG. The diffractive optical element 30 (or 30 ′) is designed so that the sine condition is unsatisfactory with respect to the laser beam L2 and the image height characteristic is minimized with respect to the second laser beam L2. And the objective lens 31 It was found that may be obtained the interval k (Fig. 1). Hereinafter, this will be described in order.

First, in FIG. 11, the first and second surfaces 31a and 31b (FIGS. 1 and 4) are aspherical based on Tables 3 and 4 shown in the objective lens 31 described above with reference to FIG. And the diffractive optical element 30 (or 30 ′) described above with reference to FIG. 2 (or FIG. 3). The inner circumferential uneven diffraction pattern portion 30a1 (or the inner circumferential stepwise diffraction pattern portion 30a3) changes the sine condition with respect to the second laser light L2, as will be described later, and the phase difference function coefficients A _{2 to} A _8. , The working distance WD 2 between the objective lens 31 and the DVD 2, and the distance k between the diffractive optical element 30 (or 30 ′) and the objective lens 31. And the center of the diffractive optical element 30 (or 30 ′) and the objective lens Shows the eccentricity characteristic due to positional deviation between the first center.

  Here, when the parallel light of the second laser beam L2 having the wavelength λ2 of 660 nm is incident on the lower surface 30b (FIGS. 9 and 10) of the diffractive optical element 30 (or 30 ′) perpendicularly, the diffractive optical element 30 The center of (or 30 ') and the center of the objective lens 31 are optimally designed in advance so as to coincide with each other without decentering, and the diffractive optical element 30 (or 30') is designed under this design condition. For example, in a state where an eccentricity of 5 μm is given between the center and the center of the objective lens 31, the distance k between the diffractive optical element 30 (or 30 ′) and the objective lens 31 is set to 1 mm, 2 mm, and 4 mm. As a result of obtaining the wavefront aberration (λ2 · rms) with respect to the working distance WD2 between the objective lens 31 and the DVD 2, the decentration characteristic is between the diffractive optical element 30 (or 30 ′) and the objective lens 31. Without depending on the interval k It is determined by the distance WD2 has been found.

  Next, FIG. 12 shows axial chromatic aberration with respect to the diffractive optical element 30 (or 30 ′) and the objective lens 31 designed under the same conditions as in FIG. 11. The above-mentioned axial chromatic aberration is wavefront aberration in the case of 661 nm, which is 1 nm wavelength shifted from the reference wavelength 660 nm of the second laser light L2. Also here, the axial chromatic aberration with respect to the working distance WD2 between the objective lens 31 and the DVD 2 is changed by changing the distance k between the diffractive optical element 30 (or 30 ′) and the objective lens 31 to 1 mm, 2 mm, and 4 mm, respectively. As a result of obtaining (λ2 · rms), it was found that the axial chromatic aberration does not depend on the distance k between the diffractive optical element 30 (or 30 ′) and the objective lens 31 but is determined by the working distance WD2.

  Next, FIG. 13 shows image height characteristics for the diffractive optical element 30 (or 30 ′) and the objective lens 31 designed under the same conditions as in FIG. 11. The image height characteristics described above refer to the diffractive optical element 30 (or 30 ′) and the parallel light of the second laser beam L2 to the lower surface 30b of the diffractive optical element 30 (or 30 ′) (FIGS. 9 and 10). ) Is a wavefront aberration (λ2 · rms) when the light beam of the second laser light L2 is tilted by, for example, 0.3 ° as a predetermined amount from the normal incidence. Also in this case, the distance k between the diffractive optical element 30 (or 30 ′) and the objective lens 31 is changed to 1 mm, 2 mm, and 4 mm, respectively, and the image height characteristic with respect to the working distance WD2 between the objective lens 31 and the DVD 2 is changed. As a result of obtaining the wavefront aberration (λ2 · rms), the image height characteristic changes when the distance k between the diffractive optical element 30 (or 30 ′) and the objective lens 31 changes. Therefore, it has been found that the image height characteristic can be optimized by appropriately setting the distance k with respect to an arbitrary working distance WD2. In other words, for the second laser light L2, the inner-side uneven diffraction pattern portion 30a1 (or inner-side step-like diffraction pattern portion 30a3) of the diffractive optical element 30 (or 30 ′) is designed to satisfy the sine condition. Above, the diffractive optical element 30 (or 30 ′) and the objective lens 31 are designed so that the image height characteristic of the second laser beam L2 tilted by a predetermined amount (0.3 °) from the optical axis of the objective lens 31 is reduced. It is only necessary to set an interval k between.

  The axial chromatic aberration described above is an important factor related to the success or failure of the system in the case of a system for recording on an optical disk. At this time, depending on the overall margin of the optical pickup device 20 (FIG. 1), the axial chromatic aberration with a wavelength error of 1 nm is within 0.03λ2 · rms, preferably 0.02λ2 · rms with respect to the wavelength λ2 of the second laser light L2. It is essential to keep it within rms. When determining the desired axial chromatic aberration, for example, in this embodiment, the axial chromatic aberration when the wavelength is shifted by 1 nm is set within 0.015λ2 · rms with a margin. At this time, as the working distance WD2 between the objective lens 31 and the DVD 2 is larger, the possibility that the objective lens 31 collides with the DVD 2 is less and the optical pickup device 20 (FIG. 1) can be easily assembled. The vicinity in which the axial chromatic aberration of about .015λ2 · rms is obtained is enlarged and simulated, so that the working distance WD2 is 0.269 mm.

The eccentric characteristic is also uniquely determined for this working distance WD2 = 0.269 mm. If the eccentricity characteristic is suppressed to within 0.012λ2 · rms with respect to the wavelength λ2 of the second laser beam L2, it is 0.019λ2 · rms when the eccentricity is 5 μm, and therefore 5 × 0.012 / 0.019 = 3. Position the diffractive optical element 30 (or 30 ′) and the objective lens 31 in the radial direction with an eccentricity tolerance within 15 μm. As described above, the image height characteristics are optimized by the distance k between the diffractive optical element 30 (or 30 ′) and the objective lens 31. At this time, in order to obtain the best image height characteristics, the distance k between the diffractive optical element 30 (or 30 ′) and the objective lens 31 is approximately 2 mm. That is, when the image height characteristic when the light beam of the second laser beam L2 is tilted by 0.3 ° is within 0.012λ2 · rms, the minimum image height in FIG. 13 with respect to the working distance WD2 = 0.269 mm. The characteristic is 0.009λ2 · rms, and the interval k may be approximately 2 mm. Table 1 previously set for the inner-side uneven diffraction pattern portion 30a1 (or the inner-side stepwise diffraction pattern portion 30a3) of the diffractive optical element 30 (or 30 ′) so as to satisfy the above characteristics. The phase difference function coefficients A _{2 to} A _{8 shown} in FIG.

  Further, unlike the case where the working distance WD2 = 0.269 mm described above, when the working distance WD2 between the objective lens 31 and the DVD 2 is set to 0.331 mm as will be described later with reference to FIG. The image height characteristic is best when the distance k (FIG. 1) between the optical element 30 (or 30 ′) and the objective lens 31 is 4 mm, while the working distance WD2 is 0.241 mm as will be described later. In this case, it is best to set the distance k between the diffractive optical element 30 (or 30 ') and the objective lens 31 to 1 mm. However, if the distance k is 1 mm, the distance k is too narrow, and the assemblability is improved. Difficult and undesirable.

  Further, when the image height characteristic is shifted by 0.001λ2 · rms with respect to the second laser beam L2, it is about 1% with respect to the Marechal criterion 0.07λ2 · rms (allowable aberration as the entire system of the optical pickup layer), Since there is no effect on the overall margin of the optical pickup device 20 (FIG. 1), the gap k is sufficiently allowed to deviate by about 10%. Accordingly, the interval k between the diffractive optical element 30 (or 30 ') and the objective lens 31 here is set to an optimum value including a deviation of about 10%. As a result, the value of the wavefront aberration when the DVD 2 is recorded or reproduced with the second laser light L2 tilted by a predetermined amount (0.3 °) from the optical axis of the objective lens 31 is less than or equal to the Marechal criterion.

Next, FIG. 14 shows the inner circumferential uneven diffraction grating pattern 30a1 (or inner circumferential stepped diffraction grating pattern 30a3) of the diffractive optical element 30 (or 30 ′) designed under the same conditions as in FIG. , Shows the minimum pitch. The above-mentioned minimum pitch is the inner circumferential uneven diffraction grating when the adjacent pitch T gradually increases or decreases among the pitch T shown in FIG. 2 (c) {or FIG. 3 (c)}. This is the minimum length of the pattern 30a1 (or the inner circumferential stepped diffraction grating pattern 30a3). Also here, when the distance k between the diffractive optical element 30 (or 30 ′) and the objective lens 31 is changed to 1 mm, 2 mm, and 4 mm, respectively, the inner circumferential concave / convex diffraction grating pattern 30a1 (or inner circumference) The minimum pitch of the side stepped diffraction grating pattern 30a3) does not depend on the distance k between the diffractive optical element 30 and the objective lens 31, and is determined only by the working distance WD2. At this time, in the case of the phase difference function coefficients A _{2 to} A _{8 in} Table 1 shown above, the minimum pitch is 69 μm.

At this time, as the minimum pitch of the inner-side uneven diffraction grating pattern 30a1 (or inner-side step-like diffraction grating pattern 30a3) of the diffractive optical element 30 (or 30 ′) increases, the loss in diffraction decreases, and the design is reduced. A diffraction efficiency close to the value can be obtained. When the diffractive optical element 30 (or 30 ′) is manufactured, there is no diffraction loss if the edge of the unevenness (or staircase) becomes an angle of 90 °, but in reality, about 80 ° is a reality. Therefore, the narrower the pitch T, the larger the number of annular zones, and the diffraction loss increases. In general, the optical components such as prisms and lenses used in the optical pickup device 20 (FIG. 1) are expected to have an efficiency loss of about 10% at maximum. Therefore, the diffraction loss of the diffractive optical element 30 (or 30 ′) is desirably 10% or less from the design value. The zone width w satisfying the diffraction loss is expressed by the following equation (5).

  At this time, as the depth d per step of the inner-side uneven diffraction grating pattern 30a1 (or inner-side step-like diffraction grating pattern 30a3) of the diffractive optical element 30 (or 30 ′) increases, the zone width increases. By taking w wide, diffraction loss can be suppressed. In the case of the present embodiment, the pitch T is binarized (or quinary) in an uneven shape (or stepped shape), so that the zone width is approximately 1/2 (or approximately 1/5) of the minimum pitch. Become.

Next, in appropriately setting the distance k (FIG. 1) between the diffractive optical element 30 (or 30 ′) and the objective lens 31, the diffractive optical element 30 (or 30) with respect to the second laser light L2. It will be described whether the inner circumferential concave / convex diffraction grating pattern 30a1 (or inner circumferential stepped diffraction grating pattern 30a3) is designed to satisfy the sine condition or to satisfy the sine condition. At this time, the sine condition unsatisfied amount OSC in the infinite system can be obtained by the following equation (6).
15 to 19 show the spherical aberration and the sine condition for the second laser light L2 based on this equation 6, and in each figure, the vertical axis indicates the numerical aperture of the second laser light L2 to the objective lens 31 ( NA) = 0.65 indicates the normalized ray height, and the horizontal axis indicates the amount of deviation (mm).

  First, in FIG. 15, the distance k between the diffractive optical element 30 (or 30 ′) and the objective lens 31 is set to 2 mm, and the diffractive optical element 30 (or 30 ′) with respect to the second laser light L2. ) Shows an aberration diagram in the case where the inner circumferential concave / convex diffraction grating pattern 30a1 (or inner circumferential stepped diffraction grating pattern 30a3) is designed to satisfy the sine condition, and as is apparent from FIG. Although not, the sine condition at infinite conjugate ratio is not satisfied.

  On the other hand, FIG. 16 shows that the distance k between the diffractive optical element 30 (or 30 ′) and the objective lens 31 is set to 2 mm as described above, but the diffractive optical element with respect to the second laser beam L2. FIG. 16 shows aberration diagrams when 30 (or 30 ′) inner-side uneven diffraction grating pattern 30a1 (or inner-side step-like diffraction grating pattern 30a3) is designed to satisfy the sine condition. Thus, there is no spherical aberration, and the sine condition at the infinite conjugate ratio is satisfied.

At this time, Table 6 below shows the inner peripheral side uneven diffraction grating pattern 30a1 (or inner peripheral stepwise diffraction grating of the diffractive optical element 30 (or 30 ') for the cases of FIG. 15 and FIG. The phase difference function coefficients A _{2 to} A ₈ for the pattern 30a3) and the characteristics at that time are compared.

As can be seen from Table 6, the inner circumferential concave / convex diffraction grating pattern 30a1 (or inner circumferential stepped diffraction grating pattern 30a3) of the diffractive optical element 30 (or 30 ′) is applied to the second laser beam L2. It can be seen that whether the sine condition is not satisfied or the sine condition is satisfied is largely caused by the value of the phase difference function coefficient A ₂ term among the phase difference function coefficients A _{2 to} A ₈ terms. Here, the sine condition becomes unsatisfied when setting the value of the phase difference function coefficient A ₂ Section example 25, whereas, the sine condition satisfied in the case of setting the value of the phase difference function coefficient A ₂ Section for example 50.6 Become. The values of the phase difference function coefficients A _{4 to} A ₈ are shown in Table 6, respectively.

  When the distance k between the diffractive optical element 30 (or 30 ′) and the objective lens 31 is 2 mm, the diffractive optical element 30 (or 30 ′) is in a sine condition with respect to the second laser light L2. The image height characteristic value obtained by unsatisfactory design is 0.009λ2 · rms, which is considerably better than the image height characteristic value (0.015λ2 · rms) obtained by designing satisfying the sine condition. Further, the value of the working distance WD2 when the diffractive optical element 30 (or 30 ′) is designed to satisfy the sine condition is 0.269 mm, and the value of the working distance WD2 when designed to satisfy the sine condition (0.297 mm). Slightly narrower than.

  Next, FIG. 17 shows that the distance k between the diffractive optical element 30 (or 30 ′) and the objective lens 31 is set to 4 mm, and the diffractive optical element 30 (or 30) with respect to the second laser light L2. ') Shows an aberration diagram in the case where the inner circumferential concave / convex diffraction grating pattern 30a1 (or inner circumferential stepped diffraction grating pattern 30a3) is designed to satisfy the sine condition, and as is apparent from FIG. 17, spherical aberration is shown. However, the sine condition at infinite conjugate ratio is not satisfied.

  On the other hand, FIG. 18 shows that the distance k between the diffractive optical element 30 (or 30 ′) and the objective lens 31 is set to 4 mm as described above, but the diffractive optical element with respect to the second laser light L2. FIG. 18 shows aberration diagrams in the case where 30 (or 30 ′) inner circumferential uneven diffraction grating pattern 30a1 (or inner circumferential stepped diffraction grating pattern 30a3) is designed to satisfy the sine condition. Thus, there is no spherical aberration, and the sine condition at the infinite conjugate ratio is satisfied.

At this time, Table 7 below shows the inner-side uneven diffraction grating pattern 30a1 (or inner-side stepped diffraction grating of the diffractive optical element 30 (or 30 ′) as compared with the cases of FIGS. The phase difference function coefficients A _{2 to} A ₈ for the pattern 30a3) and the characteristics at that time are compared.

This Table 7 also has the same tendency as in Table 6 described above. Here, when the value of the phase difference function coefficient A ₂ term is set to 85, for example, the sine condition is not satisfied, while the phase difference function coefficient A ₂ term For example, when the value is set to 103.5, the sine condition is satisfied. The values of the phase difference function coefficients A _{4 to} A ₈ are shown in Table 7, respectively.

  When the distance k between the diffractive optical element 30 (or 30 ′) and the objective lens 31 is 4 mm, the diffractive optical element 30 (or 30 ′) is sine condition with respect to the second laser light L2. Although the image height characteristic value obtained by unsatisfactory design is 0.014λ2 · rms, which is considerably better than the image height characteristic value (0.023λ2 · rms) obtained by designing satisfying the sine condition. It can be seen that the image height characteristic value is minimized (best) when the interval k is set to 2 mm. Further, the value of the working distance WD2 when the diffractive optical element 30 (or 30 ′) is designed to satisfy the sine condition is 0.331 mm, and the value of the working distance WD2 (0.0. 349 mm).

  Next, FIG. 19 shows that the distance k between the diffractive optical element 30 (or 30 ′) and the objective lens 31 is set to 1 mm, and the diffractive optical element 30 (or 30) with respect to the second laser light L2. ') Shows an aberration diagram in the case where the inner circumferential concave-convex diffraction grating pattern 30a1 (or inner circumferential stepwise diffraction grating pattern 30a3) is designed to satisfy the sine condition. As is apparent from FIG. 19, spherical aberration is shown. The sine condition at infinite conjugate ratio is satisfied.

At this time, Table 8 below shows that the diffractive optical element 30 (or 30 ′) has a concave / convex diffraction grating pattern 30a1 (or an inner peripheral stepped diffraction grating pattern 30a3) as compared with the case of FIG. The phase difference function coefficients A _{2 to} A ₈ and various characteristics at that time are shown.

  In the case of Table 8, as shown in FIG. 19, when the diffractive optical element 30 (or 30 ′) satisfies the sine condition, the image height characteristic is optimized, but as described above, when the distance k is 1 mm. Is not desirable because it is too narrow and difficult to assemble.

From the above Table 8, when the value of the phase difference function coefficient A ₂ term is near 0 or smaller, that is, the light beam of the second laser light L2 after passing through the diffractive optical element 30 (or 30 ′) is substantially parallel light. When the light is converged and emitted to the objective lens 31 side, the sine condition is satisfied, and the image height characteristic is optimal at this time. On the other hand, as shown in Tables 6 and 7, the value of the phase difference function coefficient A ₂ term is A ₂ > 0 and the value of the phase difference function coefficient A ₂ term is larger, so that the diffractive optical element 30 (or 30 ′) is large and finite correction mainly acts, that is, the sine becomes so large that the light beam of the second laser beam L2 after passing through the diffractive optical element 30 (or 30 ′) changes from substantially parallel light to diffused light. Image height characteristics can be improved by increasing the amount of unsatisfied conditions by a predetermined amount. Further, desirably, when the value of the phase difference function coefficient A ₂ term is A ₂ > 0, the minimum value of the sine condition unsatisfied amount OSC of the above-described equation 6 is 0 or more.

As described above, when reproducing, recording or erasing the DVD 2 with respect to the objective lens 31 designed for Blu-ray Disc, the inner circumference side of the diffractive optical element 30 (or 30 ′). An image height characteristic is obtained by setting the value of the phase difference function coefficient A ₂ term to the concavo-convex diffraction grating pattern 30a1 (or the inner circumferential stepped diffraction grating pattern 30a3) to A ₂ > 0 and satisfying the sine condition unsatisfactory. Is adjusted by appropriately setting the distance k between the diffractive optical element 30 (or 30 ') and the objective lens 31, and other characteristics need to be balanced. is there.

  From the margin of the optical pickup device 20 (FIG. 1), desirably, when the second laser-light L2 is incident on the diffractive optical element 30 (or 30 ′) and the objective lens 31, the second laser-light L2 The image height characteristic (0.3 °) is 0.02λ2 · rms or less with respect to the wavelength λ2, the decentering characteristic (eccentricity 5 μm with respect to the objective lens) is 0.02λ2 · rms or less, and longitudinal chromatic aberration (when the wavelength changes by 1 nm) ) Is preferably 0.02λ · rms or less.

It is the figure which showed the whole structure of the optical pick-up apparatus based on this invention. 2A to 2C are diagrams for explaining the example diffractive optical element shown in FIG. 1, wherein FIG. 2A is a top view, FIG. 2B is a longitudinal sectional view, and FIG. It is an enlarged view of the circumferential uneven | corrugated shaped diffraction pattern part. (A)-(c) is a figure for demonstrating the diffraction type optical element of the other example shown in FIG. 1, (a) is a top view, (b) is a longitudinal cross-sectional view, (c) is an inner periphery. It is an enlarged view of a side step-like diffraction pattern part and an outer peripheral side uneven | corrugated diffraction pattern part. It is the figure which expanded and showed the case where Blu-ray Disc and DVD were recorded or reproduced | regenerated using the objective lens optimized for infinite conjugate for Blu-ray Disc. FIG. 5 is a diagram showing spherical aberration and sine conditions when the first laser beam is incident on the objective lens shown in FIG. 4. It is a figure for demonstrating the sine condition in a common lens. FIG. 5 is a diagram schematically showing a case where a Blu-ray Disc is recorded or reproduced by the example diffractive optical element shown in FIG. 2 and the objective lens shown in FIG. 4. FIG. 5 is a diagram schematically showing a case where a Blu-ray Disc is recorded or reproduced by the diffractive optical element of another example shown in FIG. 3 and the objective lens shown in FIG. 4. FIG. 5 is a diagram schematically showing a case where a DVD is recorded or reproduced by the diffractive optical element of the example shown in FIG. 2 and the objective lens shown in FIG. 4. FIG. 5 is a diagram schematically showing a case where a DVD is recorded or reproduced by the diffractive optical element of another example shown in FIG. 3 and the objective lens shown in FIG. 4. It is the figure which showed the wavefront aberration at the time of decentering of the diffraction type optical element and objective lens with respect to the working distance WD2 between an objective lens and DVD. It is the figure which showed the axial chromatic aberration by the wavelength error of the 2nd laser beam to the diffraction type optical element and objective lens with respect to the working distance WD2 between an objective lens and DVD. FIG. 6 is a diagram showing image height characteristics (wavefront aberration) when a light beam of a second laser beam tilted from the optical axis is incident on the diffractive optical element and the objective lens with respect to a working distance WD2 between the objective lens and the DVD. It is the figure which showed the minimum pitch of the diffractive optical element with respect to the working distance WD2 between an objective lens and DVD. FIG. 6 is an aberration diagram when the distance k between the diffractive optical element and the objective lens is set to 2 mm, and the diffractive optical element is designed to satisfy the sine condition with respect to the second laser light. FIG. 5 is an aberration diagram when the distance k between the diffractive optical element and the objective lens is set to 2 mm, and the diffractive optical element is designed to satisfy the sine condition with respect to the second laser light. FIG. 6 is an aberration diagram when the distance k between the diffractive optical element and the objective lens is set to 4 mm, and the diffractive optical element is designed to satisfy the sine condition with respect to the second laser light. FIG. 5 is an aberration diagram when the distance k between the diffractive optical element and the objective lens is set to 4 mm, and the diffractive optical element is designed to satisfy the sine condition with respect to the second laser light. FIG. 6 is an aberration diagram when the distance k between the diffractive optical element and the objective lens is set to 1 mm, and the diffractive optical element is designed to satisfy the sine condition with respect to the second laser light. (A)-(c) is sectional drawing which each showed the specific example of the compound objective lens which consists of a diffraction type hologram and an objective lens in the conventional optical head apparatus.

Explanation of symbols

1 ... 1st optical recording medium (Blu-ray Disc),
1a: Laser beam incident surface, 1b: Signal surface,
2 ... Second optical recording medium (DVD),
2a ... laser beam incident surface, 2b ... signal surface,
10: Optical recording medium driving device (optical disk driving device),
11 ... Spindle motor, 12 ... Turntable,
20: Optical pickup device,
21 ... Pickup housing,
22 ... 1st laser light source (blue semiconductor laser),
23. Second laser light source (red semiconductor laser),
26: Phase plate for the first laser beam,
29 ... Lens holder,
30: An example aberration correction element,
30 '... an aberration correction element of another example,
30a ... upper surface, 30a1 ... inner side uneven diffraction pattern portion,
30a2 ... outer peripheral side flat part,
30a3 ... Stepped diffraction pattern part on the inner circumference side,
30a4 ... outer peripheral side uneven | corrugated diffraction pattern part,
30b ... lower surface, 30b1 ... light transmissive flat part, 30b2 ... second laser light aperture restricting part, 31 ... objective lens, 31a ... first surface, 31b ... second surface,
k: an interval between the aberration correction element in one example (or the aberration correction element in another example) and the objective lens,
L1: a first laser beam emitted from a blue semiconductor laser,
L2: a second laser beam emitted from a red semiconductor laser,
λ1: wavelength of the first laser beam emitted from the blue semiconductor laser,
λ2: The wavelength of the second laser beam emitted from the red semiconductor laser.

Claims (2)

A first optical recording medium, a second optical recording medium having a recording density lower than that of the first optical recording medium and a substrate thickness greater than that of the first optical recording medium, and the first and second optical recording media. In an optical pickup device for selectively recording or reproducing a combination type optical recording medium in which signal surfaces are combined and laminated integrally,
A first laser light source that emits a first laser beam having a wavelength of 450 nm or less corresponding to the first optical recording medium;
A second laser light source that emits a second laser beam having a wavelength longer than that of the first laser beam corresponding to the second optical recording medium;
The parallel light of the first and second laser lights is selectively incident and the first laser light is transmitted as it is, but the diffraction pattern portion formed in the inner peripheral region with respect to the second laser light. A diffractive optical element for correcting spherical aberration caused by a difference in substrate thickness of the first and second optical recording media by emitting diffracted diffracted light;
For the first optical recording medium, the numerical aperture (NA) is set to 0.75 or more, and at least one of the first and second surfaces facing each other is formed as an aspheric surface, and the first laser Provided above the diffractive optical element while satisfying a sine condition with respect to the light, and selectively collect the first and second laser beams on each signal surface of the first and second optical recording media. And at least an objective lens that emits light,
When the diffractive optical element is stored in a lower part of the lens holder and the objective lens is stored in an upper part of the lens holder, the diffraction pattern of the diffractive optical element with respect to the second laser light The diffractive optical element between the objective lens and the objective lens so that an image height characteristic to the second laser light tilted by a predetermined amount from the optical axis of the objective lens is reduced. An optical pickup device characterized in that an interval is set. The optical pickup device according to claim 1,
An optical pickup device, wherein a value of wavefront aberration when recording or reproducing the second optical recording medium with the second laser light tilted by a predetermined amount from the optical axis of the objective lens is equal to or less than a Marechal criterion.

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