JP2007102983A - Objective lens and optical pickup apparatus using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an objective lens capable of corresponding to three optical information recording media (optical disks) corresponding to the reduction of focal depth on a small spot and to provide an optical pickup apparatus using the objective lens. <P>SOLUTION: The objective lens 1 for forming an image of laser beams of a plurality of colors having mutually different wavelengths on a prescribed surface is comprised of a single lens having positive power. A diffraction grading is formed at least on one surface out of light incident and exit surfaces, a numerical aperture in the shortest wavelength is ≥0.65, and the axial chromatic aberration of the laser beam of the shortest wavelength is smaller than that of laser beams of other wavelengths. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an objective lens and an optical pickup device using the same, and in particular, is used for an optical head capable of recording and reproducing with respect to a plurality of types of optical information recording media (optical discs) having different protective layer (transparent substrate) thicknesses. The present invention relates to a high numerical aperture (NA) objective lens.

  Conventionally, the optical pickup device has been commercialized for CD-ROMs and CD-RWs corresponding to CDs (compact discs), which record and reproduce by collecting infrared laser light emitted from a laser light source on an optical disc. I came. Recently, optical pickup devices that support both DVD and CD, and that use red laser light (wavelength 650 nm) for DVD and infrared laser light (wavelength 780 nm) for CD have been commercialized. Recently, an optical pickup device compatible with the Blu-ray Disk standard and the HD-DVD standard in which the recording density is further increased by using blue laser light (wavelength 405 nm) has been put into practical use.

  At present, an optical pickup device corresponding to three types of optical disks (media) with one objective lens is not yet commercially available. The reason for this is that the three types of optical discs are made based on different standards, and the protective layer (transparent substrate) thickness of the optical disc is 1.2 mm for CD, 0.6 mm for DVD, 0.1 mm for Blu-ray Disk, HD- This is because the DVD is 0.6 mm. This is because there are many development problems, such as the necessity of dealing with laser beams having different wavelengths with different numerical apertures (NA).

  In order to cope with the different thicknesses of the protective layers, the paraxial condensing position can be moved by moving the objective lens in the optical axis direction. However, since the spherical aberration changes as the thickness of the protective layer changes, simply moving the objective lens disturbs the wavefront of the laser beam, making it impossible to converge the spot to the required diameter, and recording and reproducing information. become unable.

  For example, when an objective lens designed so that spherical aberration is corrected when using a DVD is used for reproducing a CD, the objective lens is moved in the optical axis direction so that the paraxial condensing position coincides with the recording surface. be able to. However, the spherical aberration is over and information cannot be reproduced with high accuracy.

  Therefore, various optical systems that allow laser light suitable for each optical disc to enter the objective lens according to the thickness of the protective layer have been proposed (see Patent Documents 1 and 2).

  In Patent Document 1, a hologram lens is provided in front of an objective lens to separate laser light emitted from a single semiconductor laser into zero-order light and primary light. A technique is disclosed in which zero-order light, which is parallel light, is formed as a spot for an optical disk having a thin protective layer, and primary light, which is divergent light, is formed as a spot for an optical disk having a thick protective layer.

  According to the optical system of Patent Document 1 described above, the hologram lens is designed so as to obtain an optimum laser beam according to the thickness of the protective layer, thereby suppressing the occurrence of spherical aberration and providing diffraction-limited performance for each optical disc. It is possible to obtain a spot having.

  Patent Document 2 uses an infrared laser beam having a wavelength of 780 n for a CD, a red laser beam having a wavelength of 650 nm for a DVD, and a blue laser beam having a wavelength of 400 nm for an HD-DVD. Yes. That is, Patent Document 2 discloses an objective lens for an optical pickup corresponding to three optical disks by focusing on the difference in wavelength and providing a diffraction grating on the lens surface of the objective lens.

  In the example of Patent Document 2, the spot diameter is small and the depth (depth of focus) is narrow because the NA is bright particularly in red and blue laser beams. For this reason, it is difficult to cope with the focus change due to the wavelength fluctuation in the mode hop of the laser light source. For example, laser mode hops are generated at 0.2 to 0.3 nm / ° C. for red and infrared laser light, and laser mode hops are generated at about 0.04 to 0.07 nm / ° C. for blue laser light. Considering that the depth of the red laser light is about 2 μm, it is essential to cope with the mode hop.

  Further, in an objective lens compatible with the Blu-ray Disk standard, the NA of blue laser light is 0.85, the focal depth is very narrow, about 0.7 μm, and the problem is more serious.

  The objective lens of the conventional optical pickup device that can handle two wavelengths is composed of a plastic lens, and it is configured to correct the refractive index change with respect to temperature by using the negative dispersion of the diffraction grating. .

However, the plastic lens has a large refractive index change dn / dt of -100 × 10 ⁻⁶ / ° C. with respect to temperature. For this reason, if all are corrected, the power of diffraction increases, and a moderate temperature change can be satisfactorily corrected by a change in refractive index and chromatic aberration of the diffraction lens. However, a large focus shift occurs with respect to a change in wavelength that changes on the staircase due to a laser mode hop with respect to a temperature change.

Accordingly, in practice, the power of the diffraction grating is weakened and the power of the diffraction grating is changed in the radial direction so that spherical aberration occurs when the wavelength varies. Thus, a balance is achieved by configuring the on-axis focusing movement and the marginal ray focusing movement to move in opposite directions.
JP-A-7-98431 JP 2001-195769 A

  According to the conventional method described above, it is possible to cope with up to two wavelengths, but in the case of three wavelengths, it is actually very difficult to perform the above correction after focusing on all wavelengths. Have difficulty. Also, in the Blu-ray Disk, the NA is 0.85 or higher and the spot diameter is very small, so even a slight change in wavefront aberration causes a large disturbance in the spot diameter, which can be handled by the conventional method. It was an extremely difficult situation.

  An object of the present invention is to provide an objective lens capable of supporting three media (optical information recording media) corresponding to a decrease in the depth of focus at a small spot, and an optical pickup device using the objective lens.

The objective lens of the invention of claim 1
An objective lens for imaging laser beams of different colors with different wavelengths on a predetermined surface,
The objective lens is composed of a single lens having a positive power, has a diffraction grating on at least one of the light incident / exit surfaces, has a numerical aperture of 0.65 or more at the shortest wavelength, and It is characterized in that the axial chromatic aberration is smaller than the laser light of other wavelengths with respect to the laser light of the shortest wavelength.

According to a second aspect of the present invention, in the first aspect of the present invention, the objective lens has at least one aspherical shape, and the diffraction grating has a positive power.

The invention of claim 3 is the invention of claim 1 or 2, wherein n is the refractive index of the material at the shortest wavelength of the objective lens, φ is the power other than the diffraction grating at the shortest wavelength of the objective lens, and 1 degree of the objective lens. When the refractive index change with respect to temperature is dn / dt, the numerical aperture of the objective lens at the shortest wavelength is NA, and the shortest wavelength is λ,

It is characterized by satisfying the following conditions.

According to a fourth aspect of the present invention, in the first, second, or third aspect of the present invention, the material of the objective lens is glass, and is made by glass molding.

The invention according to claim 5 is the invention according to any one of claims 1 to 4, wherein the power of the diffraction grating is Φd, and the power of the entire objective lens including the diffraction grating is Φt.
0.05 <Φd / Φt <0.5
It is characterized by satisfying the following conditions.

The invention of claim 6 is the invention of any one of claims 1 to 5, wherein the height of the grating part of the diffraction grating is H, the diffraction order is m, the refractive index of the material at the shortest wavelength of the objective lens is n, When the average wavelength of the shortest wavelength and the longest wavelength of a plurality of laser beams having different wavelengths is λa,
0.74 × λa × m / (n−1) <H <0.99 × λa × m / (n−1)
It is characterized by satisfying the following conditions.

The invention of claim 7 is the invention of any one of claims 1 to 6, wherein the refractive index of the material at the shortest wavelength of the objective lens is n, the power other than the diffraction grating at the shortest wavelength of the objective lens is φ, and the objective When the refractive index change with respect to a wavelength of 1 nm of the lens is dn / dλ, the shortest wavelength is λ, and the amount of change in axial chromatic aberration with respect to the wavelength change of the diffraction grating surface of the objective lens is SO

It is characterized by satisfying the following conditions.

The optical pickup device of the invention of claim 8
An objective lens according to any one of claims 1 to 7, and a member for recording information on and / or reproducing information recorded on the optical information recording medium using the objective lens. It is characterized by that.

The invention according to claim 9 is the invention according to claim 8, wherein the objective lens corresponds to at least three types of optical information recording media having at least two types of protective layers having different thicknesses, and the at least three types of optical information recording media. A plurality of colors of laser beams having different wavelengths are imaged on the information recording surface of the optical information recording medium.

The invention of claim 10 is the invention of claim 9, wherein the laser beams of different colors having different wavelengths are laser beams of three wavelengths of blue, red, and infrared, and the shortest wavelength is a wavelength of blue laser light. Yes, it is 0.65 for red laser light and 0.45 for infrared laser light.

The invention of claim 11 is the invention of claim 10, wherein the axial chromatic aberration at the wavelength of the blue laser light of the objective lens is L1, the axial chromatic aberration at the wavelength of the red laser light is L2, and the axial chromatic aberration at the wavelength of the infrared laser light. Is L3 L1 <L2 <L3
It is characterized by satisfying the following conditions.

  According to the present invention, the axial chromatic aberration is configured to be smaller than the laser light of other wavelengths with respect to the laser light of the shortest wavelength. This makes it possible to widen a margin for wavelength fluctuation on the short wavelength side where the focal depth becomes narrow with a small spot, and to support different media (optical information recording media) with laser beams of three wavelengths (colors), and An optical pickup device using the same can be achieved.

  Embodiments of the present invention will be described below with reference to the drawings.

  First, the configuration of the optical pickup device of this embodiment will be specifically described.

  The optical system of the optical pickup apparatus of the present embodiment condenses the collimator lens for converting the divergent light beam emitted from the light source means into a substantially parallel light beam, and the substantially parallel light beam from the collimator lens on the optical information recording surface. Objective lens.

  In this embodiment, the present invention is not limited to the above configuration, and a conversion lens (coupling lens) for converting the angle of the divergent light beam emitted from the light source means into a divergent light beam or a convergent light beam, and the light beam from the conversion lens. You may comprise so that it may have an objective lens for condensing on an optical information recording surface.

  Or you may comprise only the objective lens (finite conjugate type objective lens) for condensing the divergent light beam emitted from the light source means on the optical information recording surface.

  FIG. 1 is a schematic view of the essential portions of Embodiment 1 of an optical pickup device using the objective lens of the present invention.

  The optical pickup device in FIG. 1 is a hybrid semiconductor having a semiconductor laser 11 as a first light source for reproducing a first optical disk and second and third light sources for reproducing second and third optical disks. And a laser 12. The semiconductor laser 11 emits laser light having a wavelength of 405 nm. The hybrid semiconductor laser 12 emits laser light having a wavelength of 650 nm and a wavelength of 780 nm.

  The objective lens 1 shown in the figure is composed of a single lens having positive power, at least one surface is aspherical, and has a diffraction grating having positive power on at least one surface. The objective lens 1 has a numerical aperture of 0.65 or more at the shortest wavelength (405 nm), and the axial chromatic aberration is smaller than the laser light of other wavelengths with respect to the laser light of the shortest wavelength (the occurrence of chromatic aberration is small). It is set to be.

  The objective lens 1 is made of a glass material and is made by glass molding.

  The objective lens 1 corresponds to at least three types of optical disks (optical information recording media) having at least two types of protective layers (transparent substrates) having different thicknesses. Then, laser light of three colors of blue, red and infrared having different wavelengths is formed on the information recording surface of the optical disc 20 for at least three types of optical discs.

  Next, the case where the first, second, and third optical disks are reproduced in the configuration of FIG. 1 will be described.

  First, when reproducing the first optical disk, a laser beam (light beam) having a wavelength of 405 nm is emitted from the semiconductor laser 11 and passes through a beam splitter 7 which is a light combining unit. The beam splitter 7 has a characteristic of transmitting a light beam having a wavelength of 405 nm and reflecting a laser beam having a wavelength of 650 nm and a wavelength of 780 nm.

  The laser light (P-polarized light) that has passed through the beam splitter 7 passes through the polarizing beam splitter 6, the collimator lens 2, and the quarter-wave plate (not shown) to become a circularly polarized parallel light beam. The polarizing beam splitter 6 has a characteristic of transmitting P-polarized light and reflecting S-polarized light.

  This parallel light beam is focused by the diaphragm 3 and is focused on the information recording surface 22 by the objective lens 1 via the transparent substrate (protective layer) 21 of the first optical disk 20.

  Then, the light beam modulated and reflected by the information bit on the information recording surface 22 is transmitted again through the objective lens 1, the diaphragm 3, the quarter wavelength plate (not shown), and the collimator lens 2. The laser light (S-polarized light) that has passed through the collimator lens 2 enters the polarization beam splitter 6, and the reflected light beam is given astigmatism by a cylindrical lens (not shown) and enters the photodetector 13. . A read signal of information recorded on the first optical disc 20 is obtained using the output signal from the photodetector 13.

  Further, a change in the amount of light due to a change in the shape and position of the spot on the photodetector 13 is detected, and focus detection and track detection are performed. Based on the detection result, the objective lens 1 is moved so that the two-dimensional actuator 13a forms an image of the laser beam emitted from the semiconductor laser 11 on the information recording surface 22 of the first optical disc 20. At the same time, the objective lens 1 is moved so that the laser beam from the semiconductor laser 11 is imaged on a predetermined track.

  When reproducing the second optical disk, laser light having a wavelength of 650 nm is emitted from the second light source of the hybrid semiconductor laser 12 and reflected by the beam splitter 7 which is a light combining means. Similar to the laser light from the first semiconductor 11, the transparent substrate of the second optical disk 20 is passed through the polarizing beam splitter 6, the collimator lens 2, the quarter wavelength plate (not shown), the diaphragm 3, and the objective lens 1. The light is condensed on the information recording surface 22 via 21.

  Then, the laser beam modulated and reflected by the information bit on the information recording surface 22 is again transmitted through the objective lens 1, the diaphragm 3, the quarter wavelength plate (not shown), and the collimator lens 2 and is incident on the polarization beam splitter 6. To do. The laser beam reflected by the polarization beam splitter 6 is given astigmatism by a cylindrical lens (not shown), enters the photodetector 13, and is recorded on the second optical disc 20 using the output signal. An information read signal is obtained.

  As in the case of the first optical disk, the spot shape change on the photodetector 13 and the light quantity change due to the position change are detected, and focus detection and track detection are performed. The objective lens 1 is moved for tracking.

  When reproducing the third optical disk, laser light having a wavelength of 780 nm is emitted from the third light source of the hybrid semiconductor laser 12 as in the case of reproducing the second optical disk. The optical detector 13 performs beam detection, focus detection, and track detection using the same optical path as when reproducing the second optical disk, and the objective lens 1 is moved for focusing and tracking by the two-dimensional actuator 13a. Let

  The thickness of the transparent substrate 21 is standardized by the first, second, and third optical disks 20 as described above.

  2A and 2B are explanatory views showing the shape of the diffraction grating of the present embodiment. 2A shows a grating ring when the diffraction grating is viewed from the optical axis direction, and FIG. 2B shows a cross-sectional shape.

  2A and 2B, 51 is a diffraction grating, 52 is a substrate (lens), and 53 is a diffraction part. The diffraction grating 51 of this embodiment is composed of a single-layer diffractive portion as shown in FIG. 2B, but is not limited to this, and may be composed of a multilayered diffractive portion.

  When diffracted light of the same order is used in the single-layer diffraction grating 51, the maximum diffraction efficiency is obtained at the design wavelength as shown in the conventional example, but the diffraction efficiency deteriorates as the wavelength deviates from the design wavelength.

  In addition, the diffraction grating 51 has a special feature that its dispersion characteristics are significantly different from those of optical materials such as general glass. Specifically, Abbe number νd and partial dispersion ratio θgF are

It becomes. Therefore, it is very effective for controlling chromatic aberration.

Λ _d is the refractive index of the d line, λ _F is the refractive index of the F line, λ _C is the refractive index of the C line, and λ _g is the refractive index of the G line.

  In this embodiment, a diffraction grating is used to control chromatic aberration over a wide wavelength range exceeding the visible range of wavelengths 405 nm, 650 nm, and 780 nm.

  As shown in FIG. 2A, the diffraction grating 51 can change the power of refraction due to diffraction by changing the pitch of the grating portion 53 in the radial direction with respect to the optical axis. Therefore, an aspherical effect can be provided by changing the pitch of the grating portion 53 of the diffraction grating 51 according to the radial position.

  FIGS. 3A, 3B, and 3C are optical path diagrams at respective wavelengths when the objective lens of the reference example of the present invention is used.

  Here, the reference example has a diffraction grating on at least one of the light incident / exit surfaces, the numerical aperture at the shortest wavelength is 0.65 or more, and axial chromatic aberration with respect to the laser light with the shortest wavelength Is an objective lens that is not smaller than laser light of other wavelengths.

  3A, 3B, and 3C, reference numeral 31 denotes an objective lens, which has a diffraction grating on the first surface (light incident surface), and the first surface and the second surface (light emission surface). ) Are both aspherical. Reference numeral 32 denotes a protective layer (transparent substrate).

  In FIG. 3A, the thickness of the protective layer 32 is d = 1.2 mm, the wavelength of the laser beam (infrared laser beam) is λ = 780 nm, and the numerical aperture of the objective lens 31 is NA = 0.45.

  In FIG. 3B, the thickness of the protective layer 32 is d = 0.6 mm, the wavelength of the laser beam (red laser beam) is λ = 650 nm, and the numerical aperture of the objective lens 31 is NA = 0.65.

  In FIG. 3C, the thickness of the protective layer 32 is d = 0.1 mm, the wavelength of the laser beam (blue laser beam) is λ = 405 nm, and the numerical aperture of the objective lens is NA = 0.85.

  4A, 4B, and 4C are spherical aberration diagrams of the objective lens corresponding to FIGS. 3A, 3B, and 3C, respectively.

  4A shows a case where the wavelength λ of the laser beam shown in FIG. 3A is 780 nm and NA = 0.45. FIG. 4B shows a case where the wavelength λ of the laser beam shown in FIG. In the case of 0.65, FIG. 4C shows the case where the wavelength of the laser light shown in FIG. 3C is λ = 405 nm and NA = 0.85.

  As can be seen from FIGS. 4 (A), (B), and (C), the initial design performance is at a level where there is no problem, and the objective lens is compatible with the Blu-ray Disk standard and compatible with CDs and DVDs. It has become more.

  FIGS. 5A, 5B, and 5C are graphs showing changes in spherical aberration with respect to each wavelength corresponding to FIGS. 3A, 3B, and 3C, respectively.

  5A, 5B, and 5C, the horizontal axis represents wavelength, and the vertical axis represents longitudinal aberration (axial chromatic aberration). The reason why there are a plurality of lines in FIGS. 5 (A), 5 (B), and 5 (C) is that light rays on the pupil are shown for light rays with ratios of 0.0, 0.5, and 1.0.

  5A shows the case where the wavelength λ of the laser beam shown in FIG. 3A is 780 nm and NA = 0.45. FIG. 5B shows the case where the wavelength λ of the laser beam shown in FIG. In the case of 0.65, FIG. 5C shows the case where the wavelength λ of the laser beam shown in FIG. 3C is 405 nm and NA = 0.85.

  As is clear from FIGS. 5A, 5B, and 5C, the luminous flux is condensed at one point at each wavelength used.

  However, the light beam arrival position changes almost linearly with respect to the wavelength. In particular, when the NA = 0.85 Blu-ray Disk is supported, the designed focal depth is about 0.6 μm, and the focal depth is further reduced due to manufacturing tolerances in production.

  In the reference example, 0.3μm focus movement occurs due to the mode hop of the laser light source due to a temperature fluctuation of 1 ° C, and the change in focus movement is abrupt. Will occur.

  FIGS. 6A, 6B, and 6C are optical path diagrams at wavelengths of Example 1 of the present invention.

  The numerical values of the configuration of the present embodiment are shown in Table 1 as Numerical Embodiment 1 described later.

  6 (A), 6 (B), and 6 (C), reference numeral 1 denotes an objective lens, which has a diffraction grating on the first surface (light incident surface), and the first surface and the second surface (light emitting surface). ) Are both aspherical. Reference numeral 21 denotes a protective layer (transparent substrate), and 3 denotes a diaphragm.

  In FIG. 6A, the thickness of the protective layer 21 is d = 1.2 mm, the wavelength of the laser beam (infrared laser beam) is λ = 780 nm, and the numerical aperture of the objective lens 1 is NA = 0.45. The NA at this time may be about 0.4 to 0.5.

  In FIG. 6B, the thickness of the protective layer 21 is d = 0.6 mm, the wavelength of the laser beam (red laser beam) is λ = 650 nm, and the numerical aperture of the objective lens 1 is NA = 0.65. The NA at this time may be about 0.6 to 0.7.

  In FIG. 6C, the thickness of the protective layer 21 is d = 0.1 mm, the wavelength of the laser beam (blue laser beam) is λ = 405 nm, and the numerical aperture of the objective lens 1 is NA = 0.85.

  FIGS. 7A, 7B, and 7C are spherical aberration diagrams at the respective wavelengths in Example 1 of the present invention.

  7A shows the case where the wavelength λ of the laser beam shown in FIG. 6A is 780 nm and NA = 0.45. FIG. 7B shows the case where the wavelength λ of the laser beam shown in FIG. In the case of 0.65, FIG. 7C shows the case where the wavelength of the laser light shown in FIG. 6C is λ = 405 nm and NA = 0.85.

  As can be seen from FIGS. 7A, 7B, and 7C, the optical performance is excellent with respect to the CD, DVD, and Blu-ray Disk standards.

  8A, 8B, and 8C are graphs showing changes in spherical aberration with respect to wavelength at each wavelength in Example 1 of the present invention.

  8A, 8B, and 8C, the horizontal axis indicates the wavelength and the vertical axis indicates the longitudinal aberration. The reason why there are a plurality of lines in FIGS. 8A, 8B, and 8C is that light rays are shown at intervals of 0.1 in the light ray ratio on the pupil.

  8A shows the case where the wavelength λ of the laser light shown in FIG. 6A is 780 nm and NA = 0.45. FIG. 8B shows the case where the wavelength λ of the laser light shown in FIG. In the case of 0.65, FIG. 8C shows the case where the wavelength of the laser light shown in FIG. 6C is λ = 405 nm and NA = 0.85.

At this time, in this embodiment, when the axial chromatic aberration at the wavelength of the blue laser light of the objective lens is L1, the axial chromatic aberration at the wavelength of the red laser light is L2, and the axial chromatic aberration at the wavelength of the infrared laser light is L3, L1 < L2 <L3 (1)
Satisfy the following conditions. That is, in this embodiment, the axial chromatic aberration L1 of the blue laser beam is minimized (the least occurrence of chromatic aberration) and is corrected better than other laser beams.

  The diffraction grating has a constant negative Abbe number as described above, and the refractive index changes linearly with respect to the wavelength. On the other hand, when the refractive index change with respect to the wavelength is graphed with a normal Abbe number, the normal glass changes in a downward convex shape. Therefore, in the actual design, by combining the dispersibility of the diffraction grating and the glass, it is possible to balance so that the focus variation with respect to the wavelength becomes small in the vicinity of the wavelength of the blue laser light.

  In this embodiment, the refractive index of the material at the shortest wavelength of the objective lens is n, the power other than the diffraction grating at the shortest wavelength of the objective lens is φ, and the refractive index change with respect to the temperature of the objective lens is dn / dt. And Further, when the numerical aperture of the objective lens is NA and the shortest wavelength is λ at the shortest wavelength,

Is satisfied.

  By selecting the refractive index change dn / dt of this lens material, it is possible to select a lens material that does not cause a problem in refractive index change with respect to temperature change.

Usually, this refractive index change dn / dt is about -100 × 10 ⁻⁶ / ° C. for a plastic material, and 10 × 10 ⁻⁶ / ° C. or less in absolute value for a glass material. The depth of focus of the objective lens changes in proportion to λ / NA ² , and the selectable material range becomes a limited material as the NA becomes brighter.

  The left side of the conditional expression (2) indicates the focus fluctuation due to the refractive index fluctuation. By selecting a material that satisfies the conditional expression (2), keeping the focus variation with respect to the refractive index change of the material relatively small, and using the correction effect of the chromatic aberration of the diffraction grating, the focus movement can be reduced to a level where there is no problem. can do.

  Therefore, in this embodiment, the chromatic aberration correction effect of the diffraction grating is hardly used on the axis as in the prior art, and the focus variation due to the change in the refractive index of the material is canceled by the wavelength change of the spherical aberration correction effect of the diffractive optical element. There is no need to take a special approach. That is, in this embodiment, the axial chromatic aberration of the objective lens can be easily corrected by effectively using the correction effect of the axial chromatic aberration of the diffraction grating. Also, in the design, by concentrating the focus on optical aberration correction at three wavelengths, it is possible to provide an objective lens that can handle three wavelengths with a single lens.

  Next, specific numerical examples of the conditional expression (2) will be shown.

In this embodiment, since the glass that is the material of the objective lens is the trade name NBF1, dn / dt = 7.9e-6. Also, f (focal length of objective lens) = 1.7756mm, n = 1.7689, λ = 405nm, NA = 0.85, so the left side of the conditional expression (2) is 1.82e-5 and the right side is 2.18e-5. This satisfies the conditional expression (2). Note that ex means 10 ^−x .

In this embodiment, when the power of the diffraction grating (power at the shortest wavelength) is Φd, and the power of the entire objective lens including the diffraction grating is Φt,
0.05 <Φd / Φt <0.5 (3)
Satisfy the following conditions.

  Conditional expression (3) relates to the ratio between the power Φd of the diffraction grating and the power Φt of the entire objective lens including the diffraction grating. If the lower limit value of conditional expression (3) is exceeded, there is almost no use of correction of axial chromatic aberration of the diffraction lens, and there is a problem in focus correction particularly in NA with a short wavelength, which is not good. If the upper limit of conditional expression (3) is exceeded, chromatic aberration correction is overcorrected, which is not good.

  In this embodiment, when the conditional expression (3) is satisfied, the diffraction grating has a power that can sufficiently cope with the correction of the axial chromatic aberration of the objective lens, and the chromatic aberration can be corrected satisfactorily.

  Next, specific numerical examples of the conditional expression (3) will be shown.

  In this embodiment, Φd = 0.0533, Φt = 0.559, and Φd / Φt = 0.095. This satisfies the conditional expression (3).

  More preferably, the conditional expression (3) is set as follows.

0.05 <Φd / Φt <0.2 (3a)
In this embodiment, the refractive index of the material at the shortest wavelength of the objective lens is n, the power other than the diffraction grating at the shortest wavelength of the objective lens is φ, the refractive index change with respect to the wavelength of 1 nm is dn / dλ, and the shortest wavelength is Let λ be. Furthermore, when the amount of change in axial chromatic aberration with respect to wavelength change due to the diffraction surface of the objective lens is SO

Satisfy the following conditions.

  Conditional expression (4) defines the axial chromatic aberration change amount SO. If the upper limit value of conditional expression (4) is exceeded, overcorrection is not good when corrected by a diffraction grating. If the lower limit of conditional expression (4) is exceeded, the axial chromatic aberration correction hardly uses the power of the diffraction grating, and it is not necessary to perform chromatic aberration correction balanced with the conventional spherical aberration.

  By satisfying the conditional expression (4), the correction amount of the longitudinal chromatic aberration with respect to the wavelength variation of the refractive index can be set to a level with no problem.

  When the conditional expression (4) is verified from the numerical example, dn / dλ = −0.00026, and the left side of the conditional expression (4) is 0.00012 and the right side is 0.00109. On the other hand, when the change amount SO of axial chromatic aberration is calculated from the result of actual tracing, SO = 0.0003, which satisfies the conditional expression (4).

  More preferably, the conditional expression (4) is set as follows.

In this embodiment, the height of the grating portion of the diffraction grating is H, the diffraction order is m, the refractive index of the material at the shortest wavelength of the objective lens is n, and the average wavelength of the shortest and longest wavelengths of a plurality of laser beams having different wavelengths. Is λa,
0.74 × λa × m / (n−1) <H <0.99 × λa × m / (n−1) (5)
Satisfy the following conditions.

  Conditional expression (5) defines the height H of the grating portion of the diffraction grating. If the lower limit value of conditional expression (5) is exceeded, the diffraction efficiency at a wavelength of 780 nm is half that of the diffraction efficiency at a wavelength of 405 nm. On the other hand, if the upper limit of conditional expression (5) is exceeded, the diffraction efficiency at a wavelength of 405 nm is half that of the diffraction efficiency at a wavelength of 780 nm.

  In the present embodiment, by satisfying conditional expression (5), even in a single-layer diffraction grating, the diffraction efficiency for the three wavelengths can be balanced, and the light utilization efficiency can be increased.

  More preferably, the conditional expression (5) is set as follows.

0.84 × λa × m / (n−1) <H <0.94 × λa × m / (n−1) (5a)
FIG. 9 is a graph showing the diffraction efficiency of Example 1 of the present invention. In FIG. 9, the horizontal axis represents wavelength and the vertical axis represents diffraction efficiency.

  In FIG. 9, the average wavelength is λa = 590 nm (strictly 592 nm), and the grating height H = 0.697 μm when the first-order diffracted light is used. Balances diffraction efficiency. At this time, by setting H = 0.89 × λa × m / (n−1), the diffraction efficiency at a wavelength of 405 nm and a wavelength of 650 nm is balanced at 67%.

  At this time, the diffraction efficiency at a wavelength of 650 nm is 87%. In actual design, it is necessary to consider the coupling efficiency of the optical system and the laser light source, the optical system transmittance, the sensitivity of the detection sensor, and the like so that the optimum grating height satisfies the above conditional expression (6).

  As described above, in this embodiment, a diffraction grating is provided on one lens surface of a single objective lens as described above, and axial chromatic aberration is corrected better than laser light of other wavelengths with respect to laser light of the shortest wavelength. ing. In this way, in the chromatic aberration that occurs in principle, the objective lens for an optical pickup device that supports three types of optical discs corresponding to the depth reduction in a small spot by setting the wavelength with the smallest occurrence of chromatic aberration to the shortest wavelength side. Can be obtained.

  FIGS. 10A, 10B, and 10C are optical path diagrams at wavelengths of Example 2 of the present invention.

  The difference between the present embodiment and the first embodiment is that the thickness of the protective layer and the numerical aperture NA of the blue laser light are different. Other configurations and optical actions are substantially the same as those in the first embodiment, and the same effects are obtained.

  The numerical values of the configuration of the present embodiment are shown in Table 2 as Numerical Embodiment 2 described later.

  That is, in FIGS. 10A, 10B, and 10C, reference numeral 61 denotes an objective lens, which has a diffraction grating on the first surface (light incident surface), and has the first surface and the second surface (light emission). Both surfaces are aspherical. 62 is a protective layer (transparent substrate), and 63 is a diaphragm.

  In FIG. 10A, the thickness of the protective layer 62 is d = 1.2 mm, the wavelength of the laser beam (infrared laser beam) is λ = 780 nm, and the numerical aperture of the objective lens 61 is NA = 0.45.

  In FIG. 10B, the thickness of the protective layer 62 is d = 0.6 mm, the wavelength of the laser light (red laser light) is λ = 650 nm, and the numerical aperture of the objective lens 61 is NA = 0.65.

  In FIG. 10C, the thickness of the protective layer 62 is d = 0.6 mm, the wavelength of the laser beam (blue laser beam) is λ = 405 nm, and the numerical aperture of the objective lens 61 is NA = 0.65.

  11A, 11B, and 11C are spherical aberration diagrams at the respective wavelengths in Example 2 of the present invention.

  11A shows a case where the wavelength λ of the laser light shown in FIG. 10A is 780 nm and NA = 0.45. FIG. 11B shows a case where the wavelength λ of the laser light shown in FIG. In the case of 0.65, FIG. 11C shows the case where the wavelength of the laser beam shown in FIG. 10C is λ = 405 nm and NA = 0.65.

  As can be seen from FIGS. 11A, 11B, and 11C, images are favorably formed with respect to different substrate thicknesses. The distance between the objective lens and the optical disk is the narrowest when the substrate thickness is 1.2 mm when the wavelength is 780 nm, but it is still a sufficient amount because it is 0.54 mm.

`` Numerical examples ''
Numerical examples from the objective lens of the first and second embodiments of the present invention to the image plane (information recording surface 22 of the optical disc 20) will be described below. In the table, r is a radius of curvature, d is a surface interval, and n is a refractive index at each wavelength. In addition, the table whose interval varies depending on the wavelength is summarized in a separate table.

  In the aspherical shape, the distance (sag amount) from the tangential plane on the aspherical optical axis of the coordinate point on the aspherical surface where the height from the optical axis is h is X (h), and on the aspherical optical axis Where the curvature (1 / r) is C, the conic coefficient is K, the fourth, sixth, eighth, tenth, and twelfth aspheric coefficients are A4, A6, A8, A10, and A12. expressed.

  In Table 1, the radius of curvature of the aspheric surface is the radius of curvature on the optical axis. Table 2 shows the conical coefficient and aspheric coefficient that define the aspheric surface, and the optical path difference function coefficient that defines the diffractive lens structure.

The added amount of the optical path length due to the structure of the diffraction grating is obtained by using the height h from the optical axis, the optical path difference function coefficient Cn of the nth order (even order), and the wavelength λ.
φ (h) = (C1 · h ² + C2 · h ⁴ + C3 · h ⁶ + ...) × 2π / λ
It is represented by an optical path difference function φ (h) defined by
(Numerical example)
Table 1 shows numerical examples of the first embodiment of the present invention shown in FIG.

  Table 2 shows numerical examples of the second embodiment of the present invention shown in FIG.

1 is a schematic diagram of a main part of an optical pickup device according to a first embodiment of the present invention. Explanatory drawing of the single layer type diffraction grating of Example 1 of this invention Optical path diagram at each wavelength when the present invention is not implemented Aberration diagram at each wavelength when the present invention is not implemented Variation diagram of spherical aberration for each wavelength when the present invention is not implemented Optical path diagram at each wavelength of Example 1 of the present invention Aberration diagram at each wavelength of Example 1 of the present invention Variation diagram of spherical aberration with respect to each wavelength in Example 1 of the present invention Explanatory drawing of the diffraction efficiency of Example 1 of this invention Optical path diagram at each wavelength of Example 2 of the present invention Aberration diagram at each wavelength of Example 2 of the present invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Objective lens 2 Collimator lens 3 Aperture 4 Optical axis 6 Polarizing beam splitter 7 Beam splitter 11 Semiconductor laser 12 Hybrid semiconductor laser 13 Photo detector 13a Actuator 20 Optical disk 21 Transparent substrate 22 Information recording surface

Claims (11)

An objective lens for imaging laser beams of different colors with different wavelengths on a predetermined surface,
The objective lens is composed of a single lens having a positive power, has a diffraction grating on at least one of the light incident / exit surfaces, has a numerical aperture of 0.65 or more at the shortest wavelength, and An objective lens characterized in that axial chromatic aberration is smaller than laser light of other wavelengths with respect to laser light of the shortest wavelength.   The objective lens according to claim 1, wherein at least one of the objective lenses has an aspherical shape, and the diffraction grating has a positive power. The refractive index of the material at the shortest wavelength of the objective lens is n, the power other than the diffraction grating at the shortest wavelength of the objective lens is φ, the refractive index change with respect to a temperature of the objective lens is dn / dt, and the refractive index at the shortest wavelength is When the numerical aperture of the objective lens is NA and the shortest wavelength is λ,
The objective lens according to claim 1, wherein the following condition is satisfied.   The objective lens according to claim 1, wherein the objective lens is made of glass and is formed by glass molding. When the power of the diffraction grating is Φd and the power of the entire objective lens including the diffraction grating is Φt,
0.05 <Φd / Φt <0.5
The objective lens according to claim 1, wherein the following condition is satisfied. The height of the grating part of the diffraction grating is H, the diffraction order is m, the refractive index of the material at the shortest wavelength of the objective lens is n, and the average wavelength of the shortest and longest wavelengths of the plurality of laser beams having different wavelengths is λa. And when
0.74 × λa × m / (n−1) <H <0.99 × λa × m / (n−1)
The objective lens according to claim 1, wherein the following condition is satisfied. The refractive index of the material at the shortest wavelength of the objective lens is n, the power other than the diffraction grating at the shortest wavelength of the objective lens is φ, the refractive index change with respect to the wavelength of 1 nm of the objective lens is dn / dλ, the shortest wavelength is λ, When the amount of change in axial chromatic aberration with respect to the wavelength change of the diffraction grating surface of the objective lens is SO

The objective lens according to claim 1, wherein the following condition is satisfied.   An objective lens according to any one of claims 1 to 7, and a member for recording information on and / or reproducing information recorded on the optical information recording medium using the objective lens. An optical pickup device characterized by that.   The objective lens corresponds to at least three types of optical information recording media having at least two types of protective layers having different thicknesses, and laser beams of a plurality of colors having different wavelengths are used for the at least three types of optical information recording media. 9. The optical pickup device according to claim 8, wherein an image is formed on an information recording surface of the optical information recording medium.   The laser beams of a plurality of colors having different wavelengths are laser beams of three wavelengths of blue, red, and infrared, and the shortest wavelength is a wavelength of blue laser light. 10. The optical pickup device according to claim 9, wherein the laser beam has a value of 0.45. When the longitudinal chromatic aberration at the wavelength of the blue laser beam of the objective lens is L1, the longitudinal chromatic aberration at the wavelength of the red laser beam is L2, and the longitudinal chromatic aberration at the wavelength of the infrared laser beam is L3, L1 <L2 <L3
The optical pickup device according to claim 10, wherein the following condition is satisfied.