JP2007128654A - Multi-focal object lens, optical pickup device and optical information recording/reproducing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multi-focal objective lens capable of forming a focal point on the information recording surfaces of two high density optical disks of different standards mutually different in protection layer thickness. <P>SOLUTION: The multi-focal objective lens used in an optical pickup device having a first diffraction structure on one optical plane and condensing an m-th diffraction order light and an n-th diffraction order light (m≠n) generated when a light flux at wavelength λ<SB>1</SB>enters the first diffraction structure onto the information recording surfaces of two kinds of optical disks mutually different in protection layer thickness, wherein the absolute value of the difference between the spherical aberration value SA<SB>m</SB>of a wave front when the m-th diffraction order light is condensed via the protection layer of an optical disk having a thicker protection layer and the spherical aberration value SA<SB>n</SB>of a wave front when the n-th diffraction order light is condensed via the protection layer of an optical disk having a thicker protection layer and the absolute value of either one of the SA<SB>m</SB>and SA<SB>n</SB>are specified values, the above first diffraction structure is composed of a plurality of annular zones divided by fine step differences, and the above diffraction order m is an integer of one or larger and satisfies a specified expression. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a multifocal objective lens, an optical pickup device, and an optical information recording / reproducing device.

  In recent years, in an optical pickup device, a laser light source used as a light source for reproducing information recorded on an optical disc and recording information on the optical disc has been shortened. For example, a blue-violet semiconductor laser, A laser light source having a wavelength of 405 nm such as a blue-violet SHG laser that performs wavelength conversion of an infrared semiconductor laser using harmonic generation is being put into practical use.

  When these blue-violet laser light sources are used, when an objective lens having the same numerical aperture (NA) as that of a digital versatile disk (hereinafter abbreviated as DVD) is used, information of 15 to 20 GB is recorded on an optical disk having a diameter of 12 cm. Recording becomes possible, and when the NA of the objective lens is increased to 0.85, 23 to 27 GB of information can be recorded on an optical disk having a diameter of 12 cm. Hereinafter, in this specification, an optical disk and a magneto-optical disk using a blue-violet laser light source are collectively referred to as a “high density optical disk”.

  By the way, two standards are currently proposed as high-density optical disks. One is a Blu-ray disc (hereinafter abbreviated as BD) with an NA0.85 objective lens and a protective layer thickness of 0.1 mm, and the other is an NA0.65 to 0.67 objective lens. This is an HD DVD (hereinafter abbreviated as HD) having a protective layer thickness of 0.6 mm. In view of the possibility that these two standard high-density optical discs will be distributed in the market in the future, a high-density optical disc player / recorder capable of recording / reproducing on any high-density optical disc is desired.

  Patent Documents 1 and 2 below describe bifocal objective lenses that can perform recording / reproduction compatible with two types of optical discs having different protective layer thicknesses and the same light source wavelength.

The bifocal objective lens disclosed in two patent documents records / reproduces optical discs with different protective layer thicknesses by allocating much of the amount of incident light to the two focal points by means of a diffractive structure formed on the lens surface. Is to do.
JP-A-9-179020 Japanese Patent Laid-Open No. 9-120027

  However, the above-mentioned bifocal objective lens assumes a DVD with NA of 0.6 and a compact disc with NA of about 0.45 (hereinafter abbreviated as CD) as optical disks to be recorded / reproduced. Therefore, recording / playback cannot be performed on BD and HD having a large NA.

  An object of the present invention is to consider the above-mentioned problem, and a multifocal objective lens capable of forming a focal point on an information recording surface of two high-density optical discs having different protective layer thicknesses. It is an object of the present invention to provide an optical pickup device and an optical information recording / reproducing device used.

  In order to solve the above problems, the configuration according to the present invention is as follows.

Configuration according to claim 1 has a first diffractive structure at least on one optical surface, m order diffracted light and n-order diffracted light generated when the light flux of wavelength lambda ₁ is incident on the first diffraction structure (m ≠ n) is a multifocal objective lens used in an optical pickup device that collects light on the information recording surfaces of two types of optical discs having different protective layer thicknesses,
The spherical aberration SAm of the wave front when the _m- th order diffracted light is condensed through the protective layer of the thicker protective layer and the n-th order diffracted light through the protective layer of the thicker protective layer. The absolute value | SA _m −SA _n | of the difference from the spherical aberration value SA _n of the wavefront when the light is condensed is 0.9λ ₁ RMS or more, and one of SA _m and SA _n One absolute value is 0.07λ ₁ RMS or less,
The first diffractive structure is composed of a plurality of annular zones divided by fine steps, and the diffraction order m is an integer of 1 or more and satisfies the following expression (1):
The refractive index N of the multifocal objective lens with respect to the wavelength λ ₁ is in the range of 1.5 to 1.6, and the height d ₁ (μm) of the step closest to the optical axis among the steps is as follows: A multifocal objective lens characterized by satisfying any one of the expressions (2) to (4).

n = m−1 (1)
0.27 <d ₁ <0.48 (2)
1.01 <d ₁ <1.24 (3)
1.72 <d ₁ <2.02 (4)
In an objective lens for BD having a wavelength of 405 nm, NA of 0.85, and a protective layer thickness of 0.1 mm, if the protective layer thickness is 0.6 mm corresponding to HD, 0.9λRMS (λ = 405 nm) or more within NA 0.65 Spherical aberration occurs. Therefore, as in the configuration described in item 1, most of the light amount of the incident light beam having the wavelength λ ₁ is divided into the m-th order diffracted light and the n-th order diffracted light, and the m-th order diffracted light is condensed through the HD protective layer. a spherical aberration value SA _m of the wavefront at the time, n-order diffracted light through the protective layer of HD absolute value of the difference between the spherical aberration value SA _n of the wave front when was condensed | SA _m -SA _n | 0 and .9λ ₁ RMS or more, of the SA _m and SA _n, either one of the absolute value such that 0.07λ ₁ RMS or less, by determining the spherical aberration characteristics of the diffractive structure, the protection of the BD and HD It is possible to correct spherical aberration due to the difference in layer thickness and to form a good wavefront on the information recording surface of any high-density optical disc. In order to achieve this effect, | SA _m −SA _n | is set to 1.0λ ₁ RMS or more, and the absolute value of one of SA _m and SA _n is set to 0.05λ ₁ RMS or less. More preferably. Further, in order to cope with a case where the NA of HD becomes large in the future, it is preferable that | SA _m −SA _n | be 1.2λ ₁ RMS or more.

The condition concerning the spherical aberration of the two focal points is as follows: “The spherical aberration value SA _m of the wavefront when the m-th order diffracted light is condensed through the protective layer of the optical disk having the thinner protective layer, and the n” The absolute value | SA _m −SA _n | of the difference from the spherical aberration value SA _n of the wavefront when the next-order diffracted light is condensed through the protective layer of the optical disk having the thinner protective layer is 4.4λ ₁ RMS or more. In other words, the absolute value of one of SA _m and SA _n is 0.07λ ₁ RMS or less ”.

  In the configuration of Item 2, the combination of the diffraction order m and the diffraction order n is any one of (m, n) = (1, 0), (2, 1), (3, 2) The multifocal objective lens described in 1.

Item 3 is the multifocal objective lens for the height d ₁ (μm), the wavelength λ ₁ (μm), and the wavelength λ ₁ of the step closest to the optical axis among the steps of the first diffractive structure. The multifocal objective lens according to Item 1, wherein the refractive index N ₁ satisfies the following expression (5).

0.4 ≦ | INT (X) −X | ≦ 0.5 (5)
However, X = d ₁ · (N ₁ −1) / λ ₁ , and INT (X) is an integer closest to X.

  As in item 1, the first diffractive structure is schematically shown in FIGS. 1 (a), 1 (b), 2 (a), 2 (b), 4 (a), and 4 (b). In order to obtain a high diffraction efficiency for both the m-th order diffracted light and the n-th order diffracted light, the difference between m and n is set to 1 in the case where the structure is composed of a plurality of annular zones divided by fine steps as shown in As in item 2, the combination of the diffraction order m and the diffraction order n may be any one of (m, n) = (1, 0), (2, 1), (3, 2). preferable. There are an infinite number of combinations of the diffraction order m and the diffraction order n for obtaining high diffraction efficiency for both the m-th order diffracted light and the n-th order diffracted light. It is most preferable that the diffraction order m is 3 or less because the reduction in diffraction efficiency when the luminous flux changes increases.

Further, when the combination of the diffraction order m and the diffraction order n is (m, n) = (1, 0) as in item 2, the height d ₁ (μm) of the step closest to the optical axis is It is necessary to design so as to satisfy the formula (2), and when the combination of the diffraction order m and the diffraction order n is (m, n) = (2,1), the height of the step closest to the optical axis It is necessary to design so that d ₁ (μm) satisfies the expression (3), and when the combination of the diffraction order m and the diffraction order n is (m, n) = (3, 2), It is necessary to design such that the closest step height d ₁ (μm) satisfies the equation (4). In other words, if the height d ₁ (μm) of the step closest to the optical axis is within the range of the expression (2), the 1st-order diffracted light and the 0th-order diffracted light are used as diffracted light for recording / reproducing BD and HD. If the height d ₁ (μm) of the step closest to the optical axis is within the range of the expression (3), the second-order diffracted light and the first-order diffracted light are used for recording / reproducing BD and HD. If the height d ₁ (μm) of the step closest to the optical axis can be determined to be used as diffracted light and is within the range of the formula (4), the third-order diffracted light and the second-order diffracted light are recorded in BD and HD. / It can be determined that it is used as diffracted light for reproduction.

As in item 3, by setting the height d _{1 of the} step closest to the optical axis within the range satisfying the expression (5), the above d ₁ is approximately (q−0.5) times the wavelength λ _1. Will be set. Here, q is a natural number. As a result, most of the light amount of the light beam having the wavelength λ ₁ incident on the first diffractive structure is distributed to the two diffracted lights of the m-th order diffracted light and the n-th order diffracted light (where n = m−1). A large amount of diffracted light for recording / reproducing BD and HD can be secured.

Item 4 is the multifocal objective lens according to any one of Items 1 to 3, wherein the wavelength λ ₁ is 450 nm or less.

  In the configuration of Item 5, the diffracted light having the larger diffraction order out of the m-th order diffracted light and the n-th order diffracted light is condensed on the information recording surface of the optical disc having the thicker protective layer, and the diffraction order is smaller. The multifocus according to any one of Items 1 to 4, wherein the diffracted light is condensed on the information recording surface of the optical disk having the thinner protective layer, and the diffractive power in the paraxial axis of the first diffractive structure is negative. It is an objective lens.

  As in Item 5, the diffracted light having the larger diffraction order of the m-th order diffracted light and the n-th order diffracted light is condensed on the information recording surface of the thicker protective layer, for example, the HD, and the smaller diffraction order. Is designed such that the diffracted light of the first diffractive light is condensed on the information recording surface of the optical disk having the thinner protective layer, for example, the BD, and the diffractive power on the paraxial axis of the first diffractive structure is negative. The working distance of the multifocal objective lens for HD having a protective layer thicker than BD can be secured sufficiently.

The configuration according to Item 6, in the multifocal objective lens, a second diffractive structure including a plurality of annular zones having a stepped structure on an optical surface different from the optical surface having the first diffractive structure. further comprising a second diffractive structure, the the wavelength lambda ₁ of the light beam substantially without giving a phase difference, to claim 1 provides a phase difference to the light flux of the wavelength _{λ 2 (λ 2> λ 1} ) 5. The multifocal objective lens according to any one of 5 above.

In the configuration according to Item 7, in the second diffractive structure, the division number M ₂ of each annular zone, the height D ₂ (μm) of steps formed in each annular zone, the wavelength λ ₁ (μm ), The multifocal objective lens according to Item 6, wherein the refractive index N ₁ of the multifocal objective lens with respect to the wavelength λ ₁ substantially satisfies the following expression (8).

D ₂ · (N ₁ -1) / λ ₁ = 2 · q2 (8)
However, q2 is a natural number, M ₂ is either 4, 5, 6.

Currently, many DVDs are distributed in the market as video distribution media, so high-density optical disc players / recorders are also required to be compatible with DVDs. According to Item 6, the second diffractive structure formed on the optical surface different from the optical surface on which the first diffractive structure is formed does not substantially give a phase difference to the light beam having the wavelength λ ₁ , and the wavelength λ ₂ (λ ₂ > λ ₁ ) is given a phase difference, so that a diffraction effect can be imparted only to a light beam of wavelength λ ₂ used for optical discs other than high-density optical discs (for example, DVDs). It is possible to correct spherical aberration due to the difference in the protective layer thickness between the density optical disc and the DVD and spherical aberration due to the difference in the used wavelength. This makes it possible to record / reproduce information in a compatible manner using a common objective lens for high-density optical discs and DVDs having different protective layer thicknesses such as BD and HD.

Specifically, as described in item 7, by setting the height D ₂ of the staircase within a range that satisfies the above equation (8), the height D ₂ of the staircase is approximately a multiple of the wavelength λ _1. Will be set. When a light beam having a wavelength λ ₁ is incident on the staircase structure in which the height of the stairs is set in this way, an optical path difference of 2 × q2 × λ ₁ (μm) is generated between adjacent stairs. since substantially so that the phase difference is not given to the light flux of the wavelength lambda _1, the incident light beam of wavelength lambda ₁ is directly transmitted without being diffracted by the second diffractive structure.

On the other hand, a phase difference corresponding to the depth of the step and the number of divisions is given to the light beam having the wavelength λ ₂ and is subjected to a diffraction action, but the division number M ₂ of each annular zone is set to any one of 4, 5, and 6. By setting, diffracted light of wavelength λ ₂ having high diffraction efficiency can be obtained, and information can be recorded / reproduced with respect to the DVD by using this diffracted light.

The configuration according to Item 8 is the multifocal objective lens according to any one of Items 6 to 7, wherein the wavelength λ ₂ is in the range of 0.63 μm to 0.68 μm.

  According to Item 8, it is possible to record / reproduce information in a compatible manner using a common objective lens for high-density optical discs and DVDs having different protective layer thicknesses such as BD and HD.

The configuration according to Item 9, wherein the multifocal objective lens includes a third diffraction pattern formed of a plurality of annular zones divided by fine steps on an optical surface different from the optical surface having the first diffraction structure. The third diffractive structure further has a wavelength λ ₂ (λ ₂₎ than the diffraction order of the diffracted light having the maximum diffraction efficiency among the diffracted light generated when the light flux having the wavelength λ ₁ is incident. The multifocal objective lens according to any one of Items 1 to 8, wherein the diffraction order of the diffracted light having the maximum diffraction efficiency among the diffracted light generated when a light beam of> λ ₁ ) is incident is lower.

In the configuration of Item 10, in the third diffractive structure, the diffraction order of the diffracted light having the maximum diffraction efficiency out of the diffracted light generated when the light beam having the wavelength λ ₁ is incident is second order, and the wavelength λ Item 10. The multifocal objective lens according to Item 9, wherein the diffraction order of the diffracted light having the maximum diffraction efficiency among the diffracted light generated when the _second light beam is incident is the first order.

The configuration according to Item 11 is such that, in the third diffractive structure, the diffraction order of the diffracted light having the maximum diffraction efficiency among the diffracted lights generated when the light beam having the wavelength λ ₁ is incident is the third order, and the wavelength λ Item 10. The multifocal objective lens according to Item 9, wherein the diffracted light having the highest diffraction efficiency among the diffracted light generated when the _second light beam is incident has a second-order diffraction order.

As in Item 9, a third diffractive structure composed of a plurality of annular zones divided by fine steps is provided on an optical surface different from the optical surface having the first diffractive structure, and a light beam having a wavelength λ ₁ The largest of the diffracted light generated when a light beam having a wavelength λ ₂ (λ ₂ > λ ₁ ) is incident, compared to the diffraction order i of the diffracted light having the maximum diffraction efficiency among the diffracted light generated when the light is incident. by direction of diffraction orders j of the diffracted light becomes diffraction efficiency keep designing the third diffractive structure to be lower, the wavelength lambda ₁ of the i any of the diffraction efficiency of the diffracted light and the wavelength lambda ₂ of the j-order diffracted light In addition, it is possible to correct spherical aberration due to the difference in the protective layer thickness between the high-density optical disc and the DVD and spherical aberration due to the difference in use wavelength by using the diffraction action. Become.

  As in the terms 10 and 11, the combination of the diffraction order i and the diffraction order j is preferably (i, j) = (2,1) or (3,2). There are innumerable combinations of the diffraction order i and the diffraction order j for obtaining high diffraction efficiency for both the i-th order diffracted light and the j-th order diffracted light, but there are innumerable combinations other than those described above. The diffraction order i is most preferably set to 3 or 2 because the diffraction efficiency decreases greatly when the luminous flux changes.

The configuration described in Item 12 is the multifocal objective lens according to any one of Items 6 to 11, wherein the wavelength λ ₂ is in a range of 0.63 μm to 0.68 μm.

Configuration according to claim 13, having a first diffractive structure at least on one optical surface, m order diffracted light and n-order diffracted light generated when the light flux of wavelength lambda ₁ is incident on the first diffraction structure (m ≠ n) is a multifocal objective lens used in an optical pickup device that collects light on the information recording surfaces of two types of optical discs having different protective layer thicknesses,
The spherical aberration SAm of the wave front when the _m- th order diffracted light is condensed through the protective layer of the thicker protective layer and the n-th order diffracted light through the protective layer of the thicker protective layer. The absolute value | SA _m −SA _n | of the difference from the spherical aberration value SA _n of the wavefront when the light is condensed is 0.9λ ₁ RMS or more, and one of SA _m and SA _n One absolute value is 0.07λ ₁ RMS or less,
The first diffractive structure is a diffractive structure composed of a plurality of annular zones having a staircase structure therein, and the diffraction order m is an integer of 1 or more and satisfies the following expression (6):
The height D ₁ of the staircase formed in each ring-shaped zone of the first diffractive structure ([mu] m), the wavelength lambda ₁ ([mu] m), the refractive index N ₁ is less than or equal to the multi-focus objective lens with respect to the wavelength lambda ₁ It is a multifocal objective lens that satisfies the formula (7).

n = -m (6)
D ₁ · (N ₁ −1) / λ ₁ = q 1−0.5 (7)
However, q1 is a natural number.

  In the configuration according to item 14, the combination of the diffraction order m and the diffraction order n is any one of (m, n) = (1, −1), (2, −2), and (3, −3). Item 14. The multifocal objective lens according to Item 13.

  As in Item 13, when the first diffractive structure is a structure composed of a plurality of annular zones having stepped structures inside as schematically shown in FIGS. 3 (a) and 3 (b), In order to obtain high diffraction efficiencies for both the m-th order diffracted light and the n-th order diffracted light, the absolute values of m and n should be the same, and their signs reversed. As in item 14, the diffraction order m and the diffraction order n The combination is preferably any one of (m, n) = (1, −1), (2, −2), and (3, −3). There are an infinite number of combinations of the diffraction order m and the diffraction order n for obtaining high diffraction efficiency for both the m-th order diffracted light and the n-th order diffracted light. It is most preferable that the diffraction order m is 3 or less because the reduction in diffraction efficiency when the luminous flux changes increases.

Structure of claim 15, wherein, in the first diffractive structure is a multi-focus objective lens according to claim 13 division number M ₁ of each ring-shaped zone is even.

As in item 13, by setting the height D ₁ of the staircase within the range satisfying the expression (7), the above D ₁ is set to a depth approximately (q1-0.5) times the wavelength λ _1. It will be. Here, q1 is a natural number. Thereby, most of the light quantity of the light beam having the wavelength λ ₁ incident on the first diffractive structure is distributed to two diffracted lights of m-order diffracted light and n-order diffracted light (where n = −m). As shown in FIG. 15, by setting the division number M ₁ of each annular zone to an even number, it is possible to secure the largest amount of diffracted light for recording / reproducing BD and HD.

Item 16 is the multifocal objective lens according to any one of Items 13 to 15, wherein the wavelength λ ₁ is 450 nm or less.

  The configuration described in Item 17 is such that the diffracted light having the larger diffraction order of the m-th order diffracted light and the n-th order diffracted light is condensed on the information recording surface of the optical disc having the thicker protective layer, and the smaller diffraction order. The multifocal objective lens according to any one of Items 13 to 16, wherein the diffracted light is condensed on the information recording surface of the optical disk having the thinner protective layer, and the diffractive power in the paraxial axis of the first diffractive structure is negative. .

Item 18 is a configuration in which the multifocal objective lens includes a second diffractive structure including a plurality of annular zones having a step structure on an optical surface different from the optical surface having the first diffractive structure. Further, the second diffractive structure does not substantially give a phase difference to the light beam having the wavelength λ ₁ , and gives a phase difference to the light beam having the wavelength λ ₂ (λ ₂ > λ ₁ ). The multifocal objective lens according to any one of the above.

In the configuration according to Item 19, in the second diffractive structure, the division number M ₂ of each annular zone, the height D ₂ (μm) of steps formed in each annular zone, and the wavelength λ ₁ (μm) The multifocal objective lens according to Item 18, wherein a refractive index N ₁ of the multifocal objective lens for the wavelength λ ₁ substantially satisfies the following expression (8):

D ₂ · (N ₁ -1) / λ ₁ = 2 · q2 (8)
However, q2 is a natural number, M ₂ is either 4, 5, 6.

Item 20 is the multifocal objective lens according to Item 18 or 19, wherein the wavelength λ ₂ is in the range of 0.63 μm to 0.68 μm.

Item 21 is a third diffractive structure in which the multifocal objective lens includes a plurality of annular zones divided by fine steps on an optical surface different from the optical surface having the first diffractive structure. And the third diffractive structure has a wavelength λ ₂ (λ ₂ >) greater than the diffraction order of the diffracted light having the maximum diffraction efficiency among the diffracted light generated when the light flux having the wavelength λ ₁ is incident. Item 18. The multifocal objective lens according to any one of Items 13 to 17, wherein the diffraction order of the diffracted light having the maximum diffraction efficiency among the diffracted light generated when the light beam of λ ₁ ) is incident is lower.

The configuration according to Item 22 is that, in the third diffractive structure, the diffraction order of the diffracted light having the maximum diffraction efficiency out of the diffracted light generated when the light beam having the wavelength λ ₁ is incident is second order, and the wavelength λ ₂ Item 22. The multifocal objective lens according to Item 21, wherein the diffraction order of the diffracted light that has the maximum diffraction efficiency among the diffracted light generated when the light beam enters is first order.

The configuration according to Item 23 is that, in the third diffractive structure, the diffraction order of the diffracted light having the maximum diffraction efficiency out of the diffracted light generated when the light beam having the wavelength λ ₁ is incident is the third order, and the wavelength λ ₂ Item 22. The multifocal objective lens according to Item 21, wherein the diffraction order of the diffracted light having the maximum diffraction efficiency among the diffracted light generated when the light beam enters is second order.

The configuration according to Item 24 is the multifocal objective lens according to any one of Items 18 to 23, wherein the wavelength λ ₂ is in a range of 0.63 μm to 0.68 μm.

Item 25 is an optical pickup device for recording / reproducing information on a first optical disc having a protective layer thickness t ₁ and a second optical disc having a protective layer thickness t ₂ (t ₁ <t ₂ ). Any one of Items 1 to 24, and a first light source that emits a first light beam having a wavelength λ ₁ (350 nm ≦ λ ₁ ≦ 450 nm) used when recording / reproducing information on the first and second optical disks. When the first optical disk is recorded / reproduced, the m-th order diffracted light is included in the multifocal objective lens described in 1) and a driving unit that drives the objective lens in two directions for tracking and focusing. When one of the n-th order diffracted light and the n-th order diffracted light is condensed on the information recording surface of the first optical disc and the second optical disc is recorded / reproduced, The first optical disc And when recording / reproducing is an optical pickup device for condensing the diffracted light of a different person on the information recording surface of the second optical disk.

Item 26 is a configuration in which a first optical disc having a protective layer thickness t ₁ , a second optical disc having a protective layer thickness t ₂ (t ₁ <t ₂ ), and the first and second optical discs having a protective layer thickness t _3. An optical pickup apparatus for recording / reproducing information on a third optical disc of a different type from the first, and having a wavelength λ ₁ (λ ₁ ≦ 450 nm) for recording / reproducing of the first and second optical discs. A first light source that emits a light beam; a second light source that emits a second light beam having a wavelength λ ₂ (630 nm <λ ₂ <680 nm) for recording / reproduction of the third optical disk; Item 25. The multifocal objective lens according to any one of Items 1 to 24, wherein the light beam is condensed on the information recording surfaces of the first to third optical discs, and two directions for focusing and tracking the objective lens Drive to The objective lens is composed of a plurality of annular zones divided by steps, and the incident first light beam is converted into m-order diffracted light and n-order diffracted light (m ≠ n). A first optical surface on which a first diffractive structure to be generated is formed;
A second optical surface formed with a plurality of annular zones divided by steps and formed with a second diffractive structure for generating the incident second light flux as v-order diffracted light, and recording on the first optical disc When reproducing / reproducing, when one of the m-th order diffracted light and the n-th order diffracted light is condensed on the information recording surface of the first optical disc and recording / reproducing the second optical disc, The third optical disc is focused on the information recording surface of the second optical disc with a different order of the m-order diffracted light and the n-th diffracted light when recording / reproducing the first optical disc. When recording / reproducing is performed, the optical pickup device condenses the v-th order diffracted light generated from the second light flux on the information recording surface of the third optical disc.

The configuration according to Item 27 includes a first optical disc having a protective layer thickness t ₁ , a second optical disc having a protective layer thickness t ₂ (t ₁ <t ₂ ), and the first and second optical discs having a protective layer thickness t _3. An optical pickup device for recording / reproducing information on a third optical disc of a different type from the first optical disc and a fourth optical disc having a protective layer thickness t ₄ (t ₃ <t ₄ ), and recording on the first and second optical discs A first light source that emits a first light beam having a wavelength λ ₁ (λ ₁ ≦ 450 nm) for reproduction, and a wavelength λ ₂ (630 nm <λ ₂ <680 nm) for recording / reproduction of the third optical disc. A second light source that emits a second light beam, a third light source that emits a third light beam having a wavelength λ ₃ (λ ₂ <λ ₃ ) for recording / reproducing information on the fourth optical disk, Information on the first to fourth optical discs through the first to third light beams Item 25. The objective lens according to any one of Items 1 to 24, which is focused on a recording surface, and driving means for driving in two directions for performing focusing and tracking of the objective lens, the objective lens Is composed of a plurality of annular zones divided by steps, and a first optical surface on which a first diffractive structure for generating the incident first light beam as m-th order diffracted light and n-th order diffracted light (m ≠ n) is formed; And a second optical surface formed with a plurality of annular zones divided by steps and formed with a second diffractive structure for generating the incident second light beam as v-order diffracted light, and recording on the first optical disc When reproducing / reproducing, when one of the m-th order diffracted light and the n-th order diffracted light is condensed on the information recording surface of the first optical disc and recording / reproducing the second optical disc, The m next time Of the light and the n-th order diffracted light, the light of the order different from that when recording / reproducing the first optical disc is condensed on the information recording surface of the second optical disc, and recording / reproducing of the third optical disc is performed. Is performed, the v-th order diffracted light generated from the second light flux is condensed on the information recording surface of the third optical disk, and when recording / reproducing the fourth optical disk, the light is emitted from the third light source. In the optical pickup device, the third light beam thus made incident on the objective lens as a divergent light beam and condensed on the information recording surface of the fourth optical disk.

  According to the items 25 to 27, an optical pickup device having an effect similar to that of any one of the items 1 to 24 can be obtained.

  Item 28 is an optical information recording / reproducing apparatus equipped with the optical pickup device according to any one of Items 25 to 27.

  According to Item 28, an optical information recording / reproducing apparatus having an effect similar to that of any one of Items 25 to 27 can be obtained.

  According to the present invention, a multifocal objective lens capable of forming a focal point on an information recording surface of a high-density optical disc of two standards having different protective layer thicknesses, an optical pickup device using the same, and optical information recording / reproducing An apparatus can be provided.

The configuration according to the present invention has a first diffractive structure on at least one optical surface, and m-order diffracted light and n-order diffracted light (m ≠ e) generated when a light beam having a wavelength λ ₁ is incident on the first diffractive structure. n) is a multifocal objective lens used in an optical pickup device for condensing on the information recording surfaces of two types of optical discs having different protective layer thicknesses, the m-th order diffracted light being thicker in the protective layer The spherical aberration value SA _m of the wavefront when condensed through the protective layer of the optical disk and the spherical surface of the wavefront when the n-th order diffracted light is condensed through the protective layer of the thicker optical disk The absolute value | SA _m −SA _n | of the difference from the aberration value SA _n and any one of SA _m and SA _n is a predetermined value, and the first diffractive structure is fine. Consists of a plurality of annular zones divided by steps, the diffraction order m is an integer of 1 or more, and a multi-focus objective lens satisfying a predetermined formula.

  In the present invention, as schematically shown in FIGS. 1 (a) and 1 (b), the diffractive structure is composed of a plurality of annular zones 100, and the cross-sectional shape including the optical axis is a sawtooth shape, As schematically shown in FIGS. 2 (a) and 2 (b), the step 101 is composed of a plurality of annular zones 102 having the same direction within the effective diameter, and the cross-sectional shape including the optical axis is a stepped shape. As shown in FIG. 3 (a) and FIG. 3 (b), there are some, which are composed of a plurality of annular zones 103 in which a staircase structure is formed, FIG. 4 (a), FIG. As schematically shown in FIG. 4 (b), any one of the structures in which the direction of the step 104 is composed of a plurality of annular zones 105 that change in the middle of the effective diameter and the cross-sectional shape including the optical axis is a step shape is used. preferable. 1A to 4B schematically show the case where the diffractive structure is formed on a plane, but the diffractive structure may be formed on a spherical surface or an aspherical surface. 3A and 3B show the case where each annular zone is divided into five, the number of divisions of each annular zone is not limited to this.

  In this specification, a plurality of such as shown in FIG. 1A, FIG. 1B, FIG. 2A, FIG. 2B, FIG. 4A, and FIG. A diffractive structure composed of an annular zone is represented by the symbol “DOE”, and a diffractive structure composed of a plurality of annular zones in which a staircase structure is formed as shown in FIGS. 3 (a) and 3 (b). Is represented by the symbol “HOE”.

  Further, in this specification, the “objective lens” is arranged at a position facing the optical disc in the optical pickup device, and the light beams emitted from the light sources having different wavelengths are recorded on the respective optical discs having different recording densities. An optical system including at least a condensing element having a function of condensing on a surface. The objective optical system may be composed of only the condensing element. In such a case, a diffractive structure is formed on the optical surface of the condensing element.

  Further, when there is an optical element that is integrated with the above-described condensing element and performs tracking and focusing by an actuator, an optical system composed of the optical element and the condensing element is an objective lens. When the objective lens is composed of a plurality of optical elements in this way, a diffractive structure may be formed on the optical surface of the light condensing element. In order to reduce it, it is preferable to form a diffractive structure on the optical surface of an optical element other than the condensing element.

  In this specification, an optical disk using a blue-violet laser light source such as a blue-violet semiconductor laser or a blue-violet SHG laser as a light source for recording / reproducing information is generally referred to as a “high density optical disk” and has an NA of 0.85. In addition to a standard optical disc (for example, BD) with a protective layer thickness of 0.1 mm, information is recorded / reproduced with an objective lens with NA of 0.65 to 0.67. And a standard optical disc (for example, HD DVD) having a protective layer thickness of 0.6 mm is included. In addition to an optical disk having such a protective layer on its information recording surface, an optical disk having a protective film with a thickness of several to several tens of nanometers on the information recording surface, or the thickness of the protective layer or protective film is 0 optical disks are also included. In this specification, the high-density optical disk includes a magneto-optical disk that uses a blue-violet laser light source as a light source for recording / reproducing information.

  Further, in this specification, DVD is a general term for DVD series optical disks such as DVD-ROM, DVD-Video, DVD-Audio, DVD-RAM, DVD ± R, DVD ± RW, etc. , CD-ROM, CD-Audio, CD-Video, CD-R, CD-RW, and other CD series optical discs.

[First Embodiment]
FIG. 5 is a diagram schematically showing a configuration of the first optical pickup device PU1 capable of appropriately recording / reproducing information for both BD and HD. The optical specification of the BD is the wavelength λ ₁ = 408 nm, the thickness t ₁ of the protective layer PL _{1 is} 0.1 mm, and the numerical aperture NA _{1 is} 0.85. The optical specification of the HD is the wavelength λ ₁ = 408 nm, The thickness t ₂ of the protective layer PL2 is 0.6 mm and the numerical aperture NA ₂ is 0.67. However, the combination of the wavelength, the thickness of the protective layer, and the numerical aperture is not limited to this.

The optical pickup device PU1 is for BD / HD in which a blue-violet semiconductor laser LD that emits a light beam of 408 nm and emits a laser beam of 408 nm and a photodetector PD are integrated when information is recorded / reproduced with respect to BD and HD. The laser module LM includes an aberration correction element L1 and a light condensing element L2 having aspherical surfaces on both sides, which has a function of condensing the light beam transmitted through the aberration correction element L1 on the information recording surfaces RL1 and RL2. objective lens OBJ (multi-focus objective lens), the biaxial actuator AC1,1 axis actuator AC2, BD diaphragm STO corresponding to numerical aperture NA ₁ of, and a collimator lens COL.

  In addition to the blue-violet semiconductor laser LD, a blue-violet SHG laser can be used.

Next, the configuration of the objective lens OBJ will be described. FIG. 6 shows a schematic configuration diagram of the objective lens OBJ. The condensing element L2 is a BD-dedicated plastic lens in which spherical aberration correction is optimized with respect to the wavelength λ ₁ and the thickness t _{1 of the} protective layer. The diffractive structure formed on optical surface S1 of the laser module LM side of the aberration correcting element L1 is a plastic lens HOE 1 (first diffractive structure), the thickness t of the thickness t ₁ and the protective layer PL2 of the protective layer PL1 _The diffractive structure DOE1 formed on the optical surface S2 on the optical disc side of the aberration correction element L1 corrects the chromatic aberration of the objective lens OBJ in the blue-violet region. It is a structure to do. The aberration correction element L1 and the condensing element L2 are integrated by joining flange portions formed in the peripheral part.

As shown in FIG. 6, the diffractive structure HOE 1 is composed of a plurality of annular zones, and each annular zone is divided into two steps. The height D ₁ (μm) of the staircase formed in each annular zone of the diffractive structure HOE1 is designed to substantially satisfy the value obtained by substituting q1 = 1 into the following expression (7) ′. Here, N ₁ is a refractive index with respect to the wavelength λ ₁ of the aberration correction element L1.

D ₁ = (q 1−0.5) · λ ₁ / (N ₁ −1) (7) ′
When a light beam having a wavelength λ ₁ is incident on the staircase structure having the staircase height D ₁ set in this way, an optical path difference of 0.5 × λ ₁ (μm) is given between adjacent staircase structures. Therefore, most of the light amount of the light beam having the wavelength λ ₁ incident on the first diffractive structure is divided into two diffracted lights, a −1st order diffracted light used for BD and a 1st order diffracted light used for HD. However, by making the division number M ₁ of each annular zone of the diffractive structure HOE1 an even number (2 in the present embodiment), the largest amount of diffracted light for recording / reproducing BD and HD can be secured. It becomes possible.

  The width Λ of each annular zone of the diffractive structure HOE1 is such that the diffracted light having the higher diffraction order (first-order diffracted light) is condensed on the HD information recording surface, and the diffracted light having the smaller diffraction order (−1 (Next-order diffracted light) is focused on the information recording surface of the BD, and the paraxial diffraction power is designed to be negative. As a result, it is possible to secure a sufficient working distance for the HD having a thick protective layer.

Incidentally, the diffractive structure HOE1 is because it is formed only in the numerical aperture NA ₂ of HD, the light beam passing through the region outside the NA ₂ becomes a flare component on HD information recording surface RL2, the aperture limitation to HD It is configured to be performed automatically.

The diffraction efficiency of the first-order diffracted light of the light beam having the wavelength λ ₁ generated in the diffractive structure HOE ₁ is 40.5%, and the diffraction efficiency of the −1st-order diffracted light of the light beam having the wavelength λ ₁ is 40.5%.

In addition, as shown in FIG. 6, the diffractive structure DOE1 is a structure composed of a plurality of annular zones having a step shape in cross section including the optical axis. In the diffractive structure DOE1, the height d _A (μm) of the step closest to the optical axis is designed to substantially satisfy the following expression (9). Here, N ₁ is a refractive index with respect to the wavelength λ ₁ of the aberration correction element L1.
d _A = 1 · (N ₁ −1) / λ ₁ (9)
Since the technique for correcting the chromatic aberration of the objective lens OBJ in the blue-violet region by the diffractive structure DOE1 is a known technique, a detailed description is omitted here.

  The diffraction efficiency of the first-order diffracted light generated in the diffractive structure DOE1 is 100%.

  Further, the collimating lens COL of the present embodiment is configured such that its position can be shifted in the optical axis direction by the uniaxial actuator AC2. This makes it possible to correct the spherical aberration of the spots formed on the information recording surfaces of HD and BD, so that good recording / reproducing characteristics can always be maintained for HD and BD.

  The cause of the spherical aberration corrected by adjusting the position of the collimator lens COL is, for example, wavelength variation due to manufacturing error of the blue-violet semiconductor laser LD, refractive index change or refractive index distribution of the objective lens OBJ due to temperature change, two-layer disc, These include focus jumps between layers at the time of recording / reproduction with respect to a multi-layer disc such as a four-layer disc, and thickness variations and thickness distributions due to manufacturing errors of the protective layer PL1.

  When recording / reproducing information with respect to the BD in the optical pickup device PU1, first, the blue-violet semiconductor laser LD is caused to emit light as illustrated by the solid line in FIG. The divergent light beam emitted from the blue-violet semiconductor laser LD is converted into a parallel light beam through the collimating lens COL, the diameter of the light beam is regulated by the stop STO, and the light is condensed by the objective lens OBJ, via the protective layer PL1 of the BD. Thus, the spots are formed on the information recording surface RL1.

  The objective lens OBJ performs focusing and tracking by a biaxial actuator AC1 disposed in the periphery thereof. The reflected light beam modulated by the information pits on the information recording surface RL1 again passes through the objective lens OBJ and the collimator lens COL, and then becomes a converged light beam and converges on the light receiving surface of the photodetector PD. And the information recorded on BD can be read using the output signal of photodetector PD.

When recording / reproducing information with respect to HD in the optical pickup device PU1, first, the blue-violet semiconductor laser LD is caused to emit light, as shown by the dotted line in FIG. The divergent light beam emitted from the blue-violet semiconductor laser LD is converted into a parallel light beam through the collimating lens COL, and is collected on the information recording surface RL2 via the HD protective layer PL2 by being focused by the objective lens OBJ. Become a spot. Here, the diffractive structure HOE1 is because it is formed only in the numerical aperture NA ₂ of HD, so that the opening limitation to HD is automatically performed.

  The objective lens OBJ performs focusing and tracking by a biaxial actuator AC1 disposed in the periphery thereof. The reflected light beam modulated by the information pits on the information recording surface RL2 again passes through the objective lens OBJ and the collimator lens COL, and then becomes a convergent light beam, which converges on the light receiving surface of the photodetector PD. And the information recorded on HD can be read using the output signal of photodetector PD.

  In this embodiment, a blue-violet semiconductor laser LD that emits a 408 nm laser beam that emits light when recording / reproducing information on BD and HD is used. A configuration in which a semiconductor laser is added may be used. In this case, one of the two blue-violet semiconductor lasers emits light when recording / reproducing information with respect to the BD, and the other emits light when recording / reproducing information with respect to the HD.

[Second Embodiment]
FIG. 7 is a diagram schematically showing a configuration of an optical pickup apparatus PU2 that can appropriately record / reproduce information for any of BD, HD, DVD, and CD. The optical specification of the BD is the wavelength λ ₁ = 408 nm, the thickness t ₁ of the protective layer PL _{1 is} 0.1 mm, and the numerical aperture NA _{1 is} 0.85. The optical specification of the HD is the wavelength λ ₁ = 408 nm, The thickness t ₂ of the protective layer PL ₂ = 0.6 mm, the numerical aperture NA ₂ = 0.65, the optical specifications of the DVD are the wavelength λ ₂ = 658 nm, the thickness t ₃ of the protective layer PL _{3 is} 0.6 mm, The numerical aperture NA ₃ = 0.65, the optical specifications of the CD are the wavelength λ ₃ = 785 nm, the thickness t 4 of the protective layer PL ₄ = 1.2 mm, and the numerical aperture NA ₄ = 0.45. However, the combination of the wavelength, the thickness of the protective layer, and the numerical aperture is not limited to this.

The optical pickup device PU2 is for BD / HD, in which a blue-violet semiconductor laser LD1 that emits a light beam of 408 nm and emits a laser beam of 408 nm and a photodetector PD1 are integrated when information is recorded / reproduced with respect to the BD and HD. Laser module LM1, first emission point EP1 that is emitted when recording / reproducing information with respect to DVD and emitting a laser beam of 658 nm and light emitted when recording / reproducing information with respect to CD are 785 nm. The second light emitting point EP2 that emits the luminous flux of the first light, the first light receiving part DS1 that receives the reflected luminous flux from the information recording surface RL3 of the DVD, and the second that receives the reflected luminous flux from the information recording surface RL4 of the CD. A DVD / CD laser module LM2, a first aberration correction element L1, a second aberration correction element L1 ′ composed of a light receiving part DS2 and a prism PS, and this A condensing element L2 having aspherical surfaces on both sides, which has a function of condensing the laser beam transmitted through the first aberration correcting element L1 and the second aberration correcting element L1 ′ onto the information recording surfaces RL1, RL2, RL3, and RL4. An objective lens OBJ, a CD aperture limiting element AP, a biaxial actuator AC1, a stop STO corresponding to the numerical aperture NA _{1 of the} BD, a monoaxial actuator AC2, a polarizing beam splitter BS, a first collimating lens COL1, The second collimating lens COL2 is used.

  In addition to the blue-violet semiconductor laser LD1, a blue-violet SHG laser can also be used.

Next, the configuration of the objective lens OBJ will be described. FIG. 8 shows a schematic configuration diagram of the objective lens OBJ. The condensing element L2 is a glass lens dedicated to BD in which spherical aberration correction is optimized with respect to the wavelength λ ₁ and the thickness t _{1 of the} protective layer. Further, the diffractive structure DOE2 (first diffractive structure) formed on the optical surface S1 (first optical surface) on the laser module LM1 side of the first aberration correction element L1 which is a plastic lens has a thickness t of the protective layer PL1. ₁ is a structure for correcting spherical aberration caused by the difference between the thickness t ₂ of the protective layer PL2 and is formed on the optical surface S2 (second optical surface) on the optical disc side of the first aberration correction element L1. The diffractive structure DOE3 is a structure for correcting chromatic aberration of the objective lens OBJ in the blue-violet region, and is formed on the optical surface S3 on the laser module LM1 side of the second aberration correction element L1 ′ that is a plastic lens. structure HOE 2 (second diffractive structure) is a structure for correcting the spherical aberration due to the difference between the thickness t ₁ and the thickness t ₃ of the protective layer PL3 of the protective layer PL1, the second aberration correcting element L1 of Diffractive structure HOE3 formed on the optical surface S4 in the disk side, a structure for correcting the spherical aberration due to the difference between the thickness t ₁ of the protective layer PL1 and thickness t ₄ of the protective layer PL4. The first aberration correction element L1, the second aberration correction element L1 ′, the condensing element L2, and the aperture limiting element AP are integrated by a joining member B disposed on the side surface.

As shown in FIG. 8, the diffractive structure DOE2 is composed of a plurality of annular zones having a step shape in cross section including the optical axis, and the height d ₁ (μm) of the step closest to the optical axis is as follows. (10) Designed to substantially satisfy the equation. Here, N ₁ is a refractive index with respect to the wavelength λ ₁ of the first aberration correction element L1.
d ₁ = 1.5 · (N ₁ −1) / λ ₁ (10)
Thus, the diffractive structure DOE2 satisfies the above expressions (3) and (5), most of the light intensity of the wavelength lambda ₁ of the light beam incident on the diffractive structure DOE2, 2 2 two diffracted light diffracted light and 1-order diffracted light Will be distributed.

  Further, the width Λ of each annular zone of the diffractive structure DOE2 is such that the diffracted light having the larger diffraction order (second-order diffracted light) is condensed on the HD information recording surface, and the diffracted light having the smaller diffraction order (primary next time). (Folded light) is focused on the information recording surface of the BD, and the paraxial diffraction power is designed to be negative. As a result, it is possible to secure a sufficient working distance for the HD having a thick protective layer.

Incidentally, the diffractive structure DOE2 is because it is formed only in the numerical aperture NA ₂ of HD, the light beam passing through the region outside the NA ₂ becomes a flare component on HD information recording surface RL2, the aperture limitation to HD It is configured to be performed automatically.

Further, when the wavelength λ ₂ is incident on the diffractive structure DOE ₂ , the first-order diffracted light is generated so as to have the maximum diffraction efficiency, and when the light beam having the wavelength λ ₃ is incident, the first-order diffracted light is the maximum diffracted light. Generated to have efficiency.

The diffraction efficiency of the second-order diffracted light of the light beam with wavelength λ ₁ generated in the diffractive structure DOE 2 is 40.5%, the diffraction efficiency of the first-order diffracted light of the light beam with wavelength λ ₁ is 40.5%, and the wavelength λ The diffraction efficiency of the first-order diffracted light of the light beam ₂ is 96.4%, and the diffraction efficiency of the first-order diffracted light of the light beam having the wavelength λ ₃ is 80.3%.

Further, as shown in FIG. 8, the diffractive structure DOE3 is a structure composed of a plurality of annular zones having a stepped cross section including the optical axis. In the diffractive structure DOE3, the height d _B (μm) of the step closest to the optical axis is designed to substantially satisfy the following expression (11). Here, N ₁ is a refractive index with respect to the wavelength λ ₁ of the first aberration correction element L1.
d _B = 2 · (N ₁ −1) / λ ₁ (11)
Since the technique for correcting the chromatic aberration of the objective lens OBJ in the blue-violet region by the diffractive structure DOE3 is a known technique, a detailed description is omitted here.

Further, when a light beam having a wavelength λ ₁ is incident on the diffractive structure DOE 3, the second-order diffracted light is generated so as to have the maximum diffraction efficiency, and when a light beam having a wavelength λ ₂ is incident, the first-order diffracted light is maximum. When a light beam having a wavelength λ ₃ is incident, the first-order diffracted light is generated so as to have the maximum diffraction efficiency.

The two diffraction efficiency of diffracted light of the light flux of the wavelength lambda ₁ generated in the diffractive structure DOE3 is 100% diffraction efficiency of first order diffracted light of the light flux with wavelength lambda ₂ is 88.2%, the wavelength lambda ₃ The diffraction efficiency of the first-order diffracted light of the light beam is 100%.

As shown in FIG. 8, the diffractive structure HOE2 includes a plurality of annular zones, and each annular zone is divided into five steps. The height D ₂ (μm) of the staircase formed in each annular zone of the diffractive structure HOE2 is designed to substantially satisfy the value obtained by substituting q2 = 2 into the following equation (8) ′. Here, N ₁ is the refractive index with respect to the wavelength λ ₁ of the second aberration correction element L1 ′.

D ₂ = q ₂ · λ ₁ / (N ₁ −1) (8) ′
Since the optical path difference added to the light beam having the wavelength λ _{1 by} this staircase structure is 2 × λ ₁ , the light beam having the wavelength λ ₁ is transmitted as it is without being affected by the diffractive structure HOE 2 (0th order diffracted light). Further, since the optical path difference added to the light beam having the wavelength λ ₃ by this step structure is 1 × λ ₃ , the light beam having the wavelength λ ₃ is transmitted as it is without being affected by the step structure (0th order diffracted light). On the other hand, the optical path difference added to the light flux of wavelength λ ₂ by this staircase structure is about 0.2 × λ ₂ , and an optical path difference of exactly 1 × λ ₂ is added to one ring zone divided into five. That is, first-order diffracted light is generated. Thus, by selectively diffracts only the light flux with wavelength lambda _2, and corrects spherical aberration caused by a difference between the thickness t ₁ and the thickness t ₃ of the protective layer PL3 of the protective layer PL1.

Incidentally, the diffractive structure HOE2 is because it is formed only in the numerical aperture NA ₃ of DVD, the light beam passing through the region outside the NA ₃ becomes flare components on the DVD information recording surface RL3, aperture restriction for DVD is It is configured to be performed automatically.

The diffraction efficiency of the 0th-order diffracted light of the light beam with wavelength λ ₁ generated in the diffractive structure HOE2 is 100%, the diffraction efficiency of the 1st-order diffracted light of the light beam with wavelength λ ₂ is 87.3%, and the wavelength λ ₃ The diffraction efficiency of the 0th-order diffracted light beam is 100%.

As shown in FIG. 8, the diffractive structure HOE3 is composed of a plurality of annular zones, and each annular zone is divided into two steps. The height D ₃ (μm) of the steps formed in each annular zone of the diffractive structure HOE ₃ is designed to substantially satisfy the value obtained by substituting q = 5 into the following equation (12). Here, N ₁ is the refractive index with respect to the wavelength λ ₁ of the second aberration correction element L1 ′.

D ₃ = q · λ ₁ / (N ₁ −1) (12)
Since the optical path difference added to the light beam having the wavelength λ _{1 by} this staircase structure is 5 × λ ₁ , the light beam having the wavelength λ ₁ is transmitted as it is without being affected by the diffraction structure HOE 3 (0th-order diffracted light). Further, since the optical path difference added to the light beam having the wavelength λ ₂ by this staircase structure is 3 × λ ₂ , the light beam having the wavelength λ ₂ is transmitted as it is without being affected by the diffraction structure HOE 3 (0th order diffracted light). . On the other hand, the optical path difference added to the luminous flux of wavelength λ ₃ by this staircase structure is about 0.5 × λ ₃ , and the optical path difference is added by exactly half a wavelength in one divided ring zone. Thus, most of the light quantity of the light beam having the wavelength λ ₃ incident on the diffractive structure HOE ₃ is distributed to the first-order diffracted light and the −1st-order diffracted light. The width Λ of each annular zone of the diffractive structure HOE3 is designed so as to collect the first-order diffracted light on the information recording surface RL4 of the CD, and by this diffraction action, the thickness t ₁ of the protective layer PL1 and the protective layer PL3 The spherical aberration due to the difference from the thickness t ₃ is corrected.

Note that the diffraction efficiency of the 0th-order diffracted light of the light beam with wavelength λ ₁ generated in the diffractive structure HOE 3 is 100%, the diffraction efficiency of the 0th-order diffracted light of the light beam with wavelength λ ₂ is 100%, and the light beam with the wavelength λ ₃ The diffraction efficiency of the first-order diffracted light is 40.4%.

  Further, the first collimating lens COL1 of the present embodiment is configured such that its position can be changed in the optical axis direction by the uniaxial actuator AC2. This makes it possible to correct the spherical aberration of the spots formed on the information recording surfaces of HD and BD, so that good recording / reproducing characteristics can always be maintained for HD and BD.

  The cause of the spherical aberration to be corrected by adjusting the position of the first collimating lens COL1 is, for example, a variation in the refractive index of the objective lens OBJ or a refractive index distribution caused by a manufacturing error of the blue-violet semiconductor laser LD1 or a temperature change. These include a focus jump between layers at the time of recording / reproduction with respect to a multi-layer disk such as a disk, a four-layer disk, a thickness variation due to a manufacturing error of the protective layer PL1, a thickness distribution, and the like.

  When information is recorded / reproduced with respect to the BD in the optical pickup device PU2, first, the blue-violet semiconductor laser LD1 is caused to emit light, as shown by the solid line in FIG. The divergent light beam emitted from the blue-violet semiconductor laser LD1 is converted into a parallel light beam through the first collimating lens COL1, and after passing through the polarization beam splitter BS, the light beam diameter is regulated by the stop STO and transmitted through the aperture limiting element AP. The light is focused by the objective lens OBJ and becomes a spot formed on the information recording surface RL1 via the protective layer PL1 of the BD.

  The objective lens OBJ performs focusing and tracking by a biaxial actuator AC1 disposed in the periphery thereof. The reflected light beam modulated by the information pits on the information recording surface RL1 is transmitted again through the objective lens OBJ, the aperture limiting element AP, the polarization beam splitter BS, and the first collimating lens COL1, and then becomes a convergent light beam, which is received by the photodetector PD1. Converge on the surface. And the information recorded on BD can be read using the output signal of photodetector PD1.

When recording / reproducing information with respect to the HD in the optical pickup device PU2, first, the blue-violet semiconductor laser LD1 is caused to emit light, as shown by the two-dot chain line in FIG. The divergent light beam emitted from the blue-violet semiconductor laser LD1 is converted into a parallel light beam through the first collimating lens COL1, passes through the polarization beam splitter BS, passes through the aperture limiting element AP, and is focused by the objective lens OBJ. In response, the spot is formed on the information recording surface RL2 via the HD protective layer PL2. Here, the diffractive structure DOE2 is because it is formed only in the numerical aperture NA ₂ of HD, so that the opening limitation to HD is automatically performed.

  The objective lens OBJ performs focusing and tracking by a biaxial actuator AC1 disposed in the periphery thereof. The reflected light beam modulated by the information pits on the information recording surface RL2 again passes through the objective lens OBJ, the aperture limiting element AP, the polarization beam splitter BS, and the first collimating lens COL1, and then becomes a convergent light beam, which is received by the photodetector PD1. Converge on the surface. And the information recorded on HD can be read using the output signal of photodetector PD1.

In the optical pickup device PU2, when information is recorded / reproduced with respect to the DVD, the light emitting point EP1 is caused to emit light. The divergent light beam emitted from the light emitting point EP1 is reflected by the prism PS and converted into a parallel light beam by the second collimating lens COL2, as illustrated in FIG. The spot is reflected by the BS, passes through the aperture limiting element AP, receives a condensing action by the objective lens OBJ, and becomes a spot formed on the information recording surface RL3 via the protective layer PL3 of the DVD. Here, the diffractive structure HOE3 is because it is formed only in the numerical aperture NA ₃ of DVD, so that the aperture restriction for DVD is carried out automatically.

  The objective lens OBJ performs focusing and tracking by a biaxial actuator AC1 disposed in the periphery thereof.

  The reflected light beam modulated by the information pits on the information recording surface RL2 passes through the objective lens OBJ and the aperture limiting element AP again, is reflected by the polarization beam splitter BS, and then becomes a convergent light beam by the second collimating lens COL2, and becomes a prism PS It is reflected twice inside and condensed on the light receiving part DS1. And the information recorded on DVD can be read using the output signal of light-receiving part DS1.

  In addition, in the optical pickup device PU2, when information is recorded / reproduced with respect to the CD, the light emitting point EP2 is caused to emit light. The divergent light beam emitted from the light emitting point EP2 is reflected by the prism PS, as shown by the dotted line in FIG. 7, and then converted into a parallel light beam by the second collimating lens COL2, and is converted by the polarization beam splitter BS. Reflected, the diameter of the light beam is regulated by the aperture limiting element AP, is focused by the objective lens OBJ, and becomes a spot formed on the information recording surface RL4 via the CD protective layer PL4. The objective lens OBJ performs focusing and tracking by a biaxial actuator AC arranged around the objective lens OBJ.

  The objective lens OBJ performs focusing and tracking by a biaxial actuator AC1 disposed in the periphery thereof. The reflected light beam modulated by the information pits on the information recording surface RL4 passes through the objective optical system OBJ and the aperture limiting element AP again, is reflected by the polarization beam splitter BS, and then is reflected twice inside the prism PS and is received by the light receiving unit DS2. Condensed to And the information recorded on CD can be read using the output signal of light-receiving part DS2.

A wavelength selection filter having wavelength selectivity of transmittance is formed on the optical surface of the aperture limiting element AP. The wavelength selection filter, in the region of the NA ₃ by transmitting all wavelengths of lambda ₁ to lambda _3, outside the region from NA _3, have a wavelength selectivity of the transmission factor that blocks only the wavelength lambda ₃ Therefore, aperture limitation for CD is performed by such wavelength selectivity.

  Next, seven examples (Examples 1 to 7) of multifocal objective lenses suitable as the objective lens OBJ used in the optical pickup devices PU1 and PU2 described above will be described.

  In Examples 1 to 7, the aberration correction element is a plastic lens, and the condensing element is a glass lens dedicated to BD. The condensing element may be a plastic lens.

  The specification of the condensing element is numerical aperture = 0.85, focal length = 1.765 mm, wavelength = 405 nm, magnification = 0, and protective layer thickness = 0.1 mm.

The aspherical surface in each embodiment has a deformation amount from a plane in contact with the apex of the surface as X (mm), a height perpendicular to the optical axis as h (mm), and a radius of curvature as r (mm). The following formula 1 is expressed by a numerical formula in which the aspheric coefficient A _2i in Tables 1 to 7 is substituted. Where κ is the conic coefficient.

Further, the diffractive structure DOE and the diffractive structure HOE in each embodiment are represented by an optical path difference added to the transmitted wavefront by these structures. The optical path difference is as follows when λ is the wavelength of the incident light beam, λ _B is the manufacturing wavelength, the height in the direction perpendicular to the optical axis is h (mm), B _2j is the optical path difference function coefficient, and n is the diffraction order. It is expressed by an optical path difference function φ _b (mm) defined by Equation 2.

In the numerical data tables of the following examples, NA ₁ , f ₁ , λ ₁ , and m ₁ are the numerical aperture of the multifocal objective lens, the focal length of the multifocal objective lens, and the multifocal objective lens, respectively, when using BD. Wavelength, magnification of multifocal objective lens, NA ₂ , f ₂ , λ ₁ , m ₂ are the same values when using HD, NA ₃ , f ₃ , λ ₂ , m ₃ are using DVD NA ₄ , f ₄ , λ ₃ , and m ₄ are the same values when using a CD.

Also, r (mm) is the radius of curvature, d (mm) is the lens spacing, N ₁ , N ₂ , and N ₃ are the refractive indices of the lenses for wavelength λ ₁ , wavelength λ ₂ , and wavelength λ ₃ , respectively, and νd is d. The Abbe number of the lens of the line.

N ₁ , n ₂ , n ₃ , and n ₄ are the diffraction order of the diffracted light used for recording / reproducing BD and the recording / reproduction of HD among the diffracted light generated by the diffractive structure DOE and the diffractive structure HOE, respectively. The diffraction order of diffracted light used for reproduction, the diffraction order of diffracted light used for recording / reproduction of DVD, and the diffraction order of diffracted light used for recording / reproduction of CD.

  Table 1 shows numerical data of the multifocal objective lens of Example 1.

The multifocal objective lens of Example 1 is a BD / HD compatible lens, and corrects spherical aberration due to the difference between t ₁ and t _{2 by} the diffractive structure DOE (first diffractive structure) of the _first surface. .

  In the diffractive structure DOE, 0th order diffracted light (transmitted light) is used for BD, and 1st order diffracted light is used for HD. The diffraction efficiency of BD (0th order diffracted light) is 40.5%, The diffraction efficiency of HD (first order diffracted light) is 40.5%.

In the diffractive structure DOE, the height d _{1 of the} step closest to the optical axis is expressed by d ₁ = 0.5 · λ ₁ / (N ₁ −1) = 0.386 (μm). By changing d ₁ within the range of the expression (2), the BD (0th order diffracted light) diffraction efficiency may be emphasized, or the HD (first order diffracted light) diffraction efficiency may be emphasized.

  Furthermore, since the diffractive power at the paraxial axis of the diffractive structure DOE is negative, the working distance of HD is 0.3 mm, which is sufficiently secured.

  Table 2 shows numerical data of the multifocal objective lens of Example 2.

The multifocal objective lens of Example 2 is a BD / HD compatible lens, and corrects spherical aberration caused by the difference between t ₁ and t ₂ with the diffractive structure DOE (first diffractive structure) of the _first surface. .

  In the diffractive structure DOE, second-order diffracted light is used for BD, and third-order diffracted light is used for HD. The diffraction efficiency of BD (second-order diffracted light) is 40.5%, HD (third time (Folding light) has a diffraction efficiency of 40.5%.

In the diffractive structure DOE, the height d _{1 of the} step closest to the optical axis is expressed by d ₁ = 2.5 · λ ₁ / (N ₁ −1) = 1.93 (μm). By changing d ₁ within the range of the expression (4), a design may be made with an emphasis on the diffraction efficiency of BD (second order diffracted light) or an emphasis on the diffraction efficiency of HD (third order diffracted light).

  Furthermore, since the diffractive power at the paraxial axis of the diffractive structure DOE is negative, the working distance of HD is 0.3 mm, which is sufficiently secured.

  Table 3 shows numerical data of the multifocal objective lens of Example 3.

The multifocal objective lens of Example 3 is a BD / HD compatible lens, and corrects spherical aberration due to the difference between t ₁ and t _{2 by} the diffraction structure DOE (first diffraction structure) of the _first surface. .

  In the diffractive structure DOE, second-order diffracted light is used for BD, and first-order diffracted light is used for HD. The diffraction efficiency of BD (second-order diffracted light) is 40.5%, HD (first time (Folding light) has a diffraction efficiency of 40.5%.

In the diffractive structure DOE, the height d _{1 of the} step closest to the optical axis is expressed by d ₁ = 1.5 · λ ₁ / (N ₁ −1) = 1.16 (μm). By changing d ₁ within the range of the expression (3), the BD (second order diffracted light) diffraction efficiency may be emphasized, or the HD (first order diffracted light) diffraction efficiency may be emphasized.

  Furthermore, since the diffractive power in the paraxial direction of the diffractive structure DOE on the first surface is positive, the axial chromatic aberration in the blue-violet region can be corrected well.

  Table 4 shows numerical data of the multifocal objective lens of Example 4.

The multifocal objective lens of Example 4 is a BD / HD compatible lens, and corrects spherical aberration due to the difference between t ₁ and t ₂ with the diffraction structure HOE (first diffraction structure) of the _first surface. .

  In the diffractive structure HOE of the first surface, -1st order diffracted light is used for BD, and 1st order diffracted light is used for HD. The diffraction efficiency of BD (-1st order diffracted light) is 40.5. %, The diffraction efficiency of HD (first-order diffracted light) is 40.5%.

In the diffraction structure HOE of the first surface, the height D ₁ of the step formed in each annular zone is D ₁ = 0.5 · λ ₁ / (N ₁ −1) = 0.38 (μm). The number of divisions M ₁ of each annular zone is 2.

  Furthermore, since the diffractive power at the paraxial axis of the diffractive structure HOE on the first surface is negative, the HD working distance is 0.3 mm, which is sufficiently secured.

  Further, the diffractive structure DOE of the second surface corrects the spherical aberration change accompanying the BD wavelength change and the axial chromatic aberration in the blue-violet region.

The diffractive structure DOE on the second surface uses + _{1st order} diffracted light of a light beam having a wavelength λ ₁ for each optical disk, and its diffraction efficiency is 100%.

In the second-surface diffractive structure DOE, the height d _{A of the} step closest to the optical axis is expressed by d _A = 1 · λ ₁ / (N ₁ −1) = 0.77 (μm).

  Table 5 shows numerical data of the multifocal objective lens of Example 5.

The multifocal objective lens of Example 5 is a BD / HD compatible lens, and corrects the spherical aberration due to the difference between t ₁ and t _{2 by} the diffractive structure DOE (first diffractive structure) of the _first surface. .

  In the first-surface diffractive structure DOE, the first-order diffracted light is used for BD, and the second-order diffracted light is used for HD. The diffraction efficiency of BD (first-order diffracted light) is 40.5%, The diffraction efficiency of HD (second order diffracted light) is 40.5%.

In the first-surface diffractive structure DOE, the height d _{1 of the} step closest to the optical axis is expressed by d ₁ = 1.5 · λ ₁ / (N ₁ −1) = 1.16 (μm). However, by changing the d ₁ within the range of the expression (3), the design may be made with an emphasis on the diffraction efficiency of BD (first-order diffracted light) or the diffraction efficiency of HD (second-order diffracted light).

  Furthermore, since the diffraction power in the paraxial direction of the diffractive structure DOE on the first surface is negative, the working distance of HD is 0.3 mm, which is sufficiently secured.

  In addition, the diffractive structure DOE on the second surface corrects the spherical aberration change accompanying the BD wavelength change and the axial chromatic aberration in the blue-violet region.

The diffractive structure DOE on the second surface uses second-order diffracted light of a light beam having a wavelength λ ₁ for each optical disk, and its diffraction efficiency is 100%.

In the diffractive structure DOE of the second surface, the height d _{A of the} step closest to the optical axis is expressed by d _A = 2 · λ ₁ / (N ₁ −1) = 1.54 (μm).

  Table 6 shows numerical data of the multifocal objective lens of Example 6.

The multifocal objective lens of Example 6 is a BD / HD / DVD / CD compatible lens, and the difference between t ₁ and t _{2 in} the diffractive structure DOE (first diffractive structure) of the first surface (first optical surface). The spherical aberration due to the above is corrected.

  In the first-surface diffractive structure DOE, the first-order diffracted light is used for BD, the second-order diffracted light is used for HD, the first-order diffracted light is used for DVD, and the CD is used. The first-order diffracted light is used, the diffraction efficiency of BD (first-order diffracted light) is 40.5%, the diffraction efficiency of HD (second-order diffracted light) is 40.5%, and the diffraction efficiency of DVD (first-order diffracted light) is 96. The diffraction efficiency of CD (first order diffracted light) is 80.3%.

In the first diffractive structure DOE, the height d _{1 of the} step closest to the optical axis is expressed by d ₁ = 1.5 · λ ₁ / (N ₁ −1) = 1.16 (μm). By changing d ₁ within the range of the expression (3), the BD (first order diffracted light) diffraction efficiency may be emphasized, or the HD (second order diffracted light) diffraction efficiency may be emphasized.

  Furthermore, since the diffraction power in the paraxial direction of the diffractive structure DOE on the first surface is negative, the working distance of HD is 0.3 mm, which is sufficiently secured.

In addition, the spherical aberration due to the difference between t ₁ and t ₃ is corrected by the diffractive structure DOE (third diffractive structure) of the second surface (second optical surface).

  In the diffractive structure DOE on the second surface, second-order diffracted light is used for BD and HD, first-order diffracted light is used for DVD, and first-order diffracted light is used for CD. The diffraction efficiency of HD (second order diffracted light) is 100%, the diffraction efficiency of DVD (first order diffracted light) is 88.2%, and the diffraction efficiency of CD (+ 1st order diffracted light) is 100%.

  The total diffraction efficiency of the two diffractive structures is 40.5% for BD, 40.5% for HD, 85.0% for DVD, and 80.3% for CD.

In the second-surface diffractive structure DOE, the height d _{C of the} step closest to the optical axis is expressed as d _C = 2 · λ ₁ / (N ₁ −1) = 1.54 (μm).

In the present embodiment, the spherical aberration due to the difference between t ₁ and t ₄ is corrected by making the light beam having the wavelength λ ₃ incident in the state of diverging light.

  Table 7 shows numerical data of the multifocal objective lens of Example 7.

The multifocal objective lens of Example 7 is a BD / HD / DVD / CD compatible lens, and the difference between t ₁ and t _{2 in} the diffractive structure DOE (first diffractive structure) of the first surface (first optical surface). The spherical aberration due to the above is corrected.

  In the first-surface diffractive structure DOE, the first-order diffracted light is used for BD, the second-order diffracted light is used for HD, the first-order diffracted light is used for DVD, and the CD is used. The first-order diffracted light is used, the diffraction efficiency of BD (first-order diffracted light) is 40.5%, the diffraction efficiency of HD (second-order diffracted light) is 40.5%, and the diffraction efficiency of DVD (first-order diffracted light) is 96. The diffraction efficiency of CD (first order diffracted light) is 80.3%.

In the diffractive structure DOE of the first surface, the height d _{1 of the} step closest to the optical axis is expressed by d ₁ = 1.5 · λ ₁ / (N ₁ −1) = 1.16 (μm). In addition, by changing the above d ₁ within the range of the expression (3), the BD (first order diffracted light) diffraction efficiency may be emphasized, or the HD (second order diffracted light) diffraction efficiency may be emphasized.

  Furthermore, since the diffraction power in the paraxial direction of the diffractive structure DOE on the first surface is negative, the working distance of HD is 0.3 mm, which is sufficiently secured.

  In addition, the diffractive structure DOE on the second surface corrects the spherical aberration change accompanying the BD wavelength change and the axial chromatic aberration in the blue-violet region.

  In the diffractive structure DOE of the second surface (second optical surface), second-order diffracted light is used for BD and HD, first-order diffracted light is used for DVD, and first-order diffracted light is used for CD. The diffraction efficiency of BD and HD (second order diffracted light) is 100%, the diffraction efficiency of DVD (first order diffracted light) is 88.2%, and the diffraction efficiency of CD (first order diffracted light) is 100%.

In the second-surface diffractive structure DOE, the height d _{C of the} step closest to the optical axis is expressed as d _C = 2 · λ ₁ / (N ₁ −1) = 1.54 (μm).

Further, the spherical aberration due to the difference between t ₁ and t ₃ is corrected by the diffractive structure HOE (second diffractive structure) of the third surface.

  In the diffractive structure HOE of the third surface, 0th order diffracted light (transmitted light) is used for BD and HD, 1st order diffracted light is used for DVD, and 0th order diffracted light (transmitted light) is used for CD. ), The diffraction efficiency of BD and HD (0th order diffracted light) is 100%, the diffraction efficiency of DVD (1st order diffracted light) is 87.3%, and the diffraction efficiency of CD (0th order diffracted light) is 100.0. %.

The height D ₂ of the staircase formed in each annular zone is expressed by D ₂ = 2 · λ ₁ / (N ₁ −1) = 1.54 (μm), and the division number M of each annular zone ₂ is 5.

Further, the spherical aberration due to the difference between t ₁ and t ₄ is corrected by the diffraction structure HOE of the fourth surface.

  The diffractive structure HOE of the fourth surface uses 0th-order diffracted light (transmitted light) for BD, HD, and DVD, and uses 1st-order diffracted light for CD, BD, HD, and DVD (0 The diffraction efficiency of (second order diffracted light) is 100%, and the diffraction efficiency of CD (first order diffracted light) is 40.4%.

The height D ₃ of the staircase formed in each annular zone is represented by D ₃ = 5 · λ ₁ / (N ₁ −1) = 3.86 (μm), and the number of divisions of each annular zone is 2.

  The total diffraction efficiency of the above four diffraction structures is 40.5% for BD, 40.5% for HD, 74.1% for DVD, and 32.4% for CD.

Table 8 below shows values of each example of | SA _m −SA _n |, SA _m , and SA _n .

However, m = n ₂ , n = n ₁ , NA ₂ = 0.65, and the unit of wavefront aberration is λ ₁ RMS (λ ₁ = 405 nm).

It is a side view (a) and (b) which show an example of diffraction structure DOE. It is a side view (a) and (b) which show an example of diffraction structure DOE. It is a side view (a) and (b) which show an example of diffraction structure HOE. It is a side view (a) and (b) which show an example of diffraction structure DOE. It is a principal part top view which shows the structure of an optical pick-up apparatus. It is a top view which shows the structure of an objective lens (multifocal objective lens). It is a principal part top view which shows the structure of an optical pick-up apparatus. It is a top view which shows the structure of an objective lens (multifocal objective lens).

Explanation of symbols

DOE, HOE Diffraction structure 100, 102, 103, 105 Ring zone 101, 104 Step PU1, PU2 Optical pickup device PL1, PL2 Protective layer LD Blue semiconductor laser PD Photodetector LM Laser module L1 Aberration correction element RL1, RL2 Information recording surface L2 condensing element OBJ objective lens (multifocal objective lens)
AC1 2-axis actuator AC2 1-axis actuator STO Aperture COL Collimating lens

Claims (36)

The protective layer has m-th order diffracted light and n-th order diffracted light (m ≠ n) generated when a light beam having a wavelength λ ₁ is incident on the first diffractive structure. Used in an optical pickup device that focuses light on the information recording surfaces of two types of optical discs having different thicknesses,
The spherical aberration SAm of the wave front when the _m- th order diffracted light is condensed through the protective layer of the thicker protective layer and the n-th order diffracted light through the protective layer of the thicker protective layer. The absolute value | SA _m −SA _n | of the difference from the spherical aberration value SA _n of the wavefront when the light is condensed is 0.9λ ₁ RMS or more, and one of SA _m and SA _n In the multifocal objective lens whose _one absolute value is 0.07λ ₁ RMS or less,
The first diffractive structure is composed of a plurality of annular zones divided by fine steps, and the diffraction order m is an integer of 1 or more and satisfies the following expression (1):
The refractive index N of the multifocal objective lens with respect to the wavelength λ ₁ is in the range of 1.5 to 1.6, and the height d ₁ (μm) of the step closest to the optical axis among the steps is as follows: A multifocal objective lens characterized by satisfying any one of the expressions (2) to (4).
n = m−1 (1)
0.27 <d ₁ <0.48 (2)
1.01 <d ₁ <1.24 (3)
1.72 <d ₁ <2.02 (4)   The combination of the diffraction order m and the diffraction order n is any one of (m, n) = (1, 0), (2, 1), (3, 2). The multifocal objective lens described. Among the step of the first diffractive structure, the nearest step to the optical axis height d ₁ ([mu] m), the wavelength lambda ₁ ([mu] m), the refractive index N ₁ of the multi-focus objective lens with respect to the wavelength lambda ₁ or less The multifocal objective lens according to claim 1, wherein the following expression (5) is satisfied.
0.4 ≦ | INT (X) −X | ≦ 0.5 (5)
However, X = d ₁ · (N ₁ −1) / λ ₁ , and INT (X) is an integer closest to X. The multifocal objective lens according to claim 1, wherein the wavelength λ ₁ is 450 nm or less.   Of the m-th order diffracted light and the n-th order diffracted light, the diffracted light having the larger diffraction order is condensed on the information recording surface of the thicker protective layer, and the diffracted light having the smaller diffraction order is collected on the protective layer. 2. The multifocal objective lens according to claim 1, wherein the multifocal objective lens is focused on an information recording surface of a thinner optical disc and has a negative diffractive power in a paraxial axis of the first diffractive structure. The multifocal objective lens further includes a second diffractive structure including a plurality of annular zones having a stepped structure on an optical surface different from the optical surface having the first diffractive structure, The diffractive structure does not substantially give a phase difference to the light beam having the wavelength λ ₁ , but gives a phase difference to the light beam having the wavelength λ ₂ (λ ₂ > λ ₁ ). Multifocal objective lens. In the second diffractive structure, the division number M ₂ of each annular zone, the height D ₂ (μm) of steps formed in each annular zone, the wavelength λ ₁ (μm), and the wavelength λ ₁ The multifocal objective lens according to claim 6, wherein the refractive index N ₁ of the multifocal objective lens substantially satisfies the following expression (8).
D ₂ · (N ₁ -1) / λ ₁ = 2 · q2 (8)
However, q2 is a natural number, M ₂ is either 4, 5, 6. The multifocal objective lens according to claim 6, wherein the wavelength λ ₂ is in a range of 0.63 μm to 0.68 μm. The multifocal objective lens further includes a third diffractive structure composed of a plurality of annular zones divided by fine steps on an optical surface different from the optical surface having the first diffractive structure, 3 diffractive structure, than the diffraction order of the diffracted light having the maximum diffraction efficiency in the diffracted light which the light flux with wavelength lambda ₁ is generated when the incident wavelength lambda ₂ light beam (λ _2> λ ₁₎ is incident 2. The multifocal objective lens according to claim 1, wherein the diffraction order of the diffracted light having the maximum diffraction efficiency among the diffracted lights generated in the case is lower. In the third diffractive structure, the diffraction order of the diffracted light having the maximum diffraction efficiency secondary among diffracted light which the light flux with wavelength lambda ₁ is generated when the incident, when the light flux with wavelength lambda ₂ enters The multifocal objective lens according to claim 9, wherein the diffraction order of the diffracted light having the maximum diffraction efficiency among the generated diffracted light is first order. In the third diffractive structure, when the light beam having the wavelength λ ₁ is incident, the diffraction order of the diffracted light having the maximum diffraction efficiency out of the diffracted light generated when the light beam having the wavelength λ ₁ is incident, and the light beam having the wavelength λ ₂ is incident. The multifocal objective lens according to claim 9, wherein the diffraction order of the diffracted light having the maximum diffraction efficiency among the generated diffracted light is second order. The multifocal objective lens according to claim 6, wherein the wavelength λ ₂ is in a range of 0.63 μm to 0.68 μm. The protective layer has m-th order diffracted light and n-th order diffracted light (m ≠ n) generated when a light beam having a wavelength λ ₁ is incident on the first diffractive structure. Used in an optical pickup device that focuses light on the information recording surfaces of two types of optical discs having different thicknesses,
The spherical aberration SAm of the wave front when the _m- th order diffracted light is condensed through the protective layer of the thicker protective layer and the n-th order diffracted light through the protective layer of the thicker protective layer. The absolute value | SA _m −SA _n | of the difference from the spherical aberration value SA _n of the wavefront when the light is condensed is 0.9λ ₁ RMS or more, and one of SA _m and SA _n In the multifocal objective lens whose _one absolute value is 0.07λ ₁ RMS or less,
The first diffractive structure is a diffractive structure composed of a plurality of annular zones having a staircase structure therein, and the diffraction order m is an integer of 1 or more and satisfies the following formula (6):
The height D ₁ of the staircase formed in each ring-shaped zone of the first diffractive structure ([mu] m), the wavelength lambda ₁ ([mu] m), the refractive index N ₁ is less than or equal to the multi-focus objective lens with respect to the wavelength lambda ₁ A multifocal objective lens characterized by satisfying the expression (7).
n = -m (6)
D ₁ · (N ₁ −1) / λ ₁ = q 1−0.5 (7)
However, q1 is a natural number.   The combination of the diffraction order m and the diffraction order n is any one of (m, n) = (1, −1), (2, −2), and (3, −3). Item 14. The multifocal objective lens according to Item 13. In the first diffractive structure, the multi-focus objective lens of claim 13, the division number M ₁ of each ring-shaped zone is characterized by an even number. The multifocal objective lens according to claim 13, wherein the wavelength λ ₁ is 450 nm or less.   Of the m-th order diffracted light and the n-th order diffracted light, the diffracted light having the larger diffraction order is condensed on the information recording surface of the thicker protective layer, and the diffracted light having the smaller diffraction order is collected on the protective layer. 14. The multifocal objective lens according to claim 13, wherein the multifocal objective lens is focused on the information recording surface of the thinner optical disc and has a negative diffractive power in the paraxial axis of the first diffractive structure. The multifocal objective lens further includes a second diffractive structure including a plurality of annular zones having a stepped structure on an optical surface different from the optical surface having the first diffractive structure, The diffractive structure does not substantially give a phase difference to the light beam having the wavelength λ ₁ and gives a phase difference to the light beam having the wavelength λ ₂ (λ ₂ > λ ₁ ). Multifocal objective lens. In the second diffractive structure, the division number M ₂ of each annular zone, the height D ₂ (μm) of steps formed in each annular zone, the wavelength λ ₁ (μm), and the wavelength λ ₁ The multifocal objective lens according to claim 18, wherein the refractive index N ₁ of the multifocal objective lens substantially satisfies the following expression (8):
D ₂ · (N ₁ -1) / λ ₁ = 2 · q2 (8)
However, q2 is a natural number, M ₂ is either 4, 5, 6. The multifocal objective lens according to claim 18, wherein the wavelength λ ₂ is in a range of 0.63 μm to 0.68 μm. The multifocal objective lens further includes a third diffractive structure composed of a plurality of annular zones divided by fine steps on an optical surface different from the optical surface having the first diffractive structure, 3 diffractive structure, than the diffraction order of the diffracted light having the maximum diffraction efficiency in the diffracted light which the light flux with wavelength lambda ₁ is generated when the incident wavelength lambda ₂ light beam (λ _2> λ ₁₎ is incident 14. The multifocal objective lens according to claim 13, wherein the diffraction order of the diffracted light having the maximum diffraction efficiency among the diffracted light generated in this case is lower. In the third diffractive structure, the diffraction order of the diffracted light having the maximum diffraction efficiency secondary among diffracted light which the light flux with wavelength lambda ₁ is generated when the incident, when the light flux with wavelength lambda ₂ enters The multifocal objective lens according to claim 21, wherein the diffraction order of the diffracted light having the maximum diffraction efficiency among the generated diffracted lights is the first order. In the third diffractive structure, when the light beam having the wavelength λ ₁ is incident, the diffraction order of the diffracted light having the maximum diffraction efficiency out of the diffracted light generated when the light beam having the wavelength λ ₁ is incident, and the light beam having the wavelength λ ₂ is incident. The multifocal objective lens according to claim 21, wherein the diffraction order of the diffracted light having the maximum diffraction efficiency among the generated diffracted light is second order. The multifocal objective lens according to claim 18, wherein the wavelength λ ₂ is in a range of 0.63 μm to 0.68 μm. An optical pickup device for recording / reproducing information of a first optical disc having a protective layer thickness t ₁ and a second optical disc having a protective layer thickness t ₂ (t ₁ <t ₂ ),
A first light source that emits a first light beam having a wavelength λ ₁ (350 nm ≦ λ ₁ ≦ 450 nm) used when recording / reproducing information on the first and second optical discs;
The multifocal objective lens according to claim 1;
Driving means for driving in two directions for tracking and focusing of the objective lens;
When recording / reproducing the first optical disc, one of the m-th order diffracted light and the n-th order diffracted light is condensed on the information recording surface of the first optical disc,
When recording / reproducing the second optical disc, the diffracted light of the m-th order diffracted light and the n-th order diffracted light, which is different from that used when recording / reproducing the first optical disc, is recorded on the information on the second optical disc. An optical pickup device that collects light on the surface.   An optical information recording / reproducing apparatus comprising the optical pickup device according to claim 25. First optical disk having a protective layer thickness t _1, the protective layer thickness _{t 2 (t 1 <t 2} ) different types of third and the first and second optical disc having a second optical disk and the protective layer thickness t ₃ having An optical pickup device for recording / reproducing information on an optical disc,
A first light source that emits a first light beam having a wavelength λ ₁ (λ ₁ ≦ 450 nm) for recording / reproduction of the first and second optical discs;
A second light source for emitting a second light beam having a wavelength λ ₂ (630 nm <λ ₂ <680 nm) for recording / reproduction of the third optical disc;
The multifocal objective lens according to claim 1, wherein the first to second light beams are condensed on information recording surfaces of the first to third optical disks, respectively.
Driving means that enables driving in two directions for performing focusing and tracking of the objective lens;
The objective lens is
A first optical surface formed of a plurality of annular zones divided by a step and formed with a first diffractive structure for generating the incident first light flux as m-th order diffracted light and n-th order diffracted light (m ≠ n);
A second optical surface formed by a plurality of annular zones divided by steps and formed with a second diffractive structure for generating the incident second light flux as v-order diffracted light;
Have
When recording / reproducing the first optical disk, one of the m-th order diffracted light and the n-th order diffracted light is condensed on the information recording surface of the first optical disk,
When recording / reproducing the second optical disc, information of the second optical disc is used with light of a different order from the m-th order diffracted light and the n-th order diffracted light when recording / reproducing the first optical disc. Condensed on the recording surface,
An optical pickup device for condensing the v-th order diffracted light generated from the second light flux on the information recording surface of the third optical disc when recording / reproducing the third optical disc.   An optical information recording / reproducing apparatus comprising the optical pickup device according to claim 25. First optical disk having a protective layer thickness t _1, the protective layer thickness _{t 2 (t 1 <t 2} ) a second optical disc having a third different type from the first and second optical disc having a protective layer thickness t ₃ An optical pickup device for recording / reproducing information of an optical disc and a fourth optical disc having a protective layer thickness t ₄ (t ₃ <t ₄ ),
A first light source that emits a first light beam having a wavelength λ ₁ (λ ₁ ≦ 450 nm) for recording / reproduction of the first and second optical discs;
A second light source for emitting a second light beam having a wavelength λ ₂ (630 nm <λ ₂ <680 nm) for recording / reproduction of the third optical disc;
A third light source for emitting a third light beam having a wavelength λ ₃ (λ ₂ <λ ₃ ) for recording / reproducing information on the fourth optical disc;
The multifocal objective lens according to claim 1, wherein the first to third light beams are condensed on information recording surfaces of the first to fourth optical discs, respectively.
Driving means for driving in two directions for performing focusing and tracking of the multifocal objective lens;
The multifocal objective lens is composed of a plurality of annular zones divided by steps, and a first diffractive structure for generating the incident first light beam as m-order diffracted light and n-order diffracted light (m ≠ n) is formed. A second optical surface formed by a first optical surface and a plurality of annular zones divided by steps, and formed with a second diffractive structure for generating the incident second light flux as v-order diffracted light;
Have
When recording / reproducing the first optical disk, one of the m-th order diffracted light and the n-th order diffracted light is condensed on the information recording surface of the first optical disk,
When recording / reproducing the second optical disc, information of the second optical disc is used with light of a different order from the m-th order diffracted light and the n-th order diffracted light when recording / reproducing the first optical disc. Condensed on the recording surface,
When recording / reproducing the third optical disk, the v-th order diffracted light generated from the second light beam is condensed on the information recording surface of the third optical disk,
When recording / reproducing the fourth optical disk, the third light beam emitted from the third light source is incident on the multifocal objective lens as a divergent light beam and condensed on the information recording surface of the fourth optical disk. An optical pickup device characterized by the above.   30. An optical information recording / reproducing apparatus comprising the optical pickup apparatus according to claim 29. An optical pickup device for recording / reproducing information of a first optical disc having a protective layer thickness t ₁ and a second optical disc having a protective layer thickness t ₂ (t ₁ <t ₂ ),
A first light source that emits a first light beam having a wavelength λ ₁ (350 nm ≦ λ ₁ ≦ 450 nm) used when recording / reproducing information on the first and second optical discs;
The multifocal objective lens according to claim 13,
Drive means for driving in two directions for tracking and focusing of the multifocal objective lens;
When recording / reproducing the first optical disc, one of the m-th order diffracted light and the n-th order diffracted light is condensed on the information recording surface of the first optical disc,
When recording / reproducing the second optical disc, the diffracted light of the m-th order diffracted light and the n-th order diffracted light, which is different from that used when recording / reproducing the first optical disc, is recorded on the information on the second optical disc. An optical pickup device that collects light on the surface.   An optical information recording / reproducing apparatus comprising the optical pickup apparatus according to claim 31. First optical disk having a protective layer thickness t _1, the protective layer thickness _{t 2 (t 1 <t 2} ) different types of third and the first and second optical disc having a second optical disk and the protective layer thickness t ₃ having An optical pickup device for recording / reproducing information on an optical disc,
A first light source that emits a first light beam having a wavelength λ ₁ (λ ₁ ≦ 450 nm) for recording / reproduction of the first and second optical discs;
A second light source for emitting a second light beam having a wavelength λ ₂ (630 nm <λ ₂ <680 nm) for recording / reproduction of the third optical disc;
The multifocal objective lens according to claim 13, wherein the first to second light beams are condensed on information recording surfaces of the first to third optical disks, respectively.
Driving means that enables driving in two directions for focusing and tracking of the multifocal objective lens;
The multifocal objective lens is:
A first optical surface formed of a plurality of annular zones divided by a step and formed with a first diffractive structure for generating the incident first light flux as m-th order diffracted light and n-th order diffracted light (m ≠ n);
A second optical surface formed by a plurality of annular zones divided by steps and formed with a second diffractive structure for generating the incident second light flux as v-order diffracted light;
Have
When recording / reproducing the first optical disk, one of the m-th order diffracted light and the n-th order diffracted light is condensed on the information recording surface of the first optical disk,
When recording / reproducing the second optical disc, information of the second optical disc is used with light of a different order from the m-th order diffracted light and the n-th order diffracted light when recording / reproducing the first optical disc. Condensed on the recording surface,
An optical pickup device for condensing the v-th order diffracted light generated from the second light flux on the information recording surface of the third optical disc when recording / reproducing the third optical disc.   35. An optical information recording / reproducing apparatus comprising the optical pickup apparatus according to claim 33. First optical disk having a protective layer thickness t _1, the protective layer thickness _{t 2 (t 1 <t 2} ) a second optical disc having a third different type from the first and second optical disc having a protective layer thickness t ₃ An optical pickup device for recording / reproducing information of an optical disc and a fourth optical disc having a protective layer thickness t ₄ (t ₃ <t ₄ ),
A first light source that emits a first light beam having a wavelength λ ₁ (λ ₁ ≦ 450 nm) for recording / reproduction of the first and second optical discs;
A second light source for emitting a second light beam having a wavelength λ ₂ (630 nm <λ ₂ <680 nm) for recording / reproduction of the third optical disc;
A third light source for emitting a third light beam having a wavelength λ ₃ (λ ₂ <λ ₃ ) for recording / reproducing information on the fourth optical disc;
The multifocal objective lens according to claim 13, wherein the first to third light beams are condensed on information recording surfaces of the first to fourth optical discs, respectively.
Driving means for driving in two directions for performing focusing and tracking of the multifocal objective lens;
The multifocal objective lens is composed of a plurality of annular zones divided by steps, and a first diffractive structure for generating the incident first light beam as m-order diffracted light and n-order diffracted light (m ≠ n) is formed. A second optical surface formed by a first optical surface and a plurality of annular zones divided by steps, and formed with a second diffractive structure for generating the incident second light flux as v-order diffracted light;
Have
When recording / reproducing the first optical disk, one of the m-th order diffracted light and the n-th order diffracted light is condensed on the information recording surface of the first optical disk,
When recording / reproducing the second optical disc, information of the second optical disc is used with light of a different order from the m-th order diffracted light and the n-th order diffracted light when recording / reproducing the first optical disc. Condensed on the recording surface,
When recording / reproducing the third optical disk, the v-th order diffracted light generated from the second light beam is condensed on the information recording surface of the third optical disk,
When recording / reproducing the fourth optical disk, the third light beam emitted from the third light source is incident on the objective lens as a divergent light beam and condensed on the information recording surface of the fourth optical disk. Optical pickup device.   36. An optical information recording / reproducing apparatus comprising the optical pickup device according to claim 35.

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