JP2007172830A - Objective lens and optical pickup apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an objective lens which is used for reproduction and/or recording of information to at least two optical disks including a high density optical disk and has satisfactory wavelength characteristics, temperature characteristics and tracking characteristics, and to provide an optical pickup apparatus using the objective lens. <P>SOLUTION: The objective lens is used in the optical pickup apparatus for reproducing and/or recording information to a first optical disk having a protective substrate thickness t1 and a second optical disk having a protective substrate thickness t2 (0.8×t1≤t2). The light fluxes with the wavelengths λ1 and λ2 enters into the objective lens, and a diffractive structure is formed on the at least one optical surface of the objective lens, optical system magnifications m1 and m2 of the objective lens respectively for the light fluxes with wavelengths λ1 and λ2 have different signs and values from each other, and the diffractive structure has a positive diffractive action. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an objective lens and a pickup 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 high-density optical discs of these two standards will circulate in the market in the future, compatible light that can record and play back existing DVDs and CDs on any high-density optical disc The pick-up device is important, and it is desirable to use an objective lens for compatibility. “Compatibility with the objective lens” here means that when light having only different wavelengths is incident on the objective lens at the same angle, the light is condensed on the recording surface of the optical disc corresponding to each wavelength. It refers to what can be done.

  On the other hand, in an optical pickup device, even if the oscillation wavelength varies due to individual differences of lasers incorporated (wavelength characteristics), it is a modest hop that is called mode hopping during operation (chromatic aberration) and a moderate increase with an increase in ambient temperature. Even if a significant wavelength change (temperature characteristic) occurs, it is required to operate safely. For this purpose, it is necessary that the wavefront aberration of the objective lens does not deteriorate with respect to the environmental change as described above.

  A conventional DVD / CD compatible lens is a diffractive lens, and uses a diffraction action to achieve compatibility with a light source having a wavelength difference such as 655 nm and 785 nm. All wavelengths such as chromatic aberration, wavelength characteristics, and temperature characteristics are achieved. Aberration degradation is suppressed against environmental characteristics that accompany changes. However, when this technology is applied to compatibility between a high-density optical disk and a conventional optical disk, the following problems occur. In a DVD / CD compatible lens, the wavelength dependence of diffraction determined to achieve compatibility just satisfies the environmental characteristics, whereas in a compatible lens between a high-density optical disk and a conventional optical disk, Even when designing with only one parameter of the wavelength dependence of diffraction, there is no solution that satisfies both compatibility and environmental characteristics.

In the invention described in Patent Document 1, there is known a technique that uses an objective optical system provided with a diffractive structure and achieves compatibility by setting the optical system magnification to 0 in each of HD / DVD.
JP 2002-298422 A

  The invention described in Patent Document 1 does not cause coma aberration due to lens shift during tracking in this method, but the wavelength characteristic due to diffraction increases, and the oscillation wavelength varies between lots of lasers or temperature changes occur. There is a problem that the wavefront aberration is deteriorated.

  An object of the present invention is to consider the above-mentioned problems, and is used for reproducing and / or recording information on at least two types of optical disks including high-density optical disks, and has good wavelength characteristics, temperature characteristics, and tracking characteristics. An objective lens and an optical pickup device using the objective lens are provided.

  The above-described problems of the present invention can be achieved by the following means.

The objective lens according to claim 1,
Information is reproduced and / or recorded at least on the first optical disc having the protective substrate thickness t1 by using a light beam having the wavelength λ1 emitted from the first light source, and the protective substrate thickness t2 (0.8 × t1 ≦ t2). ) Optical pickup apparatus for reproducing and / or recording information using a light beam having a wavelength λ2 (1.5 × λ1 ≦ λ2 ≦ 1.7 × λ1) emitted from the second light source. Objective lens for
Each of the light beams having the wavelengths λ1 and λ2 is incident on the objective lens, and a diffractive structure is formed on at least one optical surface of the objective lens,
The optical system magnification m1 of the objective lens for the light beam having the wavelength λ1 and the optical system magnification m2 of the objective lens for the light of the wavelength λ2 are different from each other in sign and value.
The diffractive structure has a positive diffractive action.

  The diffractive structure formed on the optical surface of the objective lens has a spherical aberration due to the difference in thickness between the protective layer of the first optical disc and the protective layer of the second optical disc, and / or the refractive index of the objective lens accompanying an environmental temperature change. This is a structure for correcting the wavefront aberration caused by the change and the change of the oscillation wavelength.

  As the above-mentioned diffractive structure, 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, or FIG. (A) As schematically shown in 2 (b), the direction of the step 101 is composed of a plurality of annular zones 102 having the same effective diameter, and the cross-sectional shape including the optical axis is a step shape. 3 (a) and 3 (b), as shown schematically in FIG. 4 (a) and 4 (b), each of which is composed of a plurality of annular zones 103 having a staircase structure formed therein. As schematically shown, there are a plurality of annular zones 105 in which the direction of the step 104 is changed in the middle of the effective diameter, and the cross-sectional shape including the optical axis is a staircase shape. FIGS. 1A to 4B schematically show the case where each phase structure is formed on a plane, but each phase structure may be formed on a spherical surface or an aspherical surface. good. In addition, in this specification, it is comprised from several annular zones as shown to FIG. 1 (a), 1 (b), 2 (a), 2 (b), 4 (a), and 4 (b). The diffractive structure is represented by the symbol “DOE”, and the diffractive structure composed of a plurality of annular zones in which step structures are formed as shown in FIGS. 3A and 3B is represented by the symbol “HOE”. Shall.

  The “positive diffractive action” means, for example, that the third-order spherical aberration is under-performing the passing light beam in order to cancel out the third-order spherical aberration generated in the over direction due to the longer wavelength. It refers to the diffraction effect imparted when it occurs in the direction.

  By forming a diffractive structure having a positive diffractive action on the optical surface of the objective lens as in the configuration according to claim 1, spherical aberration caused by the diffractive action at the time of temperature change can be changed with the oscillation wavelength change of the laser (light source). Can be canceled by the diffraction action. The objective optical system magnification m1 for the light beam having the wavelength λ1 and the optical system magnification m2 of the objective lens for the light of the wavelength λ2 are set so that the sign and the numerical value are different so that the first optical disk and the second optical disk are compatible. Therefore, even if only the wavelength such as the oscillation wavelength variation between laser lots is changed, it operates without any problem. Further, coma aberration generated during tracking of the objective lens becomes an optical system magnification that can be recorded and reproduced.

The objective lens according to claim 2 is the objective lens according to claim 1,
The diffractive structure uses an optical path difference function φ (h),
φ (h) = C ₂ × h ² + C ₄ × h ⁴ +... + C _2i × h ²ⁱ
Defined in
C ₄ <0
It is. Here, h represents the height from the optical axis, C _2i is a coefficient of the optical path difference function, and i represents a natural number.

The objective lens according to claim 3 is the objective lens according to claim 2,
−1.0 × 10 ⁻³ <C ₄ <−1.0 × 10 ⁻⁴
Satisfying.

The objective lens according to claim 4 is the objective lens according to claim 3,
−7.0 × 10 ⁻⁴ <C ₄ <−4.5 × 10 ⁻⁴
Satisfying.

  By setting the coefficient C4 <0 as in the configuration described in claim 2, the diffracted light of wavelength λ1 generated by passing through the diffractive structure is diffracted in the opposite direction to the spherical aberration generated when the wavelength is changed by the lens material. Since this has an effect, it is possible to correct spherical aberration characteristics at the time of wavelength change or temperature change. Since the amount of spherical aberration at the time of wavelength change or temperature change is proportional to the fourth power of NA, it is effective to use this technique with a BD having a higher NA.

  In order to balance the wavelength characteristic and the temperature characteristic, it is preferable that the coefficient C4 is within the range of claim 3. The objective lens is, for example, “ZEONEX” (product name) of Nippon Zeon Co., Ltd. or Mitsui In the case where a general optical resin such as “APEL” (product name) of Petrochemical Industry Co., Ltd. is used as the material, it is more preferably within the range of claim 4.

The objective lens according to claim 5 is the objective lens according to claim 3 or 4, wherein the diffractive structure is a diffractive structure composed of a plurality of concentric annular zones around an optical axis, The cross-sectional shape including the optical axis of the diffractive structure is a sawtooth shape, and the distance d of the step in the optical axis direction of each annular zone is
(2N-1) × λ1 / (n1-1) ≦ d <2N × λ1 / (n1-1)
Satisfying. Here, n1 represents the refractive index of the objective lens with respect to the light beam having the wavelength λ1, and N represents a natural number.

  An objective lens according to a sixth aspect is the objective lens according to the fifth aspect, wherein N = 2.

  When the step distance d is set so as to give an optical path difference of almost odd number times to the light beam having the wavelength λ1 as in the configuration described in claim 5, the wavelength λ3 (1.8 × Nth-order diffracted light and N-1st-order diffracted light having substantially the same diffraction efficiency are mainly generated from the light flux of λ1 ≦ λ3 ≦ 2.2 × λ1. Here, of the two diffracted lights, the Nth order diffracted light having the smaller spherical aberration amount at the same magnification as the optical system magnification m1 is used for reproduction / recording on the third optical disk, and 2N × λ1 / (n1− 1) The step distance d is set so as to satisfy ≦ d <(2N + 1) × λ1 / (n1-1), that is, to give an optical path difference of an even number of times to the first light flux with wavelength λ1. Compared with the case where the diffracted light having the maximum diffraction efficiency among the light beams having the wavelength λ3 generated when passing through the diffractive structure is used for reproduction / recording, the former is for correcting the spherical aberration. The optical system magnification is close to 0, and the aberration generated during tracking can be reduced.

  Note that the diffraction order of the diffracted light should be low from the viewpoint of preventing the diffraction efficiency from decreasing when the wavelength fluctuates. If N = 2 as in claim 6, the optics of the objective lenses of the first optical disc and the second optical disc. The system magnification is close to 0, and the coma aberration, temperature characteristics, and wavelength characteristics during tracking are excellent.

The objective lens according to claim 7 is the objective lens according to any one of claims 1 to 6, wherein the wavefront aberration change amount ΔW [λrms] when the wavelength of the light beam having the wavelength λ1 fluctuates by +5 nm is:
ΔW ≦ 0.05
Satisfying.

  According to the configuration of the seventh aspect, the spherical aberration generated when the temperature changes due to the wavelength change characteristic of the diffractive structure is at a level at which recording and reproduction are possible.

  The objective lens according to claim 8 is the objective lens according to any one of claims 1 to 7, wherein at least one of the optical system magnification m1 and the optical system magnification m2 is zero. It is larger and 1/100 or less.

  According to the configuration of the eighth aspect, the compatibility between the first optical disc and the second optical disc can be shared by the diffraction action and the magnification change.

  An objective lens according to a ninth aspect is the objective lens according to any one of the first to eighth aspects, wherein the diffractive power of the diffractive structure is negative.

  As described in claim 9, by making the diffraction power negative, the chromatic aberration of the light beam having the wavelength λ1 or λ2 when information is reproduced and / or recorded on the first optical disk and the second optical disk is corrected satisfactorily. be able to.

The objective lens according to claim 10 is the objective lens according to claim 9, wherein the wavefront aberration minimum position change amount dfb / dλ in the optical axis direction per 1 nm wavelength change with respect to the light flux having the wavelength λ1 of the objective lens is
| Dfb / dλ | ≦ 0.1 [μm / nm]
Satisfying. Here, fb represents the distance from the objective lens to the first optical disk.

The objective lens according to claim 11 is the objective lens according to claim 9, wherein the wavefront aberration minimum position change amount dfb / dλ in the optical axis direction per 1 nm wavelength change with respect to the light flux with the wavelength λ2 of the objective lens is
| Dfb / dλ | ≦ 0.1 [μm / nm]
Satisfying. Here, fb represents the distance from the objective lens to the second optical disk.

The objective lens according to claim 12 is the objective lens according to any one of claims 1 to 11, wherein t1 = t2. According to the configuration according to claim 12, the first optical disc and the second optical disc. Is compatible only with spherical aberration correction caused by chromatic aberration between the wavelength λ1 and the wavelength λ2, and both the difference between the optical system magnifications m1 and m2 and the wavelength dependence of the diffraction action can be reduced.

The objective lens according to claim 13 is the objective lens according to any one of claims 1 to 11, wherein the numerical aperture on the exit side of the objective lens with respect to the light flux with wavelength λ1 is NA1, and the light flux with wavelength λ2. When the numerical aperture on the exit side of the objective lens is NA2,
NA1 = NA2.

  According to the configuration of the thirteenth aspect, the difference in effective diameter of the objective lens when recording / reproducing the first optical disc and the second optical disc is small, and there is no need to individually provide an aperture limit.

  The objective lens of Claim 14 is an objective lens as described in any one of Claims 1 thru | or 13. WHEREIN: The said 1st light source and the said 2nd light source are arrange | positioned separately.

  An objective lens according to a fifteenth aspect is the objective lens according to the fourteenth aspect, wherein the first light source and the second light source are respectively arranged on an optical axis.

  The objective lens according to claim 16 is the objective lens according to any one of claims 1 to 15, wherein chromatic aberration of the passing light beam is corrected on at least one of the light beams of the wavelengths λ1 and λ2. A chromatic aberration correcting element having a function is arranged.

  According to the configuration of the sixteenth aspect, even when the objective lens can correct chromatic aberration only for one of the light beams having the wavelengths λ1 and λ2, the chromatic aberration correction element is also used for the other wavelength. Chromatic aberration correction can be performed.

  The objective lens according to claim 17 is the objective lens according to claim 16, wherein the chromatic aberration correcting element is a collimating lens.

  The objective lens according to claim 18 is the objective lens according to any one of claims 1 to 11, wherein the objective lens is further applied to a third optical disc having a protective substrate thickness t3 (t2 <t3). , Used in an optical pickup device for reproducing and / or recording information using a light beam having a wavelength λ3 (1.8 × λ1 ≦ λ3 ≦ 2.2 × λ1) emitted from a third light source. In which the luminous fluxes of the wavelengths λ1, λ2, and λ3 are incident.

  An objective lens according to a nineteenth aspect is the objective lens according to the eighteenth aspect, wherein an aperture limiting element is disposed on an optical path of the light flux having the wavelength λ3.

  According to the configuration of the nineteenth aspect, it is possible to limit the aperture for the light flux having the wavelength λ3.

The objective lens according to claim 20 is the objective lens according to claim 18 or 19, wherein an optical system magnification m3 of the objective lens with respect to a light beam having the wavelength λ3 is
−1 / 10 ≦ m3 ≦ −1 / 100
Satisfying.

  According to the structure of the twentieth aspect, the tracking characteristic with respect to the light beam having the wavelength λ3 is at a level at which recording and reproduction can be performed.

  The objective lens according to claim 21 is the objective lens according to any one of claims 18 to 20, wherein the first light source, the second light source, and the third light source are arranged separately. Is.

The objective lens according to claim 22 is the objective lens according to claim 21,
Each of the first light source, the second light source, and the third light source is disposed on an optical axis.

  The objective lens according to claim 23 is the objective lens according to any one of claims 18 to 22, wherein chromatic aberration of the passing light beam is caused on at least one of the light beams of the wavelengths λ1, λ2, and λ3. A chromatic aberration correcting element having a correcting function is arranged.

  According to the configuration of claim 23, even when the objective lens can correct chromatic aberration only for one of the light beams having the wavelengths λ1, λ2, and λ3, the chromatic aberration correction element for the other wavelength as well. Thus, chromatic aberration correction can be performed.

  An objective lens according to a twenty-fourth aspect is the objective lens according to the twenty-third aspect, wherein the chromatic aberration correcting element is a collimating lens.

The objective lens according to claim 25 is the objective lens according to any one of claims 1 to 22, wherein a focal length f1 of the objective lens with respect to the light flux having the wavelength λ1 is
0.8mm ≦ f1 ≦ 4.0mm
Satisfying.

  The objective lens according to claim 26 is the objective lens according to any one of claims 1 to 25, wherein the objective lens is made of plastic.

  The objective lens according to claim 27 is the objective lens according to claim 26, wherein the plastic is a polymer having an alicyclic group-containing ethylenically unsaturated monomer unit, a phosphate structure and phenol in one molecule. It is a resin composition containing the antioxidant which has a structure.

  The objective lens according to claim 28 is the objective lens according to claim 26, wherein the plastic is a polymer having an alicyclic group-containing ethylenically unsaturated monomer unit and an oxidation represented by the general formula (1). A resin composition containing an inhibitor.

[In General Formula (1), each of R ^{19 to} R ²⁴ independently represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, a cycloalkyl group having 5 to 8 carbon atoms, or an alkylcyclohexane having 6 to 12 carbon atoms. An alkyl group, an aralkyl group having 7 to 12 carbon atoms or a phenyl group is represented, and R ^{25 to} R ²⁶ each independently represents a hydrogen atom or an alkyl group having 1 to 8 carbon atoms. X represents a single bond, a sulfur atom or a —CHR ²⁷ — group (R ²⁷ represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms or a cycloalkyl group having 5 to 8 carbon atoms). A represents an alkylene group having 2 to 8 carbon atoms or * —COR ²⁸ — group (R ²⁸ represents a simple bond or an alkylene group having 1 to 8 carbon atoms, and * represents bonding to the oxygen side). . One of Y and Z represents a hydroxyl group, an alkoxy group having 1 to 8 carbon atoms or an aralkyloxy group having 7 to 12 carbon atoms, and the other represents a hydrogen atom or an alkyl group having 1 to 8 carbon atoms. ]
The optical pickup device according to claim 29,
An optical pickup device that reproduces and / or records information on at least a first optical disk having a protective substrate thickness t1 and a second optical disk having a protective substrate thickness t2 (0.8 × t1 ≦ t2),
A first light source that emits a light beam having a wavelength λ1 for reproducing and / or recording information on the first optical disc;
A second light source that emits a light beam having a wavelength λ2 (1.5 × λ1 ≦ λ2 ≦ 1.7 × λ1) for reproducing and / or recording information on the second optical disc;
The objective lens according to any one of claims 1 to 28, wherein the light beam having the wavelength λ1 and the light beam having the wavelength λ2 are condensed on the information recording surfaces of the first optical disc and the second optical disc, respectively. Is.

  According to the present invention, an objective lens that is used for reproducing and / or recording information on at least two types of optical discs including a high-density optical disc and has good wavelength characteristics, temperature characteristics, and tracking characteristics, and light using the objective lens A pickup device can be obtained.

  The objective lens according to the present invention has a diffractive structure, and has a configuration in which the optical system magnification corresponding to two types of optical discs is different and at least one optical system magnification is not 0 as a new design freedom. By using a diffractive structure and a different optical system magnification corresponding to each optical disc, compatibility between high-density optical discs and conventional optical discs makes it possible to achieve diffraction wavelength dependency suitable for correcting environmental characteristics. Obtainable. When the optical system magnification is not 0, the light incident on the objective lens has an angle with respect to the optical axis, and there is a problem in that coma aberration occurs when the objective lens is tracked. By setting, the amount of coma can be suppressed.

  In the present specification, in addition to the above-described CD and HD, an optical disc having a protective film with a thickness of several to several tens of nanometers on the information recording surface, a protective layer or a protective film having a thickness of 0 (zero) These optical disks are also included in the high density optical disk.

  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, DVD + R, and DVD + RW. Is a general term for optical discs of CD series such as CD-ROM, CD-Audio, CD-Video, CD-R, CD-RW and the like.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings.

  FIG. 5 schematically shows a configuration of an optical pickup apparatus PU that can appropriately record / reproduce information for any of HD (first optical disk), DVD (second optical disk), and CD (third optical disk). FIG. The optical specification of HD is a wavelength λ1 = 407 nm, the thickness t1 of a protective layer (protective substrate) PL1 is 0.6 mm, and the numerical aperture NA1 = 0.65. The optical specification of a DVD is a wavelength λ2 = 655 nm, The thickness t2 of the protective layer PL2 = 0.6 mm and the numerical aperture NA2 = 0.65, and the optical specifications of the CD are the wavelength λ3 = 785 nm, the thickness t3 of the protective layer PL3 = 1.2 mm, and the numerical aperture NA3 = 0.51.

  However, the combination of the wavelength, the thickness of the protective layer, and the numerical aperture is not limited to this. Moreover, you may use BD whose thickness t1 of protective layer PL1 is about 0.1 mm as a 1st optical disk.

  The optical system magnification (first magnification m1) of the objective lens when recording and / or reproducing information on the first optical disk is m1 = 0. That is, the objective lens OBJ in the present embodiment has a configuration in which the first light flux having the wavelength λ1 is incident as parallel light.

  In addition, the optical system magnification (second magnification m2) of the objective lens when recording and / or reproducing information on the second optical disk is 0 <m2 ≦ 1/100. That is, the objective lens OBJ in the present embodiment has a configuration in which the second light beam is incident as loose convergent light.

  In the present invention, it is sufficient that at least one of the first magnification m1 and the second magnification m2 is not 0 and m1 ≠ m2, and the second light beam is collimated within the range of the optical system magnification satisfying this condition. The first light beam may be made incident as light and the first light beam and the second light beam may both be made incident as light and convergent light.

  The optical system magnification (third magnification m3) of the objective lens when recording and / or reproducing information with respect to the third optical disc is not particularly limited, but in the present embodiment, The third light beam is incident as loose divergent light (-1 / 10 ≦ m3 ≦ −1 / 100).

  The optical pickup device PU is a blue-violet semiconductor laser LD1 (first light source) that emits a 407-nm laser beam (first beam) when recording / reproducing information on the HD, and the light for the first beam. Detector PD1, red semiconductor laser LD2 (second light source) that emits a 655 nm laser beam (second beam) when recording / reproducing information on DVD, and photodetector PD2 for the second beam , An infrared semiconductor laser LD3 (third light source) that emits a 785 nm laser beam (third beam) when recording / reproducing information on a CD, a third beam photodetector PD3, A collimating lens COL through which only one light beam passes, a diffractive structure is formed on the optical surface thereof, and both surfaces having a function of condensing the laser light beam on the information recording surfaces RL1, RL2, RL3 are aspherical objectives OBJ, biaxial actuator (not shown) for moving the objective lens OBJ in a predetermined direction, first beam splitter BS1, second beam splitter BS2, third beam splitter BS3, fourth beam splitter BS4, diffraction plate G, It comprises a stop STO, sensor lenses SEN1 and SEN2, and the like.

  When information is recorded / reproduced with respect to the HD in the optical pickup device PU, first, the blue-violet semiconductor laser LD1 is caused to emit light as illustrated by the solid line in FIG. The divergent light beam emitted from the blue-violet semiconductor laser LD1 passes through the first beam splitter BS1 and reaches the collimating lens COL.

  Then, when passing through the collimating lens COL, the first light beam is converted into parallel light, passes through the second beam splitter BS2, the third beam splitter BS3, and the aperture stop STO, reaches the objective lens OBJ, and then reaches the objective lens OBJ. 1 It becomes a spot formed on the information recording surface RL1 through the protective layer PL1. The objective lens OBJ performs focusing and tracking by a biaxial actuator disposed around the objective lens OBJ.

  The reflected light beam modulated by the information pits on the information recording surface RL1 passes again through the objective lens OBJ, the third beam splitter BS3, the second beam splitter BS2, and the collimator lens COL, and is branched by the first beam splitter BS1 to be a sensor lens. Astigmatism is given by SEN1 and converges on the light receiving surface of the photodetector PD1. And the information recorded on HD can be read using the output signal of photodetector PD1.

  When recording / reproducing information with respect to a DVD, first, the red semiconductor laser LD2 is caused to emit light, as shown by the dashed line in FIG. The divergent light beam emitted from the red semiconductor laser LD2 passes through the diffraction plate G, is reflected by the third beam splitter BS3, passes through the stop STO, reaches the objective lens OBJ as loosely convergent light, and is reflected by the objective lens OBJ. 2 Spots formed on the information recording surface RL2 via the protective layer PL2. The objective lens OBJ performs focusing and tracking by a biaxial actuator disposed around the objective lens OBJ.

  The reflected light beam modulated by the information pits on the information recording surface RL2 again passes through the objective lens OBJ, is reflected by the third beam splitter BS3, and then the path is changed when passing through the diffraction plate G, and the photodetector PD2 Converges on the light receiving surface. Then, information recorded on the DVD can be read using the output signal of the photodetector PD2.

  When recording / reproducing information with respect to a CD, first, the infrared semiconductor laser LD3 is caused to emit light, as shown by the dotted line in FIG. The divergent light beam emitted from the infrared semiconductor laser LD3 passes through the fourth beam splitter BS4, is reflected by the second beam splitter BS2, passes through the third beam splitter BS3 and the stop STO, and reaches the objective lens OBJ. The spots are formed on the information recording surface RL3 via the third protective layer PL1 by the objective lens OBJ. The objective lens OBJ performs focusing and tracking by a biaxial actuator disposed around the objective lens OBJ.

  The reflected light beam modulated by the information pits on the information recording surface RL3 again passes through the objective lens OBJ and the third beam splitter BS3, is reflected by the second beam splitter BS2, is branched by the fourth beam splitter BS4, and is sensor lens SEN2. Astigmatism is given by and converges on the light receiving surface of the photodetector PD3. And the information recorded on CD can be read using the output signal of photodetector PD3.

  Next, the configuration of the objective lens OBJ will be described.

  The objective lens is a single lens made of plastic in which both the incident surface S1 (optical surface on the light source side) and the exit surface S2 (optical surface on the optical disc side) are aspherical.

  A diffractive structure DOE is formed in almost the entire area of the incident surface S1, and the exit surface S2 is a refractive surface.

The diffractive structure DOE is composed of a plurality of concentric annular zones around the optical axis, and the cross-sectional shape including the optical axis is a sawtooth shape. And the distance d of the step in the optical axis direction of each annular zone is
(2N-1) × λ1 / (n1-1) ≦ d <2N × λ1 / (n1-1)
n1: Refractive index of the objective lens with respect to the light beam having the wavelength λ1,
N: The distance d of the step is set so as to satisfy the natural number, that is, to give an optical path difference that is almost an odd multiple to the first light flux having the wavelength λ1. As a result, diffracted light having an odd diffraction order with respect to the wavelength of 407 nm (the refractive index of the objective lens on which the diffractive structure DOE is formed with respect to the wavelength of 407 nm is 1.559806) (for example, +3 when N = 2 When the diffraction efficiency of the (second order diffracted light) becomes almost 100% and the second light beam (the refractive index of the objective lens on which the diffractive structure DOE is formed has a refractive index of 1.540725) is incident on this diffractive structure DOE, +2 Since the next diffracted light is generated with a diffraction efficiency of 88%, sufficient diffraction efficiency can be obtained in any wavelength region of the first light flux and the second light flux.

  On the other hand, when the third light beam (the refractive index with respect to the wavelength of 785 nm of the objective lens on which the diffractive structure DOE is formed is 1.537237) enters the diffractive structure DOE, the + 2nd order diffracted light and the + 3rd order diffracted light are approximately the same, approximately 40%. It occurs with diffraction efficiency. Here, when the second-order diffracted light having the smaller spherical aberration amount at the same magnification as the optical system magnification m1 among the two diffracted lights is used for reproduction / recording with respect to the CD, 2N × λ1 / (n1-1) ≦ The step distance d is set so as to satisfy d <(2N + 1) × λ1 / (n1-1), that is, to provide an even-numbered optical path difference with respect to the first light flux having the wavelength λ1. Compared with the case where the diffracted light having the maximum diffraction efficiency among the third light beams generated when passing through the structure is used for reproduction / recording with respect to the CD, the former is an optical system for correcting spherical aberration. The magnification is close to 0, and aberrations that occur during tracking can be reduced.

The diffractive structure DOE is represented by an optical path difference added to the transmitted wavefront by this structure, and this optical path difference is h (mm) is a height in a direction perpendicular to the optical axis, C _2i is an optical path difference function coefficient, When i is a natural number, it is represented by an optical path difference function φ (h) (mm) defined by substituting a predetermined coefficient into the following equation (1).

Then, in this embodiment, the diffractive structure DOE is to have a positive diffracting action, are set as C ₄ in Formula in 1 is C ₄ <0.

It is preferable that −1.0 × 10 ⁻³ <C4 <−1.0 × 10 −4 is satisfied, and −7.0 × 10 ⁻⁴ <C ₄ <−4.5 × 10 ⁻⁴ is satisfied. Is more preferable.

  By setting the diffractive structure DOE in this way, it is positive with respect to at least one light beam (the first light beam having the wavelength λ1 in the present embodiment) among the light beams having the wavelengths λ1, λ2, and λ3 passing through the diffractive structure. Thus, an objective lens having excellent temperature characteristics can be obtained by suppressing the amount of change in wavefront aberration caused by wavelength fluctuation of the first light beam caused by environmental temperature change.

  Specifically, the wavefront aberration change amount ΔW [λrms] is set to satisfy ΔW ≦ 0.05 when the wavelength of the light beam having the wavelength λ1 changes by +5 nm in accordance with the environmental temperature change.

  Further, it is preferable that the diffractive structure has a negative diffractive power, so that the chromatic aberration of the light beam having the wavelength λ1 or λ2 when information is reproduced and / or recorded on the HD and DVD can be corrected. .

  Specifically, the amount of change dfb / dλ of the minimum position of wavefront aberration in the optical axis direction per 1 nm of wavelength change with respect to the light flux with wavelength λ1 of the objective lens satisfies | dfb / dλ | ≦ 0.1 [μm / nm]. By setting the diffractive structure DOE as described above, it is possible to correct the chromatic aberration of the light flux having the wavelength λ1 and obtain an objective lens having excellent wavelength characteristics. Further, diffraction is performed so that the change amount dfb / dλ of the wavefront aberration minimum position in the optical axis direction per 1 nm of wavelength change with respect to the light beam of wavelength λ2 of the objective lens satisfies | dfb / dλ | ≦ 0.1 [μm / nm]. By setting the structure DOE, it is possible to correct the chromatic aberration of the light beam having the wavelength λ2 and obtain an objective lens having excellent wavelength characteristics.

  In the present embodiment, the first light source, the second light source, and the third light source are separately provided on the optical axis, and the objective lens OBJ has a high density mainly having a narrow performance tolerance range. The sine condition is satisfied for the optical disc.

  Therefore, when using a high density optical disk, coma aberration caused by tracking of the objective lens OBJ is hardly a problem even when, for example, loosely convergent light is incident on the objective lens OBJ. On the other hand, the sine condition is not satisfied because the thickness of the protective layer and the optical system magnification of the objective lens are largely different. However, of the magnification and sine conditions that cause coma aberration when the objective lens OBJ tracks, the magnification is Since it is small, the coma aberration is at a level that can be sufficiently used for recording and reproduction, and an objective lens having excellent tracking characteristics can be obtained.

  If it is desired to further correct the coma during tracking, a coma aberration correcting element may be provided on the light source side of the objective lens OBJ, or a collimator lens or a coupling lens having a correcting function may be provided.

  Further, as an aperture limiting element for performing aperture limitation corresponding to NA3, an aperture limiting element AP is disposed in the vicinity of the optical surface S1 of the objective lens OBJ, and the aperture limiting element AP and the objective lens OBJ are integrated by a biaxial actuator. It is good also as a structure driven to tracking.

  In this case, a wavelength selection filter WF having wavelength selectivity of transmittance is formed on the optical surface of the aperture limiting element AP. This wavelength selection filter WF transmits all wavelengths from the first wavelength λ1 to the third wavelength λ3 in the region within NA3, blocks only the third wavelength λ3 in the region from NA3 to NA1, and blocks the first wavelength λ1 and the first wavelength λ1. Since it has wavelength selectivity of transmittance that transmits two wavelengths λ2, aperture restriction corresponding to NA3 can be performed by such wavelength selectivity.

  Further, as a method of limiting the aperture, not only a method using the wavelength selection filter WF, but also a method of mechanically switching the diaphragm or a method using a liquid crystal phase control element LCD described later.

  The objective lens OBJ is desirably plastic from the viewpoint of light weight and low cost, but may be made of glass in consideration of temperature resistance and light resistance. Currently, the refraction-type glass mold aspherical lens is mainly on the market, but if a low-melting-point glass being developed is used, a glass mold lens provided with a diffractive structure can be manufactured. In addition, while plastics for optical use are being developed, there are materials whose refractive index changes with temperature are small. This is to reduce the refractive index change due to the temperature of the entire resin by mixing inorganic fine particles with the opposite sign of the change in refractive index due to temperature. Some materials have a reduced dispersion, and it is even more effective if they are used in a BD objective lens.

  As these resins, a resin composition containing a polymer (A) having an alicyclic group-containing ethylenically unsaturated monomer unit and an antioxidant (B) having a phosphate structure and a phenol structure in one molecule. Product (X) can be used.

  Moreover, the resin composition (Y) containing the polymer (A) which has an alicyclic group containing ethylenically unsaturated monomer unit, and antioxidant (B) represented by General formula (1) can also be used. .

[In General Formula (1), each of R ^{19 to} R ²⁴ independently represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, a cycloalkyl group having 5 to 8 carbon atoms, or an alkylcyclohexane having 6 to 12 carbon atoms. An alkyl group, an aralkyl group having 7 to 12 carbon atoms or a phenyl group is represented, and R ^{25 to} R ²⁶ each independently represents a hydrogen atom or an alkyl group having 1 to 8 carbon atoms. X represents a single bond, a sulfur atom or a —CHR ²⁷ — group (R ²⁷ represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms or a cycloalkyl group having 5 to 8 carbon atoms). A represents an alkylene group having 2 to 8 carbon atoms or * —COR ²⁸ — group (R ²⁸ represents a simple bond or an alkylene group having 1 to 8 carbon atoms, and * represents bonding to the oxygen side). . One of Y and Z represents a hydroxyl group, an alkoxy group having 1 to 8 carbon atoms or an aralkyloxy group having 7 to 12 carbon atoms, and the other represents a hydrogen atom or an alkyl group having 1 to 8 carbon atoms. ]
It is preferable that the resins (X) and (Y) further contain a phenolic antioxidant (C).

  The resins (X) and (Y) preferably further contain a hindered amine light-resistant stabilizer (D). The hindered amine light stabilizer (D) is a polycondensate of dibutylamine, 1,3,5-triazine and N, N′-bis (2,2,6,6-tetramethyl-4-piperidyl) butylamine. , Poly {[6- (1,1,3,3-tetramethylbutyl) amino-1,3,5-triazine-2,4-diyl] [(2,2,6,6-tetramethyl-4- Piperidyl) imino] hexamethylene [(2,2,6,6-tetramethyl-4-piperidyl) imino], and dimethyl succinate and 4-hydroxy-2,2,6,6-tetramethyl-1-piperidineethanol And a substance having a number average molecular weight of 1,500 to 5,000 selected from the group consisting of polymerized products.

  As described above, a diffractive structure having a positive diffractive action is formed on the optical surface of the objective lens, and the optical system magnification m1 of the objective lens with respect to the light beam with the wavelength λ1 and the optical system magnification m2 of the objective lens with respect to the light with the wavelength λ2 By setting the magnification of at least one of the optical systems to be not 0 and m1 ≠ m2, a compatible objective lens and optical pickup device having excellent wavelength characteristics, temperature characteristics and tracking tracking characteristics can be obtained. It is done.

  Next, examples of the optical element shown in the above embodiment will be described.

Example 1
Table 1 shows lens data of Example 1.

  As shown in Table 1, the objective lens of this example is an HD / DVD / CD compatible objective lens, and has a focal length f1 = 3.20 mm at a wavelength λ1 = 407 nm, a magnification m1 = 0, and NA1 = When the wavelength λ2 = 655 nm, the focal length f2 = 3.27 mm, the magnification m2 = 1 / 185.2, NA2 = 0.65, and when the wavelength λ3 = 785 nm. Focal length f3 = 3.27 mm, magnification m3 = −1 / 55.6, and NA3 = 0.51.

The entrance surface (second surface) and the exit surface (third surface) of the objective lens are aspherical surfaces that are axisymmetric about the optical axis L and are defined by a mathematical formula in which the coefficients shown in Table 1 are substituted into the following formula 2. Is formed.

Here, x is an axis in the optical axis direction (the light traveling direction is positive), κ is a conic coefficient, and A _2i is an aspheric coefficient.

  A diffractive structure DOE is formed on the second surface and the third surface. This diffractive structure DOE is represented by an optical path difference added to the transmitted wavefront by this structure. The optical path difference is expressed as follows: h (mm) is the height in the direction perpendicular to the optical axis, B2i is the optical path difference function coefficient, n is the diffraction order of the diffracted light having the maximum diffraction efficiency out of the diffracted light of the incident light beam, and λ ( nm) is the wavelength of the light beam incident on the diffractive structure, λB (nm) is the manufacturing wavelength of the diffractive structure, and λB is the blazed wavelength of the diffractive structure DOE (1.0 mm in this embodiment), Is expressed by an optical path difference function φ (h) (mm) defined by substituting the coefficients shown in Table 1.

Note that the above equation 1 is obtained by replacing (B _2i × n × λ / λB) in equation 3 with C _2i . That is, the relationship B _2i × n × λ / λB = C _2i is established.

  In Example 1, the wavefront aberration in the HD reference state is 0.001 λrms, whereas the wavefront aberration at the time of wavelength variation (−5 nm) not accompanied by environmental temperature change is 0.042 λrms, wavelength variation accompanied by environmental temperature change. The wavefront aberration at time (+1.5 nm, + 30 ° C.) is 0.040 λrms. When the objective lens is tracking (moving 0.3 mm with respect to the direction perpendicular to the optical axis), the second light flux is incident as loosely convergent light. The wavefront aberration generated due to this is 0.001 λrms.

  From the above, it was confirmed that the objective lens of this example has good wavelength characteristics, temperature characteristics, and tracking characteristics.

Example 2
Table 2 shows lens data of Example 2.

  As shown in Table 2, the objective lens of this example is also an HD / DVD / CD compatible objective lens, and has a focal length f1 = 3.20 mm and a magnification m1 = −1 / 185 when the wavelength λ1 = 407 nm. .2, NA1 = 0.65, focal length f2 = 3.27 mm when wavelength λ2 = 655 nm, magnification m2 = 0, NA2 = 0.65, wavelength λ3 = 785 nm Focal length f3 = 3.27 mm, magnification m3 = −1 / 44.8, and NA3 = 0.51.

  The entrance surface (second surface) and the exit surface (third surface) of the objective lens are aspherical surfaces that are axisymmetric about the optical axis L and are defined by a mathematical formula obtained by substituting the coefficients shown in Table 2 into the above formula (2). Is formed.

  A diffractive structure DOE is formed on the second surface and the third surface. This diffractive structure DOE is represented by an optical path difference added to the transmitted wavefront by this structure, and this optical path difference is defined by substituting the coefficient shown in Table 2 into the above equation (3) for the optical path difference function φ (h) ( mm). In this embodiment, the blazed wavelength is 1.0 mm.

  In Example 2, the wavefront aberration in the HD reference state is 0.001 λrms, whereas the wavefront aberration at the time of wavelength variation (−5 nm) without environmental temperature change is 0.043 λrms, wavelength variation with environmental temperature change. The wavefront aberration at time (+1.5 nm, + 30 ° C.) is 0.042 λrms, and the first light beam is incident as loose divergent light when the objective lens is tracking (moving 0.3 mm with respect to the direction perpendicular to the optical axis). The wavefront aberration generated due to this is 0.001 λrms.

  From the above, it was confirmed that the objective lens of this example has good wavelength characteristics, temperature characteristics, and tracking characteristics.

Example 3
Tables 3 and 4 show lens data of Example 3.

  As shown in Tables 3 and 4, the objective lens of this example is an HD / DVD / CD compatible objective lens, and has a focal length f1 = 3.1 mm and a magnification m1 = 1 when the wavelength λ1 = 407 nm. /28.4, NA1 = 0.65, focal length f2 = 3.23 mm when wavelength λ2 = 655 nm, magnification m2 = 0, NA2 = 0.65, wavelength λ3 = The focal length f3 at 785 nm is set to 3.23 mm, the magnification m3 = −1 / 35.6, and NA3 = 0.51.

  In Example 3, the wavefront aberration in the HD reference state is 0.000 λrms, whereas the wavefront aberration at the time of wavelength variation (−5 nm) without environmental temperature change is 0.024 λrms, wavelength variation with environmental temperature change. The wavefront aberration at time (+1.5 nm, + 30 ° C.) is 0.019 λrms, and when the objective lens is tracking (moving 0.3 mm with respect to the direction perpendicular to the optical axis), the first light beam is incident as loosely convergent light. The wavefront aberration generated due to this is 0.003λrms.

  From the above, it was confirmed that the objective lens of this example has good wavelength characteristics, temperature characteristics, and tracking characteristics.

Example 4
Tables 5 and 6 show lens data of Example 4.

  As shown in Tables 5 and 6, the objective lens of the present example is an HD / DVD / CD compatible objective lens, and has a focal length f1 = 3.10 mm and a magnification m1 = 1 when the wavelength λ1 = 407 nm. /33.2, NA1 = 0.65, focal length f2 = 3.18 mm when wavelength λ2 = 655 nm, magnification m2 = −1 / 125, NA2 = 0.65, The focal length f3 = 3.20 mm when the wavelength λ3 = 785 nm, the magnification m3 = −1 / 27.9, and NA3 = 0.51 are set.

  In Example 4, the wavefront aberration in the HD reference state is 0.000 λrms, whereas the wavefront aberration at the time of wavelength variation (−5 nm) without environmental temperature change is 0.028 λrms, and the wavelength variation with environmental temperature change. The wavefront aberration at time (+1.5 nm, + 30 ° C.) is 0.040 λrms, and when the objective lens is tracking (moving 0.3 mm with respect to the direction perpendicular to the optical axis), the first light beam enters as a slowly convergent light. The wavefront aberration generated due to this is 0.002λrms.

  From the above, it was confirmed that the objective lens of this example has good wavelength characteristics, temperature characteristics, and tracking characteristics.

Example 5
Table 7 shows lens data of Example 5.

  As shown in Table 7, the objective lens of this example is an HD / DVD compatible objective lens, and has a focal length f1 = 3.10 mm and a magnification m1 = −1 / 256.4 when the wavelength λ1 = 407 nm. NA1 = 0.65, focal length f2 = 3.15 mm when wavelength λ2 = 655 nm, magnification m2 = 0, NA2 = 0.65.

  In Example 5, the wavefront aberration in the HD reference state is 0.000 λrms, whereas the wavefront aberration at the time of wavelength variation (−5 nm) with no environmental temperature change is 0.009 λrms, the wavelength variation with environmental temperature change. The wavefront aberration at time (+1.5 nm, + 30 ° C.) is 0.060 λrms, and the first light beam is incident as loose divergent light when tracking the objective lens (moving 0.3 mm with respect to the direction perpendicular to the optical axis). The wavefront aberration generated due to this is 0.000λrms.

  From the above, it was confirmed that the objective lens of this example has good wavelength characteristics, temperature characteristics, and tracking characteristics.

Example 6
Table 8 shows lens data of Example 6.

  As shown in Table 8, the objective lens of this example is an HD / DVD compatible objective lens, and has a focal length f1 = 3.10 mm when the wavelength λ1 = 407 nm, and a magnification m1 = −1 / 256.4. NA1 = 0.65, focal length f2 = 3.15 mm when wavelength λ2 = 655 nm, magnification m2 = 1 / 192.3, NA2 = 0.65.

  In Example 6, the wavefront aberration in the HD reference state is 0.000 λrms, whereas the wavefront aberration at the time of the wavelength variation (−5 nm) without the environmental temperature change is 0.014 λrms, the wavelength variation with the environmental temperature change. The wavefront aberration at time (+1.5 nm, + 30 ° C.) is 0.057 λrms, and when the objective lens is tracking (moving 0.3 mm with respect to the direction perpendicular to the optical axis), the first light beam is incident as loose divergent light. The wavefront aberration generated due to this is 0.000λrms.

  From the above, it was confirmed that the objective lens of this example has good wavelength characteristics, temperature characteristics, and tracking characteristics.

It is drawing (a) and (b) which show a phase structure. It is drawing (a) and (b) which show a phase structure. It is drawing (a) and (b) which show a phase structure. It is drawing (a) and (b) which show a phase structure. It is a principal part top view which shows the structure of an optical pick-up apparatus.

Explanation of symbols

100, 102, 103, 105 Ring zone 101, 104 Step DOE, HOE Diffraction structure PU Optical pickup device OBJ Objective lens LD1 Blue-violet semiconductor laser LD2 Red semiconductor laser LD3 Infrared semiconductor laser PD1, PD2, PD3 Photodetector COL Collimating lens BS1 First beam splitter BS2 Second beam splitter BS3 Third beam splitter STO Aperture G Diffraction plate SEN1, SEN2 Sensor lens

Claims (29)

Information is reproduced and / or recorded at least on the first optical disc having the protective substrate thickness t1 by using a light beam having the wavelength λ1 emitted from the first light source, and the protective substrate thickness t2 (0.8 × t1 ≦ t2). ) Optical pickup apparatus for reproducing and / or recording information using a light beam having a wavelength λ2 (1.5 × λ1 ≦ λ2 ≦ 1.7 × λ1) emitted from the second light source. Objective lens for
Each of the light beams having the wavelengths λ1 and λ2 is incident on the objective lens, and a diffractive structure is formed on at least one optical surface of the objective lens,
The optical system magnification m1 of the objective lens for the light beam having the wavelength λ1 and the optical system magnification m2 of the objective lens for the light of the wavelength λ2 are different from each other in sign and value.
The diffractive structure is an objective lens having a positive diffractive action. The diffractive structure uses an optical path difference function φ (h),
φ (h) = C ₂ × h ² + C ₄ × h ⁴ +... + C _2i × h ²ⁱ
Defined in
C ₄ <0
The objective lens according to claim 1, wherein
However, h is the height from the optical axis, C _2i is the coefficient of the optical path difference function, and i is a natural number. −1.0 × 10 ⁻³ <C ₄ <−1.0 × 10 ⁻⁴
The objective lens according to claim 2, wherein: −7.0 × 10 ⁻⁴ <C ₄ <−4.5 × 10 ⁻⁴
The objective lens according to claim 3, wherein: The diffractive structure is a diffractive structure composed of a plurality of concentric annular zones around the optical axis, the cross-sectional shape including the optical axis of the diffractive structure is a sawtooth shape, and the optical axis of each annular zone The distance d of the step in the direction is
(2N-1) × λ1 / (n1-1) ≦ d <2N × λ1 / (n1-1)
The objective lens according to claim 3 or 4, satisfying
n1: Refractive index of the objective lens with respect to the light beam having the wavelength λ1 N: Natural number   The objective lens according to claim 5, wherein N = 2.   The objective lens according to claim 1, wherein a wavefront aberration change amount ΔW [λrms] when the wavelength of the light beam having the wavelength λ1 fluctuates by +5 nm satisfies ΔW ≦ 0.05.   The objective lens according to any one of claims 1 to 7, wherein at least one of the optical system magnification m1 and the optical system magnification m2 has an optical system magnification greater than 0 and 1/100 or less.   The objective lens according to claim 1, wherein the diffraction power of the diffraction structure is negative. The wavefront aberration minimum position change amount dfb / dλ in the optical axis direction per 1 nm wavelength change with respect to the light flux having the wavelength λ1 of the objective lens,
| Dfb / dλ | ≦ 0.1 [μm / nm]
The objective lens according to claim 9, wherein:
fb: distance from the objective lens to the first optical disk. The wavefront aberration minimum position change amount dfb / dλ in the optical axis direction per 1 nm wavelength change with respect to the light flux having the wavelength λ2 of the objective lens,
| Dfb / dλ | ≦ 0.1 [μm / nm]
The objective lens according to claim 9, wherein:
fb: distance from the objective lens to the second optical disk.   The objective lens according to claim 1, wherein t1 = t2. When the numerical aperture on the exit side of the objective lens with respect to the light beam with the wavelength λ1 is NA1, and the numerical aperture on the output side of the objective lens with respect to the light beam with the wavelength λ2 is NA2,
The objective lens according to claim 1, wherein NA1 = NA2.   The objective lens according to claim 1, wherein the first light source and the second light source are arranged separately.   The objective lens according to claim 14, wherein the first light source and the second light source are each disposed on an optical axis.   The objective lens according to claim 1, wherein a chromatic aberration correction element having a function of correcting chromatic aberration of a passing light beam is disposed on at least one of the light beams having the wavelengths λ1 and λ2.   The objective lens according to claim 16, wherein the chromatic aberration correcting element is a collimating lens. The objective lens further emits a light beam having a wavelength λ3 (1.8 × λ1 ≦ λ3 ≦ 2.2 × λ1) emitted from the third light source with respect to a third optical disk having a protective substrate thickness t3 (t2 <t3). Is used in an optical pickup device for reproducing and / or recording information using
The objective lens according to any one of claims 1 to 11, wherein light beams having the wavelengths λ1, λ2, and λ3 are incident on the objective lens.   The objective lens according to claim 18, wherein an aperture limiting element is disposed on an optical path of the light flux having the wavelength λ3. An optical system magnification m3 of the objective lens with respect to the light beam having the wavelength λ3 is
−1 / 10 ≦ m3 ≦ −1 / 100
The objective lens of Claim 18 or 19 satisfying | fulfilling.   The objective lens according to any one of claims 18 to 20, wherein the first light source, the second light source, and the third light source are arranged separately.   The objective lens according to claim 21, wherein each of the first light source, the second light source, and the third light source is disposed on an optical axis.   23. The objective according to claim 18, wherein a chromatic aberration correction element having a function of correcting chromatic aberration of a passing light beam is disposed on at least one of the light beams having the wavelengths λ1, λ2, and λ3. lens.   The objective lens according to claim 23, wherein the chromatic aberration correcting element is a collimating lens. The focal length f1 of the objective lens with respect to the light beam having the wavelength λ1 is
0.8mm ≦ f1 ≦ 4.0mm
The objective lens as described in any one of Claims 1 thru | or 22 satisfy | filling these.   The objective lens according to any one of claims 1 to 25, which is made of plastic.   27. The resin according to claim 26, wherein the plastic is a resin composition containing a polymer having an alicyclic group-containing ethylenically unsaturated monomer unit and an antioxidant having a phosphate structure and a phenol structure in one molecule. Objective lens. 27. The objective lens according to claim 26, wherein the plastic is a resin composition containing a polymer having an alicyclic group-containing ethylenically unsaturated monomer unit and an antioxidant represented by the general formula (1).

[In General Formula (1), each of R ^{19 to} R ²⁴ independently represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, a cycloalkyl group having 5 to 8 carbon atoms, or an alkylcyclohexane having 6 to 12 carbon atoms. An alkyl group, an aralkyl group having 7 to 12 carbon atoms or a phenyl group is represented, and R ^{25 to} R ²⁶ each independently represents a hydrogen atom or an alkyl group having 1 to 8 carbon atoms. X represents a single bond, a sulfur atom or a —CHR ²⁷ — group (R ²⁷ represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms or a cycloalkyl group having 5 to 8 carbon atoms). A represents an alkylene group having 2 to 8 carbon atoms or * —COR ²⁸ — group (R ²⁸ represents a simple bond or an alkylene group having 1 to 8 carbon atoms, and * represents bonding to the oxygen side). . One of Y and Z represents a hydroxyl group, an alkoxy group having 1 to 8 carbon atoms or an aralkyloxy group having 7 to 12 carbon atoms, and the other represents a hydrogen atom or an alkyl group having 1 to 8 carbon atoms. ] An optical pickup device that reproduces and / or records information on at least a first optical disk having a protective substrate thickness t1 and a second optical disk having a protective substrate thickness t2 (0.8 × t1 ≦ t2),
A first light source that emits a light beam having a wavelength λ1 for reproducing and / or recording information on the first optical disc;
A second light source that emits a light beam having a wavelength λ2 (1.5 × λ1 ≦ λ2 ≦ 1.7 × λ1) for reproducing and / or recording information on the second optical disc;
The objective lens according to any one of claims 1 to 28, wherein the light beam having the wavelength λ1 and the light beam having the wavelength λ2 are condensed on the information recording surfaces of the first optical disc and the second optical disc, respectively. Pickup device.

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