JP4462108B2 - Objective optical system, optical pickup device, and optical disk drive device - Google Patents

Description

  The present invention relates to an objective optical system, an optical pickup device, and an optical disk drive device.

Conventionally, an optical pickup compatible with a high density optical disc, a DVD (using a red laser light source), and a CD (using an infrared laser light source) whose recording density is increased by using a blue-violet laser light source. Devices and optical elements used in such optical pickup devices are known (see, for example, Patent Documents 1 to 3).
JP 2004-079146 A JP 2002-298422 A JP 2003-207714 A

Numerical Example 7 of Patent Document 1 has a diffractive structure on the surface of an objective lens that generates second-order diffracted light with a blue-violet laser beam and first-order diffracted light with a red laser beam and an infrared laser beam. Due to the action of this diffractive structure, spherical aberration due to the difference in the protective layer thickness between the high-density optical disc and the DVD is corrected, and a divergent light beam is incident on the objective lens when recording / reproducing information with respect to the CD. An objective lens that corrects spherical aberration due to the difference in the thickness of the protective layer between a density optical disk and a CD is disclosed.
Although this objective lens can secure a high diffraction efficiency in any wavelength region, the degree of divergence of the infrared laser beam becomes too strong at the time of recording / reproducing information on a CD, and the coma when the objective lens is tracking. There is a problem in that good recording / reproducing characteristics cannot be obtained with respect to a CD because the generation of aberration becomes too large.

In Numerical Example 3 of Patent Document 2, diffraction is performed on the surface of the objective lens such that a third-order diffracted light is generated with a blue-violet laser beam and a second-order diffracted light is generated with a red laser beam and an infrared laser beam. An objective lens is disclosed which has a structure and corrects spherical aberration due to the difference in the thickness of the protective layer between the high-density optical disc and DVD and CD.
Although this objective lens can correct spherical aberration due to the difference in the protective layer thickness between the high-density optical disk and the DVD and further spherical aberration due to the difference in the protective layer thickness between the high-density optical disk and the CD due to the action of the diffractive structure. Since the diffraction efficiency of the 3rd-order diffracted light of the blue-violet laser beam and the diffraction efficiency of the 2nd-order diffracted light of the infrared laser beam are as low as about 70%, it is not possible to cope with an increase in recording / reproducing speed with respect to the optical disc. There are problems in that good recording / reproduction characteristics cannot be obtained because the S / N ratio of the detection signal is low, and the life of the laser light source is shortened because the voltage applied to the laser light source is high.

In the objective lens described in Patent Document 1, the spherical aberration due to the difference in the thickness of the protective layer between the high-density optical disk and the CD cannot be corrected by the diffractive structure, or in the objective lens described in Patent Document 2, the third time in the blue-violet wavelength region. The reason why the diffraction efficiency of the folded light and the diffraction efficiency of the second-order diffracted light in the infrared wavelength region are low is that the wavelength of the blue-violet laser light source used for the high-density optical disk Since the wavelength is approximately twice, the spherical aberration correction effect for the blue-violet laser beam and the infrared laser beam of the diffracted light generated by the diffractive structure and the diffraction efficiency of the diffracted light are in a trade-off relationship. Can be mentioned.
That is, in the objective lens of Numerical Example 7 of Patent Document 1 corresponding to a case where both the diffraction efficiency of the diffracted light of the blue-violet laser beam and the diffraction efficiency of the diffracted light of the infrared laser beam are ensured to be high, the blue-violet laser beam The diffraction angle of the diffracted light and the diffraction angle of the diffracted light of the infrared laser beam substantially coincide with each other, so that the spherical aberration due to the difference in the thickness of the high-density optical disc and the CD protective layer cannot be corrected by the diffractive structure. .

On the other hand, in the objective lens of Numerical Example 3 of Patent Document 2, which corresponds to the case where a difference is made between the diffraction angle of the diffracted light of the blue-violet laser beam and the diffraction angle of the diffracted light of the infrared laser beam, the blue-violet laser Both the diffraction efficiency of the diffracted light of the light beam and the diffraction efficiency of the infrared laser light beam are lowered.
In addition to the diffraction structures described in Patent Documents 1 and 2, not only in the technology using a phase corrector (referred to as an optical path difference providing structure in this specification) as described in Patent Document 3. Similarly to the diffraction structure, the spherical aberration correction effect for the blue-violet laser beam and the infrared laser beam by the optical path difference providing structure and the transmittance of the optical path difference providing structure are in a trade-off relationship.

In general, the accuracy of wavefront aberration required for an optical element becomes more severe as the wavelength becomes shorter and the numerical aperture becomes larger.
For example, in the objective lens for a high-density optical disk having a numerical aperture of 0.85 and a wavelength of 405 nm and the objective lens for DVD having a numerical aperture of 0.6 and a wavelength of 655 nm, the influence of the same surface accuracy error on spherical aberration is estimated. Since (655/405) · (0.85 / 0.6) ⁴ = 6.5 times, the objective lens for high-density optical discs is 6.5 times more severe than the objective lens for DVDs. Must be manufactured while maintaining.

  As described above, the shorter the wavelength and the larger the numerical aperture, the more difficult it is to obtain the performance of the optical element. Therefore, in designing an objective lens for an optical pickup device that is compatible with multiple types of optical disks, Therefore, it is necessary to prioritize the performance with respect to the light flux with the shortest wavelength among the design performance with respect to the light flux with a plurality of wavelengths. The design performance here is, for example, coma aberration generated when spherical aberration or off-axis light beam is incident.

  The present invention has been made in view of the above-described problems. Due to the action of a phase structure including a diffractive structure, spherical aberration due to a difference in protective layer thickness between a high-density optical disc and a DVD and a CD, or a high-density optical disc Spherical aberration due to the difference in operating wavelength between DVD and CD can be corrected satisfactorily, and any of a blue-violet wavelength region near 400 nm, a red wavelength region near 650 nm, and an infrared wavelength region near 780 nm An objective optical system that can obtain high light utilization efficiency even in the wavelength region and that is excellent in design performance for high-density optical discs, an optical pickup device using the objective optical system, and an optical disc equipped with the optical pickup device An object is to provide a drive device.

According to a first aspect of the present invention, information is recorded and / or reproduced with respect to a first optical disc having a protective layer having a thickness of t ₁ using a first light beam having a first wavelength λ ₁ emitted from a first light source. And recording information on the third optical disc having the protective layer having the thickness t ₃ (> t ₁ ) using the third light beam having the third wavelength λ ₃ (> λ ₁ ) emitted from the third light source. And / or an objective optical system used in an optical pickup device for performing reproduction,
The objective optical system collects the aberration correction element and the first and third light fluxes that have passed through the aberration correction element on the information recording surfaces of the first optical disc and the third optical disc, respectively. Objective lens,
The aberration correction element has a configuration in which a base lens and a resin layer are laminated on the surface of the base lens, and a first diffractive structure having a ring-shaped step is formed on the boundary surface between the base lens and the resin layer. And a base curve that is a macroscopic curvature of the first diffractive structure is configured as an aspherical surface or a spherical surface, and a difference Δν _d between the Abbe number of the d-line of the base lens and the Abbe number of the d-line of the resin layer. Satisfies the following equation (1), and the difference Δn1 between the refractive index of the base lens at the first wavelength λ _{1 and} the refractive index of the resin layer at the first wavelength λ ₁ satisfies the following equation (2): It is characterized by that.
20 <| Δν _d | <40 (1)
| Δn1 |> 0.02 (2)

As in the first embodiment, a base lens having a difference in Abbe number satisfying the equation (1) and a resin layer are laminated, and a diffractive structure is formed on the boundary surface. It is possible to achieve both a spherical aberration correction effect and a sufficient transmittance for the blue-violet laser beam (first beam) and infrared laser beam (third beam). Further, by providing the base lens and the resin layer with a difference in refractive index satisfying the expression (2) at the first wavelength λ ₁ , the step along the optical axis of each annular zone can be reduced, Manufacturing of the diffractive structure is facilitated. In addition, it is difficult to achieve both spherical aberration correction and sine condition correction with a diffractive structure in which the base curve is a flat surface. However, by configuring the base curve as an aspherical surface or a spherical surface, the first aberration correction element can be used. Both the correction of spherical aberration for the light beam and the correction of the sine condition are possible, and the design performance for the first light beam can be improved.
The “base curve” here refers to an envelope connecting the vertices of the respective sawtooths of the first diffractive structure, as indicated by dotted lines in FIG. 2 described later, and this envelope is the first diffractive structure. Represents a macroscopic curvature.
In the objective optical system of the first aspect, the pickup device further uses the second light flux having the second wavelength λ ₂ (> λ ₁ ) emitted from the second light source to have a thickness t ₂ (≧ It is effectively used when recording and / or reproducing information on the second optical disk having the protective layer of t ₁ ).

In this specification, an objective lens with a NA of 0.85 and a protective layer thickness of 0.1 mm, a Blu-ray disc with an objective lens with an NA of 0.65 to 0.67 and a protective layer thickness of 0 are used. An optical disc using a blue-violet laser light source such as HD DVD of 6 mm is collectively referred to as “high density optical disc” and is abbreviated as “HD”. In addition to the above-mentioned Blu-ray disc and HD DVD, a magneto-optical disc, an optical disc having a protective film with a thickness of several to several tens of nanometers on the information recording surface, an optical disc with a protective layer or a protective film having a zero thickness Are also included in the high density optical disk.
In this specification, DVD (Digital Versatile Disc) means DVD series optical discs such as DVD-ROM, DVD-Video, DVD-Audio, DVD-RAM, DVD-R, DVD-RW, DVD + R, DVD + RW, etc. CD (compact disc) is a generic term for CD-series optical discs such as CD-ROM, CD-Audio, CD-Video, CD-R, and CD-RW.

In the present specification, the “objective lens” is a condensing element that is disposed at a position facing the optical disk in the optical pickup device and has a function of condensing the light beam emitted from the light source on the information recording surface of the optical disk. Refers to the lens.
Further, in the present specification, the “objective optical system” refers to an optical system including the above-described condenser lens and an aberration correction element that is integrated with the condenser lens and performs tracking and focusing by an actuator. . The aberration correction element here may be composed of one lens group, or may be composed of two or more lens groups.
In this specification, “when the base curve that is the macroscopic curvature of the diffractive structure is an aspherical surface, the aspherical surface means a shape excluding a plane.

  In the objective optical system according to the first aspect, the base curve has a non-spherical deformation amount that is a distance along the optical axis from the spherical surface expressed by a paraxial radius of curvature and increases as the distance from the optical axis increases. A spherical surface is preferable.

When the aspherical surface deformation amount, which is the distance along the optical axis from the spherical surface represented by the paraxial radius of curvature, becomes larger as the distance from the optical axis increases, the spherical surface for the first light flux of the aberration correction element It becomes possible to perform aberration correction and sine condition correction more satisfactorily.
The “aspheric deformation amount” here is expressed by the following expression (10) when the aspheric shape of the base curve is expressed by [Aspheric expression] described later.
Δz = | z | − | [(y ² / R) / [1 + √ {1− (y / R) ² }]] | (10)
Here, z is an aspherical shape (mm) representing the distance along the optical axis between the plane that contacts the surface vertex and the aspherical surface, and the inside of {} is the plane and the paraxial radius of curvature. Is a spherical shape (mm) representing the distance in the direction along the optical axis with the spherical surface represented by.
Therefore, the amount of deformation of the aspheric surface expressed by the above equation (10) is “becomes larger as the distance from the optical axis is larger” that Δz asymptotically increases as y (distance from the optical axis) increases. Point to.

  In the objective optical system of the first aspect, it is preferable that the optical surface of the resin layer on the side opposite to the boundary surface is an aspherical surface having substantially the same shape as the base curve.

By designing the optical surface of the resin layer opposite to the boundary surface to be an aspherical surface having substantially the same shape as the base curve, the design performance for the first light beam can be further improved.
The “aspherical surface having substantially the same shape as the base curve” here refers to the aspherical surface shape z1 (mm) of the base curve on the resin layer side and the aspherical surface shape z2 of the optical surface of the resin layer opposite to the boundary surface. When (mm) is expressed by [Aspherical Expression] described later, this means that the following expression (11) is satisfied at an arbitrary y (distance from the optical axis) within the effective radius.
0 ≦ | z1-z2 | ≦ 0.05 (11)

In the objective optical system of the first aspect, the the paraxial diffraction power P _D of the first wavelength lambda ₁ of the first diffractive structure, the aberration correcting element entire system paraxial refractive power in the first wavelength λ1 of _PRT preferably satisfies the following formulas (3) and (4).
_{P D · P RT <0 (} 3)
0.9 <| P _D · P _RT | <1.1 (4)

By satisfying the expressions (3) and (4), the convergence (divergence) effect due to diffraction in the first diffractive structure and the divergence (convergence) due to refraction of the optical surface of the resin layer opposite to the boundary surface. The operation can be canceled, and the first light beam incident on the aberration correction element in the state of a parallel light beam can be emitted from the aberration correction element in the state of a parallel light beam. At this time, by laminating the resin layer sufficiently thin with respect to the axial thickness of the base lens, the diameter of the first light flux incident on the aberration correction element and the diameter of the first light flux emitted from the aberration correction element. The difference with can be reduced.
The “paraxial diffracted power P _{D at} the first wavelength λ ₁ ” here is the following when the optical path difference added to the first light flux by the first diffractive structure is expressed by an optical path difference function φ described later. (12). Here, λ _B is the manufacturing wavelength of the first diffractive structure, and B ₂ is a second-order diffractive surface coefficient.
P _D = −2 × λ / λ _B × M × B ₂ (12)

In the objective optical system according to the first aspect, the difference Δn2 between the refractive index of the base lens at the second wavelength λ _{2 and} the refractive index of the resin layer at the second wavelength λ ₂ and the third wavelength of the resin layer. with the difference in refractive index at lambda ₃ [Delta] n3 satisfies the following equation (5) to (7), wherein the first diffractive structure preferably has a negative paraxial diffraction power.
1.2 <| Δn2 | / | Δn1 | <2.2 (5)
1.4 <| Δn3 | / | Δn1 | <2.4 (6)
1.0 <| Δn3 | / | Δn2 | <2.0 (7)

Equations (5) to (7) are conditions for generating diffraction light of the same order for each wavelength and ensuring diffraction efficiency for each wavelength. At this time, by setting the paraxial diffraction power of the first diffractive structure to be negative, the longer the wavelength, the stronger the divergence degree and the incident light can enter the objective lens. In addition, a large working distance with respect to the third optical disk can be secured.
In the aberration correction element of the objective optical system according to the present invention, by appropriately setting the difference in refractive index between the base lens and the resin layer at each wavelength, the light beam having each wavelength is converted into the light beam having each wavelength in the first diffractive structure on the boundary surface. In contrast, it is possible to generate diffracted light of various diffraction orders. However, in order to suppress a decrease in diffraction efficiency caused by a minute change in wavelength, first-order diffracted light is generated for light beams of any wavelength. It is preferable to set the difference in refractive index between the base lens and the resin layer at each wavelength.
In general, when a light beam having a wavelength λ is incident on a diffractive structure, diffracted light of various diffraction orders is generated. However, by appropriately setting the steps of the diffractive structure, the diffraction efficiency of the diffracted light of a specific diffraction order is extremely increased. It is possible to increase it. In this specification, “M-order diffracted light is generated in a diffractive structure” means that a step is set so that the diffraction efficiency of M-order diffracted light is maximized among diffracted light of various diffraction orders generated in the diffractive structure. Refers to being.
By satisfying the equations (5) to (7), it is possible to correct spherical aberration caused by the difference between t ₁ and t ₃ , and high density optical discs and CDs typified by Blu-ray discs and HD DVDs. Compatible with can be achieved.

In the objective optical system according to the first aspect, the first diffractive structure corrects spherical aberration due to a difference between the t ₁ and the t ₃ .

In order to correct spherical aberration caused by the difference between t ₁ and t ₃ while ensuring high diffraction efficiency for the _first and third light beams, the third light beam is weaker than the objective optical system. It is preferable that the light is incident as Furthermore, if the step is set so that the diffracted light of the same order is generated for the light beams of the respective wavelengths, the degree of divergence of the third light beam incident on the objective optical system does not become too strong. Therefore, the amount of coma generated when the objective optical system is driven for tracking is sufficiently small, and good tracking characteristics can be maintained.

In the objective optical system, the first diffractive structure has a spherical aberration caused by the difference between the t ₁ and the t ₂ , or a spherical surface caused by the difference between the first wavelength λ ₁ and the second wavelength λ _2. More preferably, the aberration is corrected.

It is possible to correct the spherical aberration caused by the difference between t ₁ and t ₂ or the spherical aberration caused by the difference between the first wavelength λ ₁ and the wavelength λ ₂ , and the compatibility between the high-density optical disc and the DVD is simultaneously achieved. Can be achieved.

  In the above-described objective optical system, it is also a preferred form that a phase structure is formed on an optical surface opposite to the boundary surface among the optical surfaces of the base lens.

By forming the phase structure on the optical surface opposite to the boundary surface among the optical surfaces of the base lens, it is possible to improve the condensing characteristics of each light beam of the objective optical system. The phase structure may be a diffractive structure or an optical path difference providing structure. The aberration corrected by the phase structure may be, for example, chromatic aberration associated with a minute change in the first wavelength λ ₁ or spherical aberration generated due to a change in the refractive index of the objective lens associated with a temperature change. .

In the objective optical system, the phase structure has a characteristic of selectively diffracting the second light beam without diffracting the first light beam and the third light beam, and the phase structure allows the t ₁ and the first light beam to be diffracted. The spherical aberration due to the difference between t ₂ or the spherical aberration due to the difference between the first wavelength λ ₁ and the second wavelength λ ₂ is corrected, and the t ₁ and the t are corrected by the first diffractive structure. It is a preferable form to correct spherical aberration caused by the difference of ₃ .

Since one diffractive structure can correct only spherical aberration with respect to two light beams having different wavelengths, in an objective optical system shared by three light beams having different wavelengths, such as the objective optical system of the present invention, The spherical aberration for the three light beams cannot be corrected only by the diffraction action. As a result, when the aberration correction element has only one diffractive structure, the magnification of the remaining one light beam is uniquely determined in order to correct spherical aberration that cannot be corrected by the diffraction action. The design freedom of the pickup device is lost.
Therefore, as described above, the phase structure has a characteristic of selectively diffracting the second light beam without diffracting the first light beam and the third light beam, thereby causing the difference between t ₁ and t _2. Or spherical aberration due to the difference between the first wavelength λ ₁ and the second wavelength λ ₂ is corrected, and the first diffractive structure formed on the boundary surface causes the difference between t ₁ and t _3. By correcting the spherical aberration, it is possible to correct the spherical aberration of the light flux of each wavelength at the same magnification while ensuring high diffraction efficiency for the light flux of each wavelength.

  In the above-described objective optical system, it is preferable that a second diffractive structure is formed on a boundary surface between the base lens and the resin layer, which has a larger Abbe number in the d-line, and air. One.

Since the second diffractive structure is formed on the interface between the base lens and the resin layer having the larger Abbe number at the d-line and the air, the first light flux, the second light flux, and the third light flux. The diffraction efficiency for each of the wavelengths λ ₁ , λ ₂ , and λ ₃ can be increased.

In the above objective optical system,
The objective optical element arranged on the disk side has an Abbe number ν _{d of} d line satisfying the following formula (8), and a second diffractive structure is formed on the surface of the objective optical element. And
40 ≦ ν _d ≦ 70 (8)

If the Abbe number ν _d of the d line in the objective optical element arranged on the disk side satisfies the above equation (8) and the second diffractive structure is formed on the surface of the objective optical element, the first light flux The diffraction efficiencies for the wavelengths λ ₁ , λ ₂ , and λ ₃ of the second light beam and the third light beam can be increased.

  In the above-described objective optical system, the second diffractive structure is a diffractive structure having a plurality of steps in cross section, and it is one of preferred modes that selectively diffracts or transmits light according to a wavelength.

The second diffractive structure is a step-shaped diffractive structure having a plurality of steps (wavelength selective diffractive structure), and selectively diffracts or transmits light according to the wavelength. For example, the second diffractive structure has a first wavelength λ ₁ It is possible to diffract one beam without giving a phase difference and transmitting it without diffracting it, and by giving a phase difference to the second beam having the second wavelength λ ₂ and the third beam having the third wavelength λ ₃ . It is. If a phase difference can be given only to a light beam having a predetermined wavelength, a diffraction effect can be given only to the DVD light, and the remaining spherical aberration of the DVD can be corrected in the configuration of claim 1.

The invention according to claim 14 is the objective optical system according to claim 11 or 12,
The second diffractive structure is a blazed diffractive structure.

  Here, the blazed diffractive structure is a structure in which the cross-sectional shape including the optical axis is formed in a sawtooth shape. If the second diffractive structure is a blazed diffractive structure, it is effective for correcting chromatic aberration. Color correction means that the focal position of the objective lens does not change with respect to wavelength change. The laser used in the pickup device has a mode hop phenomenon, and the actuator of the objective lens does not catch up with the sudden wavelength change, resulting in a defocused state. Therefore, in short-wave Blu-ray and HD DVD, it is necessary to perform color correction that does not change the focal position of the objective lens even if the wavelength changes. Color correction is possible using a wavelength-selective diffractive structure, but the number of zonal zones is larger than that of a blazed diffractive structure, and DVD or CD light is transmitted, so that a color correction function can be simultaneously applied. Not suitable in that it can not.

In the objective optical system described above, it is preferable that the following expression (9) is satisfied.
0.9 × t ₁ ≦ t ₂ ≦ 1.1 × t ₁ (9)

This defines a preferable range of the thickness t2 of the protective layer of the second optical disk. If the thickness t ₂ is within this range, it is only necessary to correct the spherical aberration caused by the difference in wavelength as in the combination of HD DVD and DVD, so that the diffraction pitch can be increased and the workability can be increased. Can be increased.

  In the above-described objective optical system, it is one of preferred embodiments that the resin layer is an ultraviolet curable resin.

  In the objective optical system described above, the base lens is preferably manufactured by molding.

  As a method of laminating an optical resin on a base lens, a method (so-called insert molding) in which an optical glass having a diffraction structure formed on its surface is used as a mold and the optical resin is molded on the base lens (so-called insert molding). However, a method in which an ultraviolet curable resin is laminated on a base lens having a diffractive structure formed on the surface thereof and then cured by irradiation with ultraviolet rays is suitable for production.

  In addition, as a method of manufacturing a base lens having a diffractive structure formed on the surface thereof, a method of directly forming a diffractive structure on a base lens substrate by repeating photolithography and etching processes may be used. So-called mold molding is suitable for mass production, in which a mold (mold) having a diffractive structure is formed as a replica of the mold is produced. In addition, as a method of producing the mold in which the diffractive structure is formed, a method of forming the diffractive structure by repeating photolithography and etching processes, or a method of machining the diffractive structure with a precision lathe may be used.

In the above-described objective optical system, it is also a preferred embodiment that the base lens is made of resin.
Any optical glass or optical resin can be used as the material for the base lens, but in order to form a fine structure such as a diffractive structure or a phase structure with little shape error, a material with low viscosity in the molten state That is, an optical resin is suitable. Resin lenses are cheaper and lighter than glass lenses. In particular, when the aberration correction element is made of resin to reduce the weight, a driving force for performing focusing and tracking control at the time of recording / reproducing information with respect to the optical disk can be reduced.

In the above-described objective optical system, it is preferable that the following relationship is satisfied.
α × λ ₁ = λ ₃
K1-0.1 ≦ α ≦ K1 + 0.1 (where K1: natural number)
From this relationship, α falls within a range of ± 0.1 with respect to the natural number. That is, λ ₃ is almost a natural number multiple of λ ₁ . Here, since the wavelength relationship between the high-sensitivity optical disc and the CD is almost a natural number, the effect of the present invention can be easily achieved by satisfying this relationship.

In the objective optical system described above, it is preferable that the objective lens is optimized for spherical aberration correction with respect to a combination of the t ₁ and the first wavelength λ ₁ .

It is preferable that the aspherical shape of the objective lens is determined so that spherical aberration correction is minimized with respect to the first wavelength λ ₁ and the thickness t ₁ of the protective layer of the first optical disc. By determining the aspherical shape of the objective lens so that the spherical aberration correction is minimized with respect to the first wavelength λ ₁ and the thickness t ₁ of the first protective layer, the strictest wavefront accuracy is required. It becomes easy to obtain the condensing performance of the first light flux. Here, “the objective lens is optimized for spherical aberration correction for the combination of the t ₁ and the first wavelength λ ₁ ” means that the objective lens and the first optical disk are protected via the first protective layer. It is assumed that the wavefront aberration when the light beam is condensed is 0.05λ ₁ RMS or less.

According to a second aspect of the present invention, there is provided an optical pickup device including any one of the above-described objective optical systems.
The effect obtained by the optical pickup device of the second aspect is the same as the effect in the objective optical system described above.

A third aspect of the present invention is an optical disk drive device on which the above-described optical pickup device is mounted.
The effects obtained by the optical disk drive device of the third aspect are the same as those of the objective optical system and the optical pickup device described above.

  According to the present invention, due to the action of the first diffractive structure formed on the boundary surface, spherical aberration due to the difference in protective layer thickness between the high-density optical disc and DVD or CD, or the use of the high-density optical disc and DVD and CD. Spherical aberration due to wavelength difference can be corrected satisfactorily, and high light in any wavelength region of a blue-violet wavelength region near 400 nm, a red wavelength region near 650 nm, and an infrared wavelength region near 780 nm It is possible to obtain an objective optical system that can obtain utilization efficiency and is excellent in design performance for a high-density optical disc, an optical pickup device that uses this objective optical system, and an optical disc drive device that includes this optical pickup device.

[First embodiment]
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. First, an optical pickup device PU using an objective optical system OU according to an embodiment of the present invention will be described with reference to FIG.
FIG. 1 is a diagram schematically showing a configuration of an optical pickup apparatus PU capable of appropriately recording / reproducing information on any of a high density optical disc HD, a DVD, and a CD. The optical specification of HD is the first wavelength λ ₁ = 405 nm, the thickness t ₁ of the protective layer PL1 is 0.1 mm, and the numerical aperture NA1 = 0.85. The optical specification of the DVD is the second wavelength λ ₂ = 655 nm, protective layer PL2 thickness t ₂ = 0.6 mm, numerical aperture NA2 = 0.65, CD optical specifications are third wavelength λ ₃ = 785 nm, protective layer PL3 thickness t ₃ = 1.2 mm and numerical aperture NA3 = 0.50. 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 PU records / reproduces information to / from the blue-violet semiconductor lasers LD1, DVD that emits a 405 nm blue-violet laser beam (first beam) when recording / reproducing information on the HD. The first emission point EP1 that emits a 655 nm red laser beam (second beam) and the 785 nm infrared laser beam (first beam) that is emitted when information is recorded / reproduced on a CD. DVD / CD laser light source unit LU, HD / DVD / CD shared photodetector PD, aberration correction element SAC, and this aberration, which are formed on a single chip with a second light emitting point EP2 that emits three luminous fluxes). An objective composed of an objective lens OL having aspherical surfaces on both sides, which has a function of condensing the laser beam transmitted through the correction element SAC on the information recording surfaces RL1, RL2, and RL3. Unit OU (objective optical system), a biaxial actuator AC1, a monoaxial actuator AC2, a first lens EXP1 having a negative paraxial refractive power, and a second lens EXP2 having a positive paraxial refractive power. The reflected light beams from the expander lens EXP, the first polarizing beam splitter BS1, the second polarizing beam splitter BS2, the first collimating lens COL1, the second collimating lens COL2, the third collimating lens COL3, and the information recording surfaces RL1, RL2, and RL3. On the other hand, it is composed of a sensor lens SEN for adding astigmatism. In addition to the blue-violet semiconductor laser LD1 described above, a blue-violet SHG laser can also be used as the HD light source.

  When recording / reproducing information with respect to HD in the optical pickup device PU, 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 by the first collimating lens COL1, then reflected by the first polarization beam splitter BS1, passes through the second polarization beam splitter BS2, and passes through the first lens. After being expanded by passing through EXP1 and the second lens EXP2, the diameter of the light beam is regulated by a stop STO (not shown) and formed on the information recording surface RL1 by the objective lens unit OU via the HD protective layer PL1. Become a spot. The objective lens unit OU performs focusing and tracking by a biaxial actuator AC1 disposed in the periphery thereof.

  The reflected light flux modulated by the information pits on the information recording surface RL1 passes through the objective lens unit OU, the second lens EXP2, the first lens EXP1, the second polarizing beam splitter BS2, and the first polarizing beam splitter BS1 again, When passing through the three collimating lens COL3, it becomes a convergent light beam, is added with astigmatism by the sensor lens SEN, and 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 addition, when the optical pickup device PU records / reproduces information with respect to a DVD, the light emitting point EP1 is caused to emit light. The divergent light beam emitted from the light emitting point EP1 is converted into a parallel light beam by the second collimating lens COL2, as shown by the broken line in FIG. 1, and then reflected by the second polarization beam splitter BS2, The diameter is increased by transmitting through the first lens EXP1 and the second lens EXP2, and becomes a spot formed on the information recording surface RL2 via the protective layer PL2 of the DVD by the objective lens unit OU. The objective lens unit OU performs focusing and tracking by a biaxial actuator AC1 disposed in the periphery thereof.

  The reflected light flux modulated by the information pits on the information recording surface RL2 passes through the objective lens unit OU, the second lens EXP2, the first lens EXP1, the second polarizing beam splitter BS2, and the first polarizing beam splitter BS1 again, When passing through the three collimating lens COL3, it becomes a convergent light beam, is added with astigmatism by the sensor lens SEN, and converges on the light receiving surface of the photodetector PD. And the information recorded on DVD can be read using the output signal of photodetector PD.

  In the optical pickup device PU, when information is recorded / reproduced with respect to a CD, the interval between the first lens EXP1 and the second lens EXP2 is narrower than that during recording / reproduction of information with respect to HD. After the first lens EXP1 is driven in the optical axis direction by the uniaxial actuator AC2, the light emitting point EP2 is caused to emit light. The divergent light beam emitted from the light emitting point EP2 is converted into a loose divergent light beam by the second collimating lens COL2, as shown by the dashed line in FIG. 1, and then reflected by the second polarization beam splitter BS2. A spot which is enlarged by passing through the first lens EXP1 and the second lens EXP2 and converted into a divergent light beam and formed on the information recording surface RL3 by the objective lens unit OU via the protective layer PL3 of the CD. Become. The objective lens unit OU performs focusing and tracking by a biaxial actuator AC1 disposed in the periphery thereof.

  The reflected light flux modulated by the information pits on the information recording surface RL2 passes through the objective lens unit OU, the second lens EXP2, the first lens EXP1, the second polarizing beam splitter BS2, and the first polarizing beam splitter BS1 again, When passing through the three collimating lens COL3, it becomes a convergent light beam, is added with astigmatism by the sensor lens SEN, and converges on the light receiving surface of the photodetector PD. And the information recorded on CD can be read using the output signal of photodetector PD.

As schematically shown in FIG. 2, the objective lens unit OU in the present embodiment has the smallest spherical aberration with respect to the aberration correction element SAC, the first wavelength λ ₁ and the thickness t ₁ of the HD protective layer PL _1. The objective lens OL whose aspherical shape is designed so as to have the following configuration is integrated coaxially through the lens frame B. Specifically, the aberration correction element SAC is fitted and fixed to one end of the cylindrical lens frame B, and the objective lens OL is fitted and fixed to the other end, and these are coaxially integrated along the optical axis X. It has a configuration.

  Next, the configuration of the aberration correction element SAC and the principle of aberration correction will be described. As shown in FIG. 2, the aberration correction element SAC has a configuration in which a base lens BL that is a glass lens and a resin layer UV that is an ultraviolet curable resin are laminated on the surface of the base lens BL. A diffractive structure DOE1 having a ring-shaped step is formed on the boundary surface of the resin layer UV.

The diffraction efficiency η (λ) of the diffractive structure DOE1 formed at the boundary between the base lens BL and the resin layer UV having different Abbe numbers (dispersions) is generally the wavelength λ, and the base lens BL and the resin at the wavelength λ. As a function of the refractive index difference Δn (λ) with respect to the layer UV, the step d of the diffractive structure DOE1, and the diffraction order M (λ), it is expressed by the following equation (13).
η (λ) = sinc ² [[d · Δn (λ) / λ] −M (λ)] (13)
However, sinc (X) = sin (πX) / (πX), and the value of η (λ) is closer to 1 as the value in [] is closer to an integer.

The difference in refractive index at the first wavelength λ1 used for HD is Δn1, the diffraction order of the diffracted light of the first light beam is M1, the difference in refractive index at the second wavelength λ2 used for DVD is Δn2, and the diffracted light of the second light beam. of the difference between the refractive index of the third wavelength λ3 to use diffraction orders to M2, CD [Delta] n3, when the third light flux M3 diffraction order of the diffracted light, the diffraction efficiency in each wavelength η (λ _1), η ( λ ₂ ) and η (λ ₃ ) are expressed by the following equations (14) to (16).
η (λ ₁ ) = sinc ² [[d · Δn1 / λ ₁ ] −M1] (14)
η (λ ₂ ) = sinc ² [[d · Δn2 / λ ₂ ] −M2] (15)
η (λ ₃ ) = sinc ² [[d · Δn3 / λ ₃ ] −M3] (16)
In order to ensure high diffraction efficiency at each wavelength, the difference in refractive index Δni (i is 1, so that the values in [] in Equations (14) to (16) are close to integers. choose 2,3 or) and (i.e. have a difference .DELTA..nu _d Abbe number) based lens BL and the resin layer UV with a, and the step d, one) of the diffraction orders Mi (i is 1, 2, 3 It will be good.

The base curve BC, which is a macroscopic curvature of the diffractive structure DOE1, is configured as an aspherical surface, and as described above, the difference Δν _d between the Abbe number of the base lens BL at the d line and the Abbe number of the resin layer UV at the d line is While satisfying the above expression (1), the difference Δn1 between the refractive index of the base lens BL at the first wavelength λ _{1 and} the refractive index of the resin layer UV at the first wavelength λ ₁ satisfies the above expression (2). .
The spherical aberration due to the difference in the protective layer thickness of HD and DVD on the surface of the base lens BL where the diffractive structure DOE1 is formed (hereinafter referred to as “first diffractive surface”), and the protective layer thickness of HD and CD. Both spherical aberrations due to the difference are corrected.

Specifically, the first diffractive surface has a negative paraxial diffraction power (an effect of diverging the light beam), and all of the first, second, and third light beams that pass through the first diffractive surface are diffractive. (Divergent action) is received.
Further, the boundary surface and the optical surface of the resin layer UV opposite to the boundary surface have positive paraxial refraction power (an effect of converging the light beam).
The first light beam incident on the aberration correction element SAC as a parallel light beam is diverged by the first diffractive surface, but at the same time, converges by the refracting action of the boundary surface and the optical surface of the resin layer UV opposite to the boundary surface. The light beam goes straight without bending. That is, the above expressions (3) and (4) are satisfied.

In addition, the second light beam incident on the aberration correction element SAC as a parallel light beam is subjected to a diverging action on the first diffraction surface and at the same time a converging action due to a refraction action. Here, since the diffraction power increases in proportion to the wavelength, as described above, the first light beam travels straight without canceling the paraxial diffraction power and the paraxial refraction power. Since the paraxial diffraction power is larger than the paraxial refraction power in the light beam, the second light beam is emitted from the aberration correction element SAC as a divergent light beam. Thereby, the spherical aberration due to the difference in the protective layer thickness between HD and DVD is corrected.
In addition, the third light beam incident on the aberration correction element SAC with a loose divergent light beam is also diverged by the first diffractive surface, but for the same reason as the second light beam, the third light beam becomes a divergent light beam and the aberration correction element SAC. Is injected from. At this time, the degree of divergence of the third light flux is larger than that of the second light flux. This is because the diffraction power for the third light beam is larger than the paraxial diffraction power for the second light beam due to the relationship of λ ₃ > λ ₂ , and the third light beam is a divergent light beam that is loose relative to the aberration correction element SAC. This is due to the incident light. Thereby, the spherical aberration due to the difference in the protective layer thickness of HD and CD is corrected.

In this way, the base lens BL having a difference in Abbe number satisfying the expression (1) and the resin layer UV are laminated, and the diffractive structure DOE1 is formed on the boundary surface, which has been difficult in the prior art. Both the spherical aberration correction effect and the transmittance can be ensured for the purple laser beam (first beam) and the infrared laser beam (third beam). In addition, when the base lens BL and the resin layer UV have a refractive index difference satisfying the expression (2) at the first wavelength λ ₁ , the step along the optical axis of each annular zone can be reduced. The diffraction structure DOE1 can be easily manufactured. In addition, in a diffractive structure in which the base curve BC is flat, it is difficult to achieve both correction of spherical aberration and correction of the sine condition. However, the aberration correction element SAC is configured by configuring the base curve BC to be aspherical or spherical. The correction of the spherical aberration and the correction of the sine condition for the first light beam can be made compatible, and the design performance for the first light beam can be improved.

In the aberration correction element SAC of the present embodiment, | Δν _d | = 34.3, | Δn1 | = 0.0496, | Δn2 | / | Δn1 | = 1.44, | Δn3 | / | Δn1 | = A material satisfying 1.50, | Δn3 | / | Δn2 | = 1.05 is selected as the material of the base lens BL and the resin layer UV, and the step of the diffractive structure DOE1 is set to d = 9.14 μm. Therefore, the first-order diffracted light is generated for the light flux of any wavelength (M1 = M2 = M3 = 1). The diffraction efficiency of each primary diffraction light is 95.3% for the first light beam, 100% for the second light beam, and 94.4% for the third light beam. It has been secured.
In this embodiment, the aberration correction element SAC and the objective lens OL are integrated via the lens frame B. However, when the aberration correction element SAC and the objective lens OL are integrated, the aberration correction element SAC and It is only necessary to hold the relative positional relationship with the objective lens OL so as not to change. In addition to the method using the lens frame B as described above, each of the aberration correction element SAC and the objective lens OL is used. A method of fitting and fixing the flange portions may be used.

  Since the aberration correction element SAC and the objective lens OL are held in such a manner that the relative positional relationship between the aberration correction element SAC and the objective lens OL remains unchanged, the occurrence of aberration during focusing and tracking can be suppressed, and excellent focusing can be achieved. Characteristics or tracking characteristics can be obtained.

  Further, by driving the first lens EXP1 of the expander lens EXP in the optical axis direction by the uniaxial actuator AC2, the spherical aberration of the spot formed on the HD information recording surface RL1 can be corrected. The cause of the occurrence of spherical aberration corrected by adjusting the position of the first lens EXP1 is, for example, wavelength variation due to manufacturing error of the blue-violet semiconductor laser LD1, change in refractive index or refractive index distribution of the objective lens OL system due to temperature change, two layers For example, focus jump between information recording layers of a multi-layer disc such as a disc or a four-layer disc, thickness variation or thickness distribution due to manufacturing errors of an HD protective layer, and the like. Note that the spherical aberration of the spot formed on the HD information recording surface RL1 can be corrected even if the second lens EXP2 or the first collimating lens COL1 is driven in the optical axis direction instead of the first lens EXP1.

In the above description, the first lens EXP1 is driven in the optical axis direction to correct the spherical aberration of the spot formed on the HD information recording surface RL1, but on the DVD information recording surface RL2. Further, it is possible to correct the spherical aberration of the spot formed on the CD and further the spherical aberration of the spot formed on the information recording surface RL3 of the CD.
Further, in the present embodiment, the DVD / CD laser light source unit LU in which the first light emission point EP1 and the second light emission point EP2 are formed on one chip is used. Further, a single-chip laser light source unit for HD / DVD / CD having a light emitting point for emitting a laser beam having a wavelength of 405 nm for HD formed on the same chip may be used. Alternatively, a one-can laser light source unit for HD / DVD / CD in which three laser light sources of a blue-violet semiconductor laser, a red semiconductor laser, and an infrared semiconductor laser are housed in one housing may be used.

In the present embodiment, the light source and the photodetector PD are arranged separately. However, the present invention is not limited to this, and a laser light source module in which the light source and the photodetector are integrated may be used.
Although not shown, the optical pickup device PU shown in the above embodiment, the rotational drive device that holds the optical disc rotatably, and the control device that controls the drive of these various devices are mounted, so An optical disc drive apparatus capable of executing at least one of information recording and information reproduction on the optical disc can be obtained.
In the present embodiment, although not shown, an aperture limiting filter for performing aperture limitation corresponding to the numerical aperture NA2 and the numerical aperture NA3 is provided.

[Second Embodiment]
Hereinafter, the second embodiment of the present invention will be described with reference to the drawings. However, description of portions having the same configuration as that of the first embodiment will be omitted.
In the present embodiment, the base lens BL is made of resin, and a resin layer UV that is an ultraviolet curable resin is laminated on the surface of the base lens BL.
The present embodiment is characterized in that a phase structure different from the diffractive structure DOE1 is further added to the objective lens unit OU.
Specifically, the objective lens unit OU according to the present embodiment has an aberration correction element SAC, a first wavelength λ _1, and a thickness t ₁ of the HD protective layer PL ₁ as schematically shown in FIG. The objective lens OL whose aspherical shape is designed so that the spherical aberration is minimized is configured to be coaxially integrated via the lens frame B.

The aberration correction element SAC has a configuration in which a base lens BL and a resin layer UV are laminated on the surface of the base lens BL, and a boundary step between the base lens BL and the resin layer UV has an annular step. A diffractive structure DOE1 is formed and a diffractive structure DOE2 as a phase structure is formed on the optical surface of the base lens BL opposite to the boundary surface.
Then, spherical aberration due to the difference in protective layer thickness between HD and CD is corrected on the first diffractive surface, and HD and DVD on the surface on which the diffractive structure DOE2 of the base lens BL is formed (hereinafter referred to as “second diffractive surface”). The spherical aberration due to the protective layer thickness difference is corrected.
Specifically, the first diffractive surface has a negative paraxial diffraction power (an effect of diverging the light beam), and all of the first, second, and third light beams that pass through the first diffractive surface are diffractive. (Divergent action) is received (first-order diffraction).

Further, the second diffractive surface has positive paraxial diffraction power (an effect of converging the light beam), and only the second light beam passing through the second diffractive surface is subjected to the diffractive action (1). Next diffraction).
Here, the principle of diffracted light generation in the diffractive structure DOE2 will be described. The diffractive structure DOE2 has a characteristic of diffracting the second light beam without diffracting the first light beam and the third light beam. The diffractive structure DOE2 has a structure in which cross-sectional patterns including the optical axis are arranged in a concentric pattern, and the number of level planes is set for each predetermined number of level planes (every 5 levels in FIG. 3). The steps are shifted by the corresponding number of steps (four steps in FIG. 3). Here, one step Δ of the staircase structure is Δ = 2 · λ ₁ / (n1 _BL −1) ≈1.2 · λ ₂ / (n2 _BL −1) ≈1 · λ ₃ / (n3 _BL −1). ) Is set to satisfy the height. Here, _{n1 BL} is the refractive index of the base lens BL at the first wavelength lambda _{1, n2 BL} is the refractive index of the base lens BL at the second wavelength lambda _{2, n3 BL} base lens in the third wavelength lambda ₃ It is the refractive index of BL.

Since the optical path difference caused by this step Δ is twice the first wavelength λ _{1 and once} the third wavelength λ ₃ , the first light flux and the third light flux are not affected by the diffractive structure DOE 2. It passes through as it is.
On the other hand, since the optical path difference caused by the step Δ is 1.2 times the second wavelength λ ₂ , the phase of the second light beam passing through the level surface before and after the step is shifted by 2π / 5. Since one sawtooth is divided into five, the phase shift of the second light beam is exactly 5 × 2π / 5 = 2π for one sawtooth, and first-order diffracted light is generated.
Further, the boundary surface and the optical surface of the resin layer UV opposite to the boundary surface have positive paraxial refraction power (an effect of converging the light beam).

The first light beam incident on the aberration correction element SAC as a parallel light beam passes through the second diffractive surface as it is and undergoes a diverging action at the first diffractive surface, but at the same time, the boundary surface and the resin layer opposite to the boundary surface By receiving the convergence effect by the refractive action of the UV optical surface, the light beam goes straight without being bent. That is, the above expressions (3) and (4) are satisfied.
The third light beam incident on the aberration correction element SAC as a parallel light beam also passes through the second diffractive surface as it is, and is emitted from the aberration correction element SAC as a divergent light beam by being diverged by the first diffractive surface. As a result, spherical aberration due to the difference in the protective layer thickness between HD and CD is corrected.

The second light beam incident on the aberration correction element SAC as a parallel light beam is subjected to a converging action by receiving a diffractive action on the second diffractive surface, but is corrected for aberrations as a divergent light beam by receiving a diverging action on the first diffractive surface. Emitted from element SAC.
At this time, the divergence degree of the second light flux is smaller than the divergence degree of the third light flux. This is because the second light flux is once subjected to the convergence action by the second diffraction surface. As a result, spherical aberration due to the difference in the protective layer thickness between HD and DVD is corrected.

As described above, by forming the diffractive structure DOE2 as the phase structure on the optical surface opposite to the boundary surface among the optical surfaces of the base lens BL, the light condensing characteristics for each light flux of the objective lens unit OU can be obtained. It can be made better. The phase structure may be a diffractive structure or an optical path difference providing structure. The aberration corrected by the phase structure may be, for example, chromatic aberration associated with a minute change in the first wavelength λ ₁ or spherical aberration generated due to a change in refractive index of the objective lens OL associated with a temperature change. good.
Further, the diffractive structure DOE2 without diffracting the first light flux and the third light flux, the second light flux is selectively possible to have the property of diffracting, spherical aberration due to the difference between t ₁ and t _2, Alternatively, the spherical aberration due to the difference between the first wavelength λ ₁ and the second wavelength λ ₂ is corrected, and the spherical aberration due to the difference between t ₁ and t ₃ is corrected by the diffraction structure DOE1 formed on the boundary surface. By doing so, it is possible to correct the spherical aberration of the light flux of each wavelength with the same magnification while ensuring high diffraction efficiency for the light flux of each wavelength.

In the aberration correction element SAC of the present embodiment, | Δν _d | = 26.7, | Δn1 | = 0.0297, | Δn2 | / | Δn1 | = 1.53, | Δn3 | / | Δn1 | = 1.61, | Δn3 | / | Δn2 | = 1.05 is selected as the material of the base lens BL and the resin layer UV, and the step of the diffractive structure DOE1 is set to d = 15.06 μm. Therefore, the first-order diffracted light is generated for the light flux of any wavelength (M1 = M2 = M3 = 1). The diffraction efficiency of each first-order diffracted light is 96.5% for the first light beam, 99.3% for the second light beam, and 97.8% for the third light beam. Efficiency is secured.
In the diffractive structure DOE2, only the second light beam is selectively diffracted as described above, but the diffraction efficiency of the light beam of each wavelength is 100.0% for the first light beam (non-diffracted light), and the second light beam. The (first-order diffracted light) is 87.5% and the third light beam (non-diffracted light) is 100%, and high diffraction efficiency can be secured for light beams of any wavelength.

[Third embodiment]
Hereinafter, the third embodiment of the present invention will be described with reference to the drawings. However, description of portions having the same configuration as that of the second embodiment will be omitted.
Also in the present embodiment, as in the second embodiment, the base lens BL is made of resin, and a resin layer UV that is an ultraviolet curable resin is laminated on the surface of the base lens BL.

Similar to the second embodiment, the present embodiment is characterized in that a phase structure different from the diffractive structure DOE1 is further added to the objective lens unit OU.
Specifically, the objective lens unit OU according to the present embodiment has an aberration correction element SAC, a first wavelength λ _1, and a thickness t ₁ of the HD protective layer PL ₁ as schematically shown in FIG. The objective lens OL whose aspherical shape is designed so that the spherical aberration is minimized is configured to be coaxially integrated via the lens frame B.

The aberration correction element SAC has a configuration in which a base lens BL and a resin layer UV are laminated on the surface of the base lens BL, and a boundary step between the base lens BL and the resin layer UV has an annular step. A diffractive structure DOE1 is formed and a diffractive structure DOE2 as a phase structure is formed on the optical surface of the base lens BL opposite to the boundary surface.
Then, the spherical aberration due to the difference in the protective layer thickness of HD and CD is corrected by the refraction and divergence action of the boundary surface and the optical surface of the resin layer UV opposite to the boundary surface. The spherical aberration due to the difference in the protective layer thickness is corrected.

Specifically, the first diffractive surface has a positive diffractive power (an effect of converging the light beam), and only the first light beam passing through the first diffractive surface is subjected to the diffractive effect (convergent effect). (First-order diffraction).
Further, the second diffractive surface has a positive diffractive power (an effect of converging the light beam), and only the second light beam passing through the second diffractive surface is subjected to the diffractive action (first-order diffraction). ).
Further, the boundary surface and the optical surface of the resin layer UV on the opposite side of the boundary surface have negative refractive power (an action for diverging a light beam).

The first light beam that is incident on the aberration correction element SAC as a parallel light beam passes through the second diffractive surface as it is and is converged by the first diffractive surface. Go straight on. That is, the above expressions (3) and (4) are satisfied. The chromatic aberration of the first light beam is corrected by the action of the first diffractive surface.
Further, the third light beam incident on the aberration correction element SAC as a parallel light beam passes through the second diffractive surface and the first diffractive surface as it is, and is refracted by the boundary surface and the optical surface of the resin layer UV opposite to the boundary surface. The third light flux is emitted from the aberration correction element SAC as a divergent light flux. Thereby, the spherical aberration due to the difference in the protective layer thickness of HD and CD is corrected.

The second light beam incident on the aberration correction element SAC as a parallel light beam is subjected to a converging action by being diffracted by the second diffractive surface, but the optical interface of the resin layer UV on the side opposite to the boundary surface is not provided. By receiving a diverging action due to the refraction action of the surface, a divergent light beam is emitted from the aberration correction element SAC.
At this time, the divergence degree of the second light flux is smaller than the divergence degree of the third light flux. This is because the second light flux is once subjected to the convergence action by the second diffraction surface. As a result, spherical aberration due to the difference in the protective layer thickness between HD and DVD is corrected.
The principle of diffracted light generation of the diffractive structure DOE2 in the present embodiment is the same as the principle of the diffractive structure DOE2 in the second embodiment, so a detailed description is omitted here.

In the aberration correction element SAC of the present embodiment, | Δν _d | = 33.7, | Δn1 | = 0.0458, | Δn2 | / | Δn1 | = 0.271, | Δn3 | / | Δn1 | = A material such that 0.167, | Δn3 | / | Δn2 | = 0.617 is selected as the material of the base lens BL and the resin layer UV, and the step of the diffractive structure DOE1 is set to d = 8.84 μm. Therefore, first-order diffracted light is generated in the first light flux, and the second light flux and the third light flux are transmitted as they are without being diffracted. (M1 = 1, M2 = M3 = 0). The diffraction efficiency of the light flux of each wavelength is 100% for the first light flux (first-order diffraction), 91.2% for the second light flux (non-diffracted light), and 97.6% for the third light flux (non-diffracted light). A high diffraction efficiency can be secured for a light flux of any wavelength.
In the diffractive structure DOE2, only the second light beam is selectively diffracted as described above, but the diffraction efficiency of the light beam of each wavelength is 100.0% for the first light beam (non-diffracted light), and the second light beam. The (first-order diffracted light) is 87.5% and the third light beam (non-diffracted light) is 100%, and high diffraction efficiency can be secured for light beams of any wavelength.

Further, in the present embodiment, diffraction as a phase structure is performed on the optical surface of the base lens BL opposite to the boundary surface, that is, the boundary surface between the material having a larger Abbe number in the d-line and air. Since the structure DOE2 is formed, the diffraction efficiency with respect to the wavelengths λ ₁ , λ ₂ , and λ _{3 of} the first light beam, the second light beam, and the third light beam can be increased. Here, although the case where the diffractive structure DOE2 is a wavelength selective diffractive structure has been described as an example in the present embodiment, a blazed diffractive structure may be used as shown in FIG.
For example, if the diffractive structure DOE2 is a wavelength-selective diffractive structure, a phase difference can be given only to a light beam having a predetermined wavelength, a diffractive action can be given only to the DVD light, and the DVD remains. Spherical aberration can be corrected.
On the other hand, when the diffractive structure DOE2 is a blazed diffractive structure, chromatic aberration correction is effective.

In this embodiment, the diffractive structure DOE1 is formed on the boundary surface between the base lens BL and the resin layer UV, and the diffractive structure DOE2 is formed on the boundary surface between the material having the larger Abbe number in the d-line and air. As shown in FIG. 8, the objective lens OL arranged on the disk side has an Abbe number νd of d line satisfying 40 ≦ ν _d ≦ 70, as shown in FIG. The diffractive structure DOE3 may be formed on the surface of the OL.
Thus, since the Abbe number ν _d of the d line in the objective lens OL arranged on the disk side satisfies the above formula and the surface of the objective lens OL has a diffractive structure, the first light flux, The diffraction efficiency with respect to the wavelengths λ ₁ , λ ₂ , and λ ₃ of the two light beams and the third light beam can be increased.

Also, when thus provided a diffractive structure DOE 1, DOE 2, DOE 3, the thickness _{t 2} of the protective layer PL2 of DVD, set to satisfy _{0.9 × t 1 ≦ t 2 ≦} 1.1 × t 1 If this is the case, only the spherical aberration caused by the difference in wavelength, such as a combination of HD DVD and DVD, is corrected, so that the diffraction pitch can be increased and the workability can be improved.

Next, a specific numerical example (Example 1) of the objective lens unit OU composed of the aberration correction element SAC and the objective lens OL shown in FIG. 2 will be exemplified. The aberration correction element SAC has a configuration in which a resin layer made of an ultraviolet curable resin and a base lens made of a glass lens (BACD5 manufactured by HOYA) are laminated, and a diffractive structure DOE1 is formed on the boundary surface between the base lens and the resin layer. Has been. The objective lens OL is a glass lens whose aspherical shape as spherical aberration becomes minimum with respect to the thickness _{t 1} of the protective layer PL1 of the first wavelength λ1 and HD were designed (HOYA Co. BACD5) However, it may be a plastic lens.
Table 1 shows lens data of this example. In this numerical example, the optical path difference added to the incident light beam by the diffractive structure DOE1 is represented by an optical path difference function.

In the following Examples 2 and 3, including this embodiment, the numerical aperture NA1 of the optical density optical disk HD is 0.85, the numerical aperture NA2 of DVD is 0.65, and the numerical aperture NA3 of CD is 0.50. In Table 1 and Tables 2 and 3 described later, r (mm) is the radius of curvature, d (mm) is the lens interval, and n ₄₀₅ , n ₆₅₅ , and n ₇₈₅ are the first wavelengths λ ₁ (= 405 nm), respectively. ), The refractive index of the lens for the second wavelength λ ₂ (= 655 nm), the third wavelength λ ₃ (= 785 nm), ν _d is the Abbe number of the d-line lens, M _HD , M _DVD , M _CD are respectively These are the diffraction order of diffracted light used for recording / reproducing for HD, the diffraction order of diffracted light used for recording / reproducing for DVD, and the diffraction order of diffracted light used for recording / reproducing for CD. Further, a power of 10 (for example, 2.5 × 10 ⁻³ ) is expressed using E (for example, 2.5E-3).

The boundary surface (second surface) between the base lens and the resin layer, the optical surface on the optical disc side (third surface) of the resin layer, the optical surface on the light source side (fourth surface) of the objective lens OL, and the optical surface on the optical disc side (first surface). Each of the five surfaces is an aspherical shape, and this aspherical surface is expressed by an equation obtained by substituting the coefficient in the table into the following aspherical shape equation.
[Aspheric expression]
z = (y ² / R) / [1 + √ {1- (Κ + 1) (y / R) ² }] + A ₄ y ⁴ + A ₆ y ⁶ + A ₈ y ⁸ + A ₁₀ y ¹⁰ + A ₁₂ y ¹² + A ₁₄ y ¹⁴ + A ₁₆ y ¹⁶ + A ₁₈ y ¹⁸ + A ₂₀ y ²⁰ where z: aspherical shape (distance in the direction along the optical axis from the plane contacting the surface apex of the aspherical surface) y: distance from the optical axis R: radius of curvature Κ : conic coefficient _{A 4, A 6, A 8} , A 10, A 12, A 14, A 16, A 18, A 20: aspherical coefficients

The diffractive structure DOE1 is represented by an optical path difference added to the incident light beam by the diffractive structure DOE1. Such an optical path difference is represented by an optical path difference function φ (mm) obtained by substituting the coefficient in the table into an expression representing the next optical path difference function.
[Optical path difference function]
φ = M × λ / λ _B × (B ₂ y ² + B ₄ y ⁴ + B ₆ y ⁶ + B ₈ y ⁸ + B ₁₀ y ¹⁰ ) where φ: optical path difference function λ: wavelength of light beam incident on the diffractive structure λ _B : Production wavelength M: Diffraction order of diffracted light used for recording / reproducing with respect to optical disc y: Distance from optical axis B ₂ , B ₄ , B ₆ , B ₈ , B ₁₀ : Diffraction surface coefficient

Next, a specific numerical example (Example 2) of the objective lens unit OU composed of the aberration correction element SAC and the objective lens OL shown in FIG. 3 will be exemplified. The aberration correction element SAC has a configuration in which a resin layer made of an ultraviolet curable resin and a resin base lens are laminated, and a diffractive structure DOE1 is formed on the boundary surface between the base lens and the resin layer, and the light source side of the base lens. A diffractive structure DOE2 as a phase structure is formed on the optical surface. The objective lens OL is a glass lens (BACD5 manufactured by HOYA) whose aspheric shape is designed so that spherical aberration is minimized with respect to the first wavelength λ ₁ and the thickness t ₁ of the HD protective layer PL _1. However, it may be a plastic lens.
Table 2 shows lens data of this example. In this numerical example, the optical path difference added to the incident light beam by the diffractive structures DOE1 and DOE2 is represented by an optical path difference function.

  The boundary surface (second surface) between the base lens and the resin layer, the optical surface on the optical disc side (third surface) of the resin layer, the optical surface on the light source side (fourth surface) of the objective lens OL, and the optical surface on the optical disc side (first surface). Each of the five surfaces has an aspherical shape, and this aspherical surface is expressed by an equation obtained by substituting the coefficient in the table into the aspherical shape equation.

  The diffractive structures DOE1 and DOE2 are represented by optical path differences added to the incident light flux by each diffractive structure. Such an optical path difference is represented by an optical path difference function φ (mm) obtained by substituting the coefficient shown in Table 2 into the formula representing the optical path difference function.

Next, specific numerical value examples (Example 3) of the objective lens unit OU including the aberration correction element SAC and the objective lens OL shown in FIG. 4 will be exemplified. The aberration correction element SAC has a configuration in which a resin layer made of an ultraviolet curable resin and a resin base lens are laminated, and a diffractive structure DOE1 is formed on the boundary surface between the base lens and the resin layer, and the light source side of the base lens. A diffractive structure DOE2 as a phase structure is formed on the optical surface. The objective lens OL is a glass lens (BACD5 manufactured by HOYA) whose aspheric shape is designed so that spherical aberration is minimized with respect to the first wavelength λ ₁ and the thickness t ₁ of the HD protective layer PL _1. However, it may be a plastic lens.
Table 3 shows lens data of this example. In this numerical example, the optical path difference added to the incident light beam by the diffractive structure DOE is represented by an optical path difference function.

  The boundary surface (second surface) between the base lens and the resin layer, the optical surface on the optical disc side (third surface) of the resin layer, the optical surface on the light source side (fourth surface) of the objective lens OL, and the optical surface on the optical disc side (first surface). Each of the five surfaces has an aspherical shape, and this aspherical surface is expressed by an equation obtained by substituting the coefficients in Table 3 into the aspherical shape equation.

  The diffractive structures DOE1 and DOE2 are represented by optical path differences added to the incident light flux by each diffractive structure. Such an optical path difference is represented by an optical path difference function φ (mm) obtained by substituting the coefficient in the table into the formula representing the optical path difference function.

Next, as Example 4, Table 4 shows lens data in the case where a diffractive structure is also provided on the interface between the material having a larger Abbe number on the d line shown in FIG.

As shown in Table 4, in this embodiment, the focal length f1 = 2.60 mm at the wavelength λ ₁ 407 nm and the magnification m1 = 0 are set, and the focal length f2 = 2 at the wavelength λ ₂ = 655 nm. The focal length f3 = 2.54 mm and the magnification m3 = 0 when the wavelength λ ₃ = 785 nm are set.
Further, the refractive index nd = 1.5435 at the d-line of the base lens BL, the Abbe number ν _d = 56.7 at the d-line, the refractive index nd = 1.5600 at the d-line of the resin layer UV, and the Abbe number ν at the d-line. _d = 23.0, the refractive index nd = 1.5891 of the lens material of the objective lens OL, and the Abbe number ν _d = 61.3 of the _d -line.

The optical surface on the light source side (fifth surface) and the optical surface on the optical disc side (sixth surface) of the objective lens OL are aspherical, and this aspherical surface is expressed by the following aspherical shape formula with the coefficients in Table 4 It is expressed by a mathematical formula that substitutes.
[Aspheric expression]
z = (y ² / R) / [1 + √ {1- (Κ + 1) (y / R) ² }] + A ₄ y ⁴ + A ₆ y ⁶ + A ₈ y ⁸ + A ₁₀ y ¹⁰ + A ₁₂ y ¹² + A ₁₄ y ¹⁴

Further, each of the diffractive structure DOE1 formed on the boundary surface (third surface) between the base lens BL and the resin layer UV, and the diffractive structure DOE2 formed on the boundary surface (second surface) between the base lens BL and air, It is represented by the optical path difference added to the incident light beam by the diffractive structures DOE and DOE2. Such an optical path difference is represented by an optical path difference function φ (mm) obtained by substituting the coefficient in Table 4 into an expression representing the next optical path difference function.
[Optical path difference function]
Diffraction structure DOE
φ = λ × M × (B ₂ y ² + B ₄ y ⁴ + B ₆ y ⁶ + B ₈ y ⁸ + B ₁₀ y ¹⁰ )
Diffraction structure DOE2
φ = λ × M × (B ₂ y ² + B ₄ y ⁴ + B ₆ y ⁶ )
Here, since M is the diffraction order, in the case of the diffractive structure DOE on the third surface, 1 is substituted for HD DVD, 1 for DVD, and 1 for CD, and in the case of diffractive structure DOE2 on the second surface. Is substituted for HD DVD, 1 for DVD, and 1 for CD.

Next, as Example 5, Table 5 shows lens data when a diffraction structure is provided on the surface of the objective lens (objective optical element) shown in FIG.

As shown in Table 5, in this embodiment, the focal length f1 = 2.60 mm when the wavelength λ ₁ = 407 nm and the magnification m1 = 0 are set, and the focal length f2 = when the wavelength λ ₂ = 655 nm = 2.59 mm, magnification m2 = 0, and focal length f3 = 2.58 mm when wavelength λ ₃ = 785 nm, magnification m3 = 0.
Further, the refractive index nd = 1.5435 at the d-line of the base lens BL, the Abbe number ν _d = 56.7 at the d-line, the refractive index nd = 1.5600 at the d-line of the resin layer UV, and the Abbe number ν at the d-line. _d = 23.0, the refractive index nd = 1.5891 of the lens material of the objective lens OL, and the Abbe number ν _d = 61.3 of the _d -line.

The optical surface on the light source side (fifth surface) and the optical surface on the optical disc side (sixth surface) of the objective lens OL are aspherical, and this aspherical surface is a coefficient in Table 5 given by It is expressed by a mathematical formula that substitutes.
[Aspheric expression]
z = (y ² / R) / [1 + √ {1- (Κ + 1) (y / R) ² }] + A ₄ y ⁴ + A ₆ y ⁶ + A ⁸ y ₈ + A ₁₀ y ¹⁰ + A ₁₂ y ¹² + A ₁₄ y ¹⁴

Further, the diffractive structure DOE1 formed on the boundary surface (third surface) between the base lens BL and the resin layer UV and the diffractive structure DOE3 formed on the surface (fifth surface) of the objective lens OL are respectively diffractive structures DOE, It is expressed by the optical path difference added to the incident light beam by DOE3. Such an optical path difference is represented by an optical path difference function φ (mm) obtained by substituting the coefficient in Table 5 into an expression representing the following optical path difference function.
[Optical path difference function]
Diffraction structure DOE
φ = λ × M × (B ₂ y ² + B ₄ y ⁴ + B ₆ y ⁶ + B ₈ y ⁸ + B ₁₀ y ¹⁰ )
Diffraction structure DOE3
φ = λ × M × (B ₂ y ² + B ₄ y ⁴ + B ₆ y ⁶ )
Here, since M is the diffraction order, in the case of the diffractive structure DOE on the third surface, 1 is substituted for HD DVD, 1 for DVD, 1 for CD, and diffractive structure DOE3 on the fifth surface. Is substituted for HD DVD, 1 for DVD, and 1 for CD.

It is a principal part top view which shows the structure of an optical pick-up apparatus. It is a side view which shows an example of a structure of an objective lens unit. It is a side view which shows an example of a structure of an objective lens unit. It is a side view which shows an example of a structure of an objective lens unit. It is a side view which shows an example of a structure of an objective lens unit. It is a side view which shows an example of a structure of an objective lens unit.

Explanation of symbols

DOE diffraction structure OL objective lens OU objective lens unit PU optical pickup device SAC aberration correction element

Claims (22)

Performs recording and / or reproducing information for the first optical disk having a protective layer with a thickness of t ₁ using a first first light flux with wavelength lambda ₁ emitted from the first light source is emitted from the third light source An optical pickup device for recording and / or reproducing information on a third optical disc having a protective layer having a thickness t ₃ (> t ₁ ) using a third light beam having a third wavelength λ ₃ (> λ ₁ ) An objective optical system used for
The objective optical system collects the aberration correction element and the first and third light fluxes that have passed through the aberration correction element on the information recording surfaces of the first optical disc and the third optical disc, respectively. Objective lens,
The aberration correction element has a configuration in which a base lens and a resin layer are laminated on the surface of the base lens, and a first diffractive structure having a ring-shaped step is formed on the boundary surface between the base lens and the resin layer. And a base curve that is a macroscopic curvature of the first diffractive structure is configured as an aspherical surface or a spherical surface, and a difference Δν _d between the Abbe number of the d-line of the base lens and the Abbe number of the d-line of the resin layer. Satisfies the following equation (1), and the difference Δn1 between the refractive index of the base lens at the first wavelength λ _{1 and} the refractive index of the resin layer at the first wavelength λ ₁ satisfies the following equation (2): An objective optical system characterized by that.
20 <| Δν _d | <40 (1)
| Δn1 |> 0.02 (2) The pick-up device further uses a second light beam having a second wavelength λ ₂ (> λ ₁ ) emitted from the second light source to a second optical disc having a protective layer having a thickness t ₂ (≧ t ₁ ). The objective optical system according to claim 1, wherein information is recorded and / or reproduced.   The base curve is an aspheric surface in which an aspheric deformation amount, which is a distance along the optical axis from a spherical surface expressed by a paraxial radius of curvature, increases as the distance from the optical axis increases. 2. The objective optical system according to 2.   The objective optical system according to claim 3, wherein an optical surface of the resin layer opposite to the boundary surface is an aspherical surface having substantially the same shape as the base curve. A paraxial diffractive power P _D of the first wavelength lambda ₁ of the first diffractive structure, the aberration correcting element entire system the first wavelength λ1 paraxial refractive power P _RT in the following (3) and ( 4) The objective optical system according to any one of claims 1 to 3, wherein the expression (4) is satisfied.
_{P D · P RT <0 (} 3)
0.9 <| P _D · P _RT | <1.1 (4) The refractive index difference Δn2 of the second wavelength lambda ₂ of the refractive index between the resin layer in the second wavelength lambda ₂ of the base lens, the difference in refractive index in the third wavelength lambda ₃ of the resin layer Δn3 is less 3. The objective optical system according to claim 2, wherein the first diffraction structure has negative paraxial diffraction power while satisfying the expressions (5) to (7).
1.2 <| Δn2 | / | Δn1 | <2.2 (5)
1.4 <| Δn3 | / | Δn1 | <2.4 (6)
1.0 <| Δn3 | / | Δn2 | <2.0 (7) The objective optical system according to claim 6, wherein the diffractive structure corrects spherical aberration due to a difference between the t ₁ and the t ₃ . The first diffractive structure corrects a spherical aberration caused by a difference between the t ₁ and the t ₂ or a spherical aberration caused by a difference between the first wavelength λ ₁ and the second wavelength λ _2. The objective optical system according to claim 6 or 7.   9. The objective optical system according to claim 1, wherein a phase structure is formed on an optical surface opposite to the boundary surface among the optical surfaces of the base lens. Of the optical surfaces of the base lens, a phase structure is formed on an optical surface opposite to the boundary surface, and the phase structure does not diffract the first light beam and the third light beam, but the second light beam. Spherical aberration caused by the difference between the t ₁ and the t ₂ due to the phase structure or the spherical aberration caused by the difference between the first wavelength λ ₁ and the second wavelength λ ₂ The objective optical system according to claim 2, wherein the spherical aberration due to the difference between the t ₁ and the t ₃ is corrected by the first diffractive structure.   5. The second diffractive structure is formed on an interface between the base lens and the resin layer, which has a larger Abbe number in the d-line, and air. The objective optical system according to claim 1. The objective optical element arranged on the disk side has an Abbe number ν _{d of} d line satisfying the following formula (8), and a second diffractive structure is formed on the surface of the objective optical element. The objective optical system according to any one of claims 1 to 4.
40 ≦ ν _d ≦ 70 (8)   The objective optical system according to claim 11, wherein the second diffractive structure is a diffractive structure having a plurality of steps in cross section, and selectively diffracts or transmits light according to a wavelength.   The objective optical system according to claim 11, wherein the second diffractive structure is a blazed diffractive structure. The objective optical system according to claim 11, wherein the following expression (9) is satisfied.
0.9 × t ₁ ≦ t ₂ ≦ 1.1 × t ₁ (9)   The objective optical system according to claim 1, wherein the resin layer is an ultraviolet curable resin.   The objective optical system according to claim 1, wherein the base lens is manufactured by molding.   The objective optical system according to claim 1, wherein the base lens is made of resin. The objective optical system according to any one of claims 1 to 18, wherein the following relationship is satisfied.
α × λ ₁ = λ ₃
K1-0.1 ≦ α ≦ K1 + 0.1
K1: Natural number The objective lens, the objective optical system according to any one of claims 1 to 19, characterized in that the spherical aberration correction for a combination of the t ₁ and the first wavelength lambda ₁ is optimized .   An optical pickup device comprising the objective optical system according to any one of claims 1 to 20.   An optical disk drive device comprising the optical pickup device according to claim 21 mounted thereon.

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