JP5076747B2 - Objective lens for optical disc and optical disc apparatus - Google Patents

Description

  The present invention relates to an objective lens and an optical disc apparatus, and more particularly to a pickup lens for an optical disc such as a CD, a DVD, or a Blu-ray disc.

  In general, in an optical system, a refractive optical element (mainly a lens made of glass) that uses refraction is used to improve optical performance, in particular, to suppress the occurrence of aberrations and improve imaging performance. Yes. In this case, in order to sufficiently suppress Seidel's five aberrations and chromatic aberration related to the reference spectral line, it is necessary to increase the degree of freedom of aberration correction, and the number of lenses constituting the optical system is inevitably increased.

  Even in an objective lens (pickup lens) for an optical disk that should be reduced in size and weight, if the optical performance is to be improved, the number of lenses constituting the optical system tends to increase. In addition, in order to sufficiently correct the chromatic aberration of the optical system, it is necessary to use a plurality of optical materials having different refractive indexes and dispersions, and it is necessary to use an optical material (glass) having a large specific gravity, which increases the size of the optical system. In addition, it is easy to invite weight. Therefore, attempts have been made to correct chromatic aberration or spherical aberration by incorporating a diffractive optical element (diffraction grating) into the objective lens for an optical disk.

  However, even if a so-called single layer type diffraction grating is incorporated in an objective lens for an optical disk, the diffraction efficiency cannot be increased over a wide wavelength range. As a result, when trying to cope with a plurality of types of optical disks, the intensity of signals photoelectrically converted with respect to light of a plurality of different wavelengths is weakened, and the SN ratio is likely to deteriorate. In recent years, an increasing number of devices use light of a plurality of different wavelengths for a plurality of types of optical disks such as CDs, DVDs, and Blu-ray discs, and there is a need for objective lenses for optical disks that are well achromatic over a wide wavelength range. It has been.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a small and lightweight objective lens for an optical disc that is satisfactorily achromatic over a wide wavelength range.

In order to solve the above problems, in the first embodiment of the present invention, in an objective lens for an optical disc,
Having a numerical aperture of 0.4 or more,
A diffractive optical element comprising a positive lens having a first diffractive optical surface and a negative lens having a second diffractive optical surface;
Of the positive lens and the negative lens, one lens is formed of an optical material having a relatively high refractive index and low dispersion, and the other lens is formed of an optical material having a relatively low refractive index and high dispersion,
The first diffractive optical surface and the second diffractive optical surface have sawtooth-shaped cross sections that are rotationally symmetric with respect to the optical axis of the objective lens, and the first diffractive optical surface and the second diffractive optical surface are The grating heights h of the first and second diffractive optical surfaces are determined so as to have a diffraction efficiency of 1.0 with respect to light of a predetermined wavelength,
The positive lens is a biconvex lens;
When the thickness of the positive lens along the optical axis and the thickness of the negative lens along the optical axis is Dm,
0.05 <h / Dm <1.0
An objective lens characterized by satisfying the above conditions is provided.

  In this specification, the expression “arranged so that the first diffractive optical surface and the second diffractive optical surface face each other” means that the first diffractive optical surface and the second diffractive optical surface are in contact with each other. It corresponds to a wide concept including a state of “arranged” and a state of “arranged so that the first diffractive optical surface and the second diffractive optical surface face each other with a gap”.

  According to a second aspect of the present invention, there is provided an optical disc apparatus including the objective lens according to the first aspect.

  In the objective lens for an optical disk according to the present invention, a doublet advantageous for correcting chromatic aberration is formed by a positive lens and a negative lens. A multilayer diffractive optical surface that is advantageous for correcting chromatic aberration over a wide wavelength region is formed between the positive lens and the negative lens. As a result, in the present invention, by the achromatic action by the doublet formed by the positive lens and the negative lens, and the achromatic action by the multilayer diffractive optical surface formed between the positive lens and the negative lens, A small and lightweight objective lens for an optical disc that is satisfactorily achromatic over a wide wavelength range can be realized.

  Embodiments of the present invention will be described with reference to the accompanying drawings. Hereinafter, prior to specific description of the present invention, technical matters related to the present invention will be described. Conventionally, various attempts have been made to incorporate a diffractive optical surface into an optical system such as a pickup lens for an optical disk, for example, aiming at high performance and miniaturization that cannot be achieved by a refractive optical system and a reflective optical system. However, in the single-layer type diffractive optical element, diffraction flare is generated with respect to light having a wavelength deviating from the design wavelength, and the image quality and imaging performance are easily impaired. As a result, the application of single-layer diffractive optical elements has been limited to optical systems that use, for example, single-wavelength laser light or narrow-wavelength laser light.

  In recent years, a diffractive optical element called a multilayer type (or laminated type) has been proposed. For example, a multilayer diffractive optical element is formed by laminating a plurality of diffractive optical elements (diffractive optical members) having a diffractive optical surface formed in a sawtooth shape rotationally symmetric with respect to an optical axis in a form in which they are separated from each other or closely adhered It is comprised by. The multi-layer type diffractive optical element has a feature that high diffraction efficiency can be ensured over the entire desired wide wavelength region (for example, visible light region), and thus good wavelength characteristics can be ensured.

  For example, as shown in FIG. 1, the multilayer diffractive optical element includes a first diffractive optical member 11 that is disposed on the light incident side and formed of the first optical material, and a first diffractive optical element that is disposed on the light exit side. One optical material is composed of a second diffractive optical member 12 formed of a second optical material having a different refractive index and dispersion value. The pair of surfaces 11a and 12a on which the first diffractive optical member 11 and the second diffractive optical member 12 face each have, for example, a sawtooth cross section rotationally symmetric with respect to the optical axis (not shown). In this case, the grating height (groove height) h1 of the diffractive optical surface 11a of the first diffractive optical member 11 is determined to be a predetermined value so that the achromatic condition is satisfied simultaneously for light of two specific wavelengths. The grating height h2 of the diffractive optical surface 12a of the second diffractive optical member 12 can be determined to a predetermined value.

As a result, the multi-layer diffractive optical element has a diffraction efficiency of 1.0 for light of a specific wavelength, and can obtain a considerably high diffraction efficiency for light of other wavelengths. Here, the diffraction efficiency refers to the intensity value I _{0 of the} light incident on the diffractive optical element and the intensity value I ₁ of the first-order diffracted light included in the light transmitted through the diffractive optical element in the transmission type diffractive optical element. It is defined as the ratio η (= I ₁ / I ₀ ). By adopting such a multilayer structure, it becomes possible to apply a diffractive optical element to an optical system that uses light in a wide wavelength range, for example, for a photographic camera that uses white light. It becomes easy to use a diffractive optical element for an imaging lens or the like.

  In particular, by satisfying predetermined conditions for the first optical material and the second optical material, the grating height h1 of the diffractive optical surface 11a of the first diffractive optical member 11 and the grating height of the diffractive optical surface 12a of the second diffractive optical member 12 are increased. h2 can be made coincident with each other, and as a result, a multi-layered diffractive optical element (diffraction grating) PF arranged so that the diffractive optical surface 11a and the diffractive optical surface 12a are in contact with each other is obtained. In the close-contact multilayer diffractive optical element, the error sensitivity of the grating height of the diffractive optical surface (as compared to the separated multilayer diffractive optical element in which a pair of diffractive optical surfaces are opposed to each other with a gap therebetween ( There is a manufacturing merit that both the tolerance (tolerance) and the error sensitivity (tolerance) of the surface roughness of the diffractive optical surface become loose. That is, the multi-contact diffractive optical element has the advantages of excellent productivity, high mass productivity, and favorable cost reduction for optical products.

  One of the two diffractive optical members constituting the multilayer diffractive optical element may be made of an optical material having a relatively high refractive index and low dispersion, and the other may be made of an optical material having a relatively low refractive index and high dispersion. Although necessary, any diffractive optical member may be arranged on the light incident side. In particular, in the contact multilayer type diffractive optical element, it is important to select a combination of a relatively high refractive index and low dispersion optical material and a relatively low refractive index and high dispersion optical material. In order to reduce the manufacturing error sensitivity to a desired level for the close-contact multilayer diffractive optical element, it is important to set the difference in refractive index between the two diffractive optical members to 0.45 or less, and 0.2 or less. It is preferable to make it.

  Hereinafter, the configuration and operation of the present invention will be described. An objective lens (pickup lens) for an optical disk according to the present invention has a positive lens (corresponding to one diffractive optical member) having a numerical aperture of 0.4 or more and having a first diffractive optical surface and a second diffractive optical surface. A diffractive optical element including a negative lens (corresponding to the other diffractive optical member) is included. Of the positive lens and the negative lens, one lens is made of an optical material having a relatively high refractive index and low dispersion, and the other lens is made of an optical material having a relatively low refractive index and high dispersion. Further, the first diffractive optical surface and the second diffractive optical surface are disposed so as to face each other. That is, a multilayer diffractive optical surface is formed between the positive lens and the negative lens, and the positive lens and the negative lens constitute a contact multilayer diffractive optical element or a separated multilayer diffractive optical element. Yes.

  As described above, in the objective lens for an optical disk according to the present invention, the positive lens and the negative lens form a doublet that is advantageous for correcting chromatic aberration. A multilayer diffractive optical surface that is advantageous for correcting chromatic aberration over a wide wavelength region is formed between the positive lens and the negative lens. As a result, in the present invention, by the achromatic action by the doublet formed by the positive lens and the negative lens, and the achromatic action by the multilayer diffractive optical surface formed between the positive lens and the negative lens, A small and lightweight objective lens for an optical disc that is satisfactorily achromatic over a wide wavelength range can be realized.

  Incidentally, the numerical aperture (NA) is, for example, an amount representing the brightness of an image formed by the objective lens based on light from infinity, that is, the image plane illuminance, and is represented by Nsinθ. Here, N is the refractive index of the medium between the objective lens and the image plane, and θ is the maximum incident angle of light rays reaching the image plane. The larger the numerical aperture, the brighter the spot image. Further, a relationship of 2 × NA = 1 / FNO is established between the numerical aperture NA and the F number FNO. The present invention is intended for a bright pickup lens having an NA of 0.4 or more, but it is desirable to have a numerical aperture of 0.55 or more in order to achieve higher performance.

In the objective lens for an optical disk of the present invention, it is preferable that the following conditional expression (1) is satisfied. In conditional expression (1), f is the focal length of the objective lens, and D _T is the thickness (axial thickness) along the optical axis of the objective lens.
0.5 <f / D _T <10.0 (1)

Conditional expression (1) defines an appropriate range for the focal length f of the objective lens in proportion to the axial thickness D _T of the objective lens. Exceeding the upper limit of conditional expression (1) is not preferable because the focal length f becomes too long and it is difficult to achieve downsizing in the optical axis direction. In addition, it is not preferable to secure the required numerical aperture (NA) because the objective lens becomes large in the radial direction and large spherical aberration occurs, and good optical performance cannot be obtained.

On the other hand, if the lower limit of conditional expression (1) is not reached, the axial thickness _DT of the objective lens becomes too large, and it becomes difficult to ensure a sufficient distance between the objective lens and the object (optical disk), that is, a working distance. In addition, the possibility of adverse effects (such as spot blurring) due to birefringence of the resin material forming the lens increases, which is not preferable. In addition, in order to fully demonstrate the effect of this invention, it is desirable to set the upper limit of conditional expression (1) to 5.0. In addition, it is desirable to set the lower limit of conditional expression (1) to 0.8.

In the objective lens for an optical disk of the present invention, it is preferable that the following conditional expression (2) is satisfied. In conditional expression (2), fp is the focal length of the positive lens, and fn is the focal length of the negative lens.
0.03 <| fp / fn | <4.0 (2)

  Conditional expression (2) defines an appropriate refractive power distribution between the positive lens and the negative lens. If the upper limit value of conditional expression (2) is exceeded, the refractive power of the positive lens becomes too small, and short wavelength chromatic aberration tends to be undercorrected. For this reason, it is necessary to increase the refractive power of the diffractive optical surface. Absent. On the other hand, if the lower limit value of conditional expression (2) is not reached, the refractive power of the negative lens becomes too small, and short wavelength chromatic aberration tends to be undercorrected, so that the refractive power of the diffractive optical surface needs to be increased. Therefore, it is not preferable. In order to fully demonstrate the effects of the present invention, it is desirable to set the upper limit of conditional expression (2) to 1.0. Moreover, it is desirable to set the lower limit value of conditional expression (2) to 0.05. Moreover, in order to fully exhibit the effect of the present invention, it is more desirable to set the upper limit of conditional expression (2) to 0.45. It is more desirable to set the lower limit of conditional expression (2) to 0.09.

In the objective lens for an optical disk of the present invention, it is preferable that the following conditional expression (3) is satisfied. In conditional expression (3), Δνd is the Abbe number difference between the high refractive index and low dispersion optical material and the low refractive index and high dispersion optical material with reference to the d-line (λ = 587.6 nm). ΔNd is a refractive index difference at the d-line between the high refractive index and low dispersion optical material and the low refractive index and high dispersion optical material.
50 <Δνd / ΔNd <2000 (3)

  Conditional expression (3) defines the relationship between the high refractive index and low dispersion optical material and the low refractive index and high dispersion optical material. Exceeding the upper limit of conditional expression (3) is not preferable because it becomes difficult to obtain high diffraction efficiency over a wide wavelength range. Similarly, even if the lower limit value of conditional expression (3) is not reached, it is difficult to obtain high diffraction efficiency over a wide wavelength range, which is not preferable. In order to fully demonstrate the effects of the present invention, it is desirable to set the upper limit of conditional expression (3) to 900. In addition, it is desirable to set the lower limit of conditional expression (3) to 200.

In the objective lens for an optical disk of the present invention, it is preferable that the following conditional expression (4) is satisfied. In conditional expression (4), E _d is the diffraction efficiency of the multilayer diffractive optical element at the d-line, which is the dominant wavelength, and E _g is at the g-line (λ = 435.8 nm), which is shorter than the dominant wavelength. It is the diffraction efficiency of the diffractive optical element, and E _C is the diffraction efficiency of the diffractive optical element at C line (λ = 656.3 nm), which is longer than the dominant wavelength.
(E _g + E _C ) / (2 × E _d )> 0.8 (4)

  Conditional expression (4) defines an appropriate range for the balance of diffraction efficiency with respect to the use light having a broad band. When the lower limit of conditional expression (4) is not reached, the diffraction efficiency decreases at least one of the g-line, which is a relatively short wavelength, and the C-line, which is a long wavelength, relative to the d-line, which is the dominant wavelength. This is not preferable because the diffraction flare becomes large. In other words, light having a wavelength other than blazed light or light having an angle of view becomes unnecessary diffracted light, and the generation of diffraction flare increases. In order to fully demonstrate the effects of the present invention, it is desirable to set the lower limit of conditional expression (4) to 0.9. Moreover, it is desirable to set the upper limit of conditional expression (4) to 0.99.

In the objective lens for an optical disk of the present invention, it is preferable that the following conditional expression (5) is satisfied. In conditional expression (5), φ is the effective diameter (diameter) of the diffractive optical surface of the diffractive optical element, and f is the focal length of the objective lens as described above.
0.1 <φ / f <3.0 (5)

  Conditional expression (5) defines an appropriate range for the effective diameter (diameter) φ of the diffractive optical surface formed between the positive lens and the negative lens. If the upper limit value of conditional expression (5) is exceeded, the effective diameter φ of the diffractive optical surface becomes too large, making it difficult to manufacture the diffractive optical surface, resulting in an increase in cost of the multilayer diffractive optical element. Further, harmful light from the outside is not preferable because it easily enters the diffractive optical surface.

  On the other hand, if the lower limit of conditional expression (5) is not reached, the effective diameter φ of the diffractive optical surface becomes too small, and the tendency of the grating pitch of the diffractive optical surface to become small becomes strong. Not only is the manufacturing difficult and the cost is increased, but the generation of diffraction flare by the grating of the diffractive optical surface is increased, which is not preferable. In order to fully demonstrate the effects of the present invention, it is desirable to set the upper limit of conditional expression (5) to 2.5. In addition, it is desirable to set the lower limit of conditional expression (5) to 0.4.

In the objective lens for an optical disk according to the present invention, it is preferable that the positive lens is a biconvex lens and satisfies the following conditional expression (6). In conditional expression (6), h is the grating height of the diffractive optical surface of the diffractive optical element, and Dm is the thickness along the optical axis of the positive lens (axial thickness) and the thickness along the optical axis of the negative lens. The smaller one of (axial thickness).
0.05 <h / Dm <2.0 (6)

  Conditional expression (6) defines conditions necessary for realizing a relatively thin contact multilayered diffractive optical element. If the upper limit value of conditional expression (6) is exceeded, the grating of the diffractive optical surface becomes relatively high, and it becomes difficult to form a grating shape. Further, the step portion of the grating becomes large, and stray light is likely to be generated due to scattering of light incident on the wall surface of the lattice (surface portion forming the step: indicated by reference numeral 13 in FIG. 1).

  On the other hand, if the lower limit of conditional expression (6) is not reached, the lens as the diffractive optical member becomes too thick, and it is difficult to form a grating shape. Further, the internal absorption of light by the optical material is increased, the transmittance of the close-contact multilayer diffractive optical element, and hence the transmittance of the optical system is lowered, and coloring is likely to occur, which is not preferable. In addition, in order to fully demonstrate the effect of this invention, it is desirable to set the upper limit of conditional expression (6) to 1.0. Moreover, it is desirable to set the lower limit of conditional expression (6) to 0.07.

Furthermore, in the objective lens for an optical disk of the present invention, it is preferable to satisfy the following conditional expression (7), (8) or (9) in order to achieve better optical performance and specifications. In conditional expression (7), Pmin is the minimum pitch of the grating of the diffractive optical surface, and f is the focal length of the objective lens as described above. In conditional expressions (8) and (9), ΔW is the maximum spread width of the axial chromatic aberration of the spectral lines d, g, C, and F, and f is the focal length of the objective lens as described above. Furthermore, in the conditional expression (9), E _d , E _g and E _C are the diffraction efficiencies of the diffractive optical elements at the d-line, g-line and C-line, respectively, as described above.

0.03 <Pmin / f <2.0 (7)
0.0001 <ΔW / f <0.04 (8)
(E _g + E _d + E _C ) × f / ΔW> 200 (9)

  Conditional expression (7) defines an appropriate ratio between the focal length f of the objective lens and the minimum pitch Pmin of the grating of the diffractive optical surface. Usually, the pitch p of the grating is largest on the central axis (optical axis) and monotonously decreases toward the periphery. If the pitch p of the grating is small, the diffraction angle increases and the chromatic dispersion of the diffraction surface increases, which is effective for correcting chromatic aberration. However, if the pitch p of the grating is too small, not only is it difficult to process the diffractive optical surface, but the generation of diffractive flare is increased, which is not preferable. For this reason, it is important to define the minimum pitch Pmin of the lattice within an appropriate range.

  When the upper limit value of conditional expression (7) is exceeded, the minimum pitch Pmin of the grating becomes too small, making it difficult to process the diffractive optical surface, leading to a decrease in diffraction efficiency and generating unnecessary flare light. Since possibility becomes high, it is not preferable. On the other hand, if the lower limit value of conditional expression (7) is not reached, the minimum pitch Pmin of the lattice becomes too large, and a sufficient achromatic effect cannot be exhibited, which is not preferable. In addition, in order to fully demonstrate the effect of this invention, it is desirable to set the upper limit of conditional expression (7) to 0.2. In addition, it is desirable to set the lower limit value of conditional expression (7) to 0.001.

  Conditional expression (8) defines an appropriate correction range for axial chromatic aberration. Exceeding the upper limit value of conditional expression (8) is not preferable because chromatic aberration becomes too large and an edge or the like is colored. If the lower limit of conditional expression (8) is not reached, it is necessary to use a special optical material such as anomalous dispersion glass, which is not preferable. In order to fully demonstrate the effects of the present invention, it is desirable to set the upper limit of conditional expression (8) to 0.003. Moreover, it is desirable to set the lower limit value of conditional expression (8) to 0.002.

  Conditional expression (9) is a conditional expression that prescribes the required diffraction efficiency and chromatic aberration correction required for an optical system in which the wavelength of light used is in a wide band. Conditional expression (9) indicates that the greater the numerical value, the higher the diffraction efficiency and the lower the axial chromatic aberration for wavelengths over a wide band. If the lower limit of conditional expression (9) is not reached, it is difficult to achieve the required diffraction efficiency and chromatic aberration correction, which is not preferable. In order to fully demonstrate the effects of the present invention, it is desirable to set the lower limit of conditional expression (9) to 400.

  When actually configuring the objective lens for an optical disk of the present invention, it is preferable to satisfy the following requirements. In the present invention, the viscosity of the optical material forming the lens (uncured product viscosity) is preferably 5 mPa · s or more and 50000 mPa · s or less in order to maintain good moldability and ensure excellent mass productivity. . When a resin having a viscosity of 5 mPa · s or less is used as the optical material, the resin easily flows during molding, and workability may be deteriorated. In addition, when a resin of 50000 mPa · s or more is used, the resin is difficult to flow and workability is deteriorated, and bubbles are easily mixed.

  Therefore, in the objective lens for an optical disk of the present invention, it is preferable that at least one of the positive lens and the negative lens is formed of a UV curable resin. Furthermore, it is preferable to form both the positive lens and the negative lens with a UV curable resin from the viewpoint of production, particularly from the viewpoint of improving production efficiency. In this case, it is possible to reduce the man-hours required for manufacturing the multilayer diffractive optical element, which is advantageous because it leads to cost reduction.

  Alternatively, the positive lens may be formed of a resin for injection molding, and the negative lens may be formed of a UV curable resin. In this case, a cycloolefin resin, an acrylic resin, a polycarbonate resin, or the like can be used as the injection molding resin.

  In order to reduce the size and weight of the multi-layer diffractive optical element, and thus to reduce the size and weight of the objective lens, the specific gravity of the resin material forming the pair of lenses is preferably 2.0 or less. Since the specific gravity of resin is smaller than that of glass, forming a multilayer diffractive optical element using resin is very effective for reducing the weight of the optical system. In order to further reduce the size and weight of the optical system, it is preferable to form a multilayer diffractive optical element using a resin having a specific gravity of 1.6 or less. Furthermore, it is preferable that the interface of the positive lens or the negative lens with air is an aspherical refractive surface.

  It is also possible to add a colorant to the resin forming at least one of the positive lens and the negative lens to give a color filter effect. For example, an infrared cut filter, an ultraviolet cut filter, or the like can be configured to cut stray light.

  In addition, it is possible to arbitrarily set an aperture at an appropriate position in the optical path of the optical system, but the aperture is blocked so as to allow only effective rays that contribute to image formation to pass through (cut) unnecessary rays. It is preferable to configure. For example, the lens frame itself may function as an aperture stop, or the stop may be configured by a mechanical member positioned at a position away from the lens. The shape of the aperture opening is not limited to a circular shape, and may be, for example, an ellipse or a rectangle.

  An optical system comprising a plurality of components obtained by incorporating the objective lens for an optical disk of the present invention does not depart from the scope of the present invention. The same applies to an optical system obtained by incorporating a gradient index lens, a crystal material lens, or the like.

  When an optical disc apparatus is configured using the objective lens for an optical disc of the present invention, laser light is condensed on the information recording surface (disc surface on which pits are formed) of the optical disc via the objective lens, and reflected from the information recording surface. Light (signal light) is guided to the photoelectric sensor via the objective lens and converted into an electrical signal. Hereinafter, examples as specific numerical examples of the present embodiment will be described.

[First embodiment]
FIG. 2 is a diagram schematically showing a configuration of the objective lens for an optical disc according to the first example. Referring to FIG. 2, the objective lens PL according to the first example has an aspheric convex surface facing the light incident side and the optical disk DC side (light emission side) in order from the light incident side (left side in FIG. 2). And a negative meniscus lens (negative lens) Ln having a diffractive optical surface formed thereon, and a biconvex lens (positive lens) Lp having a diffractive optical surface formed on the light incident side. The negative meniscus lens Ln and the biconvex lens Lp are in close contact with each other so that the diffractive optical surfaces are in contact with each other. That is, the negative meniscus lens Ln and the biconvex lens Lp constitute a contact multilayer diffractive optical element PF. FIG. 2 shows a state in which light from an object at infinity (not shown) forms an image on the information recording surface of the substrate of the optical disk DC via the objective lens PL.

In the first and second embodiments, the height of the aspherical surface is y in the direction perpendicular to the optical axis, and the optical axis extends from the tangential plane at the apex of the aspherical surface to the position on the aspherical surface at height y. When the distance (sag amount) along the line is z, the apex radius of curvature is r, the cone coefficient is κ, and the n-th aspherical coefficient is C _n , the following equation (a) is expressed. In the following Tables (1) and (2), a surface formed in an aspherical shape is marked with * on the right side of the surface number.
z = (y ² / r) / [1+ {1-κ · y ² / r ² } ^1/2 ] + C ₂ · y ²
+ C ₄ · y ⁴ + C ₆ · y ⁶ + C ₈ · y ⁸ (a)

  The following table (1) lists the values of the specifications of the objective lens according to the first example. In the main specifications of Table (1), f represents the focal length of the objective lens, and NA represents the numerical aperture of the objective lens. Also, in the optical member specifications of Table (1), the surface number of the first column is the order of the surfaces from the light incident side, and r of the second column is the radius of curvature of each surface (the apex for an aspheric surface) Radius of curvature), d in the third column is the on-axis spacing of each surface, that is, the surface spacing, n (d) in the fourth column is the refractive index for the d-line (λ = 587.6 nm), and n in the fifth column. (g) is the refractive index for g-line (λ = 435.8 nm), n (C) in the sixth column is the refractive index for C-line (λ = 656.3 nm), and n (F) in the seventh column is Refractive indexes for the F line (λ = 486.1 nm) are shown. The radius of curvature r is such that the radius of curvature of the convex surface is positive toward the light incident side, and the radius of curvature of the concave surface is negative toward the light incident side.

  Further, in the specifications of the optical member in Table (1), the multi-contact diffractive optical element PF is expressed according to the ultrahigh refractive index method (UHI method). In the ultra-high refractive index method, the diffractive optical surface of the multi-layered diffractive optical element PF is regarded as a “thin lens” and the optical characteristics of the diffractive optical surface are expressed by a very high refractive index medium and an aspherical surface. To do.

  Specifically, in the first example, the first surface in the optical member specifications in Table (1) is the entrance surface of the negative meniscus lens Ln, the fourth surface is the exit surface of the biconvex lens Lp, and the fifth surface is the optical disc DC. The incident surface and the sixth surface of the substrate correspond to the information recording surface of the substrate of the optical disc DC. Then, the refractive index data of the second surface and the aspherical data of the third surface are obtained from the diffractive optical surface of the multi-contact diffractive optical element formed by the negative meniscus lens Ln and the biconvex lens Lp (reference symbol PF in FIG. 2). The optical characteristics are expressed. A, B, C, and D in the specifications of the optical member in Table (1) indicate the refractive indexes for the d-line, g-line, C-line, and F-line corresponding to the second surface, respectively. The notation in Table (1) is the same in the following Table (2).

Table (1)
(Main specifications)
f = 3.502
NA = 0.54

(Optical member specifications)
Surface number rd n (d) n (g) n (C) n (F) Optical member
1 * 3.70284 0.20000 1.527600 1.547700 1.523300 1.538500 (Ln)
2 3.00000 0.00000 ABCD (PF)
3 * 3.00000 2.20000 1.556900 1.571100 1.553700 1.564800 (PF / Lp)
4 -3.55020 2.19691
5 ∞ 0.60000 1.491080 1.501900 1.488540 1.497070 (DC)
6 (Information recording surface)
A = 0.100010000 × 10 ⁵ , B = 0.741868530 × 10 ⁴
C = 0.111704255 × 10 ⁵ , D = 0.827473110 × 10 ⁴

(Aspheric data)
One side: κ = −2.3000 C ₂ = 0
C ₄ = −1.000000 × 10 ⁻⁴ C ₆ = −1.90000 × 10 ⁻³
C ₈ = −1.03000 × 10 ⁻⁵

Three sides: κ = 1.0000 C ₂ = −8.70000 × 10 ⁻⁷
C ₄ = 0 C ₆ = 0 C ₈ = 0

(Values for conditional expressions)
f = 3.502
D _T = 2.4
fp = 3.318
fn = −33.222
Δνd = 15.46
ΔNd = 0.0293
E _g = 0.982
E _C = 0.982
E _d = 1.000
φ = 4.2
h = 0.02
Dm = 0.2
Pmin = 0.161
ΔW = 0.00989
(1) f / D _T = 1.459
(2) | fp / fn | = 0.0999
(3) Δνd / ΔNd = 527.65
(4) (E _g + E _C ) / (2 × E _d ) = 0.882
(5) φ / f = 1.199
(6) h / Dm = 0.1
(7) Pmin / f = 0.0006
(8) ΔW / f = 0.00282
(9) (E _g + E _d + E _C ) × f / ΔW = 1051.064

  3 and 4 are diagrams showing spherical aberration, astigmatism, distortion and coma in the objective lens for an optical disc of the first example. In each aberration diagram, NA is the numerical aperture, Y is the image height on the information recording surface of the substrate of the optical disk DC, d is the d-line (λ = 587.6 nm), and g is the g-line (λ = 435.8 nm). , F is the F line (λ = 486.1 nm), C is the C line (λ = 656.3 nm), H is the h line (λ = 404.7 nm), and L is the L line (λ = 780.nm). 0 nm).

  In the aberration diagrams showing astigmatism, the solid line shows the sagittal image plane, and the broken line shows the meridional image plane. The notation in FIG. 3 is the same in FIG. As is apparent from the respective aberration diagrams of FIGS. 3 and 4, the objective lens for the optical disc of the first example exhibits chromatic aberration over a wide wavelength range from the H line to the L line, despite being small and light. It can be seen that various aberrations are corrected well and excellent optical performance is ensured.

In the first embodiment, as shown in FIG. 4, excellent achromaticity is achieved over a relatively wide wavelength range from the g-line to the C-line and over a wider wavelength range from the h-line to the L-line. can do. At this time, the diffraction efficiency E _h of the diffractive optical element at the h-line is 0.914, and the diffraction efficiency E _L of the diffractive optical element at the L-line is 0.888, which is reached by the conventional single-layer diffractive optical element. Unprecedented high diffraction efficiency can be obtained.

That is, in the first embodiment, (E _h + E _L ) /2=0.901, which satisfies the following conditional expression (10).
(E _h + E _L ) / 2> 0.85 (10)

  If the lower limit value of conditional expression (10) is not reached, flare due to diffraction increases and it becomes difficult to obtain a sufficient signal intensity, and it becomes difficult to obtain a good S / N ratio. Furthermore, the axial chromatic aberration between the h line and the L line is preferably 0.1 or less. Incidentally, in the first embodiment, the axial chromatic aberration between the h line and the L line is 0.0675.

[Second Embodiment]
FIG. 5 is a diagram schematically showing a configuration of the objective lens for an optical disc according to the second embodiment. Referring to FIG. 5, the objective lens PL according to the second example has both a convex surface facing the light incident side and a diffractive optical surface formed on the optical disc DC side in order from the light incident side (left side in FIG. 2). A convex lens (positive lens) Lp and a negative meniscus lens (negative lens) Ln having a diffractive optical surface formed on the light incident side and an aspheric convex surface facing the optical disc DC side are provided. The biconvex lens Lp and the negative meniscus lens Ln are in close contact with each other so that the diffractive optical surfaces are in contact with each other. That is, the biconvex lens Lp and the negative meniscus lens Ln constitute a multi-contact diffractive optical element PF. FIG. 5 shows a state in which light from an object at infinity (not shown) forms an image on the information recording surface of the substrate of the optical disc DC via the objective lens PL.

  The following table (2) lists the values of the specifications of the objective lens according to the second example. In the second embodiment, the first surface in the optical member specifications of Table (2) is the incident surface of the biconvex lens Lp, the fourth surface is the exit surface of the negative meniscus lens Ln, and the fifth surface is the incident surface of the substrate of the optical disc DC. The sixth surface corresponds to the information recording surface of the substrate of the optical disk DC. The refractive index data of the second surface and the aspherical data of the third surface are obtained from the diffractive optical surface of the close-contact multilayer diffractive optical element formed by the biconvex lens Lp and the negative meniscus lens Ln (reference symbol PF in FIG. 5). The optical characteristics are expressed. A, B, C, and D in the specifications of the optical member in Table (2) indicate refractive indexes for the d-line, g-line, C-line, and F-line corresponding to the second surface, respectively.

Table (2)
(Main specifications)
f = 3.515
NA = 0.43

(Optical member specifications)
Surface number rd n (d) n (g) n (C) n (F) Optical member
1 3.55020 2.00000 1.525100 1.536900 1.522400 1.531700 (Lp)
2 -1.70000 0.00000 ABCD (PF)
3 * -1.70000 0.05325 1.497900 1.514500 1.494300 1.506700 (PF / Ln)
4 * -3.70284 2.47613
5 ∞ 0.50000 1.491080 1.501900 1.488540 1.497070 (DC)
6 (Information recording surface)
A = 0.100010000 × 10 ⁵ , B = 0.741868530 × 10 ⁴
C = 0.111704255 × 10 ⁵ , D = 0.827473110 × 10 ⁴

(Aspheric data)
3 sides: κ = 1.0000 C ₂ = −1.00000 × 10 ⁻⁶
C ₄ = 0 C ₆ = 0 C ₈ = 0

4 sides: κ = −5.000 C ₂ = 1.20,000 × 10 ⁻⁶
C ₄ = 1.20,000 × 10 ⁻² C ₆ = 0 C ₈ = 0

(Values for conditional expressions)
f = 3.515
D _T = 2.05325
fp = 2.520
fn = −6.369
Δνd = 16.31
ΔNd = 0.0272
E _g = 0.959
E _C = 0.982
E _d = 1.000
φ = 2.7
h = 0.0216
Dm = 0.05325
Pmin = 0.0218
ΔW = 0.00717
(1) f / D _T = 1.712
(2) | fp / fn | = 0.396
(3) Δνd / ΔNd = 599.63
(4) (E _g + E _C ) / (2 × E _d ) = 0.971
(5) φ / f = 0.768
(6) h / Dm = 0.406
(7) Pmin / f = 0.0062
(8) ΔW / f = 0.00204
(9) (E _g + E _d + E _C ) × f / ΔW = 1441.667

  FIG. 6 is a diagram showing spherical aberration, astigmatism, distortion aberration, and coma aberration in the objective lens for an optical disc of the second example. As is apparent from each aberration diagram of FIG. 6, the optical disk objective lens of the second example also has a wide wavelength from the g-line to the C-line, despite being small and light, as in the first example. It can be seen that various aberrations including chromatic aberration are well corrected over a range, and excellent optical performance is ensured.

  FIG. 7 is a diagram schematically illustrating a configuration of an optical disc apparatus including the objective lens according to the present embodiment. In the optical disk apparatus shown in FIG. 7, light emitted from a light source 71 such as a semiconductor laser light source is reflected by a half mirror 72 and enters a collimator lens 73. The light converted into parallel light by the collimator lens 73 is converged via the objective lens 74 of the present embodiment, and condensed on the information recording surface (disc surface on which pits are formed) of the optical disc DC. Reflected light (signal light) from the information recording surface of the optical disk DC enters the half mirror 72 via the objective lens 74 and the collimator lens 73. The signal light transmitted through the half mirror 72 is guided to the photoelectric sensor 75 and converted into an electrical signal.

  Note that various optical discs such as DVDs, CDs, and Blu-ray discs have different substrate thicknesses depending on their specifications. When the thicknesses of the substrates are different, spherical aberration occurs due to transmission through the inside of the substrate, but the spherical aberration is averaged over multiple types of optical disks with different substrate thicknesses due to the aspherical action of the diffractive optical element. Can be optimized to be smaller.

  In addition, the lens surface is divided into a central area and an outer ring zone area, and for multiple types of optical discs with different substrate thicknesses, the focal position of each area is determined and aberration correction is performed in each area. You can also

  As is well known, it is known that shortening the wavelength of light is effective for increasing the amount of information in an optical disk recording system. However, as in the present invention, an objective with low chromatic aberration and high diffraction efficiency over a wide wavelength range. If a lens is used, it is possible to construct an optical disk recording system with a large amount of information using a plurality of light sources (for example, a plurality of laser light sources) and a common optical system.

  As described above, if the height of the grating is set to the optimum height according to the scalar theory (= light wavelength / grating refractive index difference), the diffraction efficiency of the first-order diffracted light can be set to 100% at the reference wavelength λ. As a result, the diffraction efficiency can be increased to a value close to 100% over a wide wavelength band. On the other hand, if the height of the grating is set to, for example, half of the optimum height according to the scalar theory, about 40% can be allocated to the first-order diffracted light and the 0th-order diffracted light at each wavelength. Can be configured. As a result, it is possible to deal with a plurality of types of optical disks having different substrate thicknesses and different wavelengths of used light, such as compatible pickup lenses for CDs and DVDs.

  Further, if a recording medium appears in which the information to be picked up changes by changing the wavelength of the light used, the objective lens of the present invention can be applied to the pickup lens of this type of recording medium.

  In each of the above-described embodiments, the contact multilayered diffractive optical element, and in turn, both lenses Lp and Ln are formed of, for example, a UV curable resin, and the resin molding is performed with a mold, thereby facilitating processing and manufacturing. Therefore, the cost can be reduced.

  In each of the above-described embodiments, the close-contact multilayer diffractive optical element PF is incorporated in the objective lens for an optical disc. However, the present invention is not limited thereto, and the same can be achieved by incorporating a separate multilayer diffractive optical element. The effect of can be obtained.

  In each of the above-described embodiments, a unit of “mm” is generally used as a unit of length such as a focal length, a radius of curvature, and a center thickness (on-axis interval) in the specifications. However, since the same optical performance can be obtained even if the optical system is proportionally enlarged or reduced, the unit of length is not necessarily limited to “mm”.

It is a figure explaining the basic composition and operation of a multilayer type diffractive optical element. It is a figure which shows schematically the structure of the objective lens for optical discs concerning 1st Example. It is a 1st figure which shows the spherical aberration, astigmatism, distortion aberration, and coma aberration in the objective lens of 1st Example. It is a 2nd figure which shows the spherical aberration, astigmatism, distortion aberration, and coma aberration in the objective lens for optical discs of 1st Example. It is a figure which shows schematically the structure of the objective lens for optical discs concerning 2nd Example. It is a figure which shows the spherical aberration, astigmatism, distortion aberration, and coma aberration in the objective lens for optical discs of 2nd Example. It is a figure which shows schematically the structure of the optical disk apparatus provided with the objective lens of this embodiment.

Explanation of symbols

11, 12 Diffractive optical members 11a, 12a Diffraction optical surface 13 Wall surface of grating PL Optical lens objective lens (pickup lens)
Lp Positive lens Ln Negative lens DC Optical disk PF Adhering multilayer type diffractive optical element

Claims (10)

In objective lenses for optical discs,
Having a numerical aperture of 0.4 or more,
A diffractive optical element comprising a positive lens having a first diffractive optical surface and a negative lens having a second diffractive optical surface;
Of the positive lens and the negative lens, one lens is formed of an optical material having a relatively high refractive index and low dispersion, and the other lens is formed of an optical material having a relatively low refractive index and high dispersion,
The first diffractive optical surface and the second diffractive optical surface have sawtooth-shaped cross sections that are rotationally symmetric with respect to the optical axis of the objective lens, and the first diffractive optical surface and the second diffractive optical surface are The grating heights h of the first and second diffractive optical surfaces are determined so as to have a diffraction efficiency of 1.0 with respect to light of a predetermined wavelength,
The positive lens is a biconvex lens;
When the thickness of the positive lens along the optical axis and the thickness of the negative lens along the optical axis is Dm,
0.05 <h / Dm <1.0
An objective lens that satisfies the above conditions . When the focal length of the objective lens is f and the thickness along the optical axis of the objective lens is D _T ,
0.5 <f / D _T <10.0
The objective lens according to claim 1, wherein the condition is satisfied. The difference in refractive index at the d-line between the positive lens and the negative lens is 0.45 or less,
The objective lens according to claim 1, wherein at least one of the positive lens and the negative lens is formed of a UV curable resin. When the focal length of the positive lens is fp and the focal length of the negative lens is fn,
0.03 <| fp / fn | <4.0
The objective lens according to claim 1, wherein the condition is satisfied. The Abbe number difference based on the d-line between the high refractive index low dispersion optical material and the low refractive index high dispersion optical material is Δνd, and the high refractive index low dispersion optical material and the low refractive index high dispersion When the refractive index difference at the d-line with the optical material is ΔNd,
50 <Δνd / ΔNd <2000
5. The objective lens according to claim 1, wherein the condition is satisfied. When the diffraction efficiency of the diffractive optical element at d line is E _d , the diffraction efficiency of the diffractive optical element at g line is E _g, and the diffraction efficiency of the diffractive optical element at C line is E _C ,
(E _g + E _C ) / (2 × E _d )> 0.8
The objective lens according to claim 1, wherein the condition is satisfied. When the effective diameter (diameter) of the diffractive optical surface of the diffractive optical element is φ and the focal length of the objective lens is f,
0.1 <φ / f <3.0
The objective lens according to claim 1, wherein the condition is satisfied. The objective lens according to any one of claims 1 to 7, wherein both the positive lens and the negative lens are formed of a UV curable resin . The objective lens according to claim 1, wherein the positive lens is made of an injection molding resin, and the negative lens is made of a UV curable resin . An optical disc apparatus comprising the objective lens according to claim 1 .

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