JP2000081566A - Objective lens for optical head - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical head with high light use efficiency which enables recording and reproduction of a plurality of types of optical information recording media having different protective layer thicknesses such as DVD and CD-R with one objective lens. It is an object to provide an objective lens for use. SOLUTION: The objective lens 10 is a single lens made of resin having both aspheric surfaces, and a diffraction lens structure is formed on one lens surface 11 as a ring-shaped pattern centered on the optical axis. The diffractive lens structure has a wavelength dependency so that diffracted lights of the same order by at least two light beams having different wavelengths form good wavefronts on at least two types of optical discs having different protective layer thicknesses. I have.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high NA (numerical aperture) objective lens used for an optical head capable of recording / reproducing a plurality of types of optical disks having different protective layer thicknesses, and more particularly to a refractive lens. The present invention relates to an objective lens having a diffraction lens structure formed on a lens surface.

[0002]

2. Description of the Related Art There are a plurality of standards for optical disks having different protective layer thicknesses. For example, the protective layer of a CD (compact disk) or CD-R has a thickness of 1.2 mm, whereas the protective layer of a DVD (digital versatile disk) has a thickness of 0.60 mm. Therefore, when switching between optical discs having different standards, it is necessary to move the focusing position in the optical axis direction.

[0003] If the objective lens is moved in the optical axis direction, the paraxial focusing position can be moved. However, if the thickness of the protective layer changes, the spherical aberration changes. The laser beam wavefront is disturbed only by performing the focusing, and the spot cannot be converged to a required diameter, so that recording / reproduction of information becomes impossible. For example, if an objective lens designed to correct spherical aberration when using a DVD is used for reproducing a CD, the paraxial focusing position is made coincident with the recording surface by moving the objective lens in the optical axis direction. Even if the spherical aberration is excessive, information cannot be reproduced.

Therefore, an optical system for making a laser beam suitable for each optical disk incident on an objective lens according to the thickness of the protective layer has been conventionally known. For example, JP-A-7-98
No. 431 discloses that a hologram lens is provided in front of an objective lens to separate laser light emitted from a single semiconductor laser into zero-order light and primary light, and to convert the zero-order light, which is parallel light, into a protective layer. A technique is described in which a spot for a thin optical disk and a spot for an optical disk having a thick protective layer are formed by primary light that is divergent light. According to the optical system disclosed in the above publication, the hologram lens is designed so that an optimum laser beam can be obtained according to the thickness of the protective layer, thereby suppressing the occurrence of spherical aberration and having a diffraction-limited performance for each optical disk. You can get a spot.

[0005]

However, in the optical system described in Japanese Patent Application Laid-Open No. 7-98431, the luminous flux from the laser light source is always separated into zero-order light and primary light, and only one of them is used. For use, the efficiency of using laser light is limited to about 40% at the maximum. When information is read out by using one order of laser light, the light of the other order becomes unnecessary light that does not contribute to reading, thereby increasing noise.

Since the recording density of DVDs is higher than that of CDs, it is necessary to narrow the beam spot for recording / reproducing DVDs compared to an optical system dedicated to CDs. Since the spot diameter becomes smaller as the wavelength becomes shorter, an optical system using DVDs has a spot diameter of 780 to 830 used in an optical system dedicated to CD.
It is necessary to use a laser light source having an oscillation wavelength of 635-665 nm shorter than nm. On the other hand, when using a CD-R, it is necessary to use a laser light source having an oscillation wavelength of about 780 nm from the reflection characteristics of the recording surface.

[0007] Therefore, in the system disclosed in the above-mentioned publication, in which a single semiconductor laser is used for a plurality of types of optical disks, if a short wavelength laser light source is used to use a DVD, the CD-R is used. There is a problem that cannot be done.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and a DVD and a C / C are controlled by a single objective lens.
It is an object of the present invention to provide an objective lens for an optical head capable of recording / reproducing an optical disk having a different protective layer thickness, such as D and CD-R, and having high light use efficiency.

[0009]

In order to achieve the above object, an objective lens for an optical head according to the present invention comprises a refractive lens having a positive power and a ring-shaped minute step formed on one surface thereof. A diffractive lens structure having at least two types of optical discs having different protective layer thicknesses, wherein diffracted lights of the same order by light beams of at least two different wavelengths respectively form good wavefronts. It is characterized by having wavelength dependence so that

[0010] According to such a configuration, by switching the wavelength in accordance with the thickness of the protective layer for two disks having different protective layer thicknesses, the diffracted light of the same order by the diffractive lens structure can be obtained. A good spot can be formed by condensing light on each signal recording surface.

Further, the above-described wavelength dependence is such that short-wavelength diffracted light forms a good wavefront on an optical disk with a thin protective layer, and long-wavelength diffracted light has a good wavefront on an optical disk with a thick protective layer. Is preferably formed. More specifically, it is preferable that the diffractive lens structure has a spherical aberration characteristic in which, when the wavelength of the incident light changes to the longer wavelength side, the spherical aberration changes in a direction in which the correction becomes insufficient.

As described above, when the disk thickness increases, the spherical aberration changes in a direction in which the correction is overcorrected. Therefore, if the diffractive lens structure has a spherical aberration change characteristic with respect to a wavelength change as described above, a laser light source having a long oscillation wavelength is emitted for an optical disk having a thick protective layer, and an optical disk having a thin protective layer is emitted. On the other hand, by causing a laser light source having a short oscillation wavelength to emit light, a change in spherical aberration due to a difference in disk thickness can be canceled.

By the way, the additional amount of the optical path length due to the diffractive lens structure is calculated by using the height h from the optical axis, the n-th (even-order) optical path difference function coefficient P _n , and the wavelength λ, and φ (h) = ( P ₂ h ² + P ₄ h ⁴ + P ₆ h ⁶ +...) × λ can be represented by an optical path difference function φ (h). Here, the objective lens of the invention has a second-order optical path difference function coefficient of P ₂ , and a height at which a light beam equivalent to NA of 0.45 passes through the surface where the diffractive structure exists as h ₄₅ , where −15 <φ (h ₄₅ ) / Λ−P ₂ × (h ₄₅ ) ² <−7 (1)

Further, when the functions of the refractive lens and the diffractive lens structure are combined, there is an axial chromatic aberration characteristic such that when the wavelength of the incident light changes to the longer wavelength side, the back focus changes in the extending direction. It is desirable to satisfy the condition of -0.8 <ΔCA / ΔSA <−0.2 (2), where ΔSA is the amount of change in spherical aberration of the marginal ray with respect to change in wavelength, and ΔCA is the amount of change in axial chromatic aberration. .

Further, the objective lens of the present invention uses the second-order optical path difference function coefficient P ₂ and the wavelength λ to obtain f _D = 1 / (− P ₂
× 2 × λ), the focal length f _D of only the diffractive lens structure at the short wavelength side used wavelength, and the total focal length f at the short wavelength side combined wavelength of the refractive lens and the diffractive lens structure. _{relationship, -0.020 <f / f D <} 0.020 ... (3) satisfies the condition it is desirable that the.

The diffraction lens structure has a protective layer thickness of 0.6 m.
₁ the wavelength of the diffracted light lambda to form a good wavefront relative to m of the optical disk, the wavelength of the diffracted light forming a good wavefront with respect to the optical disk of thickness 1.2mm of the protective layer as lambda _2, 0. It is desirable that the lens is designed to satisfy the condition of 75 <λ ₁ / λ ₂ <0.87 (4).

Further, at least in a region near the optical axis,
Blazed wavelength λ of diffractive lens structure _BAre the two wavelengths
λ₁And λ_TwoPreferably, the wavelength is between
It is preferable that the conditions (5) and (6) are satisfied. 0.87 <λ_B/ Λ_Two … (5) λ_B/ Λ₁<1.13 ... (6)

It is preferable that the blazed wavelength in the peripheral region of the diffractive lens surface is set shorter than the blazed wavelength in the region near the optical axis, or that the peripheral region has a continuous aspherical shape having no step. The peripheral region is a region from a height of about 85% of the effective diameter from the optical axis to a height of 100% of the effective diameter. Also, an annular zone blazed at a short wavelength or a continuous surface corrected for aberration at a short wavelength may be provided inside the peripheral region.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an objective lens for an optical head according to the present invention will be described below. FIG. 1 is an explanatory view showing an objective lens 10 according to an embodiment, in which (A) is a front view, (B) is a longitudinal sectional view, and (C) is a partially enlarged view of the longitudinal section. The objective lens 10 is applied to an optical head of an optical information recording / reproducing device of a DVD, CD, or CD-R compatible device, and has a function of converging laser light emitted from a semiconductor laser as a light source on a medium such as a disk. are doing.

The objective lens 10 is a biconvex resin single lens having two aspheric lens surfaces 11 and 12, and has an optical axis on one lens surface 11 as shown in FIG. A diffractive lens structure is formed as a ring-shaped pattern with the center at the center. The diffractive lens structure has a step in the optical axis direction at the boundary of each annular zone like a Fresnel lens.

FIG. 2 is an explanatory diagram of an optical system of an optical head using the objective lens for an optical head according to the present invention. The optical system includes a DVD module 21, a CD module 22, a beam combiner 23, a collimator lens 24,
It comprises an objective lens 10. Each module 21,
Reference numeral 22 denotes an element in which the semiconductor laser and the sensor are integrated.

DV which is an optical disk having a 0.6 mm protective layer (hereinafter referred to as a "thin protective layer type optical disk")
To use D, red light with a wavelength of 635-665 nm is needed to create a small beam spot,
Among optical disks having a protective layer of 1.2 mm (hereinafter referred to as “thick protective layer type optical disk”), at least a CD-
In order to use R, a wavelength of 7
Near-infrared light near 80 nm is required. So DVD
Module 21 has an oscillation wavelength of 635 nm or 650 nm.
CD module 22
Is equipped with a semiconductor laser having an oscillation wavelength of 780 nm.

When using the thin protective layer type optical disk D ₁ (shown by a solid line in the figure), the DVD module 21 is operated. DVD laser beam having a wavelength 635nm or 650nm emitted from the semiconductor laser of the module 21 is converged on the information recording surface of the thin protective layer type optical disc D ₁ as indicated by a solid line in FIG. On the other hand, when using the thick protective layer type optical disk D ₂ (shown by a broken line in the figure), the CD module 22 is operated. Laser light having a wavelength of 780nm emitted from the semiconductor laser of the CD module 21 is focused on the information recording surface of the thick protective layer type optical disc D ₂ as indicated by a broken line in FIG.

The diffractive lens structure formed on the objective lens 10 is such that the diffracted light of a predetermined order, in the present embodiment, the first-order diffracted light is good for a thin protective layer type optical disc D _{1 at} a short wavelength (635 nm or 650 nm). Form a perfect wavefront,
In long wavelength (780 nm), thick protective layer type optical disc D
It is designed to have a wavelength dependency so as to form a good wavefront with respect to ₂ . More specifically, when the wavelength of the incident light changes to the longer wavelength side, it has a spherical aberration characteristic in which the spherical aberration changes in a direction in which the correction becomes insufficient.

The spherical aberration of the optical disk optical system changes in such a direction that the disk thickness increases, the correction becomes more over-corrected. On the other hand, short wavelength for a thin protective layer disc D _1, the thickness protective layer type optical disc D ₂ laser beam having a long wavelength is used.

Therefore, by providing the diffractive lens structure with such a characteristic that the spherical aberration changes in a direction in which the correction becomes insufficient when the wavelength changes to a long wavelength, the spherical aberration in which the correction is excessive due to the disk thickness is reduced. It is possible to cancel using the spherical aberration of the diffraction lens structure in the direction of undercorrection. The focusing for focusing the laser beam on the signal recording surface of each disk is performed by the objective lens 1
This is performed using a focal position adjusting mechanism that moves 0 in the optical axis direction.

The additional amount of the optical path length due to the diffractive lens structure is
Height h from the optical axis, n-th (even-order) optical path difference function coefficient P
Using n and wavelength λ, it can be represented by an optical path difference function φ (h) defined by φ (h) = (P ₂ h ² + P ₄ h ⁴ + P ₆ h ⁶ +...) × λ. P ₂ , P ₄ , P ₆ ,... Are second-order, fourth-order, sixth-order,.
Is the coefficient of The optical path difference function φ (h) is, at a point of height h from the optical axis on the diffraction surface, a virtual light beam that is not diffracted by the diffractive lens structure, and a light beam diffracted by the diffractive lens structure. 2 shows the optical path difference. In this representation, when the coefficient P ₂ of the second-order term is negative, the power is paraxially positive. When the coefficient P ₄ of the fourth-order term is positive, the negative power gradually increases toward the periphery. Become.

The actual fine shape of the lens is determined so as to have a Fresnel lens-shaped additional optical path length φ ′ in which components of an integral multiple of the wavelength are eliminated from the optical path length represented by φ (h). φ '(h) = (MOD (P 2 h 2 + P 4 h 4 + P 6 h 6 + ... + Const, l) -
Const) × λ _B λ _B is a wavelength (blazed wavelength) at which the fine step gives an optical path length difference of one wavelength, and is a wavelength that maximizes diffraction efficiency. The constant term Const is a constant for setting the phase of the boundary position of the annular zone, and takes an arbitrary number in the range of 0 ≦ Const <1. MOD
(X, Y) is a function that gives the remainder of X divided by Y. MOD
The point of h where the value of (P ₂ h ² + P ₄ h ⁴ +... + Const, 1) becomes 0 is the boundary of the annular zone. A gradient and a step are set so as to have an optical path difference of φ ′ (h) on the base shape which is the lens surface of the refractive lens.

Here, the objective lens 10 according to the embodiment has an N
The condition that 15 <φ (h ₄₅ ) / λ−P ₂ × (h ₄₅ ) ² <−7 (1) where h ₄₅ is the height at which the light beam equivalent to A0.45 passes through the surface where the diffractive structure exists. It is designed to meet.

When this condition is satisfied, the change in spherical aberration due to the difference in the thickness of the protective layer of the disk can be favorably canceled by the change in spherical aberration due to the change in the wavelength of the diffractive lens. When the value goes below the lower limit of the condition (1), the change of the spherical aberration due to the wavelength change becomes excessive. Since the oscillation wavelength of the semiconductor laser has an individual difference of about ± 5 nm, if the change in the spherical aberration due to the wavelength change is excessive, the semiconductor laser whose oscillation wavelength is shifted from the reference wavelength cannot be used. Laser sorting is required, which reduces the yield. Therefore, it is preferable that the spherical aberration correction effect due to the wavelength change of the diffractive lens structure is slightly insufficient.

When the value exceeds the upper limit of the condition (1),
The change of the spherical aberration due to the wavelength change becomes too small, and the change of the spherical aberration due to the difference in the thickness of the protective layer of the disk cannot be sufficiently canceled. When a semiconductor laser having an oscillation wavelength of 780 nm or an oscillation wavelength between 635 nm and 665 nm is selected, the value of the condition (1) must be −
About 11 is most preferable.

The oscillation wavelength of a semiconductor laser is
It changes with temperature changes. When the objective lens has axial chromatic aberration, the focal position changes due to a change in the wavelength of the semiconductor laser. However, since this change is gradual, it can be corrected by the focusing mechanism of the optical head. On the other hand, at the time of a writing operation in the optical recording device, the oscillation wavelength of the semiconductor laser rapidly changes with a change in the laser output. The change of the focus position due to the output change cannot be completely coped with even by using the focus adjustment mechanism.
Therefore, it is desirable to suppress a change in the focal position due to a wavelength change on the objective lens side.

It should be noted that this wavelength variation can generally be dealt with by correcting chromatic aberration, but the objective lens of the embodiment intentionally generates spherical aberration by wavelength switching as described above. Therefore, if the axial chromatic aberration is completely corrected, the change in the optimum writing position due to the wavelength fluctuation becomes large. Therefore, it is necessary to balance between the correction of chromatic aberration and the characteristics for wavelength switching.

For this reason, in the embodiment, when the function of the refractive lens and the structure of the diffractive lens are combined, when the wavelength of the incident light changes to the longer wavelength side, the on-axis axis changes in the direction in which the back focus extends. It has chromatic aberration characteristics, and the amount of change in spherical aberration of the marginal ray with respect to the change in wavelength is ΔSA,
Designed to satisfy the condition of -0.8 <ΔCA / ΔSA <−0.2 (2), where ΔCA is the amount of change in axial chromatic aberration.

The condition (2) is that, for example, when the wavelength shifts to the longer wavelength side and the paraxial focus moves to a position farther from the lens than before the wavelength change, the focus by the marginal ray is closer to the lens than before the wavelength change. Moving to a position. Assuming that the spherical aberration is almost corrected at the wavelength before the change, the paraxial focus at the wavelength after the change is based on the position of the paraxial focus at the wavelength before the change. The focal point of the marginal ray at the wavelength of is located at a position close to the lens. Therefore, the change in the optimum writing position averaged from the paraxial focal point to the focal point by the marginal light beam can be suppressed relatively small.

Further, in order to reduce the amount of movement of the optimum writing position due to the sudden wavelength shift, the objective lens 10 of the embodiment uses the second-order optical path difference function coefficient P ₂ and the wavelength λ to make f _D = 1 / ( −P ₂ × 2 × λ) and the focal length f _D of only the diffractive lens structure at the shorter wavelength used wavelength, and the total focal point of the refractive lens and the diffractive lens structure at the shorter wavelength used wavelength. relationship between the distance f, are satisfy the condition design _{-0.020 <f / f D <0.020} ... (3).

The condition (3) defines the degree of occurrence of longitudinal chromatic aberration. It is known that the value of the dispersion of the diffractive lens corresponding to the Abbe number of the refractive lens is -3.453. A negative value means that the sign is opposite to that of the Abbe number of the refractive lens, and a small absolute value means that the lens has a large dispersion. Therefore, chromatic aberration can be corrected by combining a low power diffractive lens with a positive refractive lens. By satisfying the condition (3), a change in the optimum writing position due to a sharp shift of the wavelength can be suppressed while maintaining the balance with the spherical aberration correction effect by the diffractive lens structure.

The wavelengths of the two laser beams to be incident are as follows: λ _{1 is} the wavelength of the laser beam used for the thin protective layer type optical disk D ₁ , and λ ₂ is the wavelength of the laser beam used for the thick protective layer type optical disk D _2. .75 <λ ₁ / λ ₂ <0.87 (4)

The condition (4) is a condition for sufficiently generating spherical aberration in the diffractive lens structure. The ratio of the two wavelengths corresponds to the amount of wavefront aberration imparted per step of the diffractive structure. For example, when the two wavelengths are selected to be 650 nm and 780 nm, 7
At 80 nm, (780-650) / 78 per step
A wavefront aberration of 0 = 0.1666λ is added. For this reason, when the difference between λ ₁ and λ ₂ becomes smaller than the upper limit of the equation (4), the number of steps of the diffractive structure required to give a predetermined wavefront aberration increases, and the light amount loss due to the edge of the step increases. Become. In addition, since the amount of change in spherical aberration per unit wavelength shift becomes too large, the spherical aberration exceeding the allowable amount changes due to the difference in oscillation wavelength due to the individual difference of the semiconductor laser, and the laser must be sorted by the oscillation wavelength. On the other hand, when λ ₁ / λ ₂ decreases below the lower limit, the difference between the two wavelengths becomes too large, and the average value of the diffraction efficiency decreases.

Blazed wavelength λ to maximize diffraction efficiency_B
Is selected when designing the microstructure. In the vicinity of the optical axis,
Lasing wavelength λ_BIncreased the average diffraction efficiency
Two wavelengths λ₁And λ_TwoIs set to a wavelength between this
According to such a setting, for example, λ ₁Is 635 nm, λ_Two7
If it is 80 nm, the blazed wavelength should be between these wavelengths.
The wavelength λ₁, Λ_TwoDiffraction efficiency at
Can be maintained at about 90% or more.

FIG. 27 shows that the blazed wavelength λ _B is 635n.
m, 710 nm, 710 nm
It is a graph which shows the diffraction efficiency at the time of selecting to nm. In each case, the diffraction efficiency at 635 nm and 780 nm is about 90% or more. Therefore, the light utilization efficiency is sufficiently higher than the efficiency of about 40% when two diffraction orders described in JP-A-7-98431 are used.

The objective lens 10 of the embodiment is set so as to satisfy the following conditions (5) and (6) in order to further increase the diffraction efficiency. 0.87 <λ _B / λ ₂ (5) λ _B / λ ₁ <1.13 (6)

When the blazed wavelength λ _B is set to a value close to _one of the wavelengths λ ₁ and λ ₂ of the semiconductor laser, as shown in FIG. 27, the diffraction efficiency at a wavelength farther from the blazed wavelength decreases. I do. On the other hand, by taking an intermediate value between the two wavelengths that satisfies the expressions (5) and (6), it is possible to maintain a diffraction efficiency of about 95% for either wavelength. .

The peripheral area of the diffractive lens surface is a diffractive surface blazed for a wavelength shorter than the blazed wavelength λ _B for determining a step near the optical axis, or a continuous non-stepped non-stepped non-stepped non-stepped surface. It is assumed to be spherical. Here, the peripheral area is
This is a region from a height of 85% of the effective diameter from the optical axis to a height of 100% of the effective diameter. An NA of 0.50 is sufficient for reproduction of CDs and CD-Rs. The area around the effective aperture for NA of 0.60 for DVDs is not only unnecessary for CDs but also has a narrower luminous flux. The recording and reproduction may be adversely affected. For this reason, it is desirable that the peripheral area be a surface whose aberration has been corrected exclusively for DVD. If the blazed wavelength in the peripheral region is shorter than that in the center, the diffraction efficiency of the laser light for CD and CD-R decreases, and the diffraction efficiency of the laser light for DVD improves. In addition, D
By correcting the aberration for VD, the peripheral region functions to converge the laser light for DVD well.

The spherical aberration of the positive refraction lens changes in a direction in which the refractive index decreases as the temperature increases, and the correction becomes overcorrected. On the other hand, the oscillation wavelength of a semiconductor laser becomes longer as the temperature rises. Therefore, as described above, by providing the diffractive lens structure with a spherical aberration characteristic in which the spherical aberration becomes insufficiently corrected as the wavelength becomes longer, the change in the spherical aberration caused by the change in the refractive index of the refractive lens caused by the temperature change. Can be canceled out by a change in the spherical aberration of the diffraction lens structure due to a change in the oscillation wavelength of the semiconductor laser due to a temperature change. For this reason, when the objective lens is made of a resin whose refractive index decreases as the temperature rises, it is preferable that the diffraction lens structure be provided to the outermost peripheral portion. However, even in this case, it is desirable to optimize the thickness of the step for a short wavelength for DVD so that the diffraction efficiency of the light flux for DVD is increased.

[0046]

EXAMPLES Next, six specific examples based on the above-described embodiment will be presented. Both have a protective layer thickness of 0.6 m
m and a protective layer having a thickness of 1.
This is an objective lens for an optical head that is also used as a CD and a CD-R using a 2 mm disk. In Examples 1 to 4, the diffractive lens structure is formed on the first surface on the light source side, and in Examples 5 and 6, the diffractive lens structure is formed on the second surface on the optical disk side.

[0047]

FIG. 3 shows an objective lens 10 according to a first embodiment.
And thin protective layer type optical disk D₁FIG. 5 shows Example 1.
Objective lens 10 and thick protective layer type optical disc D_TwoAnd show
You. Table 1 shows a specific numerical configuration of the objective lens 10 according to the first embodiment.
It is shown in FIG. The surface numbers 1 and 2 are the objective lens 10 and the surface
Numerals 3 and 4 denote protective layers of a disk as a medium.
You. In the table, NA is the numerical aperture, f is the total focal length (unit: m
m), f_DIs the wavelength used on the short wavelength side of the diffractive lens structure.
Focal length (unit: mm), ω is half angle of view (unit: degree), λ ₁
Is a thin protective layer type optical disc D₁Wavelength at the time of use (unit: nm),
λ_TwoIs a thick protective layer type optical disc D_TwoWavelength when used (unit: n)
m), h₄₅Is NA 0.45 on the surface where the diffractive lens structure exists
Passage height of the corresponding ray (unit: mm), λ_BIs blazed
The wavelength, r is the macroscopic paraxial radius of curvature of each lens surface (unit: m
m) and d1 are thin protective layer type optical disc D₁Lens thickness when used
Or lens spacing (unit: mm), d2 is a thick protective layer type optical disk
D_TwoLens thickness or lens interval (unit: mm) at use, nλ
Is the refractive index of each lens at wavelength λ nm, and ν is the refractive index of each lens.
This is the Abbe number.

The base surface of the first surface 11 of the objective lens 10 (shape as a refraction lens excluding the diffractive lens structure) and the second surface 12 are aspherical, and have a height h from the optical axis. The distance (sag amount) of the coordinate point on the aspheric surface from the tangent plane on the optical axis of the aspheric surface to X (h), the curvature (1 / r) on the optical axis of the aspheric surface to C, and the cone Coefficients are K, 4th, 6th, 8th,
10 next, the 12-order aspherical coefficients as _{A 4, A 6, A 8} , A 10, A 12, is represented by the following equation. X (h) = Ch ² / (1 + √ (1- (1 + K) C ² h ² )) + A ₄ h ⁴ + A ₆ h ⁶ + A ₈ h ⁸ + A ₁₀ h
¹⁰ + A ₁₂ h ^{12 The} radius of curvature of the aspheric surface in Table 1 is the radius of curvature on the optical axis. Table 2 shows the conic coefficient and the aspherical coefficient that define the aspherical surface, and the optical path difference function coefficient that defines the diffractive lens structure.
Is shown in

[0049]

[Table 1] λ ₁ = 650nm NA 0.60 f = 3.30mm f _D = 330.53mm ω = 1.0 ゜ h ₄₅ = 1.49mm (first surface) λ ₂ = 780nm NA 0.45 f = 3.32mm ω = 1.0 ゜ λ _B = 710nm Surface number r d1 d2 n650 n780 ν 1 2.117 2.400 2.400 1.54082 1.53677 55.6 2 -7.254 1.592 1.222 3 ∞ 0.600 1.200 4 ∞

[0050]

[Table 2]

FIG. 4 shows various aberrations at the _first wavelength λ ₁ corresponding to the thin protective layer type optical disc D ₁ of the objective lens of the first embodiment. 4A is a spherical aberration SA and a sine condition SC at a wavelength of 650 nm, FIG. 4B is a chromatic aberration represented by spherical aberration of each wavelength of 650 nm, 645 nm and 655 nm, and FIG. 4C is an astigmatism (S: sagittal, M: Meridional). Graph (A),
The vertical axis in (B) is the numerical aperture NA, and the vertical axis in (C) is the image height Y.
The horizontal axis indicates the amount of each aberration generated, and the unit is mm. FIG. 6 shows similar aberrations when λ ₂ is 780 nm.

FIGS. 4A and 6A show that the spherical aberration is favorably corrected at the _two wavelengths λ ₁ and λ ₂ . The change amount ΔCA of the axial chromatic aberration is 65 ° in FIG.
It is indicated by the movement width of the lower end of the graph of 0 nm and 655 nm, and the movement direction is the direction in which the back focus extends due to the shift of the incident light to the longer wavelength side. In addition, the variation ΔSA of the spherical aberration of the marginal ray is represented by a graph of 655 nm, the lower end of which is 650 nm.
Is shown by the width between the upper end of the graph when translated to the position overlapping the lower end of the graph and the upper end of the graph at 650 nm. When these conditions (2) are satisfied, the graph intersects the vertical axis based on the paraxial focus before the change after the wavelength change (655 nm), and the change in the optimum writing position due to the wavelength shift. Is relatively small.

In the numerical example of the first embodiment, a diffraction lens structure blazed to 710 nm is formed over the entire effective diameter. In contrast, the peripheral region of the lens surface diffractive lens structure is formed can be optimized for the thin protective layer type optical disc D _1. The effective radius of the objective lens of Example 1 is f = 3.3 mm, NA is 1.98 mm at 0.6, whereas the effective radius required for the thick protective layer type optical disk is
f = 3.32 mm, NA 0.45 and 1.49 mm. Therefore, the area outside 75.5% of the effective radius is the peripheral area.

[0054] The peripheral region to be optimized for the thin protective layer type optical disc D ₁ is a diffractive lens structure in the peripheral region
Blazing is performed at 650 nm, or a diffractive lens structure is not formed in the peripheral region, and a continuous surface with aberration correction at 650 nm can be obtained. When the peripheral region is a continuous surface, the region from the central region including the optical axis to a radius of 1.49 mm is blazed to 710 nm as described above, and the first to fifteenth annular zones are formed around the central region. I do. The peripheral region is a single sixteenth orbicular zone, and its shape is a rotationally symmetric aspheric surface represented by the following coefficient.

[0055]

[Table 3] r = 2.09903 K = -0.44 A ₄ = -8.73 × 10 ^-4 A ₆ = -1.26 × 10 ^-4 A ₈ = -6.17 × 10 ^-5 A ₁₀ = 6.67 × 10 ^-6 A ₁₂ =- 6.20 × 10 ^-6 Δ = -0.01923

Where Δ is relative to the lens surface on the optical axis.
The shift amount of the peripheral continuous surface in the optical axis direction is shown.

[0057]

Second Embodiment FIG. 7 shows an objective lens 10 according to a second embodiment.
FIG. 9 shows the objective lens 10 and the thick protective layer type optical disc D2 of the _second embodiment. Table 4 shows a specific numerical configuration of the second embodiment. Table 5 shows the conical coefficients and aspheric coefficients of the first surface and the second surface, and the optical path difference function coefficient representing the diffractive lens structure formed on the first surface. FIG. 8 shows the second embodiment.
Aberrations of the objective lens when lambda ₁ is 630 nm, 10 lambda
₂ shows various aberrations of the objective lens when 780 nm.

[0058]

[Table 4] λ ₁ = 635 nm NA 0.60 f = 3.50 mm f _D = 350.00 mm ω = 1.0 ゜ h ₄₅ = 1.58 mm (first surface) λ ₂ = 780 nm NA 0.50 f = 3.52 mm ω = 1.0 ゜ λ _B = 690nm Surface number r d1 d2 n635 n780 ν 1 2.278 2.928 2.928 1.54142 1.53677 55.6 2 -6.508 1.521 1.153 3 ∞ 0.600 1.200 4 ∞

[0059]

[Table 5]

In the numerical example of the second embodiment, a diffractive lens structure blazed to 690 nm is formed over the entire effective diameter. In contrast, the peripheral region of the lens surface diffractive lens structure is formed can be optimized for the thin protective layer type optical disc D _1. The effective radius of the objective lens of Example 2 is f = 3.5 mm, and NA = 0.6 mm, whereas the effective radius of the thick protective layer type optical disc is f = 3.5 mm.
It is 3.52 mm and 1.76 mm at NA 0.50. Therefore, the area outside the effective radius of 83.8% is the peripheral area.

[0061] The peripheral region to be optimized for the thin protective layer type optical disc D ₁ is a diffractive lens structure in the peripheral region
Blazing is performed at 635 nm, or a diffractive lens structure is not formed in the peripheral region, and a continuous surface corrected for aberration at 635 nm can be obtained.

[0062]

Third Embodiment FIG. 11 shows an objective lens 1 according to a third embodiment.
0 a thin protective layer type optical disc D ₁ and indicates, Figure 13 shows a the thickness protective layer type optical disc D ₂ objective lens 10 of Example 3. Table 6 shows a specific numerical configuration of the third embodiment. Table 7 shows the conic coefficients and aspherical coefficients of the first and second surfaces, and the optical path difference function coefficient representing the diffractive lens structure formed on the first surface.
Is shown in FIG. 12 shows various aberrations of the objective lens when λ _{1 of} Example 3 is 635 nm, and FIG. 14 shows various aberrations of the objective lens when λ ₂ is 780 nm.

[0063]

[Table 6] λ ₁ = 635 nm NA 0.60 f = 3.50 mm f _D = ∞ ω = 1.0 ゜ h ₄₅ = 1.58 mm (first surface) λ ₂ = 780 nm NA 0.50 f = 3.53 mm ω = 1.0 ゜ λ _B = 690 nm Surface number r d1 d2 n635 n780 ν 1 2.203 2.400 2.400 1.54142 1.53677 55.6 2 -8.367 1.781 1.423 3 ∞ 0.600 1.200 4 ∞

[0064]

[Table 7]

In the numerical example of the third embodiment, a diffractive lens structure blazed to 690 nm is formed over the entire effective diameter. In contrast, the peripheral region of the lens surface diffractive lens structure is formed can be optimized for the thin protective layer type optical disc D _1. The effective radius of the objective lens of the third embodiment is f = 3.5 mm, and NA = 0.6 mm, whereas the effective radius of the thick protective layer type optical disk is f = 3.5 mm.
3.53mm, NA is 1.50mm with 0.50. Therefore, the area outside the effective radius of 84.0% is the peripheral area.

[0066] The peripheral region to be optimized for the thin protective layer type optical disc D ₁ is a diffractive lens structure in the peripheral region
Blazing is performed at 635 nm, or a diffractive lens structure is not formed in the peripheral region, and a continuous surface corrected for aberration at 635 nm can be obtained.

[0067]

Fourth Embodiment FIG. 15 shows an objective lens 1 according to a fourth embodiment.
0 a thin protective layer type optical disc D ₁ and indicates, Figure 17 shows a the thickness protective layer type optical disc D ₂ objective lens 10 of Example 4. 14 shows an objective lens and a protective layer according to Example 4. Table 8 shows a specific numerical configuration of the fourth embodiment. First
Table 9 shows the conical coefficients of the surface, the second surface, the aspherical surface coefficients, and the optical path difference function coefficients representing the diffractive lens structure formed on the first surface. FIG. 16 shows various aberrations of the objective lens of Example 4 when λ ₁ is 650 nm, and FIG. 18 shows various aberrations of the objective lens when λ ₂ is 780 nm.

[0068]

[Table 8] λ ₁ = 650 nm NA 0.60 f = 3.50 mm f _D = ∞ ω = 1.0 ゜ h ₄₅ = 1.58 mm (first surface) λ ₂ = 780 nm NA 0.50 f = 3.53 mm ω = 1.0 ゜ λ _B = 710 nm Surface number r d1 d2 n650 n780 ν 1 2.193 2.300 2.300 1.54082 1.53677 55.6 2 -8.740 1.831 1.471 3 ∞ 0.600 1.200 4 ∞

[0069]

[Table 9]

In the numerical example of the fourth embodiment, the diffraction lens structure blazed to 710 nm is formed over the entire effective diameter. In contrast, the peripheral region of the lens surface diffractive lens structure is formed can be optimized for the thin protective layer type optical disc D _1. As in the third embodiment, the area outside the effective radius of 84.0% is the peripheral area. Therefore, the diffractive lens structure in the peripheral region can be blazed for 650 nm, or the diffractive lens structure can not be formed in the peripheral region, and can be a continuous surface corrected for 650 nm.

[0071]

Fifth Embodiment FIG. 19 shows an objective lens 1 according to a fifth embodiment.
0 a thin protective layer type optical disc D ₁ and indicates, Figure 21 shows an objective lens 10 and the thickness protective layer type optical disc D ₂ of Example 5. Table 10 shows a specific numerical configuration of the fifth embodiment. Table 11 shows the conical coefficients and aspherical coefficients of the first surface and the second surface, and the optical path difference function coefficient representing the diffractive lens structure formed on the second surface. FIG. 20 shows various aberrations of the objective lens of Example 5 when λ ₁ is 635 nm, and FIG. 22 shows various aberrations of the objective lens when λ ₂ is 780 nm.

[0072]

[Table 10] λ ₁ = 635 nm NA 0.60 f = 3.50 mm f _D = ∞ ω = 1.0 ゜ h ₄₅ = 1.23 mm (second surface) λ ₂ = 780 nm NA 0.50 f = 3.53 mm ω = 1.0 ゜ λ _B = 690 nm Surface number r d1 d2 n635 n780 ν 1 2.199 1.930 1.930 1.54142 1.53677 55.6 2 -9.484 2.042 1.685 3 ∞ 0.600 1.200 4 ∞

[0073]

[Table 11]

In the numerical example of the fifth embodiment, a diffraction lens structure blazed to 690 nm is formed over the entire effective diameter. In contrast, the peripheral region of the lens surface diffractive lens structure is formed can be optimized for the thin protective layer type optical disc D _1. As in the third embodiment, the area outside the effective radius of 84.0% is the peripheral area. Therefore, the diffractive lens structure in the peripheral region can be blazed at 635 nm, or the diffractive lens structure can not be formed in the peripheral region, and can be a continuous surface corrected for 635 nm.

[0075]

Embodiment 6 FIG. 23 shows an objective lens 1 according to Embodiment 6.
0 a thin protective layer type optical disc D ₁ and indicates, Figure 25 shows an objective lens 10 and the thickness protective layer type optical disc D ₂ of Example 6. Table 12 shows a specific numerical configuration of the sixth embodiment. Table 13 shows the conic coefficients and aspheric coefficients of the first surface and the second surface, and the optical path difference function coefficients representing the diffractive lens structures formed on the second surface. FIG. 24 shows various aberrations of the objective lens when λ _{1 of} Example 6 is 650 nm, and FIG. 26 shows various aberrations of the objective lens when λ ₂ is 780 nm.

[0076]

[Table 12] λ ₁ = 650 nm NA 0.60 f = 3.50 mm f _D = 202.10 mm ω = 1.0 ゜ h ₄₅ = 1.23 mm (second surface) λ ₂ = 780 nm NA 0.50 f = 3.53 mm ω = 1.0 ゜ λ _B = 710nm Surface number r d1 d2 n650 n780 ν 1 2.207 1.930 1.930 1.54082 1.53677 55.6 2 -10.066 2.042 1.685 3 ∞ 0.600 1.200 4 ∞

[0077]

[Table 13]

In the numerical example of the sixth embodiment, a diffractive lens structure blazed to 710 nm is formed over the entire effective diameter. In contrast, the peripheral region of the lens surface diffractive lens structure is formed can be optimized for the thin protective layer type optical disc D _1. As in the third embodiment, the area outside the effective radius of 84.0% is the peripheral area. Therefore, the diffractive lens structure in the peripheral region can be blazed for 650 nm, or the diffractive lens structure can not be formed in the peripheral region, and can be a continuous surface corrected for 650 nm.

Table 13 below shows the conditions (1),
The correspondence between (2), (3), (4), (5), and (6) and each embodiment is shown. The condition (1) is satisfied by all the embodiments, whereby a change in wavefront aberration due to a difference in thickness of the protective layer can be canceled by a difference in wavelength. Further, all of the embodiments satisfy the condition (2) and the condition (3), and it is possible to suppress a change in the optimum writing position due to a sharp shift of the wavelength. Further condition (4)
(5) As for (6), all the examples are satisfied.
Even if two wavelengths are used for the diffractive lens, it is possible to minimize the reduction in the diffraction efficiency.

[0080]

[Table 14] Condition (1) Condition (2) Condition (3) Condition (4) Condition (5) Condition (6) Example 1 -10.6 -0.36 0.010 0.833 0.910 1.092 Example 2 -9.7 -0.26 0.010 0.814 0.885 1.087 Example 3 -9.4 -0.50 0.000 0.814 0.885 1.087 Example 4 -10.2 -0.43 0.000 0.833 0.910 1.092 Example 5 -9.2 -0.78 0.000 0.814 0.885 1.087 Example 6 -9.3 -0.47 0.017 0.833 0.910 1.092

[0081]

As described above, according to the present invention, the change in spherical aberration caused by the difference in the thickness of the protective layer can be reduced.
It can be canceled by the change in the spherical aberration of the diffractive lens structure, and an objective lens for an optical head with high light use efficiency can be provided by a single lens. Therefore, when the present invention is applied to DVD and CD-R compatible systems, the number of movable parts around the objective lens can be reduced, and the apparatus can be made compact and high-speed.

When the diffractive lens structure is designed to satisfy the condition (1), the change in spherical aberration caused by the difference in the thickness of the protective layer is better due to the change in spherical aberration due to the wavelength change of the diffractive lens. In particular, when the thickness of the protective layer is large, a semiconductor laser having a long wavelength can be used.

When the diffractive lens structure is designed to satisfy the condition (2), when the diffractive lens structure is designed to satisfy the condition (3), when the wavelength of the laser suddenly changes. In addition, the change in the optimum writing position averaged from the paraxial focal point to the focal point by the marginal light beam can be suppressed to be relatively small.

Further, when the wavelength to be used and the wavelength for determining the diffraction lens structure are set so as to satisfy the conditions (4), (5) and (6), the shape of the diffraction structure and the diffraction efficiency depending on the wavelength are obtained. Can be kept small. Also, the light amount of the semiconductor laser can be effectively used by determining the shape of the peripheral region in accordance with the larger NA.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing an outer shape of an objective lens according to an embodiment, (A) is a front view, (B) is a longitudinal sectional view, and (C) is a partially enlarged view of the longitudinal section.

FIG. 2 is an explanatory diagram of an optical system of an optical pickup device using the objective lens according to the embodiment.

FIG. 3 is a lens diagram showing an objective lens and a thin protective layer type optical disc of Example 1.

FIG. 4 is a diagram of various aberrations when the objective lens of Example 1 is used with a thin protective layer type optical disk.

FIG. 5 is a lens diagram showing an objective lens and a thick protective layer type optical disc of Example 1.

FIG. 6 is a diagram of various aberrations when the objective lens of Example 1 is used with a thick protective layer type optical disc.

FIG. 7 is a lens diagram showing an objective lens and a thin protective layer type optical disc of Example 2.

FIG. 8 is a diagram showing various aberrations when the objective lens of Example 2 is used with a thin protective layer type optical disk.

FIG. 9 is a lens diagram showing an objective lens and a thick protective layer type optical disc of Example 2.

10 is a diagram of various aberrations when the objective lens of Example 2 is used with a thick protective layer type optical disk. FIG.

FIG. 11 is a lens diagram showing an objective lens and a thin protective layer type optical disc of Example 3.

FIG. 12 is a diagram of various aberrations when the objective lens of Example 3 is used with a thin protective layer type optical disk.

FIG. 13 is a lens diagram showing an objective lens and a thick protective layer type optical disc of Example 3.

FIG. 14 is a diagram of various aberrations when the objective lens of Example 3 is used with a thick protective layer type optical disc.

FIG. 15 is a lens diagram showing an objective lens and a thin protective layer type optical disc of Example 4.

FIG. 16 is a diagram of various aberrations when the objective lens of Example 4 is used with a thin protective layer type optical disk.

FIG. 17 is a lens diagram showing an objective lens and a thick protective layer type optical disc of Example 4.

FIG. 18 is a diagram of various aberrations when the objective lens of Example 4 is used with a thick protective layer type optical disc.

FIG. 19 is a lens diagram showing an objective lens and a thin protective layer type optical disc of Example 5.

FIG. 20 is a diagram showing various aberrations when the objective lens of Example 5 is used with a thin protective layer type optical disk.

FIG. 21 is a lens diagram showing an objective lens and a thick protective layer type optical disc of Example 5.

FIG. 22 is a diagram of various aberrations when the objective lens of Example 5 is used with a thick protective layer type optical disk.

FIG. 23 is a lens diagram showing an objective lens and a thin protective layer type optical disc of Example 6.

FIG. 24 is a diagram of various aberrations when the objective lens of Example 6 is used with a thin protective layer type optical disk.

FIG. 25 is a lens diagram showing an objective lens and a thick protective layer type optical disc of Example 6.

FIG. 26 is a diagram of various aberrations when the objective lens of Example 6 is used with a thick protective layer type optical disk.

FIG. 27 is a graph showing the relationship between diffraction efficiency and wavelength.

[Explanation of symbols]

10 objective lens 11 first surface 12 second surface D ₁ thin protective layer type optical disc D ₂ thick protective layer optical disc 21 DVD module 22 CD module 23 beam combiner 24 collimating lens

Claims (11)

[Claims] 1. A refracting lens having a positive power, and a diffractive lens structure having a ring-shaped fine step formed on at least one lens surface of the refracting lens, wherein the diffractive lens structure has at least For an optical head, diffracted light of the same order by two different wavelengths of light flux has wavelength dependence so as to form good wavefronts on at least two types of optical discs having different protective layer thicknesses. Objective lens. 2. The diffractive lens structure, wherein the short-wavelength diffracted light forms a good wavefront on an optical disk with a thin protective layer, and the long-wavelength diffracted light forms a good wavefront on an optical disk with a thick protective layer. The objective lens for an optical head according to claim 1, wherein the objective lens has a wavelength dependency to be formed. 3. The diffraction lens structure according to claim 2, wherein when the wavelength of the incident light changes to a longer wavelength side, the diffraction lens structure has a spherical aberration characteristic in which the spherical aberration changes in a direction in which the correction is insufficient. An objective lens for an optical head according to the above. 4. An addition amount of an optical path length by the diffractive lens structure is calculated by using a height h from an optical axis, an nth-order (even-order) optical path difference function coefficient P _n , and a wavelength λ to obtain φ (h) = When represented by an optical path difference function φ (h) defined by (P ₂ h ² + P ₄ h ⁴ + P ₆ h ⁶ +...) × λ,
The height NA0.45 corresponding rays pass through the existing surface of the diffractive structure as h _45, -15 <φ of _{(h 45) / λ-P} 2 × (h 45) 2 <-7 ... (1) The objective lens for an optical head according to claim 1, wherein the objective lens satisfies a condition. 5. When the function of the refractive lens and the structure of the diffractive lens are combined, an axial chromatic aberration characteristic such that when the wavelength of the incident light changes to a longer wavelength side, the back focus changes in the extending direction. Assuming that the amount of change in the spherical aberration of the marginal ray with respect to the change in wavelength is ΔSA and the amount of change in the axial chromatic aberration with respect to the change in wavelength is ΔCA, −0.8 <ΔCA / ΔSA <−0.2 (2) The objective lens for an optical head according to claim 1, wherein the objective lens satisfies a condition. 6. The diffractive lens structure has a paraxially weak path.
And the additional amount of the optical path length due to the diffractive lens structure
Is the height h from the optical axis, and the n-th (even-order) optical path difference function coefficient
P_n, Using the wavelength λ, φ (h) = (P_Twoh^Two+ P_Fourh^Four+ P₆h⁶+ ...) × λ when expressed by an optical path difference function φ (h), f_D= 1 / (-P_Two× 2 × λ) use only the diffractive lens structure on the short wavelength side
Focal length f at wavelength _DAnd the operating wavelength on the short wavelength side
The overall focus of the combined refractive and diffractive lens structures
-0.020 <f / f_D<0.020 (3) The condition of (3) is satisfied.
An objective lens for an optical head according to any one of the above. 7. The diffractive lens structure according to claim 1, wherein the wavelength of the diffracted light that forms a good wavefront for an optical disc having a protective layer thickness of 0.6 mm is λ ₁ , and the optical disc has a protective layer thickness of 1.2 mm. The wavelength of the diffracted light that forms a good wavefront is λ ₂
The objective lens for an optical head according to claim 1, wherein the objective lens is designed to satisfy the following condition: 0.75 <λ ₁ / λ ₂ <0.87 (4). 8. At least in a region near the optical axis, the blazed wavelength λ _B of the diffractive lens structure is equal to the two wavelengths λ _1.
An objective lens for an optical head according to claim 7, characterized in that a wavelength between lambda _2. 9. The objective lens for an optical head according to claim 8, wherein in a region near the optical axis, the blazed wavelength λ _B of the diffractive lens structure satisfies the following condition. 0.87 <λ _B / λ ₂ (5) λ _B / λ ₁ <1.13 (6) 10. The objective lens for an optical head according to claim 8, wherein a blazed wavelength in a peripheral portion of the diffraction lens structure is shorter than a blazed wavelength λ _B in a region near the optical axis. . 11. The surface on which the diffractive lens structure is formed,
At least 85% of the effective diameter from the optical axis to the effective diameter
The objective lens for an optical head according to any one of claims 1 to 9, wherein the peripheral region up to a height of 100% is a continuous aspheric surface having no step.