JP2000260056A - Composite objective lens, spherical aberration correction element, and optical information recording/reproducing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a composite objective lens containing only one sheet of refraction type objective lens and used for recording/reproducing plural kinds of optical information recording media with substrate (light transparent layer) having thicknesses different from each other. SOLUTION: In the spherical aberration correction element 22 used together with the refraction type objective lens 21 of this composite objective lens, a diffraction lens structure is formed on its one side lens surface 221 as a ring- shaped pattern around an optical axis. This diffraction lens structure has a spherical aberration characteristic so as to change in the direction that a spherical aberration becomes correction shortage when the wavelength of an incident beam is shifted to the long wavelength side. A laser having an oscillation wavelength corresponding to the thickness of the substrate of a disk set as a recording/reproducing object is used selectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a digital versatile disk (DVD) and a compact disk (C).
D), a high NA (numerical aperture) compound objective lens and spherical aberration correction element used for an optical head of an optical information recording / reproducing apparatus such as a CD-R, and an optical information recording / reproducing apparatus using the same; The present invention relates to a compound objective lens and a spherical aberration correction element for recording and reproducing discs having different thicknesses of substrates (light transmitting layers), and an optical information recording and reproducing apparatus using them.

[0002]

2. Description of the Related Art Hitherto, various optical systems of an optical pickup device for recording and reproducing information on a CD, which is an optical information recording medium, have been known. Larger capacity DVDs are also available. For CD the laser is from 780nm to 83
Although near-infrared light of about 0 nm was used, DVD
In order to increase the capacity, a red semiconductor laser having a relatively short wavelength of about 635 nm to 660 nm is used. The NA of the objective lens is 0.6, and the thickness of the transparent substrate is 0.
6mm, both track pitch and shortest pit length are CD
The density is higher than that of.

However, since the thickness of the transparent substrate (light transmission layer) is 0.6 mm, which is different from that of the conventional CD, if a CD having a thickness of 1.2 mm is reproduced by an optical system for DVD,
Due to the large thickness of the disk, spherical aberration is excessively corrected and a light spot shape that can be reproduced cannot be obtained.

Therefore, various types of lens systems are known as optical pickup devices for recording and reproducing information on these two optical information recording media.

For example, according to the method described in Japanese Patent Application Laid-Open No. 7-98431, a hologram is provided on an information pickup device, and each of the 0th-order light and the 1st-order light transmitted through the hologram is D
Since VD and CD are assigned, a spot having diffraction-limited performance can be obtained for each optical information recording medium. Japanese Patent Application Laid-Open No. Hei 5-303766 discloses a method of inserting an aspherical and powerless optical element into a parallel light beam so as to cancel out aberrations caused by differences in disk thickness.

[0006]

However, Japanese Patent Laid-Open No. 7-9 / 1995
In the method described in Japanese Patent Application Laid-Open No. 8431, a method in which a light beam from a laser light source is divided into light beams having different diffraction orders for DVD and CD, a plurality of light beams are always emitted toward an information recording surface of an optical information recording medium. Therefore, when information is read with one light beam, the other light beam becomes unnecessary light that does not contribute to the reading, and causes an increase in noise. Further, since the utilization efficiency of the laser light is limited to about 40% at the maximum, there is a problem that when the light amount is increased, the power consumption is increased and the laser life is shortened. Further, in a method of inserting an element having an aspherical surface according to the thickness of a disk substrate as in the method described in Japanese Patent Application Laid-Open No. Hei 5-303766, decentered coma occurs when the position of the element is shifted. In addition, there is a problem that an insertion mechanism for attaching and detaching the element with high positional accuracy must be prepared.

On the other hand, in the method using one semiconductor laser, a light source having a wavelength of 635 nm to 660 nm has to be used to form a minute spot for reproducing a DVD, so that a CD-R cannot be read. There is a problem. In view of this problem, a two-pickup method using two semiconductor lasers and two sets of objective lenses has been devised. However, when a plurality of sets of objective lenses are used,
It is inevitable that the number of parts increases and the entire pickup becomes larger. SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the related art, and has a problem that DVD and CD
An objective lens for an optical head used for recording / reproducing a plurality of types of optical information recording media having different thicknesses of substrates (light transmitting layers) such as -R, wherein the number of refractive objective lenses is reduced to one. An object of the present invention is to provide a compound objective lens having high light use efficiency, a spherical aberration correction element included in the compound objective lens, and an optical information recording / reproducing apparatus using the compound objective lens.

[0008]

In order to achieve the above object, an optical information recording / reproducing apparatus according to the present invention has a refraction type objective lens for condensing a light beam on an optical information recording medium and a rotationally symmetric phase type diffraction lens. A composite objective lens comprising a combination of a spherical aberration correction element having a structure, and a plurality of laser light sources having different oscillation wavelengths, and the laser light source is selectively used according to the thickness of the substrate of the recording optical information recording medium. At the same time, the compound objective lens converts light of the same diffraction order by the diffraction lens structure of a light beam emitted from the laser plateau at an oscillation wavelength corresponding to the thickness of the substrate of the optical information recording medium into the optical information. Light is condensed on a signal recording surface of a recording medium.

As described above, when the thickness of the substrate of the optical information recording medium (for example, a disk) increases, the spherical aberration changes in a direction in which the correction is overcorrected. Therefore, as described above, the diffraction lens structure is provided with a spherical aberration change characteristic with respect to wavelength change, and a semiconductor laser having an oscillation wavelength different depending on the type of the optical information recording medium is selectively emitted, whereby the difference in the disk thickness is caused. Changes in spherical aberration can be canceled.

At this time, the spherical aberration correcting element has a phase type diffractive lens structure on a macroscopically flat base, or has a phase type diffractive lens structure on a macroscopically aspherical refraction lens, and has a spherical aberration correction element. It is preferable that the correction element has a shape that reduces the amount of spherical aberration generated.

For reproducing a CD-R, it is preferable to use a longer wavelength light source when the substrate of the optical information recording medium is thick. It is desirable that the diffractive lens structure at that time has a spherical aberration characteristic such that when the wavelength of the incident light changes to the longer wavelength side, the spherical aberration changes in a direction in which the correction is insufficiently corrected.

By the way, the additional amount of the optical path length due to the diffractive lens structure can be calculated by using the height h from the optical axis, the n-th (even-order) optical path difference function coefficient Pn, and the wavelength λ, φ (h) = (P2h it can be represented by the ^{2 + P4h 4 + P6h 6 +} ...) × λ ... (1) optical path difference function defined by φ (h).

The spherical aberration correcting element according to the present invention has a second-order optical path difference function coefficient of P2, and a light beam having an exit angle equivalent to NA of 0.45 when condensed by the refractive objective lens has a diffraction structure. It is desirable to satisfy the condition of −15 <φ (h ₄₅ ) / λ−P ² (h ₄₅ ) ² <−7 (2), where h ₄₅ is the height passing through the existing surface.

Further, it is desirable that the peripheral portion of the diffractive lens surface has a minute step which gives an optical path difference of 1λ to a wavelength λ shorter than the wavelength determining the step near the optical axis. Or, 85% of the effective diameter from the optical axis to 100% of the effective diameter
Is preferably a continuous surface having no step.

The microscopic sectional shape of the phase type diffraction lens structure of the spherical aberration correction element having a phase type diffraction lens structure on a macroscopically aspherical refraction lens has an annular shape perpendicular to the optical axis. It is preferably a step-like shape composed of a plane.

Further, there are provided collimating means for converting light beams diverging from the plurality of light sources into parallel light, and light condensing means for condensing the parallel light passing through the collimating means on an optical recording medium. A spherical aberration corrector having a phase type diffractive lens structure may be inserted in the optical path.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an optical system in an optical recording / reproducing apparatus according to the present invention will be described below.

FIG. 1 shows a DVD and a C according to the present embodiment.
1 shows an optical information recording / reproducing apparatus using a spherical aberration correction element compatible with D and CD-R. The optical information recording / reproducing apparatus shown in FIG. 1 has a function of converging laser beams emitted from semiconductor lasers 11 and 12 as light sources on a signal recording surface of a disk D.

More specifically, laser beams emitted from two semiconductor laser light sources 11 and 12 having different oscillation wavelengths are collected on the same optical path by a prism beam combiner 13 and collimated by entering a collimator lens 14. It is luminous flux. When using this embodiment, 11 or 12 lasers are selectively turned on depending on the type of recording medium.

The compound objective lens 20 comprises a combination of a refraction type objective lens 21 and a spherical aberration correction element 22 having a rotationally symmetric phase type diffraction lens structure. This refraction type objective lens 21 has two aspherical lens surfaces 211,
It is a biconvex resin single lens having 212.

The spherical aberration correction element 22 has a diffractive lens structure formed on one of the lens surfaces 221 or 222 as an annular pattern centered on the optical axis as shown in FIGS. ing. This diffractive lens structure
Like a Fresnel lens, there is a step in the optical axis direction at the boundary of each annular zone. FIGS. 2A and 2B are a front view (a) and a vertical cross-sectional view (b) of the spherical aberration correction element 22 when a diffraction structure is formed macroscopically on a plane. FIGS. 3A and 3B are a front view (a) and a vertical cross-sectional view (b) of the spherical aberration correction element 22 when a diffractive structure is formed macroscopically on an aspherical surface. In these figures, the steps are emphasized. That is, the actual height t of the step is λB, the wavelength that maximizes the diffraction efficiency,
Assuming that the refractive index of the substrate on which the diffraction structure is provided is n, approximately t = λ
It is a very small height of about 1 μm, which is B / (n−1).
Accordingly, the number of annular zones is actually increased from tens to several tens.

A structure in which a diffractive lens structure is formed macroscopically on a plane is suitable for forming a diffractive lens structure by lithography technology, and a structure in which a diffractive lens structure is macroscopically formed on an aspherical surface is: Since the orbicular plane can be constituted by a plane perpendicular to the optical axis, it is suitable for making a diffractive lens structure on a lathe.

In any case, the diffractive lens structure formed on the spherical aberration correction element 22 changes in such a direction that the spherical aberration becomes undercorrected when the wavelength of the incident light changes to the longer wavelength side. It has a spherical aberration characteristic. The spherical aberration of the optical system that reads an optical disk changes in a direction in which the substrate (light transmission layer) of the disk becomes thicker and overcorrected.

On the other hand, as the semiconductor lasers 11 and 12 used as light sources, those emitting near-infrared light near 780 nm are required for CD-R, and a small condensed spot is formed for DVD. From 635nm to 6
One that emits 60 nm red light is used.

Therefore, as described above, the diffraction lens structure has a characteristic that the spherical aberration changes in a direction in which the correction becomes insufficient when the wavelength changes to a long wavelength, and the long wavelength changes when the disk substrate is thick. By causing the semiconductor laser to emit light selectively, spherical aberration that is overcorrected as the disk substrate becomes thicker can be canceled by utilizing the characteristic of spherical aberration that changes in the direction of undercorrection by the diffractive lens structure.

The additional amount of the optical path length due to the diffractive lens structure of the spherical aberration correction element 22 is expressed by φ (h) using the height h from the optical axis, the n-th (even-order) optical path difference function coefficient Pn, and the wavelength λ. ) = (P 2 h by ^{2 + P4h 4 + P6h 6 +} ...) × λ ... (3) an optical path difference function defined by φ (h), can be expressed. P2, P4, P6, ... are the second, fourth, sixth,
... In this expression form, the coefficient P of the quadratic term
When 2 is negative, the power is paraxially positive, and when the coefficient P4 of the fourth-order term is positive, the negative power gradually increases toward the periphery.

The fine shape of the actual diffractive lens structure of the spherical aberration correction element 22 has the following formula (4) so as to have a Fresnel lens-shaped additional optical path length φ ′ in which a component of an integral multiple of the wavelength of the optical path length is eliminated. ).

Φ ′ (h) = (MOD (P2h ² + P4h ⁴ + P6h ⁶ +... + Const, l) −Const) × λB (4) In the above equation (4), λB is an optical path length with a fine step of 1λ. This is a wavelength that gives a difference, and is a wavelength that maximizes diffraction efficiency. The constant term Const is a constant for setting the phase at 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 (P2h ² + P4h ⁴ + ... + Const, 1) becomes 0
The point h is the border of the ring zone. On the base shape, φ '(h)
The gradient and the step are set so as to have an optical path difference of.

Here, the spherical aberration correction element 22 of the embodiment
-15 <φ (h ₄₅ ) / λ−P 2 (h ₄₅ ) where h ₄₅ is the height at which the light beam equivalent to the second-order optical path difference function coefficient P 2 and NA 0.45 passes through the surface where the diffraction structure exists. ² <-7 ... Designed to satisfy the condition of (5).

When this condition is satisfied, the change in spherical aberration caused by the difference in the thickness of the disk substrate can be favorably canceled by the change in spherical aberration caused by the change in the wavelength of the diffractive lens. If the lower limit of conditional expression (5) is exceeded, the change in spherical aberration due to the wavelength change will be excessive. On the other hand, when the value exceeds the upper limit of the conditional expression (5), the change of the spherical aberration becomes insufficient. Since there are individual differences in the oscillation wavelengths of the semiconductor lasers, an appropriate value is selected in the equation (5) by selecting the oscillation wavelengths of the two lasers. Cannot handle disc thickness.

The oscillation wavelength of a semiconductor laser is
There is an abrupt change accompanying a change in laser output during a writing operation in an optical recording apparatus. Since the change in the focal position due to this cannot be handled even by using the focus adjustment mechanism, it is desirable to suppress the change in the focal position due to wavelength fluctuation on the objective lens side. Generally, for this wavelength variation,
This can be dealt with by correcting axial chromatic aberration. However, the compound objective lens 20 according to the present embodiment intentionally generates spherical aberration by wavelength switching as described above to prevent generation of wavefront aberration. When the chromatic aberration is corrected, the deviation of the optimum writing position due to the wavelength fluctuation is rather increased. Therefore, by intentionally increasing the axial chromatic aberration or making the lens with a material having a large dispersion, it is possible to balance between the spherical aberration change and the axial chromatic aberration.

The wavelength of the laser light emitted from the two lasers 11 and 12, ie, the wavelength λ1 for focusing on the signal recording surface of the optical disk whose substrate (light transmission layer) has a thickness of 0.6 mm, and the substrate ( The selection of the wavelength λ2 for focusing on the signal recording surface of an optical disk having a thickness of the light transmitting layer of 1.2 mm is as follows.
It affects the allowable width of laser oscillation wavelength and diffraction efficiency.

For example, if two wavelengths λ1 and λ2 are 657
nm and 780 nm, based on the amount of 657 nm aberration, the 780 nm
A wavefront aberration of (780-657) /780=0.1577λ is added. For this reason, when the difference between the wavelengths λ1 and λ2 of the laser light to be selected becomes small, the number of steps in the diffractive lens structure increases and the light amount loss due to the edge of the step increases. In addition, since the amount of spherical aberration generated per unit wavelength becomes too large, the spherical aberration exceeding an allowable amount changes due to the difference in the oscillation wavelength due to the individual difference of the semiconductor laser, and the laser must be sorted by the oscillation wavelength. If the ratio of λ1 / λ2 is small, the wavelength-dependent diffraction efficiency is undesirably reduced.

When there is a wavelength difference of about 20% between λ1 and λ2, there is a decrease in the wavelength-dependent diffraction efficiency. FIG. 4 shows graphs of the diffraction efficiency when the wavelength λB that maximizes the diffraction efficiency is selected to be 650 nm, 700 nm, and 770 nm. Even if λB is selected to be substantially the same as the wavelengths λ1 and λ2 of the two semiconductor lasers, as shown in FIG. 4, the diffraction efficiency with respect to the unselected wavelength is about 90%. This is because the same order is used for the thicknesses of the two types of substrates, so that the luminous flux does not have to be forcibly divided into two orders.
Therefore, the minute step near the optical axis is selected by selecting the wavelength λB between the two wavelengths λ1 and λ2 according to the power margin of the laser and the like, and when the light beam of the wavelength λB is incident, the optical path length difference of 1λ is obtained. Should be determined.

The degree of the minute step at the periphery of the surface of the spherical aberration correction element 22 where the diffraction lens structure is formed (hereinafter referred to as “diffraction lens surface”) is determined by the wavelength that determines the step near the optical axis. An optical path difference of 1λ is given to a shorter wavelength. The peripheral region from the height of 85% of the effective diameter to the height of 100% of the effective diameter from the optical axis may be a continuous surface having no step. The reason for this configuration is to prevent the focused spot on the long wavelength side from becoming too small.

For reproducing CDs and CD-Rs, NA is 0.4
5 is sufficient, and the periphery of the effective aperture for securing the NA of 0.60 required for DVDs is not only unnecessary for CDs, but on the contrary, the luminous flux is too narrow to adversely affect recording and reproduction. May be given. By setting λB to a wavelength shorter than λ1, the diffraction efficiency with respect to λ2 is further reduced, or by blazing with the second-order diffracted light of λ1, the luminous flux of wavelength λ2 is not reduced without reducing the amount of transmitted light of λ1. The amount of light in the peripheral part can be reduced. In addition, the peripheral portion may be a continuous surface having no step, and may be a surface subjected to aberration correction exclusively for DVD. On the other hand, the compound objective lens 20 according to the present embodiment in which the spherical aberration is changed by the wavelength change, the spherical aberration caused by the change in the refractive index caused by the temperature change is changed by the spherical aberration caused by the change in the oscillation wavelength of the semiconductor laser caused by the temperature change. In addition, it has the effect of canceling. Therefore, when the refraction type objective lens 21 is made of a resin whose refractive index decreases as the temperature rises, it is preferable to keep the diffraction lens structure up to the outermost part of the spherical aberration correction element 22. But even in this case,
It is desirable that the thickness of the step is optimized for a short wavelength for DVD so that the diffraction efficiency of the light beam for DVD is increased.

Further, if the light beam incident on the compound objective lens 20 is a parallel light beam, the light source unit (semiconductor laser light sources 10 and 11 and the prism beam combiner 13) and the objective lens unit (composite objective lens 21) are separated. Even if the distance between them is made variable, the aberration after exiting from the compound objective lens 20 does not change. For this reason, an optical recording / reproducing apparatus capable of high-speed tracking can be provided by fixing the light source unit and using only the objective lens unit as a movable unit.

Next, three specific examples based on the above-described embodiment will be presented. Both are substrates (light transmission layer)
Composite objective lens 20 for DVD using a disc having a thickness of 0.6 mm and for CD and CD-R using a disc having a substrate (light transmitting layer) of 1.2 mm in thickness.
It is. In each of the embodiments described below, the diffractive lens structure is formed on the side surface of the disk of the spherical aberration correction element 22, but it can be provided on the incident side.

[0039]

FIG. 5 shows a spherical aberration correction element 22, a refraction type objective lens 21, and a light transmission layer D of a disc which is an optical information recording medium according to a first embodiment. Table 1 shows a specific numerical configuration of the compound objective lens 20 according to the first embodiment.
Surface numbers 1 and 2 indicate the spherical aberration correction element 22, surface numbers 3 and 4 indicate the refraction objective lens 21, and surface numbers 5 and 6 indicate the light transmitting layer D of the disk as the medium. In the table, NA is the numerical aperture, f
Is the overall focal length (unit: mm), ω is the half angle of view (unit: degre
e), λ is the wavelength used (unit: nm), h ₄₅ is the passing height (unit: mm) of the light beam corresponding to NA 0.45 on the surface where the diffractive lens structure exists, and λB is the step setting of the diffractive structure. Wavelength for r
Is the macroscopic radius of curvature of each lens surface (unit: mm), and d1 is the lens thickness or lens spacing in the case of the configuration corresponding to the wavelength λ1.
(Unit: mm), d2 is the lens thickness or lens interval (unit: mm) in the case of the configuration corresponding to the wavelength λ2, nλ is the refractive index of each lens at the wavelength λnm, and ν is the Abbe number of each lens.

The refractive objective lens 21 has an aspherical surface on both sides, and the shape thereof is a distance between a coordinate point on the aspherical surface having a height h from the optical axis and a tangent plane on the optical axis of the aspherical surface. (Sag amount) is X, the curvature (1 / r) of the aspheric surface on the optical axis is C, the conic coefficient is K, the fourth, sixth, eighth, tenth, and twelfth aspheric coefficients are A4, A6, A8, A10 and A12 are represented by the following formula (6)
Represented by

X = Ch ² / (1 + √ (1- (1 + K) C ² h ² )) + A4h ⁴ + A6h ⁶ + A8h ⁸ +... (6) The radius of curvature is a radius of curvature on the optical axis. Conic coefficient and aspheric coefficient that define the aspheric surface,
Table 2 shows the optical path difference function coefficients defining the diffraction lens structure. Note that the notation E in Table 2 represents a power with 10 as a radix and the right number of E as an exponent. For example, the value of the fourth-order aspherical coefficient A4 of the third surface shown in Table 2 is "-
"1.2400E-3" means "-0.0012400".

[0042]

[Table 1] λ1 = 657nm NA 0.60 f = 3.50mm ω = 1.0 ゜ λ2 = 785nm NA 0.45 f = 3.52mm ω = 1.0 ゜ h ₄₅ = 1.60mm λB = 700nm Surface number r d1 d2 n657 n785 ν 1 ∞ 1.000 1.000 1.54056 1.53665 55.6 2 ∞ 0.200 0.200 3 2.084 2.400 2.400 1.54056 1.53665 55.6 4 -12.230 1.706 1.344 5 ∞ 0.600 1.200 6 ∞

[0043]

[Table 2] FIG. 6 shows various components of the compound objective lens 20 of Example 1 when a laser beam having a first wavelength λ1 = 657 nm is emitted corresponding to a first disk having a thickness of the light transmission layer D of 0.60 mm. Shows aberrations. FIG. 6A shows spherical aberration and chromatic aberration at each wavelength of 657 nm, 649 nm, and 665 nm, and FIG. 6B shows astigmatism (S: sagittal,
M: Meridional). The vertical axis of the graph (A) is the numerical aperture NA, and the vertical axis of the graph (B) is the image height Y. The horizontal axis indicates the amount of each aberration generated, and the unit is mm.

FIG. 7 shows that the thickness of the light transmitting layer D is 1.20.
4 shows a state of the compound objective lens 20 of Example 1 when a second disc having a size of mm is set. FIG. 8 shows various aberrations of the compound objective lens 20 of Example 1 when a laser beam having a second wavelength λ2 = 780 nm is emitted corresponding to the second disk. 6 and FIG. 8,
It can be seen that the spherical aberration is favorably corrected at the two wavelengths. The fine shape of the spherical aberration correction element 22 is shown in FIG.
The height h of the switching point of the annular zone from the optical axis is as shown in Table 3. Shape actual shape of the fine shape, the base surface is a plane, defined according to the equation Δx (h) = MOD (P2h 2 + P4h 4 + P6h 6 + ..., l) × λB / (n-1) ... (7) become. Here, λB is selected to be 700 nm for hoop switching points inside h ₄₅ = 1.60 mm, that is, hoop switching point numbers 1 to 11, and λB is selected to be 657 nm for the outside.

[0045]

[Table 3]

[0046]

Second Embodiment FIG. 9 shows a spherical aberration correction element 22, a refraction type objective lens 21, and a light transmission layer D of a disc as an optical information recording medium according to a second embodiment. Table 4 shows a specific numerical configuration of the second embodiment. 2nd surface, 3rd surface, 4th surface
Table 5 shows the conic coefficient of the surface, the aspherical surface coefficient, and the optical path difference function coefficient representing the diffractive lens structure formed on the second surface.

FIG. 10 shows a composite objective lens according to the second embodiment when a laser beam having a first wavelength λ1 = 657 nm is emitted corresponding to a first disk having a light transmission layer D having a thickness of 0.60 mm. 20 shows various aberrations.

FIG. 11 shows a second embodiment in which the second disk having the thickness of the light transmitting layer D of 1.20 mm is set.
3 shows the state of the composite objective lens 20 of FIG. FIG. 12 shows various aberrations of the compound objective lens 20 of Example 2 when a laser beam having the second wavelength λ2 = 780 nm is emitted corresponding to the second disk.

[0049]

[Table 4] λ1 = 657nm NA 0.60 f = 3.80mm ω = 0.9 ゜ λ2 = 780nm NA 0.50 f = 3.82mm ω = 0.9 ゜ h ₄₅ = 1.72mm λB = 700nm Surface number r d1 d2 n657 n780 ν 1 ∞ 1.500 1.500 1.54056 1.53677 55.6 2 ∞ 0.250 0.250 3 2.430 2.840 2.840 1.54056 1.53677 55.6 4 -7.826 1.862 1.500 5 ∞ 0.600 1.200 6 ∞

[0050]

[Table 5] In the second embodiment, the base surface of the second surface 221 of the spherical aberration correction element 22 (shape as a refractive lens excluding the diffractive lens structure)
Is an aspherical surface, and the combined amount of spherical aberration generated by the diffraction structure and the refraction structure is reduced. By taking this form, it is possible to minimize the bending of the light beam after passing through the spherical aberration generating element 22 and to reduce the change in aberration caused by the gap between the spherical aberration correcting element 22 and the refraction objective lens 21. it can. Further, the fine shape of the spherical aberration correction element 22 is a shape schematically shown in FIG. Actual shape of the fine shape, on the base surface of the aspherical determined according to the equation Δx (h) = MOD (P2h 2 + P4h 4 + P6h 6 + ..., l) × λB / (n-1) ... (8) With the addition of a diffractive structure, the sag is substantially constant for each zone, and all zones can be made as planes perpendicular to the optical axis. In that case, the step T of the annular zone
Is T = 0.000700 / (1.53906-1) = 0.00130 from the refractive index of 1.53906 at wavelength λB = 0.0007 mm and 700 nm. That is, it is required to be 1.3 μm.

[0051]

Third Embodiment FIG. 13 shows a spherical aberration correcting element 22, a refraction type objective lens 21, and a light transmission layer D of a disc as an optical information recording medium according to a third embodiment. Table 6 shows a specific numerical configuration of the third embodiment. Table 7 shows the conical coefficients, aspheric coefficients, and optical path difference function coefficients representing the diffractive lens structures formed on the second surface, the third surface, and the fourth surface.

FIG. 14 shows a composite objective lens according to the third embodiment when a laser beam having a first wavelength λ1 = 657 nm is emitted corresponding to a first disk having a thickness of the light transmission layer D of 0.60 mm. 20 shows various aberrations.

FIG. 15 shows a third embodiment in which the second disk having the light transmitting layer D having a thickness of 1.20 mm is set.
3 shows the state of the composite objective lens 20 of FIG. FIG. 16 shows various aberrations of the compound objective lens 20 of Example 3 when the second wavelength λ2 = 780 nm is emitted corresponding to the second disk.

[0054]

[Table 6] λ1 = 657nm NA 0.60 f = 3.06mm ω = 1.1 ° λ2 = 780nm NA 0.50 f = 3.08mm ω = 1.1 ° h ₄₅ = 1.38mm λB = 657nm surface number r d1 d2 n657 n780 ν 1 ∞ 1.500 1.500 1.54056 1.53677 55.6 2 ∞ 0.200 0.200 3 1.954 2.287 2.287 1.54056 1.53677 55.6 4 -6.293 1.421 1.500 5 ∞ 0.600 1.200 6 ∞

[0055]

[Table 7] In the third embodiment, the second surface 221 of the spherical aberration correction element 22 is also used.
The base surface (a shape as a refraction lens excluding the diffractive lens structure) is an aspheric surface, and is set so that the spherical aberration generated by the diffractive structure and the refraction structure at the wavelength λB is canceled. Also, it is possible to make all the zones as planes perpendicular to the optical axis. In that case, the step T of the annular zone has a wavelength λB = 0.000657 mm and a refractive index of 1.5 at 657 nm.
From 4056, T = 0.000657 / (1.54056-1) = 0.00122.

In this case, at 657 nm, the spherical aberration correction element 2
2 does not generate spherical aberration, so that the refractive objective lens 21
Can be used independently designed so as to have no aberration at NA 0.60.

Therefore, in this compound objective lens 20, only the peripheral portion is a flat surface having no step, and the
Can be processed so that the light of NA 0.45 or more does not contribute to the image formation due to the large spherical aberration for the 780 nm light flux without reducing the transmitted light amount.

Table 8 shows the correspondence between the above-mentioned conditional expression (5) and each embodiment. This embodiment satisfies the conditional expression (5), whereby the change in wavefront aberration due to the thickness of the substrate (light transmission layer) can be canceled by the difference in wavelength.

[0059]

[Table 8]

The optical system in the optical recording / reproducing apparatus according to the present embodiment is configured as an infinite system including the collimator lens 14, but the present invention does not include such a collimator lens. The present invention is also applicable to an optical system in which a diffuse beam is directly incident on a compound objective lens.

[0061]

As described above, according to the present invention,
The change in the spherical aberration caused by the difference in the thickness of the substrate (light transmission layer) can be canceled by the change in the spherical aberration of the diffractive lens structure. Therefore, the light use efficiency is increased, and the number of refractive lenses included in the objective lens for the optical head can be reduced to only one. As a result, when the present invention is applied to a DVD or CD-R compatible system, the number of movable parts around the objective lens can be reduced, and the device can be made compact and high-speed.

In particular, by using the same diffraction order light at a plurality of wavelengths different from each other as in claim 1, the conventional two-dimensional light is used.
The spherical aberration can be corrected by the diffractive lens without losing the light amount as much as the focal diffractive lens. In the case where the diffractive lens structure is designed to satisfy the condition of claim 6, the change in spherical aberration caused by the difference in the thickness of the substrate (light transmitting layer) is compensated for by the spherical aberration caused by the wavelength fluctuation of the diffractive lens. Can be favorably negated by the change in. In particular, when the thickness of the substrate (light transmission layer) is large, a semiconductor laser having a long wavelength can be used.

Further, when the light beam incident on the compound objective lens is formed as a parallel light beam, high-speed tracking becomes possible.

[Brief description of the drawings]

FIG. 1 is an optical configuration diagram of an optical information recording / reproducing apparatus according to an embodiment of the present invention.

FIG. 2 is a front view (a) and a side view (b) showing an example of the shape of the spherical aberration correction element in FIG. 1;

FIGS. 3A and 3B are a front view and a side view showing an example of the shape of the spherical aberration correction element shown in FIGS.

FIG. 4 is a graph showing diffraction efficiency.

FIG. 5 is a lens configuration diagram of a compound objective lens according to a first embodiment when a first disc is set.

FIG. 6 is a diagram showing various aberrations of the compound objective lens of Example 1 when a laser beam having a first wavelength is emitted.

FIG. 7 is a lens configuration diagram of the compound objective lens according to the first embodiment when a second disk is set.

FIG. 8 is a diagram illustrating various aberrations of the objective lens according to the first embodiment when a laser beam having a second wavelength is emitted.

FIG. 9 is a lens configuration diagram of a compound objective lens according to a second embodiment when a first disc is set.

FIG. 10 is a diagram illustrating various aberrations of the compound objective lens according to the second exemplary embodiment when a laser beam having a first wavelength is emitted.

FIG. 11 is a diagram illustrating a lens configuration of a compound objective lens according to a second embodiment when a second disc is set.

FIG. 12 is a diagram illustrating various aberrations of the objective lens of the second embodiment when a laser beam having a second wavelength is emitted.

FIG. 13 is a diagram illustrating a lens configuration of a compound objective lens according to a third embodiment when a first disc is set.

FIG. 14 is a diagram illustrating various aberrations of the compound objective lens according to the third example when a laser beam having a first wavelength is emitted.

FIG. 15 is a diagram illustrating a lens configuration of a compound objective lens according to a third embodiment when a second disk is set.

FIG. 16 is a diagram illustrating various aberrations of the objective lens according to the third embodiment when a laser beam having a second wavelength is emitted.

[Explanation of symbols]

 Reference Signs List 11 semiconductor laser 1 12 semiconductor laser 2 13 beam combiner 14 collimating lens 20 compound objective lens 21 refractive objective lens 22 spherical aberration correction element 221 first surface of spherical aberration correction element 222 second surface of spherical aberration correction element D disk substrate ( Light transmission layer)

Claims (10)

[Claims] 1. A compound objective lens comprising a refractive objective lens for condensing a light beam on an optical information recording medium and a spherical aberration correcting element having a rotationally symmetric phase diffractive lens structure, and a plurality of lasers having different oscillation wavelengths. A light source, and selectively using the laser light source according to the thickness of the substrate of the recording optical information recording medium, and the compound objective lens has an oscillation wavelength corresponding to the thickness of the substrate of the optical information recording medium. An optical information recording / reproducing apparatus, wherein light of the same diffraction order of the light beam emitted from the laser light source by the diffraction lens structure is focused on a signal recording surface of the optical information recording medium. 2. A compound objective lens used in the optical information recording / reproducing apparatus according to claim 1, wherein said spherical aberration correction element is provided on a light beam incident side of said refraction type objective lens. Compound objective lens. 3. A spherical aberration correcting element for use in an optical information recording / reproducing apparatus according to claim 1, wherein said spherical aberration correcting element has a phase type diffractive lens structure on a macroscopically flat base. . 4. A spherical aberration correcting element for use in the optical information recording / reproducing apparatus according to claim 1, wherein said spherical aberration correcting element has a phase type diffractive lens structure on a macroscopically aspherical refracting lens. Aberration correction element. 5. The diffraction lens structure according to claim 3, wherein when the wavelength of the incident light changes to the longer wavelength side, the spherical aberration changes to the undercorrection side. Spherical aberration correction element. 6. An additional amount of an optical path length by the diffractive lens structure is obtained by using a height h from an optical axis, an nth-order (even-order) optical path difference function coefficient Pn, and a wavelength λ to obtain φ (h) = ( P2h ² + P4h ⁴ + P6h ⁶ +...) × λ When represented by an optical path difference function φ (h),
Secondary optical path difference function coefficient P2, the height of the refraction type light becomes NA0.45 corresponding exit angle when the light is collected by the objective lens passes through the existing surface of the diffractive structure as h _45, -15 < 6. The spherical aberration correction element according to claim 5, wherein a condition of φ (h ₄₅ ) / λ−P ² (h ₄₅ ) ² <−7 is satisfied. 7. The effective diameter of the surface on which the diffraction lens structure is formed is at least 85% of the effective diameter from the optical axis to 100% of the effective diameter.
7. The spherical aberration correction element according to claim 3, wherein the peripheral region up to the height of% is a continuous surface having no step. 8. A step of the diffractive lens structure in a peripheral portion of a surface on the light beam incident side of the refraction type objective lens is 1λ for a wavelength λ shorter than a wavelength determining a step near the optical axis.
The composite objective lens according to claim 2, wherein the optical path difference is given by: 9. The phase-diffractive lens structure according to claim 4, wherein a microscopic cross-sectional shape is a step-like shape constituted by an annular plane perpendicular to the optical axis. The spherical aberration correction element according to any one of the above. 10. The thicker the substrate of the optical information recording medium is,
2. The optical information recording / reproducing apparatus according to claim 1, wherein a laser light source having a long oscillation wavelength is used.