JP2005044461A - Lens, optical system using the same, optical head, and optical disc apparatus - Google Patents

Abstract

A light beam can be focused on an information recording surface with high light utilization efficiency for each of a plurality of types of optical recording media having different thicknesses of transparent substrates.
A lens according to the present invention has different refractive powers in a common use region for all monochromatic light of one lens surface in a multi-wavelength lens for condensing a plurality of types of monochromatic light of wavelength λi. It is divided into a plurality of aspheric surfaces. More than half of the steps in the adjacent portions, where Dj is the step amount in the direction parallel to the lens optical axis in the adjacent portions of the adjacent aspheric portions of each aspheric portion. Therefore, the following equation is satisfied when the minimum value of Aij for each wavelength λi is MIN (Aij) and the maximum value is MAX (Aij).
MAX (Aij) / MIN (Aij) <3
Here, Aij = absolute value (Bij−mij), Bij = (absolute value (Dj)) * (ni−1) / λi−C, ni: wavelength λi
The refractive index of the lens at, mij: an integer closest to Bij, and C: a correction term.
[Selection] Figure 1

Description

The present invention relates to a multi-wavelength optical system that uses a plurality of types of monochromatic light.
Disc: including a CD such as a CD-R), DVD (Digital Versatile Disc), blue laser compatible disc (AOD), etc. The present invention relates to a wavelength lens, a multi-wavelength optical system, an optical head, and an optical disc apparatus.

Conventionally, compatible optical disk apparatuses have been proposed that can play back together optical disks of different types such as CDs and DVDs. A CD or DVD (hereinafter collectively referred to as an optical disk) uses a transparent substrate, and an information recording surface is provided on one surface of the transparent substrate. The optical disk has a configuration in which two transparent substrates are bonded together with their information recording surfaces facing each other, or such a transparent substrate is a transparent protective substrate, and the information recording surface of the transparent substrate is a protective substrate. It is structured to be pasted so as to face each other. When reproducing the information signal stored in the optical disk having such a configuration, it is necessary to focus the laser beam from the light source on the information recording surface of the optical disk via a transparent substrate by the optical disk device. The wavelength of the laser beam differs depending on whether it is used in a CD or a DVD as described later. In order to focus the laser beam, an objective lens is used in the optical disc apparatus. Here, the thickness of the transparent substrate used in the CD is 1.2 mm, whereas the thickness of the transparent substrate used in the DVD is 0.6 mm, which depends on the type of optical disk (difference in laser beam wavelength). The thickness of the transparent substrate provided with the information recording surface is different. In an optical disc apparatus that reproduces optical discs of different types, it is necessary to focus the laser beam on the information recording surface even if the thickness of the transparent substrate varies depending on the type of optical disc. In addition, it has been proposed that a new optical disk apparatus that has been recently proposed uses a blue laser having a wavelength of about 400 nm for information reproduction. Therefore, it is expected that such a new optical disc can be used simultaneously with the optical disc apparatus in addition to the CD and the current DVD for backward compatibility.

As such a compatible optical disc device, an objective lens is provided for each type of optical disc in the pickup, and the objective lens is exchanged according to the type of the optical disc to be used, or a pickup is provided for each type of optical disc. It is conceivable to change the pickup depending on the type. However, in order to realize cost reduction and downsizing of the apparatus, it is desirable that the same lens can be used for any kind of optical disk as the objective lens.

As a representative example of such an objective lens, there is one described in Patent Document 1. The objective lens described in this document is divided into three or more annular lens surfaces in the radial direction, and every other annular lens surface and the other annular lens surfaces have different refractive powers. ing. For every laser beam of the same wavelength, every other annular lens surface is, for example, a thin transparent substrate (0.6 mm
) Is focused on the information recording surface of the optical disc (DVD), and every other ring-shaped lens surface is, for example, a laser on the information recording surface of the optical disc (CD) of a thick transparent substrate (1.2 mm). Focus the beam.

Another representative example is described in Patent Document 2. This document uses a short wavelength (635 nm or 650 nm) laser beam for a thin transparent substrate DVD and a long wavelength (780 nm) laser beam for a thick transparent substrate CD. An optical disc apparatus is disclosed. This optical disc apparatus has an objective lens commonly used for these laser beams. In this objective lens, a diffractive lens structure is formed in which minute steps of an annular zone are densely provided on one surface of a refractive lens having a positive power. Such a diffractive lens structure condenses the diffracted light of the short wavelength laser beam on the DVD of the thin transparent substrate and the diffracted light of the long wavelength laser beam on the information recording surface of the CD of the thick transparent substrate. Designed. Each diffracted light is designed to collect the same order of diffracted light on the information recording surface. The reason for using a laser beam with a short wavelength for DVD is that the recording density of DVD is higher than that of CD, and it is therefore necessary to narrow down the beam spot. As is well known, the size of the light spot is proportional to the wavelength,
It is inversely proportional to the numerical aperture NA.

An objective lens of an annular zone correction lens system in which an annular zone phase shifter is provided on the lens surface has also been proposed (for example, Patent Document 3). In this objective lens, first, the wavelength λ used for the DVD is used.
₁ is based on a lens surface that eliminates wavefront aberration caused by a 640 nm laser beam. Further, this objective lens is divided into a plurality of zonal refracting surfaces in the radial direction, and these refracting surfaces are formed with predetermined steps from the reference lens surface (the i-th step from the lens center is d _i ). To do. Due to the level difference d _i , the DVD laser beam is phase-shifted by an integral number m _i times the wavelength λ ₁ with respect to the reference lens surface by the respective refractive surfaces, thereby reducing the wavefront aberration of the CD system.

Japanese Patent Laid-Open No. 9-145995 JP 2000-81666 A JP 2001-51192 A

In any of the above conventional examples, since a common objective lens can be used for both DVD and CD, means for exchanging members used for each DVD and CD including the objective lens becomes unnecessary, and the cost and configuration are reduced. This is advantageous in terms of simplification. However, in Patent Document 1, since the annular lens surface used in the objective lens is different for each DVD and CD, there are many portions that are ineffective with respect to the incident laser beam, and the light utilization efficiency is extremely low. .

Moreover, in the said patent document 2, since the diffracted light by a diffractive lens structure is utilized, there exists a problem that the diffraction efficiency with respect to each of a different wavelength cannot be made into 100% simultaneously. In this diffractive lens, the diffraction efficiency is 100% at a wavelength almost intermediate between the short wavelength (635 nm or 650 nm) laser beam used for DVD and the long wavelength (780 nm) laser beam used for CD. Thus, the diffraction efficiency is balanced with respect to the used laser beam. In addition, since a diffractive lens structure is provided on the lens surface, a minute step is required, but it is easily affected by manufacturing errors, and if the diffractive structure deviates from the design, the diffraction efficiency is deteriorated. As described above, the deterioration of the diffraction efficiency or the fact that the diffraction efficiency does not reach 100% means that all of the incident light cannot be condensed on the information recording surface provided on the transparent substrate of the optical disk. This is a light loss.

Furthermore, in the annular phase correction lens system disclosed in Patent Document 3, a lens surface designed so as to eliminate wavefront aberration with respect to a DVD laser beam is used as a reference surface. to reduce, and the refractive surface by depressing only the laser beam having the wavelength lambda ₁ integer m _i times the level difference d _i of the DVD from the reference plane. Further, at this time, since the condensing point position in each annular zone is shifted by the formation of the step due to the formation of the step, the curved surface shape of each annular zone is designed so as not to shift the condensing point position. . However, in the conventional example of Patent Document 3, the wavefront aberration of the DVD can be sufficiently reduced, but the wavefront aberration cannot be sufficiently reduced with respect to the CD laser beam. As for the value of wavefront aberration, it is sufficient that the RMS wavefront aberration value should be 0.07λRMS or less of the Marshallian evaluation standard (manufactured by Optronics, page 198 of optical introduction, published on November 26, 1990,
Etc.), and the patent document 3 also describes an embodiment that is 0.07λ RMS or less. However, in the case of an optical disk device, 0.07λ RMS or less is
This is a value that should be the target value for the entire optical disc apparatus, and is still insufficient as a target value for the objective lens alone. The optical disk device as a whole has many factors that degrade the RMS wavefront aberration, such as the laser astigmatism, the aberration of the collimator lens, the aberration of the reflection mirror and the transmission mirror, and the tilt shift between the optical pickup and the optical disk. Therefore, it is required that the RMS wavefront aberration value be as small as possible, not 0.07λ RMS or less. Specifically, the objective lens alone has an RM of 0.035λ RMS or less.
Although it is desirable to set the S wavefront aberration value, it is more desirable that the RMS wavefront aberration value be 0.030λ RMS or less, and even more desirably 0.025λ RMS or less. However, in Patent Document 3, the DVD is the first embodiment. 0.001λRMS, CD is 0.047λRMS, and in Embodiment 2, DVD is 0.019λRMS, CD is 0.037λRMS,
The wavefront aberration of DVD is a good value, but it is only 0.037λRMS or more in CD, and a good wavefront aberration value has not been obtained for both DVD and CD.

An object of the present invention is to eliminate such a problem and to collect a light beam on an information recording surface with a high light utilization efficiency in a state where wavefront aberration is reduced as much as possible for each of a plurality of types of optical recording media. It is an object of the present invention to provide a lens that can emit light, an optical system using the lens, an optical head, and an optical disk device.

The multi-wavelength lens according to the present invention includes at least one of the multi-wavelength lenses for condensing a plurality of types of monochromatic light having wavelengths λi (i = 1, 2, 3, 4,...) By refraction. The common use area for all the monochromatic light of the lens surface is divided into a plurality of aspherical portions having different refractive powers, and the aspherical portions adjacent to each of the divided aspherical portions are adjacent to each other. The steps in the direction parallel to the lens optical axis are set to Dj (j = 1, 2, 3, 4,
)), The minimum value of the following Aij values for each wavelength λi is MIN (Aij) and the maximum value is MAX (
Aij) satisfies the following mathematical formula.
MAX (Aij) / MIN (Aij) <3
here,
Aij = Absolute value (Bij−mij)
Bij = (absolute value (Dj)) * (ni-1) / λi−C
ni: Refractive index of lens at wavelength λi
mij: integer closest to Bij C: correction term The correction term C in the preferred embodiment is calculated based on the NA at the wavelength λi of the adjacent portion.

Further preferably, the multi-wavelength lens according to the present invention has a wavelength λi (i = 1, 2, 3, 4,
..)) A plurality of aspherical surfaces having different refractive powers in common use regions for all the monochromatic light of at least one lens surface in a multi-wavelength lens for condensing the plurality of types of monochromatic light by refraction. Dj (j = 1, 2, 3, 4, Dj (j = 1, 2, 3, 4,) in the direction parallel to the lens optical axis at the adjacent portions of the adjacent aspherical portions.
...: In the order close to the optical axis of the lens), when NAi between the j-th adjacent portions when light of wavelength .lamda.i is incident is NAij, at least half of the steps in the adjacent portions. Of the steps, the following expression is satisfied when MIN (Aij) is the minimum value and MAX (Aij) is the maximum value of the following Aij for each wavelength λi. Furthermore, it is preferable to satisfy this equation for all adjacent steps.
MAX (Aij) / MIN (Aij) <3
Where Aij = absolute value (Bij−mij)
Bij = (absolute value (Dj)) * (ni-1) / λi− (NAij ² ) * K / λi
ni: Refractive index of lens at wavelength λi
mij: integer closest to Bij
K = 0.004 mm (when NAij <0.55)
K = 0.005mm (when NAij> = 0.55)
Furthermore, it is desirable to satisfy the following formula. Moreover, it is preferable that this formula is satisfied in all adjacent steps.
MAX (Aij) / MIN (Aij) <2

Another multi-wavelength lens according to the present invention is at least one of the multi-wavelength lenses for condensing a plurality of types of monochromatic light having wavelengths λi (i = 1, 2, 3, 4,...) By refraction. The common use area for all the monochromatic light in the lens surface is divided into a plurality of aspherical portions having different refractive powers, and each of the divided aspherical portions is minute in the adjacent portion between the aspherical portions. The range has a shape different from that of the aspheric surface, and the extended non-spherical portion at the adjacent portion in a direction parallel to the optical axis of the lens when the divided aspheric surface portions are extended to the adjacent portion. The step amount between the spherical surfaces is Dj (j = 1, 2, 3, 4,...: The order close to the lens optical axis) in order from the lens optical axis, and the j-th when the light of wavelength λi is incident. When the NA between adjacent parts is NAij, the level difference in the adjacent part Of these steps, the following equation is satisfied when the minimum value of the following Aij for each wavelength λi is MIN (Aij) and the maximum value is MAX (Aij).
MAX (Aij) / MIN (Aij) <3
Where Aij = absolute value (Bij−mij)
Bij = (absolute value (Dj)) * (ni-1) / λi− (NAij ² ) * K / λi
ni: Refractive index of lens at wavelength λi
mij: integer closest to Bij
K = 0.004 mm (when NAij <0.55)
K = 0.005mm (when NAij> = 0.55)

In any one of the above-described multi-wavelength lenses, it is desirable that the shape different from the aspherical surface in a minute range has a draft in the mold drawing direction. In any of the above-described multi-wavelength lenses, the shape different from the aspherical surface in the minute range is more preferably a minute R shape.
The above-described lens is used in an optical system, an optical head, and an optical disk device.

Further, in the lens having another positive power according to the present invention and forming a light spot on the focus surface on which the light from the light source is condensed, a use area is provided on the lens surface on one side or both sides of the lens. When the light from the light source is incident on the unused area, the light spot diameter obtained when the light from the light source is incident only on the used area is provided. Also, the light spot diameter is smaller when light from the light source is incident on both the use area and the non-use area.

In this lens, the lens is a multi-wavelength lens that condenses monochromatic light of a plurality of wavelengths, and the unused area for the first wavelength is used at a second wavelength different from the first wavelength. It is preferable that it is a region. Further, in this multi-wavelength lens, one or both surfaces of the lens surface are divided in a non-use region for the first wavelength and a use region for the second wavelength. In addition, it is desirable that the phase difference based on the wavefront aberration caused by the light having the second wavelength pass through each of the divided surfaces is approximately an integral multiple.

According to the present invention, with respect to two or more types of optical discs having different thicknesses of transparent substrates, all light beams can be obtained with an aperture (NA) necessary for recording or reproduction by refraction without using a diffractive lens structure. Therefore, the light can be condensed with as little aberration as possible, and the light utilization efficiency can be further increased. Further, the present invention provides a multi-wavelength optical system using a plurality of monochromatic lights, and each of the divided aspherical surfaces has a single focal point corresponding to a unique wavelength of each monochromatic light, and the unique characteristics of each monochromatic light. The focal points corresponding to the wavelengths can be arranged at different positions, and can also be used in an optical system in optical communication or the like.

Now, with respect to a first optical disk using a transparent substrate having a thickness t1, an objective lens in an optical disk apparatus using the first optical disk is satisfactorily corrected for aberrations, and a laser beam is collected on an information recording surface provided on the substrate. It shall shine. When a second optical disk using a transparent substrate having a thickness t2 different from that of the transparent substrate is used in such an optical disk apparatus, the thickness t2 of the transparent substrate is different from the thickness t1, so Spherical aberration is caused by the transparent substrate of t2, and the laser beam is not focused well on the information recording surface provided on the transparent substrate of thickness t2.

On the other hand, when laser beams having different wavelengths are used in an optical system composed of such an objective lens and a transparent substrate, chromatic aberration occurs. Here, chromatic aberration refers to a difference in spherical aberration generated corresponding to each laser beam when the objective lens is irradiated with laser beams having different wavelengths. For example, when the objective lens is irradiated with a laser beam with a wavelength of 655 nm and a laser beam with a wavelength of 790 nm, the chromatic aberration is the spherical aberration that occurs when the objective lens is irradiated with the laser beam with a wavelength of 655 nm, and the laser beam with a wavelength of 790 nm is the objective. This is the difference in spherical aberration that occurs when the lens is irradiated.

That is, the spherical aberration when the thickness is t1 is SA (t1), the spherical aberration when the thickness is t2 is SA (t2), and the spherical aberration generated with respect to the laser beam having the wavelength λ1 is SA (λ1).
), Where SA (λ2) is the spherical aberration generated for the laser beam having the wavelength λ2, the chromatic aberration due to the difference in wavelength is expressed by the difference of the spherical aberration (SA (λ2) −SA (λ1)). At this time, it is preferable to design the lens surface so that the following numerical formula is satisfied.
S _A (t ₂ ) −S _A (t ₁ ) = − (S _A (λ ₂ ) −S _A (λ ₁ ))

This means that for any optical disc having a different substrate thickness, when a laser beam having a wavelength corresponding to the thickness of the substrate is used, all the light beams that have passed through the objective lens and the substrate of the laser beam are transmitted. The optical path length is such that the light is well condensed on the information recording surface of the substrate. At this time, the lens according to the embodiment of the present invention has a lens surface divided into a plurality of aspheric surfaces, as will be described in detail in the following embodiments. It has a single focal point corresponding to the specific wavelength of light, and the focal points corresponding to the specific wavelength of each monochromatic light are designed to be arranged at different positions.

Now, with reference to FIG. 3, a case where a laser beam is focused on the information recording surface 2a of the substrate 2 using the objective lens 1 will be described. Here, the surface A of the objective lens 1 is a light incident side surface, the surface B is a light emission side surface, and the information recording surface 2a of the substrate 2 is on the side opposite to the objective lens 1 side.

In FIG. 3, the laser beam incident on the objective lens 1 is parallel light (the optical system shown in FIG. 3 is a so-called infinite optical system), and the distance from the optical axis OA of the objective lens 1 in the direction perpendicular thereto. (Light height) This diagram schematically shows an optical path until a light beam passing through the position P1 of h reaches a point (condensing point) P5 crossing the optical axis OA. Here, the incident point to the objective lens 1 in this optical path is P
2. The exit point from the objective lens 1 is P3, the incident point on the transparent substrate 2 is P4,
Point P1 to incident point P2: Spatial distance = S1h Refractive index = n1
Incident point P2 to outgoing point P3: Spatial distance = S2h Refractive index = n2
Emitting point P3 to incident point P4: Spatial distance = S3h Refractive index = n3
Incident point P4 to condensing point P5: Spatial distance = S4h Refractive index = n4
Then, the optical path length Lh from the point P1 to the condensing point P5 is
L _h = n ₁ × S _1h + n ₂ × S _2h + n ₃ × S _3h + n ₄ × S _4h
It is represented by The optical path length Lh on the optical axis OA is a case where h = 0 in this equation.

This equation is applicable to an arbitrary light ray height h, and when aberration correction is performed, the condensing point P5 for each light ray height h is on the information recording surface 2a within each allowable range. .
That is, for example, by using a laser beam having a different wavelength for each of a plurality of substrates having different thicknesses, the chromatic aberration and the spherical aberration cancel each other, and the condensing point P5 with respect to each ray height h is within the allowable range. And preferably on the information recording surface 2a.

For example, when a monochromatic light (λ1) of 790 nm in a CD and a monochromatic light (λ2) of a 655 nm in a DVD are used, a lens surface obtained by dividing a region in which both wavelengths are used in common into a plurality of aspherical portions In this method, the optical path length of an arbitrary aspherical portion is different from the optical path length of the other aspherical portion by approximately an integer multiple of the wavelength λi of each monochromatic light, and each monochromatic light in each aspherical portion is When the difference between the maximum value and the minimum value of the wavefront aberration is ΔVd (λ1) and ΔVd (λ2) (d is an integer of 1, 2,..., Each aspherical portion), any aspherical portion In this case, the ratio of the difference of each monochromatic light is 0.4 or more and 2.5 or less, preferably 0.5 or more and 2.0 or less, so that the RMS wavefront aberration within the allowable range as a whole lens is secured at both wavelengths. Can do. The wavefront aberration referred to here is represented by the following equation, where L0 is the optical path length when the ray height (h) is h = 0, and Lh is the optical path length at each ray height. The
Vh = (Lh−L ₀ ) / λi

FIG. 10 schematically shows the wavefront aberration of the lens at the wavelengths of CD and DVD, with the horizontal axis indicating the ray height, the vertical axis indicating the wavefront aberration, and the upper side of each aspherical portion of the CD. The lower side represents the wavefront aberration obtained by the above formula of each aspherical portion of the DVD. For example, the difference between the maximum value and the minimum value of the wavefront aberration in the aspheric surface in the first region of the aspheric surface is ΔV1 (λ1
), ΔV1 (λ2). In the embodiment of the present invention, the ratio of the difference between the maximum value and the minimum value of the wavefront aberration of each wavelength is 0.4 or more and 2. 5 or less. In other words, the embodiment of the present invention forms a lens surface based on one conventional wavelength and has a phase only at the other wavelength even in the point that each aspherical portion has a constant distribution of wavefront aberration at any wavelength. This is different from the method of correcting the wavefront aberration using the deviation. The integer multiple is preferably 0 times to ± 10 times, more preferably 0 times to ± 5 times, although it depends on the number of aspheric surfaces to be divided.

Further, in the multi-wavelength lens according to the embodiment of the present invention, the difference between the maximum value and the minimum value of the wavefront aberration at each wavelength is 0.14λi or less (for example, the wavelength is 79) in each region of any aspheric part.
When the wavelength is 0 nm, ± 110.6 nm or less, and when the wavelength is 655 nm, ± 91.
7 nm or less), preferably 0.12 λi or less, more preferably 0.10 λi or less, so that even better optical characteristics can be secured at each wavelength.

Furthermore, in the embodiment of the present invention, in the case of a two-wavelength optical system, by using a multi-wavelength lens in which the wavefront aberrations of the respective wavelengths are almost symmetrical, the two wavelengths can be balanced, and further the RMS wavefront Aberration can be reduced.

In consideration of reducing the RMS wavefront aberration, the ray height 1 in FIG.
. The RMS wavefront aberration is determined from the wavefront aberration of only the common use area of DVD and CD up to 58 mm. In the case of DVD, the DVD dedicated area (light height 1.58 in FIG. 10) is outside the common use area. The RMS (Root Mean Square) wavefront aberration value can be obtained from the wavefront aberration of both the common use area and the dedicated area. Therefore, in the case of a DVD, even if the wavefront aberration in the common use area is somewhat worse, the wavefront aberration in the DVD dedicated area can be improved by ignoring the CD at all and improving only the DVD. Can be sufficiently reduced within an allowable value. For example, in the schematic diagram of FIG. 10, the wavefront aberration of DVD is 0 to −0.106λ and the wavefront aberration of CD is 0 to +0.088 in the common use region of DVD and CD. Is smaller than the wavefront aberration of DVD. The wavefront aberration in the DVD-dedicated region is -0.052λ. As a result, the RMS wavefront aberration is 0.0212λRMS for DVD, 0.0222λRMS for CD, and RMS wavefront aberration is D.
VD and CD are almost equal values. As described above, when it is desired to set the same value for both the DVD and the CD as the RMS wavefront aberration, the CD wavefront aberration is made better than the DVD wavefront aberration in the common use region of the DVD and CD. For the wavefront aberration, it is effective to compensate for the deterioration in the common use area in the DVD dedicated area. DV
Similarly, when it is desired to change the ratio of the RMS wavefront aberration between D and CD, it may be considered that the DVD can be compensated for in the dedicated area even if the wavefront aberration in the common use area is somewhat worse.

The present invention is also effective in the case of optical discs having different substrate thicknesses, such as AOD (wavelength 405 nm, substrate thickness 0.6 mm) and DVD (wavelength 655 nm, substrate thickness 0.6 mm).

According to an embodiment of the present invention, for example, for any optical disc having a different substrate thickness,
A good light spot can be formed on the information recording surface. This can be applied even if the thickness of the disk substrate is not different, that is, even when the thickness is the same and the wavelength is different, by setting the condensing point P5 within the permissible range. Further, the present invention is not limited to an optical recording medium, and can also be applied to cases where laser beams having different wavelengths are passed through the same lens or optical system in optical communication or the like.

Hereinafter, embodiments of the present invention will be described with reference to two types of optical discs having different thicknesses of transparent substrates, namely DV
An example of D and CD will be described with reference to the drawings. In addition, the lens of 1st Embodiment of this invention produced the resin which consists of amorphous polyolefin by injection molding from the ease of manufacture. The lens according to the second embodiment has a refractive index of glass. However, when the lens material is a plastic resin, the lens may be designed with the refractive index of the plastic resin.

FIG. 1 is a diagram showing the operation of the first embodiment of the objective lens according to the present invention.
Is for a DVD, and FIG. 5B is for a CD. In the figure, 1 is the objective lens of this embodiment, 2 is a DVD transparent substrate (hereinafter referred to as a DVD substrate), 3 is a CD transparent substrate (hereinafter referred to as a CD substrate), and 4 and 5 are laser beams.

First, in FIG. 1A, an objective lens 1 is provided in an optical head of an optical disk device (not shown). Then, the DVD is mounted on the optical disk apparatus, and the laser beam 4 incident as parallel light is condensed by the objective lens 1 to perform recording / reproduction. Here, the thickness t1 of the DVD substrate 2 is 0.6 mm. As the laser beam 4 at this time, a laser beam having a wavelength λ1 = 655 nm is used as a light flux having a numerical aperture NA = 0.63. Under such conditions, the laser beam is focused on the information recording surface 2a on the opposite side of the DVD substrate 2 from the objective lens 1 side.

FIG. 1B shows a case where a CD is mounted on the same optical disk apparatus as described above, and recording / reproduction is performed using the same objective lens 1. Here, the thickness t2 of the CD substrate 3 is 1.2 mm. As the laser beam 5 at this time, a laser beam having a wavelength of λ2 = 790 nm is approximately numerical aperture NA = 0.
The light beam having the numerical aperture NA = 0.47 is substantially condensed on the information recording surface 3a of the CD substrate 3 and hatched, and is substantially the light of the objective lens 1 having substantially NA = 0.47 to 0.63. Axis O
The light beam passing through the part away from A is not condensed on this information recording surface 3a. As described above, the lens area having a numerical aperture NA up to about 0.47 is a common use area for DVD and CD.

As described above, in the first embodiment, aberrations are reduced well for both DVD and CD, and a good light spot is obtained on the information recording surfaces 2a and 3a. For both DVD and CD, the lens surface shape of the objective lens 1 is set so that the optical path length Lh is within a permissible value by reducing the aberration with respect to an arbitrary beam height h. It is. Hereinafter, a specific example of the lens surface shape will be described with reference to FIG.

In FIG. 2, regarding the light exit side surface B of the objective lens 1, a point of the light beam height h is c, and this point c
If the point on the light emission side surface B in the direction parallel to the optical axis OA is d, the surface shape of the light emission side surface _{B is determined} by the distance Z _B between the points c and d with respect to an arbitrary light beam height h.

However, C = -0.12301, K = 3.312138, A4 = 0.01628151, A6 = -0.004311717, A8 = 0.000682316,
A10 = −0.00004157469

In equation 1, when the values of the coefficients C, K, A4, A6, A8, and A10 are substituted to determine the distance Z _B for an arbitrary ray height h (≠ 0), the value is a negative value. However, this is because the point d on the light emission side surface B is located at the point c, and is therefore located on the emission surface side (left side in FIG. 2) from the surface vertex e through which the optical axis OA of the light emission side surface B passes. Show. If the distance Z _B is positive,
It is located on the opposite right side.

Next, with respect to the light incident side A of the objective lens 1, the point of the light beam height h is a, and the point on the light incident side A surface in a direction parallel to the optical axis OA from this point a is b. As for the surface shape of the side surface A, the ray height h (mm) and the distance Z _A (mm) between the points a and b with respect to the ray height h are shown in Table 1 below.
The lens surface shape is set to the relationship shown in FIG.

The light emitting side surface B represented by the above-described expression 1 of the objective lens 1 and the light incident side surface A represented by the point sequence data in Table 1 also form a continuous aspherical surface. The distance between the surface vertices f and e on the optical axis of the objective lens 1, that is, the center thickness t0 is 2.2 mm, and the wavelength λ1 = 655 nm.
(DVD) has a refractive index n of 1.54014 and a wavelength λ2 = 790 nm (CD) has a refractive index n of 1.53.
65.

(I) Here, as an allowable value of the aberration for evaluating the aberration, the incident laser beam to the objective lens 1 has an incident angle of 0 ° (that is, parallel light parallel to the optical axis OA). ,
Both DVD (wavelength λ1 = 655 nm) and CD (wavelength λ2 = 790 nm) have RMS wavefront aberrations of 0.03
5λ, preferably 0.033λ, and more preferably 0.030λ. In the first embodiment, the light exit surface B and the light entrance surface A are set so that the wavefront aberration of DVD and CD is less than the allowable value.
Is set to the above-mentioned surface shape.

In the first embodiment, a case where two different wavelengths λ1 and λ2 are used is shown.
In general, n types (where n is an integer of 2 or more) of different wavelengths λi (where i = 1, 2,...).
The same applies when using., N).

(ii) Further, in the case where n types of wavelengths λi are used in this way, if each RMS wavefront aberration when the incident laser beam of these wavelengths λi has an incident angle of 0 ° is Wi · λi, these aberrations are ,

(However, the wavelength of the i-th light beam is λi (i = 1, 2,...), The sum of squares of individual RMS wavefront aberrations over all wavelengths is ΣWi2, and the light beam of wavelength λi is RMS wavefront aberration Wi
• Let λi be satisfied. The allowable value W0 at this time is 0.028, preferably 0.026, more preferably 0.025, and further preferably 0.023. In the first embodiment, the RMS wavefront aberration of DVD is W1, the RMS wavefront aberration of CD is W2, and i = 1, 2
Therefore, the above number 2 is

となる。

It becomes.

(iii) Alternatively, when using laser beams of different n types of wavelengths λi, the maximum RMS wavefront aberration of each wavelength λi is Wmax, and the minimum RMS wavefront aberration is Wmin.
1 <Wmax / Wmin <Wth
And In this case, the allowable value Wth is 1.8, preferably 1.6, and more preferably 1.4. In the case of the first embodiment, either the RMS wavefront aberration W1 of DVD or the RMS wavefront aberration W2 of CD is the maximum RMS wavefront aberration Wmax, and the other is the minimum RMS wavefront aberration Wmin.

FIG. 4 shows the calculation result of the RMS wavefront aberration in the first embodiment. The horizontal axis represents the image height (mm), and the vertical axis represents the RMS wavefront aberration.

FIG. 4A shows the RMS wavefront aberration with respect to the DVD (wavelength λ1 = 655 nm). When the image height = 0 mm, the RMS wavefront aberration = 0.02130λ1. FIG. 4B shows the RMS wavefront aberration with respect to CD (wavelength λ2 = 790 nm). When the image height = 0 mm, RM
S wavefront aberration = 0.02410λ2.

In order to evaluate such a numerical value, if inserted into the above conditional expressions,
(I) First, for DVD and CD, the RMS wavefront aberration is 0.02130λ, 0.02410λ, and the allowable value of 0.035λ, preferably 0.033λ, and more preferably smaller than 0.030λ.

  (Ii) For DVDs and CDs,

Therefore, the allowable value is 0.028, preferably 0.026, more preferably 0.025, and further preferably 0.023 or less.

(iii) For DVD and CD, looking at Wmax / Wmin,
Wmax / Wmin = 0.02410 / 0.02130 = 1.1315
Therefore, the allowable value is 1.8, preferably 1.6, and more preferably 1.4 or less.

FIG. 5 shows a light spot on the information recording surface of a DVD or CD by using the objective lens 1 having the light emitting side surface B having the surface shape shown in Equation 1 and the light incident side surface A having the surface shape shown in Table 1 above. The horizontal axis represents the position in the direction perpendicular to the optical axis with respect to the optical axis on the information recording surface as a reference point, and the vertical axis represents this reference point. The light intensity at (= 0mm) is 1
Represents the relative light intensity at each position.

FIG. 5A shows a light spot with respect to a DVD, and the relative light intensity is 1 / e ² (
= 13.5%), the light spot diameter ΦD is 0.85 μm. FIG. 5B shows a light spot with respect to the CD, and the light spot diameter ΦC at which the relative light intensity is 1 / e 2 is 1.37.
μm. Thus, a good light spot can be obtained on the information recording surface for both DVD and CD.

Next, a second embodiment of the objective lens according to the present invention will be described.
The basic configuration of the second embodiment is the same as that of the first embodiment described above, but the light incident surface A is divided into a plurality of sections in the radial direction from the optical axis, and the surface shape of each section is obtained. , DVD, CD
Both are set so that the aberration is satisfactorily reduced within an allowable value.

The surface shape of the light incident surface A of the second embodiment will be described with reference to FIG. Now, the distance between the points a and b in the j-th section from the optical axis OA side in the light beam height h direction (radial direction) of the light incident surface A is expressed by the following function ZAj:

It is represented by Note that the light source height h in Equation 5 is for the j-th section.

For both DVD and CD, the range (the range of h) and its constants B, C, K, A4, A6, A8, A10 for each section in Equation 5 for satisfactorily reducing the aberration within the allowable value. , A12, A14, and A16 are as shown in Table 2 below.

Further, the surface shape Z _B of the light emitting surface B in the second embodiment is expressed by the following equation (6).

However, C = -0.0747792, K = 15.7398, A4 = 0.012308, A6 = -0.0037652, A8 = 0.00068571, A1
0 = -0.000048284

Further, the distance between the surface vertices f and e on the optical axis of the objective lens 1, that is, the center thickness t0 is 2.2 mm, the refractive index n at the wavelength λ1 = 655 nm (DVD) is 1.604194, and the wavelength λ2 = 790nm
The refractive index n at (CD) is 1.599906.

The thickness and refractive index of the transparent substrate are 0.6 mm in thickness at a wavelength λ1 = 655 nm (DVD).
The refractive index is 1.57995 at mm, and the thickness is 1.2 mm at the wavelength λ1 = 790 nm (CD).
The refractive index is 1.573071. Also, NA for DVD is 0.60, NA for CD
Is 0.47. The focal length for DVD is 3.360 mm, and the focal length for CD is 3.
. 383 mm. As for the diaphragm of the incident parallel light beam, only the diaphragm for DVD (not shown), the diaphragm diameter = 2 × NA × focal length, and the value is φ4.032.

Even at the time of CD, the aperture diameter remains φ4.032. Accordingly, since the use area on the light incident surface in the CD is the first to sixth sections which are the common use areas of DVD / CD, φ3.178732
However, the light of φ4.032 is incident. Therefore, φ 3.178
Even though the region of 732 to φ4.032 is a DVD-dedicated region, light is also incident upon CD, so it is necessary to prevent this portion of light from acting as harmful light at the time of CD. It is clear from the light spot diagram shown in FIG. 7B that it does not act harmfully.

Further, when paying attention to the curvature (= the reciprocal of the radius of curvature), that is, C in Table 2, DV
In the 1st to 6th section, which is the common use area of D and CD, when looking at the relationship between large and small, the first section

Here, the allowable value of the aberration for evaluating the aberration is the same as that in the first embodiment.

FIG. 6 shows the calculation result of the RMS wavefront aberration in the second embodiment.
The vertical axis is the same as in FIG.

FIG. 6A shows the RMS wavefront aberration for a DVD (wavelength λ1 = 655 nm). When the image height is 0 mm, the RMS wavefront aberration is 0.01945λ1. FIG. 6B shows the RMS wavefront aberration with respect to CD (wavelength λ2 = 790 nm). When the image height = 0 mm, RM
S wavefront aberration = 0.02525λ2.

FIG. 11 shows the result of calculating the wavefront aberration in the common use region of the above-mentioned lens, and Table 3 shows the difference in wavefront aberration and the ratio thereof in each aspheric surface.

As shown in Table 3, the ratio ΔVd (λ790) / ΔVd (λ655) of the difference between the wavefront aberrations in the common use region of 790 nm and 655 nm is between 1.00 and 1.04. Also, the ratio Δ
Vd (λ655) / ΔVd (λ790) is between 0.96 and 1.00. The wavefront aberration of each region is also 0.14λ or less at both wavelengths. In this lens, the wavefront aberration appears on the + side at a wavelength of 790 nm and on the − side at a wavelength of 655 nm, and both wavefront aberrations are substantially symmetrical.

Note that there is a difference in optical path length between adjacent aspherical parts divided around the optical axis, and the difference is designed to be an integer multiple corresponding to each wavelength. In the embodiment, it is composed of an even number of divided aspheric surfaces.

In order to evaluate such numerical values, as in the first embodiment, when inserted into the above conditional expressions,
(I) First, for DVD and CD, the RMS wavefront aberration is 0.01945λ1, 0.02525λ2 and the above allowable value 0.035λ, preferably 0.033λ, and more preferably smaller than 0.030λ.

  (Ii) For DVD and CD,

Therefore, the allowable value is 0.028, preferably 0.026, more preferably 0.025, and further preferably 0.023 or less.

(iii) For DVD and CD, looking at Wmax / Wmin,
Wmax / Wmin = 0.02525 / 0.01945 = 1.298
Therefore, the allowable value is 1.8, preferably 1.6, and more preferably 1.4 or less.

FIG. 7 shows an information recording surface of a DVD or CD obtained by using the objective lens 1 having the light emitting side surface B having the surface shape shown in the above equation 6 and the light incident side surface A having the surface shape shown in the above equation 5 and Table 2. The horizontal axis and the vertical axis are the same as those in FIG. 5.

FIG. 7A shows a light spot with respect to a DVD, and the relative light intensity is 1 / e ² (
= 13.5%), the light spot diameter ΦD is 0.89 μm. FIG. 7B shows a light spot with respect to the CD, and the light spot diameter ΦC at which the relative light intensity is 1 / e ² is 1.30.
μm. At this time, the light of the same diaphragm is incident on both the CD and the DVD, and the diaphragm is set in a range that is incident on both the DVD / CD common use area and the DVD dedicated area of the lens. Thus, a good light spot can be obtained on the information recording surface for both DVD and CD. This light spot is compared with the case of NA of 0.47 with the same NA for a CD lens, which is almost an ideal lens with a wavefront aberration of 0.0000 λrms. A light spot diagram on the CD lens is shown in FIG. The relative light intensity in FIG. 22 is 1 / e ² (= 13.5%)
The light spot diameter is 1.3804 μm. That is, the CD light spot in the second embodiment is smaller than that of a normal CD lens close to an ideal lens, although the NA is the same. That is, when considering the second embodiment focusing on the CD, not only the original CD area, the first to sixth sections, but also the DVD dedicated areas of the seventh and eighth sections having the phase difference of the wavefront aberration. It is considered that a smaller CD light spot can be realized by adding. The phase difference of the wavefront aberration described above is the DVD65 in the seventh section and the eighth section of the DVD dedicated area.
This means that it has a wavefront aberration of 2λ1 at a wavelength of 5 nm. In order to prove this, for the second embodiment, the seventh and eighth sections were shielded, that is, the aperture diameter was set to Φ3.178, and the CD light spot only for the first to sixth sections was calculated. . The result is shown in FIG.

The light spot diameter of the relative light intensity of 1 / e ^{2 in} FIG. 23 is 1.3924 μm, which is about 0.01 μm larger than the CD lens shown in FIG. It is about 0.09 μm larger than CD. In other words, the use region for the other wavelength λ1 that transmits light is provided outside the lens for the wavelength λ2, and preferably the other wavelength λ1 is provided in the outer region.
By providing the phase difference of the wavefront aberration at, the light spot diameter of the wavelength λ2 is made smaller than in the case where the outer region is not provided.

In the second embodiment, the ratio in Table 3 is 0.96 to 1.04,
As the RMS wavefront aberration, DVD is 0.01945λ1 and CD is 0.02525λ2, but as described in the explanation of FIG. 10, if the wavefront aberration of DVD in the common use region is further deteriorated to improve the CD wavefront aberration, , DVD, and CD both have RMS wavefront aberrations of 0.022 to 0.023
An equivalent RMS wavefront aberration of about λ may be used.

As an example, the R of DVD and CD described in Japanese Patent Laid-Open No. 2001-51192 is described.
Looking at MS wavefront aberration,
Example 1) DVD: 0.001λ1 CD: 0.047λ2
Example 2) DVD: 0.019λ1 CD: 0.037λ2
However, λ1 = 640nm λ2 = 780nm
In any case, for both CDs, the above tolerance value of 0.03 is given.
It exceeds 5λ.

Further, when the wavefront aberration at each wavelength of the lens of Example 2) is obtained by calculation using the lens data described in the publication, the ratio is 0.03 to 33.44 as shown in Table 4 and FIG.
And out of the scope of the present invention, so the balance between them is off. Further, the wavefront aberration on the DVD side is 0.14λ or less, but the wavefront aberration on the CD side becomes large, and the RMS wavefront aberration of the entire lens also becomes large.

Is 0.0332, 0.0294 for each of Example 1 and Example 2 above, both of which are 0.02 and 0.02.
Exceeding all values of 8, 0.026, 0.025, and 0.023, these Wmax / Wmin are also shown in Example 1.
, 47 for each of Example 2 and 1.847, both of which exceed all the allowable values 1.8, 1.6, and 1.4.

As described above, in both the first and second embodiments, the aberration can be suppressed within the allowable value. However, this is because the difference in the substrate thickness is such that the aberration is within the allowable value. This is due to the lens surface shape in which spherical aberration and chromatic aberration cancel each other. On the other hand, in Japanese Patent Laid-Open No. 2001-51192, the CD aberration is reduced by simply shifting the phase of the incident laser beam by an integral multiple of the wavelength of the DVD laser beam. Even if the aberration can be suppressed to a sufficiently small value for one wavelength, the aberration cannot be simultaneously accommodated within the allowable value of the small value as described above for all wavelengths.

In the above embodiment, the spherical aberration due to the difference in substrate thickness between 0.6 mm and 1.2 mm for DVD and CD is canceled by chromatic aberration due to the difference in wavelength between 655 nm and 790 nm, and the total aberration is reduced. This is apparent from the light spot shown in FIGS. 5 and 7 and the wavefront aberration graphs shown in FIGS. In the above embodiment, the surface shape of the light incident side surface A of the objective lens 1 is given by the point sequence data shown in Table 1 above, Equation 5 and Table 2, and the surface shape of the light exit side surface B is shown in Equation 1 above. Since it is given by the aspherical expression shown in Equation 6, the diffractive lens structure as in the prior art is not used, and almost all the light flux is collected with respect to the aperture (NA) necessary for recording or reproduction. Since light can be emitted, high light utilization efficiency can be obtained.

In the above embodiment, as shown in FIG. 1, the numerical aperture NA = 0.47 to the numerical aperture NA.
= 0.63 or NA = 0.60, the outer area of the objective lens 1 is used only for DVD, C
Since it is not used in D, light having a wavelength of 655 nm is transmitted to one or both of the light incident side surface A and the light emitting side surface B in the outer region, and light having a wavelength of 790 nm is not transmitted to a CD. A diffraction grating that is thin-film treated, or does not act on light having a wavelength of 655 nm, but acts on light having a wavelength of 790 nm, either or both of the light incident side A and the light exit side B in the outer region. Forming two wavelengths 790nm without reducing the light utilization efficiency of wavelength 655nm
The light use efficiency may be reduced.

In other words, as in the above-described embodiment, when the diaphragm corresponding to the numerical aperture cannot be set when sharing the system with different numerical apertures, an extra light beam can be received in an optical system having a small numerical aperture. Therefore, it is desirable to take care that the light passing through the outer region of the lens designed to match the optical system with a large numerical aperture does not adversely affect the optical system with a small numerical aperture. For example, it is desirable that the amount of lateral aberration be 0.015 mm or more so that light that has passed through the outer region of the lens is not collected on the disk surface.

In the above embodiment, two types of optical disks, DVD and CD, are taken as an example. However, the present invention is not limited to this, and other types of optical disks may be used. It is also applicable to three or more types of optical discs with different thicknesses, and the lens surface shape is set so that chromatic aberration cancels wavefront aberration according to the wavelength of the laser beam used for each. do it. Further, even when the substrate thickness is the same, the wavelength used is different, so that a large aberration occurs in the conventional normal lens, the aberration can be reduced by applying the present invention.

FIG. 8 is a block diagram showing an embodiment of an optical head using an objective lens according to the present invention.
11 is a DVD laser, 12 is a CD laser, 13 and 14 are half prisms, 15 is a collimator lens, 16 is a detection lens, 17 is a photodetector, 18 is a diffraction grating, and 19 is an actuator, corresponding to FIG. Parts are given the same reference numerals.

In the figure, when the DVD disk 2 is recorded or reproduced, the DVD laser 11 is driven. A laser beam having a wavelength of 655 nm generated from the DVD laser 11 is reflected by the half prism 13, passes through the half prism 14, and enters the collimator lens 15. The laser beam that has passed through the collimator lens 15 and becomes parallel light is incident on the objective lens 1 and collected, and forms a light spot on the information recording surface of the DVD disk 2. Then, the reflected light reflected by the DVD disk 2 becomes parallel light by the objective lens 1 and enters the collimator lens 15. The collimator lens 15 converts the parallel light into convergent light, and the convergent light passes through the half prisms 14 and 13 and reaches the photodetector 17 through the detection lens 16. The detection output signal of the photodetector 17 is supplied to a signal processing circuit (not shown), and an information recording / reproducing signal, a focus error signal, and a tracking error signal are obtained. A system control circuit (not shown) controls an actuator drive circuit (not shown) so that the objective lens 1 is positioned at an appropriate focus position and tracking position based on the obtained focus error signal and tracking error signal. Then, the actuator 19 is driven.

When recording or reproducing the CD disk 3, the CD laser 12 is driven. A laser beam having a wavelength of 790 nm generated from the CD laser 12 passes through the diffraction grating 18, is reflected by the half prism 14, and enters the collimator lens 15. The laser beam that has passed through the collimator lens 15 and becomes parallel light is incident on the objective lens 1 and condensed to form a light spot on the information recording surface of the CD disk 3. Then, the reflected light reflected by the CD disk 3 becomes parallel light by the objective lens 1 and enters the collimator lens 15. The collimator lens 15 converts the parallel light into convergent light, and the convergent light is transmitted through the half prisms 14 and 13 to detect the detection lens 1.
6 to reach the photodetector 17. The detection output signal of the photodetector 17 is supplied to a signal processing circuit (not shown), and an information recording / reproducing signal, a focus error signal, and a tracking error signal are obtained.

Note that the tracking error signal in the case of the CD disk 3 is that the laser beam from the CD laser 12 is split into three beams of zero-order light and ± first-order light by the diffraction grating 18, and the tracking error is caused by these ± first-order light. I try to get a signal.

By using the tracking error signal and the focus error signal thus obtained, the DVD
Similarly to the disk 2, the actuator 19 is driven so that the objective lens 1 is positioned at an appropriate focus position and tracking position.

In the present invention, instead of the objective lens 1, an optical system common to both disks such as the collimator lens 15 or the half prism 14 can be optically designed to have the same function as the objective lens in the present invention. Although not shown, another optical element having a function equivalent to that of the objective lens of the present invention may be disposed in the optical path from the half prism 14 to the disk 2 or the disk 3.

The collimator lens 15 is not always necessary, and even a so-called finite optical system can be used.
The present invention is applicable.

FIG. 9 is a block diagram showing an embodiment of an optical disk apparatus using an objective lens according to the present invention, in which 20 is an actuator drive circuit, 21 is a signal processing circuit, 22 is a laser drive circuit, 2
3 is a system control circuit, and 24 is a disk discriminating means. The parts corresponding to those in FIG.

  In the figure, the optical pickup device is the same as the configuration shown in FIG.

First, the disc discriminating unit 24 discriminates the type of disc loaded. As the disc discrimination method, a method of detecting the thickness of the substrate of the disc by an optical or mechanical method, a method of detecting an identification mark recorded in advance on the disc or the cartridge of the disc, and the like can be considered. Alternatively, a method may be used in which a disc signal is reproduced assuming a disc thickness and type, and if a normal signal cannot be obtained, it is determined that the disc has a different thickness and type. The disc discrimination result is transmitted from the disc discrimination means 24 to the system control circuit 23.

When the disc is determined to be a DVD disc, a signal for turning on the DVD laser is transmitted from the system control circuit 23 to the laser drive circuit 22, and the DVD laser 11 is turned on by the laser drive circuit 22. Thereby, in the optical head, the laser beam having the two wavelengths of 655 nm reaches the photodetector 17 as in the embodiment shown in FIG. The detection signal from the photodetector 17 is sent to the signal processing circuit 21 to generate an information recording / reproducing signal, a focus error signal, and a tracking error signal, and sent to the system control circuit 23. System control circuit 2
3, the actuator drive circuit 20 is controlled based on the focus error signal and the tracking error signal. Based on this control, the actuator drive circuit 20 drives the actuator 19 to move the objective lens 1 in the focus direction and the tracking direction. Move,
It is assumed that the focus control and the tracking control are normally performed by the operation of the so-called servo circuit, and the above-described circuits and actuators 19 are operated so that the objective lens 1 is positioned at a correct position with respect to the DVD disk 2. As a result, an information recording / reproducing signal can be obtained satisfactorily.

When it is determined that the loaded disk is the CD disk 3, a signal for turning on the CD laser 12 is transmitted from the system control circuit 23 to the laser driving circuit 22. As a result, a laser beam having a wavelength of 790 nm is generated from the CD laser 12. The subsequent operation is the same as that in the case of the optical head in FIG. 8. This laser beam reaches the photodetector 17, and each circuit and the actuator 19 are operated and the servo is operated as in the case of the DVD disk 2 described above. The operation is performed, and an information recording / reproducing signal can be obtained satisfactorily.

Regarding the second embodiment, the following points could be found. That is, as the lens surface shape for obtaining the above-mentioned wavefront aberration, a plurality of types of monochromatic light having wavelengths λi (i = 1, 2, 3, 4,. The common use region for all the monochromatic light of at least one lens surface of the lens is divided into a plurality of aspherical portions having different refractive powers, and the aspherical surfaces adjacent to each of the divided aspherical portions are respectively The step amount in the direction parallel to the lens optical axis in the adjacent part between the parts is set to Dj (j = 1, 2,
3, 4,...: The order close to the lens optical axis), and NAij is the numerical aperture NA between the j-th adjacent portions when light of wavelength λi is incident. The minimum value of the following Aij values for each wavelength λi is MIN (Aij)
When the maximum is MAX (Aij), the following equation (1) should be satisfied.
MAX (Aij) / MIN (Aij) <3 (1)
here,
Aij = Absolute value (Bij−mij)
Bij = (absolute value (Dj)) * (ni-1) / λi− (NAij ² ) * K / λi
ni: Refractive index of lens at wavelength λi
mij: integer closest to Bij
K = 0.004 mm (when NAij <0.55)
K = 0.005mm (when NAij> = 0.55)
Alternatively, Aij in the above formula (1) is preferably 0.15 or less. Also, MAX (
It is desirable that Aij) / MIN (Aij) <2.5, and MAX (Aij) / MIN (Aij) <2
(Hereinafter referred to as formula (2)) is preferable.

In the above equation (1), the absolute value (Dj) * (ni-1) indicates the amount of deviation of the optical path length due to the adjacent step amount Dj. K * (NAij ² ) is a correction term for the optical path length, and in order to reduce spherical aberration, 0.0004 × NA ² (mm) in a region where NA is less than 0.55, and NA is 0.55 or more. In the region, it is necessary to correct the optical path length by 0.0005 × NA ² (mm) at the adjacent step portion. Aij indicates how much the value of Bij deviates from the nearest integer. Each A
As the value of ij decreases, the wavefront aberration value tends to decrease, and it is preferable to decrease the value of Aij in a balanced manner with respect to each wavelength λi. In addition, the absolute value of at least half of the step amount Dj should increase as the distance from the lens optical axis increases.

  Specific numerical values in the second embodiment are shown in Table 5.

FIG. 14 shows a structure in the case where adjacent portions of the respective aspheric surfaces in the second embodiment have step amounts D1 to D7. In the second embodiment, sections 1 to 6 in Table 2 are DVD / CD common use areas, and sections 7 to 8 are DVD dedicated use areas. Therefore, since the calculation targets in the equations (1) and (2) are only for the steps in the DVD / CD common use area, D1 to D5
Up to is the calculation target, and D6 and D7 are outside the calculation target of the expressions (1) and (2). Regarding the step amounts D1 to D7, a positive sign is used when the aspheric surface near the lens optical axis in the step portion is on the left side of the drawing, and a negative sign is used when the aspheric surface is on the right side. That is, this 2nd Embodiment, FIG. 14, Table 5
In this case, D1, D2, and D3 are plus sign step amounts, and D4, D5, D6, and D7 are minus sign step amounts. Further, the step D of all the DVD / CD common use area in the step amount Dj.
From 1 to D5, the absolute value increases as the distance from the lens optical axis increases. Although the shape of the stepped portion of the second embodiment is shown in FIG. 14, when the lens is actually manufactured, it is considered that the lens is made by injection molding when it is made of plastic, and is made by glass molding when it is made of glass. It is done. In either case, a mold is produced and its shape is transferred to obtain a product. The shape in consideration of the mold production and molding is shown in FIGS.

FIG. 15 is an enlarged view of the portion of the step amount D1 in FIG. 14. The shape shown in FIG. 14 is a shape in which the taper taper θ degree and the left corner are R (shown by a solid line). The above-mentioned R takes into account the bite R at the time of mold manufacture, and the draft taper θ degree is easy to mold during injection molding or glass molding. In this case, how to define the step amount D1 between the aspheric surface in the first section and the aspheric surface in the second section in this adjacent portion is indicated by a dotted line in the figure. Extending the aspheric surface of the first section and the aspheric surface of the second section to a predetermined radius, the distance between the first section aspheric surface and the second section aspheric surface in the direction parallel to the lens optical axis at the predetermined radius is a step. The amount is D1. When the predetermined radius is unknown, a portion having an arbitrary diameter from ΦE1 to ΦE2 shown in the figure may be set as the predetermined radius.

In FIG. 15, the portion of the dimension E3 becomes an ineffective region that does not contribute to optical imaging, but even the one shown in FIG. 15 is about 0.0005 to 0.001 mm in the effective region Φ of the DVD.
Compared to 4.032 mm and the effective area Φ3.179 mm of CD, this is a minute amount and is not a problem level. In order to further reduce the ineffective area, it is also effective to reduce the angle of the taper taper or to make the bite R small or zero. On the contrary, it is also effective to make the invalid area larger than that shown in FIG.

FIG. 16 is an enlarged view of the portion of the step amount D4 in FIG. 14. The shape shown in FIG. 14 has a draft taper θ degree and a shape with an R on the left side (shown by a solid line). Hereinafter, the definition of the step amount D4, the punch taper, and the cutting tool R are the same as those described with reference to FIG.

Although it has been described that the wavefront aberration shown in FIG. 11 can be obtained in the second embodiment, there may be a case where it is better to reduce the wavefront aberration in a certain part. For example, CD in the first section
In the case where it is desired to further reduce the wavefront aberration than that shown in FIG. 11, for example, the third embodiment shown in Table 6 below may be adopted.

In the third embodiment, the value of A4 in the first section is changed from the second embodiment shown in Table 2. FIG. 17 shows the wavefront aberration diagram of the third embodiment, and Table 7 shows the calculation results for the equations (1) and (2).
Shown in

The absolute values of the steps D1 to D5 in the DVD / CD common use area increase with increasing distance from the lens optical axis at steps D2, D3, D4, and D5. The absolute value of the level difference of D1 is larger than that of D2, although it is closer to the optical axis. For this reason, the DVD wavefront aberration in the first section is increased, but the RMS wavefront aberration as a whole is 0.0.
Values below 35λRMS can be kept.

FIG. 17 shows that the CD wavefront aberration in the first section is reduced. In the RMS wavefront aberration, in the second embodiment, the CD wavefront aberration in the first section takes a value from 0 to 0.095λ in the second embodiment of FIG. In three embodiments,
It can be seen that the value is improved from 0 to 0.03λ. However, the wavefront aberration of the DVD in the first section was 0 to −0.1λ in the second embodiment of FIG. 11, but deteriorated to 0 to −0.2λ in the third embodiment of FIG. It is the value. As the RMS wavefront aberration value,
DVD CD
Second Embodiment 0.01945λrms 0.02525λrms
Third Embodiment 0.02495λrms 0.02574λrms
Although the third embodiment is deteriorated with DVD, the value of 0.025λ RMS or less can still be kept. Regarding the values of the expressions (1) and (2) shown in Table 7, the value of MAX (Aij) / MIN (Aij) is 18.13 in the third embodiment in the portion corresponding to the step amount D1.
Greater than 24 and 3, and greater than 2. In the part corresponding to D2, D3, D4, D5, the value is less than 2, and for MAX (Aij) / MIN (Aij), 4 out of 5 satisfy less than 2, and 1 is unsatisfactory. . If you satisfy 4 out of 5 DVDs,
Both CDs satisfy RMS wavefront aberrations of 0.025λ RMS or less, but this is an example in which the deterioration of wavefront aberrations in DVD is recognized as compared with the case where all five are satisfied. In the third embodiment, the example of reducing the CD wavefront aberration in the first section has been described, but it is also possible to reduce the DVD wavefront aberration in the first section in the same manner. The side will deteriorate.

Furthermore, when it is desired to reduce the CD wavefront aberration in the second section, the fourth embodiment shown in Table 8 is preferably used.

In the fourth embodiment, the value of A16 in the second section is changed from the third embodiment shown in Table 6. FIG. 18 shows the wavefront aberration diagram of the fourth embodiment, and Table 9 shows the calculation results for the equations (1) and (2).

The absolute values of the steps D1 to D5 in the DVD / CD common use area increase with increasing distance from the lens optical axis at the steps D3, D4, and D5. The absolute value of the step between D1 and D2 is larger than D3 although it is closer to the optical axis. To that end, first,
Although the DVD wavefront aberration is increased in the second section, the RMS wavefront aberration as a whole can be kept at a value of 0.035λ RMS or less.

FIG. 18 shows that the CD wavefront aberration in the second section is further reduced compared to the third embodiment. Compared to the second and third embodiments, the RMS wavefront aberration in the fourth embodiment is that the CD wavefront aberration is 0 to 0 in the second embodiment in FIG. 11 and the third embodiment in FIG.
In contrast to the values up to 098λ, in the fourth embodiment of FIG. 18, the values are from 0 to 0.05λ, which is improved. However, the wavefront aberration of the DVD in the second section is 0 to −0.1λ in the second embodiment of FIG. 11 and the third embodiment of FIG. 17, but in the fourth embodiment of FIG. It is a degraded value of 0 to -0.2λ. As the RMS wavefront aberration value,
DVD CD
Second Embodiment 0.01945λrms 0.02525λrms
Third Embodiment 0.02495λrms 0.02574λrms
Fourth Embodiment 0.02926 λrms 0.02489 λrms

In the fourth embodiment, the wavefront aberration of the RMS of the DVD is further deteriorated compared to the third embodiment, but the value of 0.03λ RMS or less can still be kept. Formula (1) shown in Table 9
) And (2), MAX (Aij) / MIN in the portion corresponding to the step amounts D1 and D2.
In the fourth embodiment, the value of (Aij) is larger than 17.8242, 31.8655 and 3,
Also greater than 2. The values corresponding to D3, D4, and D5 are less than 2, and M
As for AX (Aij) / MIN (Aij), three of the five satisfy less than 2, and two are unsatisfactory. If three of the five are satisfied, both the DVD and the CD have an RMS wavefront aberration of 0.
This is an example in which the wavefront aberration deterioration in the DVD is recognized as compared with the case where all of the five are satisfied and the case where four of the five are satisfied. In the fourth embodiment, the example of reducing the CD wavefront aberration in the first and second sections has been described, but it is also possible to reduce the DVD wavefront aberration in the first and second sections in the same manner. In the RMS wavefront aberration, the CD side deteriorates.

Furthermore, when it is desired to reduce the CD wavefront aberration in the third section, the comparative example shown in Table 10 is preferable.

In this comparative example, the value of A16 in the third section is changed from the fourth embodiment shown in Table 8. FIG. 19 shows the wavefront aberration diagram of this comparative example, and Table 11 shows the calculation results for equations (1) and (2).

The absolute values of the steps D1 to D5 in the DVD / CD common use area increase with distance from the lens optical axis at the steps D4 and D5. The absolute values of the steps D1, D2, and D3 are larger than D4 although they are closer to the optical axis. To that end, first,
The DVD wavefront aberration increases in the second and third sections, and the RMS wavefront aberration as a whole is zero.
. The value is 035λRMS or more.

FIG. 19 shows that the CD wavefront aberration in the third section is further reduced as compared with the fourth embodiment. In comparison with the second, third, and fourth embodiments, in the RMS wavefront aberration, the comparative example shows that the CD wavefront aberration in the third section is the second embodiment of FIG. 11, the third embodiment of FIG. In the fourth embodiment, the value is from 0 to 0.098λ, whereas in the comparative example of FIG. 19, the value is from −0.01 to 0.04λ, which is improved. . However, the wavefront aberration of the DVD in the third section was 0 to −0.1λ in the second embodiment of FIG. 11 and the third embodiment of FIG. 17, but in the fourth embodiment of FIG. It is a degraded value of 0 to -0.2λ. As the RMS wavefront aberration value,
DVD CD
Second Embodiment 0.01945λrms 0.02525λrms
Third Embodiment 0.02495λrms 0.02574λrms
Fourth Embodiment 0.02926 λrms 0.02489 λrms
Comparative Example 0.03503λrms 0.02477λrms
Thus, in the comparative example, the wavefront aberration of the RMS of the DVD is further deteriorated compared with the fourth embodiment, and becomes 0.035λrms or more. Regarding the values of the expressions (1) and (2) shown in Table 11, the values of MAX (Aij) / MIN (Aij) in the parts corresponding to the step amounts D1, D2, and D3 are 17.8242 and 24.6772 in the comparative example. , Greater than 6.6199 and 3, and greater than 2. In the portion corresponding to D4 and D5, the value is less than 2, and MAX (Ai
For j) / MIN (Aij), 2 out of 5 are less than 2 and 3 are unsatisfactory. Since only 2 out of 5 were satisfied, both the DVD and CD had an RMS wavefront aberration of 0.
In this comparative example, 035λ RMS or less is not satisfied.

Further, the case of DVD and CD has been described so far, but the present invention is effective even when the substrate thickness is the same and the wavelength is different. As an example, so-called blue laser, wavelength 405
This is the case when the substrate thickness is 0.6 mm at nm and the DVD has a wavelength of 655 nm and the substrate thickness is 0.6 mm. This case will be described below as a fifth embodiment.

In the fifth embodiment, the basic lens configuration is the same as that of the second embodiment shown in FIG. 2, and parallel light is incident from the A surface side, and on the recording surface of a disk substrate (not shown) on the B surface side. A good light spot is formed. On the light source side A surface, the relationship between Z _A and h is expressed by Equation (4). The specific numerical values are shown in sections 1 to 9 in the upper column of Table 12. Also on the opposite side of the light source, B
The relationship between Z _B and h is expressed by equation (6). The specific numerical values are shown in the lower column of Table 12.

The distance between the surface vertices f and e on the optical axis of the objective lens, that is, the center thickness t0 is 1.94 m.
m, the refractive index n at the wavelength λ1 = 405 nm (blue) is 1.54972, and the refractive index n at the wavelength λ2 = 655 nm (DVD) is 1.53. The thickness and refractive index of the transparent substrate are 0.6 mm and a refractive index of 1.6235 at a wavelength of λ1 = 405 nm (blue), and a refractive index of 1.58 at a wavelength of λ2 = 655 nm (DVD) and a refractive index of 1.58. It is.

In addition, the NA at the wavelength of 405 nm is 0.65, the focal length is 3.1015 mm, the NA at the wavelength of 655 nm is 0.6277, and the focal length is 3.2116 mm. The effective diameter of the incident parallel light beam is φ4.032 for both blue and DVD. In the fifth embodiment, there is nothing corresponding to the DVD dedicated area in the first to fourth embodiments, and φ4.03.
2 is the blue / DVD common use area. Looking at the curvature (= the reciprocal of the radius of curvature), that is, C in Table 12, when looking at the relationship between large and small in the first to ninth sections, which are the common use areas of blue and DVD, Small, large, small relationships in sections 2, 3, 4; large, small, large relationships in sections 3, 4, 5; small, large, small relationships in sections 4, 5, 6; There are large, small, and large relationships in sections 6 and 7, and there are sections of small, large, small and large, small, and large. The curvature is designed in this way and shown in the text. Such wavefront aberration values can be realized.

FIG. 20 shows a wavefront aberration diagram of the fifth embodiment. Blue RMS wavefront aberration is 0.0315
2λrms, the RMS wavefront aberration of DVD is 0.03237λrms, blue and DVD are both 0.
035λrms or less. Table 13 shows the values of the expressions (1) and (2).

About the value of Formula (1), (2), it is 1.23 or less in all the adjacent level difference parts of D1-D8, and Formula (1), (2) is satisfied with all the level difference parts. .

FIG. 21 shows a light spot diagram of the fifth embodiment. The light spot diameter, which is the relative light intensity of 1 / e ² (= 0.135), is 0.5149 μm for 405 nm blue and 0.8606 μm for 655 nm DVD.

In the fifth embodiment, the Wmax / Wmin is
Since Wmax / Wmin = 0.03237 / 0.03152 = 1.026967, the allowable value is 1.8, preferably 1.6, and more preferably 1.4 or less. The values corresponding to Table 3 of the fifth embodiment are Δ in the first to fourth and sixth to ninth regions.
Vd (λ405) = 0.11 to 0.12λ, and ΔVd (λ655) = 0.11 to 0.12λ, and the difference ratios ΔVd (λ405) / ΔVd (λ655) and ΔVd (Λ655)
/ ΔVd (λ405), both of which are in the range of 0.9 to 1.1. For the fifth area,
ΔVd (λ405) = 0.075λ, and ΔVd (λ655) = 0.078λ. In the wavefront aberration diagram in FIG. 20, the calculation result is in increments of 0.01 relative pupil within the range of the relative pupil coordinates 0 to 1. If more detailed calculation is performed, each wavefront aberration becomes the above value. .

It is a figure which shows 1st Embodiment of the objective lens by this invention. It is a figure which shows one specific example of the lens surface shape of 1st Embodiment shown in FIG. It is a figure for demonstrating the optical path length in the optical system which consists of an objective lens and the transparent substrate of an optical disk. It is a graph which shows a specific example of the measurement result of the wavefront aberration of 1st Embodiment of 1st Embodiment shown in FIG. It is a figure which shows the calculation result of the light spot with respect to the optical disk from which the kind differs in the optical disk apparatus using 1st Embodiment shown in FIG. It is a graph which shows a specific example of the measurement result of the wavefront aberration of 2nd Embodiment of the objective lens by this invention. It is a figure which shows the calculation result of the light spot with respect to the optical disk from which the kind in the optical disk apparatus using 2nd Embodiment of the objective lens by this invention differs. It is a figure which shows one Embodiment of the optical head by this invention. It is a figure which shows one Embodiment of the optical disk apparatus by this invention. It is a schematic diagram which shows the wavefront aberration of each wavelength with respect to light ray height. It is a figure which shows the wavefront aberration of each wavelength with respect to the light ray height in 2nd Embodiment. It is a figure which shows the wave aberration of each wavelength with respect to the light height at the time of using the lens of Unexamined-Japanese-Patent No. 2001-51192. It is a figure for demonstrating the numerical formula in 2nd Embodiment. It is a figure which shows a structure in the case where the adjacent part of each aspherical part in 2nd Embodiment has level difference amount D1-D7. It is a figure which shows the lens which has a shape which considered metal mold | die preparation and shaping | molding. It is a figure which shows the lens which has a shape which considered metal mold | die preparation and shaping | molding. It is a wave aberration diagram of the lens concerning 3rd Embodiment. It is a wave aberration diagram of the lens concerning 4th Embodiment. It is a wave aberration diagram of the lens concerning a comparative example. It is a wavefront aberration figure of the lens concerning 5th Embodiment. It is a light spot figure of the lens concerning a 5th embodiment. It is a light spot figure of the lens for CD. It is a light spot figure at the time of setting the aperture diameter of the lens concerning 2nd Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Objective lens 2 Transparent substrate of DVD 2a Information recording surface 3 Transparent substrate of CD 3a Information recording surface 4, 5 Laser beam 11 DVD laser 12 CD laser 13, 14 Half prism 15 Collimator lens 16 Detection lens 17 Photo detector 18 Diffraction grating 19 Actuator 20 Actuator drive circuit 21 Signal processing circuit 22 Laser drive circuit 23 System control circuit 24 Disc discriminating means

Claims (3)

All of the monochromatic light of at least one lens surface in a multi-wavelength lens that condenses a plurality of types of monochromatic light of wavelengths λi (i = 1, 2, 3, 4,...) By refraction. Is divided into a plurality of aspherical portions having different refractive powers, and the curvature of each of the divided aspherical portions (= the reciprocal of the radius of curvature) is large, small, A multi-wavelength lens comprising a large aspherical portion. All of the monochromatic light of at least one lens surface in a multi-wavelength lens that condenses a plurality of types of monochromatic light of wavelengths λi (i = 1, 2, 3, 4,...) By refraction. The common use region is divided into a plurality of aspherical portions having different refractive powers, and the curvature (= reciprocal of the radius of curvature) of each of the divided aspherical portions is small, large, A multi-wavelength lens comprising a small aspherical portion. All of the monochromatic light of at least one lens surface in a multi-wavelength lens that condenses a plurality of types of monochromatic light of wavelengths λi (i = 1, 2, 3, 4,...) By refraction. The common use area is divided into a plurality of aspherical portions having different refractive powers, and the step amount in the direction parallel to the lens optical axis in the adjacent aspherical portions of each of the divided aspherical portions However, at least half of the steps in the adjacent portions increase as the distance from the lens optical axis increases.

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