JP5318910B2 - Objective lens - Google Patents

Description

  The present invention is a multi-wavelength optical system using a plurality of types of monochromatic light. For example, a CD (compact disc: including a CD such as a CD-R), a DVD (Digital Versatile Disc), a blue laser compatible disc (AOD). The present invention relates to a general-purpose multi-wavelength lens, a multi-wavelength optical system, an optical head, and an optical disc apparatus that can be used in compatible recording / reproducing apparatuses that can handle different types of optical recording media.

  Conventionally, compatible optical disk devices 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 structure 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 configured 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 on 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. Then, for each laser beam having the same wavelength, every other annular lens surface condenses the laser beam onto the information recording surface of an optical disk (DVD) on a thin transparent substrate (0.6 mm), for example. The laser beam is condensed on the information recording surface of the optical disk (CD) of the thick transparent substrate (1.2 mm), for example, by the annular ring-shaped lens surface.

  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 and 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, a lens surface that is used for a DVD and that eliminates wavefront aberration caused by a laser beam having a wavelength λ ₁ of 640 nm is used as a reference. 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 step di, 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 refracting 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 apparatus, 0.07λ RMS or less is a value that should be a target value for the entire optical disk 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, it is desirable that the objective lens alone has an RMS wavefront aberration value of 0.035λ RMS or less, more desirably 0.030λ RMS or less, and more desirably 0.025λ RMS or less. In Patent Document 3, DVD is 0.001λRMS and CD is 0.047λRMS in Embodiment 1, DVD is 0.019λRMS and CD is 0.037λRMS in Embodiment 2, and the wavefront aberration of DVD is good. Although it is a small value, it is only a value of 0.037λ RMS or more in CD, and a satisfactory 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.

  In the optical recording medium recording / reproducing objective lens according to the present invention, the monochromatic light having a positive power is incident on the information recording surface provided on the transparent substrate of the optical recording medium. In an optical recording medium recording / reproducing objective lens that collects light and forms a light spot, a use area and an unused area located outside the objective area are provided on one or both lens surfaces of the objective lens. In the case where the light from the light source is incident, the light spot diameter is obtained from both the use area and the non-use area from the light spot diameter obtained when the light from the light source is incident only on the use area. The light spot diameter when light is incident is smaller.

  The objective lens receives monochromatic light having a first wavelength and a second wavelength respectively corresponding to two types of optical recording media having different transparent substrate thicknesses, and is incident on the transparent substrate of the optical recording medium. The monochromatic light is condensed on the provided information recording surface to form a light spot, and the non-use area with respect to the first wavelength is a use area at a second wavelength different from the first wavelength. It is desirable. Further, in a non-use area for the first wavelength and a use area for the second wavelength, one or both surfaces of the lens surface are divided, and the divided It is desirable that the phase difference based on the wavefront aberration caused by the passage of the light having the second wavelength through each of the surfaces is approximately an integer multiple. Furthermore, the above-described objective lens for recording / reproducing optical recording media is used in an optical system optical head and an optical disc apparatus.

The multi-wavelength optical recording medium recording / reproducing objective lens design method according to the present invention has different wavelengths λi (i = 1, 2, 3, 4) for each of a plurality of types of optical recording media having different transparent substrate thicknesses. ,...) Is incident, the monochromatic light is condensed on the information recording surface provided on the transparent substrate of the optical recording medium, and all the monochromatic light of at least one lens surface is collected. A design method of an optical recording medium recording / reproducing objective lens for multi-wavelengths in which a common use area is divided into a plurality of aspherical portions, and between adjacent aspherical portions of each divided aspherical portion Dj (j = 1, 2, 3, 4,...: Order close to the lens optical axis) and light of the wavelength λi in the order of the level difference in the direction parallel to the lens optical axis in the adjacent portion. Where NAij is the NA between the jth adjacent parts when At least half of the steps are designed to satisfy the following equation when the minimum value of MIN (Aij) and the maximum value of MAX (Aij) among the following Aij values for each wavelength λi are set. This is a method for designing an objective lens for recording / reproducing for a multi-wavelength optical recording medium.
MAX (Aij) / MIN (Aij) <3
here,
Aij = Absolute value (Bij-mij)
Bij = (absolute value (Dj)) * (ni−1) / λi− (NAij2) * K / λi
ni: refractive index of lens at wavelength λi mij: integer closest to Bij K = 0.004 millimeters (when NAij <0.55)
K = 0.0005 mm (when NAij> = 0.55)

  An aspherical shape that cancels out chromatic aberration caused by a difference in wavelength λi of the monochromatic light and wavefront aberration caused by a difference in thickness of the transparent substrate of the optical recording medium is set in the plurality of aspherical portions. Is desirable. Further, it is desirable to design each monochromatic light so that the RMS wavefront aberration is converged to 0.035λ or less. Furthermore, it is desirable to design so as to satisfy the formula defined above for all adjacent steps. Furthermore, it is desirable to design so as to satisfy MAX (Aij) / MIN (Aij) <2.

  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.

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 in the lens for CD. It is a CD light spot figure of the 1st-6th section.

Now, with respect to a first optical disk using a transparent substrate having a thickness t ₁ , the objective lens in the optical disk apparatus using the optical disk is corrected with good aberration, and the laser beam is excellent on the information recording surface provided on the substrate. Condensate. When using the second optical disc using the transparent transparent substrates of different thicknesses t ₂ to the substrate in such an optical disk apparatus, for different thicknesses t ₂ of the transparent substrate and the thickness t _1, the objective lens And a transparent substrate having a thickness t ₂ causes spherical aberration, and the laser beam is not focused well on the information recording surface provided on the transparent substrate having the thickness t ₂ .

  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 that occurs 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 objective. It is the difference in spherical aberration that occurs when the lens is irradiated.

That is, the spherical aberration when the thickness is t ₁ is S _A (t ₁ ), the spherical aberration when the thickness is t ₂ is S _A (t ₂ ), and the spherical surface generated with respect to the laser beam having the wavelength λ _1. Assuming that the aberration is S _A (λ ₁ ) and the spherical aberration generated with respect to the laser beam having the wavelength λ ₂ is S _A (λ ₂ ), the chromatic aberration due to the different wavelength is the difference between the spherical aberrations (S _A (λ _2). ) -S _A (λ ₁ )). At this time, it is preferable to design the lens surface so that the following formula is established as much as possible.

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) The optical path until the light beam passing through the position P ₁ of the light beam h crosses the optical axis OA and reaches the point (condensing point) P ₅ is schematically shown. Here, the incident point to the objective lens 1 in the optical path is P ₂ , the exit point from the objective lens 1 is P ₃ , and the incident point to the transparent substrate 2 is P ₄ .
Point P ₁ to incident point P ₂ : Spatial distance = S _1h Refractive index = n ₁
Entrance point P ₂ to exit point P ₃ : Spatial distance = S _2h Refractive index = n ₂
Outgoing point P ₃ to incident point P ₄ : Spatial distance = S _3h Refractive index = n ₃
Incident point P ₄ to condensing point P ₅ : Spatial distance = S _4h Refractive index = n ₄
Then, the optical path length L _h from the point P ₁ to the condensing point P ₅ 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 L _h on the optical axis OA is a case where h = 0 in this equation.

This equation is applicable to an arbitrary light beam height h, and when aberration correction is performed, the condensing point P ₅ for each light beam height h is within the allowable range on the information recording surface 2a. is there. 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 P ₅ with respect to each ray height h is in each allowable range. It is preferable to be on the information recording surface 2a.

For example, in the case where 790 nm monochromatic light (λ ₁ ) on a CD and 655 nm monochromatic light (λ ₂ ) on a DVD are used, a region where these two wavelengths are used in common is divided into a plurality of aspherical portions. In the method of using a lens surface, the optical path length of any of the aspherical portions is made to be different from the optical path length of the other aspherical portions by substantially an integer multiple of the wavelength λi of each monochromatic light, and each of the single color in each of the aspherical portions When the difference between the maximum value and the minimum value of the wavefront aberration of light is ΔVd (λ ₁ ) and ΔVd (λ ₂ ) (d is an integer of 1, 2,... Even in the aspheric surface portion, 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 of the entire lens is acceptable at both wavelengths. Can be secured. 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

V _h = (L _h −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 aspherical portion in the first region of the aspherical portion is defined by ΔV ₁ (λ ₁ ) and ΔV ₁ (λ ₂ ). 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.

  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 of each wavelength is 0.14λi or less (for example, the wavelength is 790 nm) in each region of any aspheric surface. In some cases, ± 110.6 nm or less, and in the case where the wavelength is 655 nm, ± 91.7 nm or less), preferably 0.12 λi or less, more preferably 0.10 λi or less, and even better at each wavelength. Optical characteristics can be ensured.

  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, in the case of a CD, the RMS wavefront aberration is determined from the wavefront aberration of only the common use region of DVD and CD up to a ray height of 1.58 mm in FIG. In the case of a DVD, there is a DVD dedicated area (in the range of 1.58 to 2.02 mm in ray height in FIG. 10) outside the common use area, and from the wavefront aberration of both the common use area and the dedicated area. RMS (Root Mean Square) wavefront aberration value is obtained. 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 and 0.0222λRMS for CD, and the RMS wavefront aberration is almost equal for both DVD and CD. 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. Similarly, in the case where it is desired to change the ratio of the RMS wavefront aberration of DVD 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 the embodiment of the present invention, it is possible to form a good light spot on the information recording surface, for example, for any optical disc having a different substrate thickness. This can be applied even if the thickness of the disc substrate is not different, that is, even when the thickness is the same and the wavelength is different, by setting the condensing point P ₅ within each allowable 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, an embodiment of the present invention will be described with reference to the drawings, taking two types of optical disks having different transparent substrate thicknesses, that is, a DVD and a CD as an example. 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.

  FIGS. 1A and 1B are views showing the operation of the first embodiment of the objective lens according to the present invention. FIG. 1A is for a DVD and FIG. 1B 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 t ₁ of the DVD substrate 2 is 0.6 mm. As the laser beam 4 at this time, a laser beam having a wavelength λ ₁ = 655 nm is used as a light beam 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 t ₂ of the CD substrate 3 is 1.2 mm. As the laser beam 5 at this time, a laser beam having a wavelength λ ₂ = 790 nm is used as a light beam having a numerical aperture NA = 0.63. The light flux having a numerical aperture NA = 0.47 is condensed on the information recording surface 3a of the CD substrate 3, and the light flux passing through a portion away from the optical axis OA of the objective lens 1 having substantially NA = 0.47 to 0.63 shown by hatching. Light is not condensed on the 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.

但し、C=-0.12301、K=3.312138、A₄=0.01628151、A₆=-0.004311717、A₈=0.000682316、A₁₀=-0.00004157469で表わされるようにする。 In FIG. 2, regarding the light exit side B of the objective lens 1, if the point of the light beam height h is c, and the point on the light exit side B in a direction parallel to the optical axis OA from this point c is d, The surface shape of the exit side surface _{B is determined} by the distance Z _B between the points c and d with respect to an arbitrary ray height h.

However, C = −0.12301, K = 3.312138, A ₄ = 0.01628151, A ₆ = −0.004311717, A ₈ = 0.000682316, A ₁₀ = −0.00004157469.

In Equation 1, when the values of the coefficients C, K, A ₄ , A ₆ , A ₈ , A ₁₀ are substituted to obtain the distance Z _B for an arbitrary ray height h (≠ 0), the value is Although this is a negative value, this is because the point d on the light emission side surface B is the point c, and therefore, closer to the emission surface side (left side in FIG. 2) than the surface vertex e through which the optical axis OA of the light emission side surface B passes. It shows that it is located. When the distance Z _B is a positive value, it indicates that it is located on the opposite right side.

対物レンズ１の上記数１で表わされる光出射側面Ｂも、また、上記表１の点列データで表わされる光入射側面Ａも、連続した非球面をなすものである。また、対物レンズ１の光軸上の面頂点ｆ，ｅ間の距離、即ち、中心厚ｔ0は２.２ｍｍであって、波長λ₁＝655ｎｍ(ＤＶＤ）での屈折率ｎは1.54014であり、波長λ₂＝790ｎｍ（ＣＤ）での屈折率ｎは1.5365である。 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. The surface shape of the side surface A is set to a lens surface shape in which the ray height h (mm) and the distance Z _A (mm) between the points a and b with respect to the ray height h have the relationship shown in Table 1 below. The

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 refractive index n at the wavelength λ ₁ = 655 nm (DVD) is 1.54014. The refractive index n at the wavelength λ ₂ = 790 nm (CD) is 1.5365.

(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 λ ₁ = 655 nm) and CD (wavelength λ ₂ = 790 nm) have RMS wavefront aberrations of 0.035λ, 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 to the above-described surface shapes so that the wavefront aberration of DVD and CD is less than the allowable value.

In the first embodiment, a case where _two different wavelengths λ ₁ and λ ₂ are used is shown. In general, n types (where n is an integer of 2 or more) different wavelengths λi (where i = 1, 2,..., N) is the same.

（但し、ｉ番目の該光ビームの波長をλi（ｉ＝１，２，......）、全ての波長にわたる個々のＲＭＳ波面収差の二乗の総和をΣＷ_i ²、波長λiの光ビームのＲＭＳ波面収差をＷi・λiとする）を満足するようにする。このときの許容値Ｗ₀としては、0.028、好ましくは0.026，さらに好ましくは0.025、さらに好ましくは0.023とする。上記第１の実施形態では、ＤＶＤのＲＭＳ波面収差をＷ₁、ＣＤのＲＭＳ波面収差をＷ₂とし、かつｉ＝１，２であるから、上記数２は、

となる。 (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 ,

(Where 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 ΣW _i ² , and the light of wavelength λi. The RMS wavefront aberration of the beam is set to Wi · λi). The allowable value W _{0 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 the DVD is W ₁ , the RMS wavefront aberration of the CD is W ₂ , and i = 1, 2, the above formula 2 is

It becomes.

(iii) Alternatively, when using laser beams of different n types of wavelengths λi, assuming that 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 maximum RMS wavefront aberration Wmax next the RMS wavefront aberration W ₂ of RMS wavefront aberration W ₁ and CD of DVD, 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 for a DVD (wavelength λ ₁ = 655 nm). When the image height is 0 mm, the RMS wavefront aberration is 0.02130λ ₁ . FIG. 4B shows the RMS wavefront aberration with respect to CD (wavelength λ ₂ = 790 nm). When the image height = 0 mm, the RMS wavefront aberration = 0.02410λ ₂ .

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.

であるから、上記の許容値0.028、好ましくは0.026、さらに好ましくは0.025、さらに好ましくは0.023以下となっている。 (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 relative light intensity at each position when the light intensity at (= 0 mm) is 1 is shown.

FIG. 5A shows a light spot with respect to a DVD, and the light spot diameter ΦD at which the relative light intensity is 1 / e ² (= 13.5%) 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. Are set so that the aberration is reduced well within the allowable range for both DVD and CD.

で表わされる。なお、数５での光源高さｈは、ｊ番目の区間でのものである。 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 Z _Aj , that is,

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

また、この第２の実施形態での光出射面Ｂの面形状Ｚ_Bは、次の数６で表わされる。図１３に、一般的なレンズ形状を示している。

但し、 C=-0.0747792、K=15.7398、A₄=0.012308、A₆=-0.0037652、A₈=0.00068571、A₁₀=-0.000048284
また、対物レンズ１の光軸上の面頂点ｆ，ｅ間の距離、即ち、中心厚さｔ0は2.2ｍｍであって、波長λ₁＝655ｎｍ（ＤＶＤ）での屈折率ｎは1.604194であり、波長λ₂＝790ｎｍ（ＣＤ）での屈折率ｎは1.599906である。 For both DVD and CD, the range (range of h) and its constants B, C, K, A ₄ , A ₆ , A for each section in Equation 5 for satisfactorily reducing the aberration within the allowable value. ₈ , A ₁₀ , A ₁₂ , A ₁₄ , and A ₁₆ are shown in the following Table 2.

Further, the surface shape Z _B of the light emitting surface B in the second embodiment is expressed by the following equation (6). FIG. 13 shows a general lens shape.

However, C = -0.0747792, K = 15.7398, A ₄ = 0.012308, A ₆ = -0.0037652, A ₈ = 0.00068571, A ₁₀ = -0.000048284
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 refractive index n at the wavelength λ ₁ = 655 nm (DVD) is 1.604194. The refractive index n at the wavelength λ ₂ = 790 nm (CD) is 1.599906.

Further, the thickness and refractive index of the transparent substrate are a refractive index of 1.57995 at a wavelength of λ ₁ = 655 nm (DVD) and a refractive index of 1.57995, and a refractive index of 1.2 mm at a wavelength of λ ₁ = 790 nm (CD). Is 1.573071. The NA for DVD is 0.60, and the 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.

  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, and the horizontal axis and the vertical axis are the same as those in FIG.

FIG. 6A shows the RMS wavefront aberration for a DVD (wavelength λ ₁ = 655 nm). When the image height is 0 mm, the RMS wavefront aberration is 0.01945λ ₁ . FIG. 6B shows the RMS wavefront aberration with respect to CD (wavelength λ ₂ = 790 nm). When the image height is 0 mm, the RMS wavefront aberration is 0.02525λ ₂ .

表３に示すように、７９０ｎｍと６５５ｎｍの共通使用領域において各波面収差の差の比ΔＶｄ(λ790)／ΔＶｄ(λ655)は、１．００〜１．０４の間に入っている。また、比ΔＶｄ(λ655)／ΔＶｄ(λ790)は、０．９６〜１．００の間に入っている。そして、その各領域の波面収差自体も両波長において０．１４λ以下となっている。また、このレンズでは波面収差が７９０ｎｍの波長において＋側に、６５５ｎｍの波長において−側に現れるようにしていて、両波面収差がほぼ対称形となる。 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. Further, the ratio ΔVd (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 0.01945λ ₁ , 0.02525λ ₂ and the allowable value 0.035λ, preferably 0.033λ, and more preferably smaller than 0.030λ.

であるから、上記の許容値0.028、好ましくは0.026，さらに好ましくは0.025、さらに好ましくは0.023以下となっている。 (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 light spot diameter ΦD at which the relative light intensity is 1 / e ² (= 13.5%) 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. 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 light spot diameter at which the relative light intensity in FIG. 22 is 1 / e ² (= 13.5%) 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 aforementioned phase difference of wavefront aberration means that the seventh section and the eighth section of the DVD dedicated area have a wavefront aberration of 2λ ₁ at a wavelength of DVD655 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 that of the CD lens shown in FIG. 22, which is the same as that of the second embodiment shown in FIG. It is about 0.09 μm larger than CD. That is, the use of the other wavelength λ ₁ that allows light to pass outside the lens for the wavelength λ ₂ , and preferably the position of the wavefront aberration at the other wavelength λ ₁ in the outer region. By providing the phase difference, the light spot diameter of the wavelength λ ₂ 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, and the RMS wavefront aberration is 0.01945λ ₁ for DVD and 0.02525λ ₂ for CD. As described in the description, if the DVD wavefront aberration is improved by further degrading the DVD wavefront aberration in the common use region, the equivalent RMS of about 0.022 to 0.023λ as the RMS wavefront aberration for both DVD and CD. It can also be a wavefront aberration.

As an example, looking at the RMS wavefront aberration of DVD and CD described in JP-A-2001-51192,
Example 1) DVD: 0.001λ ₁ CD: 0.047λ ₂
Example 2) DVD: 0.019λ ₁ CD: 0.037λ ₂
However, λ ₁ = 640nm λ ₂ = 780nm
In any case, the CD exceeds the above allowable value of 0.035λ.

は、上記の例１、例２の夫々について0.0332，0.0294となり、いずれも上記の許容値0.028、0.026、0.025、0.023の全ての値を越えており、さらに、これらのＷmax／Ｗminも例１、例２の夫々について47，1.847となり、いずれも上記の許容値1.8、1.6、1.4の全ての値を越えている。 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. It is outside the scope of the invention, and therefore the balance between the two is shifted. 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 Examples 1 and 2 above, both of which exceed all of the above allowable values of 0.028, 0.026, 0.025, 0.023, and these Wmax / Wmin are also Example 1, The values of Example 2 are 47 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 embodiment described above, as shown in FIG. 1, the outer region of the objective lens 1 having a numerical aperture NA = 0.47 to a numerical aperture NA = 0.63 or NA = 0.60 is used only for a DVD. Since it is not used, thin film processing that transmits light with a wavelength of 655 nm at the time of DVD to one or both of the light incident side surface A and the light output side surface B in such an outer region and does not transmit light with a wavelength of 790 nm at the time of CD Or a diffraction grating that does not act on light having a wavelength of 655 nm, but acts on light having a wavelength of 790 nm, but either one or both of the light incident side surface A and the light exit side surface B in the outer region is formed. Then, the light utilization efficiency of the two wavelengths 790 nm may be decreased without decreasing the light utilization efficiency of the wavelength 655 nm.

  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, in which 11 is a DVD laser, 12 is a CD laser, 13 and 14 are half prisms, 15 is a collimator lens, and 16 is detection. A lens, 17 is a photodetector, 18 is a diffraction grating, and 19 is an actuator. The same reference numerals are given to portions corresponding to those in FIG.

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 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.

  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. The actuator 19 is driven so that the objective lens 1 is positioned at an appropriate focus position and tracking position in the same manner as the DVD disk 2 by using the tracking error signal and the focus error signal thus obtained.

  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 necessarily required, and the present invention can be applied to a so-called finite optical system.

  FIG. 9 is a block diagram showing an embodiment of an optical disk apparatus using an objective lens according to the present invention, wherein 20 is an actuator drive circuit, 21 is a signal processing circuit, 22 is a laser drive circuit, 23 is a system control circuit, 24 Is disc discriminating means, and 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. The system control circuit 23 controls the actuator drive circuit 20 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 By the operation of a so-called servo circuit that moves in the tracking direction, focus control and tracking control are normally performed so that the objective lens 1 is positioned at the 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 the information recording / reproducing signal is obtained satisfactorily.

  Regarding the second embodiment, the following points could be found. That is, as the lens surface shape for obtaining the wavefront aberration described above, a multi-wavelength lens for condensing a plurality of types of monochromatic light of wavelengths λi (i = 1, 2, 3, 4,...) By refraction. The common use area for all monochromatic light of at least one lens surface in FIG. 2 is divided into a plurality of aspherical portions having different refractive powers, and adjacent aspherical portions of the divided aspherical portions are adjacent to each other. The step amount in the direction parallel to the lens optical axis in the adjacent portion of Dj is Dj (j = 1, 2, 3, 4,...: The order close to the lens optical axis) and the wavelength λi. When the numerical aperture NA between the j-th adjacent portions when light is incident is NAij, the minimum one of the following Aij values for each wavelength λi in at least half of the steps in the adjacent portions Is MIN (Aij), the largest is MAX (Aij) May satisfy the following equation (1) when the.

MAX (Aij) / MIN (Aij) <3 (1)
here,
Aij = Absolute value (Bij−mij)
Bij = (absolute value (Dj)) * (ni-1) / λi− (NAij2) * 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. Further, MAX (Aij) / MIN (Aij) <2.5 is desirable, and further, MAX (Aij) / MIN (Aij) <2 (hereinafter referred to as Expression (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 * (NAij2) is a correction term for the optical path length. To reduce spherical aberration, K04 (NAij2) is 0.0004 × NA2 (mm) when NA is less than 0.55, and NA is 0.55 or more. It is necessary to correct the optical path length by the adjacent step portion by 0.0005 × NA 2 (mm). Aij indicates how much the value of Bij deviates from the nearest integer. As the value of each Aij is smaller, 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.

前記第２実施形態における各非球面部の隣接部が段差量Ｄ１〜Ｄ７を有する場合の構造を図１４に示す。なお、本第２実施形態では表２の区間１〜６までがＤＶＤ／ＣＤ共通使用領域であり、区間７〜８はＤＶＤ専用使用領域である。よって、式（１）、（２）での計算対象としては、ＤＶＤ／ＣＤ共通使用領域内の段差についてのみなので、Ｄ１〜Ｄ５までが計算対象であり、Ｄ６とＤ７は式（１）、（２）の計算対象外である。また段差量Ｄ１〜Ｄ７については、段差部においてレンズ光軸に近い側の非球面が図の左側にある場合を正符号とし、右側にある場合を負符号とする。つまり本第２実施形態、図１４、表５の場合にはＤ１、Ｄ２、Ｄ３がプラス符号の段差量で、Ｄ４、Ｄ５、Ｄ６、Ｄ７がマイナス符号の段差量である。第２実施形態の段差部の形状として図１４に示したが、レンズを実際に作製する場合、レンズがプラスチック製の場合には射出成形、ガラス製の場合にはガラス成形レンズとすることが考えられる。いずれの場合でも金型を作製してその形状を転写して製品を得ることになるが、その金型作製や成形を考慮した形状を図１５、図１６に示す。 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 are the calculation targets, and D6 and D7 are the equations (1), ( Not subject to calculation in 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, in the case of the second embodiment, FIG. 14, and Table 5, D1, D2, and D3 are plus sign step amounts, and D4, D5, D6, and D7 are minus sign step amounts. 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 step D1 in FIG. 14. The shape shown in FIG. 14 is a shape with a taper of 20 degrees and a left corner of R0.0005 mm (shown by a solid line). The above R0.0005 mm takes into account the bite R at the time of mold manufacture, and the punching taper of 20 degrees facilitates molding 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 area of dimension E3 becomes an ineffective area that does not contribute to optical imaging, but the area shown in FIG. 15 is about 0.0005 to 0.001 mm, and the effective area of DVD is 4.032 mm, CD Compared with the effective area of Φ3.179 mm, this is a minute amount and 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 of 20 degrees and a left corner of R0.0005 mm (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.

本第３実施形態は表２に示す第２実施形態から第１区間のＡ４の値を変えたものである。本第３実施形態の波面収差図を図１７に、式（１）、（２）についての計算結果を表７に示す。

図１７より、第１区間でのＣＤ波面収差が低減されていることがわかる。ＲＭＳ波面収差では、第２実施形態では、第１区間においてＣＤの波面収差が図１１の第２実施形態では０〜０．０９５λまでの値を取っていたのに対して、図１７に示す第３実施形態では、０〜０．０３λまでの値となっていて改善されていることがわかる。しかし、第１区間でのＤＶＤの波面収差については図１１の第２実施形態では０〜−０．１λの値であったが、図１７の第３実施形態では０〜−０．２λと劣化した値となっている。ＲＭＳ波面収差値としては、
ＤＶＤ ＣＤ
第２実施形態 ０．０１９４５λrms ０．０２５２５λrms
第３実施形態 ０．０２４９５λrms ０．０２５７４λrms
となっており、ＤＶＤで第３実施形態の方が劣化しているが、まだ０．０２５λＲＭＳ以下の値をキープできている。表７に示す式（１）、（２）の値については、段差量Ｄ１に相当する部分でＭＡＸ（Ａｉｊ）／ＭＩＮ（Ａｉｊ）の値が第３実施形態では１８．１３２４と３よりも大きく、また２よりも大きい。Ｄ２、Ｄ３、Ｄ４、Ｄ５に相当する部分では２未満の値となっており、ＭＡＸ（Ａｉｊ）／ＭＩＮ（Ａｉｊ）については５つのうち４つは２未満を満足し、１つは不満足である。５つのうち４つを満足していればＤＶＤ、ＣＤ共にＲＭＳ波面収差で０．０２５λＲＭＳ以下を満足できているが、５つとも満足している場合に比べるとＤＶＤでの波面収差の劣化が認められる例である。なお第３実施形態で第１区間におけるＣＤ波面収差を低減する例について記述したが、同様にして第１区間でＤＶＤ波面収差を低減させることも可能であり、その場合にはＲＭＳ波面収差においてＣＤ側が劣化してくる。 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, when it is desired to further reduce the wavefront aberration of the CD in the first section than that shown in FIG. 11, the third embodiment shown in Table 6 below may be used, for example.

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).

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 3 embodiment, it turns out that it is a value to 0-0.03 (lambda), and it turns out that it is improving. 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 larger than 18.1324 and 3 in the third embodiment in the portion corresponding to the step amount D1. , 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 four of the five are satisfied, both the DVD and CD satisfy the RMS wavefront aberration of 0.025λ RMS or less. However, the deterioration of the wavefront aberration in the DVD is recognized as compared with the case where all five are satisfied. This is an example. 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.

本第４実施形態は表６に示す第３実施形態から第２区間のＡ１６の値を変えたものである。本第４実施形態の波面収差図を図１８に、式（１）、（２）についての計算結果を表９に示す。

図１８より、第３実施形態に比べて更に第２区間でのＣＤ波面収差が低減されていることがわかる。ＲＭＳ波面収差では、第２、第３実施形態に比べて第４実施形態は、第２区間においてＣＤの波面収差が図１１の第２実施形態、図１７の第３実施形態では０〜０．０９８λまでの値を取っていたのに対して、図１８の第４実施形態では０〜０．０５λまでの値となっていて改善されていることがわかる。しかし、第２区間でのＤＶＤの波面収差については図１１の第２実施形態、図１７の第３実施形態では０〜−０．１λの値であったが、図１８の第４実施形態では０〜−０．２λと劣化した値となっている。ＲＭＳ波面収差値としては、
ＤＶＤ ＣＤ
第２実施形態 ０．０１９４５λrms ０．０２５２５λrms
第３実施形態 ０．０２４９５λrms ０．０２５７４λrms
第４実施形態 ０．０２９２６λrms ０．０２４８９λrms
となっており、第４実施形態ではＤＶＤのＲＭＳの波面収差が第３実施形態に比べて更に劣化しているが、まだ０．０３λＲＭＳ以下の値をキープできている。表９に示す式（１）、（２）の値について、段差量Ｄ１、Ｄ２に相当する部分でＭＡＸ（Ａｉｊ）／ＭＩＮ（Ａｉｊ）の値が第４実施形態では１７．８２４２、３１．８６５５と３よりも大きく、また２よりも大きい。Ｄ３、Ｄ４、Ｄ５に相当する部分では２未満の値となっており、ＭＡＸ（Ａｉｊ）／ＭＩＮ（Ａｉｊ）については５つのうち３つは２未満を満足し、２つは不満足である。５つのうち３つを満足していればＤＶＤ、ＣＤ共にＲＭＳ波面収差で０．０３０λＲＭＳ以下を満足できているが、５つとも満足している場合及び５つのうち４つで満足している場合に比べるとＤＶＤでの波面収差の劣化が認められる例である。なお第４実施形態で第１、第２区間におけるＣＤ波面収差を低減する例について記述したが、同様にして第１、第２区間でＤＶＤ波面収差を低減させることも可能であり、その場合にはＲＭＳ波面収差においてＣＤ側が劣化してくる。 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).

FIG. 18 shows that the CD wavefront aberration in the second section is further reduced compared to the third embodiment. In the RMS wavefront aberration, the fourth embodiment has a CD wavefront aberration of 0 to 0. 0 in the second embodiment of FIG. 11 and the third embodiment of FIG. 17 in the second section as compared with the second and third embodiments. 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. Regarding the values of the equations (1) and (2) shown in Table 9, the value of MAX (Aij) / MIN (Aij) is 17.8242, 31.8655 in the fourth embodiment in the portions corresponding to the step amounts D1 and D2. Greater than 3 and greater than 2. In portions corresponding to D3, D4, and D5, the value is less than 2. For MAX (Aij) / MIN (Aij), three of the five satisfy less than 2, and two are unsatisfactory. If 3 out of 5 are satisfied, both DVD and CD can satisfy RMS wavefront aberration of 0.030λ RMS or less, but 5 are satisfied and 4 out of 5 are satisfied. This is an example in which the deterioration of wavefront aberration in DVD is recognized. 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.

本比較例は表８に示す第４実施形態から第３区間のＡ１６の値を変えたものである。本比較例の波面収差図を図１９に、式（１）、（２）についての計算結果を表１１に示す。

図１９より、第４実施形態に比べて更に第３区間でのＣＤ波面収差が低減されていることがわかる。ＲＭＳ波面収差では、第２、第３、第４実施形態に比べて比較例は、第３区間においてＣＤの波面収差が図１１の第２実施形態、図１７の第３実施形態、図１８の第４実施形態では０〜０．０９８λまでの値を取っていたのに対して、図１９の比較例では−０．０１〜０．０４λまでの値となっていて改善されていることがわかる。しかし、第３区間でのＤＶＤの波面収差については図１１の第２実施形態、図１７の第３実施形態では０〜−０．１λの値であったが、図１８の第４実施形態では０〜−０．２λと劣化した値となっている。ＲＭＳ波面収差値としては、
ＤＶＤ ＣＤ
第２実施形態 ０．０１９４５λrms ０．０２５２５λrms
第３実施形態 ０．０２４９５λrms ０．０２５７４λrms
第４実施形態 ０．０２９２６λrms ０．０２４８９λrms
比較例 ０．０３５０３λrms ０．０２４７７λrms
となっており、比較例ではＤＶＤのＲＭＳの波面収差が第４実施形態に比べて更に劣化して０．０３５λrms以上となってしまっている。表１１に示す式（１）、（２）の値について、段差量Ｄ１、Ｄ２、Ｄ３に相当する部分でＭＡＸ（Ａｉｊ）／ＭＩＮ（Ａｉｊ）の値が比較例では１７．８２４２、２４．６７７２、６．６１９９と３よりも大きく、また２よりも大きい。Ｄ４、Ｄ５に相当する部分では２未満の値となっており、ＭＡＸ（Ａｉｊ）／ＭＩＮ（Ａｉｊ）については５つのうち２つは２未満を満足し、３つは不満足である。５つのうち２つしか満足できていないためにＤＶＤ、ＣＤ共にＲＭＳ波面収差で０．０３５λＲＭＳ以下を満足できていないというのが本比較例である。 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).

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 portions corresponding to D4 and D5, the value is less than 2. As for MAX (Aij) / MIN (Aij), two of the five satisfy less than 2, and three are unsatisfactory. In this comparative example, only two of the five are satisfied, and both the DVD and the CD cannot satisfy the RMS wavefront aberration of 0.035λ RMS or less.

  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. Examples include a so-called blue laser, a substrate thickness of 0.6 mm at a wavelength of 405 nm, and a DVD, a substrate thickness of 0.6 mm at a wavelength of 655 nm. This case will be described below as a fifth embodiment.

また、対物レンズの光軸上の面頂点ｆ，ｅ間の距離、即ち、中心厚さｔ0は１．９４ｍｍであって、波長λ₁＝４０５ｎｍ（ブルー）での屈折率ｎは１．５４９７２であり、
波長λ₂＝６５５ｎｍ（ＤＶＤ）での屈折率ｎは１．５３である。透明基板の厚さと屈折率は、波長λ₁＝４０５ｎｍ（ブルー）では、厚み０．６ｍｍで屈折率１．６２３５であり、波長λ_２＝６５５ｎｍ（ＤＶＤ）では厚み０．６ｍｍで屈折率は１．５８である。
また、波長４０５ｎｍのブルーの時のＮＡは０．６５、焦点距離は３．１０１５ｍｍで、波長６５５ｎｍのＤＶＤの時のＮＡは０．６２７７で、焦点距離は３．２１１６ｍｍである。入射平行光束有効直径＝焦点距離×ＮＡ×２であり、前記ＮＡと焦点距離の値からわかるように、波長４０５ｎｍのブルーの時は
入射平行光束有効直径＝３．１０１５×０．６５×２＝Φ４．０３２であり、
波長６５５ｎｍのＤＶＤの時は
入射平行光束有効直径＝３．２１１６×０．６２７７×２＝Φ４．０３２である。 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. Further, the B surface on the side opposite to the light source and on the disk side represents the relationship between Z _B and h in Expression 6. The specific numerical values are shown in the lower column of Table 12.

Further, 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 mm, and the refractive index n at the wavelength λ ₁ = 405 nm (blue) is 1.54972. Yes,
The refractive index n at the wavelength λ ₂ = 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 λ ₁ = 405 nm (blue), and 0.6 mm and a refractive index of 1 at a wavelength of λ ₂ = 655 nm (DVD). .58.
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 incident parallel beam effective diameter = focal length × NA × 2, and as can be seen from the values of NA and focal length, the incident parallel beam effective diameter = 3.1015 × 0.65 × 2 = when the wavelength is 405 nm. Φ4.032,
In the case of a DVD with a wavelength of 655 nm, the effective diameter of the incident parallel light flux is 3.2116 × 0.6277 × 2 = Φ4.032.

  In other words, the effective diameter of the incident parallel light beam is the same for both blue and DVD, and there is no equivalent to the DVD dedicated area in the first to fourth embodiments, and the entire surface of the A-side lens with Φ4.032 Blue / DVD common use area.

式（１）、（２）の値についてはＤ１〜Ｄ８の全ての隣接段差部において１．２３以下となっており、式（１）、（２）を全ての隣接段差部で満足している。 FIG. 20 shows a wavefront aberration diagram of the fifth embodiment. The blue RMS wavefront aberration is 0.03152 λrms, the DVD RMS wavefront aberration is 0.03237 λrms, and both blue and DVD are 0.035 λrms 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.

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 (6)

Laser beams having different wavelengths λi (i = 1, 2, 3, 4,...) Are incident on each of a plurality of types of optical recording media having different thicknesses of the transparent substrate,
An objective lens having a positive power for condensing the laser beam on an information recording surface provided on the transparent substrate of the optical recording medium,
On at least one lens surface,
An inner region where two types of laser beams of different wavelengths are focused correspondingly on information recording surfaces of two types of optical recording media having different transparent substrate thicknesses;
One of the two types of laser beams located outside the inner region and passing through the inner region is focused on the information recording surface of the corresponding optical recording medium. The beam comprises an outer region not focused on the information recording surface of the corresponding optical recording medium,
The inner region is divided into a plurality of sections in the radial direction from the optical axis,
Each of the plurality of sections includes an aspherical shape and an adjacent step that offset chromatic aberration caused by a difference in wavelength of the laser beam and spherical aberration caused by a difference in thickness of the transparent substrate of the optical recording medium. Is set,
Of the wavefront aberrations that occur when the laser beams of different wavelengths are condensed on the information recording surface of the corresponding optical recording medium, the maximum wavefront aberration is Wmax and the minimum wavefront aberration is Wmin. ,
Wmax ≦ 0.035λRMS
An objective lens characterized by satisfying The objective lens according to claim 1, wherein
1. Objective lens satisfying 1 ≦ Wmax / Wmin <1.8. The objective lens according to claim 2, wherein
Objective lens characterized by satisfying 1 ≦ Wmax / Wmin <1.6. The objective lens according to claim 3,
Objective lens characterized by satisfying 1 ≦ Wmax / Wmin <1.4. The objective lens according to any one of claims 1 to 4,
An objective lens characterized by satisfying W max ≦ 0.033λRMS. The objective lens according to claim 5,
An objective lens characterized by satisfying Wmax ≦ 0.030λRMS.

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