JP4316370B2 - Lens, optical system using the same, optical head, and optical disc apparatus - Google Patents

Description

  The present invention is a multi-wavelength optical system that uses a plurality of types of monochromatic light. For example, the present invention can be applied to different types of optical recording media such as CDs (compact discs including CDs such as CD-R) and DVDs (digital versatile discs). 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 compatible recording / reproducing apparatuses.

  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 typical example of such an objective lens, there is one described in Patent Document 1 (Japanese Patent Laid-Open No. 9-145959). 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 (Japanese Patent Laid-Open No. 2000-81566). 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 phase correction lens system in which an annular phase shifter is provided on the lens surface has also been proposed (Patent Document 3: Japanese Patent Application Laid-Open No. 2001-51192). In this objective lens, first, a lens surface used for a DVD so as to eliminate wavefront aberration due to a laser beam having a wavelength λ ₁ of 640 nm is used as a reference. Further, in the objective lens is divided into a refracting surface of a plurality of annular radially formed at a predetermined level difference of these refractive surfaces respectively from the reference lens surface (the i-th step difference from the lens center and 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, the light use efficiency is high, but the lens surface designed to eliminate wavefront aberration with respect to the DVD laser beam is used as a reference surface. so as to reduce the wavefront aberration for the laser beam of the CD, it has a 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. However, the wavefront aberration cannot be sufficiently reduced with respect to the laser beam of the CD simply by providing a level difference from the DVD as a reference.

  In addition, there is a Blu-ray standard that has recently been proposed using a blue laser with a shorter wavelength. In this case, backward compatibility is also required. In this case, the wavelength difference is larger than that between DVD and CD, and the difference in the refractive index of the lens is also different based on the wavelength difference. Therefore, the conventional method as described above provides good wavefront aberration in any medium. It becomes even more difficult to secure.

  An object of the present invention is to solve such a problem and to reduce the wavefront aberration as much as possible for each of a plurality of types of optical recording media having different transparent substrate thicknesses and to achieve high light utilization efficiency. It is an object of the present invention to provide a lens capable of condensing a beam on an information recording surface, an optical system using the lens, an optical head, and an optical disk device.

  In order to achieve the above object, the inventor of the present invention receives a light beam having a different wavelength for each of a plurality of types of optical recording media having different transparent substrate thicknesses, and refracts the light beam on the optical recording medium side. In a design method for an objective lens having a positive power for focusing, the lens surface is designed so as to almost cancel the spherical aberration caused by the difference in the thickness of the transparent substrate of the optical recording medium due to the chromatic aberration caused by the difference in the wavelength of the light beam. Have been filed earlier (Japanese Patent Application Nos. 2002-4993 and 2002-267451).

  The present invention improves the lens surface according to the above-described design, and a multi-wavelength lens for condensing a plurality of types of monochromatic light by refraction action using the fact that the focal position is different for each monochromatic light. As a technique for obtaining a good RMS wavefront aberration in a multi-wavelength optical system including the above, at least one lens surface of the lens is divided into a plurality of aspherical portions having different refractive powers in a common use region of all monochromatic lights. Each divided aspherical portion has a single focal point corresponding to the unique wavelength of each monochromatic light, and the focal points corresponding to the unique wavelength of each monochromatic light are arranged at different positions, respectively. The optical path length of any of the aspherical portions differs from the optical path length of the other aspherical portions by approximately an integer multiple of the wavelength λi of each monochromatic light, and the maximum value of the wavefront aberration of each monochromatic light in each of the aspherical portions is Minimum value difference ΔVd (λi) (d is an integer of 1, 2,..., Meaning each aspheric part, and i is an integer of 1, 2,...) The present invention provides for the first time a multi-wavelength optical system in which the ratio of the difference of each monochromatic light is 0.4 or more and 2.5 or less.

  In the wavefront aberration of each monochromatic light of the multi-wavelength optical system, it is preferable that the difference in wavefront aberration of the monochromatic light having the wavelength λi of each aspherical portion is 0.14λi or less. When the plurality of wavelengths are two wavelengths, for example, a two-wavelength optical system having a long wavelength near 790 nm and a short wavelength near 655 nm, or a long wavelength near 655 nm and a short wavelength near 405 nm. It is applicable to optical systems for two wavelengths, optical systems for two wavelengths having a long wavelength of about 405 nm and long wavelengths of about 790 nm, and further three-wavelength optical systems using these three wavelengths. It is preferable that the shape of the wavefront aberration is substantially symmetrical. The monochromatic light is preferably focused on the information recording surface so that the RMS wavefront aberration is 0.035λ or less, and each wavelength and each recording medium have different focal positions.

Depending on the relationship between λ ₁ and λ ₂ , the optical path length of an arbitrary aspherical portion differs from the optical path length of other aspherical portions by approximately 2 m times the wavelength λ ₁ (m is an integer, that is, m = ... -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, ...).

Further, in the above, the optical path length of any of the aspherical portions differs from the optical path length of the other aspherical portions by about 2 m times the wavelength λ ₁ (m is an integer, that is, m = ...− 4, −3, -2, -1,0,1,2,3,4,5, ...), and nearly a wavelength lambda ₂ light path length of any of the non-spherical surface portion with respect to the optical path length of the other non-spherical portion It is desirable that the difference is m times (m is an integer, that is, m = ...− 4, −3, −2, −1, 0, 1, 2, 3, 4, 5,...).

In the above preferred embodiment, the wavelength λ ₁ is 380 to 430 nm and the wavelength λ ₂ is 630 to 680 nm.

In the above, it is preferable that the difference between the lens refractive index at the wavelength λ _{1 and} the lens refractive index at λ ₂ is 0.03 or more.

  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, as can be seen from the above description, the present invention is 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 the unique wavelength of each monochromatic light. The focal points corresponding to the unique wavelength of each monochromatic light can be arranged at different positions, and can 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 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. The present invention reduces spherical aberration caused by the above-described difference in substrate thickness by utilizing such chromatic aberration. That is, a laser beam having a different wavelength is used for each optical disc having a different substrate thickness, and the spherical aberration caused by the difference in the substrate thickness is canceled by the chromatic aberration caused by the difference in the wavelength of the laser beam. In contrast, the overall aberration is within the allowable range.

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 difference in wavelength is the difference between the spherical aberrations (S _A (λ _2). ) -S _A (λ ₁ )). At this time, in the present invention, the lens surface is designed so that the following mathematical expression is established as much as possible.

  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

で表わされる。なお、光軸ＯＡ上での光路長Ｌ_hは、この数２において、ｈ＝０の場合である。 Then, the optical path length L _h from the point P ₁ to the condensing point P ₅ is

It is represented by Note that the optical path length L _h on the optical axis OA corresponds to the case where h = 0 in Equation 2.

This number 2 corresponds to an arbitrary ray height h, and when aberration correction is performed, the focal point P ₅ with respect to each ray height h is within the allowable range on the information recording surface 2a. It is in. That is, according to the present invention, 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, so that the condensing point P ₅ for each ray height h is obtained. It is intended to be on the information recording surface 2a within each allowable range.

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,..., Meaning each aspheric surface). In any of the aspherical portions, the ratio of ΔVd (λ ₁ ) to ΔVd (λ ₂ ) is 0.4 to 2.5, preferably 0.5 to 2.0 at both wavelengths. An RMS wavefront aberration within an allowable range can be secured for the entire lens.

Incidentally, the wavefront aberration mentioned here the ray height (h) an optical path length in the case of h = 0 and L _0, the optical path length of each ray height and Lh, the wavefront aberration Vh is expressed by the number 3 The

FIG. 6 schematically shows the wavefront aberration of the lens at the wavelengths of CD and DVD, with the horizontal axis representing the ray height, the vertical axis representing the wavefront aberration, and the upper side representing 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 first spherical region is defined by ΔV ₁ (λ ₁ ) and ΔV ₁ (λ ₂ ). In the present invention, as will be clarified in the following embodiments, 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 any aspherical portion. . That is, in the present invention, the lens surface is configured based on one of the conventional wavelengths and the phase shift is used only at the other wavelength even in the point that each aspheric surface has a constant distribution of wavefront aberration at any wavelength. This is different from the method of correcting wavefront aberration. The integer multiple is preferably other than 0 at least in the adjacent aspheric surface, and is preferably 0 to ± 10 times in the common use region, and preferably 0 to ± 5 times.

  In the multi-wavelength lens 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, 110 when the wavelength is 790 nm) in each region of any aspheric surface. .6 nm or less, 91.7 nm or less when the wavelength is 655 nm, 56.7 nm or less when the wavelength is 405 nm), preferably 0.12 λi or less, more preferably 0.10 λi or less. Even better optical properties can be ensured at the wavelength.

  Furthermore, in the present invention, in the case of an optical system for two wavelengths, the wavefront aberration of each wavelength can be balanced between the two wavelengths by using a multi-wavelength lens in which they are almost symmetrical, and further RMS (Root Mean Square). 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. 6) outside the common use area, and from the wavefront aberration of both the common use area and the dedicated area. The RMS 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. 6, 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, and the wavefront aberration of CD is 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.

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 disk 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.

First embodiment.
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. The lens according to the first 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, and 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 = 0.60. 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 = 0.60. 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 an approximate NA = 0.47-0.60 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 of the objective lens 1 is set so that the optical path length L _h expressed by the above equation 2 is set to a value within which the aberration is reduced to an allowable value with respect to an arbitrary light height h. The shape is set.

  In the first embodiment, 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 satisfactorily reduced within a tolerance for both DVD and CD. It is set.

で表わされる。なお、数４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 first 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 4 is for the j-th section.

For both DVD and CD, the range (the range of h) and its constants B, C, K, A ₄ , A ₆ , A for each section in Equation 4 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 1.

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 expressed by the following equation by the distance Z _B between the points c and d with respect to an arbitrary light beam height h.

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 t ₀ 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.

In Equation 5, 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.

The light incident side surface A represented by the above formula 4 and Table 1 of the objective lens 1 and the light exit side surface B represented by the above formula 5 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 t ₀ 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 a 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, however, n types (where n is an integer of 2 or more) different wavelengths λ _i (where, The same applies when i = 1, 2,..., n).

(ii) Further, when n types of wavelengths λ _i are used in this way, the RMS wavefront aberration when the incident laser beam of these wavelengths λ _i has an incident angle of 0 ° is denoted by W _i · λ _i. These aberrations are

(Where the wavelength of the i-th light beam is λ _i (i = 1, 2,...), The sum of squares of individual RMS wavefront aberrations over all wavelengths is ΣW _i ² , and the wavelength λ _{i is} (The RMS wavefront aberration of the light beam is W _i · λ _i )

To be satisfied. 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 DVD is W ₁ , the RMS wavefront aberration of CD is W ₂ , and i = 1, 2

となる。 Therefore, the above equation 6 is

It becomes.

(iii) Alternatively, when using a laser beam of different n types of wavelengths lambda _i, Wmax the maximum RMS wavefront aberration among the wavelengths of respective lambda _i, when the minimum RMS wavefront aberration and Wmin,

And In this case, the allowable value W _th 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 0.01945λ ₁ . FIG. 4B shows the RMS wavefront aberration with respect to the CD (wavelength λ ₂ = 790 nm). When the image height is 0 mm, the RMS wavefront aberration is 0.02525λ ₂ .

FIG. 7 shows the result of calculating the wavefront aberration in the common use region of the above-mentioned lens, and Table 2 shows the difference in wavefront aberration and the ratio thereof in each aspheric surface.

  As shown in Table 2, the ratio ΔVd (λ790) / ΔVd (λ655) between the wavefront aberrations in the common use region of 790 nm and 655 nm is in the range of 1.00 to 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. Table 3 shows how many times the approximate optical path length of each aspherical portion of the DVD / CD common use region is shifted by the wavelength λ when the approximate optical path length of the first section is used as a reference. In Table 3, the first to sixth sections are DVD / CD common use areas, and the seventh and eighth sections are DVD dedicated use areas.

  The difference between the approximate optical path length of the second to eighth sections and the approximate optical path length of the first section is mλ (m is an integer) for a DVD with a wavelength of 655 nm and a CD with a wavelength of 790 nm.

  In order to evaluate such a lens, 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.

(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,

  Therefore, the allowable value is 1.8, preferably 1.6, and more preferably 1.4 or less.

  FIG. 5 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 5 and the light incident side surface A having the surface shape shown in the above equation 4 and Table 1. The horizontal axis represents the position in the direction perpendicular to the optical axis with the optical axis on the information recording surface as the reference point, and the vertical axis represents the calculation result of the optical spot. The relative light intensity at each position when the light intensity at the reference point (= 0 mm) is 1 is shown.

FIG. 5A shows a light spot with respect to a DVD, and the light spot diameter φ _D at which the relative light intensity is 1 / e ² (= 13.5%) is 0.89 μm. FIG. 5B shows a light spot with respect to the CD, and the light spot diameter φ _C 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.

In the first embodiment, in Table 2, the ratio is 0.96 to 1.04, and the RMS wavefront aberration is 0.01945λ ₁ for DVD and 0.02525λ ₂ for CD. If the wavefront aberration of DVD is further deteriorated and the wavefront aberration of CD is improved, the RMS wavefront aberration of about 0.022 to 0.023λ can be obtained as the RMS wavefront aberration for both DVD and CD.

  As an example, when looking at the RMS wavefront aberration of DVD and CD described in Patent Document 3 (Japanese Patent Laid-Open No. 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.

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 ΔVd (λ655) / ΔVd (λ790) as shown in Table 4 and FIG. 8 below. Is outside the range of the present invention, and the balance between them is deviated from 0.23 to 3.44 and ΔVd (λ790) / ΔVd (λ655) are 0.03 to 4.72. 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.

は、上記夫々について、0.0332，0.0294となり、いずれも上記の許容値0.028、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を越えているし、さらに、これらのＷ_max／Ｗ_minも夫々、47，1.847となり、いずれも上記の許容値1.8、好ましくは1.6、さらに好ましくは1.4を越えている。 Also these

Is 0.0332, 0.0294 for each of the above, both of which exceed the allowable value of 0.028, preferably 0.026, more preferably 0.025, more preferably 0.023, and these W _max / W _min are also respectively 47 and 1.847, both of which exceed the allowable value of 1.8, preferably 1.6, and more preferably 1.4.

  As described above, in the first embodiment, the aberration can be suppressed within the above allowable value. This is because the spherical aberration and the chromatic aberration due to the difference in the substrate thickness are set so that the aberration is within the allowable value. This is because the lens surface shapes cancel each other. On the other hand, in Patent Document 3 (Japanese Patent Application Laid-Open No. 2001-51192), the aberration of the CD is reduced by simply shifting the phase of the incident laser beam by an integral multiple of the wavelength of the DVD laser beam. Therefore, even if the aberration can be suppressed sufficiently small for any one wavelength, the aberration cannot be simultaneously accommodated within the allowable value of the small value as described above for all wavelengths. It is.

  The distance between the surface vertices f and e on the optical axis of the objective lens, that is, the center thickness t0 is 2.2 mm. The thickness and refractive index of the transparent substrate are 0.6 mm thick and 1.58 refractive index at wavelength λ1 = 655 nm (DVD), and 1.2 mm thick and refractive index 1.57 at wavelength λ2 = 790 nm (CD). It is.

  The NA for a DVD with a wavelength of 655 nm is 0.60, the focal length is 3.36 mm, the NA for a CD with a wavelength of 790 nm is 0.47, and the focal length is 3.3833 mm. Effective diameter on the A plane side = focal length × NA × 2. As can be seen from the values of NA and focal length, when the DVD has a wavelength of 655 nm, the effective diameter on the A-side is 3.36 × 0.6 × 2 = φ4.032, and when the CD has a wavelength of 790 nm. Is the A-plane side effective diameter = 3.3833 × 0.47 × 2 = φ3.180.

  The effective diameter on the A side is φ3.18, that is, 0 to 1.589366 in the range of h, that is, the sections 1 to 6 shown in Table 2 are common use areas used for both CD and DVD, and from φ3.18 The outer section, that is, sections 7 and 8 are DVD-only use areas. However, the incident laser beam light is also incident on the DVD dedicated use area at the time of CD, and the light becomes so-called flare light with very large aberration on the information recording surface of the CD, and does not have a harmful effect. . This is also explained from the light spot diagram of FIG.

  In the first 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 the chromatic aberration due to the difference in wavelength between 655 nm and 790 nm, and the total aberration is reduced. It is clear from the light spot shown in FIG. 5 and the wavefront aberration graph shown in FIG. In the first embodiment, the surface shape of the light incident side surface A of the objective lens 1 is given by Equation 4 and Table 1, and the surface shape of the light emission side surface B is given by the aspherical formula shown in Equation 5 above. Therefore, the diffractive lens structure as in the prior art is not used, and almost all the light flux can be condensed with respect to the aperture (NA) necessary for recording or reproduction, so that high light utilization efficiency is achieved. Will be obtained.

  In the first embodiment, 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 = 0.60 is used only for DVD and not for CD. Either one or both of the light incident side surface A and the light output side surface B in the outer region is subjected to a thin film treatment that transmits light with a wavelength of 655 nm for DVD and does not transmit light with a wavelength of 790 nm for CD. In this outer region, either one or both of the light incident side surface A and the light exit side surface B do not act on light having a dual wavelength of 655 nm, but form a diffraction grating that acts on light having a wavelength of 790 nm, thereby forming a wavelength of 655 nm. The light utilization efficiency of the two wavelengths 790 nm may be decreased without reducing the light utilization efficiency.

  That is, as in the first embodiment, when the diaphragm according to the numerical aperture cannot be set when sharing the system with different numerical apertures, the optical system having a small numerical aperture also accepts an extra light beam. 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 first embodiment, two types of optical disks, DVD and CD, are taken as examples. 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 changed so that the wavelength of the laser beam used is different for each, and the chromatic aberration cancels the wavefront aberration accordingly. You only have to set 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.

Second embodiment.
In the second embodiment, the substrate thickness is different and the wavelengths are different from 405 nm and 655 nm. The case where the substrate thickness is 0.1 mm at a wavelength of 405 nm using a so-called Blu-ray or blue laser, and the case where the substrate thickness is 0.6 mm at a wavelength of 655 nm using a so-called DVD.

  In the second embodiment, the basic lens configuration is the same as that of the first embodiment shown in FIG. 2, and recording is performed on a disk substrate (not shown) on the B surface side by entering parallel light from the A surface side. A good light spot is formed on the surface.

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２．０７６ｍｍであって、波長λ₁＝４０５ｎｍ（ブルー）での屈折率ｎは１．８３１６４であり、波長λ₂＝６５５ｎｍ（ＤＶＤ）での屈折率ｎは１．７９１１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。すなわち、波長λ₁＝４０５ｎｍ（ブルー）と波長λ_２＝６５５ｎｍ（ＤＶＤ）のそれぞれの屈折率の差は０．０３以上ある。表5においてＲは曲率半径を、「小」は光軸側を、「大」は光軸から離れた側をそれぞれ示す。 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 22 in Table 5. 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 5. The specific numerical values are shown in Table 5.

Further, the distance between the surface vertices f and e on the optical axis of the objective lens, that is, the center thickness t ₀ is 2.076 mm, and the refractive index n at the wavelength λ ₁ = 405 nm (blue) is 1.83164. And the refractive index n at the wavelength λ ₂ = 655 nm (DVD) is 1.7911. 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. That is, the difference in refractive index between the wavelength λ ₁ = 405 nm (blue) and the wavelength λ ₂ = 655 nm (DVD) is 0.03 or more. In Table 5, R represents the radius of curvature, “small” represents the optical axis side, and “large” represents the side away from the optical axis.

  The NA for blue at a wavelength of 405 nm is 0.85, the focal length is 1.765 mm, the NA for DVD with a wavelength of 655 nm is 0.60, and the focal length is 1.8564 mm. The effective diameter on the A surface side = focal length × NA × 2, and as can be seen from the values of the NA and the focal length, in the case of Blu-ray having a wavelength of 405 nm, the effective diameter on the A surface side = 1.765 × 0.85 × When 2 = φ3.00 and a DVD with a wavelength of 655 nm, the A-side effective diameter = 1.8564 × 0.60 × 2 = φ2.228.

  As can be seen from Table 5, up to effective diameter φ 2.228 on the A side, that is, 0 to 1.114 in the range of h, that is, sections 1 to 21 shown in Table 5 are commonly used for both DVD and Blu-ray. An area outside φ2.228, that is, an area larger than 1.114 in the range of h, that is, an area 22 is a Blu-ray dedicated use area. However, the incident laser beam light is also incident on the Blu-ray exclusive use area at the time of DVD, and the light becomes so-called flare light with very large aberration on the information recording surface of the DVD, and does not have a harmful effect. . This is also explained from the light spot diagram of FIG.

  FIG. 9 shows a wavefront aberration diagram of the second embodiment. As the RMS wavefront aberration value, the Blu-ray RMS wavefront aberration is 0.02410 λrms, the DVD RMS wavefront aberration is 0.02753 λrms, and both Blue and DVD are 0.035 λrms or less.

Further, in each aspherical portion shown in Table 5, when the approximate optical path length of the first section is used as a reference, the approximate optical path lengths of the second to 21st common areas of Blu-ray / DVD are each shifted by about several times the wavelength λ. Table 6 shows whether or not.

As can be seen from Table 6, the second to 21st sections have a difference of 2 mλ for Blu-ray with a wavelength of 405 nm, and a difference of mλ (m is an integer) for a DVD with a wavelength of 655 nm. This is because the shorter wavelength λ ₁ is between 380 and 430 nm, and the longer wavelength λ ₂ is between wavelengths 630 and 680 nm, so that it is easy to satisfy the relationship of the above approximate optical path lengths. It is easy to obtain the favorable wavefront aberration shown in FIG. Further, since the refractive index of the lens is the value shown above, that is, the difference between the refractive index at 405 nm and the refractive index at 655 nm is larger than 0.04054 and 0.03, the difference in the approximate optical path length and the good wavefront are also obtained. It is easier to obtain aberrations.

表７に示すように、６５５ｎｍと４０５ｎｍの共通使用領域において各波面収差の差の比ΔＶｄ(λ655)／ΔＶｄ(λ405)は、０．９０〜１．６５の間に入っている。また、比ΔＶｄ(λ405)／ΔＶｄ(λ655)は、０．６０〜１．１１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。また第２の実施形態の光スポット図を図１０に示す。1/e²（＝0.135）の相対光強度となる光スポット直径は、４０５ｎｍのブルーレイのときで０．３８３６μｍで、６５５ｎｍのＤＶＤで０．８５７０μｍとなっており、問題ない光スポット形状となっている。 Table 7 shows the difference in wavefront aberration and the ratio thereof in each aspherical part shown in Table 6.

As shown in Table 7, the ratio ΔVd (λ655) / ΔVd (λ405) between the wavefront aberrations in the common use region of 655 nm and 405 nm is in the range of 0.90 to 1.65. Further, the ratio ΔVd (λ405) / ΔVd (λ655) is in the range of 0.60 to 1.11. The wavefront aberration of each region is also 0.14λ or less at both wavelengths. Moreover, the light spot figure of 2nd Embodiment is shown in FIG. The light spot diameter, which is the relative light intensity of 1 / e ² (= 0.135), is 0.3836 μm for a 405 nm Blu-ray and 0.8570 μm for a 655 nm DVD. Yes.

  In the second embodiment, the wavelength of one monochromatic light is 405 nm and the other is 655 nm, but one may be 380 to 430 nm and the other may be 630 to 680 nm.

Configuration of optical head.
FIG. 11 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, and 16 is a 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 obtained by branching the laser beam from the CD laser 12 into three beams of zero-order light and soil primary light by the diffraction grating 18, and these ± first-order light causes tracking error. 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.

Configuration of optical disk device.
FIG. 12 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, 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. As a result, in the optical head, a laser beam having 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 shown in FIG. 11. 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.

It is a figure which shows embodiment of the objective lens by this invention. It is a figure which shows one specific example of the lens surface shape of embodiment of this invention. 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. 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. 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 1st 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 graph which shows one specific example of the measurement result of the wavefront aberration of 2nd Embodiment. 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 2nd Embodiment. 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.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Objective lens 2 Transparent substrate 2a of DVD Information recording surface 3 Transparent substrate 3a of CD 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 Lattice 19 Actuator 20 Actuator drive circuit 21 Signal processing circuit 22 Laser drive circuit 23 System control circuit 24 Disc discriminating means

Claims (2)

There is a multi-wavelength lens that condenses two types of monochromatic light by refraction, respectively, and the lens has at least one lens surface on a plurality of aspherical portions having different refractive powers in a common use region for all monochromatic light. Each of the divided aspherical portions has a single focal point corresponding to the unique wavelength of each monochromatic light, and the focal points corresponding to the unique wavelength of each monochromatic light are arranged at different positions. The optical path length of an arbitrary aspheric surface portion is different from the optical path length of other aspheric surface portions by an integer multiple of the wavelength λp of each monochromatic light, and the wavefront aberration of each monochromatic light at each aspheric surface portion is different. The difference between the maximum value and the minimum value is ΔVd (λp) (d is an integer of 1, 2,..., Meaning each aspheric surface part, p is an integer of 1 or 2, and λ ₁ <λ ₂ ) when a, in any of the non-spherical portion, .DELTA.Vd (lambda ₁ The ratio of the .DELTA.Vd (lambda ₂₎ and is not less 0.4 or more and 2.5 or less,
The optical path length of an arbitrary aspherical portion differs from the optical path length of other aspherical portions by approximately 2 m times the wavelength λ ₁ (m is an integer, that is, m = ...− 4, −3, −2, − , 1, 0, 1, 2, 3, 4, 5,...), And the optical path length of any of the aspherical portions differs from the optical path length of the other aspherical portions by approximately m times the wavelength λ ₂ ( m is an integer, that is, m = ...− 4, −3, −2, −1, 0, 1, 2, 3, 4, 5,. The lens according to claim _1, wherein the wavelength λ ₁ is 380 to 430 nm, and the wavelength λ ₂ is 630 to 680 nm .

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