JP4385038B2 - Objective lens - Google Patents

Description

  The present invention is a multi-wavelength optical system using a plurality of types of monochromatic light. For example, optical recording media of different types such as CD (compact disc: including CD such as CD-R) and DVD (digital versatile disc). The present invention relates to an objective lens design method, an objective lens, a general-purpose multi-wavelength lens, a multi-wavelength optical system, an optical head, and an optical disc apparatus that can be used in a compatible recording / reproducing apparatus that can handle the above.

  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. As will be described later, the wavelength of the laser beam differs depending on whether it is used in a CD or a DVD. 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 proposed in recent years uses a blue laser having a wavelength of about 400 nm for reproducing information. 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 Patent Document 1 is divided into three or more annular lens surfaces in the radial direction. Every other annular lens surface and every other annular lens surface have refractive power. It is different. 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.

  Other representative examples include those described in Patent Document 2 or Patent Document 3. These documents use 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 device 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 (see, for example, Patent Document 4).

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 laser beam of the DVD is phase-shifted by an integer mi 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 US Pat. No. 6,118,594 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 or the said patent document 3, 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 4, the light use efficiency is high, but the lens surface designed to eliminate the 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. 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 4, 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. It is described in many prior arts that the wavefront aberration value may be 0.07λ RMS or less of the Marechal evaluation standard as the RMS wavefront aberration value. However, in the case of an optical disc apparatus, 0.07λ RMS or less is a value that should be a target value for the entire optical disc apparatus, and is still insufficient as a target value for the objective lens alone. The optical disk device as a whole has many factors that degrade the RMS wavefront aberration, such as the laser astigmatism, the aberration of the collimator lens, the aberration of the reflection mirror and the transmission mirror, and the tilt shift between the optical pickup and the optical disk. Therefore, it is required that the RMS wavefront aberration value be as small as possible, not 0.07λ RMS or less. Specifically, it is desirable that the objective lens alone has an RMS wavefront aberration value of about 0.02λ RMS. However, in Patent Document 4, in Example 1, DVD is 0.001λ RMS and CD is 0.047λ RMS. 2, DVD is 0.019λRMS and CD is 0.037λRMS, and the wavefront aberration of DVD is a good value, but it is only 0.037λRMS or more for CD, and both DVD and CD are good. The wavefront aberration value has not been obtained.

  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. An object of the present invention is to provide an objective lens design method capable of condensing a beam on an information recording surface, and an optical system, an optical head, and an optical disk device using the lens and the lens.

  In order to achieve the above object, in the present invention, a light beam having a different wavelength is incident on each of a plurality of types of optical recording media having different transparent substrate thicknesses, and the light beam is condensed on the optical recording medium side by refraction. In the design method of an objective lens having a positive power, the lens surface is designed so as to almost cancel out 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. Invented a method.

Further, according to the present invention, a light beam having a different wavelength is incident on each of a plurality of types of optical recording media having different transparent substrate thicknesses, and the light is incident on an information recording surface provided on the transparent substrate of the optical recording medium. An objective lens having a positive power for condensing the beam by refraction, and canceling spherical aberration caused by the difference in the thickness of the transparent substrate of the optical recording medium due to chromatic aberration caused by the difference in wavelength λ of the light beam. In any type of optical recording medium, the light beam is condensed on the information recording surface with an RMS wavefront aberration of 0.035λ or less, preferably 0.033λ or less, more preferably 0.030λ or less.
Alternatively, RMS wavefront aberration is applied to the information recording surface with a plurality of corresponding light beams, and the wavelength of the i-th light beam is λ _i (i = 1, 2,...),

(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 )
Preferably, the light is condensed at a value on the left side of the above expression of 0.026 or less, more preferably 0.025 or less, and even more preferably 0.023 or less, or each corresponding light beam is RMS on the information recording surface. The wavefront aberration ratio is preferably Wmax / Wmin ≦ 1.8, more preferably Wmax / Wmin ≦ 1.6, where Wmax is the maximum RMS wavefront aberration and Wmin is the minimum RMS wavefront aberration among the RMS wavefront aberrations of the light beam. Is an objective lens for condensing at Wmax / Wmin ≦ 1.4 or a light beam having a different wavelength for each of a plurality of types of optical recording media, and the information recording surface provided on the transparent substrate of the optical recording media An objective lens having a positive power for condensing a light beam by refraction, and for any type of optical recording medium, the corresponding light beam is applied to the information recording surface with an RMS wavefront aberration of 0.035λ or less. Condensation The objective lens characterized by making it provide is provided.

Further, the present invention utilizes the fact that the focal position is different for each monochromatic light, and has a good RMS wavefront in a multiwavelength optical system including a multiwavelength lens that collects a plurality of types of monochromatic light by refraction. As a method for obtaining aberration, 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 light, and any of the divided aspherical portions is used. Each of the monochromatic lights has a single focal point corresponding to a unique wavelength, the focal points corresponding to the unique wavelength of each monochromatic light are arranged at different positions, and the optical path length of any of the aspherical portions is Unlike nearly an integral multiple of the wavelength lambda _i of the optical path length of the other non-spherical portion and the respective monochromatic light, wherein the difference between the maximum value and the minimum value of the wavefront aberration of the monochromatic light at each aspheric portions △ Vd (λ _i ) (D is 1, 2, ... , I is an integer of 1, 2,...), The difference ratio of each monochromatic light is 0.4 or more and 2.5 in any aspheric part. The following multi-wavelength optical system is provided for the first time.

Further, in the wavefront aberration of each monochromatic light in the multi-wavelength optical system, it is preferable that the difference in wavefront aberration in the monochromatic light having the wavelength λ _i of each aspherical portion is 0.14λ _i or less. In the case where 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 can be applied to a wavelength optical system, a two-wavelength optical system having a long wavelength of about 405 nm and a long wavelength of about 790 nm, and a three-wavelength optical system using these three wavelengths. In such a case, it is preferable that the wavefront aberrations are substantially symmetrical.

  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 SA (t ₁ ), the spherical aberration when the thickness is t ₂ is SA (t ₂ ), and the spherical aberration that occurs with respect to the laser beam having the wavelength λ _1. SA (lambda _1), when the spherical aberration caused with respect to the laser beam having the wavelength λ2 and SA (lambda _2), chromatic aberration caused by different wavelengths, the difference of the spherical aberration _{(SA (λ 2) -SA (} λ 1 )). At this time, in the present invention, the lens surface is designed so that the following mathematical formula 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 examples. 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

It is represented by The optical path length L _h on the optical axis OA corresponds to the case where h = 0 in the equation (4).

This equation 4 corresponds to an arbitrary light beam height h, and when aberration correction is performed, the focal point P ₅ for each light beam 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 both of these wavelengths are used in common is divided into a plurality of aspherical portions. In the method of using the 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 λ _i of each monochromatic light, and each of the aspherical portions is The difference between the maximum value and the minimum value of the wavefront aberration of monochromatic light is ΔVd (λ ₁ ) and ΔVd (λ ₂ ) (d is an integer of 1, 2,. Sometimes the ratio of the difference between each monochromatic light in any aspheric part is 0.4 or more and 2.5 or less, preferably 0.5 or more and 2.0 or less. RMS wavefront aberration can be ensured. 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 5 The

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 ΔV1 (λ ₁ ) and ΔV1 (λ ₂ ). In the present invention, as will be clarified in the following examples, the ratio of the difference between the maximum value and the minimum value of the wavefront aberration at 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 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 of the present invention, the difference between the maximum value and the minimum value of the wavefront aberration at each wavelength is 0.14λ _i or less (for example, when the wavelength is 790 nm) in each region of any aspherical surface. ± 110.6 nm or less, and when the wavelength is 655 nm, ± 91.7 nm or less), preferably 0.12 λ _i or less, more preferably 0.10 λ _i or less, and even better optical characteristics at each wavelength Can be secured.

  Furthermore, in 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 are balanced, and the RMS wavefront aberration is further reduced. be able to.

  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.

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.

  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 a numerical aperture NA of about 0.47 to 0.63 is indicated 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 expressed by the above equation 4 is within the allowable value by reducing the aberration with respect to an arbitrary beam height h. Is set. Hereinafter, a specific example of the lens surface shape will be described with reference to FIG.

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 ₆ , when the values of the coefficients C, K, A ₄ , A ₆ , A ₈ , A ₁₀ are substituted to determine 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.

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 emission side surface B represented by the above equation 6 of the objective lens 1 and the light incident side surface A represented by the point sequence data of 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 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 W0 at this time is 0.028, preferably 0.026, more preferably 0.025, and further preferably 0.023. In the first embodiment, the RMS wavefront aberration of the DVD is W ₁ , the RMS wavefront aberration of the CD is W ₂ , and i = 1, 2, the above formula 7 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,
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.

(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 = 0.02410 / 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 6 and the 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 φ _D 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 φ _C 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.

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 10 is for the j-th section.

For both DVD and CD, the range (range h) and its constants B, C, K, A ₄ , A ₆ , A for each section of Equation 10 for reducing aberrations well 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 formula 11.

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.

  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 .

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 consists 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.

(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 11 and the light incident side surface A having the surface shape shown in the above equation 10 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 φ _D 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 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 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 above description, if the DVD wavefront aberration is improved by further deteriorating the DVD wavefront aberration in the common use region, the RMS wavefront aberration of both DVD and CD is equivalent to about 0.022 to 0.023λ. RMS wavefront aberration can also be used.

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.

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 present 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 the above, both of which exceed the allowable value of 0.028, preferably 0.026, more preferably 0.025, and even more preferably 0.023, and these Wmax / Wmin are 47, 1.847, both of which exceed the above tolerance of 1.8, preferably 1.6, and more preferably 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 10 and Table 2, and the surface shape of the light exit side surface B is expressed by Equation 6 above. Since it is given by the aspherical formula shown in Equation 11, the diffractive lens structure as in the prior art is not used, and almost all the light flux is collected with respect to the aperture (NA) necessary for recording or reproduction. Since light can be emitted, high light utilization efficiency can be obtained.

  In the above embodiment, as shown in FIG. 1, the outer region of the objective lens 1 having a numerical aperture NA = 0.47 to a numerical aperture NA = 0.63 is used only for DVD and not used for CD. Either one or both of the light incident side surface A and the light emitting side surface B in the 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, or In such an outer region, either one or both of the light incident side surface A and the light exit side surface B does not act on light having a dual wavelength of 655 nm, but forms a diffraction grating that acts on light having a wavelength of 790 nm, and forms a wavelength of 655 nm. The light utilization efficiency of the two wavelengths 790 nm may be decreased without decreasing the light utilization efficiency.

  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 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.

  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 an information recording / reproducing signal can be obtained satisfactorily.

The following points were found for the second embodiment. That is, as the lens surface shape for obtaining the above-mentioned wavefront aberration, all the monochromatic surfaces of at least one lens surface in a multi-wavelength lens that condenses a plurality of types of monochromatic light of DVD and CD by their respective refraction effects. D12, D23, D34, D45,..., The NA of the adjacent portion during DVD in order of the adjacent step amount between the plurality of aspherical portions having different refractive indexes in the light common use region in order from the lens optical axis. NA12, NA23, NA34, NA45,... In the order of closeness of the optical axis, and the NA at the time of CD of the adjacent part as CNA12, CNA23, CNA34, CNA45,. The lens surface shape should satisfy the following formula (14).

In Equation 14, λ ₁ is the shorter wavelength (mm), λ ₂ is the longer wavelength (mm), n ₁ is the refractive index of the lens at wavelength λ ₁ , and n ₂ is the refractive index of the lens at wavelength λ ₂ . is there. In the mathematical formula, ma is an integer closest to aij. Mb is an integer closest to bij. In other words, ma is an integer that minimizes the absolute value of ma-aij, and mb is an integer that minimizes the absolute value of mb-bij. For example, it is “1” in the case of 1.104, and “0” in the case of 0.49. The symbols used in Equation 14 are shown in FIG. As shown in FIG. 13, in this example, the lens has a DVD / CD common use area and a DVD dedicated area. The DVD / CD common use area has six aspherical surfaces (1) to (6). And the level | step difference of these aspherical surfaces is represented as D12, D23 .... In the case of a DVD, the NA between the steps D12 is NA12, and the NA between the steps D23 is NA23. In the case of a CD, the NA between the steps D12 is CNA12, and the NA between the steps D23 is CNA23. In Equation 14, Aij / Bij is in the range of 0.333 to 3, but a more preferable range is in the range of 0.5 to 2.

In addition, specific numerical values in this example are summarized in Table 5.

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 Example 2. FIG. 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. FIG. 10 is a diagram for explaining mathematical formulas in Embodiment 2.

Explanation of symbols

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

Claims (6)

For each of a plurality of types of optical recording media having different transparent substrate thicknesses , a monochromatic light beam having different wavelengths λ _i (i = 1, 2, 3,...) Is condensed through the transparent substrate, An objective lens having a positive power for forming a light spot on an information recording surface provided on the transparent substrate of an optical recording medium,
Lens surface area through which the collimated light beams of different wavelengths lambda _i which focused on the information recording surface of different kinds of optical recording medium is divided into multiple sections in a radial direction from the optical axis by the step,
Chromatic aberration caused by the difference in wavelength λ _i of the light beam and the optical recording medium in each section so that the maximum value of the wavefront aberration in the section of each optical recording medium by each light beam is less than the allowable value. an objective lens, wherein the aspherical shape of the spherical aberration generated by the difference in the transparent substrate thickness cancel is set. The allowable value is 0.14λ. _i The objective lens according to claim 1, wherein: 2. The objective lens according to claim 1, wherein an incident surface of the lens surface region is divided into a plurality of sections in a radial direction from an optical axis, and the exit surface is a single aspherical surface. For each of the two types of optical recording media with different thicknesses of the transparent substrate, two types of wavelength λ, long and short _i The light beam of monochromatic light (i = 1, 2) is condensed with light having a long wavelength through a thick transparent substrate and light with a short wavelength through a thin transparent substrate. An objective lens having a positive power for forming a light spot on an information recording surface provided on a transparent substrate,
Two types of wavelength, λ, which are condensed on the information recording surface of two types of thick and thin optical recording media _i The lens surface area through which the parallel light beam passes is divided into a plurality of sections in the radial direction from the optical axis by the step,
In each of the sections, the wavelength λ of the light beam is set so that the maximum value of the wavefront aberration in the section of each optical recording medium by each light beam is less than the allowable value. _i An objective lens having an aspherical shape in which chromatic aberration caused by the difference between the spherical aberration and spherical aberration caused by the difference in thickness of the transparent substrate of the optical recording medium cancel each other is set. Different wavelengths λ for multiple types of optical recording media with different thickness of transparent substrate _i A light beam of monochromatic light (i = 1, 2, 3,...) Is condensed through the transparent substrate to form a light spot on the information recording surface provided on the transparent substrate of the optical recording medium. An objective lens having positive power,
Different wavelengths λ focused on the information recording surface of different types of optical recording media _i The lens surface region through which the light beam passes is divided into a plurality of sections in the radial direction from the optical axis by steps,
The difference between the maximum value and the minimum value of the wavefront aberration in each section of each optical recording medium by each light beam is expressed as ΔVd (λ _i ) (D is an integer of 1, 2,..., Meaning each section and i is an integer of 1, 2,...), The ratio of the difference of each monochromatic light in any section The wavelength λ of the light beam is in each of the sections so that is 0.4 to 2.5. _i An objective lens having an aspherical shape in which chromatic aberration caused by the difference between the spherical aberration and spherical aberration caused by the difference in thickness of the transparent substrate of the optical recording medium cancel each other is set. An information recording surface provided on the transparent substrate of the optical recording medium by collecting a monochromatic light beam of a different wavelength through the transparent substrate for each of a plurality of types of optical recording media having different thicknesses of the transparent substrate. An objective lens having a positive power to form a light spot on
The exit surface consists of a single aspheric surface,
The incident surface is divided into a plurality of sections by a step in the radial direction from the optical axis through which the lens surface region through which the light beams of different wavelengths condensed on the information recording surface of different types of optical recording media pass. Also for the optical recording medium, the RMS wavefront aberration ratio is given by setting the corresponding light beam to Wmax as the maximum wavefront aberration of the wavefront aberration of each optical recording medium due to the light beam and Wmin as the minimum wavefront aberration.
1 ≦ Wmax / Wmin <1.8
And Wmax ≦ 0.035λRMS
The chromatic aberration generated due to the difference in the wavelength λ of the light beam and the wavefront aberration generated due to the difference in the thickness of the transparent substrate of the optical recording medium cancel each other so that the light is condensed under the condition satisfying An objective lens, wherein an aspherical shape is set together with a step amount.

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