JP4443368B2 - Objective lens - Google Patents

Description

  The present invention relates to an objective lens used in an optical information recording or reproducing apparatus for recording or reproducing information on or from a plurality of types of optical disks having different recording densities and protective layer thicknesses.

  There are multiple standards for optical disks with different recording densities and protective layer thicknesses. For example, a DVD (digital versatile disk) has a higher recording density and a protective layer is thinner than a CD (compact disk). Therefore, when using different optical discs for recording or reproducing information, the numerical aperture (NA) of light used for recording or reproducing information is corrected while correcting spherical aberration that changes depending on the thickness of the protective layer. Therefore, it is necessary to obtain a beam spot corresponding to the difference in recording density.

  For example, for recording or reproduction of an optical disc having a relatively thin protective layer thickness and a high recording density, it is necessary to narrow the beam spot with a higher NA than an optical system dedicated to an optical disc having a relatively thick protective layer thickness and a low recording density. Since the spot diameter becomes smaller as the wavelength is shorter, an optical system using DVD uses a laser light source having an oscillation wavelength of 635 to 665 nm shorter than 780 to 830 nm used in an optical system dedicated to CD. Therefore, in recent years, a light source unit that can oscillate laser beams having different wavelengths is used in an optical information recording / reproducing apparatus.

Further, as a means for converging laser light to the recording surface position of each optical disc in a good state for a plurality of types of optical discs having different protective layer thicknesses, for example, the following Patent Documents 1 to 3 have been conventionally used. The disclosed objective lens is known.
JP 2001-243651 A Japanese Patent Laid-Open No. 9-179021 JP 2001-76367 A

  Patent Document 1 discloses a configuration in which two types of parallel light beams having different wavelengths are incident on an objective lens provided with a diffraction structure having a ring-shaped fine step on one surface. The configurations disclosed in Patent Literature 2 and Patent Literature 3 also use two types of light fluxes having different wavelengths according to the use of two types of optical discs having different standards, as disclosed in Patent Literature 1. Patent Document 2 discloses a configuration in which a parallel light beam is incident on an objective lens when an optical disk having a high recording density such as a DVD is used, and a divergent light beam is incident on an optical disk having a low recording density such as a CD. Further, Patent Document 3 discloses that an objective lens provided with a diffractive structure having a ring-shaped fine step on one surface is used to record information on a CD rather than a divergence angle of a divergent light beam used when recording or reproducing information on a DVD. A configuration is disclosed in which the divergence angle of the divergent light beam used during recording or reproduction is smaller.

  By the way, an objective lens for realizing recording or reproduction of information on a plurality of types of optical discs having different standards corrects spherical aberration when the laser beam corresponding to each optical disc is converged on the recording surface of each optical disc. In addition, the coma aberration generated when the light beam is incident on the objective lens obliquely with respect to the optical axis is required to be well corrected. However, a configuration in which the magnification of the objective lens at the time of using the CD is the same as that at the time of using the DVD as exemplified in Patent Document 1 above, or the magnification of the objective lens at the time of using the CD with respect to the time of using DVD as exemplified in Patent Literature With a small configuration, it is difficult to satisfactorily suppress coma generated when a light beam is incident obliquely with respect to the optical axis when each optical disk is used. Conventionally, the objective lens is merely configured to adjust the balance of coma aberration generated when each optical disk is used in correspondence with the application of the optical information recording / reproducing apparatus. For example, the tolerance of aberration is low when information is recorded or reproduced on a DVD having a high recording density. For this reason, conventional objective lenses are often designed to suppress coma as much as possible when using a DVD even if the coma generated when using a CD is slightly increased. In the configuration exemplified in Patent Document 3, the distance from the light source to the disk surface is the same when using DVD and CD, and the magnification of the objective lens when using CD is slightly larger than when using DVD. Therefore, the configuration of Patent Document 3 can slightly correct the amount of coma generated when using each optical disc. However, since the magnification difference is small, the coma aberration cannot be effectively reduced.

  Accordingly, there has been a strong demand for further improvement of the objective lens so that high-precision information can be recorded or reproduced using any of a plurality of types of optical disks having different standards.

  Therefore, in view of the above circumstances, the present invention provides a coma when the light beam is incident on the objective lens obliquely with respect to the optical axis, even when information is recorded or reproduced on any optical disc of different standards. An object of the present invention is to provide an objective lens and an optical pickup device that can form a good spot on a disk recording surface while suppressing the occurrence of aberration.

を満たすように配置され、かつ、少なくとも一方の面にそれぞれの光ディスクに対して球面収差を補正するような輪帯状の回折構造を有することを特徴とする。 According to the present invention, the protective layer thickness is relatively greater than that of the first optical disk and the first optical disk by selectively using the two types of light beams having the first wavelength and the second wavelength longer than the first wavelength. The present invention relates to an optical disk objective lens capable of recording or reproducing information with respect to at least two types of optical disks having different protective layer thicknesses from a thick second optical disk. Specifically, in order to achieve the above object, the objective lens according to claim 1 is an objective lens for recording or reproducing information with respect to the first optical disk using the light beam having the first wavelength. When the magnification is M1, and the magnification of the objective lens is M2 when recording or reproducing information on the second optical disk using the second wavelength longer than the first wavelength, the following condition (1) ,

And at least one surface has an annular diffractive structure for correcting spherical aberration with respect to each optical disk.

  In order to satisfactorily suppress both the coma generated when the first optical disk is used and the coma generated when the second optical disk is used, the objective lens is configured to satisfy the condition (1) according to claim 1. Good. Condition (1) is a condition that defines the magnification of the objective lens when each optical disk is used. That is, the magnification is set so that the magnification when using an optical disk with a thick protective layer is larger than the magnification when using an optical disk with a thin protective layer, and the magnification difference is a predetermined magnitude. As a result, unlike a conventional objective lens, the light flux is oblique to the objective lens with respect to the optical axis regardless of whether a plurality of optical discs with different standards, ie, the first optical disc or the second optical disc is used. Can effectively suppress coma generated on the recording surface. In addition, by providing a diffractive structure on at least one surface of the objective lens that satisfies the condition (1), the spherical aberration generated when the light beam transmitted through the lens converges on the recording surface of each optical disk can be corrected satisfactorily. is doing. Therefore, by using the objective lens according to the first aspect, it is possible to realize highly accurate information recording or reproduction on a plurality of optical discs having different standards.

  The magnification difference between the first optical disk and the second optical disk (hereinafter simply referred to as the magnification difference) in a state where the coma generated when information is recorded or reproduced on the first optical disk is satisfactorily suppressed. If the value of M2-M1 to be expressed is equal to or greater than the upper limit of the condition (1), correction for coma generated when the second optical disk is used becomes excessive, and reverse coma is generated, which is not preferable. Further, when the value of the magnification difference M2-M1 is equal to or greater than the upper limit of the condition (1), the distance from the objective lens to the optical disc (particularly the second optical disc) (that is, the working distance) is shortened. Problems such as scratches and failures due to contact of the optical disk may occur. On the other hand, when the value of the magnification difference M2-M1 is less than or equal to the lower limit of the condition (1), correction for coma generated when the second optical disk is used is insufficient, which is not preferable.

を満たすことが望ましい（請求項２）。但し、対物レンズの焦点距離ｆは、第一の波長の光束を用いて第一の光ディスクに対する情報の記録または再生を行う際の焦点距離とする。条件（２）によれば、焦点距離ｆが長くなればなるほど、倍率差Ｍ２−Ｍ１は小さく設定することが望ましい。条件（２）において算出される値が上限以上になると、第二の光ディスク使用時に発生するコマ収差に対する補正が過剰となり、逆向きのコマ収差が発生してしまい、好ましくない。また、対物レンズから光ディスク（特に第二の光ディスク）までの距離（つまり作動距離）が短くなり、フォーカシング時などに対物レンズと該光ディスクが接触することによる傷の発生や故障などの問題も発生しかねない。また、条件（２）において算出される値が条件（２）の下限以下になると、第二の光ディスク使用時に発生するコマ収差に対する補正が不足してしまうため、好ましくない。 More specifically, the magnification that is suitable for satisfactorily correcting the coma generated when each disk is used is determined by the focal length of the objective lens. That is, if the focal length of the objective lens is f (mm), the following condition (2):

It is desirable to satisfy (Claim 2). However, the focal length f of the objective lens is a focal length when information is recorded on or reproduced from the first optical disk using a light beam having the first wavelength. According to the condition (2), it is desirable to set the magnification difference M2-M1 smaller as the focal length f becomes longer. If the value calculated in the condition (2) exceeds the upper limit, correction for coma generated when the second optical disk is used becomes excessive, and reverse coma occurs, which is not preferable. In addition, the distance from the objective lens to the optical disk (especially the second optical disk) (that is, the working distance) is shortened, and problems such as scratches and failures due to contact between the objective lens and the optical disk during focusing occur. It might be. Further, if the value calculated in the condition (2) is less than or equal to the lower limit of the condition (2), correction for coma generated when the second optical disk is used is insufficient, which is not preferable.

を満たすことが望ましい（請求項３）。 Furthermore, the diffractive structure provided on at least one surface of the objective lens is located on the inner area for securing the NA required for recording or reproducing information with respect to the second optical disk, and on the outer side of the inner optical disk. An outer region for securing an NA necessary for recording or reproducing information with respect to the optical region, and a high-order component of the optical path length addition amount in the outer region at the boundary position between the inner region and the outer region is Φ1 (λ), The high-order term component of the optical path length addition amount at the maximum effective diameter of the outer region is Φ2 (λ), the first wavelength is λ1, the second wavelength is λ2, and the numerical aperture required for recording or reproducing information on the first optical disk is Assuming NA1, the disk thickness of the first optical disk is t1 (mm), and the disk thickness of the second optical disk is t2 (mm), the following condition (3):

It is desirable to satisfy (Claim 3).

によって表現される。Ｐ_４、Ｐ_６、…はそれぞれ光路長付加量に関する４次、６次、…の係数であり、ｍは情報の記録または再生に利用する回折光の次数を表す。 Here, the high-order term component of the optical path length addition amount is the following function using the height h from the optical axis:

Is represented by P ₄ , P ₆ ,... Are fourth-order, sixth-order,... Coefficients relating to the added optical path length, and m represents the order of diffracted light used for recording or reproducing information.

  In general, when the magnification of the objective lens at the time of recording or reproducing information with respect to the second optical disk is positive, as the magnification increases, the aberration caused by the temperature change greatly deteriorates when the first optical disk is used. Therefore, it is necessary to increase the diffractive action provided by the diffractive structure provided in the objective lens, in other words, to increase the additional optical path length. Condition (3) is a condition defined from this point of view. When the calculated value exceeds the upper limit, the beam spot formed on the recording surface is recorded or reproduced as the spherical aberration changes due to temperature change. This is not preferable because it cannot be fully squeezed to a suitable level. On the other hand, if the value calculated in the condition (3) is less than the lower limit, the number of ring zones on the diffractive surface increases so much that the diffraction efficiency decreases and the light utilization efficiency decreases, which is not preferable.

  In order to satisfy the condition (1), the objective lens magnification M1 when the first optical disk is used is set to 0, and the objective lens magnification M2 when the second optical disk is used is set to be larger than 0. Alternatively, a pattern in which the magnification M1 is set smaller than 0 and the magnification M2 is set to 0 can be considered.

  Here, the condition that the magnification of the objective lens at the time of recording or reproducing information on the second optical disk is set to 0 or less, in other words, the light beam incident on the objective lens when the second optical disk is used is divergent light or parallel light. In order to satisfy (1), the absolute value of the magnification when using the first optical disk must be increased. For this reason, when the objective lens is tracking-shifted when the first optical disk is used, there is a problem that astigmatism is greatly generated. In order to avoid such a problem, the magnification of the objective lens at the time of recording or reproducing information on the second optical disk is made larger than 0, in other words, the light beam incident on the objective lens when using the second optical disk is converted into convergent light. The objective lens is configured as described above. With this configuration, it is possible to satisfactorily correct coma while suppressing astigmatism by reducing the magnification of the objective lens when using the first optical disk.

を満たすように配置される。さらに、対物レンズは、少なくとも一方の面に、それぞれのディスクに対して球面収差を補正するような輪帯状の回折構造を有する。 In order to achieve the above object, another aspect of the present invention provides a first wavelength and two kinds of light fluxes having a second wavelength longer than the first wavelength. Information can be recorded or reproduced on at least two types of optical discs having different protective layer thicknesses between the first optical disc and the second optical disc having a relatively thick protective layer thickness than the first optical disc. This optical pickup device includes an objective lens. This objective lens has a magnification of M1 when recording or reproducing information with respect to the first optical disk using a light beam having the first wavelength, and records or reproduces information with respect to the second optical disk using the second wavelength. If the magnification in the case of performing is M2, the following condition (1),

It is arranged to satisfy. Furthermore, the objective lens has an annular diffractive structure that corrects spherical aberration with respect to each disk on at least one surface.

  According to this configuration, the effect described above with respect to the objective lens that satisfies the conditional expression (1) is provided as the optical pickup device.

を満たすよう構成されていても良い。 The optical pickup device having the above-described configuration has the following condition (2), where f (mm) is the focal length of the objective lens when recording or reproducing information with respect to the first optical disk using the light beam having the first wavelength. ),

It may be configured to satisfy.

を満たすように構成されていても良い。 In the objective lens of the optical pickup device having the above-described configuration, the diffractive structure is located on the inner area for securing the NA necessary for recording or reproducing information with respect to the second optical disk, and on the outer side of the inner area. An outer region for securing an NA necessary for recording or reproducing information with respect to the optical region, and a high-order component of the optical path length addition amount in the outer region at the boundary position between the inner region and the outer region is Φ1 (λ), The high-order component of the optical path length addition amount at the maximum effective diameter of the outer region is Φ2 (λ), the first wavelength is λ1, the second wavelength is λ2, and the numerical aperture required for recording or reproducing information on the first optical disc Is NA1, the disk thickness of the first optical disk is t1 (mm), and the disk thickness of the second optical disk is t2 (mm), the following condition (3):

It may be configured to satisfy.

  In the optical pickup device, the light beam incident on the objective lens at the time of recording or reproducing information on the second optical disk may be convergent light.

  Alternatively, in the optical pickup device, the light beam incident on the objective lens at the time of recording or reproducing information on the first optical disk may be substantially parallel light.

  As described above, according to the objective lens and the optical pickup device of the present invention, by appropriately setting the magnification of the objective lens according to the standard of the optical disk used for recording or reproducing information, The coma generated on the recording surface can be corrected satisfactorily regardless of which of the plurality of optical discs having different layer thicknesses is used.

  Hereinafter, embodiments of the objective lens according to the present invention will be described. 1A and 1B show the objective lens 10 of the embodiment, the first optical disc 20A, and the second optical disc 20B for each optical path in the optical pickup device when the optical discs 20A and 20B are used. They are shown separately. The objective lens 10 is mounted on an optical pickup device of an optical information recording or reproducing device that is compatible with a plurality of types of optical discs having different standards such as protective layer thickness and recording density.

  Each of the optical disks 20A and 20B is placed on a turntable (not shown) and rotated. In this specification, an optical disc (for example, DVD) having a thin protective layer and a high recording density is referred to as a first optical disc 20A. An optical disc (eg, CD, CD-R, etc.) having a thick protective layer and a low recording density is referred to as a second optical disc 20B.

  Before describing the objective lens 10 in detail, a configuration example of an optical pickup apparatus that employs the objective lens 10 will be described with reference to FIG. The optical pickup device 100 shown in FIG. 38 includes a light source 30A and a light source 30B that output laser beams corresponding to the first optical disc 20A and the second optical disc 20B, respectively. The laser light emitted from the light source 30A passes through the cover glass 40A, the coupling lens 50A, and the beam splitter 60 and enters the objective lens 10. On the other hand, the laser light emitted from the light source 30B passes through the cover glass 40B, the coupling lens 50B, and the beam splitter 60 and enters the objective lens 10. As will be described in detail below, the objective lens 10 converges the laser beams from the light source 30A and the light source 30B on the recording surfaces of the first optical disc 20A and the second optical disc 20B, respectively.

  When recording or reproducing information on the first optical disc 20A, a short wavelength (for example, 657 nm) laser beam (hereinafter referred to as the first laser beam) is used to form a small-diameter beam spot on the recording surface. Is emitted from the light source 30A. Further, when recording or reproduction is performed on the second optical disc 20B having a low recording density, a laser having a wavelength longer than that of the first laser beam is used to form a spot having a relatively large diameter on the recording surface. Light (hereinafter referred to as second laser light) is emitted from the light source 30B.

  Irradiated from the light sources (30A, 30B), the first laser light is converted into parallel light and the second laser light is converted into convergent light via the coupling lenses (50A, 50B). Or, it converges in the vicinity of the recording surface of the optical disc 20B.

を満たすように構成される。つまり、図１に示すように、情報の記録または再生に使用される光ディスクに応じて対物レンズ１０に入射する光束の発散度を変えている。これにより、光束が光軸に対して斜めに入射することにより各光ディスクの記録面上で発生するコマ収差を良好に抑えている。 The objective lens 10 is set with a magnification corresponding to the standard of an optical disk used for recording or reproducing information. Specifically, the objective lens 10 has the following conditions when the magnification at the time of recording or reproducing information on the first optical disc 20A is M1, and the magnification at the time of recording or reproducing information on the second optical disc 20B is M2. (1),

Configured to meet. That is, as shown in FIG. 1, the divergence of the light beam incident on the objective lens 10 is changed in accordance with the optical disk used for recording or reproducing information. Thereby, coma aberration generated on the recording surface of each optical disk due to the incident light beam obliquely with respect to the optical axis is satisfactorily suppressed.

  In the objective lens 10 shown in FIG. 1, since the recording density is high, the magnification M1 is set to 0 when the first optical disc 20A having a narrow tolerance for aberration during recording or reproduction of information is used. When the magnification M1 is 0, the light beam incident on the objective lens 10 is a parallel light beam. As described above, the collimated light beam is made incident on the objective lens 10, so that when the first optical disc 20A is used, even when the objective lens 10 is tracking-shifted, coma aberration hardly occurs. When using the second optical disc 20B, which has a lower recording density than the first optical disc and has a relatively large tolerance for aberrations, the magnification M2 is set to be larger than 0 within a range that satisfies the condition (1). The When the magnification M2 is larger than 0, the light beam incident on the objective lens 10 is a convergent light beam.

このように、対物レンズ１０の焦点距離ｆを用いて求められた倍率Ｍ１、Ｍ２を備えるように対物レンズを構成することにより、コマ収差補正効果をより一層高めることができる。 Note that the magnifications M1 and M2 for satisfactorily correcting the coma aberration can be determined by the following condition (2) when the focal length f of the objective lens 10 is used.

As described above, the coma aberration correction effect can be further enhanced by configuring the objective lens to have the magnifications M1 and M2 obtained by using the focal length f of the objective lens 10.

  The objective lens 10 has a first surface 10a and a second surface 10b in order from the light source side. As shown in FIG. 1, the objective lens 10 is a biconvex plastic single lens in which both surfaces 10a and 10b are aspherical surfaces. As described above, the thickness of the protective layer differs between the first optical disc 20A and the second optical disc 20B. For this reason, the spherical aberration changes depending on the disc used. In view of this, the objective lens 10 according to the present embodiment suppresses the aberration satisfactorily by providing an annular diffractive structure having a plurality of fine steps centered on the optical axis on at least one surface. The objective lens 10 of the present embodiment has a diffractive structure on the first surface 10a. It has an inner region located around the optical axis AX and an outer region located around the inner region and extending to the outer periphery of the lens. The inner region and the outer region have different surface shapes.

  The inner region of the surface 10a has a diffractive structure that allows the first and second laser beams to converge well on the recording surfaces of the corresponding optical disks 20A and 20B.

  The following diffractive structure is formed in the outer region. That is, the diffractive structure gives the first laser light a diffractive action that converges well on the recording surface of the first optical disc 20A. Further, the diffractive structure diffuses with respect to the second laser beam and gives a diffractive action that does not contribute to the formation of spots on the recording surface of the second optical disc 20B. Specifically, the diffractive structure of the outer region is configured such that the wavefront of the first laser light transmitted through the region is substantially continuous with the wavefront of the first laser light transmitted through the inner region.

  In the second laser light transmitted through the objective lens 10 having the above configuration, only the component transmitted through the inner region converges favorably on the recording surface of the second optical disc 20B. Thereby, a relatively large-diameter spot suitable for recording or reproducing information on the second optical disc 20B is formed on the recording surface. Further, the first laser light transmitted through the objective lens 10 forms a small-diameter spot on the recording surface of the first optical disc 20A.

但し、Φ１は、対物レンズ１０の回折構造の内側領域と外側領域の境界位置における外側領域の光路長付加量の高次項成分を、
Φ２は、対物レンズ１０の回折構造の最大有効径における光路長付加量の高次項成分を、
λ１は、第一のレーザ光の波長を、
λ２は、第二のレーザ光の波長を、
ＮＡ１は、第一の光ディスク使用時に対物レンズ１０に要求される設計開口数を、
ｔ１は、第一の光ディスク２０Ａのディスク厚（ｍｍ）を、
ｔ２は、第二の光ディスク２０Ｂのディスク厚（ｍｍ）を、
それぞれ表す。 If the plastic objective lens 10 is used, the spherical aberration changes due to temperature change, and the beam spot formed on the recording surface of the optical disk may not be sufficiently narrowed. In order to maintain high utilization efficiency of the light beam transmitted through the objective lens 10 and to reduce the influence of the change in aberration due to temperature change, the objective lens 10 is provided with an optical path for expressing the diffractive action of the lens 10. The long additional amount is configured to satisfy the following condition (3).

However, Φ1 is a high-order term component of the optical path length addition amount of the outer region at the boundary position between the inner region and the outer region of the diffractive structure of the objective lens 10,
Φ2 is a high-order component of the optical path length addition amount at the maximum effective diameter of the diffractive structure of the objective lens 10,
λ1 is the wavelength of the first laser beam,
λ2 is the wavelength of the second laser beam,
NA1 is the design numerical aperture required for the objective lens 10 when the first optical disk is used.
t1 is the disc thickness (mm) of the first optical disc 20A,
t2 is the disc thickness (mm) of the second optical disc 20B.
Represent each.

  Next, five specific examples based on the above-described embodiment will be presented. Both examples relate to the objective lens 10 having compatibility between the first optical disc 20A having a protective layer thickness of 0.6 mm and the second optical disc 20B having a protective layer thickness of 1.2 mm.

  FIGS. 1A and 1B show the objective lens 10, the first optical disk 20A, and the second optical disk 20B of Example 1 separately for each optical path when the optical disks 20A and 20B are used. It is. Specific specifications of the objective lens 10 of Example 1 are shown in Table 1. Tables 2 and 3 show specific numerical configurations of the optical information recording or reproducing apparatus when information is recorded or reproduced on each of the optical disks 20A and 20B using the objective lens 10 of the first embodiment.

  In Table 1, the wavelength is the design wavelength (unit: nm) most suitable for recording or reproducing information on each optical disc, and NA is the design numerical aperture necessary for recording or reproducing information on each optical disc. The same applies to each table described later.

  In Table 2, r is the radius of curvature of each lens surface (unit: mm), d is the lens thickness or lens interval (unit: mm), n (Xnm) is the refractive index at wavelength Xnm, and the remarks are the surface numbers. The optical member shown is represented. The same applies to Table 3 and each table described later. The reason why d shown in Table 2 and Table 3 is different is that the wavelength of the laser beam used when each optical disk is used and the protective layer thickness of each optical disk are different. Further, d of the 0th surface is a distance based on the first surface. As shown in Tables 2 and 3, the first surface 10a of the objective lens 10 is divided into an inner region and an outer region with a boundary position where the height h from the optical axis AX is 1.52 mm as a boundary. Yes.

The first surface 10a and the second surface 10b of the objective lens 10 are aspherical surfaces. The shape is such that the distance (sag amount) from the tangential plane on the aspherical optical axis of the coordinate point on the aspherical surface where the height from the optical axis is h is X (h), on the aspherical optical axis. Is the curvature (1 / r) of C, the conic coefficient is K, the fourth, sixth, eighth, tenth, and twelfth aspheric coefficients are A ₄ , A ₆ , A ₈ , A ₁₀ , A ₁₂ , It is expressed by the following formula.

  Table 4 shows conical coefficients and aspheric coefficients that define each aspheric surface. As shown in Tables 2 to 4, the first surface 10a has different surface shapes (such as a radius of curvature r and an aspherical coefficient) depending on the outer region and the inner region. In addition, the notation E in Table 4 represents a power with 10 as the radix and the number to the right of E as the exponent. The same applies to each table shown below.

Further, a diffractive structure is formed on the first surface 10 a of the objective lens 10. The diffractive structure is defined using the following optical path difference function φ (h).

The optical path difference function φ (h) expresses the diffractive action given by the diffractive lens to the light flux having the wavelength λ in the form of an optical path length addition amount at a position of height h from the optical axis. P ₂ , P ₄ , P ₆ ,... Are second-order, fourth-order, sixth-order,. Table 5 shows optical path difference function coefficients P ₂ ,... Used for defining the diffractive structure of the objective lens 10. Note that m represents the order of diffracted light used for recording or reproducing information, and m = 1 in all the following embodiments including this embodiment.

  As for the objective lens 10 of Example 1, the value of the condition (1) is 0.050, the value of the condition (2) is 0.153, and the value of the condition (3) is −0.227 from the above tables. . That is, the objective lens 10 of Example 1 satisfies all the conditions (1) to (3).

  Wavefront aberration diagrams in the case where information is recorded or reproduced on the first optical disc 20A and the second optical disc 20B using the objective lens 10 of Example 1 are shown in FIG. 2 and FIG. 3 in order. FIGS. 4 and 5 sequentially show wavefront aberration diagrams when information is recorded or reproduced on the first optical disc 20A and the second optical disc 20B using the objective lens of Comparative Example 1. FIG. In each figure, (A) represents the wavefront aberration on the axis, and (B) represents the wavefront aberration at the off-axis (image height 0.06 mm). The objective lens of Comparative Example 1 is a lens having the same configuration as that of the objective lens 10 of Example 1 but having a magnification set to 0 when the second optical disk is used. When the objective lens 10 of Example 1 that satisfies all of the conditions (1) to (3) is used, coma aberration generated during recording or reproduction of information with respect to the second optical disk is corrected better than that of the objective lens of Comparative Example 1. You can see that.

  The effectiveness of coma aberration correction by the objective lens 10 of Example 1 that satisfies all the conditions (1) to (3) will be described in more detail. FIG. 6 is a graph showing the relationship between wavefront aberration and image height generated when information is recorded on or reproduced from the first optical disc 20A using the objective lens 10 of the first embodiment. FIG. 7 is a graph showing the relationship between wavefront aberration and image height generated when information is recorded on or reproduced from the second optical disc 20B using the objective lens 10 of the first embodiment. FIG. 8 is a graph showing the relationship between the wavefront aberration generated when information is recorded or reproduced on the first optical disc 20A using the objective lens of Comparative Example 1, and the image height. FIG. 9 is a graph showing the relationship between the wavefront aberration generated when information is recorded on or reproduced from the second optical disc 20B using the objective lens of Comparative Example 1 and the image height. In each graph, the vertical axis represents the amount of aberration, and the horizontal axis represents the image height. Coma3 represents third-order coma, as3 represents third-order astigmatism, and coma5 represents fifth-order coma. The same applies to graphs relating to Examples 2 to 5 described later.

  The amount of wavefront aberration generated on the recording surface of the disc when the first optical disc 20A is used will be verified while comparing FIG. 6 and FIG. When the objective lens 10 of Example 1 is used, the third-order coma aberration is suppressed to substantially the same level as when the objective lens of Comparative Example 1 is used. Further, the amount of wavefront aberration generated on the recording surface of the disc when the second optical disc 20B is used is verified by comparing FIG. 7 and FIG. When the objective lens 10 of Example 1 is used, the third-order coma is suppressed considerably better than when the objective lens of Comparative Example 1 is used, that is, the generation amount of the entire wavefront aberration is reduced. ing. As a result, when the objective lens 10 of Example 1 is used, the third-order coma aberration is corrected well regardless of whether the first optical disk 20A or the second optical disk 20B is used. Therefore, coma aberration can be sufficiently suppressed to the extent that an optimum spot for recording or reproducing information can be formed on the recording surface of each optical disc.

  FIGS. 10A and 10B illustrate the objective lens 10, the first optical disk 20A, and the second optical disk 20B of Example 2 separately for each optical path when the optical disks 20A and 20B are used. It is. Specific specifications of the objective lens 10 of Example 2 are shown in Table 6. Tables 7 and 8 show specific numerical configurations of the optical information recording or reproducing apparatus when information is recorded or reproduced on each of the optical disks 20A and 20B using the objective lens 10 of the second embodiment.

  As shown in Tables 7 and 8, the first surface 10a of the objective lens 10 is divided into an inner region and an outer region with a position where the height h from the optical axis AX is 0.82 mm. .

  The first surface 10a and the second surface 10b of the objective lens 10 of Example 2 are aspherical surfaces. Therefore, the shape of each aspherical surface is expressed by the above equation (17). Table 9 shows conical coefficients and aspherical coefficients used in Equation 17. In addition, a diffractive structure is formed on the first surface 10a of the objective lens 10 of the second embodiment. Table 10 shows optical path difference function coefficients for representing the diffractive action by the diffractive structure. As shown in Tables 7 to 10, the surfaces 10a of the objective lens 10 of Example 2 have different surface shapes (such as a radius of curvature r and an aspheric coefficient) depending on the inner region and the outer region.

  In the objective lens 10 of Example 2, the value of the condition (1) is 0.125, the value of the condition (2) is 0.225, and the value of the condition (3) is −0.046 from each table above. . That is, the objective lens 10 of Example 2 satisfies all the conditions (1) to (3).

  FIG. 11 and FIG. 12 show wavefront aberration diagrams in the case where information is recorded on or reproduced from the first optical disc 20A and the second optical disc 20B using the objective lens 10 according to the second embodiment. Further, FIG. 13 and FIG. 14 show wavefront aberration diagrams in the case where information is recorded on or reproduced from the first optical disc 20A and the second optical disc 20B using the objective lens of Comparative Example 2, respectively. In each figure, (A) represents the wavefront aberration on the axis, and (B) represents the wavefront aberration at the off-axis (image height 0.06 mm). The objective lens of Comparative Example 2 is a lens having the same configuration as that of the objective lens 10 of Example 2 but having a magnification set to 0 when the second optical disk is used. When the objective lens 10 of Example 2 that satisfies all of the conditions (1) to (3) is used, coma aberration generated during recording or reproduction of information with respect to the second optical disk is corrected better than that of the objective lens of Comparative Example 2. You can see that.

  The effectiveness of the coma aberration correction by the objective lens 10 of Example 2 that satisfies all the conditions (1) to (3) will be described in more detail. FIG. 15 is a graph showing the relationship between wavefront aberration and image height generated when information is recorded on or reproduced from the first optical disc 20A using the objective lens 10 of the second embodiment. FIG. 16 is a graph showing the relationship between wavefront aberration and image height generated when information is recorded on or reproduced from the second optical disk 20B using the objective lens 10 of the second embodiment. FIG. 17 is a graph showing the relationship between the wavefront aberration generated when information is recorded on or reproduced from the first optical disc 20A using the objective lens of Comparative Example 2 and the image height. FIG. 18 is a graph showing the relationship between the wavefront aberration generated when information is recorded on or reproduced from the second optical disc 20B using the objective lens of Comparative Example 2 and the image height.

  The amount of wavefront aberration generated on the recording surface of the disc when the first optical disc 20A is used will be verified while comparing FIG. 15 and FIG. When the objective lens 10 of Example 2 is used, the third-order coma aberration is sufficiently suppressed to about the same level as when the objective lens of Comparative Example 2 is used. Further, the amount of wavefront aberration generated on the recording surface of the disc when the second optical disc 20B is used is verified by comparing FIG. 16 and FIG. When the objective lens 10 of Example 2 is used, the third-order coma aberration is greatly suppressed as compared with the case of using the objective lens of Comparative Example 2. As a result, when the objective lens 10 of Example 2 is used, coma can be favorably corrected regardless of which of the first optical disc 20A and the second optical disc 20B is used.

  FIGS. 19A and 19B illustrate the objective lens 10, the first optical disc 20A, and the second optical disc 20B of Example 3 separately for each optical path when using the optical discs 20A and 20B. It is. Specific specifications of the objective lens 10 of Example 3 are shown in Table 11. Tables 12 and 13 show specific numerical configurations of the optical information recording or reproducing apparatus when information is recorded or reproduced on each of the optical disks 20A and 20B using the objective lens 10 of the third embodiment.

  As shown in Tables 12 and 13, the first surface 10a of the objective lens 10 is divided into an inner region and an outer region with a position where the height h from the optical axis AX is 1.14 mm as a boundary. .

  The first surface 10a and the second surface 10b of the objective lens 10 of Example 3 are aspherical surfaces. Therefore, the shape of each aspherical surface is expressed by the above equation (17). Table 14 shows conic coefficients and aspherical coefficients used in Equation 17. In addition, a diffractive structure is formed on the first surface 10a of the objective lens 10 of the third embodiment. Table 15 shows optical path difference function coefficients for representing the diffractive action of the diffractive structure. As shown in Tables 12 to 15, the surfaces 10a of the objective lens 10 of Example 3 have different surface shapes (such as a radius of curvature r and an aspherical coefficient) depending on the inner region and the outer region.

  As for the objective lens 10 of Example 3, the value of the condition (1) is 0.075, the value of the condition (2) is 0.175, and the value of the condition (3) is −0.052 from the above tables. . That is, the objective lens 10 of Example 3 satisfies all the conditions (1) to (3).

  FIG. 20 and FIG. 21 show wavefront aberration diagrams in the case where information is recorded on or reproduced from the first optical disc 20A and the second optical disc 20B using the objective lens 10 according to the third embodiment. In addition, FIG. 22 and FIG. 23 show wavefront aberration diagrams in the case where information is recorded on or reproduced from the first optical disc 20A and the second optical disc 20B using the objective lens of Comparative Example 3, respectively. In each figure, (A) represents the wavefront aberration on the axis, and (B) represents the wavefront aberration at the off-axis (image height 0.06 mm). The objective lens of Comparative Example 3 is a lens that has the same configuration as the objective lens 10 of Example 3 but has a magnification set to 0 when the second optical disk is used. When the objective lens 10 of Example 3 that satisfies all of the conditions (1) to (3) is used, coma aberration generated particularly during recording or reproduction of information on the second optical disk is corrected better than that of the objective lens of Comparative Example 3. You can see that.

  The effectiveness of coma aberration correction by the objective lens 10 of Example 3 that satisfies all the conditions (1) to (3) will be described in more detail. FIG. 24 is a graph showing the relationship between the wavefront aberration generated when information is recorded on or reproduced from the first optical disc 20A using the objective lens 10 of Example 3 and the image height. FIG. 25 is a graph showing the relationship between wavefront aberration and image height generated when information is recorded on or reproduced from the second optical disc 20B using the objective lens 10 according to the third embodiment. FIG. 26 is a graph showing the relationship between the wavefront aberration generated when information is recorded on or reproduced from the first optical disc 20A using the objective lens of Comparative Example 3, and the image height. FIG. 27 is a graph showing the relationship between the wavefront aberration and the image height that occur when information is recorded on or reproduced from the second optical disc 20B using the objective lens of Comparative Example 3.

  The amount of wavefront aberration generated on the recording surface of the disc when the first optical disc 20A is used is verified while comparing FIG. 24 and FIG. When the objective lens 10 of Example 3 is used, various aberrations including third-order coma are sufficiently suppressed to about the same level as when the objective lens of Comparative Example 3 is used. Further, the amount of wavefront aberration generated on the recording surface of the disc when the second optical disc 20B is used is verified by comparing FIG. 25 and FIG. When the objective lens 10 of Example 3 is used, the third-order coma aberration is particularly suppressed compared to the case of using the objective lens of Comparative Example 3. As a result, when the objective lens 10 of Example 3 is used, coma can be favorably corrected regardless of which of the first optical disc 20A and the second optical disc 20B is used.

  In the objective lenses 10 of Examples 1 to 3 described above, the magnification when using the first optical disc 20A was set to zero. However, the objective lens according to the present invention is not limited to the configurations of Examples 1 to 3 as long as at least the condition (1) is satisfied. For example, the objective lens 10 of Example 4 and Example 5 described below corresponds. As shown in Table 16, in the objective lens 10 of Example 4, the magnification when using the first optical disk is set to be smaller than 0 (that is, divergent light is incident on the objective lens 10), and the second optical disk is used. The magnification during use is set to 0. In the objective lens 10 of Example 5, the magnification when using the first optical disk is set to be smaller than 0, and the magnification when using the second optical disk is set to be larger than 0 (that is, the objective lens 10 has Is focused light).

  FIGS. 28A and 28B show the objective lens 10, the first optical disc 20A, and the second optical disc 20B of Example 4 separately for each optical path when using the optical discs 20A and 20B. It is. Specific specifications of the objective lens 10 of Example 4 are shown in Table 16. Tables 17 and 18 show specific numerical configurations of the optical information recording or reproducing apparatus when information is recorded or reproduced on each of the optical disks 20A and 20B using the objective lens 10 of the fourth embodiment. As shown in Tables 17 and 18, the first surface 10a of the objective lens 10 is divided into an inner region and an outer region with a position where the height h from the optical axis AX is 1.25 mm as a boundary. .

  The first surface 10a and the second surface 10b of the objective lens 10 of Example 4 are aspherical surfaces. Therefore, the shape of each aspherical surface is expressed by the above equation (17). Table 19 shows conic coefficients and aspherical coefficients used in Equation 17. In addition, a diffractive structure is formed on the first surface 10a of the objective lens 10 of the fourth embodiment. Table 20 shows optical path difference function coefficients for representing the diffractive action of the diffractive structure. As shown in Tables 17 to 20, the surfaces 10a of the objective lens 10 of Example 4 have different surface shapes (such as a radius of curvature r and an aspherical coefficient) depending on the inner region and the outer region.

  In the objective lens 10 of Example 4, the value of the condition (1) is 0.063, the value of the condition (2) is 0.147, and the value of the condition (3) is −0.115 from the above tables. . That is, the objective lens 10 of Example 4 satisfies all the conditions (1) to (3).

  FIG. 29 and FIG. 30 show wavefront aberration diagrams in the case where information is recorded on or reproduced from the first optical disc 20A and the second optical disc 20B using the objective lens 10 of the fourth embodiment. When the objective lens 10 of Example 4 that satisfies all the conditions (1) to (3) is used, coma aberration generated when information is recorded or reproduced on both the first and second optical discs is corrected well. I understand.

  The effectiveness of coma aberration correction by the objective lens 10 of Example 4 that satisfies all the conditions (1) to (3) will be further described. FIG. 31 is a graph showing the relationship between wavefront aberration and image height generated when information is recorded on or reproduced from the first optical disc 20A using the objective lens 10 of the fourth embodiment. FIG. 32 is a graph showing the relationship between the wavefront aberration generated when information is recorded on or reproduced from the second optical disc 20B using the objective lens 10 of Example 4 and the image height. When the objective lens 10 of Example 4 is used, the third-order and fifth-order coma aberration can be corrected well regardless of whether the first optical disk 20A or the second optical disk 20B is used. Recognize.

  33 (A) and 33 (B) show the objective lens 10, the first optical disc 20A, and the second optical disc 20B of Example 5 separately for each optical path when the optical discs 20A and 20B are used. It is. Specific specifications of the objective lens 10 of Example 5 are shown in Table 21. Tables 22 and 23 show specific numerical configurations of the optical information recording or reproducing apparatus when information is recorded or reproduced on the optical disks 20A and 20B using the objective lens 10 of the fifth embodiment. As shown in Tables 22 and 23, the first surface 10a of the objective lens 10 is divided into an inner region and an outer region with a position where the height h from the optical axis AX is 1.17 mm as a boundary. .

  The first surface 10a and the second surface 10b of the objective lens 10 of Example 5 are aspherical surfaces. Therefore, the shape of each aspherical surface is expressed by the above equation (17). Table 24 shows conic coefficients and aspherical coefficients used in Equation 17. In addition, a diffractive structure is formed on the first surface 10a of the objective lens 10 of the fifth embodiment. Table 25 shows optical path difference function coefficients for representing the diffractive action of the diffractive structure. As shown in Tables 22 to 25, each surface 10a of the objective lens 10 of Example 5 has different surface shapes (such as a radius of curvature r and an aspherical coefficient) depending on the inner region and the outer region.

  In the objective lens 10 of Example 5, the value of the condition (1) is 0.060, the value of the condition (2) is 0.140, and the value of the condition (3) is −0.135 from the above tables. . That is, the objective lens 10 of Example 5 satisfies all the conditions (1) to (3).

  FIG. 34 and FIG. 35 show wavefront aberration diagrams in the case where information is recorded on or reproduced from the first optical disc 20A and the second optical disc 20B using the objective lens 10 of the fifth embodiment. When the objective lens 10 of Example 5 that satisfies all of the conditions (1) to (3) is used, coma aberration generated during recording or reproduction of information on either the first or second optical disk is well corrected. I understand.

  The effectiveness of coma aberration correction by the objective lens 10 of Example 5 that satisfies all the conditions (1) to (3) will be further described. FIG. 36 is a graph showing the relationship between wavefront aberration and image height generated when information is recorded on or reproduced from the first optical disc 20A using the objective lens 10 of the fifth embodiment. FIG. 37 is a graph showing the relationship between wavefront aberration and image height generated when information is recorded or reproduced on the second optical disk 20B using the objective lens 10 of the fifth embodiment. When the objective lens 10 of Example 5 is used, the third-order and fifth-order coma aberration can be corrected well regardless of whether the first optical disk 20A or the second optical disk 20B is used. Recognize.

  The above is the embodiment of the present invention. The above embodiment is merely an example of the objective lens according to the present invention. That is, the objective lens according to the present invention is not limited to the configuration of the above embodiment.

The objective lens and each optical disk of the embodiment of the present invention are shown separately for each optical path when each optical disk is used. FIG. 6 is an aberration diagram illustrating wavefront aberration of the objective lens according to Example 1 when the first optical disc is used. FIG. 6 is an aberration diagram illustrating wavefront aberration of the objective lens according to Example 1 when the second optical disc is used. 6 is an aberration diagram illustrating wavefront aberration of the objective lens of Comparative Example 1 when using the first optical disc. FIG. FIG. 6 is an aberration diagram showing wavefront aberration of the objective lens of Comparative Example 1 when using a second optical disc. 6 is a graph showing a relationship between wavefront aberration generated when the first optical disk is used and the image height of the objective lens of Example 1; 6 is a graph showing the relationship between wavefront aberration and image height that occur when the second optical disk is used in the objective lens of Example 1; 6 is a graph showing the relationship between the wavefront aberration generated when the first optical disk is used and the image height of the objective lens of Comparative Example 1; 6 is a graph showing the relationship between wavefront aberration and image height that occur when the second optical disk of the objective lens of Comparative Example 1 is used. The objective lens and each optical disk of Example 2 of the present invention are shown separately for each optical path when each optical disk is used. FIG. 10 is an aberration diagram illustrating wavefront aberration of the objective lens according to Example 2 when the first optical disc is used. FIG. 6 is an aberration diagram illustrating wavefront aberration of the objective lens according to Example 2 when the second optical disc is used. FIG. 10 is an aberration diagram illustrating wavefront aberration of the objective lens of Comparative Example 2 when using the first optical disc. 10 is an aberration diagram showing wavefront aberration of the objective lens of Comparative Example 2 when using a second optical disc. FIG. 6 is a graph showing the relationship between wavefront aberration and image height generated when the first optical disk is used in the objective lens of Example 2. 6 is a graph showing the relationship between wavefront aberration and image height that occur when the second optical disk is used in the objective lens of Example 2. 10 is a graph showing the relationship between wavefront aberration and image height generated when the first optical disk is used in the objective lens of Comparative Example 2. 10 is a graph showing the relationship between wavefront aberration and image height generated when the second optical disk is used in the objective lens of Comparative Example 2. The objective lens and each optical disk of Example 3 of the present invention are shown separately for each optical path when each optical disk is used. FIG. 6 is an aberration diagram illustrating wavefront aberration of the objective lens according to Example 3 when the first optical disc is used. FIG. 6 is an aberration diagram illustrating wavefront aberration of the objective lens according to Example 3 when the second optical disc is used. FIG. 11 is an aberration diagram showing wavefront aberration of the objective lens of Comparative Example 3 when using the first optical disc. FIG. 11 is an aberration diagram showing wavefront aberration of the objective lens of Comparative Example 3 when using the second optical disc. 6 is a graph showing the relationship between wavefront aberration and image height generated when the first optical disk is used in the objective lens of Example 3. 6 is a graph showing the relationship between wavefront aberration and image height that occur when the second optical disk is used in the objective lens of Example 3. 10 is a graph showing the relationship between wavefront aberration and image height that occur when the first optical disk is used in the objective lens of Comparative Example 3; 10 is a graph showing the relationship between wavefront aberration and image height generated when the second optical disk is used in the objective lens of Comparative Example 3; The objective lens and each optical disk of Example 4 of the present invention are shown separately for each optical path when each optical disk is used. FIG. 10 is an aberration diagram illustrating wavefront aberration of the objective lens according to Example 4 when the first optical disc is used. FIG. 10 is an aberration diagram illustrating wavefront aberration of the objective lens according to Example 4 when the second optical disc is used. 6 is a graph showing the relationship between wavefront aberration and image height generated when the first optical disk is used in the objective lens of Example 4. 6 is a graph showing the relationship between wavefront aberration and image height generated when using the second optical disc of the objective lens of Example 4. The objective lens and each optical disk of Example 5 of the present invention are shown separately for each optical path when each optical disk is used. FIG. 10 is an aberration diagram illustrating wavefront aberration of the objective lens according to Example 5 when the first optical disc is used. FIG. 10 is an aberration diagram illustrating wavefront aberration of the objective lens according to Example 5 when the second optical disc is used. 10 is a graph showing the relationship between wavefront aberration and image height that occur when the first optical disk is used in the objective lens of Example 5. 10 is a graph showing a relationship between wavefront aberration and image height generated when the second optical disk is used in the objective lens of Example 5. It is a figure showing the structural example of the optical pick-up apparatus which employ | adopts the objective lens of this embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Objective lens 20A 1st optical disk 20B 2nd optical disk 30A, 30B Light source 40A, 40B Cover glass 50A, 50B Coupling lens 60 Beam splitter 100 Optical pick-up apparatus

Claims (10)

By selectively using two types of light fluxes having the first wavelength and the second wavelength longer than the first wavelength, the first optical disc and the first optical disc having a relatively thick protective layer than the first optical disc An optical disc objective lens capable of recording or reproducing information on at least two types of optical discs having different protective layer thicknesses from each other,
When the first wavelength is λ1 (unit: nm) and the second wavelength is λ2 (unit: nm),
635 <λ1 <665
780 <λ2 <830
And
When the protective layer of the first optical disc is t1 (unit: mm) and the protective layer of the second optical disc is t2 (unit: mm),
t1 ≒ 0.6
t2 ≒ 1.2
And
When recording or reproducing information with respect to the first optical disk using the light beam with the first wavelength, the magnification of the objective lens is M1, and recording information with respect to the second optical disk with the second wavelength. Or, when the magnification of the objective lens when performing reproduction is M2, the following condition (1),

Arranged to meet
An objective lens having an annular diffractive structure for correcting spherical aberration for each disk on at least one surface. When the focal length of the objective lens when recording or reproducing information with respect to the first optical disk using the light flux of the first wavelength is f (mm), the following condition (2):

The objective lens according to claim 1, wherein: In the objective lens, the diffractive structure is located in an inner area for securing an NA necessary for recording or reproducing information with respect to the second optical disk, and outside of the inner area. An outer area for securing the NA required for recording or reproduction, and
The high-order term component of the optical path length addition amount in the outer region at the boundary position between the inner region and the outer region is Φ1 (λ), and the high-order term component of the optical path length addition amount in the maximum effective diameter of the outer region is Φ2 (λ ), Where NA1 is a numerical aperture required for recording or reproducing information on the first optical disk, the following condition (3):

The objective lens according to claim 1, wherein:   4. The objective lens according to claim 1, wherein a light beam incident on the objective lens at the time of recording or reproducing information with respect to the second optical disc is convergent light. 5.   The objective lens according to claim 4, wherein a light beam incident on the objective lens at the time of recording or reproducing information on the first optical disc is substantially parallel light. By selectively using two types of light fluxes having the first wavelength and the second wavelength longer than the first wavelength, the first optical disc and the first optical disc having a relatively thick protective layer than the first optical disc An optical pickup device capable of recording or reproducing information with respect to at least two types of optical discs having protective layer thicknesses different from each other.
When the first wavelength is λ1 (unit: nm) and the second wavelength is λ2 (unit: nm),
635 <λ1 <665
780 <λ2 <830
And
When the protective layer of the first optical disc is t1 (unit: mm) and the protective layer of the second optical disc is t2 (unit: mm),
t1 ≒ 0.6
t2 ≒ 1.2
And
The optical pickup device includes an objective lens,
The objective lens has a magnification M1 when information is recorded or reproduced on the first optical disk using the light beam having the first wavelength, and information on the second optical disk is recorded using the second wavelength. Assuming that the magnification in recording or reproduction is M2, the following condition (1),

Arranged to meet
Furthermore, the objective lens has an annular diffractive structure which corrects spherical aberration for each disk on at least one surface. When the focal length of the objective lens when recording or reproducing information with respect to the first optical disk using the light flux of the first wavelength is f (mm), the following condition (2):

The optical pickup device according to claim 6, wherein: In the objective lens, the diffractive structure is located in an inner area for securing an NA necessary for recording or reproducing information with respect to the second optical disk, and outside of the inner area. An outer area for securing the NA required for recording or reproduction, and
The high-order term component of the optical path length addition amount in the outer region at the boundary position between the inner region and the outer region is Φ1 (λ), and the high-order term component of the optical path length addition amount in the maximum effective diameter of the outer region is Φ2 (λ ), Where NA1 is a numerical aperture required for recording or reproducing information on the first optical disk, the following condition (3):

The optical pickup device according to claim 6 or 7, wherein:   9. The optical pickup device according to claim 6, wherein a light beam incident on the objective lens at the time of recording or reproducing information on the second optical disc is convergent light.   10. The optical pickup device according to claim 9, wherein a light beam incident on the objective lens at the time of recording or reproducing information on the first optical disc is substantially parallel light.

Images