JP4504875B2 - Objective lens for optical pickup and optical pickup device - Google Patents

Description

  The present invention relates to an optical pickup device that records or reproduces data on and from a plurality of types of optical disks having different recording densities and protective layer thicknesses, and an objective lens used in the device.

  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 switching between optical discs of different standards, the spherical aberration that changes depending on the thickness of the protective layer is corrected, and the numerical aperture (NA) of the light used for recording or reproducing information is changed to change the recording density. It is necessary to obtain a corresponding beam spot.

  For example, when recording or reproducing a DVD, it is necessary to narrow the beam spot using an optical system having a higher NA than that of an optical system dedicated to CD. Since the spot diameter becomes smaller as the wavelength becomes shorter, an optical system using a DVD uses a laser light source having an oscillation wavelength of about 660 nm, which is shorter than about 780 nm used in an optical system dedicated to CD. Therefore, in recent years, an optical pickup apparatus having a light source unit that can oscillate laser beams having different wavelengths has been used in an optical information recording / reproducing apparatus. In the text, the term “optical information recording / reproducing apparatus” includes all of the information recording apparatus, the information reproduction apparatus, and the information recording / reproducing apparatus.

  As one of means for converging the laser beam to the recording surface position of each optical disc in a good state with respect to each optical disc of CD and DVD, for example, an objective lens as disclosed in the following Patent Document 1 is irradiated with light. A technique to be mounted on a pickup device is mentioned.

JP-A-9-145994

  Patent Document 1 discloses an objective lens having three or more regions on at least one surface. In the objective lens, the outermost region has a shape such that light used when an optical disc (DVD) having a thin disc thickness is used forms a good spot on the recording surface of the optical disc. In addition, in the area located between the outermost area and the innermost area, the light used when using an optical disk (CD) having a large disk thickness forms a good spot diameter on the recording surface of the optical disk. It has a shape. By configuring as described above, the objective lens described in Patent Document 1 is compatible with two types of optical disks having different recording densities. Further, the objective lens described in Patent Document 1 is configured such that a parallel light beam is incident when any of two types of optical disks having different disk thicknesses is used. Thus, off-axis aberrations such as coma and astigmatism do not occur when the objective lens is moved (tracking shift) in the direction orthogonal to the optical axis direction (circumferential direction of the optical disk) for tracking.

  By the way, in recent years, new standard optical discs with higher recording density are being put into practical use in order to realize higher capacity of information recording. Examples of the optical disc include HD DVD. Such an optical disc has a protective layer thickness equal to or less than the protective layer thickness of DVD. Further, when recording or reproducing information on the optical disc, a light beam (for example, so-called blue laser light per 405 nm) having a wavelength shorter than the wavelength used when recording or reproducing information on a DVD is used due to its high recording density. Is required.

  With the practical application of new standard optical discs such as HD DVD, it is desired to quickly realize a new optical information recording / reproducing apparatus compatible with recording or reproducing information on existing optical discs and new standard optical discs. In order to realize the apparatus at an early stage, an objective lens that favorably converges the incident light beam on the recording surface of each optical disk is required regardless of which optical disk is used. However, as described above, the conventional objective lens exemplified in Patent Document 1 is designed to be compatible with two types of optical discs with different standards, particularly CD and DVD. That is, the conventional objective lens is not supposed to use a new standard optical disc. For this reason, when blue laser light is incident on a conventional objective lens, various aberrations such as spherical aberration occur on the recording surface of the new standard optical disc, and recording or reproducing information on the new standard optical disc. A suitable spot could not be formed.

  As described above, further improvement of the optical pickup device, more specifically, the objective lens for the optical pickup mounted in the device is desired so as to be compatible with information recording or reproduction with respect to the existing optical disc and the new standard optical disc. It was rare.

  Therefore, in view of the above circumstances, the present invention forms a good spot by suppressing spherical aberration on the recording surface of each disk during recording or reproduction of information on either an existing optical disk or a new standard optical disk. An object of the present invention is to provide an objective lens for an optical pickup that can satisfactorily suppress deterioration in performance when tracking shift is performed during use, and an optical pickup device equipped with the objective lens.

In order to solve the above problems, the objective lens for an optical pickup of the present invention uses three types of light beams having first to third wavelengths for a plurality of optical discs having at least two types of protective layer thicknesses. A first optical recording apparatus that records or reproduces information using the light beam having the shortest first wavelength among the first to third wavelengths is mounted on an optical pickup device that records or reproduces information on each optical disk. Optical disk protective layer thickness t1, second optical disk protective layer thickness t2 on which information is recorded or reproduced using a light beam having a second wavelength longer than the first wavelength, out of the first to third wavelengths The protective layer thickness t3 of the third optical disc on which information is recorded or reproduced using the light beam having the longest third wavelength has a relationship of t1 ≦ t2 <t3,
The numerical aperture NA1 required for recording or reproducing information on the first optical disc, the numerical aperture NA2 required for recording or reproducing information on the second optical disc, and the numerical aperture NA3 required for recording or reproducing information on the third optical disc Are in a relationship of NA1 ≧ NA2> NA3,
The first to third light beams are incident on the objective lens as substantially parallel light,
The objective lens has, on at least one surface, a first area for securing a numerical aperture necessary for recording or reproducing information with respect to the third optical disc, and a second area located outside the first area,
The first area is composed of an inner area in the vicinity of the optical axis, and an outer area located outside the inner area and focusing light of the third wavelength on the recording surface of the third optical disc with almost no aberration. ,
When the effective radius of the first region is h1 and the effective radius of the inner region is h1a, the following condition (1),
0.75 <h1a / h1 <0.87 (1)
It is characterized by satisfying.

  Here, the first optical disc corresponds to the above-mentioned new standard optical disc, more specifically, an information disc capable of recording information with a capacity higher than that of a DVD, and an optical disc using blue laser light for recording or reproducing information. The second optical disk corresponds to, for example, a DVD. The third optical disk corresponds to, for example, a CD or a CD-R.

  With this configuration, not only information is recorded or reproduced on an existing optical disc (second optical disc, third optical disc), but also information is recorded or reproduced on an optical disc of a new standard (first optical disc). Even at this time, not only spherical aberration but also off-axis aberration at the time of tracking shift can be satisfactorily suppressed. Therefore, according to the present invention, spots suitable for recording or reproducing information can be formed on the recording surfaces of three types of optical disks having different recording densities.

  In particular, the objective lens for an optical pickup according to claim 1 is designed so that the boundary between the inner region and the outer region in the first region satisfies the condition (1) as described above. Thereby, the light beam of the third wavelength can be favorably converged on the recording surface of the third optical disk. When the value shown in the condition (1) is equal to or greater than the upper limit, the outer region contributing to the convergence of the light beam having the third wavelength is reduced. Therefore, the light cannot be made sufficiently small to a spot diameter suitable for recording or reproducing information when using the third optical disk. Moreover, when the value shown in the condition (1) is below the lower limit, the outer region becomes larger than necessary. That is, a region where spherical aberration is generated particularly for light of the first wavelength is increased. Therefore, the loss of the amount of light when using the first optical disk is undesirably increased.

  According to the objective lens for an optical pickup according to claim 2, in the inner region, the light beam having the second wavelength transmitted through the inner region converges on the recording surface of the second optical disc with substantially no aberration. It is desirable to be configured.

  When a substantially parallel light beam is incident on the objective lens, if the inner region is formed as a refracting surface, only one of the first wavelength light beam to the third wavelength light beam can improve the spherical aberration in the inner region. Correction cannot be made, and some spherical aberration occurs for the remaining two types of light beams. Therefore, according to the second aspect of the present invention, the inner region is configured in a shape that can suppress the occurrence of spherical aberration in a well-balanced manner for any light flux. Specifically, the inner region forms a good image on the recording surface of the second optical disc when a light beam having the second wavelength, which is a substantially intermediate wavelength in the first to third wavelengths, is transmitted. Configured in shape. As a result, the spherical aberration caused by the light beam having the second wavelength passing through the inner region is corrected well, and at the same time, the light beam having the first and third wavelengths is transmitted through the inner region, whereby the recording surface of each optical disc The spherical aberration that occurs above can also be kept small.

  According to the objective lens for an optical pickup according to claim 3, the outer region has a diffractive structure that converges the light beam having the second wavelength and the light beam having the third wavelength on the recording surface of the corresponding optical disc. It is desirable to have By providing the diffractive structure in the outer region, the outer region can contribute to the convergence of the light beam not only when the third optical disc is used but also when the second optical disc is used. Therefore, the loss of light quantity when using the second optical disc is suppressed, and more accurate information recording or reproduction is realized. In addition, it is desirable that the diffraction order at which the diffraction efficiency is maximized in the outer region is the first order for both the light beams of the second wavelength and the third wavelength.

  According to the objective lens for an optical pickup according to claim 4, the second region converges the light beam having the first wavelength and the light beam having the second wavelength on the corresponding recording surface of the optical disc, and It is desirable to have a diffractive structure that does not contribute to the convergence of the light flux of the three wavelengths. Specifically, the diffractive structure is configured so that the diffraction order that maximizes the diffraction efficiency of the second region is such that the light beam having the first wavelength is the third order and the light beam having the second wavelength is the second order. As a result, the third wavelength light beam transmitted through the second region can be diffused.

  Further, when the light beam diameter corresponding to the numerical aperture necessary for recording or reproducing information on each optical disk (hereinafter referred to as the effective light beam diameter) is different between the light beams of the first and second wavelengths, the effective light beam diameter is outside the second region. It is desirable to further provide a third region that efficiently converges only a light beam having a large wavelength.

Specifically, when the effective light beam diameter at the entrance surface of the objective lens when the first light beam is incident is larger than the effective light beam diameter at the entrance surface of the objective lens when the second light beam is incident, that is, When the following condition (2) is satisfied, it is desirable to provide the diffractive structure with a third region that efficiently converges only the light beam having the first wavelength outside the second region.
f1 × NA1> f2 × NA2 (2)
However, f1 is the focal length when using the first optical disc,
f2 represents the focal length when the second optical disc is used.

  In the third region, the diffraction order that maximizes the diffraction efficiency for the first wavelength light beam is set to be different from the diffraction order that maximizes the diffraction efficiency for the first wavelength light beam in the second region. (Claim 5). By providing such a third region, it is possible to diffuse the light beams having the second and third wavelengths that pass through the third region.

Further, when the effective light beam diameter at the entrance surface of the objective lens when the second light beam is incident is larger than the effective light beam diameter at the entrance surface of the objective lens when the first light beam is incident, that is, When the condition (3) is satisfied, it is desirable that the diffractive structure is provided with a third region that efficiently converges only the light beam having the second wavelength outside the second region.
f1 × NA1 <f2 × NA2 (3)

  In the third region, the diffraction order that maximizes the diffraction efficiency for the second wavelength light beam is set to be different from the diffraction order that maximizes the diffraction efficiency for the second wavelength light beam in the second region. (Claim 6). By providing such a third region, it is possible to effectively diffuse the light beams having the first and third wavelengths transmitted through the third region.

Further, when the first wavelength is λ1, the third wavelength is λ3, the refractive index of the objective lens for the first wavelength λ1 is n1, and the refractive index of the objective lens for the third wavelength λ3 is n3, the following conditions are satisfied. (4),
λ1 / (n1-1): λ3 / (n3-1) ≒ 1: 2 (4)
It is mounted on an optical pickup device provided with a light source that irradiates three types of light fluxes that satisfy the above conditions. Such an optical pickup device can record or reproduce information on any of a plurality of optical disks having at least two types of protective layer thicknesses.

  As described above, according to the present invention, the aberration generated on the recording surface of each optical disk is satisfactorily suppressed by forming at least one surface in a predetermined structure. Further, according to the present invention, a parallel light beam is used when any optical disk is used. This makes it possible to satisfactorily suppress not only spherical aberration but also off-axis aberrations that occur during tracking, regardless of whether an existing optical disc or a new standard optical disc is used. That is, there are provided an optical pickup objective lens capable of forming a good spot on the recording surfaces of three types of optical discs having different recording densities and an optical pickup device equipped with the objective lens.

  Hereinafter, an embodiment of an optical pickup objective lens 10 according to the present invention and an optical pickup device 100 in which the objective lens 10 is mounted will be described. The optical pickup device 100 is mounted on an optical information recording or reproducing device having compatibility with the first to third optical discs D1 to D3 having different protective layer thicknesses and recording densities.

  FIG. 1 is a schematic diagram illustrating a schematic configuration of an optical pickup device 100 including the objective lens 10 according to the embodiment. The optical pickup device 100 includes a light source 1A that emits a first laser beam, a light source 1B that emits a second laser beam, a light source 1C that emits a third laser beam, diffraction gratings 2A, 2B, and 2C, and a collimating lens 3A. 3B, 3C and beam splitters 41 and 42. In the optical pickup device 100, it is necessary to cope with different NAs required when using each optical disk. Therefore, in the optical pickup device 100, although not shown, an aperture limiting element that defines the light beam diameter of the third laser light may be provided.

  2A to 2C are diagrams showing the optical pickup device 100 according to the present embodiment separately for each optical path when each optical disk is used. That is, FIGS. 2A to 2C are configuration diagrams at the time of recording or reproducing information on the first optical disc D1, the second optical disc D2, and the third optical disc D3 in order. 2A to 2C, the reference axis of the optical pickup device 100 is indicated by a one-dot chain line in the drawing. In the state shown in FIGS. 2A to 2C, the optical axis of the objective lens coincides with the reference axis of the optical system. However, the optical axis of the objective lens is separated from the reference axis of the optical system by a tracking operation or the like. There is also a state that comes off.

In the present embodiment, an optical disc having the highest recording density (for example, a new standard optical disc such as HD DVD) has a relatively lower recording density (for example, DVD or DVD) than the first optical disc D1 and the first optical disc D1. -R, etc.) is referred to as the second optical disk D2, and the optical disk having the lowest recording density (eg, CD, CD-R, etc.) is referred to as the third optical disk D3. All the optical disks are placed on a turntable (not shown) and rotated when information is recorded or reproduced. Further, assuming that the protective layer thicknesses of the optical disks D1 to D3 are t1 to t3, the protective layer thicknesses have the following relationship.
t1 ≦ t2 <t3

Further, when information is recorded on or reproduced from each of the optical discs D1 to D3, it is necessary to change the required NA value so that a beam spot corresponding to the difference in recording density can be obtained. Here, assuming that the optimum design numerical apertures required for recording or reproducing information with respect to each of the optical discs D1 to D3 are NA1, NA2, and NA3, each NA has the following relationship.
NA1 ≧ NA2> NA3
That is, when recording or reproducing information on the first optical disc D1 having the highest recording density, formation of a spot with a smaller diameter is required, so that the necessary NA is increased.

  When using the optical discs D1 to D3 having different recording densities as described above, laser beams having different wavelengths are used so as to obtain beam spot diameters corresponding to the recording densities. Specifically, when information is recorded on or reproduced from the first optical disc D1, in order to form the beam spot with the smallest diameter on the recording surface of the first optical disc D1, the shortest wavelength (the first wavelength) A laser beam (hereinafter, referred to as a first laser beam) is emitted from the light source 1A. Further, when information is recorded on or reproduced from the third optical disc D3, the longest wavelength (the third wavelength is used in order to form the beam spot having the largest diameter on the recording surface of the third optical disc D3. A laser beam having a wavelength (hereinafter referred to as a third laser beam) is emitted from the light source 1C. When recording or reproducing information on the second optical disc D2, a longer wavelength than that of the first laser beam is formed in order to form a relatively small-diameter spot on the recording surface of the second optical disc D2. In addition, laser light (hereinafter referred to as second laser light) having a shorter wavelength (second wavelength) than the third laser light is irradiated from the light source 1B.

  As shown in FIG. 1, the first to third laser beams emitted from the light sources 1A to 1C are converted into parallel light beams through the collimator lenses 3A to 3C. Each laser beam emitted from the collimating lens is guided to a common optical path via the beam splitters 41 and 42 and enters the objective lens 10. Each light beam transmitted through the objective lens 10 converges in the vicinity of the recording surface of each of the optical discs D1 to D3 to be recorded or reproduced. An optical disk used for recording or reproducing information is placed on a turntable (not shown).

  The objective lens 10 has a first surface 10a and a second surface 10b in order from the light source side. As shown in FIGS. 2A to 2C, the objective lens 10 is a biconvex plastic single lens in which both surfaces 10a and 10b are aspherical surfaces. As described above, each of the optical disks D1 to D3 has a different protective layer thickness and a different wavelength of laser light used when each optical disk is used, so that the refractive index of the objective lens 10 is also different. For this reason, the spherical aberration varies depending on the optical disk used for recording or reproducing information. Therefore, in the present embodiment, at least one surface of the objective lens 10 is divided into a plurality of regions having different surface shapes. Thereby, each laser beam transmitted through the objective lens 10 has a spherical aberration suppressed on the recording surface of the corresponding optical disc, and forms a spot suitable for recording / reproducing information.

  FIG. 3 is an enlarged view of the vicinity of the surface 10 a having a cross-sectional shape on the surface including the optical axis AX of the objective lens 10. As shown in FIG. 3, the first region 1, the second region 2, and the third region 3 are formed on the surface 10 a of the objective lens 10 in order from the optical axis AX side.

  The first area 1 is an area for converging the first to third laser beams on the recording surfaces of the corresponding optical disks D1 to D3, and is optimal for recording or reproducing information on the third optical disk. It is also an area for securing a large numerical aperture NA3. Specifically, the first region 1 includes an inner region 11 and an outer region 12 in order from the optical axis AX side.

  The inner region 11 is a continuous surface. The inner region 11 is configured to converge well on the recording surface of the second optical disc D2 when the second laser light is transmitted through the region 11. In this way, the first and third lasers transmitted through the inner region 11 by applying an optimal surface shape to the second laser light having a substantially intermediate wavelength among the first to third laser lights. All of the light converges on the recording surface of each optical disc in a state where the generation of aberrations is suppressed as much as possible.

The outer region 12 has a surface shape that allows the third laser light transmitted through the region 12 to converge well on the recording surface of the third optical disc D3 without generating spherical aberration. With this configuration, the third laser beam is converged to a state suitable for recording or reproducing information with respect to the third optical disc, and is generated when another laser beam, particularly the first laser beam is used. In order to suppress the spherical aberration that occurs, the boundary between the inner region 11 and the outer region 12 is set to a position that satisfies the following condition (1).
0.75 <h1a / h1 <0.87 (1)
However, h1 is the effective radius of the first region 1,
h1a represents the effective radius of the inner region 11, respectively.

  The second region 2 has a diffractive structure in which the diffraction efficiency hardly decreases with respect to the first laser light and the second laser light. Specifically, the diffractive structure is designed so that the diffraction order that maximizes the diffraction efficiency is the third order for the first laser light and the second order for the second laser light. The phase of the wavefront of the third laser light transmitted through the second region 2 designed in this way is not the same as that of the third laser light transmitted through the first region 1. That is, the second region 2 converges the first laser light and the second laser light on the recording surfaces of the corresponding optical disks D1 and D2, and does not contribute to the convergence of the third laser light.

  4A to 4C are aberration diagrams showing spherical aberrations generated on the recording surfaces of the optical disks D1 to D3 by the respective laser beams when the objective lens 10 is used. 4A shows the spherical aberration generated when the first laser beam is used, FIG. 4B shows the spherical aberration generated when the second laser beam is used, and FIG. 4C shows the third laser beam used. Each of the spherical aberrations that sometimes occurs is represented. As shown in FIGS. 4A to 4C, the inner region 11 is configured to suppress the generation of aberrations most when the second optical disc D2 is used. For this reason, regardless of whether the first optical disk D1 or the third optical disk D3 is used, the occurrence of spherical aberration is suppressed to a sufficiently small level that does not hinder information recording or reproduction. Further, as shown in FIGS. 4A to 4C, the outer region 12 converges only the third optical disc D3 satisfactorily.

  Note that, as a mode different from the above, a diffraction structure may be formed in the outer region 12. The diffractive structure is configured such that not only the third laser beam but also the second laser beam converges on the recording surface of the second optical disc D2 after passing through the diffractive structure. At this time, the diffraction order at which the diffraction efficiency is maximized for the second and third laser beams in the outer region 12 is designed first. With this configuration, it is possible to suppress the spherical aberration that has occurred in the outer region 12 in FIG.

  Further, the third region 3 is provided when the incident light beam diameter of the first laser light on the first surface 10a of the objective lens 10 is different from the incident light beam diameter of the second laser light.

First, when the focal length when using the first optical disc D1 is f1, and the focal length when using the second optical disc D2 is f2, the following condition (2):
f1 × NA1> f2 × NA2 (2)
Is established, the third region 3 has a diffractive structure in which the first laser beam transmitted through the region 3 converges satisfactorily with almost no aberration on the recording surface of the first optical disc D1. Unlike the second region 2, the third region 3 formed when the condition (2) is satisfied does not contribute to the convergence of the second laser beam. In other words, the third region 3 has an aperture limiting function for the second laser light. Therefore, the diffraction structure has a diffraction order that maximizes the diffraction efficiency for the first laser light, and a diffraction order (third order in the present embodiment) that maximizes the diffraction efficiency for the first laser light in the second region 2. Are designed differently. At the time of designing, the third region 3 is blazed so that the diffraction efficiency with respect to the first laser beam is maximized.

In addition, the following condition (3),
f1 × NA1 <f2 × NA2 (3)
Is established, the third region 3 has a diffractive structure such that the second laser light transmitted through the region 3 converges satisfactorily with almost no aberration on the recording surface of the second optical disc D2. Unlike the second region 2, the third region 3 formed when the condition (3) is satisfied does not contribute to the convergence of the first laser beam. In other words, the third region 3 has an aperture limiting function for the first laser beam. Therefore, in the diffraction structure, the diffraction order that maximizes the diffraction efficiency for the second laser light is the diffraction order that maximizes the diffraction efficiency for the second laser light in the second region 2 (second order in this embodiment). Designed to be different. At the time of designing, the third region 33 is blazed so that the diffraction efficiency with respect to the second laser beam is maximized.

  By using the objective lens 10 in which the surface shapes of the regions 1 to 3 on the first surface 10a are designed as described above, a beam spot suitable for recording or reproducing information on each of the optical discs D1 to D3 can be obtained. .

In the optical pickup device 100 described above, when the wavelengths of the first laser beam and the third laser beam are compared in consideration of the refractive index of the objective lens 10, the aberration correction by the diffractive lens structure is difficult. Even in this case, a good spot is formed on the recording surface of each optical disc, and information can be recorded or reproduced. Specifically, the aberration correction is difficult because the first wavelength is λ1, the third wavelength is λ3, the refractive index of the objective lens 10 for the first wavelength λ1 is n1, and the objective for the third wavelength λ3. When the refractive index of the lens 10 is n3, the following relationship (4) is satisfied.
λ1 / (n1-1): λ3 / (n3-1) ≒ 1: 2 (4)

  When there is a relationship as in the condition (4), any of the first to third laser beams is incident on the objective lens 10 of the present embodiment as a parallel light beam, thereby correcting the spherical aberration, A good beam spot can be formed on the recording surface. In other words, the optical pickup objective lens 10 and the device 100 are lenses or devices that are compatible with recording or reproducing information with respect to a plurality of optical disks having the relationship as in the condition (4).

  Three specific examples based on the embodiment described above are presented. The schematic configuration of the optical pickup device 100 of each embodiment is shown in FIG. 1 and FIGS. 2 (A) to 2 (C). In each example, the objective lens for an optical pickup having compatibility between the first optical disc D1 and the second optical disc D2 having a protective layer thickness of 0.6 mm and the third optical disc D3 having a protective layer thickness of 1.2 mm. 10 relates to an optical pickup device 100 in which 10 is mounted.

  Specific specifications of the objective lens 10 mounted on the optical pickup device 100 according to the first embodiment are shown in Table 1.

  In Table 1, the design wavelength means the wavelength of each laser beam that is suitable for recording or reproducing information on each of the optical discs D1 to D3. As shown in Table 1, the magnification value indicates that in the optical pickup device of Example 1, the laser light is applied to the objective lens 10 as a parallel light beam even when any of the first to third optical disks D1 to D3 is used. Incident. Specific numerical configurations of the optical pickup device 100 including the objective lens 10 shown in Table 1 when the optical discs D1 to D3 are used are shown in Tables 2 to 4, respectively. However, in Tables 2 to 4, for the convenience of explanation, the numerical configuration regarding the members disposed between the light sources 1A to 1C and the objective lens 10 is omitted. The same applies to the tables of Example 2 and Example 3 shown below.

  In each table, r is the radius of curvature (unit: mm) of each lens surface, d is the lens thickness or lens interval (unit: mm) during information recording or reproduction, and n (Xnm) is the refractive index at wavelength Xnm. is there. Further, the first inner region indicated in the surface number indicates the inner region 11 of the first region 1, and the first outer region indicates the outer region 12 of the first region 1. Surface number 3 represents a protective layer in each optical disk, and surface number 4 represents a recording surface in each optical disk. The same applies to the tables shown in Example 2 and Example 3 described below.

According to Table 1, f1 × NA1 is 1.95 and f2 × NA2 is 1.86. That is, the optical pickup device 100 of Example 1 satisfies the condition (2). Therefore, as shown in Tables 2 to 4, in the objective lens 10 of Example 1, the third paragraph 3 is further formed outside the second paragraph 2. In addition, when the range of each area | region in the 1st surface 10a is represented by the height h from the optical axis AX,
First inner region 11... H ≦ 1.15,
First outer region 12 ... 1.15 <h ≦ 1.40,
2nd area | region 2 ... 1.40 <h <= 1.86,
Third region 3... 1.86 <h ≦ 1.95.

  As described above, the objective lens 10 of the optical pickup device 100 of Example 1 has h1a / h1 of 0.82, which satisfies the condition (1). Further, from the design wavelength shown in Table 1 and the refractive indexes shown in Tables 2 to 4, it can be seen that the condition (4) of the optical pickup device 100 of Example 1 is 1: 2.

Both surfaces 10a and 10b of the objective lens 10 are aspheric. The shape of the aspheric surface is the distance (sag amount) from the tangential plane on the optical axis of the aspherical surface at the coordinate point on the aspherical surface where the height from the optical axis is h, and X (h). The curvature (1 / r) above is C, the conic coefficient is K, the fourth, sixth, eighth, tenth, twelfth, etc. aspheric coefficients are A ₄ , A ₆ , A ₈ , A ₁₀ , A ₁₂ and so on are represented by the following equations.

  Table 5 shows conical coefficients and aspheric coefficients that define the shape of each aspheric surface of both surfaces 10a and 10b. In addition, the notation E in each table | surface represents the power which uses 10 as the radix and the number on the right of E is an exponent. As shown in Table 5, the aspherical shapes of the inner region 11, the outer region 12, the second region 2, and the third region 3 are different on the first surface 10a.

  In addition, the objective lens 10 according to the first embodiment is configured so that each laser beam converges well with a suitable NA by providing a diffractive structure in a partial region provided on the first surface 10a. Specifically, the diffractive structure is provided in the second region 2 and the third region 3 on the first surface 10a. The outer region 12 is a refractive surface that contributes only to the convergence of the first laser beam.

The diffractive structures formed in the second region 2 and the third region 3 of the objective lens 10 are represented by the following optical path difference function φ (h).

The optical path difference function φ (h) represents the function of the diffractive lens in the form of an additional optical path length at a height h from the optical axis. P ₂ , P ₄ , P ₆ ,... Are secondary, quaternary, sixth order,. The optical path difference function coefficients P ₂ ,... That define the diffraction structure are shown in Table 6. m represents the diffraction order at which the diffraction efficiency of each laser beam is maximized in each of the second region 2 and the third region 3. As described above, the diffraction order m is set to a different value for each laser beam to be used and for each region in which the diffraction structure is provided. As shown in Table 7, the second region 2 has a diffractive structure that contributes to the convergence of the first laser beam and the second laser beam. On the other hand, the third region 3 has a diffractive structure that contributes only to the convergence of the first laser beam, that is, has an aperture limiting function for the second laser beam.

  FIGS. 5A to 5C are aberration diagrams illustrating spherical aberration that occurs when the first to third laser beams pass through the objective lens 10 in the optical pickup device 100 according to the first embodiment. . 5A shows the spherical aberration generated when the first laser beam is used, FIG. 5B shows the spherical aberration generated when the second laser beam is used, and FIG. 5C shows the third laser beam used. Each of the spherical aberrations that sometimes occurs is represented. As shown in FIGS. 5A to 5C, in the outer region 12 for securing the numerical aperture when the third optical disk is used, the spherical aberration generated by the third laser beam is corrected. In the second region 2 and the third region 3 for securing a higher numerical aperture, the spherical aberration generated by the first and second laser beams is corrected. As a result, spherical aberration is satisfactorily suppressed when recording or reproducing information on any optical disc, and a beam spot suitable for recording or reproducing information is formed on the recording surface.

  6A to 6C are graphs showing beam spots formed on the recording surfaces of the optical disks D1 to D3 by the laser beams transmitted through the objective lens 10 in the optical pickup device 100 of the first embodiment. is there. 6A shows the spots formed when the first laser beam is used, FIG. 6B shows the spots formed when the second laser beam is used, and FIG. 6C shows the spots formed when the third laser beam is used. FIGS. 7A to 7C show a surface on which an optimum beam spot is formed when each of the optical disks D1 to D3 is used while adopting the same configuration as the objective lens 10 of the first embodiment. 10 is a graph showing, as Comparative Example 1, beam spots formed on the recording surfaces of optical discs D1 to D3 by laser beams transmitted through three objective lenses configured in a shape. FIG. 7A shows the use of a surface-shaped objective lens that favorably converges the first laser light, and FIG. 7B shows the use of a surface-shape objective lens that favorably converges the second laser light. (C) represents a spot formed when using a surface-shaped objective lens that favorably converges the third laser beam. In each graph, the horizontal axis indicates the spot diameter (unit: mm), and the vertical axis indicates the relative intensity when the intensity at the center of the spot is 100. Table 8 shows beam diameters up to 13.5% in intensity for Example 1 and Comparative Example 1.

  As shown in FIGS. 6 (A) to (C), FIGS. 7 (A) to (C), and Table 8, the objective lens 10 of Example 1 records or records information on any of the first to third optical disks. Even during reproduction, the spherical aberration can be corrected well, and a beam spot having substantially the same size and the same intensity as in Comparative Example 1 can be formed.

  Specific specifications of the objective lens 10 mounted on the optical pickup device 100 of Example 2 are shown in Table 9. Specific numerical configurations of the optical pickup device 100 including the objective lens 10 shown in Table 9 when the optical discs D1 to D3 are used are shown in Tables 10 to 12, respectively.

According to Table 9, both f1 × NA1 and f2 × NA2 are 1.95. That is, the objective lens 10 of Example 2 is an objective lens that does not satisfy any of the above conditions (2) and (3). Therefore, unlike the first embodiment, the third region 3 is not provided. In addition, when the range of each area | region in the 1st surface 10a is represented by the height h from the optical axis AX,
First inner region 11... H ≦ 1.20,
First outer region 12... 1.20 <h ≦ 1.46,
The second region 2... 1.46 <h ≦ 1.95.

  As described above, the objective lens 10 of the optical pickup device 100 of Example 2 has h1a / h1 of 0.82, which satisfies the condition (1). Further, from the design wavelength shown in Table 9 and the refractive indexes shown in Tables 10 to 12, it can be seen that the condition (4) of the optical pickup device 100 of Example 2 is 1: 2.

  In the second embodiment, similarly to the first embodiment, both surfaces 10a and 10b of the objective lens 10 are aspherical surfaces. Table 13 shows conical coefficients and aspheric coefficients that define the shape of each aspheric surface of both surfaces 10a and 10b. As shown in Table 13, the aspherical shapes of the inner region 11, the outer region 12, and the second region 2 are different on the first surface 10a.

  In addition, the objective lens 10 according to the second embodiment is configured such that each laser beam converges favorably with a suitable NA by providing diffraction structures in the outer region 12 and the second region 2 on the first surface 10a. The inner region 11 is a refracting surface that contributes to the convergence of the first to third laser beams. Table 14 shows optical path difference function coefficients defining the above-mentioned diffraction structures, and Table 15 shows diffraction orders.

  As shown in Table 15, the outer region 12 of Example 2 has a diffractive structure that contributes to the convergence of the second to third laser beams. The second region 2 of Example 2 has a diffractive structure that contributes to the convergence of the first laser beam and the second laser beam. Table 15 shows that the first laser beam transmitted through the outer region 12 of Example 2 has the maximum diffraction order as the first order. This only indicates that the first-order diffracted light is generated because the outer region 12 is included within the effective diameter of the first laser light, and means that the first-order diffracted light of the first laser light converges. It is not a thing.

  FIGS. 8A to 8C are aberration diagrams illustrating spherical aberration that occurs when the first to third laser beams pass through the objective lens 10 in the optical pickup device 100 according to the second embodiment. . 8A shows the spherical aberration generated when the first laser beam is used, FIG. 8B shows the spherical aberration generated when the second laser beam is used, and FIG. 8C shows the third laser beam used. Each of the spherical aberrations that sometimes occurs is represented. As shown in FIGS. 8A to 8C, the spherical aberration generated by the third laser beam is corrected in the outer region 12 for securing the numerical aperture when the third optical disk is used. In the second region 2 for securing a higher numerical aperture, the spherical aberration generated by the first and second laser beams is corrected. As a result, spherical aberration is satisfactorily suppressed when recording or reproducing information on any optical disc, and a beam spot suitable for recording or reproducing information is formed on the recording surface.

  9A to 9C are graphs showing beam spots formed on the recording surfaces of the optical disks D1 to D3 by the laser beams transmitted through the objective lens 10 in the optical pickup device 100 of the second embodiment. is there. 9A shows the spots formed when the first laser beam is used, FIG. 9B shows the spots formed when the second laser beam is used, and FIG. 9C shows the spots formed when the third laser beam is used. FIGS. 10A to 10C show a surface on which an optimum beam spot is formed when each of the optical disks D1 to D3 is used while adopting the same configuration as the objective lens 10 of the second embodiment. 10 is a graph showing, as Comparative Example 2, beam spots formed on the recording surfaces of optical discs D1 to D3 by laser beams transmitted through three objective lenses configured in a shape. FIG. 10A shows the use of a surface-shaped objective lens that favorably converges the first laser beam, and FIG. 10B shows the use of a surface-shape objective lens that favorably converges the second laser beam. (C) represents a spot formed when using a surface-shaped objective lens that favorably converges the third laser beam. In each graph, the horizontal axis indicates the spot diameter (unit: mm), and the vertical axis indicates the relative intensity when the intensity at the center of the spot is 100. Table 16 shows beam diameters up to 13.5% in intensity for Example 2 and Comparative Example 2.

  As shown in FIGS. 9A to 9C, FIGS. 10A to 10C, and Table 16, the objective lens 10 of Example 2 records information on any of the first to third optical discs. Even during reproduction, it is possible to correct the spherical aberration satisfactorily and form a beam spot having substantially the same size and the same intensity as in Comparative Example 2.

  The optical pickup device 100 of the third embodiment is the same as that of the second embodiment except that the surface shape of the objective lens 10 mounted on the device 100 is different. Therefore, the specific numerical configuration of the objective lens 10 and the specific numerical configuration of the optical pickup device 100 including the objective lens 10 when the optical discs D1 to D3 are used are described with reference to Tables 9 to 12 above. Description is omitted.

Since the objective lens 10 of Example 3 has the configuration shown in Table 9, neither of the above conditions (2) and (3) is applicable. Therefore, unlike the first embodiment, the third region 3 is not provided. In addition, when the range of each area | region in the 1st surface 10a is represented by the height h from the optical axis AX,
First inner region 11... H ≦ 1.11
First outer region 12 ... 1.11 <h ≦ 1.46,
The second region 2... 1.46 <h ≦ 1.95.

  As described above, in the objective lens 10 of the optical pickup device 100 of Example 3, h1a / h1 is 0.76, which satisfies the condition (1). In the optical pickup device 100 of the third embodiment, the condition (4) is 1: 2 as in the second embodiment.

  In the third embodiment, both surfaces 10a and 10b of the objective lens 10 are aspherical surfaces as in the other first and second embodiments. Table 17 shows conical coefficients and aspheric coefficients that define the shape of each aspheric surface of both surfaces 10a and 10b. As shown in Table 17, the aspherical shapes of the inner region 11, the outer region 12, and the second region 2 are different on the first surface 10a.

  Similarly to the second embodiment, the objective lens 10 according to the third embodiment is provided with a diffractive structure in the outer region 12 and the second region 2 on the first surface 10a so that each laser beam converges well with a suitable NA. It is configured. The inner region 11 is a refracting surface that contributes to the convergence of the first to third laser beams. Table 18 shows the optical path difference function coefficient defining each diffraction structure, and Table 19 shows the diffraction orders.

  As shown in Table 19, the outer region 12 of Example 3 has a diffractive structure that contributes to the convergence of the second to third laser beams. Further, the second region 2 of Example 3 has a diffractive structure that contributes to the convergence of the first laser beam and the second laser beam. Table 19 shows that the first laser beam transmitted through the outer region 12 of Example 3 has the maximum diffraction order as the first order. This just indicates that the first-order diffracted light is generated because the outer region 12 is included within the effective diameter of the first laser light, as in the second embodiment. It does not mean to converge.

  FIGS. 11A to 11C are aberration diagrams illustrating spherical aberration that occurs when the first to third laser beams pass through the objective lens 10 in the optical pickup device 100 according to the third embodiment. . FIG. 11A shows the spherical aberration generated when the first laser beam is used, FIG. 11B shows the spherical aberration generated when the second laser beam is used, and FIG. 11C shows the third laser beam used. Each of the spherical aberrations that sometimes occurs is represented. As shown in FIGS. 11A to 11C, the spherical aberration generated by the third laser beam is corrected in the outer region 12 for securing the numerical aperture when the third optical disk is used. In the second region 2 for securing a higher numerical aperture, the spherical aberration generated by the first and second laser beams is corrected. As a result, spherical aberration is satisfactorily suppressed when recording or reproducing information on any optical disc, and a beam spot suitable for recording or reproducing information is formed on the recording surface.

  12A to 12C are graphs showing beam spots formed on the recording surfaces of the optical discs D1 to D3 by the laser beams transmitted through the objective lens 10 in the optical pickup apparatus 100 of the third embodiment. is there. 12A shows spots formed when the first laser light is used, FIG. 12B shows the spots formed when the second laser light is used, and FIG. 12C shows the spots formed when the third laser light is used. FIGS. 13A to 13C show a surface on which an optimum beam spot is formed when each of the optical disks D1 to D3 is used while adopting the same configuration as the objective lens 10 of the third embodiment. 10 is a graph showing, as Comparative Example 2, beam spots formed on the recording surfaces of optical disks D1 to D3 by laser beams transmitted through three objective lenses configured in a shape. FIG. 13A shows the use of a surface-shaped objective lens that favorably converges the first laser light, and FIG. 13B shows the use of a surface-shape objective lens that favorably converges the second laser light. (C) represents a spot formed when using a surface-shaped objective lens that favorably converges the third laser beam. In each graph, the horizontal axis indicates the spot diameter (unit: mm), and the vertical axis indicates the relative intensity when the intensity at the center of the spot is 100. Table 20 shows beam diameters up to 13.5% in intensity for Example 3 and Comparative Example 3.

  As shown in FIGS. 12A to 12C, FIGS. 13A to 13C, and Table 20, the objective lens 10 of Example 3 records or records information on any of the first to third optical disks. Even during reproduction, the spherical aberration can be corrected well, and a beam spot having substantially the same intensity and the same size as that of the comparative example 3 can be formed.

  The above is the embodiment of the present invention. In addition, each said Example is an example of the objective lens based on this invention to the last. That is, the objective lens according to the present invention is not limited to the specific numerical configuration of each embodiment. For example, the surface on which the diffractive structure is provided may be the second surface 10b instead of the first surface 10a. Moreover, you may provide a diffraction structure in both the 1st surface and the 2nd surface.

It is a schematic diagram showing schematic structure of the optical pick-up apparatus which has the objective lens of embodiment of this invention. FIG. 3 is a diagram showing the optical pickup device of the embodiment of the present invention separately for each optical path when each optical disk is used. It is an enlarged view of the section shape in the field including the optical axis of the objective lens of an embodiment. FIG. 5 is an aberration diagram showing spherical aberration generated on the recording surface of each optical disk by each laser beam when the objective lens of the embodiment is used. FIG. 5 is an aberration diagram illustrating spherical aberration that occurs when the first to third laser beams pass through the objective lens in the optical pickup device of Example 1. 3 is a graph showing a beam spot formed on the recording surface of each optical disc by each laser beam transmitted through an objective lens in the optical pickup device of Example 1. FIG. 6 is a graph showing beam spots formed on the recording surface of each optical disc by laser beams that have passed through three objective lenses of Comparative Example 1. FIG. FIG. 6 is an aberration diagram illustrating spherical aberration that occurs when the first to third laser beams pass through the objective lens in the optical pickup device of Example 2. 6 is a graph showing beam spots formed on the recording surface of each optical disc by each laser beam transmitted through an objective lens in the optical pickup device of Example 2. 10 is a graph showing beam spots formed on the recording surface of each optical disc by laser beams transmitted through three objective lenses of Comparative Example 2. FIG. FIG. 6 is an aberration diagram illustrating spherical aberration that occurs when the first to third laser beams pass through the objective lens in the optical pickup device of Example 3. 12 is a graph showing beam spots formed on the recording surface of each optical disc by each laser beam transmitted through an objective lens in the optical pickup device of Example 3. 10 is a graph showing a beam spot formed on the recording surface of each optical disc by each laser beam transmitted through three objective lenses of Comparative Example 3.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Objective lens D1-D3 Optical disk 1A, 1B, 1C Light source 2A, 2B, 2C Diffraction element 3A, 3B, 3C Coupling lens 41, 42 Beam splitter 100 Optical pick-up apparatus

Claims (6)

An objective lens in an optical pickup device that records or reproduces information on each optical disc by using three types of light beams having first to third wavelengths for a plurality of optical discs having at least two types of protective layer thicknesses. There,
The protective layer thickness of the first optical disc on which information is recorded or reproduced using the light beam having the shortest first wavelength among the first to third wavelengths is t1, and the second is longer than the first wavelength. Information recording or reproduction is performed using a light beam with a wavelength of t2, and the thickness of the protective layer of the second optical disk is t2, and information recording is performed using a light beam with the longest third wavelength among the first to third wavelengths. Or if the protective layer thickness of the third optical disk to be played is t3,
t1 ≦ t2 <t3
And
The numerical aperture required for recording or reproducing information on the first optical disc is NA1, the numerical aperture required for recording or reproducing information on the second optical disc is NA2, and the information is recorded or reproduced on the third optical disc. If the required numerical aperture is NA3,
NA1 ≧ NA2> NA3
And
The first to third light beams are incident on the objective lens as substantially parallel light,
The objective lens is at least on one side,
A first area for ensuring a numerical aperture necessary for recording or reproducing information with respect to the third optical disc;
Outside the first region, wherein it is converged first wavelength of the light beam and the light flux of the second wavelength to each of the first optical disk and the second optical disc recording surface, and the third wavelength a second region having a diffractive structure such as not to converge the light flux,
A third region for converging only the light beam of the second wavelength outside the second region;
Have
The first region is an inner region near the optical axis, and an outer region that is located outside the inner region and converges the light of the third wavelength on the recording surface of the third optical disc almost without aberration, Consisting of
When the effective radius of the first region is h1, and the effective radius of the inner region is h1a, the following condition (1),
0.75 <h1a / h1 <0.87 (1)
Meet the,
Assuming that the focal length at the time of recording or reproducing information on the first optical disc is f1, and the focal length at the time of recording or reproducing information on the second optical disc is f2, the following condition (2):
f1 × NA1> f2 × NA2 (2)
The filling,
In the third region, the diffraction order that maximizes the diffraction efficiency for the first wavelength light beam is different from the diffraction order that maximizes the diffraction efficiency for the first wavelength light beam in the second region. A characteristic objective lens for optical pickup. An objective lens in an optical pickup device that records or reproduces information on each optical disc by using three types of light beams having first to third wavelengths for a plurality of optical discs having at least two types of protective layer thicknesses. There,
The protective layer thickness of the first optical disc on which information is recorded or reproduced using the light beam having the shortest first wavelength among the first to third wavelengths is t1, and the second is longer than the first wavelength. Information recording or reproduction is performed using a light beam with a wavelength of t2, and the thickness of the protective layer of the second optical disk is t2, and information recording is performed using a light beam with the longest third wavelength among the first to third wavelengths. Or if the protective layer thickness of the third optical disk to be played is t3,
t1 ≦ t2 <t3
And
The numerical aperture required for recording or reproducing information on the first optical disc is NA1, the numerical aperture required for recording or reproducing information on the second optical disc is NA2, and the information is recorded or reproduced on the third optical disc. If the required numerical aperture is NA3,
NA1 ≧ NA2> NA3
And
The first to third light beams are incident on the objective lens as substantially parallel light,
The objective lens is at least on one side,
A first area for ensuring a numerical aperture necessary for recording or reproducing information with respect to the third optical disc;
Outside the first region, wherein it is converged first wavelength of the light beam and the light flux of the second wavelength to each of the first optical disk and the second optical disc recording surface, and the third wavelength a second region having a diffractive structure such as not to converge the light flux,
A third region for converging only the light beam of the second wavelength outside the second region;
Have
The first region is an inner region near the optical axis, an outer region that is located outside the inner region and converges the light of the third wavelength on the recording surface of the third optical disc substantially without aberration, Consisting of
When the effective radius of the first region is h1, and the effective radius of the inner region is h1a, the following condition (1),
0.75 <h1a / h1 <0.87 (1)
Meet the,
Assuming that the focal length at the time of recording or reproducing information on the first optical disc is f1, and the focal length at the time of recording or reproducing information on the second optical disc is f2, the following condition (3):
f1 × NA1 <f2 × NA2 (3)
The filling,
In the third region, the diffraction order that maximizes the diffraction efficiency for the second wavelength light beam is different from the diffraction order that maximizes the diffraction efficiency for the second wavelength light beam in the second region. A characteristic objective lens for optical pickup. The objective lens for an optical pickup according to claim 1 or 2,
The optical pickup objective lens, wherein the inner region is configured such that the light beam having the second wavelength transmitted through the inner region converges on the recording surface of the second optical disc with substantially no aberration. . In the objective lens for optical pickups according to any one of claims 1 to 3 ,
The outer region has a diffractive structure that converges the light flux of the second wavelength and the light flux of the third wavelength on the recording surfaces of the second optical disc and the third optical disc, respectively.
The objective lens for an optical pickup, wherein the diffraction order that maximizes the diffraction efficiency in the outer region is first order for both the second wavelength light beam and the third wavelength light beam. In the objective lens for optical pickups according to any one of claims 1 to 4 ,
An optical pickup objective lens characterized in that the diffraction order at which the diffraction efficiency of the second region is maximized is that the light beam having the first wavelength is third-order and the light beam having the second wavelength is second-order. An optical pickup device that records or reproduces information on each optical disc by using three types of light beams having first to third wavelengths for a plurality of optical discs having at least two types of protective layer thicknesses,
An optical pickup objective lens according to any one of claims 1 to 5 ,
A light source for irradiating each luminous flux;
Have
When the first wavelength is λ1, the third wavelength is λ3, the refractive index of the objective lens with respect to the first wavelength λ1 is n1, and the refractive index of the objective lens with respect to the third wavelength λ3 is n3. The following formula (4),
λ1 / (n1-1): λ3 / (n3-1) ≒ 1: 2 (4)
An optical pickup device satisfying the requirements.

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