JP2005032411A - Objective lens for optical pickup and optical pickup device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an objective lens for an optical pickup which can be manufactured at a low cost and can form a good spot while suppressing spherical aberration at the time of recording or reproducing information on any of a plurality of optical discs having different standards. thing.
In the objective lens for optical pickup, substantially parallel light is incident on the first and second light beams, and divergent light is incident on the third light beam. Assuming that the imaging magnification is M1 to M3 and the focal length is f1 to f3 during recording or reproduction,
-0.02 <f1 × M1 <0.02
-0.02 <f2 × M2 <0.02
-0.29 <f3 × M3 <-0.19
And the diffraction order at which the diffraction efficiency for each light beam is maximized in the first region in which the light beam of the third wavelength is converged on at least one surface of the objective lens is the light beam of the first to third wavelengths It was set as the structure which has the diffraction structure which is 6th order, 4th order, and 3rd order in order.
[Selection] Figure 1

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.

  Further, as one of means for converging laser light to the recording surface position of each optical disk in a good state for each optical disk of CD and DVD, a diffractive structure having a ring-shaped fine step on one surface of the objective lens A technique for mounting an objective lens provided with an optical pickup device on an optical pickup device has been put into practical use. The objective lens as described above is always on the recording surface corresponding to each optical disc having a different protective layer thickness by utilizing the feature that the spherical aberration generated by the diffraction structure changes depending on the wavelength of the incident light beam. The laser beam is focused in a good state.

  An objective lens having compatibility with optical disks having different corresponding wavelengths, such as DVD and CD, and an optical pickup device equipped with the objective lens are disclosed in, for example, Patent Document 1 below.

JP 2000-81666 A

  In the objective lens described in Patent Document 1, the surface on which the diffractive structure is provided is divided in detail into an inner region located in the vicinity of the optical axis and an outer region outside the inner region. The inner area is such that light for recording or reproducing information on the CD is well converged on the recording surface of the CD, and light for recording or reproducing information on the DVD is also well converged on the recording surface of the DVD. It has a diffractive structure. The outer area is such that light for recording or reproducing information on the CD does not contribute to convergence on the recording surface of the CD, and only light for recording or reproducing information on the DVD converges favorably on the recording surface of the DVD. It has a diffractive structure.

  With the structure as described above, among the light for recording or reproducing information with respect to the CD, the light beam transmitted through the outer region diffuses on the recording surface because it has a large spherical aberration, and only the light beam transmitted through the inner region. Converge on the recording surface to form a relatively large-diameter spot. Further, the light for recording or reproducing information on the DVD has a small NA suitable for recording or reproducing information on a DVD having a high recording density because the light beam transmitted through the outer region is converged without any aberration, so that the NA increases. A spot is formed.

  In recent years, new standard optical discs with higher recording density are being put into practical use in order to achieve higher capacity for information recording. Examples of the optical disc include an 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, a so-called blue laser beam 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 as exemplified in Patent Document 1 is designed to be suitable when information is recorded or reproduced on a CD and a 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.

  An objective lens capable of recording or reproducing information on a new standard disk, DVD, and CD, and an optical pickup device equipped with the objective lens are disclosed in, for example, Patent Document 2 below.

JP 2001-195769 A

  The objective lens exemplified in Patent Document 2 has a diffractive structure that records or reproduces information by allowing substantially parallel light to enter when using a new standard disk or DVD. Also, when using a CD, diverging light is incident to record or reproduce information. However, there is no disclosure regarding what diffraction structure should be designed to obtain a spot suitable for recording or reproducing information on the recording surface.

  Further, the diffractive structure provided in the objective lens is often formed in a sawtooth shape (blazed shape) in order to increase the amount of light of a specific diffraction order light. For example, in order to efficiently extract light of the same order over a wide wavelength range, it is also known to provide a diffractive structure in which two diffraction gratings are stacked as exemplified in Patent Document 3 below.

JP 2000-75118 A

  However, when using a diffractive structure in which two diffraction gratings are stacked as in Patent Document 3, a reduction in the amount of light due to lateral shift that occurs when the diffraction gratings are stacked becomes large. In order to suppress lateral shift, manufacturing with high accuracy is required, and therefore, it is not suitable for an objective lens for an optical pickup that is inexpensive and manufactured in large quantities.

  From the above, further improvement of the objective lens has been demanded so as to be compatible with recording or reproduction of information not only on existing optical disks but also on new standard optical disks.

  Therefore, in view of the above circumstances, the present invention can be manufactured at a low cost, and can suppress spherical aberration on the recording surface of each disc when recording or reproducing information on either an existing optical disc or a new standard optical disc. An object of the present invention is to provide an objective lens for an optical pickup capable of forming a simple spot and an optical pickup apparatus equipped with the objective lens.

In order to solve the above problems, the optical pickup device of the present invention uses the first to third light fluxes having different wavelengths for a plurality of optical disks having at least two types of protective layer thicknesses, thereby An optical pickup device for recording or reproducing, comprising an objective lens, wherein information is recorded or reproduced using the first light beam having the shortest wavelength among the first to third light beams. The protective layer thickness of the optical disk is t1, the protective layer thickness of the second optical disk on which information is recorded or reproduced using a second light beam having a wavelength longer than the wavelength of the first light beam is t2, and the first to third If the protective layer thickness of the third optical disc on which information is recorded or reproduced using the third light beam having the longest wavelength among the light beams is t3, t1 to t3 are expressed by the following relationship:
t1 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
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 when recording or reproducing information on the third optical disc If the required numerical aperture is NA3, NA1 to NA3 have the following relationship:
NA1 ≧ NA2> NA3
The first wavelength and the second wavelength of the light beam are substantially parallel light, and the third wavelength of the light beam is divergent light incident on the objective lens. The imaging magnification is M1, the focal length is f1, the information is recorded or reproduced on the second optical disc, the imaging magnification is M2, the focal length is f2, and the information is recorded or reproduced on the third optical disc. When the imaging magnification is M3 and the focal length is f3, the following equations (1) to (3),
-0.02 <f1 × M1 <0.02 (1)
-0.02 <f2 × M2 <0.02 (2)
-0.29 <f3 × M3 <-0.19 (3)
Meets
Furthermore, at least one surface of the objective lens has a diffractive structure,
In the first region where the third wavelength light beam is converged on the recording surface of the third optical disk, the diffraction order that maximizes the diffraction efficiency is the sixth wavelength light beam of the first wavelength and the second wavelength. The light beam of the third wavelength is the fourth order, and the light beam of the third wavelength is the third order.

  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.

  The objective lens of the optical pickup device according to claim 1, wherein the light beams having the first wavelength and the second wavelength are incident substantially parallel light, and the light beam having the third wavelength is incident divergent light, and at least 1 The surface has a diffractive structure as described above. As a result, the light beams having the first to third wavelengths are favorably converged on each optical disk surface, and the utilization efficiency of each light beam is enhanced. Specifically, the diffraction orders of the light beams of the first to third wavelengths are configured in order of sixth order, fourth order, and third order.

  In a conventional objective lens that supports only recording or reproduction of information on CDs and DVDs, spherical aberration can be corrected for two different wavelengths by providing a diffractive structure. However, as in this patent, in the case of three different wavelengths, the spherical aberration cannot be corrected due to insufficient design freedom. Therefore, spherical aberration is corrected by diffractive structure for two of the three different wavelengths, and the divergence of the light beam incident on the objective lens is changed for the remaining one wavelength. Is corrected.

  In the objective lens in the optical pickup device according to the first aspect, the diffraction orders of the light beams having the first to third wavelengths are set in order of the sixth order, the fourth order, and the third order. In the present invention, it is assumed that the first wavelength is equivalent to 405 nm and the third wavelength is equivalent to 780 nm, and the relative spherical aberration between the first wavelength and the third wavelength cannot be corrected by the diffractive structure. . This is because the power of the diffractive lens is represented by m × λ / d, where m is the diffraction order, λ is the wavelength, and d is the grating pitch, so that the sixth-order diffracted light for the first wavelength and the third wavelength This is because the third-order diffracted light receives substantially the same power from the diffractive lens.

  When a finite system is used, aberration deterioration during tracking operation is inevitable due to off-axis coma. The higher the NA required for recording or reproducing information, the smaller the aberration tolerance. Therefore, the objective lens for an optical pickup according to claim 1 has a substantially parallel light beam incident on the objective lens when the first and second optical discs that require high NA for recording or reproducing information are used. When a third optical disk with a low NA is used, a divergent light beam enters the objective lens. Thereby, even when the objective lens is tracking-shifted, when the first and second optical disks are used, the amount of coma and astigmatism generated can be made almost negligible. As described above, the first region is a region for converging the light beams of the first to third wavelengths on the recording surfaces of the first to third optical discs, respectively, in the vicinity of the optical axis of the objective lens. Provided.

  The focal length of the coupling lens disposed between each light source and the optical disc varies depending on the refractive index due to the wavelength difference. Therefore, in the configuration in which the light beam emitted from each light source is guided to the recording surface via a common coupling lens, the light source that emits the light beam of the first wavelength is the same as the light source that emits the light beam of the second wavelength. When they are on the substrate, that is, when each light source is at the same distance from the coupling lens, at least one of the first and second wavelengths of the light beam incident on the objective lens must be convergent light or divergent light. I don't get it. Therefore, it is desirable that the objective lens according to the first aspect of the present invention has as small an imaging magnification as possible so as to satisfy the above expressions (1) and (2) when the first and second optical disks are used. Thereby, the amount of aberration generated during the tracking operation can be reduced.

  Further, the objective lens in the optical pickup device according to claim 1 is configured to satisfy the above formula (3) when the third optical disk is used. Exceeding the upper limit of formula (3) is not preferable because excessive spherical aberration remains. On the other hand, if the value is below the lower limit of the expression (3), an under spherical aberration occurs, which is not preferable.

  As described above, the present invention is not only for recording or reproducing information on an existing optical disc (second optical disc, third optical disc), but also for recording or reproducing information on an optical disc of a new standard (first optical disc). In this case, a good spot can be formed on the recording surface. In addition, as compared with a conventional objective lens in which a plurality of diffraction gratings having different shapes are laminated, molding with a single mold is possible, and thus it is possible to manufacture inexpensively and easily.

  Further, claim 2 defines the structure in one region of the diffraction grating. That is, the objective lens in the optical pickup device according to claim 2 is based on a refracting surface located on the inner side of a boundary between adjacent refracting surfaces at least a part of the first region of the diffractive structure. Then, the optical path length addition amount for the first wavelength light beam has a step of about 8 wavelengths, and the optical path length addition amount has a step of about -2 wavelengths, and the steps can be arranged alternately. preferable.

  When the diffraction lens is configured as a step having a difference in optical path length of approximately 6 wavelengths with respect to the light beam of the first wavelength for each region, a diffraction efficiency at a design wavelength is relatively high. When the oscillation wavelength of the laser changes due to changes, especially when the oscillation wavelength of the first laser is shortened, or when the oscillation wavelength of the second laser is lengthened, the amount of phase shift around the diffraction grating zone Increases, and the light utilization efficiency decreases rapidly. In order to avoid this, as long as only the first wavelength and the second wavelength are used, it is sufficient to use a structure in which the ring width is about half, using third-order diffracted light and second-order diffracted light, respectively. However, since it is assumed that the third wavelength is approximately twice the wavelength of the first wavelength, in such a configuration, the third wavelength is distributed to the first-order diffracted light and the second-order diffracted light. Therefore, only an efficiency of about 40% can be obtained.

  Therefore, based on a structure using third-order diffracted light with respect to the light beam having the first wavelength, a structure in which every other region is shifted by about 5 wavelengths with respect to the light beam having the first wavelength is used. In this way, the shift amount is about three wavelengths with respect to the second wavelength, and high diffraction efficiency is maintained. In addition, since the shift amount is about 2.5λ for the third wavelength, the original amount of about 1.5λ is added to about 4.0λ, and there is no cancellation of the light amount, resulting in high efficiency. The zone designed in this way consists of two phase regions, one zone of the diffraction grating that uses the 6th order at the first wavelength, the 4th order at the second wavelength, and the 3rd order light at the third wavelength. Can be considered.

  By designing at least a part of the first region of the diffractive structure to have a step as described above, the diffraction efficiency of each light flux of the first to third wavelengths can be further increased.

  According to a third aspect of the present invention, in the objective lens, when at least a partial range of the first region is based on a refracting surface located inside at a boundary between adjacent refracting surfaces, the first region The optical path length addition amount to the light beam may have a step corresponding to approximately −8 wavelengths, and the optical path length addition amount may include a step corresponding to approximately two wavelengths, and the steps may be alternately arranged.

  According to claim 4, the objective lens has a diffractive structure that converges the light beam of the first wavelength and the light beam of the second wavelength on the recording surfaces of the first optical disk and the second optical disk, respectively. In addition, it is preferable to provide a second region that does not contribute to the convergence of the third light flux. The diffraction orders at which the diffraction efficiency of the second region is maximized are preferably the third order light flux of the first wavelength and the second order light flux of the second wavelength.

  By providing such a second region, it is possible to diffuse the light beam having the third wavelength that passes through the second region. Further, it is possible to suppress deterioration of wavefront aberration due to a change in laser oscillation wavelength due to a temperature change or the like. Note that the second region is provided outside the first region.

  Furthermore, when the above expression (4) is satisfied, it is desirable that the diffractive structure is provided with a third region that efficiently converges only the first light flux outside the second region. In the third region, the diffraction order that maximizes the diffraction efficiency for the first light beam is set to be different from the diffraction order that maximizes the diffraction efficiency for the first light beam in the second region. Item 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.

  Moreover, when satisfy | filling said Formula (5), it is desirable to provide the said diffraction structure with the 3rd area | region which converges only the light beam of a 2nd wavelength efficiently outside the 2nd area | region. In the third region, the diffraction order that maximizes the diffraction efficiency for the light beam of the second wavelength is set to be different from the diffraction order that maximizes the diffraction efficiency for the light beam of the second wavelength in the second region. (Claim 6). By providing such a third region, it is possible to diffuse the light beams having the first and third wavelengths that pass through the third region.

In the optical pickup device according to claim 7, the first wavelength is λ1, the second wavelength is λ2, the third wavelength is λ3, the refractive index of the objective lens with respect to the first wavelength λ1, n1, When the refractive index of the objective lens for the wavelength λ2 is n2, and the refractive index of the objective lens for the third wavelength λ3 is n3, the following equations (6) and (7)
λ1 / (n1-1): λ3 / (n3-1) ≈1: 2 (6)
λ1 / (n1-1): λ2 / (n2-1) ≈3: 5 (7)
A light source that irradiates three kinds of light fluxes that satisfy both of the above is provided. 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.

  Claim 8 defines the structure in one region of the diffraction grating. That is, the optical pickup device according to claim 8 has a diffractive structure on at least one surface, and at least a part of the first region is a refraction located on the inner side at a boundary between adjacent refracting surfaces. If the surface is used as a reference, the optical path length addition amount for the light beam of the first wavelength has a step of about 8 wavelengths, and the optical path length addition amount has a step of about -2 wavelengths, and the steps are arranged alternately. It is desirable to have a diffractive element.

  Claim 9 also defines the structure in one region of the diffraction grating. That is, the optical pickup device according to claim 9 has a diffractive structure on at least one surface, and at least a partial range of the first region is located on an inner side at a boundary between adjacent refracting surfaces. Using the refracting surface as a reference, the optical path length addition amount for the light beam of the first wavelength has a step of about −8 wavelengths, and the optical path length addition amount has a step of about 2 wavelengths, and each step is arranged alternately. It is desirable to have a diffractive element provided.

  By designing at least a part of the diffractive structure to have a step as described above, the diffraction efficiency of each light flux of the first to third wavelengths can be further increased.

In the diffraction element described above, the first wavelength and the second wavelength of the light flux are substantially parallel light, and the third wavelength of the light flux is divergent light, and information is recorded or reproduced on the first optical disc. The imaging magnification is M1, the focal length is f1, and the information is recorded or reproduced on the second optical disc, the imaging magnification is M2, the focal length is f2, and the information is recorded or reproduced on the third optical disc. Where the imaging magnification is M3 and the focal length is f3, the following equations (1) to (3),
-0.02 <f1 × M1 <0.02 (1)
-0.02 <f2 × M2 <0.02 (2)
-0.29 <f3 × M3 <-0.19 (3)
And an objective lens for the second wavelength λ2, the first wavelength λ1, the second wavelength λ2, the third wavelength λ3, the objective lens refractive index n1 for the first wavelength λ1 When the refractive index is n2 and the refractive index of the objective lens with respect to the third wavelength λ3 is n3, the following equations (6) and (7),
λ1 / (n1-1): λ3 / (n3-1) ≈1: 2 (6)
λ1 / (n1-1): λ2 / (n2-1) ≈3: 5 (7)
Are mounted on an optical pickup device provided with a light source that irradiates three kinds of light fluxes that satisfy both of the above (claim 10). 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.

The objective lens for an optical pickup according to the present invention uses the three types of light beams having the first to third wavelengths for a plurality of optical discs having at least two types of protective layer thicknesses, thereby providing information on each optical disc. Protecting the first optical disc, which is an objective lens in an optical pickup device that records or reproduces information, on which information is recorded or reproduced using a light beam having the shortest wavelength among the three types of light flux The layer thickness is t1, the protective layer thickness of the second optical disc on which information is recorded or reproduced using a light beam having a second wavelength longer than the light beam having the first wavelength is t2, and three kinds of light beams When the protective layer thickness of the third optical disk on which information is recorded or reproduced using the light beam having the third wavelength which is the longest wavelength is t3, t1 to t3 have the following relationship:
t1 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
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 when recording or reproducing information on the third optical disc If the required numerical aperture is NA3, NA1 to NA3 have the following relationship:
NA1 ≧ NA2> NA3
The first wavelength and the second wavelength of the light beam are substantially parallel light, and the third wavelength of the light beam is divergent light incident on the objective lens. The imaging magnification is M1, the focal length is f1, the information is recorded or reproduced on the second optical disc, the imaging magnification is M2, the focal length is f2, and the information is recorded or reproduced on the third optical disc. When the image magnification is M3 and the focal length is f3, the following equations (1) to (3),
-0.02 <f1 × M1 <0.02 (1)
-0.02 <f2 × M2 <0.02 (2)
-0.29 <f3 × M3 <-0.19 (3)
And a diffraction order that maximizes diffraction efficiency in a first region that has a diffractive structure on at least one surface of the objective lens and converges a light beam of the third wavelength onto the recording surface of the third optical disk. Is characterized in that the light beam having the first wavelength is sixth-order, the light beam having the second wavelength is fourth-order, and the light beam having the third wavelength is third-order.

  As described above, according to the present invention, the diffraction order of the diffractive structure is appropriately set, and aberrations that cannot be removed by the diffractive structure are satisfactorily suppressed by adjusting the imaging magnification. Provided are an optical pickup objective lens capable of forming a good spot by suppressing spherical aberration on a recording surface of each disc during recording or reproduction of information on any standard optical disc, and an optical pickup device equipped with the objective lens. be able to.

  Furthermore, the objective lens according to the present invention employs a diffractive structure that can be injection-molded using a single mold. It is inexpensive and can be mass-produced.

  Hereinafter, embodiments of an optical pickup objective lens 30 according to the present invention and an optical pickup device 100 on which the objective lens 10 is mounted will be described. The optical pickup device 100 is mounted on an optical information recording / 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. The optical pickup device includes first to third light sources 10A to 10C, coupling lenses 20A to 20C, an objective lens 30, and beam splitters 41 and 42. As shown in FIG. 1, each light beam irradiated from each light source 10 </ b> A to 10 </ b> C and transmitted through each coupling lens 20 </ b> A to 20 </ b> C is guided to a common optical path by two beam splitters 41 and 42 and enters the objective lens 30. . The light beam that has passed through the objective lens converges on the recording surface of the optical discs D1 to D3 to be recorded or reproduced.

  FIGS. 2A to 2C are diagrams illustrating a schematic configuration of the optical pickup device 100 illustrated in FIG. 1 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. 1 and 2, the reference axis of the optical pickup device 100 is indicated by a one-dot chain line in the drawings. The light beam emitted from the first light source 10A is drawn with a solid line, the light beam emitted from the second light source 10B is drawn with a broken line, and the light beam emitted from the third light source 10C is drawn with a dotted line. In the state shown in FIGS. 1 and 2, the optical axis of the objective lens coincides with the reference axis of the optical system. However, there is a state where the optical axis of the objective lens deviates from the reference axis of the optical system due to a tracking operation or the like.

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) is referred to as a second optical disk D2, and the optical disk having the lowest recording density (for example, CD, CD-R, etc.) is referred to as a third optical disk D3. Further, when the protective layer thicknesses of the optical disks D1 to D3 are t1 to t3, respectively,
t1 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
It is. All optical disks are placed on a turntable (not shown) and rotated when information is recorded or reproduced.

When recording or reproducing information on 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 NAs required for recording or reproducing information on 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.

  The first light source 10A is used when information is recorded on or reproduced from the first optical disc D1. That is, the first light source 10A has a laser beam having the shortest wavelength (first wavelength) of the three light sources (hereinafter referred to as “the first wavelength”) in order to form the beam spot having the smallest diameter on the recording surface of the first optical disc D1. The first laser beam). The third light source 10C is used when information is recorded on or reproduced from the third optical disc D3. That is, the third light source 10C is a laser beam having the longest wavelength (third wavelength) of the three light sources (hereinafter referred to as “the third wavelength”) in order to form the beam spot having the largest diameter on the recording surface of the third optical disc D3. The third laser beam). The second light source 10B is used when information is recorded on or reproduced from the second optical disc D2 having a high recording density. That is, the second light source 10B has a longer wavelength than the first laser beam and shorter than the third laser beam in order to form a relatively small-diameter spot on the recording surface of the second optical disc D2. A laser beam having a wavelength (second wavelength) (hereinafter referred to as a second laser beam) is irradiated.

  In addition, each light source 10A-10C may be independently arrange | positioned in a different place, and may be arrange | positioned along with the predetermined direction on the single board | substrate. When the light sources 10A to 10C are independently arranged at different locations, the laser light emitted from the light sources 10A to 10C passes through the coupling lenses 20A to 20C as shown in FIG. The beams are combined by the beam splitters 41 and 42 and guided to the objective lens 30.

  The objective lens 30 has a first surface 30a and a second surface 30b in order from each light source side. As shown in FIGS. 2A to 2C, the objective lens 30 is a biconvex plastic single lens in which both surfaces 30a and 30b are aspherical surfaces. As described above, in each of the optical disks D1 to D3, the thickness of the protective layer is different between D1 (or D2) and D3, and the wavelength of the light beam used when using each optical disk is also different, so that the refractive index of the objective lens 30 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, an annular diffractive structure having a plurality of fine steps centered on the optical axis is provided on at least one surface of the objective lens 30 (surface 30a in the present embodiment).

  FIG. 3 is an enlarged view of the vicinity of the first surface 30 a having a cross-sectional shape on the surface including the optical axis AX of the objective lens 30. The first surface 30a of the objective lens 30 is formed as follows. The first surface 30 a includes a first region 31 positioned around the optical axis, a second region 32 positioned around the first region 31, and a lens outer peripheral portion (from the outermost periphery of the second region 33). And a third region 33 up to (not shown). Each annular zone-shaped step formed in each of the first to third regions 31 to 33 has an optical path length difference from the inner side toward the outer side of the surface 30a toward the outer side. It is formed to be doubled.

  The first region 31 has a diffractive structure that allows the first to third laser beams to converge well on the recording surfaces of the corresponding optical disks D1 to D3. Specifically, the diffractive structure has a wavelength characteristic of spherical aberration that cancels a change in spherical aberration that occurs in the refractive lens portion of the objective lens 10 due to the wavelength difference between the first and second laser beams. The first region 31 in the objective lens 30 of the present embodiment mainly has two types of modes depending on the diffraction structure.

  The first mode of the diffractive structure of the first region 31 is such that the diffraction order that maximizes the diffraction efficiency is the sixth order for the first laser light, the fourth order for the second laser light, and the third order for the third laser light. Designed to be As described above, the first region 31 in the first embodiment can be converged with high diffraction efficiency regardless of which laser beam is incident. In particular, the difference in optical path length given by the ring zone of the diffractive structure of the wavelength of the first laser beam is substantially equal to the three wavelengths of the third laser beam, so that there is an advantage that the utilization efficiency is very high.

  FIG. 4 is a cross-sectional view schematically showing a second embodiment of the diffractive structure of the first region 31 on a plane including the optical axis. As shown in FIG. 4, the second mode of the first region 31 has a plurality of annular refractive surfaces P1, P2, P3,. Further, the diffractive structure has steps S1, S2, S3,... At the boundary between the refracting surfaces adjacent to each other.

  In addition, the optical path length addition amount used below to describe the second mode of the first region 31 is all for the first laser beam unless otherwise specified. Each step has an optical path length addition amount on the outer refracting surface (for example, P2) with respect to the inner refracting surface, that is, the surface closer to the optical axis AX (for example, P1) among the refracting surfaces adjacent to each other. Designed to be -8 wavelengths or approximately 2 wavelengths. Further, as shown in FIG. 4, steps of approximately −8 wavelengths and steps of approximately 2 wavelengths are alternately and continuously arranged. In the text, the sign of the optical path length addition amount is the refractive surface adjacent to each other in FIG. 4, and the outer refractive surface is on the right side (that is, the light source side in the optical axis AX direction) with reference to the inner refractive surface. When the optical path length is added, the optical path length addition amount is defined as positive, and conversely, when the optical path length addition amount is positioned on the left side (that is, the optical disc side in the optical axis AX direction), the optical path length addition amount is defined as negative.

  The first region 31 configured as described above includes a diffractive structure (hereinafter referred to as a diffractive structure α for convenience) designed so as to have a level difference of approximately −3 wavelengths with respect to the first laser light, In this manner, the first laser beam enjoys substantially the same or more advantages than the diffractive structure designed to have approximately −6 wavelengths.

  The diffractive structure α has a plurality of annular refracting surfaces P1, P2 ', P3, P4' ... as shown by dotted lines in FIG. In the case of the diffractive structure α, the optical path length addition amount with respect to the second laser light in each annular zone is approximately −2 wavelengths, so that high efficiency can be obtained when the second laser light is incident. However, since the optical path length addition amount for the third laser light is approximately −1.5 wavelengths, when the third laser light is incident, it is distributed to the first-order diffracted light and the second-order diffracted light, which is about 40%. Only efficiency can be obtained.

  Therefore, a structure in which the first region 31 is shifted by about −5 wavelengths for every other refractive surface based on the diffraction structure α is used. Specifically, an optical path length of approximately −5 wavelengths is given to the first laser with respect to the refractive surfaces P2 ′, P4 ′, P6 ′, and P8 ′ of the annular zone of the diffractive structure α. As indicated by the solid line, an annular structure composed of refractive surfaces P1 to P8 is adopted. At this time, an optical path length difference of about −3 wavelengths is given for the second laser and about −2.5 wavelengths is given for the third laser. In this way, there are two refractive surfaces (P1 and P2, P3 and P4, P5 and P6, P7) having steps (S1, S3, S5, and S7) having an optical path length addition amount of approximately −8 wavelengths. P8) gives the optical path length difference of about −5 wavelengths to the second laser beam by adding the original about −2.0 wavelengths, and becomes highly efficient. Further, the third laser beam is added to the original approximately -1.5 wavelength to give an optical path length difference corresponding to approximately -4 wavelengths, and there is no cancellation of the amount of light, and a high diffraction efficiency is obtained.

  The diffractive structure designed in this way is one of the diffraction gratings that uses the sixth-order diffracted light for the first laser, the fourth-order diffracted light for the second laser, and the third-order diffracted light for the third laser. It can be considered that the zone is composed of two phase regions. Specifically, two sets of refracting surfaces (P1-P2 and P3-P4, P3-P4 and P5) having steps (S2, S4, S6, S8) having an optical path length addition amount of approximately −2 wavelengths. -P6, P5-P6 and P7-P8) are approximately -6 wavelengths for the first laser beam, approximately -4 wavelengths for the second laser beam, and for the third laser beam. Thus, an optical path length difference corresponding to approximately -3 wavelengths can be given. Accordingly, the first region 31 in the second mode can obtain high diffraction efficiency for all laser beams.

  The above is the configuration of the first region 31 of the second aspect. Note that the diffractive structure in the second mode in which the two types of steps as described above are continuous does not necessarily have to be provided in the entire first region 31, and may be only in a part of the range.

  Further, the first region 31 of the second mode is configured such that a step having an optical path length addition amount of about 8 wavelengths and a step having an optical path length addition amount of about -2 wavelengths alternate with respect to the inner refractive surface. The configuration may be continuous. Even with such a diffractive structure in which two types of steps are alternately continued, the same effects as described above can be obtained.

  In the objective lens 30 having the first region 31 in any of the above-described aspects, the configurations of the second and third regions 32 and 33 are substantially the same. The second region 32 shown in FIG. 3 has a diffractive structure in which the first laser beam and the second laser beam converge well with substantially no aberration on the recording surfaces of the corresponding optical disks D1 and D2, respectively. Specifically, the diffractive structure is designed so that the diffraction order that maximizes the diffraction efficiency for the first laser beam is the third order, and the diffraction order that maximizes the diffraction efficiency for the second laser beam is the second order. The The phase of the wave front of the third laser light transmitted through the second region 32 designed in this way does not match that of the third laser light transmitted through the first region 31. That is, the second region 32 does not contribute to the convergence of the third laser beam.

In the third region 33 shown in FIG. 3, the objective lens 30 has the following formula (4) or formula (5),
f1 × NA1> f2 × NA2 (4)
f1 × NA1 <f2 × NA2 (5)
It is an area provided when satisfying.

  The third region 33 provided when the objective lens 30 satisfies the formula (4) has a diffractive structure in which the first laser light is converged well with substantially no aberration on the recording surface of the first optical disc D1. . Here, unlike the second region 32, the third region 33 does not contribute to the convergence of the second laser beam. Therefore, the diffractive structure is designed such that the diffraction order that maximizes the diffraction efficiency for the first laser light is different from the diffraction order that maximizes the diffraction efficiency for the first laser light in the second region 32. . At the time of designing, the third region 33 is blazed so that the diffraction efficiency with respect to the first laser beam is maximized.

  The third region 33 provided when the objective lens 30 satisfies the formula (5) has a diffractive structure in which the second laser light is converged well with substantially no aberration on the recording surface of the second optical disc D2. . Here, unlike the second region 32, the third region 33 does not contribute to the convergence of the first laser beam. Therefore, the diffractive structure is designed such that the diffraction order that maximizes the diffraction efficiency for the second laser light is different from the diffraction order that maximizes the diffraction efficiency for the second laser light in the second region 32. . 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 designing the diffractive structure of each of the regions 31 to 33 as described above, a suitable NA (NA1 to NA3) can be obtained at the time of recording or reproducing information on each of the optical discs D1 to D3.

In addition, when the objective lens 30 having the diffractive structure as described above is on the reference axis of the apparatus 100, laser light transmitted through the objective lens 30 during recording or reproduction of information with respect to the first optical disc D1 or the second optical disc D2. Converges on the recording surface of each optical disc with substantially no aberration. However, when the objective lens 30 is displaced from the reference axis by tracking, off-axis light is incident on the objective lens 30. At this time, when divergent light enters the objective lens, coma aberration or the like occurs. In general, an optical disc that requires a high NA for recording or reproducing information has a narrower tolerance for aberration. Therefore, even when the objective lens 30 is tracking-shifted during recording or reproduction of information with respect to the first optical disc D1 or the second optical disc D2, it is substantially parallel to the objective lens 30 in order to suppress the occurrence of various aberrations. Make the light beam incident. Specifically, the imaging magnification of the objective lens 30 when using the first optical disc D1 is M1, the focal length is f1, the imaging magnification of the objective lens 30 when using the second optical disc D2 is M2, and the focal length is f2. Then, the objective lens 30 is designed so as to satisfy the following expressions (1) and (2).
-0.02 <f1 × M1 <0.02 (1)
-0.02 <f2 × M2 <0.02 (2)

  By designing the objective lens 30 so as to satisfy the expressions (1) and (2), coma and astigmatism generated during the tracking operation are good when the first optical disk D1 and the second optical disk D2 are used. Can be suppressed.

  In the present embodiment, the first light source 10A and the second light source 10B are arranged at positions where the laser light emitted from the light sources 10A and 10B is converted into parallel light beams by the coupling lenses 20A and 20B. Thus, the imaging magnification of the objective lens 30 is set to zero. That is, the coupling lenses 20A and 20B of the present embodiment function as collimating lenses for the first laser light and the second laser light.

As described above, when the objective lens 30 is designed so as to effectively suppress the aberration when using each of the optical discs D1 and D2 having a narrow tolerance for aberration, spherical aberration generated when information is recorded on or reproduced from the third optical disc D3. I can't keep it down. Therefore, the spherical aberration generated when the third optical disc D3 is used is corrected by making the light beam incident on the objective lens 30 into divergent light as shown in FIG. Specifically, when the imaging magnification of the objective lens 30 when the third optical disc D3 is used is M3 and the focal length is f3, the objective lens 30 is designed to satisfy the following expression (3).
-0.29 <f3 × M3 <-0.19 (3)

  By designing the objective lens 30 so as to satisfy the expression (3), it is possible to satisfactorily suppress spherical aberration that occurs when the third optical disc D3 is used.

  With the above-described configuration, as shown in FIGS. 2A to 2C, when recording or reproducing information on each of the optical discs D1 to D3, the laser light emitted from the light source corresponding to the optical disc to be used is It converges in the vicinity of the recording surface of the optical disc via each coupling lens 20A-20C, each beam splitter 41, 42, and objective lens 30, and forms a spot suitable for recording or reproducing information.

The optical pickup device 100 shown in FIGS. 2A to 2C described above has an aberration due to the diffractive lens structure when the wavelength of each laser beam used is compared in consideration of the refractive index of the objective lens 30. Even if the correction is difficult, good spots are formed on the recording surface of each optical disc, and information can be recorded or reproduced. Specifically, the aberration correction is difficult because the wavelength of the first laser beam is λ1, the wavelength of the second laser beam is λ2, the wavelength of the third laser beam is λ3, and the first wavelength is λ1. Assuming that the refractive index of the objective lens 30 is n1, the refractive index of the objective lens 30 for the second wavelength λ2 is n2, and the refractive index of the objective lens 30 for the third wavelength λ3 is n3, the following equations (6) and (7) refers to the relationship.
λ1 / (n1-1): λ3 / (n3-1) ≈1: 2 (6)
λ1 / (n1-1): λ2 / (n2-1) ≈3: 5 (7)

  When there is a relationship such as Equation (6) or Equation (7), if the diffraction order of the light beam with the first wavelength λ1 is set to the sixth order and the diffraction order of the light beam with the third wavelength λ3 is set to the third order, the first Since the sixth-order diffracted light for the wavelength λ1 and the third-order diffracted light for the third wavelength λ3 receive substantially the same power from the diffractive lens, the relative refractive index change due to the first and third wavelength differences and the difference in the disc protective layer thickness Spherical aberration cannot be corrected with a diffractive structure. Therefore, the objective lens 30 of the present embodiment corrects the aberration almost completely by the diffractive structure provided on the surface 30a when the first and second optical disks having a high recording density and a narrow tolerance range of the aberration are used. When using this optical disc, aberration can be corrected by the imaging magnification of the diffractive structure and the objective lens. That is, it can be said that the optical pickup objective lens 30 and the apparatus 100 are lenses or apparatuses compatible with a plurality of optical discs having a relationship such as Expression (6) and Expression (7).

  Next, three specific examples based on the embodiment described above are presented. Example 1 relates to an optical pickup device 100 including an objective lens 30 having a first region 31 of the first aspect. Example 2 and Example 3 relate to the optical pickup device 100 including the objective lens 30 having the first region 31 of the second aspect. It should be noted that the optical pickup device 100 of any of the embodiments is compatible with 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. Have.

  Schematic diagrams representing the optical pickup device 100 of Example 1 and Example 2 described later are shown in FIGS. 2 (A) to 2 (C). Specific specifications of the objective lens 30 of Example 1 are shown in Table 1.

  In Table 1, the design wavelength λ (unit: nm) is the most suitable wavelength for recording or reproducing information on each of the optical discs D1 to D3. Specific numerical configurations of the optical pickup device 100 including the objective lens 30 shown in Table 1 are shown in Tables 2 to 4.

  Tables 2 to 4 show specific numerical configurations of the optical pickup device 100 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.

As shown in the remarks in Tables 2 to 4, the surface number 0 is the light sources 10A to 10C, the surface numbers 1 and 2 are the coupling lenses 20A to 20C, and the surface numbers 3 and 4 in Tables 2 to 3 are the beams. The splitter 41, the surface numbers 5 and 6 in Tables 2 and 3, the surface numbers 3 and 4 in Table 4 are the beam splitter 42, the surface numbers 7 and 8 in Tables 2 and 3, and the surface numbers 5 and 6 in Table 4 are the objective lens 30. Surface numbers 9 and 10 in Tables 2 to 3 and surface numbers 7 and 8 in Table 4 indicate the protective layer and recording surface of each optical disk D1 to D3, which is a medium. In Tables 2 to 4, r is a radius of curvature (unit: mm) of each lens surface, d is a lens thickness or lens interval (unit: mm) at the time of recording or reproducing information, and n (Xnm) is a wavelength Xnm. Refractive index. As shown in Tables 2 to 4, the first surface 30a of the objective lens 30 includes first to third regions. When the range of each of the first to third regions 31 to 33 is expressed by a height h from the optical axis AX,
First region 31... H ≦ 1.53,
Second region 32... 1.53 <h ≦ 1.87,
The third region 33... 1.87 <h ≦ 1.95.

The second surfaces of the coupling lenses 20A to 20C and both surfaces 30a and 30b of the objective lens 30 are aspheric. 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. The curvature (1 / r) is C, the conic coefficient is K, and the fourth, sixth, eighth, tenth, and twelfth aspherical coefficients are A ₄ , A ₆ , A ₈ , A ₁₀ , A _12. Is represented by the following equation.

  Tables 5 to 7 show conical coefficients and aspheric coefficients that define the shape of each aspheric surface when information is recorded or reproduced on the first optical disc D1, the second optical disc D2, and the third optical disc D3.

  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. The same applies to Table 8 shown below.

Furthermore, the diffractive structure formed on the first surface 30a of the objective lens 30 is 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 diffractive structure are shown in Table 8. m represents the diffraction order at which the diffraction efficiency of each laser beam is maximized in each of the first to third regions 31 to 33. The diffraction order m is set to a different value for each region depending on the laser beam to be used, and details are shown in Table 9.

  The specific specifications of the objective lens 30, the specific numerical configuration of the optical pickup device 100, and various coefficients that define each surface in the second embodiment are the same as those in the first embodiment. Therefore, the description here is abbreviate | omitted with reference to Table 1-Table 9. FIG.

  Table 10 is a table showing the specific numerical configuration of the diffractive structure formed on the first surface 30a of the objective lens 30 of Example 2 and the optical path length addition amount in each annular zone of the diffractive structure. As shown in Table 10, the objective lens 30 provided in the optical pickup device 100 of Example 2 has the first region 31 of the second aspect. By forming the diffractive structure as shown in Table 10 on the first surface 30a, the objective lens 30 of Example 2 has a diffraction efficiency for the second laser beam when the diffraction efficiency for the first laser beam is 100%. Since the efficiency is about 87.2% and the diffraction efficiency for the third laser beam is very high, about 99.9%, the light amount of the spot formed on the recording surface is very large.

  The objective lens 30 of the optical pickup apparatus 100 of each embodiment described above has f1 × M1 of 0.000, f2 × M2 of 0.000, and f3 × M3 of −0.232. (3) is satisfied. Further, f1 × NA1 is 1.95 and f2 × NA2 is 1.87, which satisfies Expression (4). Accordingly, the objective lens 30 has a third region 33 having a maximum diffraction order (first order) different from the diffraction order (third order) in which the diffraction efficiency with respect to the first laser beam is maximized in the second region 32. Further, as shown in Table 1, in the optical pickup device 100 of each example, Expression (6) is 1: 2, and Expression (7) is 3: 5.

  FIG. 5 is an aberration diagram illustrating spherical aberration that occurs when the first laser light passes through the objective lens 30 in the optical pickup device 100 of each embodiment. Similarly, FIG. 6 shows spherical aberration that occurs when the second laser light passes through the objective lens 30, and FIG. 7 shows spherical aberration that occurs when the third laser light passes through the objective lens 30. Represents. As shown in FIGS. 5 to 7, the optical pickup device 100 according to each embodiment having the relations of the formulas (6) and (7) satisfies the formulas (1) to (3), so that information on which optical disk is obtained. Even during recording or reproduction, it is possible to satisfactorily correct the spherical aberration and form a spot suitable for recording or reproducing information on the recording surface.

  Schematic diagrams showing the optical pickup device 100 of the third embodiment are shown in FIGS. 8 (A) to 8 (C). As shown in FIGS. 8A to 8C, the optical pickup device 100 of the third embodiment converts the first laser beam into a parallel beam as an optical member for converting the second laser beam into a parallel beam. A collimating lens 20A ′ having the same configuration as that of the collimating lens 20A to be converted is used. Since other configurations are the same as those in the first and second embodiments, the description thereof is omitted here. Specific specifications of the objective lens 30 of Example 3 are shown in Table 11.

  Specific numerical configurations of the optical pickup device 100 including the objective lens 30 shown in Table 11 are shown in Tables 12 to 14.

  Tables 12 to 14 show specific numerical configurations of the optical pickup device 100 at the time of recording or reproducing information with respect to the first optical disc D1, the second optical disc D2, and the third optical disc D3 in order. In Tables 12 to 14, members corresponding to the respective surface numbers are shown in the remarks.

As shown in Table 11, in Example 3, the NA required when using the first optical disc D1 and the NA required when using the second optical disc D2 are relatively approximated. Therefore, the first surface 30 a of the third embodiment includes only the first region 31 and the second region 32. When the range of each region 31, 32 is represented by the height h from the optical axis AX,
First region 31... H ≦ 1.72,
The second region 32... 1.72 <h ≦ 1.95.

  The second surface of each coupling lens 20A, 20A ', 20C and both surfaces 30a, 30b of the objective lens 30 are aspheric. Table 15 shows the conical coefficient and aspheric coefficient that define the shape of each aspheric surface when recording or reproducing information on the first optical disk D1 or the second optical disk D2, and Table 15 shows information when recording or reproducing information on the third optical disk D3. Table 16 shows conical coefficients and aspheric coefficients that define the shape of each aspherical surface.

Further, the diffractive structure formed on the first surface 30a of the objective lens 30 is expressed by the above equation (2). Table 17 shows optical path difference function coefficients P ₂ ,... That define the diffractive structure. The diffraction order m set for each laser beam used is shown in Table 18.

  Table 19 is a table showing the specific numerical configuration of the diffractive structure formed on the first surface 30a of the objective lens 30 of Example 3 and the optical path length addition amount in each annular zone of the diffractive structure. As shown in Table 19, the objective lens 30 provided in the optical pickup device 100 of Example 3 has the first region 31 of the second aspect. By forming a diffractive structure as shown in Table 19 on the first surface 30a, the objective lens 30 of Example 3 has a diffraction efficiency for the second laser beam when the diffraction efficiency for the first laser beam is 100%. Since the efficiency is about 87.2% and the diffraction efficiency for the third laser beam is very high, about 98.8%, the light amount of the spot formed on the recording surface is very large.

  In the objective lens 30 of the optical pickup device 100 of the third embodiment described above, f1 × m1 is 0.000, f2 × m2 is 0.000, and f3 × m3 is −0.268. (3) is satisfied. Further, the reason why the third region 33 is not provided in the objective lens 30 of the present embodiment is derived from the fact that f1 × NA1 is 1.95 and f2 × NA2 is 1.95. Further, as shown in Table 1, in the optical pickup device 100 of Example 3, Expression (6) is 1: 2, and Expression (7) is 3: 5.

  FIG. 9 is an aberration diagram illustrating spherical aberration that occurs when the first laser light passes through the objective lens 30 in the optical pickup device 100 according to the third embodiment. Similarly, FIG. 10 shows spherical aberration that occurs when the second laser light passes through the objective lens 30, and FIG. 11 shows spherical aberration that occurs when the third laser light passes through the objective lens 30. Represents. As shown in FIG. 9 to FIG. 11, the optical pickup device 100 of each embodiment having the relationship of Expression (6) and Expression (7) satisfies which Expression (1) to Expression (3), which optical disk Even when information is recorded or reproduced, it is possible to satisfactorily correct spherical aberration and form a spot suitable for recording or reproducing information on the recording surface.

  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 30b instead of the first surface 30a. Moreover, you may provide a diffraction structure in both the 1st surface and the 2nd surface.

  Moreover, the design numerical aperture of Table 1 or Table 11 is also an example. That is, the objective lens according to the present invention is necessary for recording or reproducing information on each optical disc that can be used, in other words, NA corresponding to the function, depending on what function is given to the optical information recording / reproducing apparatus. NA can be set arbitrarily. A diffractive structure can be designed corresponding to the set NA.

  Further, the diffractive structure described above is not necessarily provided on the surface of the objective lens. FIG. 12 shows an optical pickup device 100 ′ using a diffraction element 50, which is a modification of the above embodiment. In FIG. 12, the configuration on the light source side with respect to the diffractive element 50 is the same as that of the above-described embodiment (see FIG. 1), and is not shown. As shown in FIG. 12, the optical pickup device 100 ′ is provided with a diffraction element 50 having a diffraction structure on one surface 50 a on the light source side of the objective lens 30. The other surface 50b of the diffraction element 50 is a flat surface. By using such a diffraction element 50, each surface 30a, 30b of the objective lens 30 is configured as an aspherical surface not provided with a diffraction structure. As in the modification shown in FIG. 12, an effect similar to that of the above embodiment can be obtained by providing an optical element (diffractive element) provided with the above-described diffraction structure on at least one surface on the optical axis of the objective lens. be able to.

It is a schematic diagram showing schematic structure of the optical pick-up apparatus which mounts the objective lens for optical pick-ups. The optical pickup device according to the embodiment of the present invention is shown separately for each optical path when each optical disk is used. It is sectional drawing of the 1st surface vicinity of the cross-sectional shape in the surface containing the optical axis of the objective lens for optical discs of embodiment of this invention. It is sectional drawing in the surface containing an optical axis which represented typically the 2nd aspect of the 1st area | region in an objective lens. FIG. 6 is an aberration diagram showing spherical aberration that occurs in the objective lenses of Examples 1 and 2 when the first laser beam is transmitted. FIG. 6 is an aberration diagram showing spherical aberration that occurs in the objective lenses of Examples 1 and 2 when the second laser beam is transmitted. FIG. 10 is an aberration diagram showing spherical aberration that occurs in the objective lenses of Examples 1 and 2 when the third laser beam is transmitted. The optical pickup device of Example 3 of the present invention is shown separately for each optical path when each optical disk is used. FIG. 10 is an aberration diagram illustrating spherical aberration that occurs in the objective lens according to Example 3 when the first laser beam is transmitted. FIG. 10 is an aberration diagram illustrating spherical aberration that occurs in the objective lens according to Example 3 when the second laser beam is transmitted. FIG. 10 is an aberration diagram illustrating spherical aberration that occurs in the objective lens according to Example 3 when the third laser beam is transmitted. It is a figure which shows the optical pick-up apparatus of the modification of this invention.

Explanation of symbols

10A to 10C Light source 20A to 20C Coupling lens 30 Objective lens 31 First region 32 Second region 33 Third region 41, 42 Beam splitter 50 Diffraction element D1-D3 Optical disc 100, 100 ′ Optical pickup device

Claims (16)

An optical pickup device for recording or reproducing information with respect to each optical disc by selectively 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,
With an objective lens,
The protective layer thickness of the first optical disk on which information is recorded or reproduced using the light beam having the shortest wavelength among the three kinds of light beams is t1, which is longer than the light beam having the first wavelength. The protective layer thickness of the second optical disk on which information is recorded or reproduced using a light beam of the second wavelength having the wavelength is t2, and the light beam of the third wavelength, which is the longest wavelength among the three types of light beams, If the protective layer thickness of the third optical disk on which information is recorded or reproduced is t3, t1 to t3 have the following relationship:
t1 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
The filling,
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 to NA3 have the following relationship:
NA1 ≧ NA2> NA3
The filling,
The first wavelength and the second wavelength of the light beam are substantially parallel light, and the third wavelength of the light beam is divergent light incident on the objective lens. The image magnification is M1, the focal length is f1, the imaging magnification is M2, the focal length is f2, and the information is recorded or reproduced on the third optical disc when information is recorded or reproduced on the second optical disc. When the image magnification is M3 and the focal length is f3, the following equations (1) to (3),
-0.02 <f1 × M1 <0.02 (1)
-0.02 <f2 × M2 <0.02 (2)
-0.29 <f3 × M3 <-0.19 (3)
Meets
Furthermore, at least one surface of the objective lens has a diffractive structure,
In the first region in which the light beam having the third wavelength is converged on the recording surface of the third optical disk, the diffraction order at which the diffraction efficiency is maximized is that the light beam having the first wavelength is the sixth order, and the second order The optical pickup device is characterized in that a light beam having a wavelength of quaternary and a light beam having the third wavelength is tertiary. The optical pickup device according to claim 1,
The diffractive structure of the objective lens has a structure in which at least a part of the first region corresponds to a light beam having the first wavelength when a refracting surface located at an inner side of a boundary between adjacent refracting surfaces is used as a reference. The optical path length addition amount has a step for about 8 wavelengths, and the optical path length addition amount has a step for about -2 wavelengths,
The optical pickup device is characterized in that the steps are alternately arranged. The optical pickup device according to claim 1,
The diffractive structure of the Qi objective lens has a structure in which at least a part of the first region is in a boundary between adjacent refracting surfaces with respect to a light beam having the first wavelength with respect to a refracting surface located inside. The optical path length addition amount has a level difference of about −8 wavelengths, and the optical path length addition amount has a level difference of about 2 wavelengths,
The optical pickup device is characterized in that the steps are alternately arranged. In the optical pick-up apparatus in any one of Claims 1-3,
The diffractive structure of the objective lens converges the light flux of the first wavelength and the light flux of the second wavelength on the recording surfaces of the first optical disc and the second optical disc, respectively, and the third light flux. Has a second region that does not contribute to the convergence of
An optical pickup device characterized in that the diffraction order that maximizes the diffraction efficiency of the second region is such that the light beam having the first wavelength is third-order and the light beam having the second wavelength is second-order. The optical pickup device according to claim 4,
The objective lens has the following formula (4):
f1 × NA1> f2 × NA2 (4)
Further, the diffractive structure of the objective lens has a third region that converges only the light beam of the first wavelength outside the second region,
In the third region, the diffraction order that maximizes the diffraction efficiency for the light beam of the first wavelength is different from the diffraction order that maximizes the diffraction efficiency for the light beam of the first wavelength in the second region. An optical pickup device characterized by that. The optical pickup device according to claim 4,
The objective lens has the following formula (5),
f1 × NA1 <f2 × NA2 (5)
The filling,
Furthermore, the diffractive structure of the objective lens has a third region that converges only the light beam having the first wavelength outside the second region,
In the third region, the diffraction order that maximizes the diffraction efficiency for the light beam having the second wavelength is different from the diffraction order that maximizes the diffraction efficiency for the light beam of the second wavelength in the second region. An optical pickup device characterized by that. In the optical pick-up apparatus in any one of Claims 1-6,
A light source for irradiating each of the luminous fluxes,
The first wavelength is λ1, the second wavelength is λ2, the third wavelength is λ3, the refractive index of the objective lens for the first wavelength λ1 is n1, and the objective lens for the second wavelength λ2 Where n2 is the refractive index of the objective lens and n3 is the refractive index of the objective lens with respect to the third wavelength λ3, the following equations (6) and (7):
λ1 / (n1-1): λ3 / (n3-1) ≈1: 2 (6)
λ1 / (n1-1): λ2 / (n2-1) ≈3: 5 (7)
An optical pickup device characterized by satisfying both. An optical pickup device for recording or reproducing information with respect to each optical disc by selectively 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,
Having a diffractive structure on at least one surface;
When the inner refracting surface is used as a reference at the boundary between adjacent refracting surfaces, the optical path length addition amount for the light beam having the first wavelength is approximately 8 wavelengths, and the optical path length addition amount is approximately − An optical pickup device having a step for two wavelengths, and each step has a diffraction element arranged alternately. An optical pickup device for recording or reproducing information with respect to each optical disc by selectively 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,
Having a diffractive structure on at least one surface;
When the inner refracting surface is used as a reference at the boundary between adjacent refracting surfaces, the optical path length addition amount for the light beam having the first wavelength is approximately −8 wavelengths, and the optical path length addition amount is substantially equal. An optical pickup device having a step for two wavelengths, and each step has a diffraction element arranged alternately. An optical pickup device for recording or reproducing information with respect to each optical disc by selectively 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,
A light source for irradiating the three kinds of light fluxes;
A diffractive element according to claim 8 or 9,
The first and second light fluxes are substantially parallel light, the third light flux is divergent light, and the imaging magnification at the time of recording or reproducing information on the first optical disk is determined. M1, the focal length is f1, the imaging magnification is M2 when the information is recorded or reproduced on the second optical disc, the focal length is f2, and the imaging magnification is obtained when the information is recorded or reproduced on the third optical disc. When M3 and the focal length are f3, the following equations (1) to (3),
-0.02 <f1 × M1 <0.02 (1)
-0.02 <f2 × M2 <0.02 (2)
-0.29 <f3 × M3 <-0.19 (3)
An objective lens that satisfies
Have
The first wavelength is λ1, the second wavelength is λ2, the third wavelength is λ3, the refractive index of the objective lens for the first wavelength λ1 is n1, and the objective lens for the second wavelength λ2 Where n2 is the refractive index of the objective lens and n3 is the refractive index of the objective lens with respect to the third wavelength λ3, the following equations (6) and (7):
λ1 / (n1-1): λ3 / (n3-1) ≈1: 2 (6)
λ1 / (n1-1): λ2 / (n2-1) ≈3: 5 (7)
An optical pickup device characterized by satisfying both. An objective lens in an optical pickup device that records or reproduces information on each optical disc by selectively 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 disk on which information is recorded or reproduced using the light beam having the shortest wavelength among the three kinds of light beams is t1, which is longer than the light beam having the first wavelength. The protective layer thickness of the second optical disk on which information is recorded or reproduced using a light beam of the second wavelength having the wavelength is t2, and the light beam of the third wavelength, which is the longest wavelength among the three types of light beams, If the protective layer thickness of the third optical disk on which information is recorded or reproduced is t3, t1 to t3 have the following relationship:
t1 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
The filling,
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 to NA3 have the following relationship:
NA1 ≧ NA2> NA3
The filling,
The first wavelength and the second wavelength of the light beam are substantially parallel light, and the third wavelength of the light beam is divergent light incident on the objective lens. The image magnification is M1, the focal length is f1, the imaging magnification is M2, the focal length is f2, and the information is recorded or reproduced on the third optical disc when information is recorded or reproduced on the second optical disc. When the image magnification is M3 and the focal length is f3, the following equations (1) to (3),
-0.02 <f1 × M1 <0.02 (1)
-0.02 <f2 × M2 <0.02 (2)
-0.29 <f3 × M3 <-0.19 (3)
The filling,
Furthermore, at least one surface of the objective lens has a diffractive structure,
In the first region in which the light beam having the third wavelength is converged on the recording surface of the third optical disk, the diffraction order at which the diffraction efficiency is maximized is that the light beam having the first wavelength is the sixth order, and the second order An optical pickup objective lens, wherein a light beam having a wavelength of quaternary and a light beam having the third wavelength is tertiary. 12. The objective lens for an optical pickup according to claim 11, wherein at least a part of the first region is defined by using the first refracting surface as a reference at a boundary between adjacent refracting surfaces. The optical path length addition amount for a light beam having a wavelength of approximately 8 wavelengths, and the optical path length addition amount has a step difference of approximately −2 wavelengths.
An optical pickup objective lens, wherein the steps are arranged alternately. 12. The objective lens for an optical pickup according to claim 11, wherein at least a part of the first region is defined by using the first refracting surface as a reference at a boundary between adjacent refracting surfaces. The optical path length addition amount with respect to a light beam having a wavelength of approximately -8 wavelengths and the optical path length addition amount have a step difference of approximately 2 wavelengths,
An optical pickup objective lens, wherein the steps are arranged alternately. The objective lens for an optical pickup according to any one of claims 11 to 13,
The diffractive structure converges the light flux of the first wavelength and the light flux of the second wavelength on the recording surfaces of the first optical disc and the second optical disc, respectively, and converges the third light flux. Has a second region that does not contribute,
The diffraction order in which the diffraction efficiency of the second region is maximized is such that the light beam having the first wavelength is third-order, and the light beam having the second wavelength is second-order. The objective lens for an optical pickup according to claim 14,
The following formula (4),
f1 × NA1> f2 × NA2 (4)
Furthermore, the diffractive structure has a third region that focuses only the light beam of the first wavelength outside the second region,
In the third region, the diffraction order that maximizes the diffraction efficiency for the light beam of the first wavelength is different from the diffraction order that maximizes the diffraction efficiency for the light beam of the first wavelength in the second region. An objective lens for an optical pickup. The objective lens for an optical pickup according to claim 14,
The following formula (5),
f1 × NA1 <f2 × NA2 (5)
The filling,
Furthermore, the diffractive structure has a third region outside the second region for converging only the light beam having the first wavelength,
In the third region, the diffraction order that maximizes the diffraction efficiency for the light beam having the second wavelength is different from the diffraction order that maximizes the diffraction efficiency for the light beam of the second wavelength in the second region. An objective lens for an optical pickup.

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