JP4148520B2 - 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.

  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.

  More specifically, the surface of the objective lens on which the diffractive structure is provided is divided 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 well converged on the recording surface of the DVD. It has a diffractive structure. The outer region 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.

  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 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, 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 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.

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

JP 2001-93179 A

  However, the objective lens exemplified in Patent Document 2 is designed to be suitable for recording or reproducing information on a new standard disc and DVD, but it is completely assumed that a CD is used. Not.

  From the above, further improvement of the objective lens has been desired so as to be compatible with information recording or reproduction with respect to existing discs such as CDs and DVDs and new standard optical discs.

  Therefore, in view of the above circumstances, the present invention is capable of forming 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 a pickup objective lens and an optical pickup device on which the objective lens is mounted.

In order to solve the above-described problems, the optical pickup device of the present invention uses each of 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. Is an optical pickup device that records or reproduces information with respect to the optical recording apparatus, and includes an objective lens, and records or reproduces information using the light beam having the shortest first wavelength among the first to third wavelengths. The protective layer thickness of the first optical disk is t1, the protective layer thickness of the second optical disk on which information is recorded or reproduced using a light beam having a second wavelength longer than the first wavelength, t2, When the thickness of the protective layer of the third optical disk on which information is recorded or reproduced using the light beam having the longest third wavelength among the three wavelengths is t3,
t1 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
The numerical aperture required for recording or reproducing information on the first optical disk is NA1, the numerical aperture required for recording or reproducing information on the second optical disk is NA2, and the information is recorded or reproduced on the third optical disk. If the required numerical aperture is NA3,
NA1 ≧ NA2> NA3
The first wavelength and the second wavelength light flux are substantially parallel light, and the third wavelength light flux is divergent light incident on the objective lens, 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. 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)
Further, at least one surface of the objective lens is provided with a diffractive structure, and in the first region for converging the light beam of the third wavelength of the diffractive structure on the recording surface of the third optical disc, The diffraction order that maximizes the diffraction efficiency for each light beam is characterized in that the light beam of the first wavelength is secondary, the light beam of the second wavelength, and the light beam of the third wavelength are primary.

  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.

In the objective lens in the optical pickup device according to the present invention, the light beams having the first wavelength and the second wavelength are substantially parallel light, and the light beam having the third wavelength is divergent light incident on at least one surface. It 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.

  FIGS. 1A to 1D show the diffraction efficiency when the light beams having the first to third wavelengths are incident on an objective lens having a diffractive structure. It is a graph which shows how it changes by. FIGS. 1A to 1D sequentially show diffraction efficiencies of first to third wavelengths in 0th-order diffracted light, first-order diffracted light, second-order diffracted light, and third-order diffracted light. In each graph, the first wavelength is indicated by a solid line, the second wavelength is indicated by a dotted line, and the third wavelength is indicated by a one-dot chain line. Of the three line segments spanning each graph, the line segment L1 has an annular zone height of 1λ at the first wavelength, and the line segment L2 has an annular zone height of 2λ at the first wavelength, The line segment L3 represents the ring zone height of 3λ at the first wavelength. The objective lens material having a diffractive structure is assumed to have a refractive index of d-line of 1.5434 and an Abbe number of 56. As shown in FIG. 1 (A) to FIG. 1 (D), when the diffraction order at which the diffraction efficiency is highest in the first region is set to the second order at the first wavelength, as shown in the line segment L2. It can be seen that at the second and third wavelengths, the diffraction order with the highest diffraction efficiency is the first order.

In other words, in the optical pickup device according to the present invention , the objective lens has the diffraction order with the highest diffraction efficiency in the first region, and the light flux of the first wavelength has the secondary, second, and third wavelengths. By setting the light beam to the first order, a spot having a sufficient amount of light is secured on each recording surface when information is recorded or reproduced on each optical disk.

  A conventional objective lens that only supports recording or reproduction of information on CDs and DVDs can correct spherical aberration 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 optical pickup device according to the present invention , as described above, the diffraction orders of the light beam having the first wavelength are set to the second order, and the diffraction orders of the light beams having the second wavelength and the third wavelength are set to the first 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 (diffraction structure) is expressed as m × λ / d, where m is the diffraction order, λ is the wavelength, and d is the grating pitch. This is because the first-order diffracted light for the three wavelengths 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 optical pickup device according to the present invention has a substantially parallel light flux incident on the objective lens when the first and second optical discs that require a high NA for recording or reproducing information, and a relatively low NA. When the third optical disk 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. Accordingly, in the configuration in which the light beams emitted from the respective light sources are guided to the recording surface via a common coupling lens, the light source that emits the light beam of the first wavelength and the light source that emits the light beam of the second wavelength are on the same substrate. In other words, 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. . Therefore, in the optical pickup device according to the present invention, when the first or second optical disk is used, the objective lens is arranged so that the imaging magnification is as small as possible so as to satisfy the above expressions (1) and (2). Is done . Thereby, the amount of aberration generated during the tracking operation can be reduced.

The optical pickup device according to the present invention 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, according to the present invention, not only when information is recorded or reproduced on an existing optical disc (second optical disc, third optical disc), but also when information is recorded or reproduced on an optical disc of a new standard (first optical disc). Even during reproduction, it is possible to suppress the spherical aberration and form a good spot on the recording surface.

According to the optical pickup device of the present invention, the diffraction structure of the objective lens converges the first wavelength light beam and the second wavelength light beam on the recording surfaces of the first optical disk and the second optical disk, respectively. And a second region that does not contribute to the convergence of the light beam having the third wavelength is provided . The diffraction orders at which the diffraction efficiency of the second region is maximized are 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, the light beam having the third wavelength can be diffused. Note that the second region is provided outside the first region.

  Specifically, as shown in FIG. 1A to FIG. 1D, in the second region, when the diffraction order at which the diffraction efficiency is highest is set to the third order at the first wavelength, a line segment is obtained. As indicated by L3, the first-order diffracted light and the second-order diffracted light can be dispersed at the third wavelength. Accordingly, the light beam having the third wavelength transmitted through the second region has a small contribution to the convergence on the third disk.

  In addition, by providing the second region, it is possible to suppress deterioration of wavefront aberration due to a temperature change particularly with respect to the light beams having the first wavelength and the second wavelength. In other words, a plastic refractive lens that is substantially free of aberrations at a predetermined reference temperature (reference temperature) has a higher refractive index because the laser oscillation wavelength becomes longer as the temperature increases, resulting in over spherical aberration. In order to correct the over spherical aberration with the diffraction structure, it is necessary to provide under spherical aberration.

In the optical pickup device according to the present invention , in the first region, the diffraction order in which the diffraction efficiency is maximized is set to be the second order with the first wavelength light beam and the first order with the second wavelength light beam. ing. In this case, the power of the diffractive structure is greater when the light beam having the first wavelength is used. This diffractive structure uses the second wavelength when using the first wavelength beam in order to cancel the spherical aberration that occurs in the refractive lens when using the first wavelength beam and when using the second wavelength beam. Therefore, it is necessary to set so as to generate a spherical aberration that is higher than that when using the luminous flux. However, such a diffractive structure has a characteristic that the spherical aberration changes to the over side when the wavelength used is changed to the long wavelength side, so that when the temperature is increased, it is caused by the refractive lens itself as described above. In this case, an excessive spherical aberration is given to an excessive spherical aberration that occurs at a reference temperature. Therefore, aberration deterioration due to temperature change increases.

  On the other hand, the power of the diffractive structure in the second region where the diffraction order that maximizes the diffraction efficiency is the third order for the first wavelength light beam and the second order for the second wavelength light beam is the second wavelength. Larger when using luminous flux. For this reason, if this diffraction structure is set to generate spherical aberration that is under when using a light beam with the second wavelength, the spherical aberration is under when the wavelength used changes to the longer wavelength side. It has a characteristic that changes to the side, and can correct spherical aberration that occurs when the temperature rises and is higher than that at the reference temperature. As a result, it is possible to suppress deterioration of wavefront aberration for the first wavelength and the light flux of the second wavelength by the second region.

In the optical pickup device according to the present invention, the objective lens further includes the following equation (4):
f1 × NA1> f2 × NA2 (4)
Configured to meet . Then, the above-mentioned diffractive structure, the outer side of the second region, the third region to converge only light flux of the first wavelength efficiently is provided. 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. Is done. 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.

Specifically, for example, in the third region, when the diffraction order at which the diffraction efficiency with respect to the light beam having the first wavelength is maximized is set to the first order, a line segment in FIGS. 1 (A) to 1 (D). As indicated by L1, both the second and third light beams can be dispersed into the 0th-order diffracted light and the 1st-order diffracted light. Accordingly, the second and third light fluxes transmitted through the third region have a smaller contribution to convergence on the second and third disks.

In an optical pickup device according to another aspect of the present invention, the objective lens has the following equation (5):
f1 × NA1 <f2 × NA2 (5)
Configured to meet . Such a diffractive structure of the objective lens is provided with a third region for efficiently converging only the light beam having the second wavelength outside the second 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. Is done. 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.

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, (6),
λ1 / (n1-1): λ3 / (n3-1) ≈1: 2 (6)
It is mounted on an optical pickup device having 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.

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 a 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 first wavelength among the first to third wavelengths 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 first wavelength is t2, and the thickness of the first to third wavelengths is the largest. If the thickness of the protective layer of the third optical disk on which information is recorded or reproduced using a long third wavelength light beam is t3,
t1 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
The numerical aperture required for recording or reproducing information on the first optical disk is NA1, the numerical aperture required for recording or reproducing information on the second optical disk is NA2, and the information is recorded or reproduced on the third optical disk. If the required numerical aperture is NA3,
NA1 ≧ NA2> NA3
The first and second light fluxes are substantially parallel light, and the third light flux is divergent light incident on the objective lens. The image magnification is 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. Is M3 and the focal length is f3, the following formulas (1) to (3),
−0.02 <f1 × M1 <0.02 (1)
−0.02 <f2 × M2 <0.02 (2)
−0.29 <f3 × M3 <−0.19 (3)
To meet. And this objective lens is the following formula | equation (4),
f1 × NA1> f2 × NA2 (4)
The filled, further have a diffractive structure on at least one surface. The diffractive structure has a first region for converging a third wavelength light beam on the recording surface of the third optical disc, and a first wavelength light beam and a second wavelength light beam outside the first region , respectively. Only the light beam of the first wavelength is outside the second region and the second region which are converged on the recording surfaces of the first optical disc and the second optical disc and do not contribute to the convergence of the light beam of the third wavelength. Has a third region to converge. Then, in the first region, the diffraction order at which the diffraction efficiency is maximized, the light flux secondary first wavelength, Ri luminous flux and the third light flux is the primary der wavelength of the second wavelength, the second In the region, the diffraction order at which the diffraction efficiency is maximized is that the light beam of the first wavelength is the third order, the light beam of the second wavelength is the second order, and in the third region, the diffraction efficiency for the light beam of the first wavelength is The diffraction order that is maximized is configured to be different from the diffraction order that maximizes the diffraction efficiency for the light beam having the first wavelength in the second region. In addition, an objective lens according to another aspect of the present invention has the following formula (5),
f1 × NA1 <f2 × NA2 (5)
And at least one surface has a diffractive structure. The diffractive structure has a first region for converging a third wavelength light beam on the recording surface of the third optical disc, and a first wavelength light beam and a second wavelength light beam outside the first region, respectively. Only the light flux of the second wavelength is outside the second region and the second region which are converged on the recording surfaces of the first optical disc and the second optical disc and do not contribute to the convergence of the light flux of the third wavelength. Has a third region. In the first region, the diffraction order at which the diffraction efficiency is maximized is that the light beam of the first wavelength is secondary, the light beam of the second wavelength and the light beam of the third wavelength are primary, and the second region In the third region, the diffraction order with the maximum diffraction efficiency is the third order for the light flux of the first wavelength and the second order for the light flux of the second wavelength. The diffraction order is configured to be different from the diffraction order at which the diffraction efficiency with respect to the light beam having the second wavelength is maximized in the second region.

  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.

  Hereinafter, an embodiment of an optical pickup objective lens 30 and an optical pickup apparatus 100 equipped with the objective lens 30 according to the present invention 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. 2 is a schematic diagram illustrating a schematic configuration of the 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. 2, 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. 3A to 3C are diagrams illustrating a schematic configuration of the optical pickup device 100 illustrated in FIG. 2 separately for each optical path when each optical disk is used. That is, FIGS. 3A to 3C 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. 2 and 3, 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. 2 and 3, the optical axis of the objective lens coincides with the reference axis of the optical system, but the optical axis of the objective lens may deviate 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, 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. 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 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
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, it is required to form a beam spot with a smaller diameter, so that the necessary NA becomes high.

  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. In other words, the second light source 10B has a longer wavelength than the first laser beam and more than the third laser beam in order to form a relatively small beam spot on the recording surface of the second optical disc D2. Laser light (hereinafter referred to as second laser light) having a short wavelength (second wavelength) 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. 3A to 3C, 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 each disk is used 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. 4 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 with respect to the adjacent annular zone from the inner side to the outer side of the surface 30a. It is formed to become.

  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, and The diffraction order that maximizes the diffraction efficiency is designed so that the first laser beam is secondary, and the others are primary. That is, the first region 31 has a level difference corresponding to the line segment L2 in FIGS. 1 (A) to 1 (D). As described above, the first region 31 designed to maximize the diffraction efficiency of the secondary light in the first laser light has an optical path length difference given by the annular zone of the diffraction structure of the first wavelength. It is approximately equal to the third wavelength. That is, the first region 31 of the present embodiment has an advantage that the utilization efficiency of the third laser beam is high when the third optical disc D3 is used.

  The second region 32 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 That is, the second region 32 has a level difference corresponding to the line segment L3 in FIGS. 1 (A) to 1 (D). 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, 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 beam forms a good image with substantially no aberration on the recording surface of the first optical disc D1. Have. 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, in order to suppress the occurrence of various aberrations due to off-axis light 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, the objective lens 30 is used. A substantially parallel light beam is incident on. Specifically, the imaging magnification of the objective lens 30 when the first optical disc D1 is used is M1, the focal length is f1, the imaging magnification of the objective lens 30 when the second optical disc D2 is used is M2, and the focal length is f2. Then, the objective lens 30 is disposed 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 arranging the objective lens 30 so as to satisfy the expressions (1) and (2), the 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, each coupling lens 20A, 20B of this embodiment functions as a collimating lens 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 or reproduced on the third optical disc D3. I can't keep it down. Therefore, the spherical aberration generated when the third optical disk 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 arranged so as to satisfy the following expression (3).
−0.29 <f3 × M3 <−0.19 (3)

  By arranging 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.

  As described above, the first region 31 is the only region that contributes to the convergence of the light beam when recording or reproducing information on the third optical disc D3. Therefore, it is desirable that the diffractive structure of the first region 31 is an annular structure that increases the utilization efficiency of the third laser. In this embodiment, since the wavelength of the first laser uses second-order diffracted light, the optical path length difference provided by the ring zone of the diffractive structure of the wavelength of the first laser is substantially equal to the wavelength of the third laser. . Therefore, the use efficiency of the first and third laser beams can be increased by blazing the first region 31 at a wavelength about twice the wavelength of the first laser. Further, in order to increase the utilization efficiency of the second laser beam when the second optical disc D2 is used, the annular zone in the vicinity of the optical axis of the first region 31 may be blazed at a wavelength close to the second wavelength.

  With the configuration as described above, as shown in FIGS. 3A to 3C, 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.

In the optical pickup device 100 shown in FIGS. 3A to 3C described above, when the wavelength of each laser beam used is compared in consideration of the refractive index of the objective lens 30, the aberration due to the diffractive lens structure 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 relationship is that the wavelength of the first laser light is λ1, the wavelength of the third laser light is λ3, the refractive index of the objective lens 30 with respect to the first wavelength λ1 is n1, and the third wavelength λ3. When the refractive index of the objective lens 30 with respect to n is n3, the following relationship (6) is satisfied.
λ1 / (n1-1): λ3 / (n3-1) ≈1: 2 (6)

  When there is a relationship such as Equation (6), if the diffraction order of the light beam having the first wavelength λ1 is set to the second order and the diffraction order of the light beam having the third wavelength λ3 is set to the first order, then the second order for the first wavelength λ1. Since the first-order diffracted light and the first-order diffracted light for the third wavelength λ3 receive almost the same power from the diffractive lens, it diffracts the relative spherical aberration due to the refractive index change due to the first and third wavelength differences and the disc protective layer thickness difference It cannot be corrected by the 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, the aberration is 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 disks having the relationship represented by the formula (6).

  Two specific examples based on the embodiment described above are presented. In each of the embodiments, the first optical disk D1 and the second optical disk D2 each having a protective layer thickness of 0.6 mm are compatible with the third optical disk D3 having a protective layer thickness of 1.2 mm. The present invention relates to an optical pickup device 100 on which an objective lens 30 is mounted.

  Schematic diagrams illustrating the optical pickup device 100 of the first embodiment are shown in FIGS. 3 (A) to 3 (C). Specific specifications of the objective lens 30 of Example 1 are shown in Table 1.

  In Table 1, the design wavelength is a wavelength most suitable for recording or reproducing information on each of the optical discs D1 to D3. The same applies to Table 10 shown later. 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. The same applies to each table shown in Example 2 described later.

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. Is the curvature (1 / r) of C, the conic coefficient is K, the fourth, sixth, eighth, tenth, and twelfth aspheric coefficients are A ₄ , A ₆ , A ₈ , A ₁₀ , A ₁₂ , It is expressed by the following formula.

  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 each table 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, fourth, sixth,. 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.

  In the objective lens 30 of the optical pickup device 100 according to the first exemplary embodiment, f1 × M1 is 0.000, f2 × M2 is 0.000, and f3 × M3 is −0.232. The equations (1) to (3) Meet. 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 the first embodiment, the expression (6) is 1: 2.

  FIG. 5 is an aberration diagram showing spherical aberration that occurs when the first laser light passes through the objective lens 30 in the optical pickup device 100. 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 having the relationship of the formula (6) satisfies the formulas (1) to (3), so that any optical disc can be recorded or reproduced. Spherical aberration can be corrected well, and spots suitable for recording or reproducing information can be formed on the recording surface. The above is the description of the optical pickup device 100 according to the first embodiment.

  Next, the optical pickup device 100 according to the second embodiment will be described. The optical pickup device 100 according to the second embodiment corresponds to the optical disc D3 with a relatively high NA (assuming 0.50 here) required when using a writable third optical disc (for example, a CD-R). Designed numerical aperture. Schematic diagrams showing the optical pickup device 100 of the second embodiment are shown in FIGS. 8 (A) to 8 (C). As shown in FIGS. 8A to 8C, the optical pickup device 100 according to the second 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 200A ′ having the same configuration as that of the collimating lens 200A to be converted is used. Since other configurations are the same as those of the first embodiment, description thereof is omitted here. Specific specifications of the objective lens 30 of the optical pickup device 100 according to the second embodiment are shown in Table 10. Tables 11 to 13 show specific numerical configurations of the optical pickup device 100 including the objective lens 30 shown in Table 10.

  Tables 11 to 13 are 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.

  As shown in the remarks in Tables 11 to 13, the surface number 0 is the light sources 10A to 10C, the surface numbers 1 and 2 are the coupling lenses 200A, 200A ′, 200C, the surface numbers 3 of Tables 11 and 12, 4 is a beam splitter 41, surface numbers 5 and 6 in Tables 2 and 3 and surface numbers 3 and 4 in Table 4 are beam splitters 42, surface numbers 7 and 8 in Tables 11 and 12 and surface numbers 5 and 6 in Table 13. Indicates the protective layer and the recording surface of each of the optical disks D1 to D3 in which the objective lens 30, surface numbers 9 and 10 in Tables 11 and 12 and surface numbers 7 and 8 in Table 4 are media.

As shown in Table 10, the effective diameter of the objective lens 30 is the same when the first optical disc D1 is used and when the second optical disc D2 is used. Accordingly, the first surface 30a of the objective lens 30 disposed in the optical pickup device 100 of the second embodiment is not provided with the third region, and the first region 31 and the first surface as shown in Tables 11 to 13, respectively. It consists of two regions 32. When the range of each region 31, 32 is represented by the height h from the optical axis AX,
First region 31... H ≦ 1.70,
The second region 32... 1.70 <h ≦ 1.95.

  The second surfaces of the coupling lenses 200A, 200A ′, and 200C and the both surfaces 30a and 30b of the objective lens 30 are aspheric surfaces whose shapes are defined by the above equation (1). Table 14 shows the cone coefficient and aspheric coefficient that define the shape of each aspheric surface when using the first optical disk D1 or the second optical disk D2, and Table 14 shows the cone that defines the shape of each aspheric surface when using the third optical disk D3. The coefficients and aspherical coefficients are shown in Table 15, respectively.

Further, different diffractive structures are formed on the first surface 30 a of the objective lens 30 in the regions 31 and 32. The diffractive structure is represented by the formula 2 above. The optical path difference function coefficients P ₂ ,... That define the diffraction structure are shown in Table 16. Table 17 shows the diffraction order m at which the diffraction efficiency of each laser beam is maximized in each of the regions 31 and 32.

  In the objective lens 30 of the optical pickup device 100 of the second embodiment, f1 × M1 is 0.000, f2 × M2 is 0.000, and f3 × M3 is −0.237, and the equations (1) to (3) Meet. Further, as shown in Table 1, in the optical pickup device 100 of the first embodiment, the expression (6) is 1: 2.

  FIG. 9 is an aberration diagram showing spherical aberration that occurs when the first laser light passes through the objective lens 30 in the optical pickup device 100. 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 FIGS. 9 to 11, the optical pickup device 100 having the relationship of the expression (6) satisfies the expressions (1) to (3), so that any optical disc can be recorded or reproduced. Spherical aberration can be corrected well, and spots suitable for recording or reproducing information can be formed on the recording surface.

  The above is a specific 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.

  The design numerical aperture presented in each example is also an example. In other words, in the objective lens according to the present invention, a high NA (about 0.65) necessary for the second optical disc D2 can be set to a design numerical aperture corresponding to the disc D2.

It is a graph which shows how the diffraction efficiency changes with the amount of the level | step difference of an annular zone, when each light beam which has 1st-3rd wavelength injects into the objective lens provided with the diffraction structure. 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 an enlarged view near the 1st surface of the section shape in the field including the optical axis of the objective lens for optical discs of the embodiment of the present invention. FIG. 6 is an aberration diagram illustrating spherical aberration that occurs in the objective lens according to Example 1 when the first laser beam is transmitted. FIG. 6 is an aberration diagram illustrating spherical aberration that occurs in the objective lens according to Example 1 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 1 when the third laser beam is transmitted. The optical pickup device of Example 2 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 2 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 2 when the second laser beam is transmitted. FIG. 9 is an aberration diagram illustrating spherical aberration that occurs in the objective lens according to Example 2 when the third laser beam is transmitted.

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 D1 to D3 Optical disc 100 Optical pickup device 200A, 200A ′, 200C Coupling lens

Claims (5)

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 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 having a wavelength of t2 and a protective layer thickness of the second optical disk is t2, and information recording is performed using a light beam having 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 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
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
First wavelength, and substantially parallel light flux of the second wavelength, the light flux of the third wavelength is divergent light enters the objective lens, at the time of recording or reproducing information for the first optical disc, The imaging magnification is M1, the focal length is f1, and when recording or reproducing information on the second optical disk, the imaging magnification is M2, the focal distance is f2, and when recording or reproducing information on the third optical disk, 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
The objective lens has the following formula (4):
f1 × NA1> f2 × NA2 (4)
The filling,
Furthermore, a diffractive structure is provided on at least one surface of the objective lens,
The diffractive structure is
A first region for converging the light flux of the third wavelength on the recording surface of the third optical disc ;
Outside the first region, the light flux of the first wavelength and the light flux of the second wavelength are converged on the recording surfaces of the first optical disc and the second optical disc, respectively, and the third A second region that does not contribute to the convergence of the light flux of the wavelength,
A third region that converges only the light beam having the first wavelength outside the second region.
Have
In the first region, the diffraction order at which the diffraction efficiency is maximized, Ri the first light flux having a wavelength of the secondary, the second light flux and the third light flux is the primary der of wavelengths,
In the second region, the diffraction order at which the diffraction efficiency is maximized is that the luminous flux of the first wavelength is the third order, the luminous flux of the second wavelength is the second order,
In the third region, the diffraction order that maximizes the diffraction efficiency for the light beam having 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. 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 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 having a wavelength of t2 and a protective layer thickness of the second optical disk is t2, and information recording is performed using a light beam having 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 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
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 wavelength and the second wavelength light flux are substantially parallel light, and the third wavelength light flux is divergent light incident on the objective lens, and when recording or reproducing information on the first optical disc, The imaging magnification is M1, the focal length is f1, and when recording or reproducing information on the second optical disk, the imaging magnification is M2, the focal distance is f2, and when recording or reproducing information on the third optical disk, 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
The objective lens has the following formula (5),
f1 × NA1 <f2 × NA2 (5)
The filling,
Furthermore, a diffractive structure is provided on at least one surface of the objective lens,
The diffractive structure is
A first region for converging the light flux of the third wavelength on the recording surface of the third optical disc;
Outside the first region, the light flux of the first wavelength and the light flux of the second wavelength are converged on the recording surfaces of the first optical disc and the second optical disc, respectively, and the third A second region that does not contribute to the convergence of the light flux of the wavelength,
A third region that images only the light beam having the second wavelength outside the second region.
Have
In the first region, the diffraction order at which the diffraction efficiency is maximized is that the light flux of the first wavelength is secondary, the light flux of the second wavelength and the light flux of the third wavelength are primary,
In the second region, the diffraction order at which the diffraction efficiency is maximized is that the luminous flux of the first wavelength is the third order, the luminous flux of the second wavelength is the second order,
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 Claim 1 or Claim 2 ,
A light source for irradiating each of the luminous fluxes;
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 (6),
λ1 / (n1-1): λ3 / (n3-1) ≈1: 2 (6)
An optical pickup device satisfying the requirements. 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 having a wavelength of t2 and a protective layer thickness of the second optical disk is t2, and information recording is performed using a light beam having 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 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
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
First wavelength, and, substantially parallel light flux of the second wavelength, the light flux of the third wavelength is divergent light enters the objective lens, at the time of recording or reproducing information for the first optical disc, The imaging magnification is M1, the focal length is f1, and when recording or reproducing information on the second optical disk, the imaging magnification is M2, the focal distance is f2, and when recording or reproducing information on the third optical disk, 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)
The filling,
The objective lens has the following formula (4):
f1 × NA1> f2 × NA2 (4)
The filling,
Further comprising a diffractive structure on at least one surface of the objective lens,
The diffractive structure is
A first region for converging the light flux of the third wavelength on the recording surface of the third optical disc ;
Outside the first region, the light flux of the first wavelength and the light flux of the second wavelength are converged on the recording surfaces of the first optical disc and the second optical disc, respectively, and the third A second region that does not contribute to the convergence of the light flux of the wavelength,
A third region that converges only the light beam having the first wavelength outside the second region.
Have
In the first region, the diffraction order at which the diffraction efficiency is maximized, Ri the first light flux having a wavelength of the secondary, the second light flux and the third light flux is the primary der of wavelengths,
In the second region, the diffraction order at which the diffraction efficiency is maximized is that the luminous flux of the first wavelength is the third order, the luminous flux of the second wavelength is the second order,
In the third region, the diffraction order that maximizes the diffraction efficiency for the light beam having 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 objective lens. 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 having a wavelength of t2 and a protective layer thickness of the second optical disk is t2, and information recording is performed using a light beam having 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 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
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
First wavelength, and substantially parallel light flux of the second wavelength, the light flux of the third wavelength is divergent light enters the objective lens, at the time of recording or reproducing information for the first optical disc, The imaging magnification is M1, the focal length is f1, and when recording or reproducing information on the second optical disk, the imaging magnification is M2, the focal distance is f2, and when recording or reproducing information on the third optical disk, 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)
The filling,
The objective lens has the following formula (5),
f1 × NA1 <f2 × NA2 (5)
The filling,
Furthermore, a diffractive structure is provided on at least one surface of the objective lens,
The diffractive structure is
A first region for converging the light flux of the third wavelength on the recording surface of the third optical disc;
Outside the first region, the light flux of the first wavelength and the light flux of the second wavelength are converged on the recording surfaces of the first optical disc and the second optical disc, respectively, and the third A second region that does not contribute to the convergence of the light flux of the wavelength,
A third region that images only the light beam having the second wavelength outside the second region.
Have
In the first region, the diffraction order at which the diffraction efficiency is maximized is that the light flux of the first wavelength is secondary, the light flux of the second wavelength and the light flux of the third wavelength are primary,
In the second region, the diffraction order at which the diffraction efficiency is maximized is that the luminous flux of the first wavelength is the third order, the luminous flux of the second wavelength is the second order,
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.

Images