JP2009104747A - Pickup objective lens and designing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pickup objective lens for converging laser beams within the depth of focus even if the oscillation wavelength of a laser light source changes abruptly, and to provide a method of designing the pickup objective lens. <P>SOLUTION: The pickup lens 14 for condensing luminous flux emitted from a light source 11 has a plurality of annular level differences at least on one surface, and the plurality of annular level differences have an amount of level difference for generating a phase difference in laser beams so that laser beams are converged by the pickup lens 14 within the depth of focus when the wavelength changes. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a pickup objective lens in an optical disc apparatus such as a Blu-ray or HD-DVD, and a method for designing the pickup objective lens.

  Conventionally, the oscillation wavelength of a laser light source such as an optical disk device changes due to temperature change or output change (mode hop phenomenon). For example, the oscillation wavelength of a blue laser diode used for BD (Blu-ray Disc) and HD-DVD changes by about 1 nm when the output is greatly changed. Specifically, the oscillation wavelength of a blue laser diode having a wavelength of 408 nm changes from 408 nm to 409 nm due to a change in output in switching from reproduction to recording.

In addition, the refractive index of the pickup objective lens of the optical disk device changes according to the wavelength change of the laser light source. Therefore, the position (convergence position, focal position) where the laser beam is converged by the pickup objective lens is changed by the change in output. The change in the convergence position is referred to as axial chromatic aberration (hereinafter simply referred to as chromatic aberration).
The amount of change in the convergence position varies depending on the focal length of the pickup objective lens and the glass material. When an aspherical single lens made of glass or plastic and having a focal length of 1.76 mm (design wavelength = 408 nm) is considered as a CD, DVD, HD-DVD, or BD pickup objective lens, the wavelength change of the laser light source is 1 nm. The change amount (μm / nm) of the hit convergence position is as shown in the table of FIG.

Here, when the laser beam converges on an optical disk or the like, the shape of the convergence spot changes when defocusing occurs. And the peak value of the convergence spot which caused the focus shift becomes lower than the peak value at the time of focusing (that is, the peak value when no focus shift occurs). Further, the spot diameter (1 / e ² diameter) causing the defocus is larger than the spot diameter at the time of focusing (that is, the spot diameter when the focus is not deviated). When the allowable defocus range (depth of focus) is a range where the peak value when defocusing is 95% or more of the peak value at the time of in-focus, the depth of focus is as shown in the table of FIG.

  Thus, the greater the NA (numerical aperture) of the objective lens and the shorter the light source wavelength, the shallower the depth of focus. For this reason, there may be a case where the oscillation wavelength of the light source wavelength changes due to a temperature change or an output change, and the convergence position shifts out of the focal depth range. Furthermore, since the wavelength change due to the output change occurs instantaneously, it is difficult to correct the defocus due to the wavelength change by another method. As a result, problems such as the inability to record / reproduce to / from the optical disc and the loss of focus control occur.

On the other hand, the oscillation wavelength of the blue laser diode used for BD and HD-DVD varies about ± 5 nm centering on 408 nm.
The table of FIG. 26 shows rms wavefront aberrations that occur when laser beams having different oscillation wavelengths are incident on a BD objective lens that is formed of glass or plastic and has a focal length of 1.76 mm (design wavelength = 408 nm). As shown in FIG. 26, the rms wavefront aberration does not exceed the Marshall limit regardless of whether the oscillation wavelength is 403 nm, 408 nm, or 413 nm. However, the rms wavefront aberration increases as the oscillation wavelength of the laser beam deviates from the design wavelength of 408 nm.

  Conventionally, two methods are often used as chromatic aberration correction methods for pickup objective lenses. The first is a method of correcting chromatic aberration with a collimator lens. The second is a method of correcting chromatic aberration with the objective lens itself.

  When chromatic aberration correction is performed using a collimator lens, a plurality of annular zone steps are provided on one surface of the collimator lens. Then, the chromatic aberration is corrected using the diffraction due to the annular zone step.

  Further, when chromatic aberration correction is performed by the objective lens itself, a plurality of annular zone steps are provided on one surface of the objective lens. Then, the chromatic aberration is corrected using the diffraction due to the annular zone step.

Patent Document 1 discloses a chromatic aberration correcting optical element that corrects chromatic aberration. This optical element for correcting chromatic aberration is disposed in an optical path between a light source that generates light having a wavelength of 550 nm or less and an objective lens formed of a material having an Abbe number of d-line of 95.0 or less. The chromatic aberration correcting optical element has a diffractive structure including a plurality of annular zone steps on at least one surface. The diffraction structure satisfies P ₁ <P ₀ <P ₂ (P ₀ : total paraxial power (mm ⁻¹ ) of the optical element for correcting chromatic aberration at the wavelength λ ₀ of light generated by the light source, P ₁ : Total paraxial power (mm ⁻¹ ) of the chromatic aberration correcting optical element at wavelength λ _{1 which} is 10 nm shorter than wavelength λ ₀ , P ₂ : Total near chromatic aberration correcting optical element at wavelength λ _{2 which} is 10 nm longer than wavelength λ ₀ Axis power (mm ⁻¹ )).
JP 2003-167190 A

  However, when correcting chromatic aberration with a collimator lens, a dedicated collimator lens must be designed for one objective lens. Therefore, when the objective lens is changed, there is a waste that the collimator lens must also be changed.

  Further, when chromatic aberration correction is performed by the objective lens itself, the number of annular zone steps provided on the objective lens increases. And as the number of annular zone steps increases, the area of the inclined portion between the annular zone steps increases. Therefore, unnecessary light increases and the light utilization efficiency of the objective lens decreases.

  Further, the chromatic aberration correcting optical element described in Patent Document 1 is not designed in consideration of the wavelength / temperature dependency of the refractive index of the objective lens as described above. For this reason, there is a problem that the convergence position of the laser beam converged by the objective lens does not fall within the focal depth when the wavelength of the laser light source changes abruptly by changing the output.

  The present invention has been made to solve such problems, and even if the oscillation wavelength of the laser light source changes suddenly, the pickup objective lens that can bring the convergence position of the laser light within the depth of focus. It is another object of the present invention to provide a pickup objective lens design method.

  The pickup objective lens according to the present invention is a pickup objective lens that condenses a light beam emitted from a laser light source, and the pickup objective lens has a plurality of annular zone steps on at least one surface, The ring zone step has a step amount that generates a phase difference in the laser beam so that the convergence position of the laser beam converged by the pickup objective lens is within the depth of focus when the wavelength changes.

  In the present invention, a plurality of annular zone steps are provided on at least one surface of the pickup objective lens, and the plurality of annular zone steps are focused on the convergence position of the laser beam converged by the pickup objective lens when the wavelength changes. It has a level difference that generates a phase difference that is within the depth. As a result, when the wavelength of the laser light changes, a phase difference is generated in the laser light transmitted through the annular zone so that the convergence position is within the depth of focus. And the shift | offset | difference of the convergence position accompanying a wavelength change is reduced by the said phase difference. Therefore, the laser beam is converged within the focal depth by the pickup objective lens. Therefore, even if the oscillation wavelength of the laser light source changes rapidly, the convergence position of the laser light can be within the depth of focus.

Further, the plurality of annular zone steps are steps in which the phase of transmitted light is different from each other in wavelength units in a normal state, and the convergence position of the laser beam converged by the pickup objective lens when the wavelength changes It is preferable to have a step amount that generates a phase difference in the laser light so that is within the depth of focus.
Thereby, when the wavelength changes suddenly, a phase difference is generated in the laser light so that the convergence position of the laser light is within the depth of focus. Therefore, the convergence position of the laser beam converged by the pickup objective lens can be set within the focal depth.

Furthermore, it is preferable that the plurality of annular zone steps have a step amount that reduces an aberration generated in the pickup objective lens when a wavelength changes.
As a result, when the wavelength of the laser beam changes, a phase difference that reduces the aberration generated in the pickup objective lens is generated in the laser beam that has passed through the annular zone. And the aberration which generate | occur | produces in a pick-up objective lens with a wavelength change by the said phase difference is reduced. Accordingly, it is possible to reduce the aberration that occurs according to the change in the wavelength of the laser beam.

  Further, in the adjacent annular zone steps, the thickness of the pickup objective lens in one annular zone step is higher than the thickness of the pickup objective lens in the other annular zone step inside the one annular zone step. It is preferable that it is thick in the vicinity.

The pickup objective lens according to the present invention is used in a recording / reproducing apparatus for BD and / or HD-DVD.
Thereby, for example, if the pickup objective lens according to the present invention is attached to a recording / reproducing apparatus for both BD and HD-DVD, the aberration of both BD and HD-DVD can be corrected by one pickup objective lens.

  Further, another pickup objective lens according to the present invention condenses a light beam having a wavelength λ on the information recording surface of a first optical recording medium having a transparent substrate having a thickness t1, and thickens the light beam having a wavelength λ. A pickup objective lens for focusing on an information recording surface of a second optical recording medium having a transparent substrate of t2 (t1 <t2), wherein at least one surface of the pickup objective lens transmits a transmitted light beam to the first optical recording medium; First optical recording medium region for condensing on the information recording surface of the optical recording medium, and second optical recording for condensing the transmitted light beam on the information recording surface of the second optical recording medium The first optical recording medium region is divided into medium regions, and the first optical recording medium region has a plurality of annular zone steps, and the plurality of annular zone steps are converged by the pickup objective lens when the wavelength changes. The light convergence position is within the depth of focus. As described above, the laser beam has a level difference that generates a phase difference in the laser beam.

  In the present invention, it can also be used in an optical pickup apparatus for recording / reproducing two types of optical recording media having different substrate thicknesses.

  Further, the plurality of annular zone steps are steps in which the phase of transmitted light is different from each other in wavelength units in a normal state, and the convergence position of the laser beam converged by the pickup objective lens when the wavelength changes It is preferable to have a step amount that generates a phase difference in the laser light so that is within the depth of focus.

  Furthermore, it is preferable that the plurality of annular zone steps have a step amount that reduces an aberration generated in the pickup objective lens when a wavelength changes.

  A pickup objective lens design method according to the present invention is a pickup objective lens design method, wherein a plurality of annular zone steps are formed on at least one surface of the pickup objective lens, and the plurality of annular zone steps have a wavelength Has a level difference that causes a phase difference in the laser beam so that the convergence position of the laser beam converged by the pickup objective lens is within the depth of focus.

  Further, the plurality of annular zone steps are steps in which the phase of transmitted light is different from each other in wavelength units in a normal state, and the convergence position of the laser beam converged by the pickup objective lens when the wavelength changes It is preferable to have a step amount that generates a phase difference in the laser light so that is within the depth of focus.

  Furthermore, it is preferable that the plurality of annular zone steps have a step amount that reduces an aberration generated in the pickup objective lens when a wavelength changes.

  According to the present invention, even if the oscillation wavelength of the laser light source changes rapidly, the convergence position of the laser light can be within the depth of focus.

  Hereinafter, specific embodiments to which the present invention is applied will be described in detail with reference to the drawings. Note that the present invention is not limited to the following embodiments. FIG. 1 shows an example of an optical pickup device 1 according to an embodiment of the present invention. The optical pickup device 1 includes a light source 11 (laser light source), a beam splitter 12, a collimator lens 13, a pickup lens 14 (pickup objective lens), a detection system 16, and the like.

  The light source 11 includes a blue laser diode used for HD-DVD and BD.

  A beam splitter 12 is provided on the optical path of the laser light emitted from the light source 11.

  A collimator lens 13 is provided on the optical path of the laser light emitted from the beam splitter 12. The collimator lens 13 converts the laser light emitted from the light source 11 from divergent light into substantially parallel light.

  A pickup lens 14 is provided on the optical path of the laser light that has passed through the collimator lens 13.

The pickup lens 14 has a function of condensing incident light on the information recording surface of the optical disc 15 to near the diffraction limit. The pickup lens 14 further has a function of guiding the laser beam reflected by the information recording surface of the optical disc 15 to the detection system 16.
Further, the pickup lens 14 has a plurality of annular zone steps on at least one surface. In other words, at least one surface of the pickup lens 14 is formed with a plurality of annular zones that are concentric with the center (optical axis) of the pickup lens 14. The pickup lens 14 is made of a plastic material.

As will be described later, the position in the optical axis OA (described later) direction of the plurality of annular zone steps formed on the pickup lens 14 is the phase of the laser beam transmitted normally (when the wavelength of the laser beam is not changed). Are set to be different from each other in the wavelength unit between adjacent annular zones.
Further, the plurality of annular zone steps formed on the pickup lens 14 cause a phase difference in the laser beam so that the convergence position of the laser beam converged by the pickup lens 14 is within the depth of focus when the wavelength changes. Has a level difference. At the same time, the step amount is set so as to reduce the aberration generated in the pickup lens 14 when the wavelength changes. Specifically, in adjacent annular zone steps, the thickness of the pickup lens 14 at one annular zone step is higher than the thickness of the pickup lens 14 at the other annular zone step inside the one annular zone step. An annular step is formed to be thick in the vicinity. That is, the annular zone step is formed so that the surface of the pickup lens 14 having the annular zone step has a saw-like shape from the center toward the outside.

  The oscillation wavelength of the light source 11 changes by about 1 nm due to a change in the output of the light source 11. For example, the oscillation wavelength of the light source 11 changes from 408 nm to 409 nm due to a change in output in switching from reproduction to recording. When the oscillation wavelength of the light source 11 changes due to a change in output, the refractive index of the pickup lens 14 also changes. And the convergence position of the laser beam of the light source 11 converged by the pickup lens 14 also changes. However, when the laser light passes through a plurality of annular zones provided in the pickup lens 14, a phase difference is generated in the laser light so that the convergence position is within the depth of focus. Therefore, even if the oscillation wavelength of the light source 11 changes, the convergence position of the laser light converged by the pickup lens 14 is within the depth of focus.

  At the time of focus servo and tracking servo, the pickup lens 14 is operated by an actuator (not shown).

  In the present embodiment, the transparent substrate of the optical disc 15 for HD-DVD and BD is polycarbonate, and the transparent substrate thickness of the optical disc 15 for HD-DVD and BD is 0.6 mm and 0.0875 mm, respectively. did.

  Next, the behavior until the laser beam emitted from the light source 11 is reflected by the information recording surface of the optical disc 15 and detected by the detection system 16 will be described. The laser light emitted from the light source 11 passes through the beam splitter 12 and enters the collimator lens 13.

  The collimator lens 13 converts the laser light emitted from the light source 11 from divergent light into substantially parallel light.

  The laser light that has passed through the collimator lens 13 enters the pickup lens 14. Here, in the present embodiment, when the wavelength of the laser beam changes, the plurality of annular zones provided in the pickup lens 14 are such that the convergence position of the laser beam converged by the pickup lens 14 is within the depth of focus. The phase of the laser beam is corrected so that Then, the pickup lens 14 focuses the corrected laser beam on the information recording surface of the optical disc 15 to near the diffraction limit. The laser beam reflected by the information recording surface of the optical disc 15 enters the detection system 16 via the pickup lens 14 and is detected. The detection system 16 detects the laser beam and performs photoelectric conversion to generate a focus servo signal, a track servo signal, a reproduction signal, and the like.

  Next, the pickup lens 14 used in the optical pickup device 1 according to the embodiment of the present invention will be described in detail. 2A and 2B are diagrams showing the pickup lens 14 in the optical pickup device 1 according to the present embodiment. FIG. 2A shows the wavefront (phase) of the laser beam in a normal state, and FIG. FIG. 2C shows the wavefront (phase) of the laser beam when the wavelength of the laser beam is increased. FIG. 2C shows the wavefront (phase) of the laser beam when the wavelength of the laser beam is increased. In the present embodiment, the plurality of annular zone steps described above are provided on the surface of the pickup lens 14 on the light source 11 side. The position of the annular zone step of the pickup lens 14 in the direction of the optical axis OA (described later) is set so that the phase of the laser beam transmitted in the normal time differs in wavelength units from the adjacent annular zones. Further, the annular step of the pickup lens 14 has a step amount that causes a phase difference in the laser beam so that the convergence position of the laser beam converged by the pickup lens 14 is within the depth of focus when the wavelength changes.

  That is, when the laser light is normally incident on the pickup lens 14, the phases of the laser light transmitted through the annular zone steps are different from each other by an integral multiple of the wavelength. Therefore, as shown in FIG. 2A, no phase difference occurs in the laser light transmitted through different annular zone steps in the normal state. Therefore, the laser light incident on the pickup lens 14 is emitted with the same phase. Accordingly, at the normal time, the convergence position of the laser beam converged by the pickup lens 14 is substantially the same position as when no annular zone step is formed. In the present embodiment, the desired position so that the laser beam converges in the best state is the information recording surface of the optical disc 15 (not shown). That is, the laser beam is well focused on the information recording surface of the optical disc 15 at the time of focusing.

  On the other hand, as shown in FIGS. 2 (b) and 2 (c), when the laser beam whose wavelength changes due to a change in output or the like and enters the pickup lens 14, the laser which has passed through each annular zone step. The difference in light phase is not an integral multiple of the wavelength. Therefore, as shown in FIGS. 2B and 2C, when the wavelength is changed, a phase difference is generated in the laser light transmitted through different annular zone steps. In the present invention, the phase difference is adjusted so that the convergence position of the laser beam converged by the pickup lens 14 is within the depth of focus. Therefore, when the wavelength changes, conventionally, the convergence position of the laser beam converged by the pickup lens 14 changes, but in the present invention, due to the phase difference of the laser beam transmitted through each annular zone of the pickup lens 14. , The change of the convergence position of the laser light accompanying the wavelength change is suppressed. Then, the laser beam emitted from the pickup lens 14 is favorably condensed on the information recording surface of the optical disc 15.

Further, the annular step of the pickup lens 14 has a step amount that reduces the aberration that occurs in the pickup lens 14 when the wavelength of the laser light changes.
Actually, the oscillation wavelength of the light source 11 varies about ± 5 nm around 408 nm. FIG. 3A shows rms wavefront aberration generated in a pickup lens in which no annular zone is formed when the wavelength of the light source 11 is 403 nm, and FIG. 4A shows a case where the wavelength of the light source 11 is 413 nm. The rms wavefront aberration that occurs in a pickup lens in which no annular zone is formed is shown. 3B shows rms wavefront aberration generated in the pickup lens 14 when the wavelength of the light source 11 is 403 nm, and FIG. 4B shows the rms wavefront aberration in the pickup lens 14 when the wavelength of the light source 11 is 413 nm. The rms wavefront aberration which generate | occur | produces is shown. 3 and 4, the vertical axis represents the magnitude of the rms wavefront aberration, and the horizontal axis represents the position in the radial direction of the pickup lens.

As shown in FIGS. 3A and 4A, when the oscillation wavelength of the light source 11 is 403 nm and 413 nm, the rms wavefront aberration becomes very large in a pickup lens in which no annular zone is formed.
On the other hand, as shown in FIGS. 3B and 4B, even if the oscillation wavelength of the light source 11 is 403 nm and 413 nm, the rms wavefront aberration is suppressed in the pickup lens 14 in which the annular zone is formed. I can do it. Specifically, as shown in FIGS. 3B and 4B, a phase difference is generated in the laser light transmitted through each annular zone due to the annular zone step formed in the pickup lens 14. The phase difference reduces the aberration that occurs in the pickup lens 14 due to wavelength fluctuation. Thereby, even if the oscillation wavelength of the laser beam 11 varies about ± 5 nm around 408 nm, the aberration generated in the pickup lens 14 is reduced. Therefore, the laser beam emitted from the pickup lens 14 is well focused on the information recording surface of the optical disk 15.

  According to the pickup lens 14 according to the present embodiment configured as described above, a plurality of annular zone steps are provided on at least one surface of the pickup lens 14, and the plurality of annular zone steps are formed when the wavelength changes. There is a level difference that generates a phase difference such that the convergence position of the laser beam converged by the pickup lens 14 is within the depth of focus. As a result, when the wavelength of the laser light changes, a phase difference is generated in the laser light transmitted through the annular zone so that the convergence position is within the depth of focus. And the shift | offset | difference of the convergence position accompanying a wavelength change is reduced by the said phase difference. Therefore, the laser beam is converged within the focal depth by the pickup lens 14. Therefore, even if the oscillation wavelength of the laser light source changes rapidly, the convergence position of the laser light can be within the depth of focus.

Specifically, the plurality of annular zone steps are steps in which the phase of transmitted light in the normal state differs from each other in wavelength units, and the convergence of the laser beam converged by the pickup lens 14 when the wavelength changes. There is a step amount that generates a phase difference in the laser beam so that the position is within the depth of focus.
Thereby, when the wavelength changes suddenly, a phase difference is generated in the laser light so that the convergence position of the laser light is within the depth of focus. Therefore, the convergence position of the laser beam converged by the pickup lens 14 can be within the depth of focus.

Furthermore, the plurality of annular zone steps have a level difference that reduces the aberration generated in the pickup lens 14 when the wavelength changes.
As a result, when the wavelength of the laser beam changes, a phase difference that reduces the aberration generated in the pickup lens 14 is generated in the laser beam that has passed through the annular zone. And the aberration which generate | occur | produces in the pick-up lens 14 with a wavelength change is reduced by the said phase difference. Accordingly, it is possible to reduce the aberration that occurs according to the change in the wavelength of the laser beam.

  Further, in the adjacent annular zone step, the thickness of the pickup lens 14 at one annular zone step is closer to the step position than the thickness of the pickup lens 14 at the other annular zone step inside the one annular zone step. It is preferable to be thick.

The pickup lens 14 according to the present invention is used in the optical pickup device 1 for BD and HD-DVD.
Thereby, for example, if the pickup lens 14 according to the present invention is attached to an optical pickup device for both BD and HD-DVD, the aberration of both BD and HD-DVD can be corrected by one pickup lens 14.

  In the present embodiment, the pickup lens 14 is made of a plastic material, but may be made of a glass material.

  Next, Example 1 of the present invention will be described in comparison with Comparative Example 1 that is out of the scope of the present invention. As Example 1, as shown in FIG. 5, a plurality of annular zone steps were provided on the surface of the pickup lens 14 on the light source 11 (not shown) side. In the table shown in FIG. 14, the zone number of the pickup lens 14 according to the first embodiment, the zone position (position where the zone is formed in a direction perpendicular to the optical axis OA (described later)), and the step difference on the axis. Indicates. FIG. 6 shows a side view of the pickup lens 14 according to the present invention. Here, the on-axis step amount is an intersection point X1 at which the surface shape virtually intersects the lens optical axis when the surface shape of each annular zone is virtually extended to the optical axis OA side in FIG. And the distance B1 from the intersection point X2 where the surface shape of the first annular zone intersects the optical axis OA. Since the curvature, conical coefficient, and aspheric coefficient of each annular zone are different, the surface shape of each annular zone is slightly different. For this reason, the distance B1 and the step B2 of each annular zone do not always coincide with each other. Therefore, using the distance B1 between the intersection point X1 where the surface shape of each annular zone virtually intersects the lens optical axis and the intersection point X2 where the surface shape of the first annular zone intersects the optical axis OA, Represents the amount of step. When the axial step amount is a positive value, it means that the virtual intersection X1 is on the optical disk 15 side of the pickup lens 14. Further, when the axial step amount is a negative value, it means that the virtual intersection X1 is on the light source 11 side of the pickup lens 14. As shown in FIG. 14, in adjacent annular zone steps, the axial step amount at one annular zone step is a negative value smaller than the axial step amount at the other annular zone step inside the one annular zone step. An annular step is formed so that Specifically, the annular zone step is formed so that the axial step amount becomes a negative value smaller by 0.006229 mm as it goes from the center of the lens toward the outer edge. In other words, the annular zone step is formed such that the virtual intersection point X1 moves 0.006229 mm toward the light source 11 side of the pickup lens 14. Further, the number of annular zone steps is 15. The position of the annular step in the optical axis OA direction is designed so that the phase of the laser light transmitted through the adjacent annular step is different in units of wavelength when the wavelength of the light source 11 does not change. .

  The tables shown in FIGS. 15, 16, and 17 show the data of the optical system according to the first example. In FIG. 15, the objective lens R1 surface is a surface of the pickup lens 14 on the light source 11 side. The objective lens R2 surface is a surface of the pickup lens 14 on the optical disc 15 side. As shown in FIG. 15, a plastic lens was used as the pickup lens 14. Also, the coefficient values shown in FIG. 16 are the coefficient values used in Equations (1) and (2) that define the surface shape of the pickup lens 14. The coefficient values shown in FIG. 17 are the coefficient values used in Expression (2) that defines the surface shape of the pickup lens 14. In FIG. 16, the coefficient value of surface number 2 (that is, the surface of the pickup lens 14 on the light source 11 side) is the first annular zone of the pickup lens 14 (the first annular zone from the center of the pickup lens 14). The surface shape is defined. In FIG. 16, the value of the coefficient of surface number 3 (that is, the surface of the pickup lens 14 on the optical disk 15 side) defines the surface shape of the entire surface of the pickup lens 14 on the optical disk 15 side. Further, the coefficient values shown in FIG. 17 define the surface shapes of the second to fifteenth annular zones of the pickup lens 14.

で表されるように、光出射面Ｒ２の面形状が形成される。
そして、数式（１）と図１６に示す面番号３の係数の値とにより、ピックアップレンズ１４の光ディスク１５側の面全体の面形状が規定される。 Equations (1) and (2) will be described with reference to FIG. FIG. 7 is a side view showing an example of a pickup lens.
First, the surface shape of the light exit surface R2 (objective lens R2 surface) of the pickup lens will be described. In FIG. 7, the height of the light beam is h, the vertex of the light exit surface R2 of the objective lens 1 is e, the point of the light beam height h on the tangent surface in contact with the vertex e is c, and the point c is parallel to the optical axis OA. If the point on the light exit surface R2 in any direction is d, the distance Z _B between the points c and d with respect to an arbitrary light height h is

As shown, the surface shape of the light exit surface R2 is formed.
Then, the surface shape of the entire surface of the pickup lens 14 on the optical disk 15 side is defined by Equation (1) and the value of the coefficient of surface number 3 shown in FIG.

Incidentally, in Equation (1), the coefficient C, K, A4, A6, A8, and substitutes the value of the A10 obtains distances _{Z B} for any ray height h (≠ 0), the value is a negative value In this case, it is indicated that the point d is located on the emission surface side (left side in FIG. 7) from the surface vertex e through which the optical axis OA of the light emission surface R2 passes. When the distance Z _B is a positive value, it indicates that the point d is located on the right side of the vertex e.

で表されるように、光入射面Ｒ１の面形状が形成される。 Next, the surface shape of the light incident surface R1 (objective lens R1 surface) of the pickup lens will be described. In FIG. 7, the vertex of the light incident surface R1 of the objective lens 1 is f, the point of the ray height h on the tangent surface in contact with the vertex f is a, and the light incident surface in a direction parallel to the optical axis OA from this point a. When a point on R1 is is b, a point for any ray height h a, the distance Z _a between b

As shown, the surface shape of the light incident surface R1 is formed.

  When the surface shapes of the first annular zone, the second annular zone,..., The 15th annular zone of the pickup lens 14 are defined, the coefficient h in the formula (2) is shown in the table of FIG. Substitute the values of the annular zone positions of the first annular zone, the second annular zone, ..., the 15th annular zone. Further, when the surface shapes of the first annular zone, the second annular zone,..., And the 15th annular zone of the pickup lens 14 are defined, the coefficient B in the formula (2) is the first shown in the table of FIG. The value of the axial step amount of the 1st annular zone, the 2nd annular zone, ..., the 15th annular zone is substituted. The surface shapes of the first to fifteenth annular zones of the pickup lens 14 are defined by the mathematical formula (2), the value of the coefficient of the surface number 2 shown in FIG. 16, and the value of the coefficient shown in FIG. .

On the other hand, a pickup lens having no annular zone step is referred to as Comparative Example 1. FIG. 8A shows the rms wavefront aberration when the wavelength of the light source 11 is changed from 403 nm to 404 nm in Comparative Example 1. FIG. 8B shows the wavelength of the light source 11 from 408 nm to 409 nm in Comparative Example 1. FIG. 8C shows the rms wavefront aberration when the wavelength of the light source 11 is changed from 413 nm to 414 nm in the first comparative example. The scale of the vertical axis is 0.5λ.
As shown in FIG. 8, in the case of a pickup lens having no annular zone step, when the wavelength of the light source 11 is changed from 403 nm to 404 nm, 408 nm to 409 nm, and 413 nm to 414 nm, the light spot formed on the information recording surface of the optical disk 15 The rms wavefront aberrations of are 113.3 mλ, 106.4 mλ, and 110.8 mλ, respectively.
In addition, when the wavelength of the light source 11 is 403 nm and 413 nm, in the pickup lens having no annular zone step, the deviation amount of the laser beam convergence position from the in-focus position (information recording surface of the optical disc 15) is − 1.634 μm and +1.570 μm.
When the wavelength of the light source 11 is 403 nm, 408 nm, and 413 nm, the rms wavefront aberration at the convergence position is 35.3 mλ, 2.0 mλ, and 35.4 mλ in the pickup lens that does not have the annular step. .

On the other hand, FIG. 9A shows rms wavefront aberration when the wavelength of the light source 11 is changed from 403 nm to 404 nm in Example 1. FIG. 9B shows the wavelength of the light source 11 in Example 1 from 408 nm. The rms wavefront aberration when changing to 409 nm is shown, and FIG. 9C shows the rms wavefront aberration when the wavelength of the light source 11 is changed from 413 nm to 414 nm in Example 1. The scale of the vertical axis is 0.2λ.
As shown in FIG. 9, in the case of the pickup lens 14 having a plurality of annular zone steps, it was formed on the information recording surface of the optical disc 15 when the wavelength of the light source 11 changed from 403 nm to 404 nm, 408 nm to 409 nm, and 413 nm to 414 nm. The rms wavefront aberrations of the light spot are 29.7 mλ, 12.3 mλ, and 38.5 mλ, respectively.
In addition, when the wavelength of the light source 11 is 403 nm and 413 nm, in the pickup lens 14 having a plurality of annular zone steps, the amount of deviation from the focal position (information recording surface of the optical disc 15) of the convergence position of the laser light is respectively , −0.206 μm and +0.139 μm.
Further, when the wavelength of the light source 11 is 403 nm, 408 nm, and 413 nm, the rms wavefront aberration at the convergence position in the pickup lens 14 is 32.5 mλ, 1.3 mλ, and 31.7 mλ, respectively.

  When the convergence position is deviated from the focal position (information recording surface of the optical disc 15) (that is, when the focus is deviated), the peak value of the laser beam at the convergence position of the light spot is the peak value at the focal position (that is, the focal position). The peak value when there is no defocus is low. In other words, when the convergence position of the laser beam is deviated from the information recording surface of the optical disk 15 (when defocusing), the peak value of the light spot formed on the information recording surface of the optical disk 15 is the same as the convergence position of the laser beam. This is lower than the peak value of the light spot formed on the information recording surface of the optical disc 15 in the case of 15 information recording surfaces (when there is no defocus). When the allowable range of defocus (depth of focus) is a range in which the peak value when defocusing is 95% or more of the peak value at the time of in-focus, the depth of focus is 0.21 μm for HD-DVD. In the case of BD, it is 0.11 μm.

In the pickup lens having no annular step according to the comparative example 1, as shown in FIG. 8, when the wavelength of the light source 11 is changed by 1 nm, the aberration generated in the pickup lens increases and exceeds the marshal limit. .
Moreover, in the comparative example 1, when the wavelength of the light source 11 is 403 nm and 413 nm, the convergence position becomes out of the focal depth by changing the wavelength by 1 nm.
On the other hand, in the pickup lens 14 having a plurality of annular zone steps according to Example 1, as shown in FIG. 9, even if the wavelength of the light source 11 changes by 1 nm, the aberration generated in the pickup lens 14 is The marshal limit is not exceeded.
Moreover, in Example 1, when the wavelength of the light source 11 is 403 nm and 413 nm, even if the wavelength changes by 1 nm, the convergence position is within the depth of focus. In other words, the convergence position of the laser light is within the focal depth even if the wavelength of the light source 11 varies due to the annular step provided in the pickup lens 14.

  According to the above-described first embodiment, the number of annular zone steps formed on the pickup lens 14 can be reduced as compared with the conventional case of correcting the aberration of the pickup lens using diffraction. For example, the number of annular zone steps provided in the pickup lens 14 of the present invention is 15 annular zones (step = 8th order). On the other hand, when aberration correction is performed using conventional diffraction, for example, the number of annular zone steps provided in the pickup lens is 30 annular zones (8th order). And when the number of annular zone steps decreases, the area of the inclined portion between the annular zone steps decreases. Therefore, unnecessary light is reduced and the light utilization efficiency of the pickup lens 14 can be improved.

  Next, Example 2 of the present invention will be described in comparison with Comparative Example 2 that is out of the scope of the present invention. The pickup lens 17 according to Example 2 is a plastic BD / HD-DVD compatible lens. A plurality of annular zone steps were provided on the surface of the pickup lens 17 on the light source 11 (not shown) side. The pickup lens 17 is shown in FIG.

  The surface on the light source 11 side of the pickup lens 17 according to the second embodiment has a BD region (for the first optical recording medium) for condensing the transmitted light beam on the information recording surface of the BD (first optical recording medium). Area) and an HD area (second optical recording medium area) for condensing the transmitted light beam on the information recording surface of the HD-DVD (second optical recording medium). The BD area and the HD area are alternately arranged in the pickup lens 17. In FIG. 10, only light rays that pass through the BD region are displayed.

  The pickup lens 17 according to the second example has an aberration correction function. Specifically, the pickup lens 17 has a plurality of annular zone steps in the BD region. The table shown in FIG. 18 shows the zone number, the zone position, and the axial step amount of the pickup lens 17 according to the second embodiment. Note that the axial step amount shown in FIG. 18 indicates the position of the intersection X1 where the surface shape virtually intersects the optical axis OA when the surface shape of each annular zone is virtually extended to the optical axis OA side. This is the amount of change. As shown in FIG. 18, in adjacent annular zone steps, an axial step amount at one annular zone step is a negative value smaller than an axial step amount at another annular zone step inside the one annular zone step. An annular step is formed so that Specifically, the annular zone step is formed so that the axial step amount becomes a negative value as it goes from the center of the lens toward the outer edge. In other words, the annular zone step is formed so that the virtual intersection X1 moves to the light source 11 side of the pickup lens 14. The number of annular zone steps is 19. The position of the annular step in the optical axis OA direction is designed so that the phase of the laser light transmitted through the adjacent annular step is different in units of wavelength when the wavelength of the light source 11 does not change. . The HD region of the pickup lens 17 has a radius of curvature that becomes a convex lens surface. Therefore, the surface on the light source 11 side of the pickup lens 17 according to the second embodiment is shaped so as to be a substantially convex lens surface as a whole.

  Moreover, the table | surface shown in FIG.19, FIG.20, FIG.21 shows the data of the optical system in the BD area | region of the present Example 2. FIG. In FIG. 19, the objective lens R1 surface is a surface of the pickup lens 17 on the light source 11 side. The objective lens R2 surface is a surface of the pickup lens 17 on the optical disk 15 side. Also, the coefficient values shown in FIG. 20 are the coefficient values used in Equation (1) and Equation (2) that define the surface shape of the pickup lens 17. The coefficient values shown in FIG. 21 are the coefficient values used in Expression (2) that defines the surface shape of the pickup lens 17. In FIG. 20, the coefficient value of surface number 2 (that is, the surface on the light source 11 side of the pickup lens 17) is the first annular zone in the BD region of the pickup lens 17 (first from the center of the pickup lens 17). This defines the surface shape of the annular zone. In FIG. 20, the coefficient value of surface number 3 (that is, the surface of the pickup lens 17 on the optical disk 15 side) defines the surface shape of the entire surface of the pickup lens 17 on the optical disk 15 side. The coefficient values shown in FIG. 21 define the surface shapes of the second to nineteenth annular zones in the BD region of the pickup lens 17.

Then, the surface shape of the entire surface of the pickup lens 17 on the optical disk 15 side is defined by Equation (1) and the value of the coefficient of surface number 3 shown in FIG.
Further, when the surface shapes of the first annular zone, the second annular zone,..., The 19th annular zone of the pickup lens 17 are defined, the coefficient h of the formula (2) is shown in the table of FIG. The value of the ring zone position of the 1st ring zone, the 2nd ring zone, ..., the 19th ring zone is substituted. In addition, when the surface shapes of the first annular zone, the second annular zone,..., The 19th annular zone of the pickup lens 17 are defined, the coefficient B in Expression (2) is shown in the table of FIG. The values of the axial step amounts of the first annular zone, the second annular zone, ..., the 19th annular zone are substituted. The surface shape of the first to nineteenth annular zones of the pickup lens 17 is defined by the mathematical formula (2), the coefficient value of surface number 2 shown in FIG. 20, and the coefficient value shown in FIG. .

The tables shown in FIGS. 22 and 23 show optical system data in the HD region of the present example. In FIG. 22, the objective lens R1 surface is a surface of the pickup lens 17 on the light source 11 side. The objective lens R2 surface is a surface of the pickup lens 17 on the optical disk 15 side. The coefficient values shown in FIG. 23 are the coefficient values used in Expression (1) and Expression (2) that define the surface shape of the pickup lens 17. In FIG. 23, the coefficient value of surface number 2 (that is, the surface of the pickup lens 17 on the light source 11 side) defines the surface shape of the HD region of the pickup lens 17. In FIG. 23, the coefficient value of surface number 3 (that is, the surface of the pickup lens 17 on the optical disk 15 side) defines the surface shape of the entire surface of the pickup lens 17 on the optical disk 15 side.
Further, when defining the surface shape of the HD region of the pickup lens 17, a value of “0” is substituted for the coefficient B in Equation (2). This is because in this embodiment, no annular zone step is formed in the HD region.
Then, the surface shape of the entire surface of the pickup lens 17 on the optical disk 15 side is defined by Equation (1) and the value of the coefficient of surface number 3 shown in FIG. Further, the surface shape of the HD region of the pickup lens 17 is defined by Equation (2) and the coefficient of surface number 2 shown in FIG.

  On the other hand, the pickup lens 18 according to the comparative example 2 is a BD / HD-DVD compatible lens having no annular zone step. The pickup lens 18 was formed of the same plastic material as the pickup lens 17 according to the second example. The pickup lens 18 is shown in FIG.

  The surface on the light source 11 side of the pickup lens 18 according to the comparative example 2 is divided into a BD area and an HD area as in the second embodiment. The BD area and the HD area are alternately arranged in the pickup lens 18. In FIG. 11, only the light rays that pass through the BD region are displayed. Further, the pickup lens 18 according to the comparative example 2 does not have a phase correction function. That is, unlike the pickup lens 17 according to the second embodiment, in the BD region of the pickup lens 18, the phase of the laser light transmitted through the annular zone step is the same between the annular zones. The BD area and the HD area of the pickup lens 18 have surface shapes that are different convex lens surfaces. Specifically, the BD region of the pickup lens 18 has a surface shape defined by the value of the coefficient of surface number 2 in the table shown in FIG. 20 and Equation (2). The HD region of the pickup lens 18 has a surface shape defined by the value of the coefficient of surface number 2 in the table shown in FIG. When defining the surface shapes of the BD area and the HD area of the pickup lens 18, a value of “0” is substituted for B, which is the on-axis step amount, in Equation (2). This is because the position of the surface shape between the annular zones has not changed in the BD region and the HD region of the pickup lens 18. Further, the surface on the disk 15 side of the pickup lens 18 has a surface shape defined by the value of the coefficient of the surface number 3 in the table shown in FIG. 20 or FIG. 23 and the mathematical expression (1). Since the BD region and the HD region of the pickup lens 18 have different aspherical shapes, the respective light beams are condensed at appropriate positions for both the BD and the HD-DVD.

  Note that in the BD / HD-DVD compatible lens according to Example 2 and Comparative Example 2, since the incident light beam is split, a super-resolution phenomenon occurs and the spot diameter becomes too small. Therefore, the normal NA (numerical aperture) in BD is 0.85, but in Example 2 and Comparative Example 2, the NA is corrected to about 0.79.

FIG. 12A shows the rms wavefront aberration when the wavelength of the light source 11 is changed from 403 nm to 404 nm in Comparative Example 2, and FIG. 12B shows the wavelength of the light source 11 from 408 nm to 409 nm in Comparative Example 2. FIG. 12C shows the rms wavefront aberration when the wavelength of the light source 11 is changed from 413 nm to 414 nm in the second comparative example. The scale of the vertical axis is 0.5λ.
As shown in FIG. 12, in the case where the pickup lens 18 according to Comparative Example 2 is used, when the wavelength of the light source 11 is changed from 403 nm to 404 nm, 408 nm to 409 nm, and 413 nm to 414 nm, it is formed on the information recording surface of the optical disc 15. The rms wavefront aberrations of the light spot are 95.6 mλ, 91.1 mλ, and 91.5 mλ, respectively.
Further, when the wavelength of the light source 11 is 403 nm and 413 nm, when the pickup lens 18 of Comparative Example 2 is used, the deviation amount of the laser beam convergence position from the focal position (information recording surface of the optical disk 15) is respectively −1.577 μm and +1.516 μm.
When the wavelength of the light source 11 is 403 nm, 408 nm, and 413 nm, the rms wavefront aberration at the convergence position is 20.4 mλ, 1.8 mλ, and 21.9 mλ when the pickup lens 18 of Comparative Example 2 is used. Become.

On the other hand, FIG. 13A shows the rms wavefront aberration when the wavelength of the light source 11 is changed from 403 nm to 404 nm in Example 2, and FIG. 13B shows the wavelength of the light source 11 in Example 2 from 408 nm. The rms wavefront aberration when changing to 409 nm is shown, and FIG. 13C shows the rms wavefront aberration when the wavelength of the light source 11 is changed from 413 nm to 414 nm in Example 2. The scale of the vertical axis is 0.1λ.
As shown in FIG. 13, when the pickup lens 17 according to Example 2 is used, the light formed on the information recording surface of the optical disk 15 when the wavelength of the light source 11 changes from 403 nm to 404 nm, 408 nm to 409 nm, and 413 nm to 414 nm. The rms wavefront aberration of the spot is 18.8 mλ, 4.7 mλ, and 27.4 mλ, respectively.
Further, when the wavelength of the light source 11 is 403 nm and 413 nm, when the pickup lens 17 of Example 2 is used, the amount of deviation from the focal position (information recording surface of the optical disk 15) of the convergence position of the laser light is respectively −0.044 μm and +0.019 μm.
Further, when the wavelength of the light source 11 is 403 nm, 408 nm, and 413 nm, when the pickup lens 17 of Example 2 is used, the rms wavefront aberration at the convergence position is 23.5 mλ, 1.0 mλ, and 22.9 mλ, respectively. Become.

In Comparative Example 2, as shown in FIG. 12, when the wavelength of the light source 11 changes by 1 nm, the aberration generated in the pickup lens 18 increases and exceeds the marshal limit.
In Comparative Example 2, when the wavelength of the light source 11 is 403 nm and 413 nm, the convergence position is outside the depth of focus.
On the other hand, in Example 2, as shown in FIG. 13, even if the wavelength of the light source 11 changes by 1 nm, the aberration generated in the pickup lens 17 does not exceed the Marechal limit.
In Example 2, even if the wavelength of the light source 11 is 403 nm and 413 nm, the convergence position is within the depth of focus. That is, due to the aberration correction function due to the annular zone step formed in the pickup lens 17, even if the wavelength of the light source 11 varies, the convergence position of the laser light is within the depth of focus.

In Examples 1 and 2, a plastic pickup lens is used, but the same effect can be obtained by using a glass pickup lens.
In the second embodiment, the aberration correction function is provided only in the BD region of the pickup lens 17, but a function for appropriately correcting aberration may be provided in the HD region as well.

1 shows an example of an optical pickup device according to an embodiment of the present invention. FIG. 2A is a diagram (FIG. 2A) showing a wavefront (phase) of laser light that is transmitted through the pickup lens according to the present embodiment in a normal state, and the pickup lens according to the present embodiment when the wavelength of the laser light is shortened. FIG. 2B shows the wavefront (phase) of the laser beam that passes through the laser beam, and shows the wavefront (phase) of the laser beam that passes through the pickup lens according to the present embodiment when the wavelength of the laser beam increases. It is a figure (FIG.2 (c)). FIG. 3A shows a rms wavefront aberration generated in a pickup lens having a light source wavelength of 403 nm and no annular step, and is generated in a pickup lens having a light source wavelength of 403 nm and a annular step. It is a figure (FIG.3 (b)) which shows rms wavefront aberration to do. FIG. 4A shows a rms wavefront aberration generated in a pickup lens having a light source wavelength of 413 nm and no annular step (FIG. 4A), and occurs in a pickup lens having a light source wavelength of 413 nm and a annular step. FIG. 4B is a diagram showing rms wavefront aberration (FIG. 4B). It is a figure which shows the pick-up lens concerning Example 1 of this invention. It is a side view which shows an example of the lens surface shape of the pick-up lens concerning this invention. It is a side view which shows an example of the lens surface shape of a pickup lens. The figure which shows the rms wavefront aberration which generate | occur | produces in a pick-up lens when the wavelength of a light source changes from 403 nm to 404 nm in the comparative example 1 (FIG. 8A), The case where the wavelength of a light source changes from 408 nm to 409 nm in the comparative example 1 FIG. 8 shows rms wavefront aberration generated in the pickup lens (FIG. 8B) and FIG. 8 shows rms wavefront aberration generated in the pickup lens when the wavelength of the light source is changed from 413 nm to 414 nm in Comparative Example 1 (FIG. 8). (C)). FIG. 9A shows the rms wavefront aberration that occurs in the pickup lens when the wavelength of the light source changes from 403 nm to 404 nm in Example 1. FIG. 9A shows the case where the wavelength of the light source changes from 408 nm to 409 nm in Example 1. FIG. 9 shows rms wavefront aberration generated in the pickup lens (FIG. 9B), and shows rms wavefront aberration generated in the pickup lens when the wavelength of the light source is changed from 413 nm to 414 nm in Example 1 (FIG. 9). (C)). It is a figure which shows the pick-up lens concerning Example 2 of this invention. 6 is a view showing a pickup lens according to Comparative Example 2. FIG. The figure which shows the rms wavefront aberration which generate | occur | produces in a pick-up lens when the wavelength of a light source changes from 403 nm to 404 nm in the comparative example 2 (FIG. 12A), The case where the wavelength of a light source changes from 408 nm to 409 nm in the comparative example 2 FIG. 12 is a diagram showing rms wavefront aberration generated in the pickup lens (FIG. 12B), and is a diagram showing rms wavefront aberration generated in the pickup lens when the wavelength of the light source is changed from 413 nm to 414 nm in Comparative Example 2 (FIG. 12). (C)). FIG. 13A shows the rms wavefront aberration generated in the pickup lens when the wavelength of the light source is changed from 403 nm to 404 nm in Example 2 (FIG. 13A), and the case where the wavelength of the light source is changed from 408 nm to 409 nm in Example 2. FIG. 13 shows rms wavefront aberration generated in the pickup lens (FIG. 13B), and shows rms wavefront aberration generated in the pickup lens when the wavelength of the light source is changed from 413 nm to 414 nm in Example 2 (FIG. 13). (C)). It is a table | surface which shows the ring zone number of the pick-up lens concerning Example 1 of this invention, a ring zone position, and an axial level | step difference amount. It is a table | surface which shows the data of the optical system concerning Example 1 of this invention. It is a table | surface which shows the data of the optical system concerning Example 1 of this invention. It is a table | surface which shows the data of the optical system concerning Example 1 of this invention. It is a table | surface which shows the ring zone number of the pick-up lens concerning Example 2 of this invention, a ring zone position, and an axial level | step difference amount. It is a table | surface which shows the data of the optical system in the BD area | region of Example 2 of this invention. It is a table | surface which shows the data of the optical system in the BD area | region of Example 2 of this invention. It is a table | surface which shows the data of the optical system in the BD area | region of Example 2 of this invention. It is a table | surface which shows the data of the optical system in HD area | region of Example 2 of this invention. It is a table | surface which shows the data of the optical system in HD area | region of Example 2 of this invention. It is a table | surface which shows the variation | change_quantity (micrometer / nm) of the convergence position per 1 nm wavelength change of a laser light source at the time of making a glass aspherical single lens into a pick-up objective lens. It is a table | surface which shows the depth of focus when the tolerance | permissible_range of a focus shift is made into the range from which the peak value at the time of a focus shift becomes 95% or more of the peak value at the time of focusing. It is a table | surface which shows the rms wavefront aberration which arises when the laser beam from which an oscillation wavelength differs parallelly enters into the objective lens which is formed with a glass material and has a focal distance of 1.76 mm (design wavelength = 408 nm).

Explanation of symbols

11 Light source (laser light source)
14, 17 Pickup lens (Pickup objective lens)
15 Optical disc (optical recording medium)

Claims (11)

A pickup objective lens that collects a light beam emitted from a laser light source,
The pickup objective lens has a plurality of annular zone steps on at least one surface,
The plurality of annular zone steps have a step amount for generating a phase difference in the laser beam so that a convergence position of the laser beam converged by the pickup objective lens is within a focal depth when a wavelength is changed. lens.   The plurality of annular zone steps are steps in which the phase of transmitted light in a normal state is different from each other in wavelength units, and the focal position of the laser beam converged by the pickup objective lens when the wavelength changes The pickup objective lens according to claim 1, wherein the pickup objective lens has a step amount that generates a phase difference in the laser light so as to be within a depth.   The pickup objective lens according to claim 1, wherein the plurality of annular zone steps have a step amount that reduces an aberration generated in the pickup objective lens when a wavelength changes.   In the adjacent annular zone step, the thickness of the pickup objective lens in one annular zone step is closer to the step position than the thickness of the pickup objective lens in the other annular zone step inside the one annular zone step. The pickup objective lens according to any one of claims 1 to 3, wherein the pickup objective lens is thick.   5. The pickup objective lens according to claim 1, which is used in a recording / reproducing apparatus for BD and / or HD-DVD. The light beam having the wavelength λ is condensed on the information recording surface of the first optical recording medium having the transparent substrate having the thickness t1, and the light beam having the wavelength λ is the first having the transparent substrate having the thickness t2 (t1 <t2). A pickup objective lens that focuses light onto the information recording surface of the optical recording medium of No. 2,
At least one surface of the pickup objective lens has a first optical recording medium region for condensing the transmitted light beam on the information recording surface of the first optical recording medium, and the transmitted light beam is a second optical recording medium. Divided into a second optical recording medium area for condensing on the information recording surface,
The first optical recording medium region has a plurality of annular zone steps,
The plurality of annular zone steps have a step amount for generating a phase difference in the laser beam so that a convergence position of the laser beam converged by the pickup objective lens is within a focal depth when the wavelength is changed. .   The plurality of annular zone steps are steps in which the phase of transmitted light in a normal state is different from each other in wavelength units, and the focal position of the laser beam converged by the pickup objective lens when the wavelength changes The pickup objective lens according to claim 6, wherein the pickup objective lens has a step amount that generates a phase difference in the laser light so as to be within a depth.   The pickup objective lens according to claim 6 or 7, wherein the plurality of annular zone steps have a step amount that reduces an aberration generated in the pickup objective lens when a wavelength changes. A method of designing a pickup objective lens that collects a light beam emitted from a laser light source,
Forming a plurality of annular zone steps on at least one surface of the pickup objective lens;
The plurality of annular zone steps have a step amount for generating a phase difference in the laser beam so that a convergence position of the laser beam converged by the pickup objective lens is within a focal depth when a wavelength is changed. Lens design method.   The plurality of annular zone steps are steps in which the phase of transmitted light in a normal state is different from each other in wavelength units, and the convergence position of the laser beam converged by the pickup objective lens when the wavelength changes The method of designing a pickup objective lens according to claim 9, wherein the pickup objective lens has a step amount for generating a phase difference in the laser light so as to be within a depth of focus.   11. The pickup objective lens design method according to claim 9, wherein the plurality of annular zone steps have a step amount that reduces an aberration generated in the pickup objective lens when a wavelength is changed.

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