Objective lens element and optical pickup device

An optical pickup device is provided which is compatible with at least two types of optical disc standards having different NAs and which controls an effective NA when a light beam for an optical disc standard having a relatively small NA is converged, thereby forming a desired spot. An inner part 131B and an outer part 131F of an objective lens element 143 are provided with diffraction structures different from each other. A condition (1), DO11×DO12>0, and a condition (2), DO21×DO22<0, are satisfied (DO11 (DO21) is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ1 (λ2) diffracted by the diffraction structure on the inner part; and DO12 (DO22) is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ1 (λ2) diffracted by the diffraction structure on the outer part).

CROSS REFERENCE TO RELATED APPLICATION

The disclosure of Japanese Patent Application No. 2010-219877 filed on Sep. 29, 2010 and Japanese Patent Application No. 2011-164628 filed on Jul. 27, 2011 are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an objective lens element used for performing at least one of recording, reproducing, or erasing of information on an information recoding surface of an optical information storage medium, and an optical pickup device including the objective lens element.

2. Description of the Background Art

In recent years, researches and developments have been actively carried out concerning high-density optical discs that have an increased recording density by using a blue laser beam with a wavelength of about 400 nm and thus have an improved storage capacity. One of the standards of such high-density optical discs is Blu-Ray Disc (registered trademark; hereinafter, referred to as “BD”) in which the image side numerical aperture (NA) of an objective lens is set to about 0.85 and the thickness of a protective base plate on an information recoding surface of an optical disc is set to about 0.1 mm.

Other than BD, DVD (protective base plate thickness: about 0.6 mm) for which a red laser beam with a wavelength of about 680 nm is used, and CD (protective base plate thickness: about 1.2 mm) for which an infrared laser beam with a wavelength of about 780 nm is used also exist. Various objective lenses that are compatible with three types of standards of these discs have been proposed.

For example, Japanese Patent No. 3993870 discloses an optical element and an optical pickup device that are compatible with the three types of the standards of BD, DVD, and CD. An objective lens disclosed in Japanese Patent No. 3993870 is provided with a stair-like diffraction structure (also referred to as binary type diffraction structure) in which stair-like steps are periodically arranged. The height of each step is set such that a difference in optical path of about 1.25 wavelengths is provided to a light beam having a shortest designed wavelength. In addition, one periodic structure consists of four steps that are consecutive in a radial direction (the height from a base surface is 0 to 3 times that of a unit step).

Since such a step structure is provided, the diffraction efficiency of a +1st order diffracted light beam can be at its maximum when a light beam of a wavelength for BD is used, and the diffraction efficiency of a −1st order diffracted light beam can be at its maximum when a light beam of a wavelength for DVD is used. Thus, use of change in an angle of diffraction with respect to a wavelength makes it possible to compensate for a spherical aberration that occurs due to differences in wavelength and disc base material thickness when changing between BD and DVD.

Further, Japanese Laid-Open Patent Publication No. 2005-243151 discloses an objective lens that is compatible with DVD and a high-density optical disc for which a violet light semiconductor laser is used. The objective lens disclosed in Japanese Laid-Open Patent Publication No. 2005-243151 is provided with a plurality of ring-shaped steps on an optical function surface thereof. The depth of each ring-shaped step is set such that the diffraction efficiency of a +3rd order diffracted light beam is at its maximum when a light beam of a wavelength for the high-density optical disc is used and the diffraction efficiency of a +2nd order diffracted light beam is at its maximum when a light beam of a wavelength for DVD is used.

When the numerical aperture (NA) for the high-density optical disc is higher than the NA for DVD, the outer circumferential portion of the optical function surface of the objective lens is a region dedicated for the high-density optical disc. Thus, it is necessary to prevent a light beam for DVD that is incident on the region from contributing to spot formation. When the light beam for DVD that is incident on the region dedicated for the high-density optical disc is converged on an information recoding surface of an optical disc, it leads to deterioration of spot performance. However, in Japanese Laid-Open Patent Publication No. 2005-243151, designing that takes into consideration the difference in NA between the optical disc standards, such as limiting convergence of the light beam for DVD that is incident on the region dedicated for the high-density optical disc, is not performed. Therefore, in order to adjust an effective NA, an additional optical component having an aperture limiting function is required, but it is not preferred to provide such a component, since the number of parts and the cost are increased.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide: an objective lens element that is compatible with at least two types of optical disc standards having different NAs and that controls an effective NA when a light beam for an optical disc standard having a relatively small NA is converged, thereby forming a desired spot; and an optical pickup device including the objective lens element.

The present invention is directed to an objective lens element that has optical function surfaces on an incident side and an exit side, that converges a first incident light beam of a wavelength λ1through a base plate having a thickness t1to form a spot, and that converges a second incident light beam of a wavelength λ2longer than the wavelength λ1through a base plate having a thickness t2larger than the thickness t1to form a spot. At least either one of the optical function surfaces is divided into a first region that includes a rotational symmetry axis and through which the first and second incident light beams that substantially contribute to spot formation pass, and a second region that is a ring-shaped region surrounding the first region and through which only the first incident light beam that substantially contributes to spot formation passes. The first region and the second region are provided with diffraction structures different from each other. The objective lens element satisfies the following conditions.
DO11×DO12>0  (1)
DO21×DO22<0  (2)
Here,

DO11is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ1that are diffracted by the diffraction structure on the first region,

DO21is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ2that are diffracted by the diffraction structure on the first region,

DO12is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ1that are diffracted by the diffraction structure on the second region, and

DO22is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ2that are diffracted by the diffraction structure on the second region.

In addition, the present invention is directed to an objective lens element that has optical function surfaces on an incident side and an exit side, that converges a first incident light beam of a wavelength λ1through a base plate having a thickness t1to form a spot, that converges a second incident light beam of a wavelength λ2longer than the wavelength λ1through a base plate having a thickness t2larger than the thickness t1to form a spot, and that converges a third incident light beam of a wavelength λ3longer than the wavelength λ2through a base plate having a thickness t3larger than the thickness t2to form a spot. At least either one of the optical function surfaces is divided into a first region that includes a rotational symmetry axis and through which the first to third incident light beams that substantially contribute to spot formation pass, a second region that is a ring-shaped region surrounding the first region and through which only the first and second incident light beams that substantially contribute to spot formation pass, and a third region that is a ring-shaped region surrounding the second region and through which only the first incident light beam that substantially contributes to spot formation passes. The first to third regions are provided with diffraction structures different from each other. The objective lens element satisfies the following conditions.
DO12×DO13>0  (5)
DO22×DO23<0  (6)
Here,

DO12is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ1that are diffracted by the diffraction structure on the second region,

DO22is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ2that are diffracted by the diffraction structure on the second region,

DO13is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ1that are diffracted by the diffraction structure on the third region, and

DO23is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ2that are diffracted by the diffraction structure on the third region.

In addition, the present invention is directed to an optical pickup device that converges a first incident light beam of a wavelength λ1through a base plate having a thickness t1to form a spot and that converges a second incident light beam of a wavelength λ2longer than the wavelength λ1through a base plate having a thickness t2larger than the thickness t1to form a spot. The optical pickup device includes: a first light source for emitting a light beam of the wavelength λ1; a second light source for emitting a light beam of the wavelength λ2; any one of the above-described objective lens elements; and a detection element for detecting a light beam reflected by an information storage medium that is an optical disc.

According to the present invention, when a light beam for an optical disc standard having a small NA is converged, the outermost region exerts an aperture limiting function. Thus, an objective lens element that is compatible with at least two types of optical disc standards having different NAs and that can form a desired spot, and an optical pickup device including the objective lens element can be realized.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a schematic configuration diagram of an objective lens element according to Embodiment 1.

An objective lens element143according to Embodiment 1 is compatible with the BD standard (NA=0.85) and the DVD standard (NA=0.65), converges a blue light beam of a wavelength λ1(about 400 nm) on an information recoding surface through a base plate having a thickness of 0.1 mm to form a spot thereon, and converges a red light beam of a wavelength λ2(about 680 nm) on an information recoding surface through a base plate having a thickness of 0.6 min to form a spot thereon. An incident side optical function surface of the objective lens element143is divided into an inner part131B including a rotational symmetry axis (optical axis) and a ring-shaped outer part131F surrounding the inner part131B. The inner part131B is provided with a stair-like diffraction structure that consists of periodic stair-like steps, and the outer part131F is provided with a sawtooth-like diffraction structure.

FIG. 2is a diagram illustrating the stair-like diffraction structure provided on the inner part of the objective lens element shown inFIG. 1.FIG. 2(a) shows a theoretical shape of the stair-like step structure provided on the optical function surface of the objective lens element.FIG. 2(b) shows an amount of phase change provided to the light beam of the wavelength λ1for BD, andFIG. 2(c) shows an amount of phase change provided to the light beam of the wavelength λ2for DVD.

FIG. 3is a diagram illustrating diffraction structures provided near the boundary between the inner part and the out part of the objective lens element shown inFIG. 1.FIG. 3(a) is a diagram in which a base aspheric surface is removed and only the diffraction structures are provided on a planar surface for easier understanding.FIG. 3(b) is a diagram in which the diffraction structures are provided on the base aspheric surface of the objective lens element.

The stair-like diffraction structure shown inFIG. 2(a) is a periodic structure in which one cycle consists of consecutive 4-level steps. The height of one step is set such that a difference in optical path that is about 1.25 times that of the wavelength λ1is provided to the blue light beam for BD. When the light beam of the wavelength λ1is incident on the stair-like diffraction structure, a phase difference of about 0.25 wavelength (about ½π) is provided to the light beam of the wavelength λ1each time the step height is increased by one step, as shown inFIG. 2(b). The diffraction structure for one cycle inFIG. 2(b) can be regarded as a diffraction grating in which 4 steps each providing a phase difference of 0.25 wavelength are consecutively arranged in a stair-like manner. Thus, the diffraction order having the highest diffraction efficiency is +1st order.

Meanwhile, when the light beam of the wavelength λ2is incident on the stair-like diffraction structure shown inFIG. 2(a), one step provides a difference in optical path of about 0.75 wavelength to the light beam of the wavelength λ2. Thus, the stair-like diffraction structure provides a phase difference of about −0.25 wavelength (about −½π) to the light beam of the wavelength λ2each time the step height is increased by one step, as shown inFIG. 2(c). When the light beam of the wavelength λ2is used, the stair-like diffraction structure shown inFIG. 2(a) can be regarded as a diffraction grating in which 4 steps each providing a phase difference of −0.25 wavelength are consecutively arranged in a stair-like manner. Thus, the diffraction order having the highest diffraction efficiency is −1st order.

The outer part131F is a region dedicated for BD, and thus has an aperture limiting function for adjusting an effective NA, with respect to the light beam of the wavelength λ2for DVD. In other words, the outer part131F is designed such that a light beam of the wavelength λ2incident on the outer part131F is not converged at a position largely distant from a spot formed by a light beam of the wavelength λ2incident on the inner part131B and a great defocus component and a great spherical aberration component are generated. In addition, the sawtooth depth is set such that the diffraction efficiency at the outer part131F which is provided when the light beam of the wavelength λ2is used is lower than that which is provided when the light beam of the wavelength λ1is used.

Specifically, in the objective lens element143according to the present embodiment, the step depth is determined such that the diffraction efficiency of a +3rd order diffracted light beam is at its maximum when the light beam of the wavelength λ1for BD is used. In this case, the diffraction order having the highest diffraction efficiency at the wavelength λ2for DVD is +2nd order, but the diffraction efficiency of an +2nd order diffracted light beam is relatively low. In addition, the diffraction order of the light beam of the wavelength λ1is +1st order at the inner part131B and +3rd order at the outer part131F, while the diffraction order of the light beam of the wavelength λ2is −1st order at the inner part131B and +2nd order at the outer part131F. The power of diffraction provided to the light beam of the wavelength λ2experiences a substantial change from negative to positive. Thus, a great difference in focal point occurs between a light beam having passed through the inner part131B and a light beam having passed through the outer part131F, and an amount of a generated spherical aberration is also increased. In this manner, the outer part131F substantially prevents the incident light beam of the wavelength λ2from contributing to spot formation, and exerts the aperture limiting function.

In another example, the sawtooth-like diffraction structure may be designed such that the diffraction efficiency of a +1st order diffracted light beam among light beams of the wavelength λ1that are diffracted by the outer part131F is at its maximum. In this case, when the light beam of the wavelength λ2for DVD is used, the diffraction efficiency of a +1st order diffracted light beam is at its maximum, but the diffraction efficiency becomes about +60% at most. In addition, since the diffraction order also experiences a substantial change from −1st order at the inner part131B to +1st order at the outer part131F, a difference in focal point and a great spherical aberration occurs. Thus, the outer part131F can exert the aperture limiting function similarly to the above example.

DO11is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ1that are diffracted by the diffraction structure on the inner part,

DO21is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ2that are diffracted by the diffraction structure on the inner part,

DO12is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ1that are diffracted by the diffraction structure on the outer part, and

DO22is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ2that are diffracted by the diffraction structure on the outer part.

The conditions (1) and (2) define sign changes of the diffraction order at the outer part131F and the diffraction order at the inner part131B. When the conditions (1) and (2) are satisfied, the diffraction direction of the light beam of the wavelength λ2for DVD can be greatly different between the inner part131B and the outer part131F, and the aperture limiting function can be provided to the outer part131F.

The objective lens element143according to Embodiment 1 preferably satisfies the following condition (3).
1.5≦(DO22/DO12)−(DO21/DO11)≦3.0  (3)

The condition (3) defines the relation between the diffraction order at the outer part131F and the diffraction order at the inner part131B. When the relation is out of the numerical range of the condition (3), the outer part131F cannot sufficiently exert the aperture limiting function with respect to the light beam of the wavelength λ2.

The objective lens element143according to Embodiment 1 preferably satisfies the following condition (4).
−1.0≦(DO22/DO12)/(DO21/DO11)≦−0.3  (4)

The condition (4) defines the relation between the diffraction order at the outer part131F and the diffraction order at the inner part131B. In order that the outer part131F can sufficiently exert the aperture limiting function with respect to the light beam of the wavelength λ2, it is preferred to satisfy these conditions.

Table 1 shows combinations of diffraction orders provided by the diffraction structures on the inner part131B and the outer part131F. Note that as long as the conditions (1) and (2) are satisfied, the diffraction structures may be designed such that other combinations of diffraction orders are provided.

FIG. 4is a schematic configuration diagram of an optical pickup device including the objective lens element according to Embodiment 1. The optical pickup device shown inFIG. 4is compatible with the two optical disc standards of BD and DVD.

A blue light beam61emitted from a laser beam source1passes through a relay lens2, is reflected by a beam splitter4, and then is converted into a substantially parallel light beam by a collimating lens8. The collimating lens8is movable in an optical axis direction. By moving in the optical axis direction, the collimating lens8compensates for a spherical aberration caused by an error of a base material thickness of an optical disc and a difference in base material thickness between information recoding surfaces. The blue light beam61having passed through the collimating lens8is reflected by an upward reflection mirror12, enters the objective lens element143, and is converged on an information recoding surface of an optical disc9to form a desired spot thereon. The blue light beam61reflected by the information recoding surface of the optical disc9passes through the objective lens element143again, is reflected by the upward reflection mirror12, and passes through the collimating lens8and the beam splitter4in order. The blue light beam61outputted from the beam splitter4is reflected by a beam splitter16, is converged on a photodetector33by a detection lens32, and is detected as an optical signal.

A red light beam62emitted from a laser beam source20passes through the beam splitter16and the beam splitter4, enters the collimating lens8, and is converted into diffused light. The collimating lens8can adjust the parallelism of the red light beam62by moving in the optical axis direction. In addition, similarly to the case where the optical disc9is used, by moving in the optical axis direction, the collimating lens8compensates for a spherical aberration caused by a difference in disc base material thickness, a temperature change, a wavelength change, and the like. The red light beam62having passed through the collimating lens8is reflected as diverging light by the upward reflection mirror12, enters the objective lens element143, and is converged on an information recoding surface of an optical disc10to form a desired spot thereon. The red light beam62reflected by the information recoding surface of the optical disc10passes through the objective lens element143again, is reflected by the upward reflection mirror12, and passes through the collimating lens8and the beam splitter4in order. The red light beam62outputted from the beam splitter4is reflected by the beam splitter16, is converged on the photodetector33by the detection lens32, and is detected as an optical signal.

Since the optical pickup device shown inFIG. 4includes the objective lens element143according to Embodiment 1, the outer part131F, which is the region dedicated for BD, exerts the aperture limiting function for adjusting the effective NA, when the light beam of the wavelength λ2is used. Thus, in the optical pickup device according to the present embodiment, it is possible to form a desired spot on an optical disc of either standard.

FIG. 5is a schematic configuration diagram of an objective lens element according to Embodiment 2.

An objective lens element163according to Embodiment 2 is compatible with the optical disc standards of BD, DVD, and CD, converges a blue light beam of a wavelength λ1(about 400 nm) on an information recoding surface through a base plate having a thickness of 0.1 mm to form a spot thereon, converges a red light beam of a wavelength λ2(about 680 nm) on an information recoding surface through a base plate having a thickness of 0.6 mm to form a spot thereon, and converges an infrared light beam of a wavelength λ3(about 780 nm) on an information recoding surface through a base plate having a thickness of 1.2 mm to form a spot thereon.

An incident side optical function surface of the objective lens element163is divided into three regions each having a center at a symmetry axis (optical axis), namely, an inner part151B including the symmetry axis, a ring-shaped intermediate part151M surrounding the inner part151B, and a ring-shaped outer part151F surrounding the intermediate part151M. The inner part151B is provided with a stair-like diffraction structure, the intermediate part151M is provided with a stair-like diffraction structure different from that on the inner part151B, and the outer part151F is provided with a sawtooth-like diffraction structure.

FIG. 6is a diagram illustrating the stair-like diffraction structure provided on the inner part of the objective lens element shown inFIG. 5.FIG. 6(a) shows a theoretical shape of the stair-like step structure provided on the optical function surface of the objective lens element.FIG. 6(b) shows an amount of phase change provided to the light beam of the wavelength λ1for BD,FIG. 6(c) shows an amount of phase change provided to the light beam of the wavelength λ2for DVD, andFIG. 6(d) shows an amount of phase change provided to the light beam of the wavelength λ3for CD.

FIG. 7is a diagram illustrating diffraction structures provided near the boundary between the inner part and the intermediate part of the objective lens element shown inFIG. 5.FIG. 7(a) is a diagram in which a base aspheric surface is removed and only the diffraction structures are provided on a planar surface for easier understanding.FIG. 7(b) is a diagram in which the diffraction structures are provided on the base aspheric surface of the objective lens element.

The inner part151B is a region shared by the light beams of the three wavelengths for BD, DVD, and CD. The stair-like diffraction structure provided on the inner part151B is a periodic structure in which one cycle consists of 8-level steps whose height is monotonically increased step by step. The height of one step is set such that a difference in optical path that is about 1.25 times as long as the wavelength λ1is provided to the blue light beam for BD. When the light beam of the wavelength λ1is incident on the stair-like diffraction structure, a phase difference of about 0.25 wavelength (about ½π) is provided to the light beam of the wavelength λ1each time the step height is increased by one step. When the light beam of the wavelength λ1is used, the stair-like diffraction structure inFIG. 6(a) can be regarded as a diffraction grating in which 4 steps each providing a phase difference of 0.25 wavelength are consecutively arranged in a stair-like manner, as shown inFIG. 6(b). Thus, the diffraction order having the highest diffraction efficiency is +2nd order.

In addition, when the light beam of the wavelength λ2is incident on the stair-like diffraction structure shown inFIG. 6(a), one step provides a difference in optical path of about 0.75 wavelength to the light beam of the wavelength λ2. Thus, the stair-like diffraction structure provides a phase difference of about −0.25 wavelength (about −½π) to the light beam of the wavelength λ2each time the step height is increased by one step, as shown inFIG. 6(c). When the light beam of the wavelength λ2is used, the stair-like diffraction structure shown inFIG. 6(a) can be regarded as a diffraction grating in which 4 steps each providing a phase difference of −0.25 wavelength are consecutively arranged in a stair-like manner. Thus, the diffraction order having the highest diffraction efficiency is −2nd order.

Moreover, when the light beam of the wavelength λ3is incident on the stair-like diffraction structure shown inFIG. 6(a), one step provides a difference in optical path of about 0.625 wavelength to the light beam of the wavelength λ3. Thus, the stair-like diffraction structure provides a phase difference of about −0.375 wavelength to the light beam of the wavelength λ3each time the step height is increased by one step, as shown inFIG. 6(d). When the light beam of the wavelength λ3is used, substantially 3 steps each providing a phase difference of −0.375 wavelength can be regarded as one diffraction grating, in the stair-like diffraction structure shown inFIG. 6(a). Thus, the diffraction order having the highest diffraction efficiency is −3rd order.

One cycle of the stair-like diffraction structure provided on the inner part151B does not necessarily need to consist of 8-level steps, and may consist of 5-, 6-, 7-, or 9-level steps.

The intermediate part151M is a region shared by the light beams of the two wavelengths for BD and DVD. The stair-like diffraction structure provided on the intermediate part151M is a periodic structure in which one cycle consists of 4-level steps whose height is monotonically increased step by step. The height of one step is set such that a difference in optical path that is about 1.25 times as long as the wavelength λ1is provided to the light beam of the wavelength λ1for BD. Thus, the diffraction efficiency of a +1st order diffracted light beam is at its maximum when the blue light beam of the wavelength λ1is used, and the diffraction efficiency of a −1st order diffracted light beam is at its maximum when the red light beam of the wavelength λ2is used. An infrared light beam for CD incident on the intermediate part151M diffuses without contributing to a spot and entering a photodetector as stray light. In other words, the intermediate part151M exerts an aperture limiting function with respect to the light beam of the wavelength λ3for CD. One cycle of the stair-like diffraction structure provided on the intermediate part151M does not necessarily need to consist of 4-level steps, and may consist of steps other than 4-level steps.

The outer part151F is a region dedicated for BD, and thus has an aperture limiting function for adjusting an effective NA, with respect to the light beam of the wavelength λ2for DVD and the light beam of the wavelength λ3for CD. In other words, the outer part151F is designed such that a light beam of the wavelength λ2incident on the outer part151F is not converged at a position largely distant from a spot formed by a light beam of the same wavelength incident on the inner part151B and a great defocus component and a great spherical aberration component are generated. In addition, the sawtooth depth is set such that the diffraction efficiency at the outer part151F which is provided when the light beam of the wavelength λ2or λ3is used is lower than that which is provided when the light beam of the wavelength λ1is used.

Specifically, in the objective lens element163according to the present embodiment, the step depth is determined such that the diffraction efficiency of a +3rd order diffracted light beam is at its maximum when the light beam of the wavelength λ1for BD is used. In this case, the diffraction order having the highest diffraction efficiency among diffracted light beams of the wavelength λ2for DVD and the diffraction order having the highest diffraction efficiency among diffracted light beams of the wavelength λ3for CD are +2nd order. In addition, the diffraction efficiency of the +2nd order diffracted light beam of the wavelength λ2and the diffraction efficiency of the +2nd order diffracted light beam of the wavelength λ3are about 84% and about 40%, respectively, and are lower than the diffraction efficiency of the light beam of the wavelength λ1for BD.

The diffraction order of the light beam of the wavelength λ1for BD is +2nd order at the inner part151B, +1st order at the intermediate part151M, and +3rd order at the outer part151F. Thus, the phase gently increases and decreases. Thus, when a BD is used, light beams incident on the inner part151B, the intermediate part151M, and the outer part151F can form a desired spot.

The diffraction order of the light beam of the wavelength λ2for DVD is −2nd order at the inner part151B and −1st order at the intermediate part151M, and the phase gently changes. Thus, when a DVD is used, light beams incident on the inner part151B and the intermediate part151M can form a desired spot. Here, the diffraction order of a light beam of the wavelength λ2that is diffracted by the outer part151F is +2nd order. The power caused by diffraction experiences a substantial change from negative to positive over the inner part151B to the outer part151F, and thus a convergence spot from the outer part151F is displaced from a convergence spot from the intermediate part151M to cause a difference in focal point of about 0.17 mm. In addition, since the diffraction order experiences a substantial change from −1st order to +2nd order, a great spherical aberration occurs. When a DVD is used, the outer part151F substantially prevents the incident light beam of the wavelength λ2from contributing to spot formation, and exerts the aperture limiting function.

The diffraction order of the light beam of the wavelength λ3for CD is −3rd order at the inner part151B and −1st order at the intermediate part151M, but the change rates of these diffraction orders are different from that of the light beam of the wavelength λ1for BD. Thus, the light beam incident on the intermediate part151M does not contribute to desired spot formation. In addition, in the case of the stair-like diffraction structure in which one cycle consists of 4 steps, the diffraction efficiency of the light beam of the wavelength λ3is about 35%, and the intensity of the diffracted light beam is low. In this manner, when a CD is used, the intermediate part151M has a sufficient aperture limiting function with respect to the incident light beam of the wavelength λ3. Further, the diffraction order of the light beam of the wavelength λ3that is diffracted by the outer part151F is +2nd order. The power caused by diffraction changes from negative to positive over the intermediate part151M to the outer part151F, and thus a convergence spot from the outer part151F is displaced from a convergence spot from the intermediate part151M to cause a difference in focal point. In addition, since the diffraction order experiences a substantial change from −1st order to +2nd order, a great spherical aberration occurs. Therefore, when a CD is used, the outer part151F also substantially prevents the incident light beam of the wavelength λ3from contributing to spot formation, and exerts the aperture limiting function.

In another example, the sawtooth-like diffraction structure may be designed such that the diffraction efficiency of a +1st order diffracted light beam among light beams of the wavelength λ1that are diffracted by the outer part151F is at its maximum. In this case, when the light beam of the wavelength λ2for DVD or the light beam of the wavelength λ3for CD is used, the diffraction efficiency of a +1st order diffracted light beam is at its maximum. However, the diffraction efficiency of the light beam of the wavelength λ2becomes about +60% at most, and the diffraction efficiency of the light beam of the wavelength λ3becomes about +42% at most. In addition, in either one of the cases of a DVD and a CD, since the diffraction order also experiences a substantial change from −1st order at the inner part151B to the +1st order at the outer part151F, a difference in focal point and a great spherical aberration occurs. Thus, the outer part151F can exert the aperture limiting function similarly to the above example.

DO12is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ1that are diffracted by the diffraction structure on the intermediate part,

DO22is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ2that are diffracted by the diffraction structure on the intermediate part,

DO13is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ1that are diffracted by the diffraction structure on the outer part, and

DO23is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ2that are diffracted by the diffraction structure on the outer part.

The conditions (5) and (6) define sign changes of the diffraction order at the outer part151F and the diffraction order at the intermediate part151M. When the conditions (5) and (6) are satisfied, the diffraction direction of the light beam of the wavelength λ2for DVD can be greatly different between the inner part151B and the outer part151F, and the aperture limiting function can be provided to the outer part151F.

The objective lens element163according to Embodiment 2 satisfies the following condition (7) in addition to the conditions (5) and (6).
DO32×DO33<0  (7)
Here,

DO32is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ3that are diffracted by the diffraction structure on the intermediate part, and

DO33is the diffraction order of the diffracted light beam having the highest diffraction efficiency among light beams of the wavelength λ3that are diffracted by the diffraction structure on the outer part.

Further, the condition (7) defines sign changes of the diffraction order at the outer part151F and the diffraction order at the intermediate part151M. When the condition (7) is satisfied, the diffraction direction of the light beam of the wavelength λ3for CD can be greatly different between the inner part151B and the outer part151F, and the aperture limiting function can be provided to the outer part151F.

The objective lens element163according to Embodiment 2 preferably satisfies the following condition (8).
1.5≦(DO23/DO13)−(DO22/DO12)≦3.0  (8)

The objective lens element163according to Embodiment 2 preferably satisfies the following condition (9).
−1.0≦(DO23/DO13)/(DO22/DO12)≦−0.3  (9)

The objective lens element163according to Embodiment 2 preferably satisfies the following condition (10).
1.5≦(DO33/DO13)−(DO32/DO12)≦3.0  (10)

Further, the objective lens element163according to Embodiment 2 preferably satisfies the following condition (11).
−3.0≦(DO33/DO13)/(DO32/DO12)≦−1.0  (11)

The conditions (8) to (11) define the relation between the diffraction order at the outer part151F and the diffraction order at the intermediate part151M. In order that the outer part151F can sufficiently exert the aperture limiting function with respect to the light beams of the wavelengths λ2and λ3, it is preferred to satisfy these conditions.

Table 2 shows combinations of diffraction orders provided by the diffraction structures in the inner part151B, the intermediate part151M, and the outer part151F. Note that as long as the conditions (5) to (7) are satisfied, the diffraction structures may be designed such that other combinations of diffraction orders are provided.

FIG. 8is a schematic configuration diagram of an optical pickup device including the objective lens element according to Embodiment 2. The optical pickup device shown inFIG. 8is compatible with the three optical disc standards of BD, DVD, and CD.

A blue light beam61emitted from a laser beam source1passes through a relay lens2, is reflected by a beam splitter4, and then is converted into a substantially parallel light beam by a collimating lens8. The collimating lens8is movable in an optical axis direction. By moving in the optical axis direction, the collimating lens8compensates for a spherical aberration caused by an error of a base material thickness of an optical disc and a difference in base material thickness between information recoding surfaces. The blue light beam61having passed through the collimating lens8is reflected by an upward reflection mirror12, enters the objective lens element163, and is converged on an information recoding surface of an optical disc9to form a desired spot thereon. The blue light beam61reflected by the information recoding surface of the optical disc9passes through the objective lens element163again, is reflected by the upward reflection mirror12, and passes through the collimating lens8and the beam splitter4in order. The blue light beam61outputted from the beam splitter4is reflected by a beam splitter16, is converged on a photodetector33by a detection lens32, and is detected as an optical signal.

A laser beam source according to the present embodiment is a two-wavelength laser beam source that selectively emits a red light beam and an infrared light beam. A red light beam62emitted from a laser beam source20passes through the beam splitter16and the beam splitter4, enters the collimating lens8, and is converted into diffused light. The collimating lens8can adjust the parallelism of the red light beam62by moving in the optical axis direction. In addition, similarly to the case where the optical disc9is used, by moving in the optical axis direction, the collimating lens8compensates for a spherical aberration caused by a difference in disc base material thickness, a temperature change, a wavelength change, and the like. The red light beam62having passed through the collimating lens8is reflected as diverging light by the upward reflection mirror12, enters the objective lens element163, and is converged on an information recoding surface of an optical disc10to form a desired spot thereon. The red light beam62reflected by the information recoding surface of the optical disc10passes through the objective lens element163again, is reflected by the upward reflection mirror12, and passes through the collimating lens8and the beam splitter4in order. The red light beam62outputted from the beam splitter4is reflected by the beam splitter16, is converged on the photodetector33by the detection lens32, and is detected as an optical signal.

An infrared light beam63emitted from the laser beam source20passes through the beam splitter16and the beam splitter4, enters the collimating lens8, and is converted into diffused light. The infrared light beam63outputted from the collimating lens8is reflected by the upward reflection mirror12, enters the objective lens element163, and is converged on an information recoding surface of an optical disc11to form a desired spot thereon. The infrared light beam63reflected by the information recoding surface of the optical disc11passes through the objective lens element163again, is reflected by the upward reflection mirror12, passes through the collimating lens8and the beam splitter4in order, and is reflected by the beam splitter16. Then, the infrared light beam63is converged by the detection lens32and detected as an optical signal by the photodetector33.

Since the optical pickup device shown inFIG. 8includes the objective lens element163according to Embodiment 2, the outer part151F, which is the region dedicated for BD, exerts the aperture limiting function for adjusting the effective NA, when the light beam of the wavelength λ2or λ3is used. Thus, in the optical pickup device according to the present embodiment, it is possible to form a desired spot on an optical disc of any one of the standards.

EXAMPLES

Hereinafter, Numerical Examples of the present invention will be specifically described with construction data, aberration diagrams, and the like. Note that in each Numerical Example, a surface to which an aspheric coefficient is provided indicates a refractive optical surface having an aspherical shape or a surface (e.g., a diffractive surface) having a refraction function equal to that of an aspheric surface. The surface shape of an aspheric surface is defined by the following equation.

X is the distance from an on-the-aspheric-surface point at a height h relative to the optical axis to a tangential plane at the top of the aspheric surface,

h is the height relative to the optical axis,

Cjis the radius of curvature at the top of an aspheric surface of a lens jth surface (Cj=1/Rj),

Kjis the conic constant of the lens jth surface, and

Aj,nis the nth-order aspheric constant of the lens jth surface.

Further, a phase difference caused by a diffraction structure added to an optical surface is provided by the following equation.
φ(h)=ΣPj,mh2m
Here,

Φ(h) is a phase function,

h is the height relative to the optical axis, and

Pj,mis the 2mth-order phase function coefficient of the lens jth surface.

Numerical Example 1

Numerical Example 1 corresponds to Embodiment 1. A first surface of an objective lens element according to Numerical Example 1 is divided into an inner part including a symmetry axis and an outer part surrounding the inner part. The inner part of the first surface is provided with a stair-like diffraction structure, and the outer part is provided with a sawtooth-like diffraction structure. A second surface of the objective lens element is also divided into an inner part and an outer part that consist of different aspheric surfaces, respectively. The objective lens element according to Numerical Example 1 is a BD/DVD compatible lens. With regard to designed values for BD, the wavelength is 408 nm; the focal length is 2.24 mm; the numerical aperture (NA) is 0.86; and the protective layer thickness of an information storage medium is 0.1 mm. With regard to designed values for DVD, the wavelength is 658 nm; the focal length is 1.74 mm; the NA is 0.6; and the protective layer thickness of an information storage medium is 0.6 mm.

Tables 3 and 4 show construction data of the objective lens element according to Numerical Example 1.

TABLE 3BDDVDWavelength0.4080.658Diameter of aperture2.241.74NA0.860.6Working distance (WD)0.40.3Disc thickness (DT)0.10.6Focal length1.31.4Diffraction order of inner part of the first surface2−2Diffraction order of outer part of the first surface3—Object point (OP)∞100Sur-Radius offacecurvatureNo.at the topThicknessMaterialRemarks column0OPInner part (diffractive surface)Outer part (diffractive surface)10.86235961.53761n1Inner part (aspheric surface)Outer part (aspheric surface)2−1.4180252WD3∞DTdiskPlane4∞PlaneWavelength0.4080.658n11.521831.50399disk1.616421.57829

FIG. 9is an optical path diagram of the objective lens element according to Numerical Example 1.FIG. 10is graphs each showing a spherical aberration when a parallel light beam is incident on the objective lens element according to Numerical Example 1.FIG. 11is graphs each showing a sine condition when a parallel light beam is incident on the objective lens element according to Numerical Example 1. FromFIGS. 10 and 11, it is recognized that aberrations are favorably compensated.

Table 5 shows ring zone cycles of the stair-like step structure provided on the inner part of the first surface, and cycles of steps arranged in each ring zone.

On the inner part of the Numerical Example 1, one ring zone cycle consists of consecutive 4-level steps. Each ring zone cycle in Table 5 indicates the width of a ring zone in a radial direction (in a direction perpendicular to the optical axis) as indicated by an arrow inFIG. 3(b). On the inner part, a first ring zone, a second ring zone, a third ring zone, . . . , and a twenty-ninth ring zone are provided in order from the optical axis toward the outer circumference of the objective lens element. Further, each step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow inFIG. 3(b). In each ring zone, the steps are referred to as a first step, a second step, a third step, and a fourth step in order from the optical axis side toward the outer circumference.

Table 6 shows ring zone cycles of the sawtooth-like diffraction structure provided on the outer part of the first surface.

Each ring zone cycle in Table 6 indicates the width of a ring zone in the radial direction (in the direction perpendicular to the optical axis) as indicated by an arrow inFIG. 3(b). Specifically, each ring zone cycle indicates the distance between points (adjacent points) where the contour line of a lens effective surface (except wall surfaces of steps that are parallel to the optical axis) intersects an alternate long and short dashed line representing a curved surface MG2in the cross-section shown inFIG. 3(b). On the outer part, a first ring zone, a second ring zone, a third ring zone, a fourth ring zone, . . . , and a ninth ring zone are provided in order from the optical axis toward the outer circumference of the objective lens element.

Table 7 shows step heights of the stair-like diffraction structure provided on the inner part of the first surface. In one cycle of the stair-like diffraction structure, the height of each of the first to third steps is set such that a phase difference of 1.25 wavelengths is provided to a light beam of a designed wavelength for BD, and the height of the fourth step is set such that a phase difference of 3.75 wavelengths is provided in the opposite direction.

Table 8 shows step heights of the sawtooth-like diffraction structure provided on the outer part of the first surface. The step heights of the sawtooth-like diffraction structure are set such that a phase difference of 3 wavelengths is provided to the light beam of the designed wavelength for BD, and a +3rd order diffracted light beam is used.

FIG. 12is a graph showing a longitudinal aberration when a light beam having a wavelength of 658 nm for DVD and a diameter equal to the effective diameter of a BD is incident on an incident surface of the objective lens element according to Numerical Example 1.

It is seen that the focal point of a light beam having passed through the outer part of the objective lens element is displaced in the optical axis direction by about 0.13 mm and an excessive spherical aberration occurs. The outer part favorably exerts an aperture limiting function such that the incident light beam for DVD is not converged and does not become stray light.

Numerical Example 2

Numerical Example 2 corresponds to Embodiment 2. A first surface of an objective lens element according to Numerical Example 2 is divided into an inner part including a symmetry axis, an intermediate part surrounding the inner part, and an outer part surrounding the intermediate part. The inner part of the first surface is provided with a stair-like diffraction structure. The intermediate part is provided with a stair-like diffraction structure different from that on the inner part. The outer part is provided with a sawtooth-like diffraction structure. A second surface of the objective lens element consists of an aspheric surface. The objective lens element according to Numerical Example 2 is a BD/DVD/CD compatible lens. With regard to designed values for BD, the wavelength is 408 nm; the focal length is 1.8 mm; the numerical aperture (NA) is 0.86; and the protective layer thickness of an information storage medium is 87.5 μm. With regard to designed values for DVD, the wavelength is 658 nm; the focal length is 2.0 mm; the NA is 0.6; and the protective layer thickness of an information storage medium is 0.6 mm. With regard to designed values for CD, the wavelength is 785 nm; the focal length is 2.1 mm; the NA is 0.47; and the protective layer thickness of an information storage medium is 1.2 mm.

Tables 9 and 10 show construction data of the objective lens element according to Numerical Example 2.

TABLE 9BDDVDCDWavelength0.4080.6580.785Diameter of aperture3.082.372.05NA0.860.60.47Working distance (WD)0.530.430.3Disc thickness (DT)0.08750.61.2Focal length1.82.02.1Diffraction order of inner part of2−2−3the first surfaceDiffraction order of intermediate1−1—part of the first surfaceDiffraction order of outer part of3——the first surfaceObject point (OP)∞−76100Sur-Radius offacecurvatureNo.at the topThicknessMaterialRemarks column0OPInner part (diffractive surface)Intermediate part (diffractivesurface)Outer part (diffractive surface)11.17174322.185991n1Aspheric surface2−1.983364WD3∞DTdiskPlane4∞PlaneWavelength0.4080.6580.785n11.521731.503891.50072disk1.616421.578291.57203

FIG. 13is an optical path diagram of the objective lens element according to Numerical Example 2.FIG. 14is graphs each showing a spherical aberration when a parallel light beam is incident on the objective lens element according to Numerical Example 2.FIG. 15is graphs each showing a sine condition when a parallel light beam is incident on the objective lens element according to Numerical Example 2. FromFIGS. 14 and 15, it is recognized that aberrations are favorably compensated.

Table 11 shows ring zone cycles of the stair-like step structure provided on the inner part of the first surface, and cycles of steps arranged in each ring zone.

On the inner part of Numerical Example 2, one ring zone cycle consists of consecutive 8-level steps. Each ring zone cycle in Table 12 indicates the width of a ring zone in a radial direction (in a direction perpendicular to the optical axis) as indicated by an arrow inFIG. 7(b). On the inner part, a first ring zone, a second ring zone, a third ring zone, . . . , and a sixteenth ring zone are provided in order from the optical axis toward the outer circumference of the objective lens element. Further, each step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow inFIG. 7(b). In each ring zone, the steps are referred to as a first step, a second step, a third step, . . . , and an eighth step in order from the optical axis side toward the outer circumference.

Table 12 shows ring zone cycles of the stair-like diffraction structure provided on the intermediate part of the first surface, and cycles of steps arranged in each ring zone.

On the intermediate part of Numerical Example 2, one ring zone cycle consists of consecutive 4-level steps. Each ring zone cycle in Table 12 indicates the width of a ring zone in the radial direction (in the direction perpendicular to the optical axis) as indicated by an arrow inFIG. 7(b). On the intermediate part, a first ring zone, a second ring zone, a third ring zone, . . . , and a eleventh ring zone are provided in order from the optical axis toward the outer circumference of the objective lens element. Further, a step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow inFIG. 7(b). In each ring zone, the steps are referred to as a first step, a second step, a third step, and a fourth step in order from the optical axis side toward the outer circumference.

Table 13 shows ring zone cycles of the sawtooth-like diffraction structure provided on the outer part of the first surface.

Each ring zone cycle in Table 13 is defined similarly to Numerical Example 1, and indicates the width of a ring zone in the radial direction (in the direction perpendicular to the optical axis) as indicated by the arrow inFIG. 3(b). On the outer part, a first ring zone, a second ring zone, a third ring zone, a fourth ring zone, . . . , and a thirty-fifth ring zone are provided in order from the optical axis toward the outer circumference of the objective lens element. Further, step heights of the sawtooth-like diffraction structure are set such that a phase difference of 3 wavelengths is provided to a light beam of a designed wavelength for BD, and a +3rd order diffracted light beam is used.

Table 14 shows step heights of the stair-like diffraction structure provided on the inner part of the first surface.

Table 15 shows step heights of the stair-like diffraction structure provided on the intermediate part of the first surface.

Table 16 shows the step heights of the sawtooth-like diffraction structure provided on the outer part of the first surface.

FIG. 16is a graph showing a longitudinal aberration when a light beam having a wavelength of 658 nm for DVD and a diameter equal to the effective diameter of a BD is incident on an incident surface of the objective lens element according to Numerical Example 2.

It is seen that the focal point of a light beam having passed through the outer part of the objective lens element is displaced in the optical axis direction by about 0.18 mm and an excessive spherical aberration occurs. Thus, the outer part favorably exerts an aperture limiting function such that the incident light beam for DVD is not converged and does not become stray light.

FIG. 17is a graph showing a longitudinal aberration when a light beam having a wavelength of 785 nm for CD and a diameter equal to the effective diameter of a BD is incident on the incident surface of the objective lens element according to Numerical Example 2.

It is seen that the focal point of a light beam having passed through the intermediate part of the objective lens element is displaced in the optical axis direction by about 0.1 mm and the focal point of a light beam having passed through the outer part of the objective lens element is displaced in the optical axis direction by about 0.13 mm. In addition, it is seen that an excessive spherical aberration occurs. Thus, each of the intermediate part and the outer part favorably exerts an aperture limiting function such that the incident light beam for CD is not converged and does not become stray light.

The present invention is applicable to objective lens elements used for performing for performing at least one of recording, reproducing, or erasing of information on optical discs of a plurality of standards for which light beams having different wavelengths are used, and optical pickup devices including the objective lens elements.

While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It will be understood that numerous other modifications and variations can be devised without departing from the scope of the invention.