JP5706839B2 - Objective lens and optical pickup using the same - Google Patents

Description

  The present invention relates to an optical pickup for recording / reproducing information on / from an optical information recording medium and an objective lens used therefor, and more particularly to an objective lens having compatibility with a plurality of information recording media having different recording densities.

  Optical information recording media represented by optical disks have been increased in capacity from CD (Compact Disc) to DVD (Digital Versatile Disc) and BD (Blu-ray Disc) for consumer use such as music and video. . CD is mainly used for music, DVD is used for standard-definition video, and BD is used for high-definition video. It is backward compatibility that has always been considered in the industry when DVDs and BDs are newly commercialized. The medium structure and apparatus have been devised so that past contents can always be referred to in a new system so that a DVD recording / reproducing apparatus can record / reproduce a CD and a BD recording / reproducing apparatus can record / reproduce a DVD or CD. It was.

  In DVD, an annular phase shift structure for phase shift and diffraction action has been introduced to the objective lens so that compatibility with CD can be achieved. As a result, even if there are two laser light sources, red and infrared, it is possible to record and reproduce both DVD and CD optical disks with a single objective lens. However, in BD, it is still common to mount two objective lenses, a BD dedicated lens and a DVD / CD compatible lens, on a lens actuator. This is because it is difficult for the annular phase shift structure to satisfy all three wavelengths even if the aberration performance of two types of wavelengths can be achieved. However, there is an increasing demand for a three-wavelength compatible lens that can be handled by a single objective lens including a BD in order to increase the transfer speed, reduce the size of the pickup device, and the drive device, and reduce the price.

  In order to meet this requirement, for example, Patent Document 1 and Patent Document 2 propose a configuration in which the degree of divergence and convergence of light incident on the objective lens is varied depending on the wavelength and the diffractive lens element is combined. In particular, Patent Document 2 describes that the first optical path difference providing structure and the second optical path difference providing structure are introduced, and the optical path difference is given to each of the odd and even multiples of the shortest wavelength with an error within 0.1 wavelength. It has been. Patent Document 3 proposes a configuration in which parallel light is incident on all three wavelengths and an annular diffractive surface is combined.

  Further, Patent Document 4 basically uses a non-periodic phase structure that gives an optical path difference that is an integral multiple of the wavelength of the BD in a phase compensation plate that is assumed to be a separate body from the BD-dedicated objective lens. In addition to reducing the aberration of the CD, by making the secondary aperiodic phase structure and the aperiodic phase structure an aspheric surface for each ring zone, the aberration in the BD is deteriorated within an allowable range. A method for reducing the aberration of CD has been proposed.

  In Patent Document 5, the first optical path difference providing structure and the second optical path difference providing structure similar to Patent Document 2 are used, and parallel light is incident on the lens in Example 1 to be condensed on the BD, DVD, and CD. Lens design examples have been proposed. Here, two diffractive structures represented by different optical path difference functions in the central area corresponding to the BD / DVD / CD common area are overlapped on the incident surface side of the lens as the first optical path difference providing structure, and correspond to the BD / DVD common area. It is shown that three diffractive structures represented by different optical path difference functions in the intermediate region are also superimposed on the incident surface side of the lens as the second optical path difference providing structure. The exit surface side is a single aspherical structure without an annular phase shift structure.

Japanese Patent No. 4062742 Japanese Patent No. 3957003 Japanese Patent No. 4099662 Japanese translation of PCT publication No. 2008-524639 JP 2011-96350 A

  In the configurations disclosed in Patent Documents 1 and 2, there may be a problem that the light beam incident on the objective lens is not parallel light. When the optical disk is mounted on the drive device, the center of rotation of the spindle motor that rotates the disk does not necessarily coincide with the center of curvature of the information track of the disk. This is unavoidable in an information recording apparatus using a replaceable medium such as an optical disk, and accordingly, the information track of the stationary optical pickup and the rotating optical disk is relatively displaced.

  In order to compensate for this relative positional deviation, the optical pickup detects the positional deviation between the information track and the focused spot as a tracking error signal, and drives the actuator equipped with the objective lens by this signal. As a result, the light spot can always follow the information track during signal reproduction, thereby enabling continuous information reproduction. However, when moving the lens with an actuator to move the spot, if the light beam incident on the lens is parallel light, no wavefront aberration occurs even if the lens moves, but the light beam incident on the lens converges. When light or diverging light is used, coma aberration occurs because the principal ray of the objective lens tilts as the lens moves.

  In particular, the influence is serious in a lens with a large NA such as BD. Therefore, it is desirable that the incident light is parallel light. In Patent Document 1, Example 6 is parallel light with all three wavelengths being a magnification of 0, but the diffraction surface for correcting aberration is an example in the coupling lens, and the light beam incident on the objective lens is divergent. Although it is not light or convergent light, it is considered that the wavefront has spherical aberration. In this case, movement of the lens causes coma aberration, which is not preferable. In addition, a compatible lens element for a plurality of wavelengths using a diffractive action has a problem that the diffraction efficiency cannot be made 100% at a plurality of wavelengths at the same time, and the utilization efficiency is lowered.

  Patent Document 2 states that two types of optical path difference providing structures are provided with an optical path difference that is an odd multiple of the shortest wavelength and an even multiple of the shortest wavelength. When all the incident light is parallel light with a single lens to be reproduced, a solution under such conditions could not be found. In each of the examples of Patent Document 2, a diffractive lens surface is required, and a decrease in utilization efficiency at a plurality of wavelengths is also a problem.

  In the configuration disclosed in Patent Document 3, the light beam incident on the lens is parallel light for all three wavelengths, but none of the examples corresponds to the wavelength and NA of BD, DVD, and CD. A design example corresponding to the problem to be solved is not disclosed. In each of the examples of Patent Document 3, a diffractive lens surface is required, and a decrease in utilization efficiency at a plurality of wavelengths is also a problem.

  In the configuration disclosed in Patent Document 4, although a secondary step structure that degrades BD aberration is introduced, a staircase structure that provides an optical path difference that is an integral multiple of the BD wavelength is fundamental. There is little freedom of design. Furthermore, there is no guideline about what kind of aspheric surface the step surface of the stairs is. For this reason, there is a concern that a sufficient aberration reduction effect cannot be obtained. Moreover, there is no disclosure of setting of variables for automatic design when the annular phase shift structure is aspherical. Furthermore, Patent Document 4 has a problem that the amount of aberration that can be corrected is essentially small from the viewpoint of reducing aberration by phase shift. In order to secure a substantial working distance (working distance), the defocus amount of the CD with respect to the BD is still insufficient at 5λ, and about 30λ may be necessary.

  In the configuration disclosed in Patent Document 5, an optical path difference providing structure is basically configured by using a diffractive structure, and the light utilization efficiency due to the dispersion of light energy to diffraction orders that are basically unnecessary in the diffractive structure. There is a problem that decline is inevitable. By using a so-called blazed grating structure, it is possible in principle to set the diffraction efficiency to 100% for a specific wavelength. However, in order to share light of different wavelengths, the blaze conditions are compatible at all wavelengths. It is not possible.

  The present invention has been made in order to solve the above-mentioned problems in the prior art, and a single objective that sufficiently reduces aberrations when parallel light is incident on any of BD, DVD, and CD optical disks. The object is to provide a lens. At that time, without using a diffraction element whose use efficiency is expected to decrease due to the wavelength dependence of diffraction efficiency, compatibility is achieved by using a phase shift by an annular phase shift structure, and a sufficient CD working distance is secured. Objective.

  In order to achieve the above object, the objective lens of the present invention has the following configuration. First, as a premise, laser beams having three wavelengths λ1, λ2, and λ3 (λ1 <λ2 <λ3) are incident as parallel lights, respectively, and the thicknesses of the first, second, and third optical information recording media are respectively determined. Light is condensed with numerical apertures NA1, NA2, and NA3 (NA1> NA2> NA3) through the transparent medium layers t1, t2, and t3 (t1 <t2 <t3), respectively.

  The innermost first lens region is composed of a plurality of annular zones divided by a plurality of concentric circles with the optical axis as the center, and gives different phase differences depending on the wavelength to adjacent regions, and is uniform within the region. The first, second, and third annular phase shift structures that give a large phase difference are superimposed on one lens surface.

  The second lens region arranged annularly outside the first lens region is composed of a plurality of annular zones divided by a plurality of concentric circles with the optical axis as the center, and the adjacent region is dependent on the wavelength. A fourth annular phase shift structure is provided which gives different phase differences and gives a uniform phase difference in the region.

  Further, in the third lens region that is annularly arranged outside the second lens region, the wavefront aberration so that the laser beam having the wavelength λ1 can be incident and condensed as parallel light through the transparent medium having the thickness t1. Is an optimized aspherical lens shape.

  At this time, in the first lens region, the lens action excluding the phase difference added by the first, second, and third annular phase shift structures causes the thickness t1 of the transparent medium and the transparent medium. Optimized so that the laser beam with the wavelength λ1 can be focused without a third-order spherical wavefront aberration when the laser beam with the wavelength λ1 is focused through a transparent medium having a thickness t4 (t1 <t4 <t2) in the middle of the thickness t2 Make it face. Here, it is assumed that the refractive index of the virtual transparent substrate having the thickness t4 is the same as that of the transparent medium of the first optical information recording medium. In this specification, it is assumed that the lens has a surface shape optimized so that the third-order spherical aberration is zero when the lens collects light through a substrate having a certain thickness. The thickness is referred to as “corresponding substrate thickness”, “optimal substrate thickness”, etc. as one of the intrinsic parameters of the lens.

  Further, in the second lens region, the lens action excluding the phase difference added by the fourth annular phase shift structure is intermediate between the thickness t1 of the transparent medium and the thickness t2 of the transparent medium. The lens surface has a thickness t5 (t1 <t5 <t2) corresponding to the substrate thickness. Here, it is assumed that the refractive index of the virtual transparent substrate having the thickness t4 is the same as that of the transparent medium of the first optical information recording medium.

  The first, second, and third annular phase shift structures of the first lens region combine the phase shift functions of these annular zones to obtain the wavelengths of λ1, λ2, and λ3, and t1, t2 respectively. For any of the transparent medium layers having a thickness of t3, a combination of phase shift structures with depth steps that can reduce the wavefront aberration to a peak to peak value of less than 1λ is selected. Further, the wavefront aberration before adding the phase shift by the annular phase shift structure has a defocus component larger than the third-order spherical aberration component in the RMS wavefront aberration value, and the peak of the wavefront aberration including the defocus component is included. An annular phase shift structure in which the to-peak value is 5λ or more, and the sign of the wavefront aberration is the same sign for the wavelengths λ2 and λ3, and these are mutually opposite to the sign of the wavefront aberration of the λ1 wavelength And

  The fourth annular phase shift structure of the second lens region has a peak-to-peak value of wavefront aberration for each of the wavelengths λ1 and λ2 and the transparent medium having a thickness of t1 and t2. The structure can be reduced to less than 1λ. Furthermore, it is assumed that the wavefront aberration before adding the phase shift to be compensated by the annular phase shift structure has a defocus component larger than the third-order spherical aberration component in the RMS wavefront aberration value. Here, for the convenience of calculating the third-order spherical aberration component and the defocus component of the RMS wavefront aberration, the function of the wavefront aberration of the second lens region continues to the first lens region with virtually the same function. Calculate as a thing. At this time, the Peak to Peak value of the wavefront aberration including the defocus component is 10λ or more, and the sign of the wavefront aberration is inverted between the wavelength of λ1 and the wavelength of λ2. Further, the wavefront aberration has an annular phase shift structure in which, at the wavelength of λ3, the aberration at the focal point of the wavelength of λ3 in the first lens region is not reduced to less than 1λ by the Peak to Peak value.

  By configuring the annular phase shift structure as described above, the laser light having the wavelength λ1 incident on the first, second, and third lens regions is condensed on the first optical information recording medium, and The laser beam having the wavelength λ2 incident on the first and second lens regions is condensed on the second optical information recording medium, and the laser beam having the wavelength λ3 incident on the first lens region is the third light. Condensed on the information recording medium.

  According to the present invention, it is possible to realize a single-lens objective lens capable of recording and reproducing signals by entering parallel light on any of the BD, DVD, and CD optical discs. Even when decentered, coma does not occur. At that time, by using the phase shift structure to form the lens without using the diffractive structure, it is possible to solve the problem of lowering the utilization efficiency that occurs when the diffractive structure is used. Further, by introducing defocus to the wavefront aberration to be corrected, the working distance of the CD can be secured at a practical value and the angle of view characteristic can be improved. Furthermore, even if the steps due to the phase shift structure are used extensively, it is possible to suppress an increase in chromatic aberration and a deterioration in angle of view characteristics.

It is a figure explaining the basic concept of this invention. It is a figure explaining the optical path difference produced by a level | step difference. It is a schematic diagram of the state in which the light beam is incident on the lens of FIG. It is a schematic diagram explaining correction | amendment of the wavefront aberration by an annular zone phase shift structure. It is a figure which shows the design flow in a BD / DVD area | region. It is a figure which shows the design flow in a BD / DVD / CD area | region. It is a calculation result of the phase difference in each wavelength with respect to a step depth. It is a figure which shows the aberration wavefront shape to correct | amend designed with the annular zone phase shift structure which does not add a defocus (comparative example). It is a figure which shows the lens shape and wavefront aberration which were designed with the annular zone phase shift structure which does not add a defocus (comparative example). It is a figure explaining the concept of the phase shift lens which correct | amends the wavefront aberration containing a defocus. It is a figure which shows the wavefront aberration which should be corrected in the lens of Example 1 (design example 1). It is a figure which shows the effect which added the phase shift to the flare light countermeasure. It is a figure which shows the case where the phase shift of a flare light countermeasure is not added (for comparison). It is a figure which shows the wavefront aberration of BD correct | amended with the phase shift structure of the design example 1. FIG. It is a figure which shows the wavefront aberration of DVD correct | amended with the phase shift structure of the design example 1. FIG. It is a figure which shows the wavefront aberration of CD correct | amended by the phase shift structure of the design example 1. FIG. It is a figure explaining composition of a narrow zone shape. It is a figure which shows the lens design shape by the design example 1, and a wavefront aberration distribution. It is the figure which displayed the axial wavefront aberration of the lens of the design example 1 by the interference fringe. It is a figure which shows spot intensity distribution of the exclusive lens used as a reference | standard (for comparison). It is a figure which shows the spot intensity distribution of the lens of the design example 1 (with a limiting aperture). It is a figure which shows the spot intensity distribution of the lens of the design example 1 (there is no restriction | limiting opening). It is a figure which shows the wavefront aberration which should be corrected in the lens of Example 2 (design example 2). It is a figure which shows the wavefront aberration of BD correct | amended with the phase shift structure of the design example 2. FIG. It is a figure which shows the wavefront aberration of DVD correct | amended with the phase shift structure of the design example 2. FIG. It is a figure which shows the wavefront aberration of CD correct | amended with the phase shift structure of the design example 2. FIG. It is a figure which shows the lens design shape by the design example 2, and wavefront aberration distribution. It is the figure which displayed the axial wavefront aberration of the lens of the design example 2 by the interference fringe. It is a figure which shows the spot intensity distribution of the lens of the design example 2 (with a restriction | limiting opening). It is a figure which shows spot intensity distribution of the lens of the design example 2 (there is no restriction | limiting opening). It is an Example of the optical pick-up using the objective lens of Example 1,2.

Hereinafter, first, the basic principle of the embodiment of the present invention will be described.
FIG. 1 is a diagram for explaining the basic concept of the present invention. The objective lens 100 according to the present invention is basically a single lens made of a certain material, and the surface on the left side of the drawing (hereinafter referred to as the first surface 101) and the light on the right side of the drawing. It is comprised from the surface (henceforth the 2nd surface 102) radiate | emitted. The figure shows a cross-sectional view above the optical axis 130, and the actual shape of the objective lens is assumed to be rotationally symmetric with respect to the optical axis 130. Basically, a lens that collects parallel light has a large curvature on the surface on which parallel light is incident (small radius of curvature) and a small curvature on the surface on which convergent light is emitted (large radius of curvature). Is well known.

  The first surface 101 and the second surface 102 have three regions (lens regions) divided by concentric circles with the optical axis 130 as the center. The first surface 101 has the first region 103 adjacent to the innermost region. The annular region is the second region 104, and the outermost outermost region is the third region 105.

  Similarly, the second surface 102 has three regions (lens regions), which are a first region 106, a second region 107, and a third region 108 from the inside. The first region 103 on the first surface and the first region 106 on the second surface, the second region 104 on the first surface and the second region 107 on the second surface, the third region 105 on the first surface and the second region on the second surface. It is assumed that the three regions 108 are regions through which the same light beam is transmitted basically, and the boundaries thereof are light rays 109, 110, and 111.

  The first region 103 of the first surface 101 includes first, second, and third annular phase shift structures 112, 113, and 114, and the optical path difference distributions 112 ′, 113 ′, and 114 ′ are caused by these step structures. The optical path difference represented by is generated. It is assumed that each annular phase shift structure basically produces the same optical path difference at any step in the annular phase shift structure in the regions on both sides across the step boundary. The actual lens surface shape 116 (shown by a solid line) is formed by shifting the phase of the aspherical shape 115 (shown by a broken line) of the lens before adding the annular phase shift structure in the optical axis direction. A phase shift amount is generated as a result of adding the phase differences 112 ′, 113 ′, 114 ′ of the annular phase shift structures 112, 113, 114.

  Similarly, in the second region 104 of the first surface 101, there is a fourth annular phase shift structure 117, and an optical path difference represented by an optical path difference distribution 117 'is generated by the step structure. Here again, it is assumed that the same optical path difference is basically generated in the regions on both sides of the step boundary in any step in the annular phase shift structure. In order to generate such a phase difference, the aspherical shape 115 before adding the annular phase shift structure is phase-shifted in the optical axis direction to form an actual lens surface shape 116.

  On the second surface 102, the lens surface shape is different in the first, second, and third regions 106, 107, and 108, and the actual lens surface shape 120 (shown by a solid line) has a step at the boundary. That is, the lens surface shape expanded from the first region 106 is indicated by a broken line 118 between the first region 106 and the second region 107, and between the second region 107 and the third region 108 (expanded from the second region 107). The lens surface shape indicated by a broken line 119) has a sag level difference of 5λ or more at the maximum.

  On the other hand, the lens surface shape 115 before adding the annular phase shift structure of the first surface 101 is the third-order spherical aberration with a specific substrate thickness at the BD wavelength, together with the lens surface shape 120 of the second surface 102. The lens shape is such that becomes zero. In particular, the specific substrate thickness t3 for the first regions 103 and 106 and the specific substrate thickness t4 for the second regions 104 and 107 are both specific thicknesses between the DVD substrate thickness and the BD cover layer thickness. However, t3 and t4 are generally different and t3 is made thicker than t4. This is because the BD, DVD, and CD need to be focused on the CD having the thickest substrate in the first areas 103 and 106, and the second areas 104 and 107 do not need to be focused on the CD. .

  FIG. 2 is a diagram for explaining an optical path difference generated in incident light due to a step formed on the lens surface. A thick polygonal line AEGD is a cross-sectional profile obtained by enlarging the vicinity of a certain step in the lens surface. When a horizontal parallel light beam enters from the left side of the drawing and is refracted by the lens surface represented by AEGD, the optical path difference generated between the upper light beam and the lower light beam with the step EG as a boundary is represented by the equation in the figure. .

  Here, it is assumed that the lens surface is enlarged in the vicinity of the step so that the curvature of the surface is not visible. The line segments AE and GD are parallel, and these line segments that are lens surfaces are inclined with respect to the vertical direction ( It is assumed that the tangent angle) is θ and at the same time the incident angle of the incident light with respect to the lens surface is θ.

  Further, it is assumed that the step EG is parallel to the optical axis and the incident light beam. In the formula, n represents the refractive index of the lens. When the optical path difference given in this way is expressed as mλ + φ (where −λ / 2 <φ ≦ λ / 2) using the wavelength λ and the integer m, φ is defined as a phase difference in this specification. (It may be more convenient when φ is expressed in units of λ. In this case, (m + φ) λ (where −1/2 <φ ≦ 1/2). Definition). In the propagating light wave, if the optical path difference due to the step is within the range of the optical path difference that is sufficiently shorter than the coherence length of the light, the behavior of the light after passing through the step depends on the phase difference φ, not m. It is well known that there is. The aberration correction amount in this case is also corrected by the amount of the phase difference φ.

  Further, FIG. 2 shows that the optical path difference and the phase difference generated between adjacent annular zones due to a certain step are functions of the step amount EG and the tilt angle θ of the surface with respect to the light beam. Furthermore, when the optical path difference due to a certain step is expressed by m1λ1 + φ1, m2λ2 + φ2, m3λ3 + φ3 at wavelengths λ1, λ2, and λ3, naturally, m1, m2, and m3 are likely to be different from each other, and φ1, φ2, and φ3 are further similarly. Different values with high probability. It can also be seen that one step results in different aberration correction amounts at different wavelengths.

  The optical path difference and phase difference applied by the first and second annular phase shift structures 112 and 113 in FIG. 1 are substantially constant within the same annular phase shift structure and the same region regardless of the inclination angle θ of the surface. To form. However, even in the same region, the optical path difference and the phase difference are generally different between the first annular phase shift structure and the second annular phase shift structure.

  Further, in the third region 105, the annular phase shift structure is not formed in the example of FIG. This is because the third region is a BD-dedicated region, so that a BD-dedicated lens structure can be obtained. However, an annular phase shift structure that does not cause a phase difference in the BD is added as necessary so that the light incident on the third region 105 can be sufficiently diffused on the recording film surface during reproduction of the DVD or CD. You can also

  FIG. 3 is a schematic diagram of a state where a light beam is incident on the lens 100 of FIG. The transparent medium layers 204, 205, and 206 of BD, DVD, and CD are shown on the right side of the drawing.

  A CD incident light beam 203 having a wavelength of 785 nm is incident on a region corresponding to the first region 103 of the first surface 101 of the lens 100 shown in FIG. The incident light beam 203 has its aberration corrected by the first, second, and third annular phase shift structures 112, 113, and 114, exits the lens 100, and then passes through the transparent medium layer 206 having a thickness t3 for CD. The light is condensed on the surface of the CD recording film on the surface opposite to 100. The boundary radius position of the first region 106 of the second surface 102 is set so that the light beam 203 incident on the first region 103 of the first surface is emitted.

  Further, a DVD incident light beam 202 having a wavelength of 660 nm is incident on a region corresponding to the range of the first region 103 and the second region 104 of the first surface 101 of FIG. In the incident light beam 202, the aberration of the first region 103 is corrected by the first, second, and third annular phase shift structures 112, 113, and 114 of the first region 103, and the fourth ring of the second region 104 is corrected. The band phase shift structure 117 corrects the aberration of the second region 104. After exiting the lens 100, the light is condensed on the DVD recording film surface on the surface opposite to the lens 100 through the transparent medium layer 205 having a thickness t2 for DVD. From the vicinity of the outer boundary of the second area 107 of the second surface 102, the boundary radius position of the light beam 202 incident on the second area 104 of the first surface 101 is emitted so that light incident near the outer boundary is emitted. Is set.

  Further, a BD incident light beam 201 having a wavelength of 405 nm is incident on a region corresponding to the range of the first region 103, the second region 104, and the third region 105 of the first surface 101 of FIG. In the incident light beam 201, the aberration in the first region 103 is corrected by the first, second, and third annular phase shift structures 112, 113, and 114 in the first region 103, and the fourth ring in the second region 104 is corrected. The band phase shift structure 117 corrects the aberration of the second region 104. The light incident on the third region 105 is emitted from the third region 108 of the second surface 102 without any aberration correction. Then, after exiting the lens 100, the light is condensed on the surface of the BD recording film on the surface opposite to the lens 100 through the transparent medium layer 204 having a thickness t1 for BD.

  Here, corresponding to actual BD, DVD, and CD, t1 is a depth of 0.0875 mm from the surface of the intermediate position of the two layers of BD, t2 is 0.6 mm of the substrate thickness of the DVD, and t3 is the substrate of the CD. A lens is formed with a thickness of 1.2 mm. The BD is generally a two-layer disc, and each of the two layers has a recording film formed at a depth corresponding to a cover layer thickness of 0.1 mm and 0.075 mm from the surface. As a general rule, it is generally set to 0.0875 mm, which is the middle.

  In the above description, the example in which the first, second, and third annular phase shift structures 112, 113, 114 in the first region 103 are all formed on the first surface 101 has been shown. In general, even if any of the first, second, and third annular phase shift structures 112, 113, 114 is formed on the second surface 102, the same phase shift effect can be provided. However, if formed on two surfaces, the aberration correction performance may be deteriorated due to mutual displacement, and it is preferable to form them integrally on one surface so that such displacement is less likely to occur. On the other hand, the fourth annular phase shift structure 117 in the second region 104 is formed on the same first surface 101 as the first, second, and third annular phase shift structures 112, 113, and 114 in the first region 103. However, this may be formed on the second surface 102. This is because they do not act on the same light flux, and the effect is only the shift of the edge portion, and the area of the region affected by the shift is not large. If the first to fourth annular phase shift structures are formed on the second surface (exit side) 102, when the refracted light is emitted at a large refraction angle, some of the light rays are shielded by the wall surface of the annular step. Sometimes. Therefore, by setting a step on the first surface (incident side) 101, the step and the incident light can be made parallel to each other, which can reduce the influence of light shielding.

  Based on the concept of the annular zone phase shift structure as described above, in the lens of the present invention, the first, second, and third annular zone phase shift structures 112, 113, and 114 are synthesized in the first region 103. The second region 104 has at least one fourth annular phase shift structure 117, and the third region 105 has no annular phase shift structure. However, the second region 104 and the third region 105 may be configured by adding a fifth annular phase shift structure and a sixth annular phase shift structure, respectively.

  Next, with the three annular phase shift structures in the first region 103 and the one annular phase shift structure in the second region 104, it is possible in principle to correct aberrations at three wavelengths of BD, DVD, and CD. explain.

  FIG. 4 is a schematic diagram for explaining correction of wavefront aberration by the annular phase shift structure. Here, the relationship between the wavefront aberration to be corrected, one annular phase shift structure determined therefrom, and the corrected wavefront aberration by the annular phase shift structure is shown. FIG. 4 is a three-dimensional graph of cylindrical coordinates, where the radius is the radius coordinate within the effective aperture of the lens, and the vertical axis represents the wavefront aberration when reproducing a certain disk at the corresponding wavelength.

  When this wavefront aberration is corrected by the phase step of the phase difference φ corresponding to the lens step, the N axis is engraved at the phase interval of φ with the same aberration of the vertical axis, and the annular zone at the radial position corresponding to the engraved boundary. The boundary will be decided. The engraved wavefront aberration curve is folded around the wavefront aberration value near 0, that is, along the radial coordinate axis in accordance with the phase shift amount. This is the wavefront aberration after aberration correction. In this way, the number of annular zones N is determined so that an aberration correction amount of φ × N obtained by multiplying the phase difference φ of the step by the number of annular zones N substantially corresponds to the Peak to Peak value of the wavefront aberration to be compensated.

  When such an annular phase shift structure is operated at a different wavelength, an aberration compensation amount is obtained by multiplying the corresponding different phase difference φ by the same common annular zone number N at all wavelengths, and a similar wavefront aberration is corrected. it can. By using these as the three annular phase shift structures of the first, second, and third annular phase shift structures, the BD, DVD, and CD are all applied to the first region where aberration correction is performed. In addition, by using one annular phase shift structure of the fourth annular phase shift structure, both the BD and the DVD need to be corrected for aberrations, and the CD is applied to the second region that does not need to be corrected for aberrations. Thus, aberration correction can be performed for any of BD, DVD, and CD.

  As described above, the wavefront aberration that can be compensated by one annular phase shift structure has a similar wavefront aberration shape at different wavelengths, but each of the multiple annular phase shift structures does not necessarily have a similar shape. do not have to. Accordingly, when a plurality of annular phase shift structures are combined, different aberration shapes can be compensated with different distributions depending on the wavelengths, and therefore the wavefront aberration to be compensated for at different wavelengths does not necessarily have a similar shape. That is, in the first region, the working distance of the CD is expanded independently of the working distance of the BD and DVD by combining the annular phase shift structure so as to compensate for a large amount of defocus wavefront aberration only in the CD. It is also possible in principle. On the other hand, in the second region where only one annular phase shift structure is used, the wavefront aberration of BD and DVD needs to be similar to each other.

The above design principle will be specifically described with reference to a flowchart.
FIG. 5 is a diagram showing a design flow in the BD / DVD area. First, in the second region (BD / DVD region), the base substrate thickness t ₅ before adding the fourth annular phase shift structure and the amount of defocus amount δ _B to be added in the BD are initially set. Assuming appropriate values. From these values, the wavefront aberration shape W _B (ρ) to be compensated in the BD is determined. A wavefront aberration function W (ρ) that is rotationally symmetric with respect to the optical axis is expressed as in equation (1).

Here _{A m} is the wavefront aberration coefficients of the Zernike of the m terms, m = 4 defocus, m = 9 is third-order spherical aberration, m = 16 is the fifth-order spherical aberration, m = 25 is the seventh order spherical aberration Is the coefficient. On the other hand, Z _m (ρ) is a Zernike polynomial, and is expressed as in equation (2).

Z ₄ is defocused, Z ₉ is third-order spherical aberration, Z ₁₆ is fifth-order spherical aberration, and Z ₂₅ is Zernike polynomial with seventh-order spherical aberration. Using these, the wavefront aberration W _B (ρ) of the BD of FIG. 5 is developed and displayed. _ABm is the Zernike wavefront aberration coefficient for BD. Here, the Zernike wavefront aberration coefficient A _Bm is a function of the substrate thickness error t ₁ -t ₅ and the defocus δ _B.

Next, so that the shape of the wavefront aberration is substantially similar between BD and DVD, that is, the ratio of the defocused Zernike wavefront aberration coefficient and the third-order spherical aberration Zernike wavefront aberration coefficient are equal between BD and DVD. determine the defocus amount δ _D of on DVD. Assuming that the substrate thickness deviation in DVD is t ₂ -t ₅ , it is easy to determine the ratio equally by searching for the defocus amount. In general, since wavefront aberration is dominated by low-order terms, the shape of wavefront aberration including spherical aberration can be almost determined by the ratio of defocused Zernike wavefront aberration coefficient and third-order spherical aberration Zernike wavefront aberration coefficient. . If the defocus amount is always selected so that the value of this ratio is constant, the wavefront aberration shapes of BD and DVD can always be similar, regardless of the substrate thickness error. This wavefront aberration of BD and DVD is determined with respect to the corresponding substrate thickness t ₅ of a given base lens. If the Peak to Peak value of the wavefront aberration of the BD and DVD at that time is obtained, the value is empirically proportional to the respective substrate thickness errors t ₁ -t ₅ and t ₂ -t _5. α _B and α _D are obtained in advance.

Next, if the on-axis step difference d of the annular phase shift structure is appropriately assumed, φ _B and φ _D can be determined by the phase shift amounts of BD and DVD. A simultaneous equation for compensating the Peak to Peak value of the wavefront aberration before adding the BD and DVD ring zones can be established using the α _B and α _D. . Solving this for N and t _5, it is possible to determine the corresponding substrate thickness t ₅ of the base lens, it is possible to compensate for the wavefront aberration of BD and DVD at that time in the N zones.

  Here, the axial step amount is the distance between the surfaces on the optical axis when the adjacent aspheric surfaces constituting the annular surface are virtually extended to the optical axis using the same surface coefficient. Means that. The farther the radial position of the annular zone is from the optical axis, the greater the refraction angle of the light beam, and the greater the change in the step amount that achieves the same phase shift amount. However, if each ring zone surface is designed so that predetermined light can be collected at a predetermined position in a range wider than the actual ring zone region including the optical axis, those ring zone surfaces can be obtained at any radial position. It is possible to generate a predetermined optical path difference reflecting the surface interval on the optical axis. As a result, the step amounts at all radial positions can be handled in a centralized manner by being represented by the axial step amount.

  Here, the number N of ring zones is easy to understand as an integer value from the point of view, but in practice, the solution of the simultaneous equations is not necessarily an integer value, and may be a real value and a negative value. The fractions after the decimal point appear as residual aberrations that cannot be corrected when wavefront aberrations are cut in equal steps. Therefore, it is desirable that the decimal value after the decimal point is small. When the value is negative, it means a solution when the unevenness is inverted with respect to the sign of the given annular zone step.

  If the number of ring zones N is too large, the zone width becomes narrow and difficult to manufacture, and if it is too small, it means that the phase difference is large, and the Peak to Peak value of the corrected wavefront is large. Become. Therefore, aberration compensation is performed as described in FIG. 4 using the obtained wavefront aberration shape, and the RMS wavefront aberration amount is obtained from the obtained wavefront aberration shape after the phase shift, and the value is confirmed while checking the value. What is necessary is just to search the level | step difference of an annular phase shift structure so that it may become a value. As described above, the annular phase shift structure of the second region, that is, the BD / DVD region can be determined.

  Next, FIG. 6 is a diagram showing a design flow in the BD / DVD / CD area. In the first region (BD / DVD / CD region), an annular phase shift structure of a three-wavelength shared region is designed. Similarly to the second region, it is assumed that the wavefront aberration compensated by the plurality of annular phase shift structures is similar in the first region. Then, from the simultaneous equations that the sum of the phase shift amounts of each annular phase shift structure at each wavelength is equal to the peak-to-peak value of aberration, the number of annular zones shared by each annular phase shift structure can be obtained. it can. It is also possible to compensate for aberrations by a plurality of annular phase shift structures and to allow each of the plurality of annular phase shift structures to compensate for different wavefront aberration shapes. The method will be described below.

In FIG. 6, first, the value of the corresponding substrate thickness t ₄ of the aspheric lens before adding the annular phase shift structure and the values of defocus amounts δ _B , δ _D , and δ _C in BD, DVD, and CD are selected. To do. Here, the value of t ₄ is actually larger than t ₅ obtained in the second region and is preferably set to a value between the substrate thickness of the BD and the DVD, but can be selected independently from t _{5 in} terms of design. . The [delta] _B and [delta] _D is, [delta] _B the difference is determined in the second _region, it is necessary to select to match the difference of [delta] _D. This is because it is necessary to match the focal positions of the BD and DVD in the second area and the first area. However, it is not necessary to match the defocus values themselves. This is because, at the stage of the optical design of the zonal surface shape, the focal position of the base lens is previously set to the first area and the first area so that the focal position after the addition of the zonal phase shift structure matches between the first area and the second area. This is because they can be shifted in two areas.

By selecting the above values, the wavefront aberration shapes W _B (ρ), W _D (ρ), and W _C (ρ) for BD, DVD, and CD to be compensated by the annular phase shift structure are determined. Can be expanded and displayed with the Zernike polynomials already described.

Next, the axial step differences of the three types of annular zone phase shift structures are assumed. Accordingly, the phase shift amounts φ _Bn , φ _Dn , and φ _Cn (n = 1, 2, 3) at the BD, DVD, and CD wavelengths by the three annular zone phase shift structures can be respectively determined.

Next, using the three phase shift amounts at these three wavelengths, a total of nine phase shift amounts, the terms A _Bm , A _Dm , A _Cm (m = 4, 9, 16, 25) of the Zernike aberration expansion term. In each case, a ternary linear simultaneous equation can be established from the condition for matching the distribution of aberration compensation amounts shared by the respective phase shift amounts. Since the wavefront aberration shape of each expansion term is determined by the equation (2), it is guaranteed that the expansion term component of different wavelengths always has a similar shape. Accordingly, simultaneous equations similar to those applied to the Peak to Peak value in the second region can be established for each development component. However, the value of the Zernike coefficient itself does not necessarily correspond to the Peak to Peak value of the expansion component, but as long as the function is fixed, there is no doubt that the coefficient is proportional to the Peak to Peak value, and the proportional coefficient is also There is no problem in practice if we consider that the variable of N is defined.

In this way, when the ternary simultaneous equations are solved for each order m of the Zernike expansion coefficient and the distribution value N _nm of each annular zone phase shift structure (n = 1, 2, 3) is obtained, eventually The shape of wavefront aberration that can be compensated for three wavelengths with the band phase shift structure can be determined.

  Therefore, aberration compensation is performed as described in FIG. 4 using the obtained wavefront aberration shape, and the RMS wavefront aberration amount is obtained from the obtained wavefront aberration shape after the phase shift, and the value is confirmed while checking the value. What is necessary is just to search the amount of on-axis level | step differences of a ring zone phase shift structure so that it may become a value. As described above, the annular phase shift structure in the first region, that is, the three-wavelength shared region can be determined.

  In the above description, when the corresponding substrate thickness at the BD wavelength of the base lens is t4 or t5, the corresponding substrate thicknesses at the DVD / CD wavelength are also t4 and t5, and the generated spherical aberration is the substrate thickness of the DVD / CD. For t2 and t3, it was assumed that spherical aberrations of substrate thickness deviation of t2-t4, t3-t4, t2-t5, and t3-t5 occurred. However, since an actual objective lens has a different refractive index depending on the wavelength, spherical aberration generated in the objective lens is mixed due to the wavelength difference. In the lens design of the present invention, as will be described later, the deviation is designed by correcting the deviation in advance as follows according to the initial setting value of the disc substrate thickness. First, as a result of designing the phase shift structure using the original disk substrate thickness and designing the lens surface shape, the spherical aberration generated in the lens due to the wavelength shift cannot be corrected in the DVD or CD, and remains in the design result. The disk substrate thickness at a DVD or CD wavelength that compensates for the spherical aberration is examined. When the disc substrate obtained as a result of the investigation is thinner than the original disc substrate thickness, the initial set value of the disc substrate thickness of the DVD or CD used for the initial phase structure design is increased by the substrate thickness difference. When the lens shape is designed again, the corresponding substrate thickness at the DVD and CD wavelengths is effectively reduced in the same manner under the influence of spherical aberration due to the wavelength difference, so that it matches the actual DVD and CD substrate thickness. Can do.

  Furthermore, in the above description, basically, whatever the phase difference of the annular phase shift structure input to each simultaneous equation is, unless the determinant of the so-called simultaneous equations becomes zero, There will be a satisfactory solution. However, considering that the wavefront aberration after aberration correction shown in FIG. 4 is a sawtooth-shaped curved waveform with a step difference of the phase difference φ, it is better to engrave many annular zones with as small a phase shift value as possible. It can be easily imagined that the final RMS wavefront aberration is reduced.

  Therefore, an approximate value of the phase difference input to the simultaneous equations is determined using FIG. FIG. 7 shows the calculation result of the phase difference at each wavelength with respect to the step depth. Here, the refractive index of the medium constituting the lens is as shown in the figure, and the incident angle to the medium is 0 ° (perpendicular incidence). The reason why the curve at each wavelength is serrated is that the optical path difference exceeding ± λ / 2 is folded within ± λ / 2 by subtracting the rounded off integer wavelength. Thus, at the respective wavelengths, the same step amount appears to be a different phase difference due to the difference in wavelength.

  A curve 301 (open square plot point) shown in a legend named “3-wavelength RMS” is a root mean square value of phase difference values of three wavelengths. That is, the RMS value of the wavefront aberration in the three-wavelength shared region may be searched in the vicinity of the step depth where this value is small. Further, in the curve 302 (open circular plot point) shown in the legend named “red and blue two-wavelength RMS”, the root mean square value of the phase difference value between the red wavelength and the blue wavelength is similarly obtained. . What is necessary is just to search the level | step difference amount of BD / DVD area | region in the vicinity of this small level | step difference depth.

  In this way, in the lens shape of the present case, three types of annular phase shift structures are used in the three-wavelength shared region and one type in the BD / DVD shared region, so that aberrations can be simultaneously applied to three optical discs of BD, DVD, and CD. Can be corrected.

  In practice, it may be necessary to add an annular phase shift structure for removing unnecessary flare light in addition to this. In the three-wavelength shared region, it is necessary to use all three wavelengths of light, and therefore no ring zone for removing flare light is necessary. Since CD light is not required in the BD / DVD area, if there is no limiting aperture for selectively blocking the CD light, the CD light transmitted through this area causes a deterioration of the necessary light collection state of the CD light. There is a possibility. Furthermore, in the BD-dedicated area, both DVD light and CD light are not required, so unnecessary DVD light that passes through this area of the DVD condensing spot deteriorates and unnecessary light that passes through the CD condensing spot is unnecessary. CD light may be deteriorated. In order to reduce these effects, an annular phase shift structure that diffuses CD light without affecting BD light and DVD light transmitted through the BD / DVD area (hereinafter referred to as fifth annular phase shift). Structure) and an annular phase shift structure that diffuses DVD light and CD light without affecting the BD light transmitted through the BD dedicated region (hereinafter referred to as sixth annular phase shift structure) may be added. .

  In FIG. 7, for example, if the annular phase shift structure has a depth of about 0.8 μm, the phase difference between DVD and CD can be shifted by about 0.5λ while the phase difference between BDs is kept at zero. What is necessary is just to add this as a ring zone of the flare light countermeasure of a BD exclusive area | region. For example, if the depth is about 3.9 μm, it is possible to add a phase difference of about 0.5λ only to the CD light while keeping both the phase differences of the BD light and the DVD light substantially zero. What is necessary is just to add this as a ring zone of the flare countermeasure of a BD / DVD area.

  Since the annular zone for preventing flare does not affect the necessary light in principle, the relative displacement of the annular zone for aberration correction does not matter so much. Therefore, it is not particularly necessary to form these annular zones on the same plane as the aberration correcting annular zone. For this reason, you may form a flare countermeasure ring zone in a 2nd surface.

  In forming the annular zone as described above, in the present invention, defocus is introduced into the wavefront aberration to be corrected by the annular phase shift structure. Hereinafter, the necessity of introducing defocus will be described using a comparative example.

  FIG. 8 shows, as a comparative example, the aberration wavefront shape to be corrected when designed with an annular phase shift structure without adding defocus. For each of BD, DVD, and CD, the aberration wavefront in the three-wavelength shared area and the aberration wavefront in the BD / DVD area are shown side by side. The focal positions of the aberration wavefront shapes in each region are the same, and the focal position is set to a position that gives the best image point for the generated spherical aberration. For this reason, the Peak to Peak value of the aberration amount to be corrected is not so large as 1 to 2λ. There are two types of overlapping zones, 1.46 μm and 6.3 μm in the 3-wavelength shared region, 3.88 μm and 5.33 μm in the BD / DVD region, and the phase difference is 8λ at the BD wavelength. It reaches. There are 54 such zones, and the lens thickness corresponding to the axis constituting each zone (axial lens thickness) roughly changes from the inner circumference to the outer circumference by about 0.5 mm.

  However, in this comparative example, unlike the design principles described with reference to FIGS. 5 and 6, both the three-wavelength shared region and the BD / DVD region have a structure in which two types of annular phase shift structures are overlapped. In this case, the annular specifications of the three-wavelength shared region are first determined by three unknowns, which are the combined thickness of the base lens and the number of annular zones of the two types. Then, assuming the corresponding substrate thickness obtained, the annular zone specifications of the BD / DVD region are determined by using the two types of annular zones to be superimposed on the BD / DVD region as unknowns. In this way, the design of the BD / DVD area cannot be actually obtained in a very good way, and in the present invention, the design method is changed to that shown in FIGS.

  FIG. 9 is a diagram showing a lens shape and wavefront aberration designed with an annular phase shift structure without adding defocus. The wavefront aberration indicates the shape of a wavefront aberration having an axial wavefront aberration and an angle of view. The annular zone phase shift structure was basically designed corresponding to the on-axis wavefront aberration of FIG. In order to keep the peak-to-peak value of the corrected wavefront aberration small, in FIG. 7, the depth at which the phase shift amount is relatively small at all three wavelengths is set to, for example, 1.5 μm or 6.3 μm. Select the depth of the combination. Further, since the phase shift amount becomes small with respect to the wavefront aberration amount to be corrected, the number of annular zones increases, and the number of deep annular zones increases. As a result, the shape change with respect to the base lens becomes large, and irregular irregularities are generated in the lens shape of FIG. In addition, although the lens shape is displaying only the range of NA of a design with BD, DVD, and CD, respectively, the shape currently displayed is a common lens. The disc substrate thickness is different for each disc. The working distance (WD) of the CD is about 0.41 mm. In order to secure this, the base lens is designed so that the WD of the BD is about 1 mm. For this reason, the thickness of the base lens is inevitably reduced as compared with the original lens for BD, and the lens that cannot sufficiently secure the angle-of-view characteristic is inevitably used as the base.

  With respect to these lens shapes, the axial wavefront aberration is well suppressed to about ± 0.1λ for each of BD, DVD, and CD as expected. However, for wavefront aberration with angle of view, BD, DVD, and CD are 0.05λ and 0.07λ even for very small angles of view such as 0.03 °, 0.07 °, and 0.15 °. A wavefront aberration amount equivalent to that of the Marechal standard has occurred. In addition to the insufficient angle-of-view characteristics of the base lens, the difference in effective on-axis lens thickness of each zone from the inner circumference It is thought that it is also caused by the fact that it has changed greatly over the outer periphery.

  In view of the above, the present invention has studied to reduce the level difference of the annular zone. However, if the step becomes shallower, a small value cannot be selected for the phase shift, and thus the Peak to Peak value of the residual aberration must be increased. For this reason, the RMS wavefront aberration is expected to increase.

  Therefore, the present applicant has come to consider introducing a part of the idea of the diffractive lens system into the design of the annular phase shift lens described above. In the following description, the relationship between the phase shift lens and the diffractive lens will be described first.

  In the diffractive lens system, a phase step similar to that of the phase shift system is determined by a periodic structure (diffractive lens structure) having a corrected aberration as a phase function. The phase function is the phase difference of the first-order light with respect to the 0th-order light of the diffracted light wavefront generated by the diffractive lens structure, and the phase difference of the n-th order diffracted light (n is an integer value) is n times that. If the phase structure has periodicity, the complex amplitude distribution of the light wave transmitted through the diffractive element including the diffractive lens can be developed with a power series of a complex exponential function having a phase function as a phase value. At this time, the phase of the nth-order diffracted light Corresponds to n times the primary light. This is because the operation of the power series to the nth power is equal to n times the phase value when applied to an exponential function. Here, the phase difference itself does not depend on the wavelength, and the optical path difference that is actually corrected is obtained by multiplying the phase difference by the wavelength, and is proportional to the wavelength. The phase function is a function that substantially determines only the position of the annular zone, and is irrelevant to the phase structure of each annular zone, and is also independent of the diffraction efficiency. The diffraction efficiency corresponds to the square value of the absolute value of the coefficient of the complex exponential function of light of each diffraction order in the previous power series expansion. This diffraction efficiency is determined by the phase structure before power series expansion for one period of each annular zone, and is nothing but the additional phase distribution added to the incident light immediately after passing through the diffraction element. The phase of the basis function that expands this to the power series is the phase function, which is independent of the phase structure of each annular zone. Conversely, the phase structure of the phase shift lens is a phase addition to the incident light of the light immediately after passing through the element before being separated into the respective orders, and directly affects the intensity distribution.

  The difference that the phase function of the diffractive lens is the phase value of the power series expansion basis function and the phase shift amount itself is corrected, and the difference in interpretation of the corrected phase amount linked. In the diffractive lens, the phase amount generated in the first-order diffracted light in one annular zone is always one wavelength regardless of the step, but in the phase shift lens, the phase shift amount corrected in one annular zone is the wavelength from the optical path difference. It is understood that the value of the phase difference is within ± λ / 2 except for an integer multiple of. Usually, this is set to about 0.1λ corresponding to the Peak to Peak value of the residual aberration, so that the phase shift amount corrected by the phase shift lens is the same as that of the diffractive lens for the same number of annular zones. It becomes about 1/10 of the amount. However, if this is interpreted as a diffractive lens, the phase addition amount is a value corresponding to the removed integer wavelength, and the light of the diffraction order corresponding to the integer value is a phase shift amount integrated by a number of steps. It is understood that the phase shift effect of the envelope occurs for zero order light generated by an aspheric surface added to the base aspheric surface of the phase shift lens. Thus, the phase shift lens can also be interpreted as a diffractive lens under the condition that diffracted light is separated. Conversely, it is possible to unconditionally interpret a diffractive lens as a phase shift lens. The base aspherical surface of the phase shift lens is not the envelope surface of the annular phase shift structure, which is the base aspherical surface of the diffractive lens, but a surface in which the optical path difference from the optical axis is removed over the entire lens surface, and the phase shift is stepped. It is interpreted as a value obtained by subtracting an integer wavelength from the optical path difference due to. Therefore, for example, in a diffractive lens to which a blazed grating having a diffraction efficiency of 100% in which the phase difference is an integral multiple of the wavelength, the phase shift amount as the phase shift lens is zero, and the lens thickness is reduced, the wavelength shift or the temperature is increased. Although there is an effect of reducing the influence of deviation, the lens has no aberration correction effect with respect to the base lens under the basic conditions.

  Such a difference in interpretation between the phase function of the diffractive lens and the phase shift of the phase shift lens is also reflected in the difference in evaluation of residual aberration and light utilization efficiency. In other words, residual aberrations are essentially unavoidable because phase aberration lenses correct continuous aberration wavefronts with discontinuous phase steps, but diffractive lenses have a continuous phase with a specific diffraction order component by design. Since the aberration is corrected with the diffracted light of the function, no apparent residual aberration appears. However, if this is interpreted in terms of the phase shift lens, the unnecessary wave order is eliminated on the focal plane even though there is residual aberration in the wavefront immediately after passing through the element due to the mixture of other order components. Thus, it can be considered that the residual aberration is removed. The unnecessary diffraction order component that has been excluded is evaluated as a reduction in diffraction efficiency. That is, the residual aberration of the phase shift lens and the reduction of the diffraction efficiency of the diffractive lens are substantially equivalent, and it is misunderstood that the diffractive lens generally has better aberration performance. Conversely, the phase shift lens does not separate the diffracted light, so the diffraction efficiency does not decrease and the high light utilization efficiency is true for the integrated light quantity around the focused spot, but the intensity of the focused spot center due to aberrations is true. It will appear in the decline. However, there is a demand for higher utilization efficiency if the aberration is within an allowable range, which has led to the superiority of recent phase shift lenses.

  Another point of the diffractive lens system is that the diffractive element itself included in the diffractive lens has a lens action. The diffractive lens generates nth order diffracted light, which is used in an optical device. In order to generate the n-th order diffracted light, it is necessary to spatially separate from other orders of light in the far field. In a diffractive lens rotationally symmetric with respect to the optical axis, the spatial separation is on the optical axis. This means that light of each order is separated at different focal positions. The action of separating the diffracted light at different positions on the optical axis is nothing but the lens action. Therefore, the diffractive lens is literally an element having a lens action on the diffractive element itself. In a diffractive element in which n-th order diffracted light is not separated, it cannot be said that diffraction as a general diffractive element occurs, and it is not usually called a diffractive element. Even in an optical element having a phase structure not called a diffraction element, a phase shift occurs. In other words, the phase shift element is not limited to a lens and is a phase shift element regardless of whether diffraction occurs or not, but the phase shift element in which diffraction occurs is particularly called a diffraction element. Although it has been described above that the phase shift lens is evaluated not as diffraction efficiency but as residual aberration, a phase shift lens in which diffracted light is not separated cannot be evaluated based on diffraction efficiency, and therefore usually has to be evaluated as residual aberration. On the contrary, under the condition that the diffracted light tends to be separated, there may be a case where the phase shift lens can be evaluated by the light use efficiency. On the other hand, although it has not been carried out much as a residual aberration on the wavefront immediately after transmitting the diffractive lens from which diffracted light is separated, it should be possible in principle.

  Whether the diffractive element has a lens action enough to separate diffracted light depends on the defocus component of the phase function. For example, since the focal depth of the focused spot is expressed by λ / NA ^ 2 when the numerical aperture NA and the wavelength λ, the defocus coefficient of the phase function that gives more defocus is W20> (λ / ( 2 × NA ^ 2)) × (NA ^ 2) = λ / 2, that is, a defocus wavefront aberration of 0.5λ or more is sufficient. However, this is a case of a non-aberrated focused spot, and if there is an aberration in the phase function, a larger defocus is required.

  Even if the diffractive element does not have a defocus component that sufficiently separates diffracted light, it may be said that the order is substantially separated. That is when diffracting with 100% diffraction efficiency. This is the case when the annular phase shift structure has an integer wavelength phase step at the annular boundary, but this is a matter of interpretation and it is also understood that the phenomenon occurs as an integer wavelength phase shift. it can. However, in the case of a phase shift element used for the purpose of compatibility of a plurality of wavelengths, such a case can be excluded from consideration in the present invention because the phase difference is not made an integral multiple of the wavelength at all wavelengths at the same time.

  Another factor that determines whether the diffractive element spatially separates the diffracted light according to the order is the periodicity of the annular phase shift structure. Even if it seems that a phase shift lens can be interpreted as a diffractive lens element with a defocus component that separates the diffracted light into a phase function, the phase shift lens actually has a phase structure of each period of the annular phase shift structure Without periodicity, power series expansion cannot be performed with a complex exponential function having the phase function as a phase value. That is, the diffracted light is not spatially separated in the optical axis direction for each order. That is, in this case as well, since the annular phase shift structure does not generate diffraction, it can be said that such a phase shift lens is not a diffraction lens.

  In the above description, the lens action and periodicity of the phase structure have been described as the conditions for the diffractive lens. However, the lens action is also a form of periodicity in a sense. In general, in wave and signal theory based on Fourier analysis, it is common knowledge that the frequency spectrum of a signal represented by a periodic function becomes discrete, but light wave propagation from the near-field pupil plane to the far-field focal plane The same applies to wave optics on the premise that the Fraunhofer diffraction is expressed by Fourier transform. That is, it can be interpreted that the diffracted light is separated into the respective orders in the far field and the phase structure that caused it is periodic. Therefore, it can be understood that the defocus component of the phase function causing the lens action is also one of the manifestations of the periodicity of the phase structure. From the above, the diffractive lens can be considered to be a phase shift element having a periodicity in the phase structure in such a broad sense.

  In contrast to such a diffractive lens, the phase shift lens has mainly focused on correcting aberration itself by phase shift due to a step. Therefore, in order to correct the aberration wavefront itself that does not include the defocus component, the function corresponding to the phase function of the diffractive lens essentially does not include the defocus component. That is, the phase shift lens that is not a diffractive lens has no periodicity in the phase structure unlike the diffractive lens, and the diffracted light is not spatially separated from the condensed spot. For this reason, the wavefront aberration after the phase shift by the annular phase shift structure is the RMS wavefront aberration itself including a discontinuous phase step, which is a predetermined value (0.07λ, 0.05λ, or 0.03λ of the Marechal condition). Etc.) has been designed to be:

  However, as described above, when it was attempted to realize a three-wavelength compatible lens by phase shift, it became clear that the small phase shift amount and the angle of view characteristic of the lens are not compatible. We decided to introduce defocus to the corrected wavefront aberration. This aims to effectively reduce a large phase shift generated by shallowing the step of the annular phase shift structure and a large residual wavefront aberration resulting therefrom. That is, the aim is to spatially separate light components having large residual aberrations by a pseudo diffraction effect. For this reason, the direct evaluation index of the phase shift lens for introducing defocus is not a wavefront aberration but a spot shape. Of course, wavefront aberration can also be used indirectly as an evaluation index, but it is not a total RMS wavefront aberration as in the past, but is approximated by a finite order term (for example, up to 37 general terms) developed by Zernike expansion. The combined value of only the components is used.

  Of course, the question arises that a phase shift lens designed based on this idea may be a diffractive lens. As described above, it is a diffractive lens that separates diffracted light into a plurality of orders, and a lens that has undergone an optical design based on a phase function based on this is naturally a diffractive lens. However, in the present invention, the phase structure of each annular zone is designed so that the phase function of the diffractive lens is not included. In the process of designing and synthesizing multiple annular phase shift structures separately, the spatial periodicity of the entire annular phase shift structure becomes sparse, and components with large aberrations as energy are required in the optical system. Even if it is unevenly distributed from the focused spot, it cannot be an intensity distribution in which each order is spatially discrete like a diffractive lens. Therefore, a phase shift lens designed by synthesizing a plurality of annular zones cannot have the periodicity necessary as a condition of the diffractive lens just because the aberration wavefront to be corrected includes defocus. Therefore, the lens of the present invention is a phase shift lens having a defocus component in the aberration to be corrected.

  FIG. 10 is a diagram illustrating the concept of a phase shift lens that corrects wavefront aberration including defocus, which is a basic concept of the present invention. (A) is before adding the annular phase shift structure, (b) is after adding the annular phase shift structure, the upper stage is a conceptual diagram of the lens shape and light rays incident thereon, and the lower stage is wavefront aberration ( The distribution in the pupil plane of (optical path difference) is shown.

  Before adding the annular phase shift structure of (a), the BD light beam L1, the DVD light beam L2, and the CD light beam L3 are basically condensed at the same focal position P0 (here, chromatic aberration is ignored). doing. On the other hand, when the curvature centers of the reference spheres of BD, DVD, and CD are placed at the positions indicated by P1, P2, and P3 in the vicinity of the focal point, the wavefront aberration with respect to the reference wavefront is like W1, W2, and W3 in the lower stage. Suppose that Here, since the wavefront aberration is relative to a reference focal position different from the actual focal position (original focal position), a wavefront aberration is generated by adding the respective defocused wavefront aberrations.

  In contrast, in (b), the wavefront aberrations W1, W2, and W3 generated in (a) are corrected by adding an annular phase shift structure. That is, the wavefront aberration is folded by the phase shift due to the annular step, and approaches 0 (corresponding to the horizontal axis) as indicated by W1 ', W2', and W3 '. When such a wavefront aberration distribution is obtained, considering that the focus position originally set as the reference in (a) is a position shifted from the original focus position P0 such as P1, P2, and P3, (b) As shown by, the light is actually condensed at the respective positions P1, P2, and P3. That is, the lens action is caused by the phase shift. If this residual aberration is small enough, it is ideal, but even if it is large, if the converging spot shape remains within the allowable range, the component with large aberration is substantially separated spatially from the necessary condensing spot. It is equivalent to being in the form.

  After designing the annular phase shift structure including the defocus component as described above, the actual annular lens surface shape is designed. In this case, the lens surface is focused on the corresponding substrate thicknesses t4 and t5 so that the lens surface can be focused without third-order spherical aberration, and the lens thickness is set on the axial step of each annular surface of the phase structure design. Design by changing the amount. At this time, the paraxial focal length of each annular zone determined by each annular zone including the second surface changes for each annular zone. For this reason, it is necessary to correct the radial position of each annular zone so that light can be collected at a predetermined light collection angle with respect to the optical axis.

  Furthermore, since the paraxial focal position of each annular surface changes for each annular zone, the focal length of the entire lens becomes unclear. Therefore, in the present design, when a light beam incident at an angle θ with respect to the designed lens surface shape is condensed with a height h with respect to the optical axis, the condensing focal position is expressed as f = Calculate and confirm by h / tan θ. The numerical aperture (NA) of the objective lens is also determined for the effective diameter using this focal length. Therefore, the NA value is also fed back as necessary so that the final value becomes the target value with respect to the initial design value of the phase structure design.

  The embodiments of the present invention based on the principle of the present invention described with reference to FIGS. 4, 5, 6, and 10 will be described in more detail below.

First, a first embodiment (design example 1) of the objective lens of the present invention will be described.
Table 1 shows the design results of the annular zone phase shift structure in design example 1. In the design example 1, the wavefront aberration shape to be compensated is made similar in all annular phase shift structures.

  (A) is the table | surface which put together the basic parameter for designing an annular zone phase shift structure. As described above, in designing the annular phase shift structure, the substrate thickness setting value for the phase structure design is corrected from the actual value for DVD and CD so as to correct the spherical aberration generated by the objective lens due to the wavelength shift. Shift in advance. Here, when the DVD is 0.87 mm and the CD is 1.98 mm, the designed spherical aberration including the influence of the wavelength shift in the objective lens is compensated. The NA values were designed with initial values of 0.85, 0.6373, and 0.5014, with the target values of 0.85 for BD, 0.65 for DVD, and 0.53 for CD, respectively.

  Under these preconditions, the BD / DVD area (C) will be described first. At the depth of the fourth annular zone shown in (C1) of 0.98 μm, the corresponding substrate thickness of the base lens and the number of annular zones of the fourth annular zone are determined by solving the simultaneous equations of the BD / DVD region shown in FIG. It was. At this time, the ratio of the defocus term A4 of the Zernike expansion term of the wavefront aberration to the third-order spherical aberration term A9 was set to −50 using the result of trial calculation with respect to an appropriate initial value. The solution of the simultaneous equations under such conditions was a corresponding substrate thickness of 0.318 mm and a ring zone number of 24.3. Thus, the corresponding substrate thickness of the base lens was a value between 0.0875 mm for BD and 0.6 mm for DVD. At this time, the defocus amount, aberration amount, and phase difference at each wavelength of BD, DVD, and CD were as shown in (C2). The BD defocus amount of −0.055 mm corresponds to the focal length of the base lens being shortened by 0.055 mm due to the annular phase shift structure. The defocus amount at this time was 0.106 mm for DVD and 0.272 mm for CD. The phase difference of the 4th annular zone is 0.27λ for BD, −0.25λ for DVD, and −0.37λ for CD. Both are large values as a phase shift lens. I decided to confirm this and adopted it.

  Next, the three-wavelength shared region (B) will be described. In this design example, as shown in (B1), the corresponding substrate thickness was set to 0.318 mm, which is the same as the BD / DVD area. Further, as shown in (B2), the defocus amount was also set to the same value as the BD / DVD area. This determined the wavefront aberration shape to be corrected. At this time, the aberration of BD was 23.3λ, DVD was −21.6λ, and CD was −39.1λ. Next, by changing the depth of the first, second, and third annular zones in the range of 0 to 3 μm, the simultaneous equations of the three-wavelength shared region shown in FIG. 6 are solved for various combinations of depths. The phase shift structure was determined, and a combination of depths where the corrected wavefront aberration amount at that time was as small as possible was sought. As a result, the solutions of 0.09 μm, 1.495 μm, and 2.375 μm were selected for the depths of the first, second, and third annular zones, respectively. The phase difference for each ring zone of BD, DVD, and CD is a value exceeding 0.3λ at the maximum. These ring zone depths are very shallow values with respect to the depth of 6 μm of the comparative example having no defocus component shown in FIG. For this reason, the added integer wavelength component of the optical path difference is very small as compared with the comparative example including 1 to 2 wavelengths and 8 wavelengths. For this reason, the change in the axial lens thickness of the lens surface forming the annular zone is reduced, and the angle of view characteristic can be improved.

  The annular phase shift structure of the BD / DVD region and the three-wavelength shared region designed as described above were integrated to design an annular phase shift structure as shown in Table 1 (D). The number of ring zones was 126 in the three-wavelength shared region, 25 in the BD / DVD region, and 1 in the BD dedicated region, for a total of 152.

  FIG. 11 is a diagram showing a calculation result of wavefront aberration to be corrected in the lens of Example 1 (Design Example 1) based on the phase structure design result of Table 1. The calculation is performed in the same manner as in FIG. 8 of the comparative example. The left side is the three-wavelength shared area, and the right side is the outer BD / DVD area. Here, a focal position shift of about 0.32 mm is added to the focal position shift between BD and CD, which is merely caused as chromatic aberration, by the annular phase shift structure. This shows that the relative defocus amount between BD and DVD / CD is larger than that in FIG. In FIG. 8, the wavefront aberration has an extreme value near the boundary between the three-wavelength shared region and the BD / DVD region, whereas the extreme value in FIG. 11 shifts outside the BD / DVD region. This is also clear from the fact that the wavefront slope of the / DVD region has the same sign as the three-wavelength shared region. The amount of wavefront aberration in the three-wavelength shared region reaches 13λ or more for BD, −9λ for DVD, and −17λ for CD. Therefore, it can be seen that the defocus of the CD with respect to the BD has roughly reached 30λ. Including the BD / DVD area, the BD is about 23λ, the DVD is −21λ, and the CD is about −39λ. Such a large amount of aberration has not been considered to be corrected by phase shift. Further, the RMS wavefront aberration value is shown in the graph. In the three-wavelength shared region and the region including the BD / DVD region, the RMS wavefront aberration value (defocus) of the defocus component is larger than the RMS wavefront aberration value (SA3) of the third-order spherical aberration. Here, a comparison is again made with the comparative example of FIG. 8 in which the defocus is not included in the wavefront aberration to be corrected. In FIG. 8, in the region including the BD / DVD region, the RMS wavefront aberration value (defocus) of the defocus component is almost zero, and the RMS wavefront aberration value (SA3) of the third-order spherical aberration component is the defocus component. On the other hand, it is very large. This is the basic design of the conventional phase shift lens, and this embodiment shown in FIG. 11 is greatly different from this.

  Furthermore, in this embodiment, when such a phase shift is applied, a phase structure for reducing flare light of CD light is added to the BD / DVD area. This corresponds to the fifth annular phase shift structure described above. The phase shift is almost insensitive to BD and DVD, and a step having a depth that has a phase shift effect only on CD is alternately applied to the annular phase shift structure designed above. Specifically, in FIG. 7, for example, a depth in the vicinity of 3.88 μm is a depth that gives such a phase shift.

  FIG. 12 is a diagram illustrating the effect of adding a phase shift to the flare light countermeasure. FIG. 13 is a diagram showing a case where a phase shift for preventing flare light is not added for comparison. In each figure, the upper row is BD, the middle row is DVD, the lower row is CD, the left column is a phase shift including an integer wavelength, and the center column is a phase shift not including an integer wavelength. The right column shows the wavefront aberration after the phase shift, and is a result of performing aberration compensation by subtracting the phase shift of the center column of FIGS. 12 and 13 from the aberration curve of the right column of FIG. The horizontal axis represents the normalized radius of the BD / DVD area, and the vertical axis represents the phase shift amount or wavefront aberration.

  In this case, a phase shift of 5.01λ for BD, 2.97λ for DVD, and 2.48λ for CD occurs. In FIG. 12, the wavefront aberration including a phase shift for an integer wavelength repeats unevenness in any of BD, DVD, and CD. This is a phase step which is insensitive to the BD and DVD described above and causes a phase shift in the CD. In FIG. 13, such a phase step does not appear. Since the center column is displayed excluding the integer wavelengths, this phase shift hardly appears in BD and DVD in FIG. 12, and almost becomes a phase step due to only the original aberration compensation phase shift. It becomes. However, with respect to CD, a phase difference of about 0.5λ is added in the center row of FIG. 12, so that the slope of the entire phase shift curve is substantially reversed from that of FIG. The wavefront aberration of FIG. 11 is substantially compensated by the curve in the center row. Therefore, the wavefront aberration of the CD of FIG. 11 is a curve that falls to the right in the BD / DVD region. By subtracting the phase shift, the wavefront aberration will increase. As a result, in the wavefront aberration after aberration compensation, the number of sections in which ± 0.5λ is swung by CD is increased to 14 sections in FIG. As a result, the inclination of the wavefront is increased, and the light is diffused to the necessary condensing spot, thereby reducing the influence.

  14 to 16 show the result of correcting the wavefront aberration of FIG. 11 by applying the design result shown in Table 1 using the phase shift structure. FIG. 14 shows the wavefront aberration of BD, FIG. 15 shows the wavefront aberration of DVD, FIG. 16 shows the wavefront aberration of CD, and the horizontal axis is the normalized pupil radius, and only the effective ranges of BD, DVD and CD are displayed. ing. Based on the wavefront aberration of the BD, the wavefront aberration is determined by engraving the wavefront aberration shape shared by each annular phase shift structure by phase shift. For this reason, the overall fluctuation of the envelope of wavefront aberration is small in BD, but the envelope is slightly fluctuating in DVD and CD. The reason is estimated as follows. That is, in the present embodiment, the wavefront aberration shape to be compensated is made similar in all annular phase shift structures that are superimposed not only in the BD / DVD region but also in the three-wavelength shared region. At that time, since the wavefront aberration component considered for the similar shape was only the defocus component and the third-order spherical aberration component, the ratio of the higher-order spherical aberration component did not always match and appeared as a compensation residual component. It is seen. As described in the second embodiment, these can be improved by a method of determining the compensation distribution of each annular phase shift structure for each expansion term component.

  The amount of wavefront aberration after compensation is 0.3λp-p (pp means Peak to Peak) in the range of −0.17λ to 0.13λ in the BD of FIG. 14, and −0. Aberrations are suppressed within a range of 0.33λp-p from 21λ to 0.12λ, and 0.6λp-p from −0.38λ to 0.22λ in the CD of FIG. When the periodicity of the waveform is confirmed in each figure, in FIG. 14, the waveform undulates every two to three annular zones from the center position of the normalized radius 0, but there is not necessarily periodicity due to the number or arrangement of the waveforms. . In the BD / DVD region, since the annular phase shift structure is only the fourth annular zone, it seems that there is periodicity. However, light transmitted through the BD / DVD area and used in the optical disc optical system is DVD light using the three-wavelength shared area and the BD / DVD area, and BD using the three-wavelength shared area, the BD / DVD area, and the BD dedicated area. Since it is light, it cannot be overemphasized that there is no periodicity as a whole use area. Therefore, it is considered that the condensing spot is not separated for each order in the optical axis direction, but becomes a phase shift lens instead of a diffraction lens.

  In determining the annular position of the three-wavelength shared region in the annular phase shift structure described above, it is not so much when the annular positions are determined separately in the three annular phase shift structures and then combined on one surface. Avoid narrow ring zones. Therefore, in an annular region that is not adjacent to the boundary between the first, second, and third lens regions, a narrow annular zone projected on the entrance pupil plane is weighted by the phase shift value at the annular boundary on both sides thereof. Adjustment was made so that a phase shift amount obtained by combining the phase shifts on both sides was added to the weighted average radial position.

  FIG. 17 is a diagram for explaining synthesis of a narrow annular zone shape. When the phase shift shapes of the two annular zone phase shift structures are 401 and 402, the boundary between the two phase shifts is close in the region 405. Here, the horizontal direction in the figure indicates the spatial coordinates in the radial direction of the lens, and the vertical direction indicates the phase shift amount due to the annular zone. When the phase shifts of these adjacent annular zones are combined as they are, a narrow annular zone 406 is generated in the proximity region 405 as indicated by reference numeral 403. When the width of the annular zone 406 is equal to or smaller than the actually processable dimension, it is necessary to synthesize the adjacent annular zone boundary as one step 407 as indicated by 404. Here, in the two adjacent phase shift amounts, the annular zone phase shift structure 402 is larger than the annular zone phase shift structure 401, so the new annular zone boundary 407 obtained by synthesizing the two annular zones is the ring of the original 402. It is close to the belt boundary. Specifically, a radius position that internally divides two adjacent annular zone radii by the ratio of the absolute values of the two phase shift amounts is set as a new annular zone boundary. Since an annular phase shift structure with a large phase shift naturally has a large amount of aberration to be compensated, if the annular zone boundary position deviates from the ideal state, the residual aberration amount is smaller than when the annular zone boundary with a small phase shift shifts. growing. Therefore, it is desirable to arrange the annular zone boundary having a large phase shift so that the shift is smaller than the annular zone boundary having a small phase shift. By the above method, the annular zone width was made to be at least 3 μm.

  Next, the optical design of the lens surface shape having the phase structure shown in Table 1, FIG. 11, and FIGS. In optical design, a BD-dedicated objective lens without a uniform annular zone having a lens surface corresponding to the BD-dedicated region is first optimally designed using optical design software with a wider working distance as long as the angle of view characteristics allow. Next, with the distance from the second surface of the lens to the focal point (back focus) and the distance between the first and second surfaces fixed, the wavefront aberration of BD and DVD is the same as the wavefront aberration of BD and DVD in FIG. The first surface and the second surface are optimally designed so that Next, the second surface is fixed, and the axial lens thickness is changed so that the phase shift amount of each annular zone of the BD / DVD region designed for phase structure is obtained, and all the rings in the BD / DVD region of the first surface are changed. The optimum design is made so that the wavefront aberration of FIG.

  Next, the wavefront aberration of BD, DVD, and CD is fixed while the distance from the second surface to the focal point (back focus) and the surface distance between the first surface and the second surface of the lens in the original BD area are fixed. The surface shape of the first surface is optimally designed so as to have each wavefront aberration of FIG. At this time, the surface of the BD / DVD area is used as the second surface. Next, the wavefront of FIG. 11 is the same for all the annular surfaces in the three-wavelength shared region of the first surface by changing the axial lens thickness so that the phase shift amount of each annular zone in the three-wavelength shared region designed by the phase structure is obtained. Optimum design to obtain aberrations. The ring zone boundary position at each ring zone boundary so that the refracted light beam emitted from the second surface is condensed at a predetermined inclination with respect to the paraxial focal length calculated from the surface shape of each ring zone obtained. Tweak the.

  Next, the aspherical shape corresponding to the phase shift at the radial position is subjected to the least-square fitting with the radius of curvature and the conic constant, or the second-order aspheric coefficient and the fourth-order aspheric coefficient. These parameters are input as initial values to the optical design software, and further optimal design is performed with the wavefront aberration of FIGS. In this case, the first surface has an annular phase shift surface with the number of the annular zones designed by the phase structure, the second surface has a three-wavelength shared region and a BD / DVD region compatible region, and an aspheric shape different in the BD dedicated region. Become.

  Table 2 shows lens basic specification values (basic parameters) in Example 1 (Design Example 1). Basic specification values include the wavelength, the spacing between the lens and disk surfaces, the refractive index of the lens and disk, and the effective beam diameter. Here, the effective light beam diameter of BD indicates the outer diameter of the BD dedicated area, the effective light beam diameter of DVD indicates the outer diameter of the BD / DVD common area, and the effective light diameter of CD indicates the outer diameter of the three-wavelength common area.

  Tables 3A and 3B show the annular phase shift structure of the first lens surface. There are 152 ring zones, the 0th to 104th ring zones are the 3-wavelength shared area, the 105th to 150th ring zones are the BD / DVD shared area, and the 151st ring zone is the BD dedicated area. Here, the on-axis sag amount is A0 of the aspherical surface expressed by the equation (3).

  The axial sag amount is 0 in the 0th annular zone. The 0th zone to the 104th zone are represented only by the radius of curvature (the reciprocal of the curvature c) and the conic constant κ. On the other hand, from the 105th zone to the 150th zone, the curvature c of the aspherical surface is 0 and is represented only by A2 and A4. The reason why the number of terms is expressed in such a small number is that the width of the annular zone is narrow and the necessary shape can be sufficiently described even with a small number of terms. The unit of on-axis sag amount, opening radius, and curvature radius in the table is mm.

  Table 4 shows coefficients up to A4 of the aspherical surface of the second lens surface. The parameters are the same as in Tables 3A and 3B. The unit of on-axis sag amount, opening radius, and curvature radius in the table is mm. On the second surface, the three-wavelength shared region and the BD / DVD region have the same aspherical surface as the 0th ring zone (ring zone number 0), and the BD dedicated region has the 1st ring zone (ring zone number 1). ) Surface shape.

  Table 5 shows the aspherical coefficients after A6 for the 0th to 150th and 151st annular zones of the first lens surface and the 0th and first annular zones of the second lens surface. The 151st zone of the 1st surface is a BD exclusive area, and since a field is wide, many aspherical coefficients are required with the 1st and 2nd zone of the 2nd surface.

  FIG. 18 shows the results of analyzing the lens design shape of design example 1, the axial wavefront aberration distribution, and the wavefront aberration distribution of the image height by ray tracing for each of BD, DVD, and CD. Although the lens shape is displayed including BD, DVD, and CD up to the BD dedicated area, the effective diameter of DVD and CD is narrower than the displayed range. That is, DVD and CD are displayed including unnecessary flare rays. Further, in the drawing, values of effective diameter, focal length (f), numerical aperture (NA), and working distance (WD) are shown.

  Here, in the annular phase shift lens of the present embodiment, the focal length is determined as follows. In the annular phase shift lens, the aspherical shape of each annular surface is determined so that the wavefront aberration at the predetermined focal position becomes a predetermined shape by locally changing the lens thickness by the amount of the axial step to be phase shifted. To do. At this time, as the lens thickness is changed, the geometric optical focal length is strictly different in each annular zone. Since the conventional annular phase shift lens does not include defocus in the wavefront aberration, the wavefront aberration to be compensated for by the annular phase shift structure is small, and the number of annular zones is smaller than that of the lens of this embodiment. The band width was also wide. Therefore, there is no problem even if the effective focal length is substituted with the geometric optical focal length of the center 0th annular zone. However, the annular phase shift lens of this embodiment has a large number of annular zones, and the geometrical optical focal length of each annular zone changes by several tens of μm. Therefore, the effective focal length of the objective lens is quantified as follows according to the design shape. That is, it is considered that the image height at which the wave optical condensing spot center intensity of incident light with a slight angle of view forms an image is a value obtained by multiplying the focal length by tan θ of the angle of view θ. The image height value is divided by tan θ to obtain the focal length. In the design example of FIG. 18, the focal lengths of BD, DVD, and CD are 1.788 mm, 1.972 mm, and 2.080 mm, respectively.

  The NA value is determined for each effective diameter in accordance with the focal lengths of BD, DVD, and CD. In FIG. 18, BD is 0.850, DVD is 0.635, and CD is 0.519. The effective diameter is 3.04 mm in BD, which is smaller than 3.74 mm in FIG. 9 (comparative example). Generally, 3.74 mmφ is for a drive device having a thickness called half height, and 3 mmφ is for a thin drive device called slim type. The working distance (WD) is 0.626 mm for BD, 0.498 mm for DVD, and 0.244 mm for CD. Since the effective diameter is different from the comparative example of FIG. 9, it is difficult to simply compare with the value of WD, but the focal lengths of BD, DVD, and CD are 1.788 mm, 1.972 mm, and 2.080 mm in FIG. It is. Compared with 2.2 mm, 2.259 mm, and 2.44 mm in FIG. 9, the ratio of the focal length of CD and the focal length of BD increases from 1.11 times to 1.16 times. It can be seen that the focal length expansion effect by the phase shift structure appears. The NAs of DVD and CD are 0.635 and 0.519, which are larger than 0.611 and 0.466 in FIG. This is because the design is intended to be compatible with DVD and CD recording.

  In the ray diagram, even light rays in the effective region of each wavelength seem to be not necessarily condensed on the recording film surface of the disc. However, this is a geometric optical path of the light beam and does not reflect the actual intensity distribution of the focused spot. Actually, it should be evaluated by wavefront aberration and wave optical condensing spot distribution.

  The wavefront aberration similarly displays the effective range of BD. The on-axis wavefront aberration of the BD varies finely with an amplitude of about 0.3λp-p in the three-wavelength shared region and the BD / DVD region. This corresponds to the phase structure design of FIG. 14, and a surface shape that produces a wavefront aberration almost as intended can be designed. The wavefront of the BD dedicated area of the BD is naturally flat and this also corresponds to FIG. The on-axis wavefront aberration of DVD has a variation of about 0.3λp-p in the three-wavelength shared region and the BD / DVD region, and a variation exceeding ± 0.5λ has begun suddenly in the BD dedicated region. In the figure, it seems that it does not necessarily reach ± 0.5λ, but this is due to the setting of the number of sampling points, and this also substantially corresponds to FIG. On-axis wavefront aberration of CD fluctuates with an amplitude of about 0.4λp-p in the three-wavelength shared region, but suddenly exceeds ± 0.5λ in the BD / DVD region and the BD-dedicated region. This also substantially corresponds to FIG. These aberrations are much larger than those in the comparative example of FIG. 9, but the influence is evaluated by the subsequent spot distribution.

  Next, the angle of view characteristic (image height characteristic) of FIG. 18 will be described. The angle of view is set to 0.1 ° for BD, 0.2 ° for DVD, and 0.3 ° for CD, because the display of wavefront aberration needs to be within an effective range of ± 0.5λ. These values have already spread from 2 to 3 times the 0.03 °, 0.07 °, and 0.15 ° of the comparative example of FIG. 9, and the effective aberration amount is sufficient as described later. It is a practical value.

  The wavefront aberration distribution shown in FIG. 18 is suitable for numerically grasping the amount of aberration within the effective diameter, but it is difficult to confirm the influence of flare light outside the effective diameter. Accordingly, FIG. 19 shows an interference fringe image of axial wavefront aberration synthesized. This is the case of BD, DVD, and CD from the left, and all are shown in the range of the BD effective diameter. In order to visualize the amount of aberration, a tilt wavefront aberration of 10λ is added to each image in the horizontal direction of the screen. Thereby, it is an ideal state that ten straight and clear vertical stripes are arranged at equal intervals, and the deviation from that represents the amount of aberration. Flare light has less influence as the interference fringes are finer, and a rough portion is a portion that may cause a problem. However, moiré fringes generated when the interference fringes are close to the pixel pitch are also partially mixed, so care must be taken (local thin thin fringes that are difficult to think from symmetry are considered to be moire. ). As a result, almost good interference fringes appear within the effective diameter in the BD and DVD, and the sharpness of the interference fringes slightly falls within the effective diameter in the CD, indicating a reduction in efficiency. The flare light areas on the outer periphery of DVD and CD are basically sufficiently fine interference fringes, but there are thin interference fringes in the BD / DVD area of CD, and it is difficult to judge whether these are moire fringes. Further, there is a place where interference fringes are slightly rough and clear at the outermost periphery of the BD dedicated area of the CD, and a flare countermeasure ring zone having a depth insensitive to BD may be added to this area.

  Next, Table 6 shows the Zernike expansion analysis results of wavefront aberration. Here, based on the ray tracing results of FIG. 18, the wavefront aberration on the axis and the wavefront aberration with respect to the angle of view for each of BD, DVD, and CD are shown by the RMS wavefront aberration component value of the Zernike expansion term and the wavefront aberration value synthesized from them. ing.

(A) in Table 6 is the axial wavefront aberration, and shows the spherical aberration and Fit error of rotationally symmetric term numbers 9, 16, 25, 36, and 37 and their combined values. Total is the RMS wavefront aberration before fitting, and 5 to 37Total means the composite value of the Zernike coefficients from the fifth term to the 37th term, but in the case of wavefront aberration on the axis where the angle of view is zero The combined value of only five spherical aberrations that are centrosymmetric. According to this, the total wavefront aberration value before fitting is as large as 0.058 λ rms for BD, 0.073 λ rms for DVD, and 0.118 λ rms for CD, but in the objective lens using the annular phase shifter of this embodiment, This can be interpreted as an indicator of light utilization efficiency. For example, 0.07λrms, which is a Marechal evaluation criterion, is known as the amount of wavefront aberration when the center intensity of the focused spot is reduced to 80% with respect to no aberration. Such center intensity is called Strehl intensity and can be approximated by 1− (2πWrms / λ) ² with respect to the RMS wavefront aberration value Wrms. The Strehl intensity is the ratio of the center intensity of the spot, but if the spot diameter does not change with no aberration, it can be interpreted as being almost equivalent to the light utilization efficiency. When the Strehl strength is calculated from the above in this embodiment, it is estimated that BD is 87%, DVD is 79%, and CD is 45%. On the other hand, the wavefront aberration value of 5 to 37 Total is an evaluation index as a normal wavefront aberration reflecting the quality of the light spot that is actually condensed in this embodiment. These are sufficiently small for BD, DVD, and CD, respectively, 0.009λrms, 0.021λrms, and 0.003λrms. These will be verified by evaluation of spot distribution described later.

  In interpretation of these total RMS wavefront aberrations, the fluctuation components of the residual aberration shown in FIG. 18 fluctuate so severely that fitting with normal wavefront aberration is impossible. Corresponds to the fact that it diffuses on the focal plane and has no practical effect. This intensely varying component is reflected in the Fit error. In the conventional phase shift lens, this component is also taken into consideration as an evaluation index. However, in this embodiment in which a defocus component is introduced into the correction aberration, it is estimated that the component is unevenly distributed on the optical axis. Then, it is thought that it may be excluded and evaluated.

  Further, (B) of Table 6 shows the wavefront aberration development result of the light with the angle of view. Because of obliquely incident light, a non-rotationally symmetric aberration component that was not included in the on-axis evaluation result is added, and the number of displayed items is increased. As with axial aberration, the aberration amount of 5 to 37 Total is 0.020 λrms for a BD with an angle of view of 0.1 °, 0.030 λrms for a DVD with an angle of view of 0.2 °, and a CD with an angle of view of 0.3 °. It is 0.026λrms. In either case, the amount of wavefront aberration expected in practice is somewhat large, but is greatly improved over the comparative example. As the aberration component, the third-order coma aberration (Coma3) in the eighth term is dominant, and is 0.015λ rms for BD, 0.022λ rms for DVD, and 0.023λ rms for CD. Since the third-order coma aberration is proportional to the angle of view, when converted to 0.3 °, the BD is 0.045λrms, the DVD is 0.033λrms, the CD is 0.023λrms, and the coma has a large BD.

  Hereinafter, the evaluation result of the spot intensity distribution will be described. FIG. 20 shows a calculation result of the spot intensity distribution of the dedicated lens which is a reference for comparison. BD has a wavelength of 0.405 μm, NA of 0.85, DVD has a wavelength of 0.66 μm, NA of 0.65, CD has a wavelength of 0.785 μm, and NA of 0.53. NA value for recording. It is also assumed that the incident light intensity is flat across the entire pupil plane. The upper row is a bird's eye view of a spot shape, and the display range is 2 μm □ for BD, 4 μm □ for DVD, and 6 μm □ for CD. In the figure, WEX indicates the spot diameter in μm unit where the intensity becomes 1 / e ^ 2 with respect to the center intensity. SDX is the ratio of the sidelobe intensity to the center intensity. S. I. Is the Strehl intensity, which is the center intensity normalized by the non-aberration center intensity, which is 1 in an ideal state with a dedicated lens. The lower part shows the spot center intensity value in the optical axis direction, and it is considered that a peak appears if the flare light is condensed at a focal position shifted in the optical axis direction. Since this is an ideal spot here, naturally no unnecessary intensity peak appears.

  FIG. 21 shows the calculation result of the spot intensity distribution of the lens according to design example 1, and shows the condensing spot distribution in the focal plane and the center intensity distribution in the optical axis direction. Here, it is assumed that both DVD and CD are provided with a limiting aperture for shielding an area other than a predetermined effective light beam diameter. That is, it is assumed that unnecessary light outside the effective aperture range is ideally blocked. In the case of BD, the calculation result is almost the same as that of the dedicated lens of FIG. 20 (comparative example), which is good. However, since the sidelobe ratio SDX is good, it is estimated that this is because the NA value has decreased from 0.65 in the ideal state to 0.635 as shown in FIG. . In practice, this NA ratio is 0.977, which is supported by the fact that it matches the reciprocal of the spot diameter ratio. In other words, this can be compensated by feeding back to the initial setting value of the NA of the DVD, which means that this is not an essential problem. Similarly, the spot diameter WEX of the CD is increased from 1.22 μm to 1.23 μm, which is also considered to be due to the NA being decreased from the ideal value of 0.53 to 0.52. Therefore, from the above, it was confirmed that the design result almost intended in principle was obtained. Strehl strength I. Is 0.87 for BD, 0.81 for DVD, and 0.56 for CD, and it can be expected that the light utilization efficiency compared to each NA dedicated lens will be approximately these values.

  However, the calculation result described in FIG. 21 corresponds to the case where flare light is removed by an ideal limiting aperture in DVD and CD. Such a limiting aperture that makes the effective diameter variable is theoretically possible with a liquid crystal element or the like, but in an actual optical pickup, it is not preferable because of an increase in cost. Therefore, FIG. 22 shows a spot distribution in the case where there is no restriction aperture and light is incident on the entire effective diameter of the BD in both DVD and CD. When this is compared with FIG. 21 having a limiting aperture, the spot diameter and the central intensity distribution on the optical axis hardly change in DVD. That is, it can be seen that the DVD flare light in the BD dedicated area has little influence. However, the condensing spot diameter WEX of the CD is smaller from 1.23 μm to 1.17 μm as compared with FIG. This is considered to be due to the CD light in the BD exclusive area where no flare countermeasure is taken in this embodiment, but the sidelobe intensity ratio SDX is good, and the flare light in the optical axis direction intensity distribution is also considered to be an allowable range. .

Next, a second embodiment (design example 2) of the objective lens of the present invention will be described.
Table 7 shows the design results of the annular zone phase shift structure in design example 2. In the design example 2, the wavefront aberration shapes to be compensated are not similar in each annular phase shift structure.

  (A) is the table | surface which put together the basic parameter for designing an annular zone phase shift structure. In order to correct the spherical aberration generated by the objective lens due to the wavelength shift, the substrate thickness setting value for the phase structure design is shifted in advance from the actual value in DVD and CD. Here, when the DVD is 0.77 mm and the CD is 1.8 mm, the designed spherical aberration including the influence of the wavelength shift in the objective lens is compensated. The NA values were designed at initial values of 0.85, 0.64, and 0.51, respectively, with BD being 0.85, DVD being 0.65, and CD being 0.53.

  Under these preconditions, the BD / DVD area (C) will be described first. At the depth of the fourth annular zone shown in (C1) of 0.976 μm, the corresponding substrate thickness of the base lens and the number of annular zones of the fourth annular zone are determined by solving the simultaneous equations of the BD / DVD region shown in FIG. It was. At this time, the ratio of the defocus term A4 of the Zernike expansion term of the wavefront aberration to the third-order spherical aberration term A9 was set to −114 using the result of trial calculation with respect to an appropriate initial value. The solution of the simultaneous equations under such conditions was a corresponding substrate thickness of 0.285 mm and a ring zone number of 29.3. Thus, the corresponding substrate thickness of the base lens was a value between 0.0875 mm for BD and 0.6 mm for DVD. At this time, the defocus amount, aberration amount, and phase difference at each wavelength of BD, DVD, and CD were as shown in (C2). The BD defocus amount of −0.065 mm corresponds to the focal length of the base lens being shortened by 0.065 mm due to the annular phase shift structure. The defocus amount at this time was 0.135 mm for DVD and 0.364 mm for CD. The phase difference of the fourth annular zone was 0.26λ for BD, −0.25λ for DVD, and −0.38λ for CD. The defocus amount is larger than that in Table 1 (Design Example 1), and the working distance is not reduced even if the NA of DVD and CD is increased.

  Next, the three-wavelength shared region (B) will be described. In design example 2, as shown in (B2), the corresponding substrate thickness of the base lens is 0.39 mm, which is different from 0.285 mm in the BD / DVD area shown in (C1). However, at this time, the interval between the defocus amounts of the BD and the DVD is made equal so that the focal positions of the BD and DVD coincide in the three-wavelength shared area and the BD / DVD area. That is, the defocus interval between the BD and the DVD is 0.135 + 0.065 = 0.200 mm in (C2), and 0.140 + 0.060 = 0.200 mm in (B2). In this way, at the stage of the optimum design of the surface shape, the focal position of the base lens is effectively adjusted between the three-wavelength shared area and the BD / DVD area so that the focal positions coincide. As a result, in the three-wavelength compatible region where CD reproduction is also necessary, the substrate thickness of the base lens is brought close to 1.2 mm of the CD, and the amount of aberration compensation in a CD that requires a large defocus component to secure WD is reduced. Yes.

  Furthermore, in the design example 2, the wavefront aberrations compensated by the first, second, and third annular phase shift structures are not similar to each other. In other words, in the design example 1 (Table 1), the three annular zone phase shift structures to be combined are such that the Zernike third-order spherical aberration coefficient A9 and the defocus coefficient A4 are all equal. On the other hand, in the design example 2, as shown in (B1), the values of the A4 phase ratio and the A9 phase ratio are made different in each annular zone phase shift structure. Here, the phase ratio is an amount proportional to the number of annular zones allocated to each aberration order component, which is obtained as a solution of the simultaneous equations described in FIG. Since the value of the Zernike coefficient itself is different from the Peak to Peak value of the aberration amount, the phase ratio is not the number of zones. Therefore, here, the value of the coefficient of each Zernike expansion component of the wavefront aberration that can be compensated by each annular phase shift structure is referred to as a value obtained by dividing the value by the phase shift amount of one step. That is, since the product of the phase ratio multiplied by the phase difference of the step becomes the Zernike coefficient, the phase ratio of the Zernike coefficient of the same annular phase shift structure is equal to the ratio of the Zernike coefficient. In (B1), it is obvious that the ratios of the A4 phase ratio and the A9 phase ratio of the first, second, and third annular phase shift structures are different.

  As described above, the value of “phase sum including integers” in (B2) can be significantly reduced as compared with design example 1 (Table 1). This is a value reflecting the sum of the steps actually applied to the lens surface. When this value is reduced, chromatic aberration that occurs when the wavelength changes can be reduced.

  The annular phase shift structure of the BD / DVD region and the three-wavelength shared region designed as described above were integrated to design an annular phase shift structure as shown in Table 7 (D). The number of annular zones was 148 in the three-wavelength shared area, 29 in the BD / DVD area, and 1 in the BD dedicated area, for a total of 178. At this time, the zone boundaries were integrated as described in FIG. 17 so that the narrowest zone width would not be too narrow.

  FIG. 23 is a diagram showing a calculation result of wavefront aberration to be corrected in the lens of Example 2 (Design Example 2) based on the phase structure design result of Table 7. Similarly to FIG. 11 (Design Example 1), the defocused RMS wavefront aberration value (defocus) and the third-order spherical aberration RMS wavefront aberration value (SA3) are also shown. Compared with FIG. 11 in the three-wavelength shared region, the ratio of defocus and third-order spherical aberration is greatly different for BD, DVD, and CD, respectively. In Table 7, it has been explained that the wavefront aberrations compensated by the three annular zone phase shift structures to be superimposed are not similar to each other, but as a result, the total wavefront aberration of BD, DVD and CD is not similar either. However, as a matter of course, the wavefront aberration compensated by one of the three annular zone phase shift structures is similar in BD, DVD, and CD.

  24 to 26 show results obtained by applying the design results shown in Table 7 to the wavefront aberration of FIG. FIG. 24 shows the wavefront aberration of BD, FIG. 25 shows the wavefront aberration of DVD, FIG. 26 shows the wavefront aberration of CD, and the horizontal axis is the normalized pupil radius, and only the effective ranges of BD, DVD and CD are displayed. ing. Based on the wavefront aberration of the BD, the wavefront aberration is determined by engraving the wavefront aberration shape shared by each annular phase shift structure by phase shift. Compared with FIGS. 14 to 16 of the design example 1, the fluctuation of the envelope in DVD and CD is improved. In design example 1, the compensation wavefront aberration of DVD and CD is determined so that the ratio of the third-order spherical aberration and the defocus only is the same based on the wavefront aberration of BD, and the Peak to Peak value is compensated at three wavelengths. The zonal phase shift structure was determined as described. However, in design example 2, the zonal phase shift structure is determined so that the third, fifth, and seventh order Zernike coefficient values are compensated at three wavelengths from the defocus instead of the Peak to Peak value. Has improved. For this reason, it is considered that the envelope is flattened. When the envelope is flattened, the final wavefront aberration is also reduced and the focused spot performance is improved. However, in Design Example 2, since only one annular zone is used in the BD / DVD shared area, the wavefront aberration compensated at the three wavelengths must be made similar in the same manner as in Design Example 1, Since only defocus and third-order spherical aberration are taken into account, a slight envelope variation remains in the outermost peripheral area of the DVD.

  In design example 2, one annular zone phase shift structure is used in the BD / DVD region, but it is also possible in principle to design a plurality of the same in the three-wavelength shared region.

  Next, the aspherical shape of each zone surface having the zone phase shift structure designed in this way was designed. Table 8 shows basic lens specification values (basic parameters) in Example 2 (Design Example 2). Basic specification values include the wavelength, the spacing between the lens and disk surfaces, the refractive index of the lens and disk, and the effective beam diameter.

  Tables 9A and 9B show the annular phase shift structure of the first lens surface. The number of annular zones is 178, the 0th to 147th annular zones are the three-wavelength shared region, the 148th to 176th annular zones are the BD / DVD shared region, and the 177th annular zone is the BD dedicated region. Here, the axial sag amount is A0 in the equation (3) as in the first design example, and is 0 in the 0th annular zone on the axis. The 0th zone to the 115th zone are represented only by the radius of curvature (the reciprocal of the curvature c) and the conic constant κ. On the other hand, from the 116th zone to the 176th zone, the aspherical curvature c is 0, and is represented only by A2 and A4. The reason why the number of terms is expressed in such a small number is that the width of the annular zone is narrow and the necessary shape can be sufficiently described even with a small number of terms. The unit of on-axis sag amount, opening radius, and curvature radius in the table is mm.

  Table 10 shows coefficients up to A4 of the aspherical surface of the second lens surface. The parameters are the same as those in Tables 9A and 9B. The unit of on-axis sag amount, opening radius, and curvature radius in the table is mm. On the second side, the three-wavelength shared area is the 0th ring zone (ring zone number 0), the BD / DVD area is the 1st ring zone (ring zone number 1), and the BD dedicated area is the 2nd ring zone (ring zone number 2). ) Surface shape. Unlike design example 1, the second surface has a three-band configuration because the wavefront aberration to be corrected is changed between the three-wavelength shared region and the BD / DVD region, so that the second surface is a common surface in these regions. This is because it is difficult to achieve both the post-compensation wavefront aberration of the phase structure design at three wavelengths. By using these surface coefficients and the surface coefficients shown in Table 11, the sag of the surface shape obtained by extending the shape of the second surface of the three-wavelength shared region to the BD / DVD region with respect to the surface shape of the BD / DVD region. The amount difference was in the range of -9λ to + 12λ with respect to the BD wavelength. Similarly, the difference of the sag amount with respect to the surface shape of the BD dedicated region in the surface shape obtained by expanding the surface shape of the BD / DVD region to the BD dedicated region is from −10λ to −137λ. Thus, the surface shapes of the three regions of the second surface are substantially different surfaces.

  Table 11 shows aspherical coefficients after A6 for the 0th to 176th and 177th annular zones of the lens first surface and the 0th, 1st and 2nd annular zones of the lens second surface. The 177 ring zone on the first surface is a BD-dedicated region, and since the region is wide, a large number of aspheric coefficients are required together with each zone on the second surface.

  FIG. 27 shows the lens design shape, axial wavefront aberration distribution, and image height wavefront aberration distribution of design example 2 for BD, DVD, and CD, respectively. The lens shape is shown including the BD exclusive area, and DVD and CD are displayed including unnecessary flare light. Further, in the drawing, values of effective diameter, focal length (f), numerical aperture (NA), and working distance (WD) are shown.

  Although the NA of the DVD is 0.646, which is almost the target value, the NA of the CD is as large as 0.57. Further, as can be seen from the wavefront aberration of the BD, the light transmission position is mismatched at the boundary between the BD dedicated area and the BD / DVD area, and the wavefront aberration is locally increased. This is considered to be the effect of making the second surface a three-ring zone. However, even in design example 1, the boundary between the BD dedicated area and the BD / DVD shared area should have been discontinuous on the second surface, so it can be solved by adjusting the sag amount so as to reduce the level difference. Since the wavefront aberration of DVD and CD is also shown up to the effective diameter range of BD, the outside of the effective diameter fluctuates greatly within the range of ± 0.5λ or more. However, the wavefront aberration within the effective diameter is in the phase structure design result. The envelope amplitude is almost the same. Spherical aberrations remain somewhat in the DVD, indicating that it is necessary to refeed back to the DVD phase structure design substrate thickness.

  FIG. 28 shows an interference fringe image of axial wavefront aberration in design example 2. Compared with the design example 1 (FIG. 19), the sharpness of the interference fringes in the compatible area of the BD is decreased, which reflects the decrease in utilization efficiency. Moreover, it is considered that an annular streak is formed at the boundary between the BD dedicated area of the BD and the BD / DVD area because of an inconsistency of the annular area on the second surface. In the case of CD interference fringes, two rough interference fringes in the interchangeable region are seen in the middle and outermost periphery compared to design example 1, and it is effective to add a flare countermeasure ring zone that is insensitive to BD in these regions. It is.

  Next, Table 12 shows the Zernike expansion analysis results of wavefront aberration. Here, from the ray tracing results of FIG. 27, the wavefront aberration on the axis and the wavefront aberration with respect to the angle of view for each of BD, DVD, and CD are shown by the RMS wavefront aberration component value of the Zernike expansion term and the wavefront aberration value synthesized from them. ing.

  (A) is the on-axis wavefront aberration, and the total on-axis wavefront aberration is 0.079 λrms for BD, 0.106 λrms for DVD, and 0.125 λrms for CD, which is larger than that of design example 1 (Table 6). It has become. Along with this, the Strehl strength is also 0.75, 0.55, and 0.38, respectively, which means that the light utilization efficiency is slightly lowered as compared with the design example 1. On the other hand, in the composite value of the RMS wavefront aberration from the fifth term to the 37th term that reflects the substantial focused spot quality, the BD is 0.010 λrms, the DVD is 0.025 λrms, and the CD is 0.028 λrms. Is also within the allowable range. The reason why the values are large for DVD and CD is mainly the third-order spherical aberration (SA3) in the ninth term, so recovery is expected by refeedback to the phase structure design substrate thickness in design. In addition, even an actual lens can be absorbed by a spherical aberration compensation mechanism essential for BD. As a spherical aberration compensation mechanism, a collimating lens that collimates light incident on an objective lens is generally finely moved in the optical axis direction to change the degree of light divergence and convergence.

  On the other hand, the combined wavefront aberrations of items 5 to 37 in the case of (B) having an angle of view are 0.018 λrms when BD is 0.1 ° and 0.047 λrms when DVD is 0.2 ° and CD is 0.27. Is 0.042λrms at 0.3 °. In any case, the factor of the increase with respect to the on-axis aberration is the third-order spherical aberration (Coma3) of the eighth term, but −0.007λrms of the BD is −0.021λrms in terms of the angle of view of 0.3 °, which is acceptable. 0.034λrms of DVD is 0.051λrms in terms of angle of view of 0.3 °, and needs to be improved slightly. Since CD is 0.024λrms at an angle of view of 0.3 °, it can be said to be an allowable range.

  FIG. 29 shows the calculation result of the spot intensity distribution of the lens according to design example 2. Here, it is assumed that DVD and CD have a limited opening in their effective diameter ranges. Compared with the dedicated lens of FIG. 20 (comparative example), the BD is excellent with almost no difference between the in-plane distribution and the optical axis direction distribution. In DVD, the spot diameter WEX is considerably large from 0.835 μm to 0.873 μm of the dedicated lens. Compared to design example 1, NA is close to the target value of 0.65, but it seems that the wavefront aberration has increased. In the CD, the spot diameter WEX is reduced from 1.22 μm to 1.13 μm of the dedicated lens. As described with reference to FIG. 27, this is considered to be because the NA has increased to 0.57. However, since this spot diameter is just inversely proportional to the ratio of NA and there is no increase in side lobes, this is not an essential problem, and it can be considered that correction can be performed by design feedback.

  FIG. 30 is a spot distribution calculated for a DVD and a CD including flare light in the BD effective diameter range without a limiting aperture. When this is compared with FIG. 29 having a restricted aperture, the DVD shows almost no difference in the spot diameter on the focal plane, and the influence of flare light does not appear. Looking at the central intensity distribution in the optical axis direction, stray light with a peak intensity ratio of about 5% is generated at a position near the lens of about 100 μm from the focal plane, but there is no problem because the peak intensity is small and the lens side. it is conceivable that. In CD, the spot diameter on the focal plane is further reduced. In the central intensity distribution in the optical axis direction, flare light appears in a wide range from the focal point to the lens side. However, since the intensity is small, the influence as flare light is considered to be small.

The aberration amounts of the two examples (design example 1 and design example 2) described above are compared.
Table 13 compares the amount of chromatic aberration generated. Here, the unit of chromatic aberration is μm / nm, and shows the amount of movement of the best image point position when the wavelength changes by 1 nm. Chromatic aberration in design example 1 is 0.9, BD is 0.72, and CD is 0.92, whereas design example 2 is reduced to 0.67, 0.52, and 0.57, respectively. doing. This is due to the fact that the total phase value including the integer of design example 2 (table 7) is significantly smaller than design example 1 (table 1). As described above, in the design example 2, the focusing spot performance is generally inferior to the design example 1, but the effect of the originally expected chromatic aberration is great.

  Table 14 compares the generation amount of the third-order coma aberration with respect to various deviation factors. The coma aberration is caused by the angle of view, lens tilt, and disk tilt. From the geometrical relationship, the coma of the angle of view has a relationship that is approximately the sum of the coma aberration of the lens tilt at the same angle and the coma aberration of the disk tilt. is there. The table shows that this relationship is almost true. Here, in an optical pickup equipped with a lens, it is desirable to adjust the relative tilt of the pickup with respect to the disc. However, the relationship varies from disc to disc depending on the warpage of the disc. It is desirable to make adjustments. In order to do this, in recent optical pickups, the drive axis of the objective lens can be driven in addition to the radial direction and the focal direction of the disc, and the tilt can be driven, and this tilt is adjusted by the jitter of the reproduction signal. Has been. Therefore, it is necessary to be able to correct the coma caused by the tilt of the disk by the tilt of the lens. In that sense, it is desirable that the coma aberration ratio (ratio of the coma aberration amount of the lens tilt to the coma aberration amount of the disc tilt) shown in the table has a certain magnitude (absolute value). In the case of a three-wavelength compatible lens, the value of the coma aberration ratio tends to be small in DVD, and is 0.22 in Design Example 1, but is restored to 0.71 in Design Example 2.

  The advantage of design example 2 as described above is that the corresponding substrate thickness in the three-wavelength shared region is made larger than that in the BD / DVD region, and the shape of the wavefront aberration to be compensated for by the three annular phase shift structures superimposed in the three-wavelength shared region is obtained. This is possible because they are not similar to each other.

  Next, Table 15 summarizes the effect of expanding the CD working distance (WD) in Design Examples 1 and 2. Here, for BD, DVD, and CD, the actual working distance and the converted value of the working distance when there is no defocus component in the corrected wavefront aberration (comparative example) are shown. In both design examples 1 and 2, the CD WD has a negative value when there is no defocus wavefront aberration, but the actual design value can achieve a positive value of 0.23 mm or more. ing. This is an effect obtained by including a defocus component in the corrected wavefront aberration. From this, the effectiveness of the present embodiment with respect to the problem of securing the working distance of the CD can be confirmed.

  In the lens design examples shown in the above embodiments, the performance is not always sufficient, and it cannot be denied that there is room for improvement. However, in principle, the approach of this embodiment has the effect of improving individual performance individually, and the remaining tasks can be expected to be improved by design feedback. Therefore, the effectiveness of this embodiment has been confirmed.

  FIG. 31 is a block diagram showing an embodiment of an optical pickup using the objective lens of the first and second embodiments. The optical disk is rotated by a spindle motor 2, and the optical pickup 1 transmits a light beam to the transparent medium layer (BD: 204, DVD: 205, CD: 206) of the optical disk via the objective lens 100 compatible with BD, DVD, and CD. Light is collected and information is recorded and reproduced.

  In the case of BD recording / reproduction, the BD divergent light emitted from the blue-violet semiconductor laser chip 12 mounted in the blue-violet laser package 11 passes through the tracking three-beam generating grating 13 and the dichroic prism 14. The beam is reflected by the beam splitter 15 and is converted into substantially parallel light by the collimating lens 16. The collimating lens 16 can be finely moved in the optical axis direction by a moving element 17, and the moving element 17 can be moved and fixed to a predetermined position by a screw shaft rotated by a stepping motor 18.

  The collimating lens 16 is moved in the optical axis direction because it is necessary to change the collimating lens position according to the recording / reproducing layer. That is, the effective cover glass thickness for the objective lens 100 changes depending on which layer of the BD two-layer recording film is selected for recording and reproduction, thereby generating spherical aberration. By changing the collimating lens position, the degree of divergence / convergence of the light beam incident on the objective lens 100 is changed, and spherical aberration generated in the objective lens is canceled. Furthermore, since it is assumed that the objective lens 100 is molded by plastic injection molding for cost reduction, spherical aberration is also generated by a refractive index change due to a temperature change. This can also be corrected by the collimating lens position. Therefore, a temperature sensor 28 is disposed near the objective lens 100.

  The blue-violet laser beam converted into parallel light by the collimating lens 16 is reflected by the rising prism 19 and is focused on the recording film by the objective lens 100 through the BD transparent medium layer 204. In practice, in the case of the two-layer BD, the thickness of the BD transparent medium layer 204 varies substantially depending on the selected layer. The light reflected from the recording film again passes through the objective lens 100, the rising prism 19, and the collimating lens 16, passes through the beam splitter 15, and is condensed on the OEIC (photoelectric conversion integrated circuit) 23 through the concave lens 22.

  A sub beam (not shown) generated by the grating 13 is also incident on the OEIC 23, and the light amount is detected as an individual photocurrent signal by a predetermined divided light detection region. As the detection signal for each region, a predetermined defocus signal, tracking error signal, and reproduction signal are generated by an arithmetic circuit (not shown). The generated defocus signal and tracking error signal are fed back to the objective lens actuator 20 by the recording / reproducing apparatus on which the optical pickup 1 is mounted, and controlled so that the focused spot follows the information track even when the disk rotates. The

  On the other hand, in the case of DVD / CD recording / reproduction, a red laser beam or an infrared laser beam is emitted from the red semiconductor laser chip 25 for DVD or the infrared semiconductor laser chip 26 for CD in the two-wavelength semiconductor laser package 24. . However, these are not radiated at the same time, and only one of them is radiated according to the disc to be recorded / reproduced. These laser beams are generated as sub-beams for tracking by the two-wavelength grating 27 for generating three beams for DVD / CD tracking, and are reflected by the dichroic prism 14 and the beam splitter 15 and converted into parallel light by the collimating lens 16. The The parallel light is reflected by the rising prism 19 and enters the objective lens 100, and the red laser light is focused on the recording film through the DVD transparent medium layer 205 and the infrared laser light is passed through the CD transparent medium layer 206.

  The red or infrared laser light reflected from the recording film returns to the optical path of the objective lens 100, the rising prism 19, and the collimator lens 16, passes through the beam splitter 15, passes through the concave lens 22, and is used for BD signal reproduction. It is condensed on the OEIC 23. Then, similarly to the BD, a defocus signal, a tracking error signal, and a reproduction signal are generated by the photocurrent signal from the divided light detection region.

  Here, beam diameters for BD, DVD, and CD will be described. The parallel beams for DVD and CD that are incident on the objective lens 100 are irradiated to a range wider than the effective diameter of the DVD and CD, respectively, but only the light in the effective diameter range is effectively condensed on the recording film surface. So that That is, light outside the effective diameter range is not condensed but diffused over a wide range on the disk, so that there is practically no problem in recording and reproduction. However, if the amount of light outside the effective diameter range is too large, the light utilization efficiency of the semiconductor laser light is reduced, and it becomes difficult to collect high power for recording. In order to avoid this, it is desirable that the divergence angle of the emitted light from the semiconductor laser chip is reduced in proportion to the effective diameter of the DVD or CD in proportion to the effective diameter. In other words, since the effective diameter has a relationship of BD> DVD> CD, the light use efficiency with respect to the effective diameter can be made uniform at three wavelengths by setting the emission angle of the semiconductor laser light to BD> DVD> CD.

  The objective lens 100 mounted in the present embodiment is an objective lens that collects and collimates parallel light for BD, DVD, and CD, and no aberration occurs even if the objective lens is decentered from the optical axis in accordance with the tracking operation. Since one optical lens can record and reproduce three optical recording media (BD, DVD, CD), it is effective for reducing the size and weight of the optical pickup and reducing the price.

1: Optical pickup
2: Spindle motor
11: Blue-violet semiconductor laser package,
12: Blue-violet semiconductor laser chip (for BD),
13: Grating,
14: Dichroic prism,
15: Beam splitter,
16: Collimating lens
17: Moving element,
18: Stepping motor,
19: Launch prism,
20: Objective lens actuator,
22: concave lens,
23: OEIC,
24: two-wavelength semiconductor laser package,
25: Red semiconductor laser chip (for DVD),
26: Infrared semiconductor laser chip (for CD),
27: Two-wavelength grating,
28: temperature sensor,
100: objective lens,
101: First side,
102: Second side,
103: 1st area | region (3 wavelength shared area | region) of the 1st surface,
104: 2nd area | region (BD / DVD shared area) of the 1st surface,
105: 3rd area | region (BD exclusive area | region) of the 1st surface,
106: 1st area | region (3 wavelength shared area | region) of a 2nd surface,
107: 2nd area | region (BD / DVD shared area) of the 2nd surface,
108: 3rd area | region (BD exclusive area | region) of the 2nd surface,
109: a light beam incident on the outermost periphery of the first region
110: Light incident on the outermost periphery of the second region,
111: Rays incident on the outermost periphery of the third region,
112, 112 ′: the first annular phase shift structure and the optical path difference distribution thereby,
113, 113 ′: second annular phase shift structure and optical path difference distribution thereby,
114, 114 ′: Third annular phase shift structure and optical path difference distribution by the third annular phase shift structure,
115: The aspherical shape of the lens before adding the annular phase shift structure,
116: Actual lens surface shape,
117, 117 ′: Fourth annular zone phase shift structure and optical path difference distribution thereby,
118: a lens surface shape obtained by extending the lens surface shape of the first region of the second surface to the second region;
119: a lens surface shape obtained by extending the lens surface shape of the second region of the second surface to the third region,
120: Lens surface shape of the third region of the second surface,
130: optical axis,
201: incident light flux for BD,
202: incident light flux for DVD,
203: incident luminous flux for CD,
204: BD transparent medium layer (thickness t1),
205: DVD transparent medium layer (thickness t2),
206: CD transparent medium layer (thickness t3),
401: Phase shift distribution of first annular phase shift structure before synthesis;
402: Phase shift distribution of second annular phase shift structure before synthesis,
403: Phase shift distribution when two phase shift distributions are simply combined,
404: Phase shift distribution adjusted to eliminate the narrow ring zone resulting from the synthesis;
405: the region where the two phase shift boundaries are close,
406: narrow ring zone,
407: Stepped portion after adjustment.

Claims (18)

A single recording / reproducing information by condensing laser beams of three wavelengths λ1, λ2, λ3 (λ1 <λ2 <λ3) on the first, second, and third optical information recording media, respectively. In the objective lens,
The three wavelengths of laser light are respectively incident as parallel light, and transparent medium layers having thicknesses t1, t2, and t3 (t1 <t2 <t3) of the first, second, and third optical information recording media, respectively. Are condensed with numerical apertures of NA1, NA2, and NA3 (NA1>NA2> NA3), respectively.
First, second, which are composed of a plurality of annular zones divided by a plurality of concentric circles centered on the optical axis, give different phase differences depending on the wavelength to adjacent regions, and give uniform phase differences in the regions A third lens zone phase shift structure is superimposed on one lens surface, the innermost first lens region;
An annular arrangement arranged outside the first lens region, comprising a plurality of annular zones divided by a plurality of concentric circles centered on the optical axis, and giving different phase differences depending on the wavelength to adjacent regions. A second lens region to which a fourth annular phase shift structure that gives a uniform phase difference is added;
A third lens having an aspherical lens shape arranged in an annular shape outside the second lens region and having a wavefront aberration set so as to collect the laser beam having the wavelength λ1 through the transparent medium having the thickness t1; An area, and
In the first lens region, the lens action excluding the phase difference added by the first, second, and third annular phase shift structures has the same effect as the transparent medium layer of the first optical information recording medium. A third order laser beam having the same refractive index and having a wavelength λ1 is passed through a transparent medium having a thickness t4 (t1 <t4 <t2) intermediate between the thickness t1 of the transparent medium layer and the thickness t2 of the transparent medium layer. It is a lens surface set to condense without spherical aberration,
In the second lens region, the lens action excluding the phase difference added by the fourth annular phase shift structure has the same refractive index as that of the transparent medium layer of the first optical information recording medium. The laser beam having the wavelength λ1 is condensed without a third-order spherical aberration through a transparent medium having a thickness t5 (t1 <t5 <t2) intermediate between the thickness t1 of the transparent medium layer and the thickness t2 of the transparent medium layer. A lens surface set to
A laser beam having a wavelength λ2 incident on the first, second, and third lens regions is focused on the first optical information recording medium, and is incident on the first lens region. An objective lens, wherein light is condensed on the second optical information recording medium, and laser light having a wavelength λ3 incident on the first lens region is condensed on the third optical information recording medium. The objective lens according to claim 1,
The first, second, and third annular phase shift structures of the first lens region combine the phase shift functions of these annular zones to obtain the wavelengths of λ1, λ2, and λ3, respectively, and t1, For both of the transparent medium layers having thicknesses t2 and t3, the wavefront aberration before the phase shift due to the annular phase shift structure is reduced while the wavefront aberration is reduced to a peak to peak value of less than 1λ. In the RMS wavefront aberration value, the defocus component is larger than the third-order spherical aberration component, the peak to peak value of the wavefront aberration including the defocus component is 5λ or more, and the wavefront aberration is the wavelength of λ1, λ2, and λ3. It is a zonal phase shift structure in which the sign is inverted with respect to the wavelength,
The fourth annular phase shift structure of the second lens region has a peak-to-peak value for wavefront aberration for each of the wavelengths λ1 and λ2 and the transparent medium having a thickness of t1 and t2. And the wavefront aberration before adding the phase shift due to the annular phase shift structure in the region combined with the first lens region is smaller than the third-order spherical aberration component in the RMS wavefront aberration value. The focus component is large, and the peak to peak value of the wavefront aberration including the defocus component is 10λ or more. The sign of the wavefront aberration is inverted between the wavelength of λ1 and the wavelength of λ2, and at the wavelength of λ3 An objective lens having an annular phase shift structure in which a wavefront aberration at a focal position of a wavelength of λ3 in the first lens region is not reduced to a Peak to Peak value of less than 1λ. The objective lens according to claim 1 or 2,
An optical path difference (φ) within ± (1/2) wavelength is obtained by removing an optical path length (mλ) corresponding to an integer wavelength from the optical path difference (mλ + φ) added in the first to fourth annular phase shift structures. An objective lens, wherein the absolute value of the integer value (m) is 3 or less. A single recording / reproducing information by condensing laser beams of three wavelengths λ1, λ2, λ3 (λ1 <λ2 <λ3) on the first, second, and third optical information recording media, respectively. In the objective lens,
The three wavelengths of laser light are respectively incident as parallel light, and transparent medium layers having thicknesses t1, t2, and t3 (t1 <t2 <t3) of the first, second, and third optical information recording media, respectively. Are condensed with numerical apertures of NA1, NA2, and NA3 (NA1> NA2> NA3), respectively.
First, second, which are composed of a plurality of annular zones divided by a plurality of concentric circles centered on the optical axis, give different phase differences depending on the wavelength to adjacent regions, and give uniform phase differences in the regions A third lens zone phase shift structure is superimposed on one lens surface, the innermost first lens region;
An annular arrangement arranged outside the first lens region, comprising a plurality of annular zones divided by a plurality of concentric circles centered on the optical axis, and giving different phase differences depending on the wavelength to adjacent regions. A second lens region to which a fourth annular phase shift structure that gives a uniform phase difference is added;
A third lens having an aspherical lens shape arranged in an annular shape outside the second lens region and having a wavefront aberration set so as to collect the laser beam having the wavelength λ1 through the transparent medium having the thickness t1; An area, and
In the second lens region, an optical path difference that is substantially an integral multiple of both of the wavelengths λ1 and λ2 is given, and an absolute value of a substantial phase difference obtained by removing the integer wavelength for the wavelength of λ3. A fifth annular phase shift structure in which the phase difference is adjusted so that is equal to or greater than 0.3λ,
A laser beam having a wavelength λ2 incident on the first, second, and third lens regions is focused on the first optical information recording medium, and is incident on the first lens region. An objective lens, wherein light is condensed on the second optical information recording medium, and laser light having a wavelength λ3 incident on the first lens region is condensed on the third optical information recording medium. A single recording / reproducing information by condensing laser beams of three wavelengths λ1, λ2, λ3 (λ1 <λ2 <λ3) on the first, second, and third optical information recording media, respectively. In the objective lens,
The three wavelengths of laser light are respectively incident as parallel light, and transparent medium layers having thicknesses t1, t2, and t3 (t1 <t2 <t3) of the first, second, and third optical information recording media, respectively. Are condensed with numerical apertures of NA1, NA2, and NA3 (NA1> NA2> NA3), respectively.
First, second, which are composed of a plurality of annular zones divided by a plurality of concentric circles centered on the optical axis, give different phase differences depending on the wavelength to adjacent regions, and give uniform phase differences in the regions A third lens zone phase shift structure is superimposed on one lens surface, the innermost first lens region;
An annular arrangement arranged outside the first lens region, comprising a plurality of annular zones divided by a plurality of concentric circles centered on the optical axis, and giving different phase differences depending on the wavelength to adjacent regions. A second lens region to which a fourth annular phase shift structure that gives a uniform phase difference is added;
A third lens having an aspherical lens shape arranged in an annular shape outside the second lens region and having a wavefront aberration set so as to collect the laser beam having the wavelength λ1 through the transparent medium having the thickness t1; An area, and
In the third lens region, an optical path difference that is substantially an integral multiple of the wavelength of λ1 is given, and an absolute value of a substantial phase difference obtained by removing the integer wavelength for the wavelengths of λ2 and λ3 A sixth annular phase shift structure in which the phase difference is adjusted to be 0.3λ or more is added,
A laser beam having a wavelength λ2 incident on the first, second, and third lens regions is focused on the first optical information recording medium, and is incident on the first lens region. An objective lens, wherein light is condensed on the second optical information recording medium, and laser light having a wavelength λ3 incident on the first lens region is condensed on the third optical information recording medium. The objective lens according to claim 4 or 5,
5. The objective lens according to claim 5, wherein the fifth or sixth annular phase shift structure has phase shift signs alternately reversed in adjacent annular zones. The objective lens according to claim 1 or 2,
The objective lens, wherein the first to fourth annular phase shift structures are formed on an incident side surface of the objective lens. The objective lens according to claim 4 or 5,
The objective lens, wherein the fifth or sixth annular phase shift structure is formed on an emission side surface of the objective lens. A single recording / reproducing information by condensing laser beams of three wavelengths λ1, λ2, λ3 (λ1 <λ2 <λ3) on the first, second, and third optical information recording media, respectively. In the objective lens,
The three wavelengths of laser light are respectively incident as parallel light, and transparent medium layers having thicknesses t1, t2, and t3 (t1 <t2 <t3) of the first, second, and third optical information recording media, respectively. Are condensed with numerical apertures of NA1, NA2, and NA3 (NA1> NA2> NA3), respectively.
First, second, which are composed of a plurality of annular zones divided by a plurality of concentric circles centered on the optical axis, give different phase differences depending on the wavelength to adjacent regions, and give uniform phase differences in the regions A third lens zone phase shift structure is superimposed on one lens surface, the innermost first lens region;
An annular arrangement arranged outside the first lens region, comprising a plurality of annular zones divided by a plurality of concentric circles centered on the optical axis, and giving different phase differences depending on the wavelength to adjacent regions. A second lens region to which a fourth annular phase shift structure that gives a uniform phase difference is added;
A third lens having an aspherical lens shape arranged in an annular shape outside the second lens region and having a wavefront aberration set so as to collect the laser beam having the wavelength λ1 through the transparent medium having the thickness t1; An area, and
The zone width is 3 μm or less so that the minimum value of the zone width projected on the entrance pupil plane of the zone area that is not adjacent to the boundary between the first, second, and third lens regions is not 3 μm or less. Annulus is an annulus that combines with the adjacent annulus and gives a phase difference that is the sum of the phase differences of the annulus.
A laser beam having a wavelength λ2 incident on the first, second, and third lens regions is focused on the first optical information recording medium, and is incident on the first lens region. An objective lens, wherein light is condensed on the second optical information recording medium, and laser light having a wavelength λ3 incident on the first lens region is condensed on the third optical information recording medium. The objective lens according to claim 4,
The objective lens, wherein the fifth annular phase shift structure of the second lens region is added to a different surface from the fourth annular phase shift structure. The objective lens according to claim 5,
The objective lens, wherein the sixth annular phase shift structure of the third lens region is added to the same surface as the fifth annular phase shift structure of the second lens region. The objective lens according to claim 4,
The fifth annular phase shift structure of the second lens region is added to the same plane as the fourth annular phase shift structure, and the annular zone position is aligned and only the annular zone height is flare-removed. An objective lens that is adjusted to have a phase difference of. A single recording / reproducing information by condensing laser beams of three wavelengths λ1, λ2, λ3 (λ1 <λ2 <λ3) on the first, second, and third optical information recording media, respectively. In the objective lens,
The three wavelengths of laser light are respectively incident as parallel light, and transparent medium layers having thicknesses t1, t2, and t3 (t1 <t2 <t3) of the first, second, and third optical information recording media, respectively. Are condensed with numerical apertures of NA1, NA2, and NA3 (NA1> NA2> NA3), respectively.
First, second, which are composed of a plurality of annular zones divided by a plurality of concentric circles centered on the optical axis, give different phase differences depending on the wavelength to adjacent regions, and give uniform phase differences in the regions A third lens zone phase shift structure is superimposed on one lens surface, the innermost first lens region;
An annular arrangement arranged outside the first lens region, comprising a plurality of annular zones divided by a plurality of concentric circles centered on the optical axis, and giving different phase differences depending on the wavelength to adjacent regions. A second lens region to which a fourth annular phase shift structure that gives a uniform phase difference is added;
A third lens having an aspherical lens shape arranged in an annular shape outside the second lens region and having a wavefront aberration set so as to collect the laser beam having the wavelength λ1 through the transparent medium having the thickness t1; An area, and
The thickness of the transparent medium layer that can be condensed through the transparent medium without third-order spherical aberration by the lens action excluding the phase difference added by the first to third annular phase shift structures in the first lens region. The thickness of the transparent medium layer t4 can be condensed through the transparent medium layer without third-order spherical aberration by the lens action excluding the phase difference added by the fourth annular phase shift structure in the second lens region. It is set to be at least t5,
A laser beam having a wavelength λ2 incident on the first, second, and third lens regions is focused on the first optical information recording medium, and is incident on the first lens region. An objective lens, wherein light is condensed on the second optical information recording medium, and laser light having a wavelength λ3 incident on the first lens region is condensed on the third optical information recording medium. A single recording / reproducing information by condensing laser beams of three wavelengths λ1, λ2, λ3 (λ1 <λ2 <λ3) on the first, second, and third optical information recording media, respectively. In the objective lens,
The three wavelengths of laser light are respectively incident as parallel light, and transparent medium layers having thicknesses t1, t2, and t3 (t1 <t2 <t3) of the first, second, and third optical information recording media, respectively. Are condensed with numerical apertures of NA1, NA2, and NA3 (NA1> NA2> NA3), respectively.
First, second, which are composed of a plurality of annular zones divided by a plurality of concentric circles centered on the optical axis, give different phase differences depending on the wavelength to adjacent regions, and give uniform phase differences in the regions A third lens zone phase shift structure is superimposed on one lens surface, the innermost first lens region;
An annular arrangement arranged outside the first lens region, comprising a plurality of annular zones divided by a plurality of concentric circles centered on the optical axis, and giving different phase differences depending on the wavelength to adjacent regions. A second lens region to which a fourth annular phase shift structure that gives a uniform phase difference is added;
A third lens having an aspherical lens shape arranged in an annular shape outside the second lens region and having a wavefront aberration set so as to collect the laser beam having the wavelength λ1 through the transparent medium having the thickness t1; An area, and
A virtual area expanded to a region where light transmitted through the second lens region is emitted using a coefficient describing an aspherical shape of a region where light transmitted through the first lens region is emitted on the emission surface. With respect to the shape of the exit surface, the actual aspheric shape of the region from which the light transmitted through the second lens region exits is an aspheric shape having a significant difference such that the sag amount is 5λ or more. There,
A laser beam having a wavelength λ2 incident on the first, second, and third lens regions is focused on the first optical information recording medium, and is incident on the first lens region. An objective lens, wherein light is condensed on the second optical information recording medium, and laser light having a wavelength λ3 incident on the first lens region is condensed on the third optical information recording medium. A single recording / reproducing information by condensing laser beams of three wavelengths λ1, λ2, λ3 (λ1 <λ2 <λ3) on the first, second, and third optical information recording media, respectively. In the objective lens,
The three wavelengths of laser light are respectively incident as parallel light, and transparent medium layers having thicknesses t1, t2, and t3 (t1 <t2 <t3) of the first, second, and third optical information recording media, respectively. Are condensed with numerical apertures of NA1, NA2, and NA3 (NA1> NA2> NA3), respectively.
First, second, which are composed of a plurality of annular zones divided by a plurality of concentric circles centered on the optical axis, give different phase differences depending on the wavelength to adjacent regions, and give uniform phase differences in the regions A third lens zone phase shift structure is superimposed on one lens surface, the innermost first lens region;
An annular arrangement arranged outside the first lens region, comprising a plurality of annular zones divided by a plurality of concentric circles centered on the optical axis, and giving different phase differences depending on the wavelength to adjacent regions. A second lens region to which a fourth annular phase shift structure that gives a uniform phase difference is added;
A third lens having an aspherical lens shape arranged in an annular shape outside the second lens region and having a wavefront aberration set so as to collect the laser beam having the wavelength λ1 through the transparent medium having the thickness t1; An area, and
The virtual surface is extended to the region where the light transmitted through the third lens region is emitted using a coefficient describing the aspherical shape of the region where the light transmitted through the second lens region is emitted on the emission surface. With respect to the shape of the exit surface, the actual aspheric shape of the region where the light transmitted through the third lens region exits is an aspheric shape having a significant difference such that the sag amount is 5λ or more. There,
A laser beam having a wavelength λ2 incident on the first, second, and third lens regions is focused on the first optical information recording medium, and is incident on the first lens region. An objective lens, wherein light is condensed on the second optical information recording medium, and laser light having a wavelength λ3 incident on the first lens region is condensed on the third optical information recording medium. In the optical pickup equipped with the objective lens according to any one of claims 1 to 15,
A semiconductor laser that emits laser light having three wavelengths λ1, λ2, and λ3;
A collimating lens that converts the laser light into a parallel beam and guides it to the objective lens, and is movable in the optical axis direction;
A beam splitter that separates the reflected light reflected and reflected by the objective lens on the first, second, and third optical information recording media from the optical path from the semiconductor laser;
An optical pickup comprising a photodetector for detecting the signal by collecting the separated reflected light. The optical pickup according to claim 16, wherein
A temperature sensor is arranged around the objective lens, and the position of the collimating lens is controlled according to the detected temperature. The optical pickup according to claim 16, wherein
Light having a relationship of θ1> θ2> θ3, where θ1, θ2, and θ3 are divergence angles of laser beams of three wavelengths λ1, λ2, and λ3 emitted from the semiconductor laser. pick up.

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