JP2007305212A - Light condensing optical system for optical pickup and optical pickup apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light condensing optical system for an optical pickup suppressing reduction of light utilization efficiency and reducing change of performance due to focusing. <P>SOLUTION: The light condensing optical system for the optical pickup is so constituted that a luminous flux 131 of a first wavelength λ of a first optical information recording medium is made incident in an objective lens 112 as nearly parallel light, and a luminous flux 132 of a second wavelength λ of a second optical information recording medium is made incident in the objective lens 112 as finite divergent light. A spherical aberration generated due to the difference between thicknesses of transparent protective substrates of the first and the second optical information recording mediums is corrected by a non-periodical step difference part 141 having annular multiple stages formed on the objective lens 112. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention provides reproduction and recording using light of a first wavelength and light of a second wavelength different from the first wavelength for each of two types of optical information recording media having different thicknesses of transparent protective substrates. The present invention relates to a condensing optical system for an optical pickup and an optical pickup device that are used in an optical pickup that performs at least one of them.

  An optical information recording medium having a transparent protective substrate includes, for example, an optical disc. Two types of optical disks with different transparent protective substrate thicknesses are DVD (digital versatile disk) and CD (compact disk). DVD includes a recording type such as DVD-R, while CD includes a recording type such as CD-R. The thickness of the transparent protective substrate of DVD is 0.6 mm, the numerical aperture on the optical disc side required for reproduction is 0.6, and the numerical aperture on the optical disc side required for recording is 0.65. On the other hand, the transparent protective substrate thickness of the CD is 1.2 mm, the numerical aperture on the optical disc side required for reproduction is 0.43, and the numerical aperture on the optical disc side required for recording is 0.51. However, when information recorded on a CD-R or the like needs to be reproduced, it is generally set to 0.47 or more.

  Various techniques for recording and / or reproducing such different optical disks with a single condensing optical system have been proposed.

  As one of them, a technique for imparting a diffractive action to an objective lens of an optical pickup device has been proposed (see, for example, Patent Document 1 and Patent Document 2).

  Here, Patent Document 1 discloses that two types of diffraction structures are formed on the surface of the objective lens so as to cancel out spherical aberration caused by the difference in thickness of the transparent protective substrate with wavelength dependency caused by the difference in laser wavelength. Compatible with optical disks. Patent Document 2 improves the temperature characteristics of a DVD by providing a diffractive structure in an area necessary for DVD.

  The diffractive structure constituting the objective lens is generally a kinoform shape. The kinoform shape of the objective lens is generally a large number of ring zones with a minute width.

In addition to this, there is a condensing optical system for an optical pickup in which both surfaces of the objective lens are aspherical surfaces, making DVD and CD compatible and enabling reproduction or recording on any optical information recording medium. . In such a condensing optical system for an optical pickup, there is one in which spherical aberration is corrected in both DVD and CD by appropriately setting an imaging magnification of the objective lens alone (see, for example, Patent Document 3). .
Japanese Patent No. 3689266 Japanese Patent No. 3687328 JP-A-9-63100

  However, when achieving compatibility using an objective lens that utilizes diffractive action, the diffractive structure in manufacturing cannot be realized completely because the zone width is very small. For this reason, in a condensing optical system for an optical pickup that uses an objective lens having a diffractive structure and utilizing a diffractive action, there is a problem that the use efficiency of light decreases as the number of ring zones increases.

  On the other hand, in the focusing optical system for an optical pickup using an aspheric objective lens that enables compatibility between DVD and CD by the imaging magnification of the objective lens alone, when focusing the objective lens, There is a problem that the performance changes due to the change in the numerical aperture corresponding to the light condensed on the recording surface of the DVD or CD as the objective lens moves.

  Accordingly, an object of the present invention is to provide a condensing optical system and an optical pickup device for an optical pickup capable of suppressing a decrease in performance due to focusing of an objective lens while suppressing a decrease in light utilization efficiency. To do.

  In order to solve this problem, a condensing optical system for an optical pickup according to the first aspect of the present invention includes a first light source that emits a light beam having a first wavelength used in a first optical information recording medium. A second light source that emits a light beam of a second wavelength used in the second optical information recording medium, a collimator lens that converts the light beam of the first wavelength into substantially parallel light, and the light converted into substantially parallel light The light beam having the first wavelength is condensed on the first optical information recording medium with a first numerical aperture, and the light beam having the second wavelength is condensed on the second optical information recording medium with a second numerical aperture. A condensing optical system for an optical pickup comprising an objective lens for condensing, wherein the thickness of the transparent protective substrate of the first optical information recording medium is t1, and the transparent protective substrate of the second optical information recording medium , The first wavelength is λ1, the second wavelength is λ2, and the first optical information recording When the first numerical aperture corresponding to the light beam condensed on the recording medium is NA1, and the second numerical aperture corresponding to the light beam condensed on the second optical information recording medium is NA2, t1 <t2, λ1 <Λ2, NA1> NA2 are satisfied, and the second light source converts the light beam having the second wavelength into the finite divergent light after passing through the collimator lens to the objective lens. When the imaging magnification of the objective lens alone with respect to the incident light beam at this time is β, the relational expression −0.05 <β <−0.01 holds, A non-periodic first step portion having an annular shape having multiple stages is provided in a region corresponding to the second numerical aperture on at least one surface of the objective lens.

  As a result, the imaging magnification β of the objective lens alone with respect to the light beam of the second wavelength is made small as “−0.05 <β <−0.01”. Changes can be reduced. For example, as the absolute value of the difference in imaging magnification between the objective lens alone of the DVD as the first optical information recording medium and the CD as the second optical information recording medium increases, at least one off-axis characteristic deteriorates. However, extreme deterioration of off-axis characteristics can be suppressed by suppressing the CD imaging magnification as described above. In addition, since the wavefront aberration is corrected by the first stepped portion (a non-periodic annular zone-shaped stepped portion including multiple steps) formed in the region corresponding to the second numerical aperture, the light use efficiency is reduced. Can be suppressed. Furthermore, in the optical pickup optical system, the first optical information recording medium and the second optical recording medium having different transparent protective substrate thicknesses with a single objective lens that has high light utilization efficiency and can reduce a change in performance due to focusing. The optical information recording medium can be reproduced or recorded with light beams having different wavelengths.

  According to a second aspect of the present invention, a condensing optical system for an optical pickup of the present invention includes a first light source that emits a light beam having a first wavelength used in a first optical information recording medium, and a second optical information. A second light source that emits a light beam having a second wavelength used in a recording medium; a collimator lens that converts the light beam having the first wavelength into substantially parallel light; and a space between the second light source and the collimator lens. A divergence angle conversion lens disposed in the optical path for converting the divergence angle of the light beam having the second wavelength, and the light beam having the first wavelength converted into substantially parallel light is supplied to the first optical information recording medium. And a focusing optical system for an optical pickup, comprising: an objective lens that collects the light beam having the second wavelength on the second optical information recording medium with the second numerical aperture while condensing the light beam with the numerical aperture of 1 The thickness of the transparent protective substrate of the first optical information recording medium is t1, The thickness of the transparent protective substrate of the second optical information recording medium is t2, the first wavelength is λ1, the second wavelength is λ2, and the first light beam corresponds to the light beam condensed on the first optical information recording medium. Is NA1, and the second numerical aperture corresponding to the light beam condensed on the second optical information recording medium is NA2, the relational expressions of t1 <t2, λ1 <λ2, NA1> NA2 are obtained. In addition, the light flux of the second wavelength is converted into finite divergent light after passing through the divergence angle conversion lens and the collimator lens, and is incident on the objective lens. When the imaging magnification of the objective lens alone is β, a relational expression of −0.05 <β <−0.01 is established, and the second aperture on at least one surface of the objective lens A non-periodic ring-shaped acyclic zone consisting of multiple stages in the area corresponding to the number Having one of the stepped portion, characterized in that.

  As a result, the imaging magnification β of the objective lens alone with respect to the light beam of the second wavelength is made small as “−0.05 <β <−0.01”. Changes can be reduced. For example, as the absolute value of the difference in imaging magnification between the objective lens alone of the DVD as the first optical information recording medium and the CD as the second optical information recording medium increases, at least one off-axis characteristic deteriorates. However, extreme deterioration of off-axis characteristics can be suppressed by suppressing the CD imaging magnification as described above. In addition, since the wavefront aberration is corrected by the first stepped portion (a non-periodic annular zone-shaped stepped portion including multiple steps) formed in the region corresponding to the second numerical aperture, the light use efficiency is reduced. Can be suppressed. Furthermore, in the optical pickup optical system, the first optical information recording medium and the second optical recording medium having different transparent protective substrate thicknesses with a single objective lens that has high light utilization efficiency and can reduce a change in performance due to focusing. The optical information recording medium can be reproduced or recorded with light beams having different wavelengths. The use efficiency of the second optical information recording medium is greatly increased by arranging the divergence angle conversion lens. As a result, recording on the second optical information medium is very advantageous, and when used for reproduction, the output of the second light source can be reduced and power consumption can be reduced.

  According to a third aspect of the present invention, in the first or second aspect of the present invention, the first step portion generates a phase difference that is an integer multiple of 2π with respect to the light beam having the first wavelength. A predetermined phase difference different from a phase difference of an integral multiple of 2π is generated for the light beam having the second wavelength.

  As a result, the light beam having the first wavelength that passes through the first stepped portion has a phase difference that is an integral multiple of 2π with respect to the light beam having the first wavelength before passing through, so that the first light beam that passes through the first stepped portion. The wavefront of the light beam having the first wavelength is the same as that when there is no first step portion. In other words, the wavefront aberration of the light beam having the first wavelength is not affected. On the other hand, the light beam having the second wavelength that passes through the first step portion has a predetermined phase difference that is different from the phase difference that is an integral multiple of 2π with respect to the light beam having the second wavelength before the passage. The wavefront aberration for the light beam having the second wavelength can be corrected using a predetermined phase difference.

  According to a fourth aspect of the present invention, in the invention according to any one of the first to third aspects, the region corresponding to the first numerical aperture on the one surface corresponds to the second numerical aperture. In order to improve the temperature characteristics of the light flux having the first wavelength in the difference region from the region to be performed, the second step portion having a non-periodic annular zone having multiple stages is provided. Features.

  Accordingly, the second step portion provided in the difference area between the area corresponding to the first numerical aperture and the area corresponding to the second numerical aperture causes a temperature change with respect to the light flux of the first wavelength. The generated aberration can be suppressed.

  According to a fifth aspect of the present invention, in the invention according to any one of the first to fourth aspects, the thickness (t1) of the transparent protective substrate of the first optical information recording medium and the second light The thickness (t2) of the transparent protective substrate of the information recording medium is expressed by a relational expression of t1 = 0.6 mm and t2 = 1.2 mm, and the first wavelength (λ1) and the second wavelength (λ2). ) Is expressed by a relational expression of 630 nm <λ1 <700 nm and 750 nm <λ2 <820 nm, and the first numerical aperture (NA1) and the second numerical aperture (NA2) are 0.59 <NA1 <0. .68, 0.43 ≦ NA2 <0.56.

  Thereby, a condensing optical system for an optical pickup suitable for reproduction or recording of the first optical information recording medium and the second optical information recording medium can be realized.

  A sixth aspect of the present invention includes the condensing optical system according to any one of the first to fifth aspects, and light receiving means for receiving reflected light from the information recording surface of the optical information recording medium. An optical pickup device.

  Thereby, an optical pickup device with high light utilization efficiency can be provided.

  According to the present invention, since the imaging magnification β of the objective lens alone in the light beam of the second wavelength is made small as “−0.05 <β <−0.01”, focusing of the objective lens is performed. It is possible to reduce the change in performance due to, and to suppress the extreme deterioration of off-axis characteristics. In addition, since the wavefront aberration is corrected by the first stepped portion (a non-periodic annular zone-shaped stepped portion including multiple steps) formed in the region corresponding to the second numerical aperture, the light use efficiency is reduced. Can be suppressed.

  Further, according to the present invention, since the amount of light emitted from the second light source can be sufficiently secured by using the divergence angle conversion lens, it is possible to increase the use efficiency in the second recording medium, or in some cases. The power used by the laser can be reduced.

  Further, according to the present invention, the light beam having the second wavelength passing through the first step portion has a predetermined phase difference different from the phase difference of an integer multiple of 2π with respect to the light beam having the second wavelength before passing. Therefore, the wavefront aberration for the light beam having the second wavelength can be corrected using the predetermined phase difference.

  Furthermore, according to the present invention, since the wavefront aberration is corrected by the first step portion formed in the region corresponding to the second numerical aperture, it is possible to suppress a decrease in light utilization efficiency.

  Further, according to the present invention, the second stepped portion provided in the difference area between the area corresponding to the first numerical aperture and the area corresponding to the second numerical aperture allows the light beam having the first wavelength to be Aberrations that occur with temperature changes can be suppressed.

  Furthermore, according to the present invention, an optical pickup device with high light utilization efficiency can be obtained by incorporating the condensing optical system into the optical pickup device.

  Hereinafter, the best mode for carrying out the present invention will be described more specifically with reference to the drawings. Here, the overlapping description of the same components is omitted in the accompanying drawings. In addition, since description here is the best form by which this invention is implemented, this invention is not limited to the said form.

  (Embodiment 1)

  The condensing optical system and the optical pickup device for an optical pickup according to the present invention have a first wavelength light beam and a first wavelength for each of two types of optical information recording media having different thicknesses of transparent protective substrates. It is used in an optical pickup that performs at least one of reproduction and recording using light beams having different second wavelengths.

  A first optical information recording medium having a first transparent protective substrate having a predetermined thickness, and a second optical information recording medium having a second transparent protective substrate having a thickness different from that of the first transparent protective substrate are: For example, an optical disk. Here, the first optical information recording medium is a DVD (Digital Versatile Disc) that is reproduced or recorded using a light beam having the first wavelength, and the second optical information recording medium is the second optical information recording medium. A CD (compact disc) that is reproduced or recorded using light of a wavelength.

  Here, the thickness of the first transparent protective substrate is t1, the thickness of the second transparent protective substrate is t2, the first wavelength is λ1, the second wavelength is λ2, and the light is collected on the first optical information recording medium. The first numerical aperture corresponding to the luminous flux is denoted by NA1, and the second numerical aperture corresponding to the luminous flux condensed on the second optical information recording medium is denoted by NA2.

  FIG. 1 is a diagram for explaining a condensing optical system for an optical pickup (hereinafter referred to as a condensing optical system), and FIGS. 2 to 4 illustrate an objective lens for the optical pickup (hereinafter referred to as an objective lens). The figure to show. 2 is a front view of the objective lens 112, FIG. 3 is a cross-sectional view of the objective lens 112, and FIG. The figure shown in FIG. The objective lens 112 in FIG. 2 is a front view of the objective lens 112 in FIG. 1 viewed from the surface on the light source side.

  As shown in FIG. 1, the condensing optical system has a light source (first light source) 11 that emits a light beam 131 having a wavelength λ1 of DVD, and the emitted light beam 131 includes a cover glass 101, a dichroic prism 102, and a collimator. After passing through the lens 103, it is converted into substantially parallel light. Further, a light beam having a width of the numerical aperture NA1 of the DVD is incident on the objective lens 112 by the diaphragm 111 and is condensed on the recording surface 122 via the transparent protective substrate 121 of the DVD. A CD light source (second light source) 12 is a light source that emits a light beam 132 having a wavelength λ2, and is converted into finite divergent light through a dichroic prism 102 and a collimator lens 103. The light flux of the CD becomes a light flux having a numerical aperture NA1 at the stop 111 and enters the objective lens, and is condensed on the recording surface 124 via the transparent protective substrate 123 of the CD. The diaphragm 111 and the objective lens 112 are moved together, and when focusing on the DVD recording surface, the first position 113, and when condensing on the CD recording surface, the second. To position 114.

  As shown in FIG. 3, the objective lens 112 has both a light source side surface and an optical disc side surface (second surface) formed as aspherical surfaces.

  The objective lens 112 is, for example, a resin-made biconvex aspheric lens, and has an area corresponding to the second numerical aperture NA2 required for reproduction or recording on a CD (that is, a value equal to or smaller than the second numerical aperture NA2). Area corresponding to the first numerical aperture NA1 required only for reproduction or recording on the DVD and an area corresponding to the second numerical aperture NA2 (that is, the second numerical aperture NA2). Area corresponding to a larger value).

  An area corresponding to the second numerical aperture NA2 (an area corresponding to a value equal to or smaller than the second numerical aperture NA2) on either one of the two surfaces of the objective lens 112, for example, the surface on the light source side, is shown in FIG. As shown, a first step 141 having, for example, nine ring zones is formed on the surface 142 of the reference lens.

  The reference lens described above is an aspherical lens before forming a step, and corresponds to a DVD.

  In addition, since the reference lens is compatible with DVD, the light flux of the CD that passes through the difference area that is larger than NA2 of the objective lens and smaller than NA1 becomes flare light that does not substantially contribute to the spot of the CD.

  Note that the number of annular zones formed in the area corresponding to the second numerical aperture NA2 (area corresponding to a value equal to or smaller than the second numerical aperture NA2) is the numerical aperture required for CD reproduction or recording. It is increased or decreased according to the imaging magnification of the objective lens alone.

  By the way, each annular zone of the first stepped portion 141 has a structure that generates a phase difference of an integral multiple of 2π with respect to the light flux having the first wavelength λ1.

The shift amount Dλ1 on the optical axis from the reference lens that causes a phase difference of 2π in absolute value with respect to the light beam having the first wavelength λ1 is expressed by the following mathematical formula 1.

Here, n _o λ1 is the refractive index of the original medium for the light flux with wavelength λ1, and n _n λ1 is the refractive index of the new medium for the light flux with wavelength λ1.

  The original medium is a medium that has filled the first stepped portion 141 before the first stepped portion 141 is formed and after the first stepped portion 141 is formed. On the other hand, the new medium is a medium that fills the first stepped portion 141 after the first stepped portion 141 is formed and after the first stepped portion 141 is formed.

  Now, the above-mentioned dimension “| Dλ1 |” shown in FIG. 4 is the absolute value of Dλ1 obtained by calculating the above equation 1. The wavefront of the light beam having the first wavelength λ1 passing through the first stepped portion 141 having the step as shown in FIG. 4 described above is substantially the same as the case where there is no first stepped portion 141.

  In the objective lens 112 according to the first embodiment, “thickness t1 of the first transparent protective substrate <thickness t2 of the second transparent protective substrate” = relational expression 1, “first wavelength λ1 <second wavelength λ2”. “= Relational expression 2,“ First numerical aperture NA1> Second numerical aperture NA2 ”= Relational expression 3 holds, and the light beam having the first wavelength λ1 is incident on the objective lens 112 as substantially parallel light. At the same time, when the light beam having the second wavelength λ2 is incident as finite divergent light and the imaging magnification of the objective lens alone in the light beam having the second wavelength λ2 is β, .05 <β <−0.01 ”on the condition that the relational expression 4 is satisfied, the second of the two surfaces of the objective lens 112 (for example, the light source side surface = the light source side surface) In a region corresponding to the numerical aperture NA2, a non-periodic annular zone-shaped step portion (first step) And so as to form a stepped portion) 141.

  The first step 141 generates a phase difference that is an integer multiple of 2π with respect to the light beam having the first wavelength λ1, and is different from the phase difference that is an integral multiple of 2π with respect to the light beam having the second wavelength λ2. It has a structure that generates a phase difference.

  In the first embodiment, each annular zone of the first step portion 141 is formed by a curved surface having a predetermined width with respect to the optical axis and a step having a predetermined length in the optical axis direction. . That is, each annular zone has a predetermined width W and a predetermined level difference H, and is formed in an annular shape.

  The predetermined step H is a value that is an integral multiple of the above-mentioned Dλ1. In FIG. 3, the predetermined step H is formed in the right direction (direction in which the light source side surface of the objective lens 112 is cut) and in the left direction (the light source of the objective lens 112). In some cases, it is formed in a direction in which the side surface is raised. Here, the predetermined level difference H is formed in the direction of cutting the light source side surface of the objective lens 112 (right direction in FIG. 3).

  The predetermined width W is not the same for all the annular zones, and at least one of the nine annular zones (annular zones 1 to 9) is different from the widths of the other annular zones.

  That is, the first step portion 141 (each ring zone) is formed in a non-periodic ring zone shape including multiple stages as described above.

By the way, the phase difference Φ2 generated in the light beam having the second wavelength λ2 when the light beam having the second wavelength λ2 enters the first stepped portion 141 is expressed by the following mathematical formula 2.

Here, n _o λ2 is the refractive index of the original medium for the light flux with wavelength λ2, and n _n λ2 is the refractive index of the new medium for the light flux with wavelength λ2.

  From the above formula 2, when the original medium or the new medium is air, the phase generated in the light flux of the second wavelength λ2 is not an integer multiple of 2π (a predetermined difference that is different from the phase difference of an integer multiple of 2π). Phase difference). By utilizing this phase shift, it becomes possible to correct the wavefront aberration of the light beam having the second wavelength λ2. Further, by making the first step portion 141 multistage and non-periodic, the wavefront aberration of the light beam having the second wavelength λ2 can be within the diffraction limit.

By the way, the aspherical shape of the reference lens is expressed by the following mathematical formula 3.

The aspherical shape is considered in an orthogonal coordinate system in which the vertex of the surface is the origin and the optical axis direction is the Z axis. In Equation 3, h is a height from the optical axis (unit: mm), c is a paraxial curvature (c = 1 / r, r is a paraxial radius of curvature (unit: mm)), k is a conical coefficient, A _2i Denotes a 2i-order aspheric coefficient, and Z _j denotes a Sag amount (unit: mm) of the surface j at a height h from the optical axis. In Equation 3, the numerical value i is an integer.

Here, the optical constants of the condensing optical system shown in FIG. 1 are shown in Table 1, the arrangement of the optical elements is shown in Table 2, the aspherical data of the collimator lens 103 is shown in Table 3, and FIGS. The aspherical data of the reference lens in the objective lens 112 shown in Table 4 is shown in Table 4, and the data representing the annular structure (the first step portion 141 composed of the nine annular zones 1 to 9) is shown. As shown in FIG.

  In Table 1, the numerical aperture of the CD is 0.51, and this numerical aperture is the size of the numerical aperture that contributes substantially to light collection in the spot formation of the CD as described above. From Table 1, it can be seen that the above-described relational expressions 1 to 4 are established. Incidentally, the fact that these relational expressions hold is specifically exemplified below.

  Regarding transparent protective substrate thickness (mm), t1 = 0.6 <t2 = 1.2

  For wavelength (nm), λ1 = 660 <λ2 = 785

  Regarding the numerical aperture, NA1 = 0.65> NA2 = 0.51

-0.05 <β = −0.02 <−0.01 with respect to the imaging magnification of the objective lens alone

In the DVD arrangement of Table 2, the stop and the objective lens are at the first position 113 in FIG. In the CD arrangement, the position of the stop and the objective lens is the second position 114 in FIG.

  In Tables 3 and 4, in the first embodiment, the aspheric coefficients of A12 to A20 are zero.

Furthermore, in the columns corresponding to A4 to A10 in Tables 3 and 4, “E and the subsequent number” represent “power of 10”, and the numerical value is multiplied by the immediately preceding numerical value (Tables 8 and 9 described later). The same applies to Tables 13 and 14.) For example, “−9.029E-5” in A4 of the surface number 3 represents (−9.029) × 10 ⁻⁵ .

  In Table 5, the unit of “the height at which the annular zone starts” is “mm”, the unit of “the height at which the annular zone ends” is “mm”, and “on the optical axis from the reference lens” The unit of “shift amount” is “μm”. Further, | Dλ1 | = 1.219 μm.

  The sign in “shift amount on the optical axis from the reference lens” in Table 5 means a step that is positive in the direction of cutting with respect to the surface 142 of the reference lens (for example, the right direction in FIG. 3), Negative means a step in the direction of raising with respect to the surface 142 of the reference lens (for example, the left direction in FIG. 3).

  In the first embodiment, as can be seen from Table 5, the steps are formed in the direction of cutting (for example, the right direction in FIG. 3), but the steps are formed in the rising direction (for example, the left direction in FIG. 3). In some cases.

  FIG. 5 shows the axial OPD aberration in the condensing optical system. (A) shows the OPD aberration with respect to the light beam having the first wavelength λ1, and (b) shows the second wavelength λ2. The OPD aberration with respect to the luminous flux is shown. In FIG. 5, WFA represents the magnitude of wavefront aberration in λrms.

  Here, it can be seen that the wavefront relating to the light beam having the first wavelength λ1 shown in FIG. 5A is continuous. From this, it can be seen that the wavefront is not affected even if a non-periodic annular zone-shaped step portion (first step portion 141) having multiple stages is formed on the reference lens of the objective lens corresponding to the DVD.

  On the other hand, the wavefront for the light flux having the second wavelength λ2 shown in FIG. 5B is a discontinuous wavefront. A non-periodic annular zone-shaped first step portion 141 having multiple stages is formed on the reference lens, whereby a predetermined phase difference due to a difference in wavelength is generated, and the wavefront is corrected discontinuously. The wavefront aberration at the continuous wavefront before correction was 0.200 λrms, but is corrected to 0.042 λrms as shown in FIG.

  As described above, the light flux having the first wavelength λ1 (DVD light flux) enters the objective lens 112 as substantially parallel light, and the second wavelength λ2 light flux (CD light flux) is incident as finite divergent light. When the imaging magnification of the objective lens alone in the light beam having the second wavelength λ2 is β, “−0.05 <β <−0.01” is satisfied and the second wavelength λ2 By correcting the wavefront aberration with respect to the light beam by the first step portion 141, the wavefront aberration with respect to the light beam with the first wavelength λ and the light beam with the second wavelength λ2 is within the diffraction limit in the range of both numerical apertures. be able to.

  By the way, in the first embodiment, the first step portion 141 is composed of nine ring zones, but the number of ring zones is necessary when reproducing or recording a CD. The numerical aperture is increased or decreased according to the image forming magnification of the objective lens alone, and is not limited to nine.

  In the first embodiment, the first step portion 141 is formed on the light source side, that is, the light source side surface. However, the present invention is not limited to this, and the first step portion 141 is formed. It may be formed on the surface of the optical information recording medium (optical disc) side, that is, the optical disc side.

  As described above, according to the first embodiment, the following effects (1) to (4) can be obtained.

  (1) Since the imaging magnification β of the objective lens alone in the light beam having the second wavelength λ2 is made small as “−0.05 <β <−0.01”, the performance of the objective lens due to focusing is reduced. Can be reduced.

  (2) Since the wavefront aberration is corrected by the first step portion 141 formed in the region corresponding to the second numerical aperture NA2, it is possible to suppress a decrease in light utilization efficiency.

  (3) Since the light beam having the first wavelength λ1 that passes through the first stepped portion 141 has a phase difference that is an integral multiple of 2π with respect to the light beam having the first wavelength λ1 before passing, the first stepped portion 141 The wavefront of the light beam having the first wavelength λ1 that passes through is the same as that in the case where the first step 141 is not provided. In other words, the wavefront aberration of the light beam having the first wavelength λ1 is not affected. On the other hand, the light beam having the second wavelength λ2 passing through the first stepped portion 141 has a predetermined phase difference different from the phase difference of an integer multiple of 2π with respect to the light beam having the second wavelength λ2 before passing through. Therefore, the wavefront aberration of the light beam having the second wavelength λ2 can be corrected using the predetermined phase difference.

  That is, the wavefront aberration with respect to the light beam with the first wavelength λ1 and the wavefront aberration with respect to the light beam with the second wavelength λ2 can be within the diffraction limit. This is very advantageous when recording on a medium.

  (4) In the condensing optical system for the optical pickup using the objective lens 112, the use efficiency of light is high, and one objective lens that can reduce the change in performance due to focusing has different transparent protective substrate thicknesses. The first optical information recording medium (for example, DVD) and the second optical information recording medium (for example, CD) can be reproduced or recorded with light beams having different wavelengths.

  (Embodiment 2)

  Next, a condensing optical system for an optical pickup according to Embodiment 2 (hereinafter referred to as a condensing optical system) will be described.

  Also in the second embodiment, it is assumed that the relational expressions 1 to 4 described in the first embodiment are satisfied.

  FIG. 6 is a diagram for explaining the condensing optical system, and FIGS. 7 to 10 are diagrams for explaining the objective lens for the optical pickup (hereinafter referred to as objective lens). 7 is a front view of the objective lens 212, FIG. 8 is a sectional view of the objective lens 212, and FIG. 9 is an enlarged view (schematic) of a portion (ring zone portion on the light source side) indicated by reference numeral B in FIG. FIG. 10 is an enlarged view (schematically illustrated) of a portion (ring zone portion on the light source side) indicated by reference sign D in FIG.

  The objective lens 212 in FIG. 7 is a front view of the objective lens 212 in FIG. 6 viewed from the surface on the light source side.

  The condensing optical system has the same configuration as the first embodiment as shown in FIG. However, in FIG. 6, with respect to the reference numerals assigned to the respective constituent elements, the light flux 231 having the wavelength λ1 of the DVD, the light source (first light source) 21 that emits the light flux, the light flux 232 having the wavelength λ2 of the CD, and the light flux Light source (second light source) 22, cover glass 201, dichroic prism 202, collimator lens 203, aperture 211 with numerical aperture NA1, objective lens 212, first position 213, second position 214, and transparent protection of DVD A substrate 221, a DVD recording surface 222, a CD transparent protective substrate 223, and a CD recording surface 224 are formed.

  Here, the second embodiment is a condensing optical system having the same configuration as the first embodiment, in which the objective lens is characterized.

  As in the case of Embodiment 1 described above, the objective lens 212 is aspherical on both the light source side surface (first surface) and the optical disk side surface (second surface), as shown in FIG. Is formed.

  The objective lens 212 is, for example, a resin-made biconvex aspheric lens, and has a region corresponding to the second numerical aperture NA2 required for reproduction or recording on a CD (that is, a value equal to or smaller than the second numerical aperture NA2). Area corresponding to the first numerical aperture NA1 required only for reproduction or recording on the DVD and an area corresponding to the second numerical aperture NA2 (that is, the second numerical aperture NA2). Area corresponding to a larger value).

  On one of the two surfaces of the objective lens 212, for example, the surface on the light source side, a phase difference that is an integral multiple of 2π is generated in the region corresponding to the second numerical aperture NA2 with respect to the light beam having the first wavelength λ1. A non-periodic annular zone-shaped first step portion 241a is formed, and a difference area between a region corresponding to the first numerical aperture NA1 and a region corresponding to the second numerical aperture NA2 Is for improving the temperature characteristics of the light beam having the first wavelength λ1, and is a non-periodic multi-stage circuit that generates a phase difference of an integral multiple of 2π with respect to the light beam having the first wavelength λ1. An annular zone-like second step portion 241b is formed.

  Specifically, the surface on the light source side of the objective lens 212 includes a first step portion 241 a having nine annular zones 1 to 9 and two zones 10 to 11. A second stepped portion 30b having a ring zone and eleven non-periodic stepped portions (a first stepped portion 241a and a second stepped portion 241b) are formed on the surface 242 of the reference lens. Has been. Here, as shown in FIG. 9, the first step portion 241a is a step portion having a step ring zone in the direction of cutting with reference to the surface 242 of the reference lens, and the second step portion 241b is shown in FIG. As shown in FIG. 4, the step portion has a stepped annular zone in the direction of raising with respect to the surface 242 of the reference lens.

  The first step portion 241a and the second step portion 241b have the same structure as the first step portion 141 of the first embodiment.

  In addition, as in the first embodiment, the reference lens is compatible with DVD. Since the second stepped portion is created to improve the temperature characteristics, even if the second stepped portion is configured as in the present embodiment, the wavefront of the CD that has passed through the stepped portion contributes to imaging. Not corrected enough. That is, the light flux of the CD that passes through the differential region that is larger than NA2 of the objective lens and smaller than NA1 becomes flare light that does not substantially contribute to the spot of the CD.

Here, optical constants of the condensing optical system shown in FIG. 6 are shown in Table 6, optical element arrangements are shown in Table 7, aspherical data of the collimator lens 203 are shown in Table 8, and FIGS. Table 9 shows the aspherical data of the reference lens in the objective lens 212 shown, and Table 10 shows the data representing the step portion 241 composed of the ring zone structure (11 zones of the zone 1 to zone 11).

  From Table 6, it can be seen that the above-described relational expressions 1 to 4 are established. Incidentally, the fact that these relational expressions hold is specifically exemplified below.

  Regarding transparent protective substrate thickness (mm), t1 = 0.6 <t2 = 1.2

  For wavelength (nm), λ1 = 660 <λ2 = 785

  Regarding the numerical aperture, NA1 = 0.65> NA2 = 0.51

-0.05 <β = −0.03 <−0.01 with respect to the imaging magnification of the objective lens alone

In the DVD arrangement of Table 7, the aperture and the objective lens are at the first position 213 in FIG. In the CD arrangement, the position of the aperture and the objective lens is the second position 214 in FIG.

In Tables 8 and 9, in Embodiment 2, the aspheric coefficients of A12 to A20 are zero.

  In Table 10, the unit of “the height at which the annular zone starts” is “mm”, the unit of “the height at which the annular zone ends” is “mm”, and “on the optical axis from the reference lens” The unit of “shift amount” is “μm”. Further, | Dλ1 | = 1.219 μm.

  The sign in “shift amount on the optical axis from the reference lens” in Table 10 means a step in the direction in which the positive is shaved with reference to the surface 242 of the reference lens (for example, the right direction in FIG. 8), Negative means a step in the direction of raising with respect to the surface 242 of the reference lens (for example, the left direction in FIG. 8).

  That is, the first stepped portion 241a is a stepped portion having a ring-shaped stepped portion in the direction of cutting with respect to the surface 242 of the reference lens (see FIG. 9). The second stepped portion 241b is a stepped portion having a stepped annular zone that is raised with respect to the surface 242 of the reference lens (see FIG. 10).

  FIG. 11 shows the axial OPD aberration in the condensing optical system, where (a) shows the OPD aberration with respect to the light beam having the first wavelength λ1, and (b) shows the second wavelength λ2. The OPD aberration with respect to the luminous flux is shown. In FIG. 11, WFA represents the magnitude of wavefront aberration by λrms.

  Here, it can be seen that the wavefront relating to the light flux having the first wavelength λ1 shown in FIG. 11A is continuous. From this, it can be seen that even if a non-periodic annular zone-shaped step portion (step portion 241) having multiple stages is formed on a reference lens corresponding to DVD, the wavefront is not affected.

  On the other hand, the wavefront relating to the light flux having the second wavelength λ2 shown in FIG. 11B is a discontinuous wavefront. By forming the first step portion 241 with a non-periodic annular zone having multiple stages on the reference lens, a predetermined phase difference due to a difference in wavelength is generated, and the wavefront is corrected discontinuously. The wavefront aberration at the continuous wavefront before correction was 0.200 λrms, but is corrected to 0.042 λrms as shown in FIG.

  By the way, generally, when a laser light source is used as the light source of the first wavelength λ1, the laser light source changes in wavelength by 0.2 nm / deg depending on the temperature.

  Therefore, in the second embodiment, by using this wavelength change, as described above, the difference area between the area corresponding to the first numerical aperture NA1 and the area corresponding to the second numerical aperture NA2 (that is, the first numerical aperture). By forming the second stepped portion 241b in a region corresponding to a value greater than the numerical aperture NA2 of 2, the temperature characteristics with respect to the light beam having the first wavelength λ1 are improved.

  FIG. 12 shows the light flux having the first wavelength λ1 when the temperature change is +50 deg from the aberration state shown in the first embodiment (FIG. 5) and the aberration state shown in the second embodiment (FIG. 11). FIG. 12A shows the OPD aberration with respect to the light flux having the first wavelength λ1 when the second stepped portion 241b is not present on the light source side surface as comparison data. (B) shows the OPD aberration with respect to the light beam having the first wavelength λ1 when the second step portion 241b is on the light source side surface. In FIG. 12, WFA represents the magnitude of wavefront aberration by λrms.

  Here, when the wavefront aberration for the light beam having the first wavelength λ1 shown in FIG. 12A is compared with the wavefront aberration for the light beam having the first wavelength λ1 shown in FIG. 12B, the former wavefront is compared. The aberration is 0.039 λrms, the latter wavefront aberration is 0.018 λrms, and the wavefront aberration in the case where the non-periodic annular zone-shaped second step portion 241b including multiple steps is formed on the reference lens is Wavefront aberration when the step 241b is not formed on the reference lens is corrected from 0.039 λrms to 0.018 λrms.

  As described above, the second embodiment has the following advantages in addition to the effects (1) to (4) of the first embodiment described above.

  That is, the second region is provided in a difference region between the region corresponding to the first numerical aperture NA1 and the region corresponding to the second numerical aperture NA2, and improves the temperature characteristic of the light flux having the first wavelength λ1. The stepped portion 241b can suppress aberration that occurs due to a temperature change with respect to the light flux having the first wavelength λ1.

  (Embodiment 3)

  Next, a condensing optical system for an optical pickup according to Embodiment 3 (hereinafter referred to as a condensing optical system) will be described.

  Also in the third embodiment, it is assumed that relational expressions 1 to 4 described in the first embodiment are satisfied.

  FIG. 13 is a diagram for explaining the condensing optical system, and FIGS. 14 to 17 are diagrams for explaining the objective lens for the optical pickup (hereinafter referred to as objective lens). 14 shows a front view of the objective lens 312, FIG. 15 shows a cross-sectional view of the objective lens 312, and FIG. 16 shows an enlarged view (schematic view) of a portion (ring zone portion on the light source side) indicated by reference numeral D in FIG. FIG. 17 is an enlarged view (schematically illustrated) of a portion (ring zone portion on the light source side) indicated by E in FIG.

  The objective lens 312 in FIG. 14 is a front view of the objective lens 312 in FIG. 13 viewed from the surface on the light source side.

  As shown in FIG. 13, the condensing optical system has a light source (first light source) 31 that emits a light beam 331 having a wavelength λ 1 of DVD. The emitted light beam 331 passes through a dichroic prism 301 and a collimator lens 302. After that, it is converted into substantially parallel light. Further, a light beam having a width of the numerical aperture NA1 of the DVD is incident on the objective lens 312 by the diaphragm 311 and is condensed on the recording surface 322 through the transparent protective substrate 321 of the DVD. A CD light source (second light source) 32 is a light source that emits a light beam 332 having a wavelength λ2, and is converted into finite divergent light through a cover glass 303, a divergence angle conversion lens 304, a dichroic prism 301, and a collimator lens 302. Converted. The light flux of the CD becomes a light flux having a numerical aperture NA1 at the stop 311 and enters the objective lens, and is condensed on the recording surface 324 through the transparent protective substrate 323 of the CD. The aperture 311 and the objective lens 312 are moved together, and the first position 313 is used when condensing on the DVD recording surface, and the second operation is used when condensing on the CD recording surface. The position 314 is moved.

  The objective lens 312 has the same configuration as that of the second embodiment as shown in FIGS.

Here, the optical constants of the condensing optical system shown in FIG. 13 are shown in Table 11, the arrangement of the optical elements is shown in Table 12, the aspherical data of the collimator lens 302 is shown in Table 13, and FIGS. Table 14 shows the aspherical data of the reference lens in the objective lens 312 shown, and Table 15 shows data representing the step portion 341 composed of the annular zone structure (the eleven annular zones 1 to 11.

  From Table 11, it can be seen that the relational expressions 1 to 4 described above are established. Incidentally, the fact that these relational expressions hold is specifically exemplified below.

  Regarding transparent protective substrate thickness (mm), t1 = 0.6 <t2 = 1.2

  For wavelength (nm), λ1 = 660 <λ2 = 785

  Regarding the numerical aperture, NA1 = 0.65> NA2 = 0.51

  -0.05 <β = −0.03 <−0.01 with respect to the imaging magnification of the objective lens alone

In Table 11, the NA on the light source side of the CD is 0.12. In Table 6 in Embodiment 2, since the NA on the light source side of the CD is 0.087, it can be seen that more light from the light source can be used by using the divergence angle conversion lens.

In the DVD arrangement of Table 12, the aperture and the objective lens are at the first position 313 in FIG. In the CD arrangement, the position of the stop and the objective lens is the second position 314 in FIG.

In Tables 13 and 14, in the third embodiment, the aspheric coefficients of A12 to A20 are zero.

  In Table 15, the unit of “the height at which the annular zone starts” is “mm”, the unit of “the height at which the annular zone ends” is “mm”, and “on the optical axis from the reference lens” The unit of “shift amount” is “μm”. Further, | Dλ1 | = 1.219 μm.

  The sign in “shift amount on the optical axis from the reference lens” in Table 15 means that the positive is a step in the direction of cutting with reference to the surface 342 of the reference lens (for example, the right direction in FIG. 15), Negative means a step in the direction of raising with respect to the surface 342 of the reference lens (for example, the left direction in FIG. 15).

  That is, the first stepped portion 341a is a stepped portion having a stepped annular zone in the direction of cutting with respect to the surface 342 of the reference lens (see FIG. 16). The second stepped portion 341b is a stepped portion having a stepped annular zone that is raised with respect to the surface 342 of the reference lens (see FIG. 17).

  FIG. 18 shows the axial OPD aberration in the condensing optical system, where (a) shows the OPD aberration with respect to the light beam having the first wavelength λ1, and (b) shows the second wavelength λ2. The OPD aberration with respect to the luminous flux is shown. In FIG. 18, WFA represents the magnitude of wavefront aberration by λrms.

  Here, it can be seen that the wavefront relating to the light beam having the first wavelength λ1 shown in FIG. 18A is continuous. From this, it can be seen that even if a non-periodic annular zone step portion (first step portion 341) having multiple stages is formed on a reference lens corresponding to DVD, the wavefront is not affected.

  On the other hand, the wavefront relating to the light flux having the second wavelength λ2 shown in FIG. 18B is a discontinuous wavefront. By forming the first non-periodic annular zone step 341 composed of multiple stages in the reference lens, a predetermined phase difference accompanying a difference in wavelength is generated, and the wavefront is corrected discontinuously. The wavefront aberration at the continuous wavefront before correction was 0.200 λrms, but is corrected to 0.042 λrms as shown in FIG.

  As described above, the third embodiment has the following advantages in addition to the effects (1) to (4) of the first embodiment described above.

  By using the divergence angle conversion lens, the utilization efficiency of the light source of the CD can be further increased.

  (Embodiment 4)

  Next, an optical pickup device (hereinafter referred to as an optical pickup device) according to Embodiment 4 will be described.

  The optical pickup device utilizes the condensing optical system in the present invention. In the fourth embodiment, an example of an optical pickup device using the condensing optical system in the third embodiment will be described.

  FIG. 19 is a diagram for explaining an optical pickup device according to the fourth embodiment.

  As shown in FIG. 19, the optical pickup device has a light source (first light source) 41 that emits a light beam 431 having a wavelength λ1 of DVD, and the emitted light beam 431 is reflected by the beam splitter 43 to be dichroic prism 401. After passing through the collimator lens 402, the light is converted into substantially parallel light. Further, a light beam having a width of the numerical aperture NA1 of the DVD is incident on the objective lens 412 by the stop 411 and is condensed on the recording surface 422 through the transparent protective substrate 421 of the DVD. The light beam reflected by the recording surface is received by the light receiving unit (light receiving unit) 44 through the objective lens 412, the collimator lens 402, the dichroic prism 401, and the beam splitter 43.

  A light beam 432 having a wavelength λ2 of CD is emitted from an integrated element 42 having a laser and a light receiving portion (light receiving means), and passes through a cover glass 403, a divergence angle conversion lens 404, a dichroic prism 401, and a collimator lens 402. Converted to finite divergent light. The light beam of the CD enters the objective lens 412 as a light beam having a numerical aperture NA1 through the stop 411 and is condensed on the recording surface 424 through the transparent protective substrate 423 of the CD. The aperture 411 and the objective lens 412 move together, and when focusing on the DVD recording surface, the first position 413, and when focusing on the CD recording surface, the second. The position 414 is moved. The light beam reflected by the recording surface is received by the light receiving portion of the integrated element 42 after passing through the objective lens 412, the collimator lens 402, the dichroic prism 401, the divergence angle conversion lens 404, and the cover glass 403.

  The light source 41 and the light receiving unit 44 of the DVD according to the fourth embodiment may be integrated elements. The beam splitter 43 may be a dichroic prism.

  The integrated element 42 of the CD may separate the light source and the light receiving unit.

  Further, the integrated element 42 of the CD may be a light source of the CD, and the light receiving unit 44 may be a light receiving unit used in common by the DVD and the CD.

  With such a configuration, an optical pickup device with high light utilization efficiency can be obtained.

  The present invention is a drive that performs at least one of reproduction and recording on an optical information recording medium such as a DVD-ROM, DVD-R, DVD-RW, DVD-RAM, CD-ROM, CD-R, and CD-RW. The present invention can be applied to a condensing optical system for an optical pickup used in the apparatus or an optical pickup apparatus.

1 is a diagram illustrating a condensing optical system for an optical pickup according to Embodiment 1. FIG. It is the front view which looked at the objective lens for optical pick-up shown in FIG. 1 from the light source side. It is sectional drawing which shows the cross section of the objective lens for optical pick-up shown in FIG. It is the enlarged view to which the 1st level | step-difference part of the objective lens for optical pick-up shown in FIG. 3 was expanded. FIG. 6 is a diagram illustrating an axial OPD aberration in the condensing optical system for the optical pickup according to the first embodiment. 6 is a diagram illustrating a condensing optical system for an optical pickup according to Embodiment 2. FIG. It is the front view which looked at the objective lens in the condensing optical system for optical pick-ups shown in FIG. 6 from the light source side. It is sectional drawing which shows the cross section of the objective lens in the condensing optical system for optical pick-ups shown in FIG. It is the enlarged view to which the 1st level | step difference part of the objective lens in the condensing optical system for optical pick-ups shown in FIG. 8 was expanded. It is the enlarged view to which the 2nd level | step-difference part of the objective lens in the condensing optical system for optical pick-ups shown in FIG. 8 was expanded. FIG. 10 is a diagram illustrating an axial OPD aberration in a condensing optical system for an optical pickup according to a second embodiment. OPD aberration of the light beam having the first wavelength λ1 when the temperature change is +50 deg with respect to the condensing optical system for the optical pickup according to the first embodiment and the condensing optical system for the optical pickup according to the second embodiment. It is a figure which shows FIG. 6 is a diagram illustrating a condensing optical system for an optical pickup according to a third embodiment. It is the front view which looked at the objective lens in the condensing optical system for optical pick-ups shown in FIG. 13 from the light source side. It is sectional drawing which shows the cross section of the objective lens in the condensing optical system for optical pick-ups shown in FIG. FIG. 16 is an enlarged view of a first step portion of the objective lens in the condensing optical system for the optical pickup shown in FIG. 15. FIG. 16 is an enlarged view of an enlarged second step portion of the objective lens in the condensing optical system for the optical pickup shown in FIG. 15. FIG. 10 is a diagram illustrating an axial OPD aberration in a condensing optical system for an optical pickup according to a second embodiment. FIG. 10 is a diagram for explaining an optical pickup device according to a fourth embodiment.

Explanation of symbols

11, 21, 31, 41 Light source (first light source)
12, 22, 32 Light source (second light source)
42 Integrated element 43 Beam splitter 44 Light receiving part (light receiving means)
101, 201 Cover glass 102, 202, 301, 401 Dichroic prism 103, 203, 302, 402 Collimator lens 303, 403 Cover glass 304, 404 Divergence angle conversion lens 111, 211, 311, 411 Aperture 112, 212 with numerical aperture NA1 , 312, 412 Objective lens 113, 213, 313, 413 First position 114, 214, 314, 414 Second position 121, 221, 321, 421 DVD transparent protective substrate 122, 222, 322, 422 DVD recording Surfaces 123, 223, 323, and 423 CD transparent protective substrates 124, 224, 324, and 424 CD recording surfaces 131, 231, 331, and 431 First wavelength beams 132, 232, 332, and 432 Second wavelength beams 141, 241a, 341, 341 Numerical aperture corresponding to the first step portion 241b, 341b second numerical aperture corresponding to the surface NA1 first wavelength light of the stepped portions 142,242,342 reference lens NA2 second wavelength light

Claims (6)

A first light source that emits a light beam of a first wavelength used in a first optical information recording medium; a second light source that emits a light beam of a second wavelength used in a second optical information recording medium; A collimator lens that converts a light beam having a wavelength of 1 into substantially parallel light, and a light beam having the first wavelength that has been converted into substantially parallel light is condensed on the first optical information recording medium with a first numerical aperture. An objective lens for condensing the light beam having the second wavelength on the second optical information recording medium with a second numerical aperture,
The thickness of the transparent protective substrate of the first optical information recording medium is t1, the thickness of the transparent protective substrate of the second optical information recording medium is t2, the first wavelength is λ1, the second wavelength is λ2, When NA1 is the first numerical aperture corresponding to the light beam condensed on the first optical information recording medium, and NA2 is the second numerical aperture corresponding to the light beam condensed on the second optical information recording medium. ,
t1 <t2,
λ1 <λ2,
NA1> NA2,
Each relational expression of
The second light source is disposed at a position where the light beam having the second wavelength is converted into finite divergent light after passing through the collimator lens and is incident on the objective lens, and the light beam with respect to the incident light beam at this time When the imaging magnification of the objective lens alone is β,
−0.05 <β <−0.01,
Is established,
Furthermore, it has an annular non-periodic first step portion consisting of a plurality of steps in a region corresponding to the second numerical aperture on at least one surface of the objective lens.
A condensing optical system for an optical pickup. A first light source that emits a light beam of a first wavelength used in a first optical information recording medium; a second light source that emits a light beam of a second wavelength used in a second optical information recording medium; A collimator lens that converts a light beam of one wavelength into substantially parallel light; and a divergence angle that is disposed in an optical path between the second light source and the collimator lens to convert a divergence angle of the light beam of the second wavelength. The conversion lens and the light beam having the first wavelength converted into substantially parallel light are condensed on the first optical information recording medium with a first numerical aperture, and the light beam having the second wavelength is condensed to the second light beam. A condensing optical system for an optical pickup comprising an objective lens for condensing the optical information recording medium with a second numerical aperture,
The thickness of the transparent protective substrate of the first optical information recording medium is t1, the thickness of the transparent protective substrate of the second optical information recording medium is t2, the first wavelength is λ1, and the second wavelength is λ2, NA1 is a first numerical aperture corresponding to the light beam condensed on the first optical information recording medium, and NA2 is a second numerical aperture corresponding to the light beam condensed on the second optical information recording medium. When
t1 <t2,
λ1 <λ2,
NA1> NA2,
Each relational expression of
The light beam having the second wavelength is converted into finite divergent light after passing through the divergence angle conversion lens and the collimator lens and enters the objective lens, and the objective lens alone with respect to the incident light beam at this time When the imaging magnification at is β,
−0.05 <β <−0.01,
Is established,
Furthermore, it has an annular non-periodic first step portion consisting of a plurality of steps in a region corresponding to the second numerical aperture on at least one surface of the objective lens.
A condensing optical system for an optical pickup. The first step portion is
Generating a phase difference of an integer multiple of 2π for the light flux of the first wavelength and generating a predetermined phase difference different from a phase difference of an integer multiple of 2π for the light flux of the second wavelength;
3. A condensing optical system for an optical pickup according to claim 1 or 2. For improving the temperature characteristics of the light flux of the first wavelength in the difference area between the area corresponding to the first numerical aperture and the area corresponding to the second numerical aperture on the one surface The condensing optical system for an optical pickup according to any one of claims 1 to 3, further comprising a non-periodic annular zone-shaped second step portion including multiple stages. The thickness (t1) of the transparent protective substrate of the first optical information recording medium and the thickness (t2) of the transparent protective substrate of the second optical information recording medium are:
t1 = 0.6 mm, t2 = 1.2 mm,
It is expressed by the relational expression of
The first wavelength (λ1) and the second wavelength (λ2) are:
630 nm <λ1 <700 nm, 750 nm <λ2 <820 nm,
It is expressed by the relational expression of
The first numerical aperture (NA1) and the second numerical aperture (NA2) are:
0.59 <NA1 <0.68, 0.43 ≦ NA2 <0.56,
Represented by the relational expression
The condensing optical system for an optical pickup according to claim 1, wherein the condensing optical system is an optical pickup. The condensing optical system according to any one of claims 1 to 5,
A light receiving means for receiving reflected light from the information recording surface of the optical information recording medium;
An optical pickup device comprising:

Images