JP2009104781A - Optical pickup device and objective lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an objective lens and an optical pickup device, for reproducing/recording data to/from two types of optical disks having transparent substrates different in thickness by light sources of two wavelengths different from each other, simultaneously reducing color aberrations for the optical disk of a thin transparent substrate, and halving residual aberrations for the optical disk of a thick transparent substrate. <P>SOLUTION: In the optical pickup device, light fluxes from first and second light sources 111 and 112 are converged on an information recording surface 22 by an objective lens 16 including a diffraction pattern via the transparent substrate 21 of a first or second optical disk 20 to record/reproduce information. The tertiary spherical aberration component of a wavefront aberration of a light flux via the transparent substrate of the second optical disk is over-corrected, the light flux coming from the second light source through the objective lens, where the numerical aperture on the optical disk side is equal to or less than NA2 necessary for reproducing/recording of the second optical disk having a thick transparent substrate. When the absolute value thereof is WSA2λ3rms, 0.02λ2rms≤WSA2≤0.06λ2rms is satisfied. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical pickup device and an objective lens capable of recording and / or reproducing a plurality of optical information recording media having different transparent substrate thicknesses by a condensing optical system including the objective lens.

  In recent years, with the practical use of short-wavelength red lasers, DVDs, which are high-density optical information recording media (also referred to as “optical discs”) that have the same size and large capacity as CDs (compact discs), have been commercialized. ing. In the DVD recording / reproducing apparatus, the numerical aperture NA on the optical disk side of the objective lens when a 650 nm semiconductor laser is used is set to 0.6 to 0.65. DVD has a track pitch of 0.74 μm and a shortest bit length of 0.4 μm, and has a density less than half that of a CD track pitch of 1.6 μm and a shortest pit length of 0.83 μm. Also, in DVD, the transparent substrate thickness is 0.6 mm, which is half of the CD transparent substrate thickness (1.2 mm), in order to suppress the coma aberration that occurs when the optical disk is tilted with respect to the optical axis. .

  In addition to the above-mentioned CD and DVD, optical discs of various standards such as different light source wavelengths and transparent substrate thicknesses, such as CD-R, RW (recordable compact disc), VD (video disc), MD (mini disc). ), MO (magneto-optical disk), etc. are also commercialized and popularized. Further, as the wavelength of semiconductor lasers has been shortened, a short wavelength blue laser having an oscillation wavelength of about 400 nm is being put to practical use. Since the wavelength is shortened, the capacity of the optical information recording medium can be further increased even if the same numerical aperture as that of the DVD is used.

  In addition, the thickness and recording of the transparent substrate on the recording surface, such as a CD-R that can be recorded and reproduced, and a DVD that has a higher recording density, are approximately the same size as a CD that is a conventional optical information recording medium as described above. Development of a plurality of optical information recording media having different wavelengths of reproduction laser light has progressed, and it has been demanded that these optical information recording media can be recorded and reproduced with the same optical pickup. For this reason, various types of optical pickups that have a plurality of laser light sources according to the wavelength used and converge the laser light with the necessary numerical aperture on the recording surface with the same objective lens have been proposed (for example, Japanese Patent Laid-Open No. 9-92). No. 5,4973, JP-A-11-96585, JP-A-11-86319, etc.).

  Among these, in JP-A-9-54973, 635 nm is transmitted light (0th order diffracted light), 785 nm is an optical system using a hologram optical element using -1st order diffracted light, and 635 nm is + 1st order diffracted light, 785 nm is An optical system using a hologram optical element using transmitted light (0th order diffracted light) is disclosed. However, according to this hologram optical element, the steps of the hologram are deep and it is difficult to integrate with the objective lens.

  Japanese Patent Laid-Open No. 11-96585 provides three division surfaces on the light source side refractive surface of the objective lens, and the first division surface and the third division surface are provided when reproducing the first optical disk. An optical pickup device that utilizes a light beam that passes through the first divided surface and a light beam that passes through the second divided surface when reproducing a second optical disk having a transparent substrate thickness different from that of the first optical disk. It is disclosed. However, according to this objective lens, the residual aberration is increased in the optical disk (for example, CD) with the thicker transparent substrate.

  In addition, in the Japanese Patent Application No. 11-312701, the present inventors previously used an objective lens provided with a diffractive ring zone on a refracting surface to form a diffraction surface and a refracting surface with respect to a plurality of light sources having different wavelengths. An optical pickup device has been proposed in which the action is canceled and the spherical aberration is corrected. In this case, chromatic aberration may occur if the wavelength changes in the light source having a shorter wavelength.

  An object of the present invention is to reproduce or record at least two kinds of optical information recording media having transparent substrates having different thicknesses by using light sources having at least two wavelengths different from each other, and to reduce chromatic aberration for an optical information recording medium having a thin transparent substrate. At the same time, it is an object of the present invention to provide an objective lens that can reduce the residual aberration by half for an optical information recording medium having a thick transparent substrate, and an optical pickup device having a condensing optical system including the objective lens.

In order to achieve the above object, an optical pickup device according to the present invention condenses a light beam from a light source on an information recording surface through a transparent substrate of an optical information recording medium by a condensing optical system including an objective lens. Is an optical pickup device configured to record or reproduce information on at least two types of optical information recording media having different transparent substrate thicknesses and recording densities, and having a wavelength λ1 (nm). 1 light source, a second light source having a wavelength λ2 (nm) (λ2> λ1), and light that receives reflected light from the optical information recording medium of the light flux emitted from the first light source and the second light source. NA1 is a required numerical aperture on the optical information recording medium side of the condensing optical system necessary for recording or reproducing the first optical information recording medium having a transparent substrate thickness t1 with a wavelength λ1. The thickness of the transparent substrate is t2. The required numerical aperture on the optical information recording medium side of the condensing optical system necessary for recording or reproducing the second optical information recording medium (t2> t1) at the wavelength λ2 is NA2 (NA2 <NA1) In addition, a diffraction pattern is provided on at least one surface of the condensing optical system, and m-order diffracted light from the diffraction pattern of the condensing optical system of the light beam from the first light source (where m is an integer) Is used to record and / or reproduce the first optical information recording medium having a transparent substrate thickness t1, and n from the diffraction pattern of the condensing optical system of the light beam from the second light source. By using at least the next diffracted light (where n is an integer and n = m = 0 is excluded), the second optical information recording medium having a transparent substrate thickness of t2 (where t2> t1) is obtained. Optical pick-up for recording and / or playback In the up-apparatus, when the numerical aperture on the optical information recording medium side of the light beam from the second light source that has passed through the objective lens passes through the transparent substrate of the second optical information recording medium at a portion of NA2 or less When the third-order spherical aberration component of the wavefront aberration is over and the absolute value is WSA2λ2rms,
0.02λ2rms ≦ WSA2 ≦ 0.06λ2rms
It is characterized by being.

  According to this optical pickup device, chromatic aberration can be reduced even with a diffraction pattern, and spherical aberration in the second optical information recording medium having a thick transparent substrate can be reduced.

  The m is an integer other than 0, and preferably n = m. The objective lens is preferably a single lens, and the diffraction pattern is preferably provided on the single lens.

  Further, when the information is recorded or reproduced on the first optical information recording medium, the imaging magnification viewed from the optical information medium side of the objective lens is M1, and the information is recorded or reproduced on the second optical information recording medium. It is preferable that M2 and M1 are substantially equal when the imaging magnification of the objective lens when viewed from the optical information recording medium side is M2. According to this, it becomes possible to unify the light receiver and to make one package of the first light source and the second light source, and the apparatus can be configured compactly. Further, it is preferable that M1 and M2 are substantially 0. According to this, it is easy to adjust the position of the light source.

  Further, the position where the light beam farthest from the optical axis among the light beams from the second light source transmitted through the objective lens converges through the transparent substrate of the second optical information recording medium is transmitted through the objective lens. From the position where the wavefront aberration when the numerical aperture on the optical information recording medium side is less than NA2 through the transparent substrate of the second optical information recording medium among the light flux from the second light source is minimized. It is preferable that the distance between the objective lens and the objective lens is 5 μm or more. If the difference is 5 μm or more, out of the light flux from the second light source, the aberration of the light flux having a numerical aperture of near NA1 increases, so that the beam spot is not excessively narrowed by the objective lens.

  The position at which the light beam farthest from the optical axis among the light beams from the second light source that has passed through the objective lens converges via the transparent substrate of the second optical information recording medium is Of the transmitted light flux from the second light source, the wavefront aberration when the numerical aperture on the optical information recording medium side is NA2 or less through the transparent substrate of the second optical information recording medium is minimized. More preferably, the position is farther from the objective lens than the position, and the difference is 15 μm or more. If the difference is 15 μm or more, among the light beams from the second light source, a light beam having a numerical aperture greater than about NA2 becomes a flare, so that the beam spot is not limited too much by the objective lens, and the aperture is limited. Is eliminated, and the condensing optical system is simplified.

Another optical pickup device according to the present invention condenses the light flux from the light source on the information recording surface via the transparent substrate of the optical information recording medium by the condensing optical system including the objective lens, and records or reproduces the information. And an optical pickup device for recording or reproducing information on at least two types of optical information recording media having different thicknesses and recording densities of a transparent substrate, comprising: a first light source having a wavelength λ1 (nm); A second light source having a wavelength λ2 (nm) (λ2> λ1), and a photodetector for receiving reflected light from the optical information recording medium of the luminous flux emitted from the first light source and the second light source. Provided, the required numerical aperture on the optical information recording medium side of the condensing optical system necessary for recording or reproducing the first optical information recording medium having a thickness t1 of the transparent substrate at a wavelength λ1 is NA1, and the transparent substrate Thickness is t2 (where t2> t 1) NA2 (where NA2 <NA1) is set as the required numerical aperture on the optical information recording medium side of the condensing optical system necessary for recording or reproducing the second optical information recording medium at wavelength λ2. A substantially ring-shaped diffraction pattern is provided on at least one surface of the objective lens of the optical system, and m-order diffracted light from the diffraction pattern of the condensing optical system of the light beam from the first light source (where m is one) (Integer) is used to record and / or reproduce the first optical information recording medium having a transparent substrate thickness of t1, and from the diffraction pattern of the condensing optical system of the light beam from the second light source. N-th order diffracted light (where n is an integer, excluding n = m = 0), at least the second optical information recording with a transparent substrate thickness of t2 (where t2> t1) Record and / or play back media In the optical pickup apparatus, a numerical aperture of the optical information recording medium side of the light ray passing through the periphery of the diffraction pattern of the substantially annular shape including the optical axis is taken as NAX,
0.2 ≦ NAX / NA2 ≦ 0.9
It is characterized by being.

  In this case, m is an integer other than 0, and it is preferable that n = m. The objective lens is preferably a single lens.

  Further, when the information is recorded or reproduced on the first optical information recording medium, the imaging magnification when viewed from the optical information recording medium side of the objective lens is M1, and the information on the second optical information medium is recorded or reproduced. It is preferable that M2 and M1 are substantially equal when the imaging magnification of the objective lens as viewed from the optical information medium side is M2. The M1 and M2 are preferably substantially zero.

  Further, the number of ring zones of the diffraction pattern is preferably 7 to 30, and the number of ring zones can be reduced as compared with the case where spherical aberration is completely corrected by diffraction, and manufacture is facilitated.

  The light beam incident on the information recording surface is at least three of a first light beam near the optical axis, a second light beam outside the first light beam, and a third light beam outside the second light beam. The second light beam is prevented from reaching the vicinity of the information recording surface by the shielding means, and the light beam from the first light source from the diffraction pattern of the condensing optical system is divided. Of the m-order diffracted light, a beam spot is formed mainly by the first light flux and the third light flux to record and / or reproduce the first optical information recording medium, and the light flux from the second light source. Of the n-th order diffracted light from the diffraction pattern of the condensing optical system, it is preferable that a beam spot is formed mainly by the first light flux to record and / or reproduce the second optical information recording medium. .

  The objective lens is preferably a single lens, and the diffraction pattern is preferably provided on the single lens.

  Moreover, it is preferable that the objective lens is a single lens, and the shielding means is provided on the single lens.

  The light beam incident on the information recording surface is at least three of a first light beam near the optical axis, a second light beam outside the first light beam, and a third light beam outside the second light beam. Of the light beams from the first light source, the first light beam and the third light beam are beams by using at least the m-order diffracted light from the diffraction pattern of the condensing optical system. Forming a spot, recording and / or reproducing the first optical information recording medium, and the nth-order diffracted light from the diffraction pattern of the condensing optical system of the first light flux of the light flux from the second light source; It is preferable to record and / or reproduce the second optical information recording medium by forming a beam spot by using at least the second light flux.

  Further, it is preferable that a convergence position of a portion of the first light flux of the light flux from the second light source that is farthest from the optical axis is different from a convergence position of the second light flux.

  The objective lens is preferably a single lens, the diffraction pattern is preferably provided on the single lens, and the second light beam is preferably diffracted by the diffraction pattern.

  In addition, the second light beam can pass through a portion without the diffraction pattern. Further, among the light beams from the second light source, a light beam having a numerical aperture of NA3 (NA2 ≦ NA3 <NA1) or more on the optical information recording medium side can be prevented from reaching the vicinity of the information recording surface by the shielding means. .

  The shielding means is preferably an annular dichroic filter that transmits a light beam having a wavelength λ1 and reflects a light beam having a wavelength λ2.

  In addition, a beam spot is formed by a light beam of a portion whose numerical aperture on the optical information recording medium side of the n-th order diffracted light from the diffraction pattern of the condensing optical system of the light beam from the second light source is approximately NA2 or less. The second optical information recording medium can be recorded and / or reproduced so that the portion having a numerical aperture of approximately NA2 or more is flare light.

  Further, it is preferable that the first light source and the second light source are unitized, and the photodetector is common to the first light source and the second light source.

The objective lens according to the present invention is an objective lens for an optical pickup device that records or reproduces information on an optical information recording medium, and has a diffraction pattern on at least one surface, and a parallel light beam having a wavelength of 780 nm is incident thereon. The portion of the light flux that has passed through the objective lens when the numerical aperture on the optical information recording medium side is 0.45 or less is passed through a transparent substrate having a thickness of 1.2 mm and a refractive index of 1.57. The third-order spherical aberration component of the wavefront aberration is over, and the absolute value thereof is WSA2λ2rms, and the numerical aperture on the optical information recording medium side of the light beam that has passed through the objective lens when a parallel light beam having a wavelength of 650 nm is incident. The absolute value of the third-order spherical aberration component of the wavefront aberration when passing through a transparent substrate having a thickness of 0.6 mm and a refractive index of 1.58 for a portion of 0.6 or less is WSA1λ1rms. And when
0.02λ2rms ≦ WSA2 ≦ 0.06λ2rms and WSA1 ≦ 0.04λ1rms
It is characterized by satisfying.

The objective lens for an optical pickup according to the present invention has a substantially ring-shaped diffraction pattern over the entire effective diameter of at least one surface, and the substantially ring-shaped diffraction pattern including the optical axis from the peripheral green optical axis. When the height is HX and the height of the outermost ring is HMAX,
0.15 ≦ HX / HMAX ≦ 0.65
It is characterized by satisfying.

  Moreover, it is preferable that the objective lens is a single lens.

  Another objective lens for an optical pickup according to the present invention is provided with a substantially ring-shaped diffraction pattern on the entire effective diameter of at least one surface, and is discontinuous with at least two spherical aberrations for a light beam having a certain wavelength. It is characterized by being.

  Further, another objective lens for an optical pickup according to the present invention is a single lens, in which a substantially ring-shaped diffraction pattern is provided on the entire effective diameter of one surface, the other surface is a continuous surface, and has at least a certain wavelength. Spherical aberration is discontinuous at two or more places with respect to the luminous flux.

  Further, in another objective lens for optical pickup according to the present invention, a plurality of annular diffraction patterns are provided around the optical axis portion of at least one surface and the effective diameter, and between the annular zone and the adjacent annular zone. Is a refractive surface, and spherical aberration is discontinuous at the boundary between the refractive surface and the diffraction pattern. In this case, it is preferable that the objective lens is a single lens.

  Further, the number of ring zones of the diffraction pattern can be 7 to 30.

  Note that the condensing optical system in the present invention is one or more sets of optical systems capable of recording or reproducing, for example, CDs and DVDs, and recording and / or information on an information recording medium. It may mean not only the entire optical system for making it possible to reproduce information on the recording medium, but also a part of the optical system, and includes an objective lens.

  Examples of the information recording medium in the present invention include various CDs such as CD, CD-R, CD-RW, CD-Video, and CD-ROM, DVD, DVD-ROM, DVD-RAM, DVD-R, and DVD. Examples include various DVDs such as -RW, and disk-shaped information recording media such as MD. Generally, a transparent substrate exists on the information recording surface of the information recording medium.

  Further, recording and reproduction of information with respect to the information recording medium means recording information on the information recording surface of the information recording medium as described above and reproducing information recorded on the information recording surface. The condensing optical system in the present invention may be used for performing only recording or reproduction, or may be used for performing both recording and reproduction. Further, it may be used for recording on a certain information recording medium and reproducing on another information recording medium, or may be used for recording or recording on a certain information recording medium. It may be used for performing reproduction and recording and reproduction on another information recording medium. Note that reproduction here includes simply reading information.

  In the present invention, the first light source (wavelength λ1) and the second light source (wavelength λ2) each emit light having a wavelength satisfying λ2> λ1 and having a sufficient wavelength difference. Such light of different wavelengths from the first and second light sources, in addition to the difference in the type and recording density of the information recording medium described above, for example, the difference in the thickness of the transparent substrate of the information recording medium and the recording. Used for differences from playback.

  The objective lens, in a narrow sense, is a single condensing function arranged to face the optical information recording medium at the position closest to the optical information recording medium when the optical information recording medium is loaded in the optical pickup device. The term “lens” refers to a lens group that can be actuated at least in the optical axis direction by an actuator together with the lens. Here, such a lens group indicates at least one lens, and includes only a single lens. Therefore, in this specification, the numerical aperture NA of the objective lens on the optical information recording medium side refers to the numerical aperture NA of the lens surface located closest to the optical information recording medium of the objective lens. The numerical aperture NA is a numerical aperture NA defined as a result of the light flux from the light source being limited by a component or member having a diaphragm function such as a diaphragm or a filter provided in the optical pickup device.

  The diffraction pattern refers to a pattern in which a relief is provided on the surface of a lens, for example, to have a function of condensing or diverging a light beam by diffraction. There are regions where diffraction occurs on the same optical surface and regions where it does not occur In the case, it refers to a region where diffraction occurs. As the shape of the relief, for example, it is formed as a concentric ring zone centered on the optical axis on the surface of the optical element, and each ring zone has a sawtooth shape when viewed in cross section in a plane including the optical axis. Although known, it includes such shapes.

  According to the present invention, at least two types of optical information recording media having transparent substrates having different thicknesses can be reproduced or recorded by light sources having at least two wavelengths different from each other, and chromatic aberration can be reduced for an optical information recording medium having a thin transparent substrate. At the same time, it is possible to provide an objective lens that can reduce the residual aberration by half for an optical information recording medium having a thick transparent substrate, and an optical pickup device having a condensing optical system including the objective lens.

1 is an optical path diagram illustrating a configuration of an optical pickup device according to an embodiment of the present invention. It is a spherical aberration figure with respect to 1st reference wavelength (lambda) 1 = 650nm of the objective lens of Example 1 concerning embodiment of this invention. FIG. 3 is an optical path diagram of the objective lens of Example 1 in the case of FIG. 2. FIG. 6 is a spherical aberration diagram of the objective lens according to Example 1 with respect to a second reference wavelength λ2 = 780 nm. FIG. 5 is an optical path diagram of the objective lens of Example 1 in the case of FIG. 4. It is a spherical aberration figure with respect to 1st reference wavelength (lambda) 1 = 650nm of the objective lens of Example 2 concerning embodiment of this invention. FIG. 7 is an optical path diagram of the objective lens in Example 2 in the case of FIG. 6. FIG. 6 is a spherical aberration diagram of the objective lens according to Example 2 with respect to the second reference wavelength λ2 = 780 nm. It is a spherical aberration figure with respect to 1st reference wavelength (lambda) 1 = 650nm of the objective lens of Example 3 concerning embodiment of this invention. FIG. 10 is an optical path diagram of the objective lens in Example 3 in the case of FIG. 9. FIG. 10 is a spherical aberration diagram of the objective lens according to Example 3 for the second reference wavelength λ2 = 780 nm. It is a spherical aberration figure with respect to 1st reference wavelength (lambda) 1 = 650nm of the objective lens of Example 4 concerning embodiment of this invention. FIG. 13 is an optical path diagram of the objective lens of Example 4 in the case of FIG. 12. FIG. 10 is a spherical aberration diagram of the objective lens according to Example 4 with respect to the second reference wavelength λ2 = 780 nm.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, an optical pickup device according to a preferred embodiment of the invention will be described with reference to the drawings. FIG. 1 is a diagram showing a schematic configuration of an objective lens according to the present embodiment and an optical pickup device including the objective lens.

  The optical pickup device of FIG. 1 is configured to read information from the information recording surface with light having wavelengths of 650 nm and 780 nm from the first and second light sources for both CD and DVD, which are optical information recording media, for example. Has been.

  As shown in FIG. 1, the optical pickup device is unitized with a first semiconductor laser 111 that emits light having a wavelength of 650 nm for DVD and a second semiconductor laser 112 that emits light having a wavelength of 780 nm for CD as light sources. Has been. A beam splitter 120 is disposed between the collimator 13 and the objective lens 16, and the light substantially parallelized by the collimator 13 passes through the beam splitter 120 toward the objective lens 16. Further, the optical path is changed so that the light beam reflected from the information recording surface 22 of the optical disk 20 having the transparent substrate 21 is directed to the photodetector 30 by the beam splitter 120 as the optical path changing means. The objective lens 16 has a flange portion 16a on the outer periphery thereof, and the objective lens 16 can be easily attached to the optical pickup device by the flange portion 16a. Further, since the flange portion 16a has a surface extending in a direction substantially perpendicular to the optical axis of the objective lens 16, attachment with higher accuracy can be facilitated.

  When reproducing the first optical disk (DVD), the light beam emitted from the first semiconductor laser 111 passes through the collimator 13 and becomes a parallel light beam, as indicated by the solid line in the figure. Further, the light is focused by the diaphragm 17 through the beam splitter 120, and is condensed on the information recording surface 22 by the objective lens 16 through the transparent substrate 21 of the first optical disk 20. Then, the light beam modulated and reflected by the information pits on the information recording surface 22 is reflected again by the beam splitter 120 through the objective lens 16 and the diaphragm 17, and astigmatism is given by the cylindrical lens 180. Then, a read signal of information recorded on the first optical disc 20 is obtained using a signal incident on the light detector 30 and output from the light detector 30.

  In addition, focus detection and track detection are performed by detecting a change in the amount of light due to a change in the shape and position of the spot on the photodetector 30. Based on this detection, the two-dimensional actuator 150 moves the objective lens 16 so that the light beam from the first semiconductor laser 111 forms an image on the information recording surface 22 of the first optical disc 20, and also moves the first semiconductor laser. The objective lens 16 is moved so that the light beam from the 111 is imaged on a predetermined track.

  Next, when reproducing the second optical disk (CD), the light beam emitted from the second semiconductor laser 112 passes through the collimator 13 and becomes a parallel light beam, as indicated by the broken line in the figure. Further, the light is focused by the diaphragm 17 through the beam splitter 120, and is condensed on the information recording surface 22 by the objective lens 16 through the transparent substrate 21 of the second optical disk 20. Then, the light beam modulated and reflected by the information pits on the information recording surface 22 is reflected again by the beam splitter 120 through the objective lens 16 and the diaphragm 17, and astigmatism is given by the cylindrical lens 180. After that, a read signal of information recorded on the second optical disc 20 is obtained using a signal incident on the photodetector 30 and output from the photodetector 30. In addition, focus detection and track detection are performed by detecting a change in the amount of light due to a change in the shape and position of the spot on the photodetector 30. Based on this detection, the objective lens 16 is moved so that the light beam from the two-dimensional actuator 15 or the first semiconductor laser 112 is imaged on the information recording surface 22 of the second optical disk 20, and the second semiconductor laser 112 is used. The objective lens 16 is moved so that the light beam from the light beam is imaged on a predetermined track.

The objective lens 16 shown in FIG. 1 is a single lens provided with a diffraction pattern. Of the light flux from the second semiconductor racer 112 that has passed through the objective lens 16, the second numerical aperture on the optical disc side is NA2 or less. The third-order spherical aberration component of the wavefront aberration when passing through the transparent substrate of the optical disc is over (overcorrection), and when the absolute value is WSA2λ2rms,
0.02λ2rms ≦ WSA2 ≦ 0.06λ2rms
There is a residual aberration in this range. NA1 is a required numerical aperture on the first optical disc side, and NA2 is a required numerical aperture on the second optical disc side.

  Further, each diffracted light generated when the light flux from the first semiconductor laser 111 and the second semiconductor laser 112 passes through the diffraction pattern is a diffracted light of the same order other than the 0th order. The imaging magnification M1 on the optical disk side of the objective lens 16 at the time of reproduction of the first optical disk and the imaging magnification M2 on the optical disk side of the objective lens 16 at the time of reproduction of the second optical disk are substantially equal and substantially zero. For this reason, the optical pickup apparatus of FIG. 1 requires only one photodetector, and the first semiconductor laser 111 and the second semiconductor laser 112 can be configured as an integral part, and can be unitized. .

  In FIG. 1, the first optical disc is a DVD (light source wavelength 650 nm) and the second optical disc is a CD (light source wavelength 780 nm). However, the present invention is not limited to this. The optical disc may be a next-generation high-density optical disc (light source wavelength 400 nm), the second optical disc may be a DVD (light source wavelength 650 nm), or the like.

  Next, the above objective lens will be described. The condensing optical system in the present embodiment is a double-sided aspherical single lens, and a diffraction ring zone (ring-shaped diffraction surface) is provided as a diffraction pattern on one aspherical surface.

  That is, the refractive surface of the objective lens is formed in an aspherical shape expressed by the following [Equation 1].

  However, Z is an axis in the optical axis direction, h is an axis perpendicular to the optical axis (height from the optical axis: the light traveling direction is positive), R0 is a paraxial radius of curvature, κ is a cone coefficient, Ai Is the aspheric coefficient, and Pi is the power of the aspheric surface.

  In general, the pitch of the diffraction zone is defined using a phase difference function or an optical path difference function. Specifically, the phase difference function ΦB is expressed by the following [Equation 2] with the unit as radians, and the optical path difference function Φb is expressed by [Equation 3] with the unit as mm. The optical path difference function is for the first reference wavelength λ1.

  Although these two representation methods are different in unit, they are equivalent in terms of representing the pitch of the diffraction zone. That is, if the principal wavelength λ (unit: mm) is multiplied by the phase difference function coefficient B by λ / 2π, it can be converted to the optical path difference function coefficient b, and conversely, the optical path difference function coefficient b is 2π / λ. Can be converted to the coefficient B of the phase difference function.

  Next, Examples 1, 2, 3, and 4 will be described as specific examples of the objective lens according to the present embodiment. Each example is a single lens, and each single lens has a first reference wavelength λ1 = 650 nm, a focal length f = 3.3 mm, a numerical aperture NA1 = 0.6, and a thickness t1 of the transparent substrate of the first optical disk. = 0.6 mm, the second reference wavelength λ2 = 780 nm, the numerical aperture NA2 = 0.45, and the thickness t2 = 1.2 mm of the transparent substrate of the second optical disk, sufficient imaging performance is obtained. Further, there is almost no aberration with respect to the first reference wavelength λ1 having a short wavelength and the transparent substrate thickness t1. In the following, the image side means the optical information recording medium side.

<Example 1>
Table 1 shows lens data of Example 1. In Tables 1 to 4 below, for example, “2.2E-02” means “2.2 × 10 ⁻² ”.

  FIG. 2 is a spherical aberration diagram of the lens of Example 1 at the first reference wavelength λ1 = 650 nm and the transparent substrate thickness t1 = 0.6 mm. FIG. 3 is an optical path diagram of the lens of Example 1 in this case. It is. It can be seen from FIG. 2 that the spherical aberration is sufficiently corrected at the first reference wavelength λ1. Similarly, FIG. 4 is a spherical aberration diagram of the lens of Example 1 at the second reference wavelength λ2 = 780 nm and the transparent substrate thickness t1 = 1.2 mm. FIG. 5 is the lens of Example 1 in this case. FIG. As can be seen from FIG. 4, the spherical aberration remains at the second reference wavelength λ2, but the spherical aberration due to the difference in the thickness of the transparent substrate generated in the normal refractive lens and the chromatic spherical aberration generated due to the wavelength difference. Compared to the added spherical aberration, the aberration is corrected to some extent by the diffraction effect of the diffraction ring zone.

  This lens is a second light source that emits light of a second reference wavelength λ2 by disposing an annular dichroic filter (not shown) in FIG. 1 between the diaphragm 17 and the objective lens 16 perpendicular to the optical axis. It is preferable not to reach the information recording surface 22 by reflecting a light beam having a numerical aperture NA3 or more on the image side out of the light beam from. It is preferable not to reach the information recording surface 22 by a shielding means such as the shielding unit 10.

<Example 2>
Table 2 shows lens data of Example 2.

  6 is a spherical aberration diagram of the lens of Example 2 at the first reference wavelength λ1 = 650 nm and the transparent substrate thickness t1 = 0.6 mm. FIG. 7 is an optical path diagram of the lens of Example 2 in this case. It is. It can be seen from FIG. 6 that the spherical aberration is sufficiently corrected at the first reference wavelength λ1. FIG. 8 is a spherical aberration diagram of the lens of Example 2 at the second reference wavelength λ2 = 780 nm and the transparent substrate thickness t1 = 1.2 mm. As can be seen from FIG. 8, spherical aberration remains at the second reference wavelength λ2, but as in Example 1, the aberration is corrected to some extent by the diffraction effect. Also, as shown in FIG. 7, the necessary imaging performance can be obtained by providing an annular shield 10 on the lens surface. The shielding unit 10 can be configured by forming an annular cutout or an annular dichroic filter on the lens surface, for example. The shielding unit 10 has little influence on the imaging performance of the light having the first reference wavelength λ1.

<Example 3>
Table 3 shows lens data of Example 3.

  FIG. 9 is a spherical aberration diagram of the lens of Example 3 at the first reference wavelength λ1 = 650 nm and the transparent substrate thickness t1 = 0.6 mm. FIG. 10 is an optical path diagram of the lens of Example 3 in this case. It is. It can be seen from FIG. 10 that the spherical aberration is sufficiently corrected at the first reference wavelength λ1. FIG. 11 is a spherical aberration diagram of the lens of Example 3 at the second reference wavelength λ2 = 780 nm and the transparent substrate thickness t1 = 1.2 mm. As can be seen from FIG. 11, in Example 3, in order to obtain the required imaging performance at the second reference wavelength λ2, the spherical aberration is over (overcorrected) when the numerical aperture is large (NA 0.45 or more). Yes.

  Further, it is preferable that a beam spot is formed on the information recording surface 22 by a light beam having a numerical aperture of about NA2 or less on the image side, and a portion having a numerical aperture of about NA2 or more is flare light.

<Example 4>
Table 4 shows lens data of Example 4.

  FIG. 12 is a spherical aberration diagram of the lens of Example 4 at the first reference wavelength λ1 = 650 nm and the transparent substrate thickness t1 = 0.6 mm. FIG. 13 is an optical path diagram of the lens of Example 4 in this case. It is. It can be seen from FIG. 12 that the spherical aberration is sufficiently corrected at the first reference wavelength λ1. FIG. 14 is a spherical aberration diagram of the lens of Example 4 at the second reference wavelength λ2 = 780 nm and the transparent substrate thickness t1 = 1.2 mm. As can be seen from FIG. 14, in the same way as in the third embodiment, in the fourth embodiment, in order to obtain the required imaging performance at the second reference wavelength λ2, the spherical aberration is over (overcorrected) at a large numerical aperture. Yes. In Example 4, one surface of the lens is further divided into three parts, and the first, second, and third light fluxes are obtained from the optical axis. As shown in FIG. By providing a discontinuous portion of spherical aberration at the reference wavelength λ2, a better imaging performance can be obtained compared to the third embodiment.

  In addition, the convergence position of the portion of the first light beam that is farthest from the optical axis does not coincide with the convergence position of the second light beam, so that residual aberration can be reduced.

  In the present embodiment, the first divided surface and the third divided surface are the same surface shape data, but may not be the same.

  Moreover, although the olefin resin was used for the lens material of each Example and the polycarbonate resin (PC) was used for the transparent substrate, the refractive index of each material is shown in Table 5 for each reference wavelength.

  Next, a plurality of annular zones provided in each lens of the above-described embodiment will be described. A plurality of annular zones are formed substantially concentrically around the optical axis on the lens surface, the annular zone pitch PH corresponding to the maximum numerical aperture on the image side of the lens, and the numerical aperture that is 1/2 of the maximum numerical aperture. An example of the ring zone pitch PH corresponding to is shown together with a comparative example in the case of no residual aberration.

(Example)
1st ring zone: 990.7μm
Minimum pitch: 22.0 μm
PH: 990.7μm
PF: 22.0 μm
Number of rings: 20

(Comparative example)
1st ring zone: 850.2μm
Minimum pitch: 11.5 μm
PH: 55.6 μm
PF: 11.5 μm
Number of rings: 38

  As described above, in this embodiment, although the spherical aberration remains with respect to the second reference wavelength λ2 (650 nm), the number of ring zones can be reduced as compared with the comparative example lens having no residual aberration. It is easy to form a diffraction pattern of the lens, and the manufacturing cost of the lens can be reduced.

Further, when the diffraction ring zone is provided on the entire effective diameter of the lens surface, the height HX from the optical axis at the periphery of the diffraction ring zone and the height HMAX of the outermost ring zone are:
0.15 ≦ HX / HMAX ≦ 0.65
Is preferably satisfied.

10 Shielding part (shielding means)
13 Collimator 16 Objective Lens 17 Aperture (Aperture Limiting Means)
20 optical disk 21 transparent substrate 22 information recording surface 30 photodetector 111 semiconductor laser (first light source)
112 Semiconductor laser (second light source)
120 Polarizing beam splitter 150 Two-dimensional actuator

Claims (33)

The light beam from the light source is condensed on the information recording surface via the transparent substrate of the optical information recording medium by a condensing optical system including an objective lens, and information is recorded or reproduced. An optical pickup device for recording or reproducing information of at least two types of optical information recording media having different recording densities,
A first light source having a wavelength λ1 (nm);
A second light source of wavelength λ2 (nm) (λ2>λ1);
A photodetector for receiving reflected light from the optical information recording medium of the luminous flux emitted from the first light source and the second light source;
The required numerical aperture on the optical information recording medium side of the condensing optical system necessary for recording or reproducing the first optical information recording medium with the transparent substrate thickness t1 at the wavelength λ1 is NA1, and the thickness of the transparent substrate Is the required numerical aperture on the optical information recording medium side of the condensing optical system necessary for recording or reproducing the second optical information recording medium of t2 (where t2> t1) at the wavelength λ2 (where NA2 < NA1), a diffraction pattern is provided on at least one surface of the condensing optical system, and m-order diffracted light from the diffraction pattern of the condensing optical system of the light beam from the first light source (where m is one) (Integer) is used to record and / or reproduce the first optical information recording medium having a transparent substrate thickness of t1, and from the diffraction pattern of the condensing optical system of the light beam from the second light source. N-order diffracted light (where n is An optical pickup that records and / or reproduces a second optical information recording medium having a transparent substrate thickness of t2 (where t2> t1) by using at least two integers, excluding n = m = 0 In the device
Of the light flux from the second light source that has passed through the objective lens, the wavefront aberration when the numerical aperture on the optical information recording medium side is NA2 or less through the transparent substrate of the second optical information recording medium When the third-order spherical aberration component is over and its absolute value is WSA2λ2rms,
0.02λ2rms ≦ WSA2 ≦ 0.06λ2rms
An optical pickup device characterized by that. The optical pickup device according to claim 1, wherein the m is an integer other than 0, and n = m. The optical pickup device according to claim 1, wherein the objective lens is a single lens, and the diffraction pattern is provided on the single lens. The imaging magnification seen from the optical information medium side of the objective lens at the time of recording or reproducing information on the first optical information recording medium is M1, and at the time of recording or reproducing information on the second optical information recording medium 4. The optical pickup device according to claim 1, wherein M <b> 2 and M <b> 1 are substantially equal when the imaging magnification of the objective lens viewed from the optical information recording medium side is M <b> 2. 5. The optical pickup device according to claim 4, wherein M1 and M2 are substantially zero. Of the luminous flux from the second light source that has passed through the objective lens, the position at which the light beam farthest from the optical axis converges through the transparent substrate of the second optical information recording medium passes through the objective lens. Of the light flux from the second light source, the objective from the position where the wavefront aberration is minimized when the numerical aperture on the optical information recording medium side is NA2 or less through the transparent substrate of the second optical information recording medium. The optical pickup device according to claim 1, wherein the optical pickup device is far from the lens and has a difference of 5 μm or more. Of the luminous flux from the second light source that has passed through the objective lens, the position where the light beam farthest from the optical axis converges through the transparent substrate of the second optical information recording medium has passed through the objective lens. From the position where the wavefront aberration when the numerical aperture on the optical information recording medium side through the transparent substrate of the second optical information recording medium of the luminous flux from the second light source passes through the transparent substrate is minimized. The optical pickup device according to claim 1, wherein the optical pickup device is far from the objective lens and has a difference of 15 μm or more. The light beam from the light source is condensed on the information recording surface via the transparent substrate of the optical information recording medium by a condensing optical system including an objective lens, and information is recorded or reproduced. An optical pickup device for recording or reproducing information of at least two types of optical information recording media having different recording densities,
A first light source having a wavelength λ1 (nm);
A second light source of wavelength λ2 (nm) (λ2>λ1);
A photodetector for receiving reflected light from the optical information recording medium of the luminous flux emitted from the first light source and the second light source;
The required numerical aperture on the optical information recording medium side of the condensing optical system necessary for recording or reproducing the first optical information recording medium with the transparent substrate thickness t1 at the wavelength λ1 is NA1, and the thickness of the transparent substrate Is the required numerical aperture on the optical information recording medium side of the condensing optical system necessary for recording or reproducing the second optical information recording medium of t2 (where t2> t1) at the wavelength λ2. NA1), a substantially ring-shaped diffraction pattern is provided on at least one surface of the objective lens of the condensing optical system, and m-order diffracted light from the diffraction pattern of the condensing optical system of the light beam from the first light source (Where m is an integer), at least, the first optical information recording medium having a transparent substrate thickness t1 is recorded and / or reproduced, and the light flux from the second light source is collected. From diffraction pattern of optical optics A second optical information recording medium in which the thickness of the transparent substrate is t2 (where t2> t1) by using at least n-th order diffracted light (where n is an integer and excluding n = m = 0) In an optical pickup device for recording and / or reproducing
When the numerical aperture on the optical information recording medium side of the light beam passing through the periphery of the substantially ring-shaped diffraction pattern including the optical axis is NAX,
0.2 ≦ NAX / NA2 ≦ 0.9
An optical pickup device characterized by that. 9. The optical pickup device according to claim 8, wherein m is an integer other than 0, and n = m. The optical pickup device according to claim 8, wherein the objective lens is a single lens. The imaging magnification seen from the optical information recording medium side of the objective lens at the time of recording or reproducing information on the first optical information recording medium is M1, and at the time of recording or reproducing information on the second optical information medium 11. The optical pickup device according to claim 8, wherein M <b> 2 and M <b> 1 are substantially equal when an imaging magnification as viewed from the optical information medium side of the objective lens is M <b> 2. 12. The optical pickup device according to claim 11, wherein M1 and M2 are substantially zero. The optical pickup device according to any one of claims 8 to 12, wherein the number of ring zones of the diffraction pattern is 7 to 30. The light beam incident on the information recording surface is at least three of a first light beam near the optical axis, a second light beam outside the first light beam, and a third light beam outside the second light beam. The second light flux is divided so that it does not reach the vicinity of the information recording surface by the shielding means, and m next time from the diffraction pattern of the condensing optical system of the light flux from the first light source. Of the folded light, a beam spot is formed mainly by the first light flux and the third light flux to record and / or reproduce the first optical information recording medium, and the light flux from the second light source Of the n-th order diffracted light from the diffraction pattern of the condensing optical system, a beam spot is mainly formed by the first light flux to record and / or reproduce the second optical information recording medium. Claim 1, 2, 8 or 9 The optical pickup device. The optical pickup device according to claim 14, wherein the objective lens is a single lens, and the diffraction pattern is provided on the single lens. 16. The optical pickup device according to claim 14, wherein the objective lens is a single lens, and the shielding means is provided on the single lens. The light beam incident on the information recording surface is at least three of a first light beam near the optical axis, a second light beam outside the first light beam, and a third light beam outside the second light beam. Of the light beams from the first light source, the first light beam and the third light beam are divided into beam spots by using at least the m-order diffracted light from the diffraction pattern of the condensing optical system. Forming and recording and / or reproducing the first optical information recording medium, and the nth-order diffracted light from the diffraction pattern of the condensing optical system of the first light flux of the light flux from the second light source and the first light flux. 10. The optical pickup device according to claim 1, wherein a beam spot is formed by using at least two light beams to record and / or reproduce the second optical information recording medium. . 18. The light according to claim 17, wherein a convergence position of a portion of the first light flux of the light flux from the second light source that is farthest from the optical axis is different from a convergence position of the second light flux. Pickup device. The optical pickup device according to claim 17, wherein the objective lens is a single lens, and the diffraction pattern is provided on the single lens. The optical pickup device according to claim 17, wherein the second light beam is diffracted by the diffraction pattern. 20. The optical pickup device according to claim 17, 18 or 19, wherein the second light beam passes through a portion without the diffraction pattern. Among the light beams from the second light source, a light beam having a numerical aperture of NA3 (NA2 ≦ NA3 <NA1) or more on the optical information recording medium side is prevented from reaching the vicinity of the information recording surface by the shielding means. The optical pickup device according to claim 1, 2, 8, or 9. 23. The optical pickup device according to claim 22, wherein the shielding means is an annular dichroic filter that transmits a light beam having a wavelength [lambda] 1 and reflects a light beam having a wavelength [lambda] 2. A beam spot is formed by the light beam of the portion of the n-th order diffracted light from the diffraction pattern of the condensing optical system of the light beam from the second light source whose numerical aperture on the optical information recording medium side is approximately NA2 or less. 24. The optical pickup device according to claim 1, wherein the optical information recording medium is recorded and / or reproduced, and a portion having a numerical aperture of approximately NA2 or more is flare light. 25. The unit according to claim 1, wherein the first light source and the second light source are unitized, and the photodetector is common to the first light source and the second light source. The optical pick-up apparatus of any one of Claims. An objective lens for an optical pickup device for recording or reproducing information with respect to an optical information recording medium, having a diffraction pattern on at least one surface and passing through the objective lens when a parallel light beam having a wavelength of 780 nm is incident The third-order spherical aberration component of the wavefront aberration when the numerical aperture on the optical information recording medium side of the light flux passes through a transparent substrate with a thickness of 1.2 mm and a refractive index of 1.57 is as follows: The absolute value is WSA2λ2rms, and the portion of the light beam that has passed through the objective lens when a parallel light beam having a wavelength of 650 nm is incident has a numerical aperture of 0.6 or less on the optical information recording medium side, When the absolute value of the third-order spherical aberration component of the wavefront aberration when passing through a transparent substrate having a thickness of 0.6 mm and a refractive index of 1.58 is WSA1λ1 rms,
An objective lens satisfying 0.02λ2rms ≦ WSA2 ≦ 0.06λ2rms and WSA1 ≦ 0.04λ1rms. At least one of the effective diameters of at least one surface has a substantially annular diffraction pattern, the height of the substantially annular diffraction pattern including the optical axis from the peripheral green optical axis is HX, and the height of the outermost annular zone is When S is HMAX,
0.15 ≦ HX / HMAX ≦ 0.65
An objective lens for an optical pickup characterized by satisfying The objective lens for an optical pickup according to claim 27, wherein the objective lens is a single lens. An objective lens for an optical pickup, characterized in that a substantially ring-shaped diffraction pattern is provided on the entire effective diameter of at least one surface, and spherical aberration is discontinuous at two or more locations with respect to a light beam having at least a certain wavelength. A single lens is provided with a substantially ring-shaped diffraction pattern over the entire effective diameter of one surface, the other surface is a continuous surface, and is discontinuous with at least two spherical aberrations for a light beam of a certain wavelength. An objective lens for an optical pickup, characterized in that there is. A plurality of zonal diffraction patterns are provided around the optical axis portion of the at least one surface and the effective diameter, and a refracting surface is formed between the zonal zone and the adjacent zonal zone. An objective lens for an optical pickup, characterized in that spherical aberration is discontinuous at the boundary. The objective lens for an optical pickup according to claim 31, wherein the objective lens is a single lens. The objective lens according to any one of claims 26 to 32, wherein the number of annular zones of the diffraction pattern is 7 to 30.

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