JP2006251494A - Objective and optical pickup device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an objective for recording/reproducing holographic optical information, which forms an image at a desired focal position with servo light having wavelength different from wavelength for recording/reproducing information and makes wave aberration to be equal to or under Marechal's criterion for evaluation. <P>SOLUTION: A first lens group LG1 is obtained by sticking lenses L11 and L12 having different dispersions, and makes first wavelength light LT1 and second wavelength light LT2 incident on an information recording layer REL and a tracking information surface TIL as parallel beams or a minute spot, and makes spherical aberration changed due to the difference of the wavelength between the first wavelength light LT1 and the second wavelength light LT2 very small. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an objective lens mounted on an optical information recording / reproducing apparatus using holography, and an optical pickup apparatus using the objective lens.

  2. Description of the Related Art In recent years, an optical pickup device for hologram recording / reproduction that records / reproduces information with respect to an optical information recording medium using holography has been developed. As an objective lens mounted on this optical pickup device, there is an objective lens that achieves low aberration with a one-group one-element configuration (see Patent Document 1).

In addition, a technique for optimizing the distance between the objective lens and the optical information recording medium in an optical pickup device that utilizes a wavelength different from the wavelength used for recording / reproducing information for the astigmatism method for servo has been reported. (See Patent Document 2). In such an optical pickup device, information recording / reproduction and servo can be realized in parallel without changing the distance between the objective lens and the optical information recording medium, that is, the working distance.
JP 2004-177839 A JP 2001-256654 A

  However, the former document does not mention optimizing the interval between the objective lens and the optical information recording medium assuming the use at different wavelengths, and moreover, it considers correction of chromatic aberration. Not.

  The latter document does not disclose a method for setting the focal positions of different wavelengths to desired depth positions in the optical information recording medium, nor does it disclose a reduction in chromatic aberration. In other words, in the method of matching the working distances at different wavelengths, it is extremely important to set a pair of focal points of different wavelengths corresponding to information recording / reproduction and servo to the desired depth position. There is no specific disclosure about such a technique. Further, in order to realize a method of matching the working distances at different wavelengths, it is necessary to make the wavefront aberration below the Marechal evaluation standard for each wavelength, but there is no disclosure of a technique for reducing such wavefront aberration.

  Accordingly, the present invention provides a holographic light that forms an image of servo light having a wavelength different from the wavelength at which information is recorded / reproduced at a desired focal position, and that allows wavefront aberration to be below the Marshallian evaluation standard. An object of the present invention is to provide an objective lens for recording / reproducing information.

  Another object of the present invention is to provide an optical pickup device incorporating the objective lens as described above.

  In order to solve the above problems, an objective lens according to the present invention is an objective lens for recording and / or reproduction for an optical information recording medium using holography, and a concave surface is directed to the light source side in order from the light source side. The first lens group having a meniscus shape and having different dispersions bonded together, and the second lens group having a meniscus shape with a concave surface facing the image side and having a positive refractive power.

  According to the objective lens described above, the first lens group is formed by bonding materials having different dispersions, so that light beams with different wavelengths can be focused at desired positions, and the wavelengths are different. The changing spherical aberration can be made smaller than the Marechal criterion. Further, since the first lens group has a meniscus shape with the concave surface facing the light source, the Petzval sum of the entire objective lens can be reduced, and the field curvature can be reduced. Furthermore, since the second lens group is a meniscus lens having a concave surface facing the image side, the convergence angle of the light beam emitted from the objective lens can be reduced, and the working distance can be increased.

  In a specific aspect or aspect of the present invention, the objective lens further includes a third lens group that is disposed between the first lens group and the second lens group and has a positive refractive power. In this case, positive refractive power can be shared by the second lens group and the third lens group, and aberrations can be further reduced.

を満たす。この場合、第１レンズ群を構成する２つのレンズのｄ線における屈折率ｎ１，ｎ２、アッベ数ν１，ν２を式（１）の下限以上にすることで、２つのレンズの分散の差を利用し、異なる波長を所望の位置に合焦することができ、式（１）の上限以下にすることで、比較的容易に入手できる硝材の組み合わせを使用することが可能となる。 In another specific aspect of the present invention, when the refractive indexes at the d-line of the two lenses constituting the first lens group are n1 and n2 and the Abbe numbers are ν1 and ν2,

Meet. In this case, the difference in dispersion between the two lenses is used by setting the refractive indices n1, n2 and Abbe numbers ν1, ν2 at the d-line of the two lenses constituting the first lens group to be equal to or higher than the lower limit of Equation (1). Then, different wavelengths can be focused on a desired position, and by making it below the upper limit of the formula (1), it becomes possible to use a combination of glass materials that can be obtained relatively easily.

In another specific aspect of the present invention, when the refractive power of the first lens group is P1 and the refractive power of the objective lens is Pt, the following expression −5.56 <P1 / Pt <10 (2)
Meet. In the objective lens as in the present invention, if the refractive power of the first lens group is negative, the diameter of the light beam can be increased. Therefore, the working distance tends to be long, but when the negative refractive power increases. The positive refractive power of the second lens group and the like must be increased, and the manufacturing error sensitivity and the assembly error sensitivity of each lens constituting the second lens group and the like are increased. Under such circumstances, the lens surface eccentricity sensitivity and the lens assembly error sensitivity can be reduced by making the relationship between the powers P1 and Pt larger than the lower limit of the expression (2). Further, if the refractive power of the first lens group is positive, the working distance tends to be short. However, by making the relationship between the powers P1 and Pt less than the upper limit of the expression (2), a sufficiently long working distance. Can be secured.

In another specific aspect of the present invention, when the radius of curvature of the optical surface on the light source side of the first lens group is r1 and the focal length of the objective lens is ft, the following equation −1.3 <r1 / ft <−0 64 (3)
Meet. In the objective lens according to the present invention, when the absolute value of the radius of curvature r1 of the optical surface is small on the light source side of the first lens group, the light beam is diverged on the surface, and the numerical aperture is increased while keeping the working distance large. However, if the absolute value of the radius of curvature r1 is made too small, there is a problem that the sensitivity related to eccentricity and error in manufacturing the first lens group increases. Therefore, by setting the ratio r1 / ft within the range of the expression (3), the sensitivity of eccentricity and error is sufficiently small, and the numerical aperture can be increased while keeping the working distance large.

  In another specific aspect of the present invention, the first lens group is made of glass, and at least the optical surface on the light source side is spherically polished. In general, aspherical lenses manufactured by glass molding, injection molding, etc. are caused by local errors from the design value at the time of mold molding compared to spherical polished glass manufactured by polishing optical surfaces. As a result, a slight distortion (surface roughness) is generated on the optical surface. From the above, when the first lens group is made of glass and at least the optical surface on the light source side is spherically polished, optical recording / reproduction can be performed with high accuracy.

  In another specific aspect of the present invention, all the lens groups constituting the objective lens are made of glass. In this case, it is possible to use a relatively short wavelength near 400 nm as the light source by making all the lens groups glass.

  An optical pickup apparatus according to the present invention includes the above-described objective lens capable of forming a spot image on a guide layer of an optical information recording medium while forming a hologram image on the recording layer of the optical information recording medium. Information on the medium can be read or information can be written on the optical information recording medium.

  In the optical pickup device, a hologram image and a spot image can be formed on the recording layer and the guide layer with little chromatic aberration, respectively, so that optical information can be recorded and / or reproduced with high accuracy.

  FIG. 1 illustrates the structure of an objective lens according to an embodiment of the present invention. FIG. 1 (a) shows a light beam in the case of recording / reproducing, and FIG. 1 (b) shows tracking and focusing. The luminous flux in the case of is shown.

  The objective lens 10 of the present embodiment is a recording and / or reproducing objective lens for an optical information recording medium using holography, and includes a first lens group LG1 and a second lens group LG2 in order from the light source side. And a third lens group LG3 is further provided between the lens groups LG1 and LG2. A stop ST is provided on the light source side of the objective lens 10, that is, immediately before the first lens group LG1, and an image of the spatial light modulator is formed by an appropriate relay lens at this position, or spatial light. The modulator is placed directly.

  The first lens group LG1 has a meniscus shape with a concave surface facing the light source, and has a structure in which the first lens L11 and the second lens L12 are bonded together. Here, the first lens L11 is a biconcave lens and has negative refractive power, and the second lens L12 is a biconvex lens and has positive refractive power. Further, the first lens L11 and the second lens L12 are formed of glass or the like having different dispersions, and color correction is possible.

  The second lens group LG2 has a meniscus shape with a concave surface facing the image side, that is, the optical disc OD side which is an optical information recording medium, and has a positive refractive power. The second lens group LG2 is made of, for example, glass and is composed of a single double-sided aspheric lens.

  The third lens group LG3 is a biconvex lens in which both the light source side and the image side are convex surfaces, and has positive refractive power. The third lens group LG3 is made of, for example, glass and is composed of a single double-sided aspheric lens.

  As shown in FIG. 1A, when the recording / reproducing first wavelength light LT1 is used, the first wavelength light LT1 emitted from each point of the aperture stop ST is emitted so as to diverge from the first lens group LG1. Then, after passing through the third lens group LG3 and the second lens group LG2, it becomes a parallel light beam and enters the information recording layer REL of the optical disc OD as the parallel light beam. At this time, the diffraction angle of the light beam diffracted by each cell constituting the spatial light modulator disposed at the position of the stop ST depends on the size of the cell and is set to 2 °, for example. As a whole, the first wavelength light LT1 is converged light in which the light beam diameter is increased in the first lens group LG1, and the light beam diameter is decreased in the second lens group LG2, and the light beam is further increased in the third lens group LG3. The convergent light is reduced in diameter.

  As shown in FIG. 1B, when the second wavelength light LT2 for tracking and focusing is used, the second wavelength light LT2 emitted from the position of the aperture stop ST has a light beam diameter increased by the first lens group LG1, The second lens group LG2 is converged light whose beam diameter is reduced, and the third lens group LG3 is converged light whose beam diameter is further reduced. The second wavelength light LT2 that has passed through the third lens group LG3 is focused on the tracking information surface TIL that is the guide layer of the optical disc OD, and forms a fine spot on the tracking information surface TIL.

  According to the objective lens 10 of FIG. 1, the first lens group LG1 is formed by bonding lenses L11 and L12 having different dispersions, and the first wavelength light LT1 and the second wavelength light LT2 are combined with the information recording layer REL. The tracking information surface TIL can be made incident as a parallel light beam or a fine spot, and the spherical aberration that changes due to the difference in wavelength between the first wavelength light LT1 and the second wavelength light LT2 is made sufficiently small. be able to. Moreover, in the objective lens 10 of the present embodiment, since the first lens group LG1 has a meniscus shape with the concave surface facing the light source, the Petzval sum of the objective lens 10 as a whole can be reduced, and the field curvature can be reduced. can do. Thereby, a highly accurate hologram image can be formed on the information recording layer REL, and diffracted light can be read from the information recording layer REL with high accuracy. Furthermore, in the objective lens 10 of the present embodiment, since the second lens group LG2 is a meniscus lens having a concave surface directed toward the optical disc OD, the first wavelength light LT1 and the second wavelength finally emitted from the objective lens 10 are used. The convergence angle of the light LT2 can be reduced, and the working distance corresponding to the distance to the optical disc OD can be made relatively long. By using the first lens group LG1 as a cemented lens, it is possible to efficiently correct chromatic aberration.

が満たされている。屈折率ｎ１，ｎ２や、アッベ数ν１，ν２を上記式（１）の下限を満たすように設定することで、２つのレンズＬ１，Ｌ２の分散の差を利用し、異なる波長を所望の位置に合焦することができる。また、屈折率ｎ１，ｎ２や、アッベ数ν１，ν２を式（１）の上限を満たすように設定することで、比較的容易に入手できる硝材の組み合わせを使用することが可能となる。 Further, in the objective lens 10 of the present embodiment, when the refractive indexes at the d-line of the two lenses L1 and L2 constituting the first lens group LG1 are n1 and n2, and the Abbe number is ν1 and ν2, the following conditions are satisfied.

Is satisfied. By setting the refractive indexes n1 and n2 and the Abbe numbers ν1 and ν2 so as to satisfy the lower limit of the above formula (1), the difference in dispersion between the two lenses L1 and L2 can be used to set different wavelengths to desired positions. You can focus. Further, by setting the refractive indexes n1 and n2 and the Abbe numbers ν1 and ν2 so as to satisfy the upper limit of the expression (1), it is possible to use a combination of glass materials that are relatively easily available.

In the objective lens 10 of the present embodiment, when the refractive power of the first lens group LG1 is P1, and the refractive power of the objective lens 10 as a whole is Pt, the following condition −5.56 <P1 / Pt <10 (2)
Is satisfied. By making the refractive power of the first lens group LG1 negative, the beam diameters of the first and second wavelength lights LT1 and LT2 can be increased, and the working distance can be increased. On the other hand, if the negative refracting power is made too large, the positive refracting power has to be increased for the third lens group LG3 and the second lens group LG2 in the subsequent stage, and manufacturing errors of the second and third lens groups LG2 and LG3. Sensitivity and assembly error sensitivity increase, resulting in increased manufacturing costs or difficulty in maintaining accuracy. In such a situation, by making the refractive power ratio P1 / Pt larger than the lower limit of the expression (2), it is possible to reduce the eccentricity and error sensitivity, and to increase the allowable error in manufacturing, assembling and the like. it can. If the refractive power of the first lens group LG1 is positive, the working distance corresponding to the distance to the optical disc OD tends to be short, but the refractive power ratio P1 / Pt is less than the upper limit of the expression (2). By doing so, it is possible to secure a sufficiently long working distance.

In the objective lens 10 of the present embodiment, when the radius of curvature of the optical surface on the light source side of the first lens group LG1 is r1 and the focal length of the objective lens is ft, the following condition −1.3 <r1 / ft <−0.64 (3)
Is satisfied. If the absolute value of the radius of curvature r1 on the light source side of the first lens L11 constituting the first lens group LG1 is small, the light beam diverges on the surface, and the working distance corresponding to the distance to the optical disc OD is kept large, while the objective Although the numerical aperture of the lens 10 can be increased, if the absolute value of the radius of curvature r1 is too small, there is a problem that the sensitivity of the first lens group LG1 to eccentricity and error increases. For this reason, by setting the ratio r1 / ft within the range of the expression (3), the sensitivity of eccentricity and error can be sufficiently reduced, and the numerical aperture is increased to a certain level or more while keeping the working distance large. Can do.

  FIG. 2 is a diagram for explaining the structure of an optical pickup device incorporating the objective lens 10 of FIG. The optical pickup device 100 is a hologram recording / reproduction type device, and includes a focusing objective lens 10 facing an optical disk OD that is an optical information recording medium, and a first laser light source 31 that is a first light source for recording / reproduction. A second laser light source 32 that is a second light source for servo, a first photodetector 41 that receives the information light IL from the optical disc OD, and a second photodetector that receives the servo light SL from the optical disc OD. 42.

  The optical pickup device 100 also includes a collimator lens 51 that converts the information light IL from the first laser light source 31 into a parallel light beam, a spatial light modulator 53 that provides an appropriate two-dimensional light distribution to the information light IL, and information recording. And a movable mirror 55 for switching between reproduction and reproduction, and a biaxial actuator 57 for focus and tracking. Here, between the first laser light source 31 and the collimator lens 51, a half mirror 61 that branches the reference light RL by reflection is disposed, and the reference light RL is reflected on the reflected light path of the half mirror 61. A pair of mirrors 65 and 67 are disposed which reflect and guide from the side of the objective lens 10 to the optical disc OD.

  Further, the optical pickup device 100 includes a dichroic mirror 71 that branches the servo light SL from the optical path of the information light IL, a mirror 73 for bending the optical path of the servo light SL, a collimator lens 75 for forming a parallel light beam, and a servo. A half mirror 77 that divides the light SL into an optical path on the second laser light source 32 side and an optical path on the second photodetector 42 side, and the like, are disposed on the optical path on the second photodetector 42 side to form astigmatism. A cylindrical lens 79 is provided.

  Further, although not shown in the drawing, the optical pickup device 100 has a light source driving circuit for appropriately operating the first and second laser light sources 31 and 32, and a sensor for appropriately operating the first and second photodetectors 41 and 42. A drive circuit, a displacement drive circuit for operating the biaxial actuator 57, and the like are included.

  In the optical pickup device 100 of FIG. 2, the objective lens 10 has first to third lens groups LG1 to LG3, and these first to third lens groups LG1 to LG3 are integrated and fixed by a holder 12. It is driven by the biaxial actuator 57 and is slightly displaced in the optical axis direction and the tracking direction perpendicular thereto. The objective lens 10 is a condensing optical system for individually condensing the light beams from the laser light sources 31 and 32 at different depths of the optical disc OD.

  The first laser light source 31 generates a light beam having a first wavelength λ1 (specifically, for example, blue object light OL or reference light RL) as recording / reproducing light, and is formed on the surface side of the optical disc OD. The hologram image information recorded on the information recording layer REL can be reproduced and / or the hologram image information can be recorded on the information recording layer REL. As the first laser light source 31, for example, a YAG laser second harmonic or a blue-violet semiconductor laser whose frequency is stabilized by using an external resonator can be used. For the optical path in the objective lens 10 when recording or reproducing hologram image information, see the first wavelength light LT1 shown in FIG.

  The second laser light source 32 generates a light beam having a second wavelength λ2 (specifically, for example, red servo light SL), and is recorded on the tracking information surface TIL formed on the back side of the optical disc OD. Enables detection of servo pit position information, and enables focus servo and tracking servo. For example, a red semiconductor laser or the like can be used for the second laser light source 32. For the optical path in the objective lens 10 during the focus servo or tracking servo, see the second wavelength light LT2 shown in FIG.

  The first photodetector 41 is an image sensor for detecting the information light IL that has returned from the information recording layer REL of the optical disc OD. Detect and output as image information. As the first photodetector 41, a CCD image sensor, a CMOS image sensor, or the like can be used.

  The second photodetector 42 is a quadrant sensor or the like for detecting the servo light SL reflected by the tracking information surface TIL of the optical disc OD, and detects a focus error signal and a tracking error signal based on the servo light SL. Output.

  In the above, the first laser light source 31, the first photodetector 41, the collimator lens 51, the spatial light modulator 53, the objective lens 10 and the like used for recording / reproducing information are referred to as a first optical system. Further, the second laser light source 32, the second photodetector 42, the collimator lens 75, the cylindrical lens 79, the objective lens 10 and the like used for the servo are referred to as a second optical system.

  Hereinafter, the operation of the optical pickup device 100 shown in FIG. 2 will be described. At the time of information recording, the light beam emitted from the first laser light source 31 for information recording / reproducing is made into a parallel light beam by the collimator lens 51 and then demultiplexed into the object light OL and the reference light RL by the half mirror 61. . The object light OL is converted into two-dimensional page data by the spatial light modulator 53, passes through the mirrors 55 and 71, enters the objective lens 10, and becomes a parallel light beam with a narrowed beam diameter. The object light OL emitted as a parallel light beam from the objective lens 10 enters the information recording layer REL that is a hologram recording medium, and interference fringes are formed here by interference with the reference light RL incident from the side of the objective lens 10. Record.

  On the other hand, at the time of information reproduction, an aluminum vapor deposition type movable mirror 55 is disposed on the optical path of the object light OL. The light beam emitted from the first laser light source 31 is collimated by the collimator lens 51 and then demultiplexed into the object light OL and the reference light RL by the half mirror 61. However, at the time of reproducing information, the movable mirror 55 is arranged on the optical path, so that the object light OL does not reach the hologram formed on the information recording layer REL, and only the reference light RL is a hologram on the information recording layer REL. And the wavefront recorded here is reproduced. The information light IL reproduced by the information recording layer REL is reflected by a dielectric multilayer film provided on the back surface of the information recording layer REL, enters the objective lens 10, passes through the dichroic mirror 71, and is reflected by the movable mirror 55. Is incident on the first photodetector 41. That is, the two-dimensional page data recorded on the information recording layer REL is detected by the first photodetector 41.

  At the time of recording / reproducing information as described above, the objective lens 10 is held by the biaxial actuator 57 and performs focusing and tracking. At that time, the servo light SL emitted from the second laser light source 32 is reflected by the half mirror 77, converted into a parallel light beam by the collimator lens 75, then reflected by the dichroic mirror 71 through the mirror 73, and incident on the objective lens 10. To do. The light beam condensed by the objective lens 10 passes through the information recording layer REL and the dielectric multilayer film provided on the back surface thereof, and is condensed on the tracking information surface TIL on which servo pits are recorded. That is, the servo light SL having the wavelength λ2 for tracking and focusing is recorded on the information recording layer REL when the objective lens 10 makes the object light OL having the recording / reproducing wavelength λ1 incident on the information recording layer REL as a parallel light beam. Passes through the dielectric multilayer film on the back surface of the lens and focuses on the tracking information surface TIL. The servo light SL modulated and reflected by the servo pits is incident on the half mirror 77 again through the objective lens 10, the dichroic mirror 71, the mirror 73, and the collimator lens 75. The servo light SL transmitted through the half mirror 77 is given astigmatism by the cylindrical lens 79 and is incident on the second photodetector 42. By detecting a change in the amount of light due to a change in the shape or position of the spot on the second photodetector 42, focus detection or track detection can be performed. Based on these detection results, the biaxial actuator 57 incorporated in the optical head moves the objective lens 10 in the optical axis direction so that the servo light SL from the second laser light source 32 forms an image on the tracking information surface TIL of the optical disc OD. And the objective lens 10 is moved in a direction perpendicular to the optical axis so that the servo light SL emitted from the second laser light source 32 forms an image on a predetermined track in the tracking information surface TIL. As a result, the object light OL from the spatial light modulator 53 illuminated by the first laser light source 31 enters the predetermined area of the information recording layer REL via the objective lens 10, and from the predetermined area of the information recording layer REL. Information light IL is collected by the objective lens 10 and is incident on the first photodetector 41.

  When reproducing information, a series of sequences including, for example, tracking, focusing, confirmation of written information, and the like are executed. Such a sequence may be appropriately changed according to the use and specifications of the optical pickup device 100. it can.

以上の表１において、第６面、第７面、第８面、及び第９面は非球面となっている。各面の円錐定数κ、非球面係数Ａ_２ｉは、以下の表２で与えられる。

以上の表２において、非球面の形状は、
ｘ ：光軸からの高さがｈの非球面上の点の非球面頂点の接平面からの距離
ｈ ：光軸からの高さ
ｃ ：非球面頂点の曲率（＝１／ｒ）
κ ：円錐定数
Ａ_２ｉ：第２ｉ次（ｉは２以上の自然数）の非球面係数として、以下の式

で与えられる。 A specific example of the objective lens 10 shown in FIG. 1 will be described below. In the objective lens of the embodiment, the wavelength λ1 of the light beam emitted from the first laser light source 31 for hologram recording / reproduction is assumed to be 405 nm, and the wavelength of the light beam emitted from the second laser light source 32 for tracking and focusing. λ2 was assumed to be 650 nm. Table 1 below shows specifications of the examples. In Table 1, r is a paraxial radius of curvature, d is a surface separation, n (λ1) is a refractive index at λ1, and n (λ2) is a refractive index at λ2. The object height in the first optical system for recording / reproducing information (wavelength λ1) is 0.45 mm, and the aperture diameter φ in the second optical system for servo (wavelength λ2) is 1.0 mm. It was supposed to be. Further, the diffraction angle of the light beam diffracted by each cell constituting the spatial light modulator imaged at the position of the stop ST is 2 °.

In Table 1 above, the sixth surface, the seventh surface, the eighth surface, and the ninth surface are aspherical surfaces. The conic constant κ and the aspheric coefficient A _{2i for} each surface are given in Table 2 below.

In Table 2 above, the shape of the aspheric surface is
x: distance from the tangent plane of the aspherical vertex of the point on the aspherical surface having a height from the optical axis h: height from the optical axis c: curvature of the aspherical vertex (= 1 / r)
κ: conic constant A _2i : 2nd order (i is a natural number of 2 or more) aspheric coefficient, the following formula

Given in.

FIG. 3 is a graph showing the curvature of field on the light receiving element during hologram reproduction for the first optical system using the objective lens 10 of the present embodiment, and FIG. It is a graph which shows the distortion aberration. Table 3 below shows the wavefront aberration at the image height divided by 10 at the position of the stop ST. In the second optical system using the objective lens 10 of this example, the wavefront aberration is 0.003 λrms.

  In the present embodiment described above, the value of the above formula (1) is 0.014, the value of the above formula (2) is -0.045, and the value of the above formula (3) is -0. .758. That is, the objective lens 10 of the present embodiment satisfies all the conditional expressions (1), (2), and (3) described above, and enables optical information recording / reproduction using high-precision and high-density holography. become.

  As described above, the present invention has been described according to the embodiment, but the present invention is not limited to the above embodiment. For example, the objective lens 10 can be composed of only the first lens group LG1 and the second lens group LG2, and in this case, the positive refractive power of the second lens group LG2 is increased to supplement the third lens group LG3. To do. Conversely, a lens having an appropriate refractive power can be added between the first lens group LG1 and the third lens group LG3, or between the second lens group LG2 and the third lens group LG3.

(A), (b) is sectional drawing of the objective lens which concerns on one Embodiment of this invention, and has shown the state used by different wavelength, respectively. It is a schematic block diagram of the optical pick-up apparatus incorporating the objective lens of FIG. It is a graph which shows the field curvature on the photodetector at the time of hologram reproduction using the objective lens of an Example. It is a graph which shows the distortion aberration on the photodetector at the time of hologram reproduction using the objective lens of an Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Objective lens 31 ... 1st laser light source 32 ... 2nd laser light source 41 ... 1st photo detector 42 ... 2nd photo detector 53 ... Spatial light modulator 55 ... Movable mirror 71 ... Dichroic Mirror, 79 ... Cylindrical lens, 100 ... Optical pickup device, L11 ... First lens, L12 ... Second lens, LG1 ... First lens group, LG2 ... Second lens group, LG3 ... Third lens group, LT1 ... First Wavelength light, LT2 ... Second wavelength light, ST ... Aperture, OD ... Optical disk, REL ... Information recording layer, IL ... Information light, OL ... Object light, RL ... Reference light, SL ... Servo light, TIL ... Tracking information surface

Claims (8)

An objective lens for recording and / or reproducing for an optical information recording medium using holography,
In order from the light source side, a meniscus shape having a concave surface facing the light source side, a first lens group in which materials having different dispersions are bonded together, and a meniscus shape having a concave surface facing the image side, have positive refractive power. An objective lens comprising a second lens group.   The objective lens according to claim 1, further comprising a third lens group that is disposed between the first lens group and the second lens group and has a positive refractive power. When the refractive indices at the d-line of the two lenses constituting the first lens group are n1 and n2, and the Abbe numbers are ν1 and ν2,

The objective lens according to any one of claims 1 and 2, wherein: When the refractive power of the first lens group is P1 and the refractive power of the objective lens is Pt, the following formula −5.56 <P1 / Pt <10 (2)
The objective lens according to any one of claims 1 to 3, wherein: When the radius of curvature of the optical surface on the light source side of the first lens group is r1 and the focal length of the objective lens is ft, the following equation −1.3 <r1 / ft <−0.64 (3)
The objective lens according to any one of claims 1 to 4, wherein:   The objective lens according to any one of claims 1 to 5, wherein the first lens group is made of glass, and at least an optical surface on a light source side is polished by a spherical surface.   All the lens groups which comprise an objective lens are glass, The objective lens as described in any one of Claims 1-6 characterized by the above-mentioned. The objective lens according to any one of claims 1 to 7, wherein a spot image can be formed on the guide layer of the optical information recording medium while forming a hologram image on the recording layer of the optical information recording medium,
An optical pickup device capable of reading information on an optical information recording medium or writing information on the optical information recording medium.

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