JP2005302236A - Optical pickup apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical pickup apparatus composed of a simple optical system in which a polarization beam splitter or the like is omitted. <P>SOLUTION: To suppress the occurrence of spherical aberration when a converged spot is formed, it is enough to lower a collimate function by making the refractive index of a coupling lens 5 small such that the longer the length is in the wavelength of a laser beam, and the heavier the thickness is in a protecting substrate, the reduction of the reflective index becomes larger. Thereby, the radiated luminous flux of the laser beam is made incident on an objective lens 1, consequently spherical aberration acts in an under state. Consequently, the aberration is canceled. On the other hand, it is enough to make the refractive index of the coupling lens 5 large such that the shorter the length is in the laser beam, and the smaller the thickness is in the protecting substrate, the enlargement of the refractive index becomes large. That is, a material of which the refractive index is large is selected for a laser beam L1 having a short wavelength, and a material of which the refractive index is small is selected for a laser beam L2 having long a wavelength as the coupling lens 5. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical pickup device for recording and / or reproducing an optical information recording medium.

Up to now, various optical pickup devices capable of reproducing / recording information on a plurality of optical information recording media such as CDs and DVDs have been developed and manufactured, and are widely spread. As such an optical pickup device, for example, there is an optical pickup device in which light beams from a pair of light sources having different wavelengths are combined by a polarization beam splitter and guided to the same optical path and condensed at substantially the same position (see Patent Document 1). In this optical pickup device, substantially the same optical characteristics for different wavelengths are realized by an objective lens composed of a pair of closely contacted glass lenses having different Abbe numbers.
JP 2004-29050 A

  However, when the light beams from the pair of light sources having different wavelengths are combined by the polarization beam splitter as described above, it is necessary to align the optical axes, and it takes time to adjust the position of the light source. Moreover, it is not easy to manufacture an objective lens composed of a pair of glass lenses that are in close contact with each other. Furthermore, when the substrate thicknesses of a plurality of optical information recording media are different, it is necessary to optimize the imaging characteristics regarding both light sources for each wavelength.

  There is also an optical pickup device in which a pair of light sources having different wavelengths are arranged close to each other and a focused spot for recording / reproducing information is formed at a close position using the same optical system. In such an optical pickup device, a wavelength selection element such as a diffractive structure is provided on, for example, an objective lens so that the imaging characteristics at both wavelengths are matched, or the objective lens is moved to a certain degree to move the condensed spots of different wavelengths. Are formed on recording surfaces of a plurality of optical information recording media, respectively. However, when the wavelength difference is compensated by providing the objective lens with a diffractive structure as in the former, the light quantity is reduced due to the presence of the diffractive structure, and the objective lens is not easily manufactured. Further, when the wavelength difference is compensated by the movement of the objective lens, an apparatus such as an actuator for compensating characteristics is separately required depending on the degree of the wavelength difference, and the optical system and the driving mechanism are complicated.

  Accordingly, an object of the present invention is to provide an optical pickup device including a simple optical system in which a polarizing beam splitter or the like for combining light beams from a pair of light sources having different wavelengths is omitted.

  It is another object of the present invention to provide an optical pickup device that avoids light loss due to a diffraction structure that realizes wavelength switching, and that does not require an actuator or other device for wavelength switching.

  In order to solve the above-mentioned problems, an optical pickup device according to the present invention includes: (a) a first light beam that emits a first wavelength light beam to be irradiated on an information recording surface of a first optical information recording medium set to a first substrate thickness. A first light source, and (b) a second light beam that is arranged close to the first light source and emits a light beam having a second wavelength to be irradiated on the information recording surface of the second optical information recording medium set to the second substrate thickness. Two light sources, and (c) the distance from the first light source to the surface of the protective layer of the first optical information recording medium is substantially the same as the distance from the second light source to the surface of the protective layer of the second optical information recording medium. And a certain condensing optical system. Here, the condensing optical system includes at least one optical component, and the first and second optical information recording media generated by the condensing optical system and the first and second optical information recording media due to the refractive index characteristic of the at least one optical component. The imaging characteristics of the two-wavelength light beam are relatively adjusted.

  In addition, that the light sources are arranged close to each other means that the interval between the light emitting points is generally 0.2 mm or less.

  In the optical pickup device, the first and second wavelength light fluxes generated by the condensing optical system and the first and second optical information recording media due to the refractive index characteristic of at least one optical component constituting the condensing optical system. Since the imaging characteristics are relatively adjusted, the imaging characteristics for recording / reproducing information on the first optical information recording medium and the imaging characteristics for recording / reproducing information on the second optical information recording medium Can be individually set to a desired state. Therefore, it is possible to record / reproduce information conforming to the respective standards on both the first and second optical information recording media, and to perform highly accurate and stable reading corresponding to a plurality of standards.

  In a specific aspect of the present invention, in the optical pickup device described above, at least one optical component causes the aberration generated by the condensing optical system and the first and second optical information recording media to be a light beam having the first and second wavelengths. Relative adjustment. In this case, it is possible to adjust the aberration generation amount for the first and second wavelengths by giving different refractive indexes at the first and second wavelengths to the optical components such as the objective lens and the collimator lens. Both optical information recording media can be recorded and reproduced with high accuracy and low aberration.

In another specific aspect of the present invention, the first substrate thickness is smaller than the second substrate thickness, and the first wavelength is shorter than the second wavelength.
Note that when the objective lens whose spherical aberration is corrected with the first substrate thickness is made thicker than the first substrate thickness, spherical aberration in an over state occurs. Further, when the light beam incident on the objective lens is divergent in the objective lens whose spherical aberration is corrected with parallel light with the first substrate thickness, spherical aberration in an under state occurs. Further, when the refractive index of the objective lens increases in the objective lens in which the spherical aberration is corrected by the first substrate thickness, an under-state spherical aberration occurs. In such a case, if different refractive indexes are intentionally given to the above-described optical component at the first and second wavelengths, it is possible to compensate for the amount of spherical aberration generated at the first and second wavelengths. Both the generation amounts of spherical aberration with respect to the first and second wavelengths can be minimized.

  In the above description, the spherical aberration is “under state” or “over state” as shown in FIG. 1 in the spherical aberration with the paraxial image point position as the origin as compared with the paraxial image point. The case of crossing the optical axis on the near side (condensing optical system side) is referred to as “under state”, and the case of crossing the optical axis at a position far from the paraxial image point is referred to as “over state”.

  In still another specific aspect of the present invention, at least one optical component forms an optical spot on one of the information recording surfaces of the first and second optical information recording media with respect to the objective lens. It is a collimator lens for supplying a parallel light beam. In this case, the adjusting optical component is a collimator lens for adjusting the incident light beam to the objective lens.

  In still another specific aspect of the present invention, the collimator lens has a focal length longer than the first wavelength with respect to the second wavelength. In this case, since the light beam having the second wavelength is incident on the objective lens in a relatively diverging state, the spherical aberration of the second wavelength condensed by the second optical information recording medium via the objective lens is relatively Acts under to cancel the spherical aberration.

  In still another specific aspect of the present invention, the collimator lens is composed of a single optical element, and the Abbe number of the optical element is in the range of 0.8 to 1.7. In this case, aberration correction by the collimator lens is optimized.

  In yet another specific aspect of the present invention, at least one optical component forms a light spot on one of the information recording surfaces of the first and second optical information recording media. It is an objective lens arranged facing the two-optical information recording medium. In this case, the adjustment optical component is an objective lens for forming a focused spot.

  In yet another specific aspect of the present invention, the objective lens has a shorter focal length with respect to the second wavelength than the first wavelength. In this case, since the light beam having the second wavelength is converged relatively close by the objective lens, the spherical aberration of the second wavelength condensed by the second optical information recording medium acts relatively under, and the spherical surface Aberrations are canceled out.

  In yet another specific aspect of the present invention, the objective lens is composed of a single optical element, and the Abbe number of the optical element is in the range of −2.0 to −0.8. In this case, aberration correction by the objective lens is optimized.

[First Embodiment]
FIG. 2 is a diagram for explaining the structure of an optical pickup device according to an embodiment of the present invention. This optical pickup device includes an objective lens 1 having one lens element per group, a laser light emitting device 2 that emits laser light as a light source, and a group that converts the divergence angle of the laser light emitted from the laser light emitting device 2. A single coupling lens 5 and a photodetector 8 that receives reflected light from a pair of optical disks 6 and 7 set in an optical information reproducing device including an optical pickup device as a sensor when reproducing or recording information. With. Here, the laser beam emitting device 2 is installed in the vicinity of the first semiconductor laser 3 that is a generation source of the laser beam L1 (solid line in the figure) having a specific wavelength, and the laser beam L1. And a second semiconductor laser 4 which is a generation source of laser light L2 (broken line in the figure) having a different wavelength. Note that one of the pair of optical disks 6 and 7 is an optical information recording medium for the laser beam L1, and the other second optical disk 7 is an optical information recording medium for the laser beam L2.

  The optical pickup device of FIG. 2 is further disposed between a polarizing beam splitter 9 that emits reflected light from the optical disks 6 and 7 toward the photodetector 8, and the objective lens 1 and the coupling lens 5. A quarter-wave plate 10, a stop 11 disposed in front of the objective lens 1, an astigmatism optical system 12 including a cylindrical lens and a concave lens, and a biaxial actuator 13 for focusing and tracking are provided.

  In the optical pickup device, the objective lens 1, the coupling lens 5, the polarization beam splitter 9, the quarter wavelength plate 10, the diaphragm 11, the astigmatism optical system 12, and the actuator 13 are respectively The laser beams L1 and L2 are appropriately converged to form a condensing spot or function as a condensing optical system for guiding return light from the condensing spot to the photodetector 8.

  In FIG. 2, the semiconductor lasers 3 and 4 are depicted separately for the sake of clarity, but as long as there is no obstacle in receiving and detecting reflected light in the photodetector 8, for example, It is also possible to adopt a configuration in which two semiconductor lasers are formed close to each other in one chip. In this case, the difference in the position in the direction perpendicular to the optical axis of each light emitting portion in each semiconductor laser 3 and 4 is slight. Further, the positions of the light emitting portions in the semiconductor lasers 3 and 4 in the optical axis direction can be easily matched.

  In the optical pickup device of FIG. 2, it is assumed that the short wavelength side is the laser light L1 and the long wavelength side is the laser light L2. The first optical disk 6 on which one laser beam L1 is incident has a protective substrate 61 thinner than the protective substrate 71 of the second optical disk 7 on which the other laser beam L2 is incident. The laser beams L1 and L2 can be used to reproduce information recorded on the optical discs 6 and 7 at a high density, or information can be recorded on the optical discs 6 and 7 at a high density. The recording / reproducing information with respect to the optical disc 6 has a higher density.

  More specifically, for example, as a numerical value conforming to the standard of a normal disk recorder, the wavelength of the laser beam L1 is 655 nm corresponding to DVD (Digital Versatile Disc), and the wavelength of the laser beam L2 is CD (Compact Disc). It is assumed that the wavelength is 785 nm. That is, the first optical disc 6 for reproducing and / or recording information by the laser beam L1 is adapted to the DVD standard, and for reproducing and / or recording information by the laser beam L2. The second optical disc 7 conforms to the CD standard. Therefore, the thickness of the protective substrate 61 of the first optical disc 6 is 0.600 mm, whereas the protective substrate 71 of the second optical disc 7 is thicker, 1.200 mm. Thus, when there is a difference in the thicknesses of the protective substrates 61 and 71, a difference also occurs in the occurrence of spherical aberration and the like. More specifically, first, in general, in an optical pickup device, the substrate thickness of an optical disk increases as the wavelength of the laser beam used increases. That is, as described above, the short-wavelength laser light L1 enters the thin first optical disc 6, and the long-wavelength laser light L2 enters the thick second optical disc 7. In the case of the long wavelength laser beam L2, for example, the spherical aberration is over. Therefore, when information is recorded or reproduced on the optical disks 6 and 7 having different substrate thicknesses using the laser beams L1 and L2 having different wavelengths in the same mechanism as in the optical pickup device of the present embodiment. It is necessary to appropriately suppress aberrations that appear differently depending on the wavelength and the substrate thickness. Details of how to suppress the aberration will be described later.

  In FIG. 2, the objective lens 1 is drawn doubly by a solid line and a broken line. When the laser beam L1 is used, the objective lens 1 is positioned at, for example, a solid line portion. When the laser beam L2 is used, the objective lens 1 is shifted to a position at a broken line portion, for example. Thereby, the distance from the coupling lens 5 can be optimized according to the laser light emitted from the laser light emitting device 2, and the optical disks 6 and 7 that are alternatively arranged at the same position can be optimized. A focused spot can be formed. At this time, the distance from the first semiconductor laser 3 to the surface of the protective substrate 61 that is the protective layer of the first optical disc 6 and the surface from the second semiconductor laser 4 to the surface of the protective substrate 71 that is the protective layer of the second optical disc 7. The distance is almost the same. As a result, it is possible to read and record information from the optical disks 6 and 7 by the same optical system.

  The coupling lens 5 serves as a collimator lens that collimates the light beams of the laser beams L1 and L2 emitted from the laser beam emitting device 2. In the present embodiment, the objective lens 1, the coupling lens 5, and the like have appropriate refractive index characteristics to suppress different aberrations for each wavelength, which is a problem. The focus is on. In order to suppress the occurrence of spherical aberration when forming the focused spot, the collimating function may be lowered by decreasing the refractive index of the coupling lens 5 as the wavelength of the laser beam is longer and the protective substrate is thicker. As a result, a more divergent laser beam is incident on the objective lens 1, and as a result, the spherical aberration acts in an under state. That is, by increasing the refractive index of the coupling lens 5 on the long wavelength side, it is possible to provide a function that cancels out aberrations caused by the substrate thickness. On the contrary, for a laser beam having a short wavelength and a thin protective substrate, the refractive index of the coupling lens 5 may be increased. That is, a material having a high refractive index for the laser light L1 having a short wavelength and a material having a low refractive index for the laser light L2 having a long wavelength are selected as the coupling lens 5. When the lens material of the coupling lens 5 is determined, the suppression of spherical aberration as described above can be effectively achieved by setting the Abbe number range to, for example, 0.8 to 1.7. FIG. 3 is a graph in which the Abbe number of the coupling lens 5 in which the lens material is appropriately changed is plotted on the horizontal axis and the residual amount of wavefront aberration is plotted on the vertical axis in the optical pickup device according to the present embodiment. As is apparent from FIG. 3, when the Abbe number is in the above-described range, the coupling lens 5 is appropriate for the laser beam L1 having a wavelength of 655 nm and the laser beam L2 having a wavelength of 785 nm. It can be seen that the lens has refractive power and the spherical aberration at the focused spot is sufficiently suppressed.

  Hereinafter, the operation of the optical pickup device shown in FIG. 2 will be described. First, when reproducing information from the first optical disc 6, the laser beam L 1 is emitted only from the first semiconductor laser 3 among the semiconductor lasers installed in the laser beam emitting device 2. The light beam emitted from the first semiconductor laser 3 passes through the polarization beam splitter 9, the coupling lens 5, and the quarter wavelength plate 10 and becomes a circularly polarized parallel light beam. This light beam is focused by the diaphragm 11, and a focused spot is formed on the information recording surface 62 by the objective lens 1 through the protective substrate 61 of the first optical disk 6.

  The light beam modulated and reflected by the information bit on the information recording surface 62 is transmitted again through the objective lens 1, the stop 11, the quarter wavelength plate 10, and the coupling lens 5, and enters the polarization beam splitter 9. The light beam incident on the polarization beam splitter 9 is reflected here and is given astigmatism when passing through the astigmatism optical system 12 and is incident on the photodetector 8. By using the output signal of the photodetector 8, a read signal for information recorded on the first optical disc 6 can be obtained.

  At this time, a spot shape change on the photodetector 8 and a light amount change due to a position change are detected to perform focus detection and track detection. Based on this detection, the objective lens 1 is moved in the optical axis direction so that the biaxial actuator 13 incorporated in the controller 14 forms an image of the light beam from the first semiconductor laser 3 on the information recording surface 62 of the first optical disk 6. At the same time, the objective lens 1 is moved in a direction perpendicular to the optical axis so that the light flux from the first semiconductor laser 3 is imaged on a predetermined track.

  Next, a case where information is reproduced from the second optical disk 7, this is to emit the laser beam L 2 from only the second semiconductor laser 4 among the semiconductor lasers installed in the laser beam emitting device 2. As a result, it is executed in the same manner as when information is reproduced from the first optical disc 6, and is condensed on the information recording surface 72 via the protective substrate 71 of the second optical disc 7, and the reflected light from the information recording surface 72 is detected by light. Incident on the vessel 8. The operation of the following optical pickup device is the same as that of the first optical disc 6. However, in this operation, the reference position of the objective lens 1 is previously shifted to the position indicated by the broken line in FIG.

  The above has been a description of the case where information is reproduced from each of the optical discs 6 and 7, but when the information is recorded on each of the optical discs 6 and 7, the same operation is performed for the optical system shown in FIG. However, the intensities of the laser beams L1 and L2 emitted from the semiconductor lasers 3 and 4 are set to a predetermined threshold value or more for recording. In addition, the sequence of tracking, focusing, confirmation of writing information, and the like at the time of writing can be appropriately changed according to the use and specifications of the optical pickup device.

以上の表１において、第４面から第６面は非球面となっている。各面の円錐定数κ，非球面係数Ａｍは、以下の表２で与えられる。尚、第５面は、対物レンズ１表面であり、中心部と周辺部とでは異なる特性を持つ。特に、周辺部は、輪帯状の回折面であり、これを第５´面とする。

以上の表２において、非球面は、
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
ｒ ：非球面頂点の曲率半径
κ ：円錐定数
Ａ_２ｔ：第２ｔ次（ｔは２以上の自然数）の非球面係数
として次式

で与えられる。 With the optical pickup device according to the present embodiment described above, it is possible to obtain good properties for reproducing and / or recording information on both the first optical disc 6 and the second optical disc 7.
[First embodiment]
This example relates to the one based on the first embodiment, in particular, the coupling lens 5 having a refractive index characteristic. In this case, the objective lens 1 is selected so that the refractive index is substantially constant regardless of the wavelength of the laser light. For example, an Abbe number that minimizes the residual amount of wavefront aberration is selected with reference to FIG. 3 so that the refractive index characteristic of the coupling lens 5 is optimized. Specifically, the refractive index nd = 1.8303 for the d-line (wavelength 587.6 nm) of the coupling lens 5 is set, and the Abbe number νd is 1.078. Table 1 shows the lens data of the condensing optical system determined as described above. In Table 1, “i-th surface” is the surface number of the lens surface from the light source side, “ri” is the radius of curvature of the corresponding lens surface, and “di” is from the corresponding lens surface to the next lens surface. “Ni” indicates the refractive index from the corresponding lens surface to the next lens surface.

In Table 1 above, the fourth to sixth surfaces are aspheric. The conic constant κ and the aspheric coefficient Am of each surface are given in Table 2 below. The fifth surface is the surface of the objective lens 1 and has different characteristics in the central portion and the peripheral portion. In particular, the peripheral portion is an annular diffractive surface, which is the 5 ′ surface.

In Table 2 above, the aspherical surface is
x: distance from the tangent plane of the aspherical vertex of the point on the aspherical surface whose height from the optical axis is h: height from the optical axis r: radius of curvature κ of the aspherical vertex: conic constant A _2t : As an aspherical coefficient of 2t order (t is a natural number of 2 or more)

Given in.

で与えられる。 Furthermore, regarding the 5 ′ surface, it is an annular diffractive surface, and the optical path difference function Φ for describing this is
p: The order of the diffracted light having the maximum amount of diffraction among the diffracted light generated on the diffractive surface. In this case, 1
C _2j : Coefficient of the optical path difference function of the 2jth order (j is a natural number of 1 or more) h: The following expression as the height perpendicular to the optical axis

Given in.

4A and 4B show spherical aberration amounts with respect to respective numerical apertures on the DVD side (ie, laser light L1) and CD side (ie, laser light L2) in the optical pickup device of the present embodiment. is there. As is clear from FIG. 4A, the DVD side includes diffractive imaging by the 5 ′ surface, so the amount of aberration is suppressed over a wide range, and a sufficient numerical aperture can be obtained during imaging. On the other hand, as shown in FIG. 4B, since the protective substrate 71 is thick on the CD side and the numerical aperture originally required may be smaller than that on the DVD side, an image is formed in a certain range or more. There is no need. Correspondingly, as shown in FIG. 4B, in this embodiment, image formation by diffraction is not performed on the CD side. In either case, it can be seen that the amount of aberration is sufficiently suppressed with respect to the required range.
[Second Embodiment]
In the first embodiment, the problem is solved by providing the refractive index characteristic particularly with respect to the coupling lens 5. However, in the present embodiment, the problem is provided by providing the refractive index characteristic with respect to the objective lens 1. To solve the problem. That is, spherical aberration or the like caused by a difference in thickness between the protective substrates 61 and 71 is suppressed by providing the objective lens 1 with a refractive index difference. More specifically, when the long-wavelength laser light L2 is used, the substrate thickness of the optical disk 7 is increased, so that, for example, the spherical aberration is over, so that the spherical surface can be increased by increasing the refractive index of the objective lens 1. Make the aberration work in the under state. On the contrary, when the laser beam L1 is used, since the substrate thickness of the optical disk 7 becomes small, the refractive index of the objective lens 1 needs to be made smaller.

以上の表３において、第１実施例と同様に第４面から第６面は非球面となっている。各面の円錐定数κ，非球面係数Ａｍは、以下の表４で与えられ、非球面は上述の数１で与えられる。尚、第５面は、対物レンズ１表面であり、中心部と周辺部とでは異なる特性を持つ。特に、周辺部は、輪帯状の回折面であり、これを第５´面とする。第１実施例同様、光路差関数は数２で与えられる。

図６（ａ）及び（ｂ）は、本実施例の光ピックアップ装置におけるＤＶＤ側（即ちレーザ光Ｌ１）及びＣＤ側（即ちレーザ光Ｌ２）のそれぞれの開口数に対する球面収差量を示したものである。図６（ａ）から明らかなように、ＤＶＤ側は第５´面による回折結像を含むので広い範囲にわたって収差量が抑えられており、結像に十分な開口数が得られる。一方、図６（ｂ）に示すように、ＣＤ側は、保護基板７１が厚く、もともと必要とされる開口数がＤＶＤ側に比べ小さくて良いので、ある一定以上の範囲に関しては結像される必要がない。これに対応して、図６（ｂ）に示すように、本実施例において、回折による結像をＣＤ側に関しては行っていない。いずれの場合も必要な範囲に関して十分に収差量が抑えられていることが分かる。 When the lens material of the objective lens 1 is determined, the above-described suppression of spherical aberration can be effectively achieved by setting the Abbe number range to -2.0 to -0.8. FIG. 5 is a graph in which the Abbe number of the objective lens 1 is taken on the horizontal axis and the residual amount of wavefront aberration is taken on the vertical axis. From FIG. 5, if the Abbe number is in the above range, the objective lens 1 has appropriate refractive power for the laser beam L1 having a wavelength of 655 nm and the laser beam L2 having a wavelength of 785 nm, It can be seen that the spherical aberration at the focused spot is sufficiently suppressed. The mechanism of the optical pickup device and the standard of the disk recorder in the present embodiment are the same as those in the first embodiment, and are omitted.
[Second Embodiment]
This example relates to the objective lens 1 having a predetermined refractive index characteristic based on the second embodiment. In this case, the refractive index of the coupling lens 5 is assumed to be substantially constant regardless of the wavelength of the laser light. For example, an Abbe number that minimizes the residual amount of wavefront aberration is selected with reference to FIG. 5 so that the refractive characteristics of the objective lens 1 are optimized. Specifically, the refractive index of the objective lens 1 with respect to the d-line (wavelength 587.6 nm) = 1.4372, and the Abbe number νd = −1.194. Table 3 shows the lens data of the condensing optical system thus determined. In Table 3, “i-th surface” is the surface number of the lens surface from the light source side, “ri” is the radius of curvature of the corresponding lens surface, and “di” is from the corresponding lens surface to the next lens surface. “Ni” indicates the refractive index from the corresponding lens surface to the next lens surface.

In Table 3 above, the fourth to sixth surfaces are aspheric as in the first embodiment. The conic constant κ and the aspherical coefficient Am of each surface are given by the following Table 4, and the aspherical surface is given by the above-mentioned formula 1. The fifth surface is the surface of the objective lens 1 and has different characteristics in the central portion and the peripheral portion. In particular, the peripheral portion is an annular diffractive surface, which is the 5 ′ surface. As in the first embodiment, the optical path difference function is given by Equation 2.

FIGS. 6A and 6B show the amount of spherical aberration with respect to the respective numerical apertures on the DVD side (ie, laser beam L1) and CD side (ie, laser beam L2) in the optical pickup device of this embodiment. is there. As apparent from FIG. 6 (a), the DVD side includes diffractive image formation by the 5 'surface, so that the amount of aberration is suppressed over a wide range, and a sufficient numerical aperture for image formation can be obtained. On the other hand, as shown in FIG. 6B, since the protective substrate 71 is thick on the CD side and the numerical aperture originally required may be smaller than that on the DVD side, an image is formed in a certain range or more. There is no need. Correspondingly, as shown in FIG. 6B, in this embodiment, image formation by diffraction is not performed on the CD side. In either case, it can be seen that the amount of aberration is sufficiently suppressed with respect to the required range.

  In the first embodiment and the present embodiment, either one of the objective lens 1 and the coupling lens 5 has a refractive index characteristic. However, both of them have a refractive index characteristic as described above. It is possible to optimize the imaging by the same adjustment. Further, laser light having a wavelength other than that of the above embodiment can be dealt with by appropriately setting the Abbe number of the objective lens 1 and the coupling lens 5 at the corresponding wavelength. It can be applied to other than CD.

  Further, the objective lens 1 and the coupling lens 5 which are optical elements are both configured as one lens per group, but may be a lens group consisting of a plurality of lenses, and an optical element having the refractive index characteristic in the lens group. Due to the presence of at least one component, the same effect as in the above embodiment can be obtained.

It is a figure explaining the under and over of spherical aberration. It is a figure explaining the structure of the optical pick-up apparatus which concerns on embodiment of this invention. It is a graph which shows the relationship between the Abbe number of the coupling lens in 1st Embodiment, and a residual aberration. (A) is a graph showing the relationship between spherical aberration on the DVD side and pupil coordinates (numerical aperture) in the first embodiment. (B) is a graph showing the relationship between spherical aberration on the CD side and pupil coordinates (numerical aperture) in the first embodiment. It is a graph which shows the relationship between the Abbe number of the objective lens in 2nd Embodiment, and a residual aberration. (A) is a graph showing the relationship between spherical aberration on the DVD side and pupil coordinates (numerical aperture) in the second embodiment. (B) is a graph showing the relationship between spherical aberration on the CD side and pupil coordinates (numerical aperture) in the second embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Objective lens, 3, 4 ... Semiconductor laser, 5 ... Coupling lens, 6, 7 ... Optical disk, 8 ... Photodetector, L1, L2 ... Laser beam

Claims (9)

A first light source that emits a light beam having a first wavelength to be irradiated on an information recording surface of a first optical information recording medium set to a first substrate thickness;
A second light source that is disposed in the vicinity of the first light source and emits a light beam having a second wavelength to be irradiated onto the information recording surface of the second optical information recording medium set to the second substrate thickness;
The distance from the first light source to the surface of the protective layer of the first optical information recording medium is substantially the same as the distance from the second light source to the surface of the protective layer of the second optical information recording medium. With an optical system,
The condensing optical system includes at least one optical component, and the first and second optical information recording media generated by the condensing optical system and the first and second optical information recording media due to a refractive index characteristic of the at least one optical component. An optical pickup device characterized by relatively adjusting an imaging characteristic of a light beam having a second wavelength.   The at least one optical component adjusts the aberration generated by the condensing optical system and the first and second optical information recording media relatively with respect to the light fluxes of the first and second wavelengths. The optical pickup device according to claim 1.   3. The optical pickup device according to claim 1, wherein the first substrate thickness is smaller than the second substrate thickness, and the first wavelength is shorter than the second wavelength. 4.   The at least one optical component is a collimator lens for supplying a parallel light beam to an objective lens in order to form a light spot on one of the information recording surfaces of the first and second optical information recording media. The optical pickup device according to any one of claims 1 to 3, wherein:   The optical pickup device according to claim 4, wherein the collimator lens has a focal length that is longer than the first wavelength with respect to the second wavelength.   6. The optical pickup device according to claim 5, wherein the collimator lens is composed of a single optical element, and the Abbe number of the optical element is in the range of 0.8 to 1.7.   The at least one optical component is disposed to face the first and second optical information recording media in order to form a light spot on one of the information recording surfaces of the first and second optical information recording media. The optical pickup device according to claim 1, wherein the optical pickup device is an objective lens.   The optical pickup device according to claim 7, wherein the objective lens has a focal length that is longer than the first wavelength with respect to the second wavelength. 9. The optical pickup device according to claim 8, wherein the objective lens is made of a single optical element, and the Abbe number of the optical element is in the range of -2.0 to -0.8.

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