JP2008052881A - Optical pickup apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical pickup apparatus in which unnecessary diffraction light emitted from diffraction structure can be eliminated effectively. <P>SOLUTION: In a luminous flux passing through a first wavelength plate OE1 and a second wavelength plate OE2, since diffracted light (sub-luminous flux) with a diffraction order other than the prescribed diffraction order is a noise component and a polarization direction is not changed, and as it passes through a second polarization beam splitter BS2, it does not reach a photodetector PD, thereby, occurrence of errors or the like can be suppressed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical pickup device that records and / or reproduces information using diffracted light.

  In recent years, in an optical pickup device, the reproduction of information recorded on an optical disk and the shortening of the wavelength of a laser light source used as a light source for recording information on the optical disk have progressed. When using an objective lens with the same numerical aperture (NA) as (Digital Versatile Disc), it is possible to record 15 to 20 GB of information on an optical disc with a diameter of 12 cm, and the NA of the objective lens is up to 0.85. When the height is increased, information of about 25 GB can be recorded on an optical disk having a diameter of 12 cm. Hereinafter, in this specification, an optical disk and a magneto-optical disk using a blue-violet laser light source are collectively referred to as a “high density optical disk”.

  In a high-density optical disk using an NA 0.85 objective lens, coma aberration generated due to the inclination (skew) of the optical disk increases, so the protective layer is designed thinner than in the case of DVD (0 of DVD). Some have reduced the amount of coma due to skew. By the way, it cannot be said that the value as a product of an optical disk player / recorder is sufficient just to be able to appropriately record / reproduce information on such a high-density optical disk. At present, based on the reality that a wide variety of DVDs and CDs (compact discs) are sold and widely used, an optical pickup device mounted on an optical disc player / recorder for high-density optical discs includes high-density optical discs and DVDs. Furthermore, it is desired to have a performance capable of appropriately recording / reproducing information while maintaining compatibility with any CD.

  As a method for recording / reproducing information appropriately while maintaining compatibility with both high-density optical discs and DVDs, and even CDs, optical systems for high-density optical discs and optical systems for DVDs and CDs are used. A method of selectively switching the system to and from the recording density of an optical disk for recording / reproducing information is conceivable, but a plurality of optical systems are required, which is disadvantageous for miniaturization and increases the cost.

  Therefore, in order to simplify the configuration of the optical pickup device and reduce the cost, the optical system for high-density optical discs and the optical system for DVDs and CDs must be shared in compatible optical pickup devices. It is preferable to reduce the number of optical components constituting the optical pickup device as much as possible. In addition, it is most advantageous for simplifying the configuration of the optical pickup device and reducing the cost to make the objective optical system arranged opposite to the optical disk in common. In order to obtain a common objective optical system for a plurality of types of optical disks having different recording / reproducing wavelengths, it is desirable to form a phase structure having a wavelength dependency of spherical aberration in the objective optical system.

Patent Document 1 describes an objective optical system that has a diffractive structure as a phase structure and can be used in common for high-density optical discs and conventional DVDs and CDs, and an optical pickup device equipped with the objective optical system. Has been.
European Published Patent No. 1304689

  Here, in the optical pickup device disclosed in Patent Document 1, the diffractive structure of the objective optical system is designed so that the diffraction angle varies depending on the wavelength of the light beam passing therethrough, so that different optical disks can be compatible. It has become. However, in general, such a diffractive structure is often designed to exhibit good utilization efficiency when, for example, a high-density optical disk or DVD is used. In particular, when CD information recording light is incident on the diffractive structure, in addition to a predetermined order of diffracted light used for recording / reproducing information, other orders of unnecessary diffracted light may be generated. If the unnecessary diffracted light is incident on the photodetector, there is a risk of hindering correct signal reading.

  The present invention has been made in view of the problems of the prior art, and an object thereof is to provide an optical pickup device that can effectively remove unnecessary diffracted light generated from a diffractive structure.

The optical pickup device according to claim 1 is configured to perform a first optical information recording with a thickness t1 of the protective layer on a light beam having a wavelength λ1 emitted from a first light source via a condensing optical system including an objective lens. In an optical pickup device that records and / or reproduces information by focusing on an information recording surface of a medium,
A first optical element comprising a first optical region and a second optical region across the optical axis;
A second optical element comprising a third optical region and a fourth optical region across the optical axis;
A condensing element for condensing a light beam between the first optical element and the second optical element;
Polarization splitting means;
A photodetector,
The main light beam that has passed through the first optical region and the fourth optical region, and the main light beam that has passed through the second optical region and the third optical region have a polarization direction in the first direction. Yes,
The sub-light beam that has passed through the first optical region and the third optical region and the sub-light beam that has passed through the second optical region and the fourth optical region have a polarization direction in the first direction. Is a second direction different from
A diffractive structure is provided on the optical surface of the condensing optical system;
When a light beam reflected from the information recording surface of the first optical information recording medium enters the diffractive structure, diffracted light of a predetermined diffraction order emitted from the diffractive structure serves as a main light beam, and the first optical element After passing through, the light is condensed between the first optical element and the second optical element, passes through the second optical element, and further enters the photodetector via the polarization branching means. On the other hand, the diffracted light of orders other than the predetermined diffraction order emitted from the diffractive structure is condensed between the first optical element and the second optical element as a sub-beam. Therefore, the sub-light beam incident on the polarization branching unit is branched by the polarization branching unit so as not to enter the photodetector. In the present specification, the “optical axis” refers to the optical axis of the objective lens.

  The principle of the present invention will be described with reference to FIG. 1, the first optical element OE1 has a first optical region A above the optical axis X and a second optical region B below the optical axis. Further, the second optical element OE2 that is separated from the first optical element OE1 by a predetermined distance d along the optical axis has a third optical region C above the optical axis X, and is below the optical axis. Has a fourth optical region D. Each of the optical regions A to D has a function of generating a phase difference of λ / 4 in the light beam passing therethrough, but the directions of the optical regions A and B and C and D are different from each other. As a specific example, a phase difference of + λ / 4 is generated in the light beam that has passed through the first optical region A, a phase difference of −λ / 4 is generated in the light beam that has passed through the second optical region B, It is conceivable that a phase difference of −λ / 4 is generated in the light beam that has passed through the third optical region C, and a phase difference of + λ / 4 is generated in the light beam that has passed through the fourth optical region D. Further, a phase difference of + λ / 2 is generated in the light beam that has passed through the first optical region A, and the light beam that has passed through the second optical region B has passed through the third optical region C without causing a phase difference. A phase difference of + λ / 2 may be generated in the light beam, and the light beam that has passed through the fourth optical region D may not cause a phase difference.

  Here, it is assumed that reflected light in a linearly polarized state is emitted from the information recording surface of the optical information recording medium from the left side of FIG. A marginal ray α in a diffracted light beam of a predetermined order that has passed through the diffractive structure of the objective lens is indicated by a solid line, a marginal light beam β in a diffracted light beam of a non-predetermined order is indicated by a dotted line, and a marginal ray γ is indicated by a one-dot chain line. That is, here, the light ray α is a regular information recording light, and the light rays β and γ are noise components.

  When the light beam α is collected at the position X in the optical axis direction between the first optical element OE1 and the second optical element OE2, the light beam β is closer to the optical information recording medium side than the position X ( The light beam γ is condensed at the position Y on the light detector side (right side in FIG. 1) with respect to the position X.

  However, the portion of the light ray α that passes through the first optical region A necessarily passes through the fourth optical region D, and the portion that passes through the second optical region B always passes through the third optical region D. It will pass through region C. Therefore, the polarization direction of the light beam α after being emitted from the second optical element OE2 is 90 degrees different from the light beam α before being incident on the first optical element OE1 (first polarization direction).

  On the other hand, the portion of the light beam β that passes through the first optical region A passes through the third optical region C, and the portion that passes through the second optical region B further passes through the fourth optical region C. It will pass through region D. Therefore, the polarization direction of the light beam β after exiting from the second optical element OE2 is invariable with respect to the light beam β before entering the first optical element OE1 (a second direction different from the first polarization direction). Polarization direction).

  Similarly, the portion of the light ray γ that passes through the first optical region A passes through the third optical region C, and the portion that passes through the second optical region B further passes through the fourth optical region. D will be passed. Accordingly, the polarization direction of the light beam γ after exiting from the second optical element OE2 is invariable with respect to the light beam γ before entering the first optical element OE1 (a second direction different from the first polarization direction). Polarization direction).

  As described above, since the normal diffracted light (also referred to as the main light beam) and the noise component light (also referred to as the sub light beam) have different polarization directions, for example, by 90 degrees, the light emitted from the second optical element OE1 is polarized. By passing through a polarization branching unit such as a beam splitter, for example, regular diffracted light can be reflected and noise component light can be transmitted, thereby removing the noise component. In addition, the regular diffracted light may be transmitted and guided to the photodetector, and the noise component light may be reflected. The polarization splitting means is not limited to the polarization beam splitter, and a linear polarizing plate that allows only regular information recording light in a predetermined polarization state to pass therethrough may be used.

  According to a second aspect of the present invention, there is provided the optical pickup device according to the first aspect, wherein the condensing optical system emits a light beam having a wavelength of λ2 (λ2 <λ1) emitted from the second light source. Since the light is focused on the information recording surface of the second optical information recording medium of t2 (t2 <t1), information can be recorded and / or reproduced in a manner compatible with different optical information recording media. Yes.

  According to a third aspect of the present invention, in the optical pickup device according to the first or second aspect, at least one reflective element is disposed in an optical path between the first optical element and the photodetector. It is characterized by.

  According to a fourth aspect of the present invention, in the invention according to any one of the first to third aspects, the first optical element and the second optical element have a structural birefringence structure. And

Here, structural birefringence will be described. Structural birefringence refers to birefringence generated by the direction of fine structure. For example, flat plates with different refractive indexes that do not have birefringence characteristics are arranged in parallel with a period sufficiently smaller than the wavelength of light (<λ / 2). In addition, it is known that a fine periodic structure (so-called line and space structure) generates birefringence characteristics (see Principle of Optics, Max Born and Emi Wolf, PERGAMON PRESS LTD.). The refractive index n _{p of the} light whose polarization direction is parallel to the groove and the refractive index n _v of the perpendicular light are respectively
n _p = (tn ₁ ² + (1−t) n ₂ ² ) ^1/2 (1)
n _v = 1 / (t / n ₁ ² + (1−t) / n ₂ ² ) ^1/2 (2)
It becomes. n ₁ and n ₂ are the refractive index of the material (line) in which the fine periodic structure is formed and the refractive index of the material (space) filling the groove, respectively, and t is the duty ratio of the fine periodic structure. If the width of w is w ₁ and the width of the space is w ₂ ,
t = w ₁ / (w ₁ + w ₂ ) (3)
It is.

The birefringence characteristics of quartz and calcite are unique to the substance and can hardly be changed. On the other hand, the birefringence of fine periodic structures can be changed by changing the material and shape. Can be easily controlled. Further, the phase difference (delay amount) Re between the light whose polarization direction is parallel to the groove and the light perpendicular to the groove is d, where the birefringence height (groove depth) of the fine periodic structure is d.
Re = (n _p −n _v ) d (4)
It becomes. From these equations, the phase difference (delay amount) Re can be changed by making the birefringence duty ratio t of the fine periodic structure and the birefringence height (groove depth) d of the fine periodic structure variable. I understand.

  For example, when trying to form a λ / 4 wavelength plate for 400 nm laser light as an optical element, a resin material having a refractive index of about 1.5 at room temperature is used, the line width is 100 nm, and the space width is Is 90 nm, the fine structure height d needs to be 1200 nm. That is, the aspect ratio is about 12. According to the fine periodic structure, it is easy to integrally manufacture wave plates having the same phase difference but different optical axis orientations, and the loss area generated at the boundary between the wave plates can be reduced to several μm or less. Light loss can be reduced and unnecessary light blocking performance can be improved. In particular, in order to obtain an optical pickup device compatible with a plurality of types of optical discs such as CD, DVD, and BD, it is desired to compensate the performance of the optical element with a plurality of types of wavelengths used for the respective optical discs. In contrast, when forming a staircase structure in an optical element, even if a complicated structure (such as a wall height) is designed to cope with a plurality of wavelengths used for each optical disk, the resin is molded with the mold. According to the nanoprint method, batch production is possible. Other structural birefringence structures include, for example, self-cloning photonic crystals formed by a self-cloning method (see JP 2001-51122 A).

  ADVANTAGE OF THE INVENTION According to this invention, when using the optical information recording medium provided with the multilayer information recording surface, the optical pick-up apparatus which can remove a noise effectively can be provided.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 2 shows a book that can appropriately record / reproduce information to / from a BD (Blu-ray Disc) that is a second optical information recording medium (also called an optical disk) and a CD that is the first optical information recording medium. It is a figure which shows schematically the structure of optical pick-up apparatus PU1 of embodiment. Needless to say, the present invention can be applied to HD DVD, DVD and other optical disks.

  FIG. 3 is a perspective view showing a first wave plate OE1 that is a first optical element and a second wave plate OE2 that is a second optical element. In FIG. 3, the first optical region A and the second optical region B are formed on the optical surface of the thin plate-shaped first wave plate OE1 with the optical axis (not shown) interposed therebetween. In the first optical region A, a plurality of fine walls WA are arranged at equal intervals. In the second optical region B, a plurality of fine walls WB are arranged at equal intervals, but the end portions are joined with each wall WB and the wall WA being orthogonal to each other. A structural birefringence structure is formed by the walls WA and WB.

  Similarly, a third optical region C and a fourth optical region D are formed on the optical surface of the thin plate-like second wavelength plate OE2 with an optical axis (not shown) interposed therebetween. In the third optical region C, a plurality of fine walls WC are arranged at equal intervals, but are orthogonal to the wall WA facing the optical axis direction when viewed in the optical axis direction. A plurality of fine walls WD are arranged at equal intervals in the fourth optical region D, but are orthogonal to the walls WB facing the optical axis direction when viewed in the optical axis direction. Each wall WC and wall WD are joined to each other in an orthogonal state, and a structural birefringence structure is formed by the walls WC and WD.

  Among the light beams that have passed through the first wave plate OE1 and the second wave plate OE2, the light beams that have passed through the first optical region A and the fourth optical region D, and the second optical region B and the third optical region C. And the second optical region B and the fourth optical region D. The light beams that have passed through the first optical region A and the third optical region C, and the second optical region B and the fourth optical region D. The direction of polarization of the light beam that has passed through is unchanged. Here, by adjusting the interval and height of the walls WA to WD, structural birefringence is generated when a light beam having a wavelength λ2 (350 nm ≦ λ2 ≦ 450 nm) passes, or the wavelength λ1 (750 nm ≦ λ1 ≦). It can be configured to react only to a specific wavelength such that structural birefringence is generated when a light beam of 800 nm) passes.

  In FIG. 2, the spherical surface aberration caused by the difference between the thickness t2 = 0.1 mm of the BD protective layer and the thickness t1 = 1.2 mm of the CD protective layer is formed on the optical surface of the objective lens OBJ. A diffractive structure DS for correction is formed. When recording and / or reproducing information on the BD, when the first semiconductor laser (second light source) LD1 emits light, the divergent light beam having the wavelength λ2 emitted from the semiconductor laser LD1 is shown in FIG. As described above, after passing through the first coupling lens CL1 and converted into a parallel beam, it is reflected by the first polarization beam splitter BS1, and further passed through the λ / 4 wavelength plate QWP, and then the beam diameter is reduced by a diaphragm (not shown). Is controlled, passes through the diffraction structure DS of the objective lens OBJ, and the 0th-order diffracted light becomes a spot formed on the information recording surface RL1 of the BD. The objective lens OBJ performs focusing and tracking by a biaxial actuator (not shown) arranged around the objective lens OBJ.

  The reflected light beam modulated by the information pits on the information recording surface RL1 passes through the objective lens OBJ and the λ / 4 wavelength plate QWP again, and then passes through the first polarizing beam splitter BS1 and the third polarizing beam splitter BS3. The light beam emitted from the third polarization beam splitter BS3 is converted into a convergent light beam by the lens L3 that is a condensing element, passes through the first wavelength plate OE1, and is between the first wavelength plate OE1 and the second wavelength plate OE2. After condensing, it passes through the second wave plate OE2 and is converted into a parallel light beam by the lens L4.

  Here, the light beam that has passed through the first wave plate OE1 and the second wave plate OE2 is only the reflected light (main light beam) from the information recording surface RL1, and the polarization direction is inclined by 90 degrees. Are reflected by the second polarization beam splitter BS2, condensed by the lens L5, added with astigmatism by the sensor lens SEN, and converged on the light receiving surface of the photodetector PD. And the information recorded on BD can be read using the output signal of photodetector PD.

  On the other hand, when recording and / or reproducing information on a CD, when the second semiconductor laser LD2 emits light, the divergent light beam having the wavelength λ1 emitted from the semiconductor laser (first light source) LD2 is shown in FIG. As shown in FIG. 2, after passing through the second coupling lens CL2 and converted into a parallel light beam, it is reflected by the third polarizing beam splitter BS3, passes through the first polarizing beam splitter BS1, and further is a λ / 4 wavelength plate. After passing through QWP, the beam diameter is regulated by a diaphragm (not shown), passes through the diffraction structure DS of the objective lens OBJ, and the first-order diffracted light becomes a spot formed on the information recording surface RL2 of the CD. The objective lens OBJ performs focusing and tracking by a biaxial actuator (not shown) arranged around the objective lens OBJ.

  The reflected light beam modulated by the information pits on the information recording surface RL2 passes through the objective lens OBJ and the λ / 4 wavelength plate QWP again, and then passes through the first polarizing beam splitter BS1 and the third polarizing beam splitter BS3. The light beam emitted from the third polarization beam splitter BS3 is converted into a convergent light beam by the lens L3 that is a condensing element, passes through the first wavelength plate OE1, and is between the first wavelength plate OE1 and the second wavelength plate OE2. After condensing, it passes through the second wave plate OE2 and is converted into a parallel light beam by the lens L4.

  Here, depending on the diffractive structure DS, even when the intensity of the + 1st order diffracted light becomes the highest when the light beam having the wavelength λ1 passes, for example, as shown in FIG. -1st order diffracted light may be generated. This is likely to occur due to the characteristics of an objective lens that is commonly used for light beams having different wavelengths λ1 and λ2 that are used when BD and CD are used interchangeably. In such a case, there is a risk of causing a reading error when the −1st order diffracted light enters the photodetector PD.

  However, in the present embodiment, in the light beams that have passed through the first wave plate OE1 and the second wave plate OE2, the first order diffracted light (the main light beam that is diffracted light of a predetermined order) emitted from the diffractive structure DS is polarized. Since the direction is inclined by 90 degrees, it is reflected by the second polarization beam splitter BS2 which is a polarization branching means, condensed by the lens L5, added with astigmatism by the sensor lens SEN, on the light receiving surface of the photodetector PD. Converge to. And the information recorded on CD can be read using the output signal of photodetector PD. However, the −1st order diffracted light emitted from the diffractive structure DS (a sub-beam which is a diffracted light of an order other than the predetermined order) is not condensed between the first wave plate OE1 and the second wave plate OE2, and accordingly Since the polarization direction does not change, the light does not pass through the second polarization beam splitter BS2 which is a polarization branching unit, and does not enter the photodetector PD. The compatible optical disk is not limited to the combination of BD and CD, but may be a combination of BD, DVD, and CD, or a combination of BD, HD DVD, DVD, and CD.

  As described above, according to the present embodiment, in the light beams that have passed through the first wave plate OE1 and the second wave plate OE2, the diffracted light (sub-light beam) of an order other than the predetermined order is a noise component, and the polarization direction Is unchanged, and therefore does not reach the photodetector PD because it passes through the second polarization beam splitter BS2, thereby suppressing the occurrence of errors and the like.

FIG. 6 is a perspective view showing a first wave plate OE1 ′ as a first optical element and a second wave plate OE2 ′ as a second optical element according to another embodiment. In FIG. 6, a first optical part A and a second optical part B are formed on a thin plate-like first wave plate OE <b> 1 ′ with an optical axis (not shown) interposed therebetween. The first optical part A has a cycle formed by repeating a plurality of chevron shapes where two slopes SLa (one slope is hidden and cannot be seen on the uppermost surface) across each of a plurality of ridge lines RLa extending in parallel. Structural birefringence structure. Such a chevron shape has a multilayer structure in which a high refractive material HR such as Si or Ta ₂ O ₂ and a low refractive material LR such as SiO ₂ are alternately stacked in a micron order by a sputtering method or the like. There are 8 layers (4 cycles). On the other hand, in the second optical part B, two slopes SLb (one slope is hidden and cannot be seen on the uppermost surface) across each of the plurality of ridge lines RLb orthogonal to the ridge line RLa and extending in parallel with each other. This is a periodic structural birefringence structure formed by repeating a plurality of chevron shapes. Such a chevron shape also has a multilayer structure in which a high refractive material HR such as Si or Ta ₂ O ₂ and a low refractive material LR such as SiO ₂ are alternately stacked on the micron order by a sputtering method or the like. There are 8 layers (4 cycles). The first optical part A and the second optical part B are joined to each other at the abutted ends.

  Similarly, a third optical part C and a fourth optical part D are formed on the optical surface of the second wavelength plate OE2 ′ with an optical axis (not shown) interposed therebetween. The third optical unit C also has a similar periodic structural birefringence structure, but is arranged so that its ridgelines RLa and RLc are orthogonal to the first optical unit A facing in the optical axis direction. The The fourth optical unit D has a similar periodic structural birefringence structure, but is arranged so that the ridge lines RLb and RLd are orthogonal to the second optical unit B facing in the optical axis direction. The Each of the third optical part C and the fourth optical part D has an eight-layer (four-period) structure, and the end portions that are abutted are joined to each other. As the above periodic birefringence structure, there is a so-called self-cloning photonic liquid crystal, which is described in, for example, JP-A-2001-51122.

  Of the light beams that have passed through the first wave plate OE1 ′ and the second wave plate OE2 ′, the light beams that have passed through the first optical region A and the fourth optical region D, and the second optical region B and the third optical device. The direction of polarization of the light beam that has passed through the region C changes by 90 degrees, the light beam that has passed through the first optical region A and the third optical region C, and the second optical region B and the fourth optical region. The direction of polarization of the light beam that has passed through the region D remains unchanged. Here, by adjusting the ridge line pitch and the layer thickness, structural birefringence occurs when a light beam having a wavelength λ2 (350 nm ≦ λ2 ≦ 450 nm) passes, or a wavelength λ1 (750 nm ≦ λ1 ≦ 800 nm). It can be configured to react only to a specific wavelength such that structural birefringence is generated when the light beam passes.

  FIG. 4 is a schematic diagram illustrating an optical pickup device according to a modification. In this modification, one surface of the prism PS is the entrance surface IP and the exit surface OP, and the other two surfaces are the reflection surfaces R1 and R2. A first structural birefringence structure OE1 including a first region A and a second region B made up of a plurality of fine walls (see FIG. 3) is formed on the entrance surface IP, and a plurality of structures are formed on the exit surface OP. A second structural birefringent structure OE2 including a third region C and a fourth region D, each of which is a fine wall (see FIG. 3), is formed. Note that the first structural birefringent structure OE1 and the second structural birefringent structure OE2 include the first optical region A, the second optical region B, and the third optical region in the direction perpendicular to the paper surface with the optical axis in between. It is assumed that C and the fourth optical region D are arranged side by side.

  In FIG. 4, the light beam reflected from the CD and passed through the expander lens EXP passes through the polarizing beam splitter BS, becomes a convergent light beam by the lens L3 that is a light condensing element, and is formed on the incident surface IP of the prism PS. After passing through the birefringent structure OE1, the light is condensed after being reflected by the reflecting surface R1, and further reflected by the second reflecting surface R2, and then passes through the structural birefringent structure OE2 formed on the exit surface OP. Then, the light is emitted from the prism PS, converted into a parallel light beam by the same lens L3, further passed through the λ / 2 wave plate HWP, and then incident on the same polarization beam splitter BS, and information to be recorded and / or reproduced. Only the reflected light from the recording surface RL1 is directed to the photodetector PD. Since other configurations are the same as those of the above-described embodiment, the description thereof is omitted. The lens that allows the incident light beam to the first wavelength plate OE1 and the lens that allows the outgoing light beam from the second wavelength plate OE2 to pass may be separated. The optical axis of the light beam traveling from the polarization beam splitter BS to the lens L3 and the optical axis of the light beam traveling from the lens L3 to the polarization beam splitter BS may be shifted in parallel. In such a case, the distance between the optical axes may be as long as the incident and outgoing light beams can be separated.

  The present invention has been described above with reference to the embodiments. However, the present invention should not be construed as being limited to the above-described embodiments, and can be modified or improved as appropriate. For example, the present invention is not limited to an optical pickup device for CD.

It is a figure for demonstrating the principle of this invention. In the present embodiment, information can be appropriately recorded / reproduced with respect to a BD (Blu-ray Disc) which is a first optical information recording medium (also referred to as an optical disc) and a CD which is a second optical information recording medium. It is a figure which shows schematically the structure of optical pick-up apparatus PU1. It is a perspective view which shows 1st wavelength plate OE1 which is a 1st optical element, and 2nd wavelength plate OE2 which is a 2nd optical element. It is a figure which shows a part of optical pick-up apparatus concerning a modification. It is a figure which shows the example of the longitudinal spherical aberration figure concerning this Embodiment. It is a perspective view which shows 1st wavelength plate OE1 'which is a 1st optical element concerning another example, and 2nd wavelength plate OE2' which is a 2nd optical element.

Explanation of symbols

A to D Optical region BS1 First polarization beam splitter BS2 Second polarization beam splitter BS3 Third polarization beam splitter CL1 First coupling lens CL2 Second coupling lens DS Diffraction structure L3 Lens L4 Lens L5 Lens LD1 First semiconductor laser LD2 Second semiconductor laser M Mirror OBJ Objective lens OE1 First optical element OE1, OE1 ′ First wave plate OE2, OE2 ′ Second optical element PD Photodetector PU1 Optical pickup device QWP λ / 4 wavelength plate
HWP λ / 2 wave plate RL1 Information recording surface RL2 Information recording surface SEN Sensor lenses WA to WD Wall

Claims (4)

By condensing the light beam having the wavelength λ1 emitted from the first light source onto the information recording surface of the first optical information recording medium having the thickness t1 of the protective layer via the condensing optical system including the objective lens. In an optical pickup device for recording and / or reproducing information,
A first optical element comprising a first optical region and a second optical region across the optical axis;
A second optical element comprising a third optical region and a fourth optical region across the optical axis;
A condensing element for condensing a light beam between the first optical element and the second optical element;
Polarization splitting means;
A photodetector,
The main light beam that has passed through the first optical region and the fourth optical region, and the main light beam that has passed through the second optical region and the third optical region have a polarization direction in the first direction. Yes,
The sub-light beam that has passed through the first optical region and the third optical region and the sub-light beam that has passed through the second optical region and the fourth optical region have a polarization direction in the first direction. Is a second direction different from
A diffractive structure is provided on the optical surface of the condensing optical system;
When a light beam reflected from the information recording surface of the first optical information recording medium enters the diffractive structure, diffracted light of a predetermined diffraction order emitted from the diffractive structure serves as a main light beam, and the first optical element After passing through, the light is condensed between the first optical element and the second optical element, passes through the second optical element, and further enters the photodetector via the polarization branching means. On the other hand, the diffracted light of orders other than the predetermined diffraction order emitted from the diffractive structure is condensed between the first optical element and the second optical element as a sub-beam. Accordingly, the sub-beam incident on the polarization branching unit is further branched by the polarization branching unit so as not to enter the photodetector.   The condensing optical system converts the light beam having the wavelength λ2 (λ2 <λ1) emitted from the second light source into the information recording surface of the second optical information recording medium having the thickness t2 (t2 <t1) of the protective layer. The optical pickup device according to claim 1, wherein the optical pickup device condenses light.   3. The optical pickup device according to claim 1, wherein at least one reflection element is disposed in an optical path between the first optical element and the photodetector. 4.   The optical pickup device according to claim 1, wherein the first optical element and the second optical element have a structural birefringence structure.