JP2007293955A - Diffraction element, objective lens unit, optical pickup, and optical disk device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diffraction element 20 capable of showing satisfactory characteristics. <P>SOLUTION: A cover layer 20D of the diffraction element 20 is constituted of a cover lower layer 20Du made of a second UV cured resin 100B and joined to a diffraction pattern PTc for a CD and a cover upper layer 20Do made of a third UV cured resin 100C and joined to the cover lower layer 20Du. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a diffractive element, and is suitably applied to a diffractive element corresponding to a plurality of wavelengths, for example.

  Conventionally, in an optical pickup of an optical disc apparatus, a diffraction element is used for the purpose of correcting an aberration of a light beam. A diffraction element forms a fine groove-shaped diffraction grating on the surface of a transparent plate-shaped element substrate, and diffracts a light beam by an optical path difference when passing through the peaks and valleys of the diffraction grating (for example, , See Patent Document 1).

Here, the diffraction efficiency of the diffractive element changes according to the depth of the groove of the diffraction pattern forming the diffraction grating and the wavelength of the light beam, and the diffraction angle changes according to the period of the diffraction pattern (or the width of the groove). To do.
JP 2005-339762 A

  As a method for manufacturing a diffraction element, injection molding in which a mold is filled with an optical resin to produce a molded product is widely used. However, when a diffractive element having a deep groove and a narrow diffraction pattern is manufactured by injection molding, the molded product is distorted due to a large mold release resistance, and the manufactured diffractive element generates aberration.

  Therefore, a molded product having a diffraction pattern as a base is formed by injection molding, and this diffraction is performed by a so-called UV replica method in which a flat plate mold is pressed against an ultraviolet curable resin applied on the diffraction pattern and cured. Some patterns are covered with the ultraviolet curable resin so as to form an optical plane to exhibit a desired diffraction performance.

  However, when this molded product has a diffraction pattern with a fine structure, if a high-viscosity resin is used as the ultraviolet curable resin, the resin cannot be filled into the fine structure, or the structure can be Since the filled resin cannot flow, it cures without increasing the density, and the refractive index of the cured resin changes from the design value.

  On the other hand, if a low-viscosity resin is used, the applied resin will flow out to the outside. Therefore, it is not possible to apply the necessary amount of resin to fill the unevenness of the diffraction pattern and reduce the effect of this unevenness. The plane cannot be formed flat.

  For this reason, when an optical plane is formed by covering a molded product having a diffraction pattern with a fine structure with an ultraviolet curable resin, there is a problem that the diffraction characteristics deteriorate.

  The present invention has been made in consideration of the above points, and intends to propose a diffraction element, an objective lens unit, an optical pickup, and an optical disc apparatus that can exhibit good characteristics.

  In order to solve this problem, in the present invention, a diffractive element that diffracts an incident light beam according to the wavelength of the light beam, a base part on which a diffraction pattern is formed, and a cover part that covers the base part, The cover portion includes a first layer made of a first material bonded to the diffraction pattern and a second layer made of a second material different from the first material and bonded to the first layer. To have.

  The diffraction pattern has a fine groove shape with a width of 10 μm or less, the first material has fluidity in the diffraction pattern before solidification, and the second material solidifies. Previously, the viscosity was higher than that of the first material.

  As a result, the first material is surely filled in the diffraction pattern so that no gap is formed between the first layer and the base portion, and the refractive index of the first layer is changed to the design value by changing the density when solidifying. The surface of the second layer can be formed flat using a sufficient amount of the second material.

  Further, since the refractive index of the second material is higher than the refractive index of the first material, the aberration caused by the unevenness formed in the first layer is canceled at the interface between the second layer and air. Thus, the aberration can be further reduced.

  According to the present invention, since the cover layer is formed in two layers, the inside of the diffraction pattern is firmly filled so that no gap is formed between the first layer and the base portion, and the density is changed when solidifying. The refractive index of the first layer can be made as designed, and thus a diffractive element, an objective lens unit, an optical pickup, and an optical disk device exhibiting good characteristics can be realized.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

(1) Configuration of Optical Disc Device (1-1) Overall Configuration of Optical Disc Device In FIG. 1, reference numeral 1 denotes an optical disc device using a diffractive element according to the present invention, which is a CD (Compact Disc) system or a DVD (Digital Versatile Disc) system. Alternatively, the optical disc 100 can be played back by any one of three methods such as a BD (Blu-ray Disc, registered trademark) method.

  The optical disc device 1 is configured to be controlled in an integrated manner by the control unit 2. When a reproduction instruction or the like from an external device (not shown) is received in a state where the optical disc 100 is loaded, a drive unit is received from the control unit 2. 3 and the signal processing unit 4 are controlled to read information recorded on the optical disc 100.

  In practice, the drive unit 3 rotates the optical disc 100 at a desired rotational speed by the spindle motor 5 based on the control of the control unit 2, and the sled motor 6 increases the optical pickup 7 in the tracking direction which is the radial direction of the optical disc 100. Further, the biaxial actuator 8 finely moves the objective lens unit 9 in two directions, ie, a focus direction and a tracking direction, which are directions in which the objective lens unit 9 approaches or separates from the optical disc 100.

  In parallel with this, the signal processing unit 4 irradiates the desired track of the optical disc 100 with a predetermined light beam from the objective lens unit 9 by the optical pickup 7, and generates a reproduction signal based on the detection result of the reflected light. The reproduction signal is sent to an external device (not shown) via the control unit 2.

  The optical pickup 7 is a so-called three-wavelength compatible type, and light having a wavelength of about 780 [nm] from the objective lens unit 9 to the optical disk 100 of the CD system (hereinafter referred to as the CD system disk 100c). The optical disc 100 is irradiated with a beam, and an optical beam 100 having a wavelength of about 650 [nm] is irradiated onto an optical disc 100 made of DVD (hereinafter referred to as a DVD disc 100d). This is called a BD type disc 100b) and is irradiated with a light beam having a wavelength of about 405 [nm].

  As described above, the optical disc apparatus 1 can reproduce the optical disc 100 by irradiating the optical disc 100 of the CD format, DVD format, or BD format with a light beam suitable for each format. .

(1-2) Configuration of Optical Pickup As shown in FIG. 2, the optical pickup 7 uses a light beam having a wavelength of about 780 [nm] for a CD system as a light beam light source (hereinafter referred to as a CD light beam). Lc) and a laser diode 11 capable of emitting a light beam having a wavelength of about 650 [nm] for DVD system (hereinafter referred to as a DVD light beam Ld), and a wavelength of about 405 [nm] for the BD system. ] And a laser diode 12 that can emit a light beam (hereinafter referred to as a BD light beam Lb).

  The coupling lens 13 converts the optical magnification of the light beam emitted from the laser diode 11.

  The beam splitter 14 reflects or transmits the light beam according to the wavelength at the reflection / transmission surface 14A. The light beam for CD Lc having a wavelength of about 780 [nm] is reflected on the reflection / transmission surface 14A. The DVD light beam Ld having a wavelength of about 650 [nm] is reflected, and the BD light beam Lb having a wavelength of about 405 [nm] is transmitted.

  The polarization beam splitter 15 reflects or transmits the light beam according to the polarization direction on the polarization plane 15A, transmits the light beam incident from the beam splitter 14 side, and adjusts the polarization direction. Thus, the light beam incident from the collimator lens 16 side is reflected.

  The collimator lens 16 converts the light beam incident from the polarization beam splitter 15 and made of divergent light into parallel light, and converts the light beam made incident from the rising mirror 17 and made of parallel light into convergent light. .

  The rising mirror 17 reflects the horizontal light beam incident from the collimator lens 16 and raises it in the vertical direction, that is, the direction in which the light beam is incident substantially perpendicularly to the optical disc 100. A vertically incident light beam is reflected in the horizontal direction.

  The ¼ wavelength plate 18 converts the light beam incident from the rising mirror 17 from linearly polarized light to circularly polarized light by delaying the phase of a part of the component in the light beam by ¼ wavelength, or an objective lens. The light beam incident from the unit 9 is converted from circularly polarized light to linearly polarized light.

  As shown in the perspective view of FIG. 3, the objective lens unit 9 has a flat disk-like diffraction element 20 attached to the lower part of a substantially cylindrical lens barrel portion 19 (partially cut surface is shown in FIG. 3). In addition, a spindle-shaped portion having a slightly smaller diameter than that of the disk is integrally formed on the lower surface of a flat disk-shaped portion having the same size as that of the diffraction element 20 from the upper portion to the center portion of the lens barrel portion 19. The objective lens 21 having the shape as described above is attached.

  The objective lens unit 9 converts the light beam incident from the quarter-wave plate 18 and made of parallel light into convergent light by the diffraction element 20 and the objective lens 21, and focuses it on the signal recording surface of the optical disc 100. Has been made.

  The optical pickup 7 converts the light beam reflected and diverged from the signal recording surface of the optical disc 100 into parallel light by the objective lens 21 and the diffraction element 20 of the objective lens unit 9, and is converted by the quarter wavelength plate 18. The circularly polarized light is converted into linearly polarized light, reflected by the rising mirror 17 in the horizontal direction, that is, in the direction in which the polarization beam splitter 15 is provided, and converted from parallel light to convergent light by the collimator lens 16, and then the polarization beam splitter 15. To enter.

  In this case, the polarization beam splitter 15 reflects the light beam on the polarization plane 15A in accordance with the polarization direction of the light beam and makes it incident on the conversion lens 22.

  The conversion lens 22 converts the optical magnification of the CD light beam Lc, the DVD light beam Ld, and the BD light beam Lb. The optical axis synthesizing element 23 matches the optical axes of the CD light beam Lc and DVD light beam Ld emitted from the laser diode 11 with the optical axis of the BD light beam Lb emitted from the laser diode 12. Has been made.

  A plurality of detection regions each having a predetermined shape are formed at predetermined positions on the upper surface of the photodetector 24 (that is, the surface irradiated with the light beam that has passed through the conversion lens 22 and the optical axis combining element 23). A plurality of detection signals are generated by detecting and photoelectrically converting the respective light amounts of the light beams, and supplying the detection signals to the signal processing unit 4 (FIG. 1).

  In response to this, the signal processing unit 4 generates a reproduction RF signal by performing predetermined arithmetic processing using the detection signal from the photodetector 24 (FIG. 2), and performs predetermined decoding based on the reproduction RF signal. A reproduction signal is generated through processing, demodulation processing, and the like.

  The signal processing unit 4 (FIG. 1) generates a drive control signal such as a tracking error signal and a focus error signal by performing predetermined arithmetic processing using the detection signal from the photodetector 24 (FIG. 2). These are supplied to the control unit 2. In response to this, the control unit 2 performs tracking control, focus control, and the like via the drive unit 3 and can adjust the irradiation state of the light beam to the optical disc 100 to normally generate a reproduction signal.

(1-2-1) In the case of a CD-type disc In practice, when the control unit 2 (FIG. 1) recognizes that the optical disc 100 is a CD-type disc 100c by a predetermined disc type determination method, the optical pickup 7 ( A CD light beam Lc composed of divergent light is emitted from the light emitting point 11 A of the laser diode 11 in FIG. 2) and is incident on the beam splitter 14 via the coupling lens 13.

  The optical pickup 7 reflects the CD light beam Lc at the reflection / transmission surface 14 A by the beam splitter 14, transmits the polarization beam splitter 15, converts the divergent light into parallel light by the collimator lens 16, and After rising from the horizontal direction to the vertical direction and converted from linearly polarized light to circularly polarized light by the quarter wavelength plate 18, the CD light beam Lc is incident on the objective lens unit 9.

  The objective lens unit 9 converts the CD light beam Lc incident from the quarter-wave plate 18 into convergent light by the diffraction element 20 and the objective lens 21, and focuses this on the signal recording surface of the CD disc 100c. .

  The objective lens unit 9 converts the CD light beam Lc reflected on the signal recording surface of the CD disc 100c into divergent light into parallel light by the objective lens 21 and the diffractive element 20, and the quarter-wave plate 18 is converted. To enter.

  Thereafter, the optical pickup 7 converts the CD light beam Lc incident on the quarter-wave plate 18 from circularly polarized light to linearly polarized light, reflects it in the horizontal direction by the rising mirror 18, and converges from parallel light by the collimator lens 16. After being converted into light, the light is reflected on the polarization plane 15A by the polarization beam splitter 15, and sequentially passes through the conversion lens 22 and the optical axis synthesizing element 23 to irradiate the photodetector 24.

  The photodetector 24 detects the light amount of the irradiated CD light beam Lc by a plurality of detection regions, generates a plurality of detection signals, and supplies the detection signals to the signal processing unit 4 (FIG. 1).

  In response to this, the signal processing unit 4 generates a reproduction RF signal based on a plurality of detection signals, generates a reproduction signal, and generates a drive control signal such as a tracking error signal and a focus error signal. Has been made.

(1-2-2) In the case of a DVD type disc When the controller 2 (FIG. 1) recognizes that the optical disc 100 is a DVD type disc 100d by a predetermined disc type determination method, the optical pickup 7 (FIG. 2) The DVD light beam Ld made of divergent light is emitted from the light emitting point 11B of the laser diode 11 and is incident on the beam splitter 14 via the coupling lens 13.

  Thereafter, as in the case of the CD system disc 100c, the optical pickup 7 converts the DVD light beam Ld into the coupling lens 13, the beam splitter 14, the polarization beam splitter 15, the collimator lens 16, the rising mirror 17 and the quarter wavelength plate. Then, the light beam Ld for DVD is converted into convergent light by the diffraction element 20 and the objective lens 21 in the objective lens unit 9, and is focused on the signal recording surface of the DVD type disc 100d.

  Similarly to the case of the CD disc 100c, the objective lens unit 9 converts the DVD light beam Ld, which has been reflected and diverged from the signal recording surface of the DVD disc 100d, into parallel light by the objective lens 21 and the diffraction element 20. After the conversion, the ¼ wavelength plate 18, the rising mirror 17, the collimator lens 16, the polarization beam splitter 15, the conversion lens 22, and the optical axis synthesizing element 23 are passed or reflected in this order to irradiate the photodetector 24.

  As in the case of the CD system disc 100c, the photodetector 24 generates a plurality of detection signals by detecting the amount of light of the irradiated DVD light beam Ld using a plurality of detection areas, and generates these detection signals from the signal processing unit 4 (FIG. 1). ).

  In response to this, the signal processing unit 4 generates a reproduction RF signal, generates a reproduction signal, and generates a drive control signal such as a tracking error signal and a focus error signal.

(1-2-3) In the case of a BD disc When the control unit 2 (FIG. 1) recognizes that the optical disc 100 is a BD disc 100b by a predetermined disc type determination method, the optical pickup 7 (FIG. 2) A BD light beam Lb made of divergent light is emitted from the light emitting point 12 A of the laser diode 12 and is incident on the beam splitter 14.

  In this case, the beam splitter 14 transmits the BD light beam Lb through the reflection / transmission surface 13 </ b> A and makes it incident on the polarization beam splitter 15.

  Thereafter, the optical pickup 7 passes or reflects the BD light beam Lb in the order of the polarizing beam splitter 15, the collimator lens 16, the rising mirror 17 and the quarter wavelength plate 18 in the same manner as in the case of the CD disc 100c, and the objective lens. In the unit 9, the BD light beam Lb is converted into convergent light by the objective lens 21, and this is focused on the signal recording surface of the BD disc 100b.

  Incidentally, the diffractive element 20 of the objective lens unit 9 transmits the BD light beam Lb as it is without being diffracted (details will be described later).

  Similarly to the case of the CD disc 100c, the objective lens unit 9 converts the BD light beam Lb, which is reflected from the signal recording surface of the BD disc 100b and becomes divergent light, into parallel light by the objective lens 21. The quarter-wave plate 18, the rising mirror 17, the collimator lens 16, the polarization beam splitter 15, the conversion lens 22, and the optical axis synthesizing element 23 are passed or reflected in this order, and are irradiated to the photodetector 24.

  As in the case of the CD system disc 100c, the photodetector 24 generates a plurality of detection signals by detecting the amount of light of the irradiated BD light beam Lb by a plurality of detection areas, and generates these signals as a signal processing unit 4 (FIG. 1). ).

  In response to this, the signal processing unit 4 generates a reproduction RF signal, generates a reproduction signal, and generates a drive control signal such as a tracking error signal and a focus error signal.

  As described above, the optical pickup 7 has a CD light beam Lc, a DVD light beam Ld, or a DVD light beam Ld by the objective lens unit 9 regardless of whether the optical disk 100 is a CD disk 100c, a DVD disk 100d, or a BD disk 100b. The BD light beam Lb can be focused on the signal recording surface of the optical disc 100, and the reflected light can be detected by the photodetector 24.

(1-3) Structure of Objective Lens Unit Next, FIG. 4 shows an enlarged cross-sectional view of the CD lens 100c, the DVD disk 100d, the BD disk 100b, and the objective lens unit 9.

  Incidentally, although the biaxial actuator 8 (FIG. 1) is attached to the objective lens unit 9, it is omitted in FIG.

  In general, in the CD system, DVD system, and BD system, from the viewpoint of compatibility and the like, the wavelength of the light beam for reading information, the numerical aperture for condensing the light beam, and signal recording from the lower surface of each optical disc 100 The thickness of the portion up to the surface (so-called cover layer) is defined by the standards.

  Specifically, in the CD system, the wavelength is specified to be about 780 [nm], the numerical aperture is about 0.45, and the thickness of the cover layer is 1.2 [mm]. In the DVD system, the wavelength is about 650 [nm]. nm], a numerical aperture of about 0.6, and a cover layer thickness of 0.6 [mm]. In the BD method, the wavelength is about 405 [nm], the numerical aperture is about 0.85, and the cover The thickness of the layer is defined as 0.1 [mm].

  In the objective lens unit 9, due to the characteristics of the objective lens 21, the focal lengths when the CD light beam Lc, the DVD light beam Ld, and the BD light beam Lb are respectively irradiated from the objective lens 21 are different. .

  For this reason, in the optical disc apparatus 1, the height of the objective lens unit 9 (that is, the distance from the optical disc 100) is adjusted via the biaxial actuator 8 (FIG. 1) while the height of the optical disc 100 is actually fixed. Each light beam is focused on the signal recording surface of each optical disk.

  In FIG. 4, for convenience of explanation, the objective lens unit 9 is fixed and the height of the optical disc 100 is changed. As a result, in FIG. 4, the height of the lower surface of each optical disk is different. In FIG. 4, only the cover layer is shown for the CD disc 100c, DVD disc 100d, and BD disc 100b.

  By the way, the objective lens 21 has priority over the CD light beam Lc and the DVD light beam Ld from the viewpoint of the relative intensity of the BD light beam Lb and the numerical aperture defined by the BD method. The design is optimized for the light beam Lb.

  For this reason, the objective lens 21 of the objective lens unit 9 converts the BD light beam Lb into convergent light when the BD light beam Lb is incident as parallel light from the lower side of the objective lens 21, and converts the BD light beam Lb into convergent light. It is possible to focus on the signal recording surface of 100b.

  However, since the objective lens 21 is optimized for the BD light beam Lb, if the CD light beam Lc or the DVD light beam Ld is incident as parallel light from the lower side of the objective lens 21, it is converted into convergent light. Although it can be converted, aberration is generated and the signal recording layer of the optical disc 100 cannot be correctly focused.

  Therefore, the objective lens unit 9 selectively diffracts only the CD light beam Lc or the DVD light beam Ld by the diffraction element 20 and makes it enter the objective lens 21 as non-parallel light, and the BD light beam Lb is converted into parallel light. The light is incident on the objective lens 21 as it is.

  Specifically, the diffraction element 20 has a CD diffraction grating DGc formed on the upper layer portion 20A, which is a hologram that diffracts the CD light beam Lc and does not diffract the DVD light beam Ld and the BD light beam Lb. As shown in FIG. 4, the CD light beam Lc is diffracted slightly outward by the CD diffraction grating DGc.

  That is, the upper layer portion 20A of the diffraction element 20 can transmit the DVD light beam Ld and the BD light beam Lb as they are, and can selectively diffract only the CD light beam Lc, in other words, the CD light beam. It functions as a lens that corrects only the aberration of Lc.

  In response to this, as shown in FIG. 4, the objective lens 21 refracts the CD light beam Lc incident from the diffractive element 20 on its lower surface and upper surface, and converts it into convergent light. As a result, the objective lens unit 9 can correct the aberration of the CD light beam Lc, and can focus the CD light beam Lc irradiated from the objective lens 21 on the signal recording surface of the CD system disc 100c. .

  Further, the diffraction element 20 is formed with a DVD diffraction grating DGd formed of a hologram that diffracts the DVD light beam Ld and does not diffract the CD light beam Lc and the BD light beam Lb in the lower layer portion 20B. As shown in FIG. 4, the DVD light beam Ld is slightly diffracted outward by the DVD diffraction grating DGd.

  That is, the lower layer portion 20B of the diffraction element 20 can transmit the CD light beam Lc and the BD light beam Lb as they are, and can selectively diffract only the DVD light beam Ld, in other words, the DVD light beam. It functions as a lens that corrects only the Ld aberration.

  In response to this, the objective lens 21 refracts the DVD light beam Ld incident from the diffraction element 20 on the lower surface and the upper surface, respectively, and converts it into convergent light, as shown in FIG. As a result, the objective lens unit 9 can correct the aberration of the DVD light beam Ld, and can focus the DVD light beam Ld irradiated from the objective lens 21 on the signal recording surface of the DVD disc 100d. .

  In this way, the objective lens unit 9 corrects the aberration by diffracting only the CD light beam Lc by the upper layer portion 20A of the diffractive element 20, and diffracts only the DVD light beam Ld by the lower layer portion 20B of the diffractive element 20. By correcting the aberration, the CD light beam Lc, the DVD light beam Ld, and the BD light beam Lb irradiated from the objective lens 21 optimized for the BD light beam Lb are converted into the CD disk 100c and the DVD disk. Each of the signal recording surfaces of the disc 100d and the BD disc 100b can be focused.

(1-4) Configuration of Diffraction Element As shown in FIG. 5A, the diffraction element 20 is configured around a base layer 20C formed in a generally flat disk shape, as described above. A CD diffraction grating DGc is formed in the upper layer portion 20A, and a DVD diffraction grating DGd is formed in the lower layer portion 20B.

  The base layer 20C is made of a transparent synthetic resin having a predetermined refractive index, and refracts the light beam at the interface with another substance having a different refractive index or air.

  On the other hand, as shown in FIG. 5B, the upper layer portion 20A has a stepwise CD diffraction pattern PTc periodically formed on the upper surface 20Ca of the base layer 20C, and transparent on the upper side. A cover layer 20D made of a hardened resin is joined.

  The CD diffraction pattern PTc as the first diffraction pattern is formed in three steps, and the overall height, that is, the distance between the first step and the third step is about 12 [μm]. The period for each CD diffraction pattern PTc is about 18 [μm] at the shortest portion. Further, as shown in FIG. 3, the CD diffraction pattern PTc is formed concentrically in a range from the center to about half of the outer diameter on the upper surface of the diffraction element 20.

  The cover layer 20D as the third member is made of a material having a refractive index different from that of the base layer 20C as the first member, and the lower surface thereof is spaced from the CD diffraction pattern PTc as the first diffraction pattern. It is formed so as to be joined together, and its upper surface is formed in a planar shape.

  As described above, in the upper layer portion 20A of the diffraction element 20, the CD diffraction pattern PTc having a step shape is periodically formed on the upper surface 20Ca of the base layer 20C, and a cover layer having a refractive index different from that of the base layer 20C is formed on the upper side. As a CD diffraction grating DGc that diffracts or transmits a light beam according to its wavelength, thereby diffracting only the CD light beam Lc as the light beam of the first wavelength. It is made to function.

  On the other hand, the lower layer portion 20B has a diffraction pattern layer 20E in which a diffraction grating DGd for DVD is formed on a lower surface 20Cb of a base layer 20C formed in a planar shape, as shown in a partially enlarged sectional view in FIG. It is formed by bonding.

  The diffraction pattern layer 20E as the second member is made of a transparent resin having a refractive index substantially equal to that of the base layer 20C as the first member, and the stepped DVD diffraction pattern PTd is periodically formed on the lower surface side. Is formed. In addition, since the lower surface of the diffraction pattern layer 20E is not covered with other substances, it directly touches the air.

  The DVD diffraction pattern PTd as the second diffraction pattern is formed in five steps, and the overall height, that is, the distance between the first step and the fifth step is about 6 [μm]. The period for each DVD diffraction pattern PTd is about 170 [μm]. Further, as shown in FIG. 3, the DVD diffraction pattern PTd is formed concentrically in a range from the center to about 2/3 of the outer diameter on the lower surface of the diffraction element 20.

  In this way, in the lower layer portion 20B of the diffraction element 20, the diffraction pattern PTd for DVD as the second diffraction pattern having a step shape is periodically formed on the diffraction pattern layer 20E bonded to the lower side of the base layer 20C. Thus, the light beam is diffracted or transmitted according to the wavelength, and as a result, functions as a DVD diffraction grating DGd that diffracts only the DVD light beam Ld as the light beam of the second wavelength. ing.

(2) Method for Manufacturing Diffraction Element Next, a method for manufacturing the diffraction element 20 will be described. As described above, the diffraction element 20 is configured by bonding the cover layer 20D and the diffraction pattern layer 20E to the upper side and the lower side of the base layer 20C, respectively.

  In this diffractive element 20, a cover layer 20D is formed on an upper surface 20Ca of a base layer 20C made of an injection-molded product with an ultraviolet curable resin (hereinafter referred to as UV (Ultra Violet ray) curable resin) having a different refractive index. At the same time, by forming the diffraction pattern layer 20E with the UV curable resin on the lower surface 20Cb (so-called UV replica method), the distortion of the base layer 20C is optically offset by the cover layer 20D and the diffraction pattern layer 20E. The optical characteristics of the diffractive element 20 are kept good.

  That is, when manufacturing the diffractive element 20, first, as shown in FIG. 6 (A), the lower surface 20Cb of the base layer 20C by injection molding has a refractive index substantially equal to that of the injection molding resin forming the base layer 20C. The first UV curable resin 100A having is applied.

  Subsequently, as shown in FIG. 6B, a UV curable resin 100A in which a DVD diffraction grating mold 101A having a mold shape obtained by inverting the DVD diffraction pattern PTd (FIG. 5C) is applied to the lower surface 20Cb. In addition, the ultraviolet light source (not shown) is used to irradiate UV light for resin curing from the upper surface side of the base layer 20C to transfer the first shape of the DVD diffraction grating mold 101A. Thus, as shown in FIG. 6C, a diffraction pattern layer 20E accurately reproducing the DVD diffraction pattern PTd is formed in close contact with the lower surface 20Cb of the base layer 20C.

  Here, as described above, the base layer 20C has distortion caused by injection molding, and therefore, distortion is also generated on the lower surface 20Cb that should be flat. However, since the first UV curable resin 100A forming the diffraction pattern layer 20E has a refractive index substantially equal to that of the base layer 20C, the diffraction pattern layer 20E and the base layer 20C are optically integrated, As a result, the distortion of the lower surface 20Cb serving as the interface between the two has no optical influence.

  Next, a cover layer 20D having an optical surface 20Da is formed on the upper surface 20Ca of the base layer 20C by a similar procedure using the second UV curable resin 100B. In this case, the flat mold 101B having a flat mold surface is pressed against the UV curable resin applied on the CD diffraction pattern PTc.

  Here, the CD diffraction pattern PTc formed on the upper surface 20Ca of the base layer 20C has a very fine structure, and the aspect ratio R, which is the ratio of the depth d to the width w of the staircase, is R = 12 [μm] / 6 [μm] = 2 is relatively large. Therefore, attention is paid to the fluidity when the UV curable resin is cured in a fine diffraction pattern having a large aspect ratio (that is, a deep and narrow groove).

  In the curing treatment for curing the UV curable resin, a low molecular weight by-product accompanying the crosslinking of the resin component evaporates. Therefore, the cured UV curable resin is generally about 3% to 10% compared to before curing. [%] Shrink. As a result, since the density of the UV curable resin slightly changes (generally increases), the refractive index also changes accordingly.

  Here, considering the fluidity of the fluid in the fine structure, as shown in FIG. 7, for example, the Reynolds number representing the ratio of the inertia to the viscosity of the fluid LQ in the groove FN of 10 [μm] square is the same on the plane. Compared to the Reynolds number of the fluid LQ, it is 2 to 3 orders of magnitude smaller, and the viscous action of the fluid LQ becomes very strong in the groove FN.

  In other words, in the diffraction pattern PTc for CD (FIG. 5 (B)) having a groove of a smaller 6 [μm] square (depth ds = 6 [μm], width w = 6 [μm]), the fluid is The viscosity action of certain UV curable resins becomes very strong and the fluidity becomes low. Furthermore, in the case of a UV curable resin, the viscosity of the resin itself increases with the progress of curing, so that the fluidity of the UV curable resin during the curing process becomes extremely low.

  When the curing process proceeds in such a low fluidity state, the UV curable resin cannot be shrunk and cured without changing the density. As a result, the refractive index of the resin cannot be changed, and the refractive index near the CD diffraction pattern PTc becomes a value different from the design value.

  Accordingly, it is preferable to select a resin having the lowest possible viscosity as the UV curable resin used in the CD diffraction pattern PTc to improve the fluidity.

  However, when the viscosity of the UV curable resin is lowered until it can sufficiently flow in the CD diffraction pattern PTc, the UV curable resin flows out to the outside when the UV curable resin is applied on the base layer 20C. UV curable resin cannot be applied. As a result, the thickness of the cover layer 20D cannot be set to a thickness necessary for ensuring the flatness of the CD diffraction pattern PTc, and the flat optical surface 20Da cannot be formed.

  Therefore, in the diffraction element 20 of the present invention, after filling the groove of the CD diffraction pattern PTc with the second UV curable resin 100B having a viscosity that can flow sufficiently in the CD diffraction pattern PTc, a third viscosity having a higher viscosity is obtained. The optical surface 20Da is formed by the UV curable resin 100C.

  That is, as shown in FIG. 8A, after applying the second UV curable resin 100B having a viscosity capable of sufficiently flowing in the groove of the CD diffraction pattern PTc on the CD diffraction pattern PTc, By pressing the mold 101B against the second UV curable resin 100B, the second UV curable resin 100B is brought into close contact with the CD diffraction pattern PTc formed on the upper surface 20Ca of the base layer 20C. The groove is filled in the CD diffraction pattern PTc (FIG. 8B). Furthermore, the cover lower layer 20Du covering the diffraction pattern PTc for CD is formed by irradiating ultraviolet rays from the lower surface side of the base layer 20C to cure and solidify the second UV curable resin 100B (FIG. 8C). ).

  Thus, when cured, the second UV curable resin 100B can sufficiently flow in the CD diffraction pattern PTc, and therefore can shrink in accordance with the inherent curing shrinkage rate and change its density. As a result, the cover lower layer 20Du having a refractive index close to the design value can be formed.

  Further, as shown in FIG. 8D, after applying an appropriate amount of the third UV curable resin 100C having a higher viscosity than the second UV curable resin 100B on the cover lower layer 20Du, the flat plate mold 101B is attached to the third UV curable resin 100B. By pressing against the cured resin 100C, the third UV cured resin 100C is brought into close contact with the cover lower layer 20Du (FIG. 8E), and further, ultraviolet rays are irradiated from the lower surface side of the base layer 20C to form a third. The UV curable resin 100C is cured and solidified to form the cover upper layer 20Do having the flat optical surface 20Da so as to cover the cover lower layer 20Du (FIG. 8F).

  Accordingly, a sufficient amount of the third UV curable resin 100C can be applied to the cover lower layer 20Du, and the cover upper layer 20Do having a sufficient thickness that can reduce the influence of the unevenness of the CD diffraction pattern PTc can be formed. The optical surface 20Da that is the surface of the cover upper layer 20Do can be flattened.

  Here, as shown in FIG. 9, the interface IF of the cover lower layer 20Du and the cover upper layer 20Do is formed in accordance with the thickness of the second UV curable resin 100B applied when the cover lower layer 20Du is formed. The UV curable resin 100B has unevenness caused by curing shrinkage.

  However, the refractive index (405 nm, the same applies hereinafter) of the second UV curable resin 100B is 1.66062, whereas the refractive index of the third UV curable resin 100C is 1.66194, and the cover lower layer 20Du and the cover The difference in refractive index from the upper layer 20Do is as small as 0.00132. For this reason, the cover layer 20D can pass the light beam almost without being refracted by the unevenness formed on the plane of the cover lower layer 20Du, and almost all aberrations occur at the cover interface IF of the cover lower layer 20Du and the cover upper layer 20Do. It is made not to let you.

  Further, distortion caused by injection molding occurs in the CD diffraction pattern PTc formed on the upper surface 20Ca of the base layer 20C, and aberration occurs due to this distortion. Since the diffraction of light by the diffraction grating is caused by the difference in refractive index between the two layers in contact with each other through the diffraction pattern, the greater the difference in refractive index, the greater the aberration due to distortion of the diffraction pattern.

In the conventional diffraction element, the diffraction pattern is formed so as to be in direct contact with air, whereas the refractive index difference between the refractive index of the material forming the diffraction element and the refractive index of air (n _AIR = 1) is large. In the diffractive element 20 of the present invention, the CD diffraction pattern PTc (refractive index of about 1.5) of the base layer 20C is covered with the cover lower layer 20Du (refractive index is 1.60662). The refractive index difference between the base layer 20C and the cover lower layer 20Du that are in contact with each other via the pattern PTc can be set to be smaller than that of the conventional diffraction element, and thereby aberration due to distortion of the CD diffraction pattern PTc can be reduced. .

(3) Operation and Effect In the above configuration, the cover layer 20D of the diffractive element 20 is made of the second UV curable resin 100B, which is the first material, and the cover lower layer 20Du joined to the CD diffraction pattern PTc; The first material is a third UV curable resin 100C, which is a second material having different physical properties, and is constituted by a cover upper layer 20Do joined to the cover lower layer 20Du.

  That is, as the second UV curable resin 100B, by selecting a resin having fluidity in a fine CD diffraction pattern PTc having a groove of about 6 [μm] that is 10 [μm] or less, it is for CD. The inside of the diffraction pattern PTc is surely filled to form the cover lower layer 20Cu as the first layer.

  Furthermore, a sufficient amount of the first upper cover layer 20Do is formed by using the third UV curable resin 100C having a higher viscosity and lower fluidity than the second UV curable resin 100B. 3 can be used to sufficiently reduce the influence of the unevenness of the CD diffraction pattern PTc. As a result, the flat optical surface 20Da can be formed on the cover upper layer 20Do, and the diffractive element 20 exhibiting good characteristics can be manufactured.

  Further, the difference between the refractive index (1.66194) of the third UV curable resin 100C forming the cover upper layer 20Do and the refractive index (1.66062) of the second UV curable resin 100B forming the cover lower layer 20Du. By limiting the ratio to less than 3.5%, the aberration at the cover interface IF between the cover lower layer 20Du and the cover upper layer 20Do can be made less than λ / 20 (λ = 405 nm), and no practical problems can be caused. .

  Here, since the CD diffraction pattern PTc has a structure with a large aspect ratio and severe irregularities, when the diffraction cover lower layer 20Du is formed, the second UV curing applied above the first step of the staircase. The thickness of the resin 100B and the thickness of the second UV curable resin 100B applied above the third stage are different.

  As described above, the second UV curable resin 100B has a curing shrinkage rate of 3 [%] to 10 [%], and the thickness of the second UV curable resin 100B applied to the cover lower layer 20Du. Concavities and convexities corresponding to are formed on the surface.

  The third UV curable resin 100C applied on the cover lower layer 20Du also has the same curing shrinkage rate, the shrinkage amount of the thickly applied portion is large, and the shrinkage amount of the thinly applied portion is small. On the surface of the cover upper layer 20Do, although the size thereof becomes very small in accordance with the curing shrinkage rate, unevenness having a pattern substantially the same as the unevenness formed on the cover lower layer 20Du is formed.

  Therefore, in the diffraction element 20, the third UV curable resin 100C is selected so that the refractive index of the cover upper layer 20Do is higher than the refractive index of the cover lower layer 20Du.

  As a result, as shown in FIG. 10, the direction in which the light beam BA passing through the cover layer 20 is refracted at the cover interface IF and the direction in which the light beam BA is refracted at the air interface AR between the cover upper layer 20Do and the air can be reversed. The aberration of the light beam BA generated at the cover interface IF can be canceled. As a result, the aberration generated in the light beam passing through the cover layer 20D can be further reduced.

  That is, the diffraction element 20 is refracted in the same direction at the cover interface IF and the air interface AR, unlike the light beam BB representing the case where the refractive index of the cover upper layer 20Do is lower than the refractive index of the cover lower layer 20Du. The aberration of the passing light beam can be suppressed to a low level.

  Furthermore, the same chemical and physical characteristics of the cover upper layer 20Do and the cover lower layer 20Du are obtained by using the same acrylic resin as the second UV curable resin 100B and the third UV curable resin 100C. Therefore, it is possible to improve the adhesion between the cover upper layer 20Do and the cover lower layer 20Du, approximate the dimensional change rate with respect to temperature and humidity, and firmly bond the cover upper layer 20Do and the cover lower layer 20Du. .

  Similarly, the use of the ultraviolet curable resin as the second UV curable resin 100B and the third UV curable resin 100C may make the chemical and physical characteristics of the cover upper layer 20Do and the cover lower layer 20Du more similar. it can.

  According to the above configuration, the cover layer 20D of the diffraction element 20 is formed on the cover lower layer 20Du that needs to be embedded in a fine structure by forming the cover upper layer 20Do and the cover lower layer 20Du using two different types of materials. Can select a resin having a low viscosity, and a resin having a high viscosity can be selected for the cover upper layer 20Do that needs to form the flat optical surface 20Da. As a result, the resin embedded in the CD diffraction pattern PTc can be cured and shrunk in accordance with its curing shrinkage rate to obtain a refractive index as designed, while the optical surface 20Da having high planarity can be formed. Good characteristics can be exhibited.

(4) Other Embodiments In the above-described embodiment, the difference between the refractive index (1.66062) of the cover lower layer 20Du and the refractive index (1.66194) of the cover upper layer 20Do is reduced, and the cover interface IF is changed. Although the case where the generated aberration is reduced has been described, the present invention is not limited to this. For example, as shown in FIG. 11, the refractive index of the cover upper layer 20Do is set to be different from the refractive index of the cover lower layer 20Do (1.66062). It may be set high so that most of the aberration generated at the cover interface IF is canceled by the air interface AR.

  In this case, as shown in FIG. 12, the depth of the CD diffraction pattern PTc is d, and the unevenness of the cover interface IF and the air interface AR is large according to the curing shrinkage rate α (100 [%] is 1). Is determined, the unevenness depth di of the cover interface IF and the unevenness depth da of the air interface AR are expressed by Equations (1) and (2). In the equations (1) and (2), the second shrinkable UV resin 100B and the third UV hardened resin 100C used are calculated to have the same shrinkage.

  di = α × d (1)

da = α ² × d (2)

  Here, when the refractive index of the cover lower layer 20Du is n1, the refractive index of the cover upper layer 20Do is n2, and the refractive index of air is 1, the difference value of the refractive index at the cover interface IF is expressed as shown in Equation (3). When the multiplication value obtained by multiplying the depth of the unevenness of the cover interface IF by the di is equal to the product of the difference value of the refractive index at the air interface AR multiplied by the depth da of the unevenness of the air interface AR. The aberration generated at the interface IF can be canceled out at the air interface AR.

  (N2-n1) * di = (n2-1) * da (3)

  When the equations (1) and (2) are applied to the equation (3) and deformed, the equation (4) is obtained.

  n2 = (n1-α) / 1-α (4)

  For example, when the refractive index n1 of the cover lower layer 20Du is 1.66062 and the curing shrinkage α of the second UV curable resin 100B and the third UV curable resin 100C is 0.05 (5 [%]), the cover When the refractive index n2 of the upper layer 20Do is calculated according to the equation (4), it is 1.69539.

  Therefore, by selecting a resin having a refractive index n2 of 1.695539 as the third UV curable resin 100C, the aberration generated when the light beam passes through the cover layer 20D can be made almost zero.

  Incidentally, the curing shrinkage rate α ′ of the third UV curable resin 100C may be different from the cure shrinkage rate α of the second UV curable resin 100B used. In this case, the above-described formula (1) Then, the refractive index n2 of the third UV curable resin 100C100B can be calculated in the same manner by fitting the following expression (5) to the expression (3).

  da = α × α ′ × d (5)

  In the above-described embodiment, the case where the refractive index of the third UV curable resin 100C is higher than the refractive index of the second UV curable resin 100B has been described. However, the present invention is not limited to this. By setting the difference in refractive index to less than 3.5%, even if the refractive index of the third UV curable resin 100C is lower than the refractive index of the second UV curable resin 100B, the aberration is reduced to less than λ / 20. It is possible to avoid causing problems in practical use.

  Further, in the above-described embodiment, the case where the second UV curable resin 100B and the third UV curable resin 100C are combined with an acrylic resin of the same system has been described, but the present invention is not limited thereto. Alternatively, different series of resins may be combined.

  Furthermore, in the above-described embodiment, the case where both the second UV curable resin 100B and the third UV curable resin 100C are UV curable resins has been described, but the present invention is not limited to this, You may make it select suitably from curable resin and a thermoplastic resin.

  Further, in the above-described embodiment, the case where the CD diffraction pattern PTc has a groove of about 6 [μm] square has been described, but the present invention is not limited to this, and the shape and depth of the groove are not limited thereto. Regardless, for example, the present invention can be applied to a pattern having a recess of 10 [μm] or less.

  Furthermore, in the above-described embodiment, the aspect ratio between the depth ds (6 [μm]) and the width w (6 [μm]) per step in the CD diffraction pattern PTc is about 1. However, the present invention is not limited to this, and the present invention may be applied to various shapes in which the aspect ratio between the opening portion and the depth of the pattern having a concave shape is, for example, 0.8 or more.

  Further, in the above-described embodiment, the case where the cover upper layer 20Do forms the optical plane 20Da has been described. However, the present invention is not limited to this, and for example, a predetermined diffraction pattern is provided on the cover upper layer 20Do. In this case, a resin that satisfies the requirements corresponding to the diffraction pattern is selected as the third UV curable resin 100C.

  Further, in the above-described embodiment, the case where the cover layer 20D is configured by two layers of the cover upper layer 20Do and the cover lower layer 20Du has been described. However, the present invention is not limited to this, and three layers or three layers are provided. The above structure may be adopted. Even in this case, resins that satisfy the requirements according to the respective layers are selected as the third and fourth UV curable resins.

  Furthermore, in the above-described embodiment, a cover having a base layer 20C as a base portion having a CD diffraction pattern PTc which is a fine groove, a cover lower layer 20u as a first layer, and a cover upper layer 20o as a second layer. The case where the diffractive element 20 as the diffractive element is constituted by the cover part as the layer has been described, but the present invention is not limited to this, and the present invention is not limited to this, and the base part and the cover part having various configurations. The diffraction element may be configured.

  The present invention can be used, for example, in an optical disc apparatus mounted on various electronic devices.

1 is a schematic diagram illustrating an overall configuration of an optical disc device. It is a basic diagram which shows the structure of an optical pick-up. It is a basic diagram which shows the structure of an objective lens unit. It is a basic diagram which shows the optical path in an objective lens unit. It is a basic diagram which shows the structure of a diffraction element. It is a basic diagram with which it uses for description of the manufacturing method of a base layer. It is a basic diagram with which it uses for description of the fluid in a microstructure. It is a basic diagram with which it uses for description of the manufacturing method of a cover layer. It is a basic diagram which shows the structure of a cover layer. It is a basic diagram with which it uses for description of the correction effect of an aberration. It is a basic diagram with which it uses for description of the correction | amendment of the aberration in other embodiment. It is a basic diagram which shows the structure of the cover layer in other embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Optical disk apparatus, 2 ... Control part, 3 ... Drive part, 4 ... Signal processing part, 7 ... Optical pick-up, 9 ... Objective lens unit, 11 ... Laser diode, 19 ... Lens-barrel part 20 …… Diffraction element 20A …… Upper layer portion, 20B …… Lower layer portion, 20C …… Base layer, 20D …… Cover layer, 20Du …… Cover upper layer, 20Do …… Cover lower layer, 20E …… Diffraction pattern layer, 21 ... Objective lens, DGc ... CD diffraction element, DGd ... DVD diffraction element, Lb ... BD light beam, Ld ... DVD light beam, Lc ... CD light beam, 100A ... No. 1 UV curable resin, 100B... 2nd UV curable resin 100B, 100C... 3rd UV curable resin 100C, IF... Cover interface, AR.

Claims (9)

A diffraction element that diffracts an incident light beam according to the wavelength of the light beam,
A base portion on which a diffraction pattern is formed;
A cover portion covering the base portion,
The cover part
A first layer made of a first material bonded to the diffraction pattern; and a second layer made of a second material different from the first material and bonded to the first layer. And a diffraction element. The diffraction pattern is
It has a fine groove shape with a width of 10 [μm] or less,
The first material is
In the state before solidification has fluidity in the diffraction pattern,
The second material is
The diffractive element according to claim 1, wherein the fluidity is lower than that of the first material in a state before solidification. The refractive index of the second material is
The diffraction element according to claim 1, wherein the refractive index is higher than a refractive index of the first material. The first material and the second material are:
The diffraction element according to claim 1, wherein the difference in refractive index is less than 3.5%. The first material and the second material are:
The diffraction element according to claim 1, wherein the diffraction element is made of a resin of the same system. The first material and the second material are:
The diffraction element according to claim 1, wherein the diffraction element is made of an ultraviolet curable resin. An objective lens unit that irradiates an optical disk with a light beam emitted from a light source,
A diffractive element having a base part on which a diffraction pattern is formed and a cover part covering the base part, and diffracting an incident light beam according to the wavelength of the light beam;
An objective lens configured integrally with the diffractive element, condensing the light beam incident from the diffractive element and irradiating the optical disc;
The cover part
A first layer made of a first material bonded to the diffraction pattern; and a second layer made of a second material different from the first material and bonded to the first layer. Objective lens unit. A light source that emits a light beam;
An objective lens for condensing the light beam and irradiating the optical disc;
A diffractive element having a base part on which a diffraction pattern is formed and a cover part covering the base part, and diffracting an incident light beam according to the wavelength of the light beam;
With
The cover part
A first layer made of a first material bonded to the diffraction pattern, and a second layer made of a second material different from the first material and bonded to the first layer. An optical pickup characterized by that. A light source that emits a light beam;
An objective lens for condensing the light beam and irradiating the optical disc;
A diffraction element having a base part on which a diffraction pattern is formed and a cover part covering the base part, and diffracting an incident light beam according to the wavelength of the light beam;
A drive unit for driving the objective lens to follow a desired track of the optical disc,
A first layer made of a first material bonded to the diffraction pattern; and a second layer made of a second material different from the first material and bonded to the first layer. An optical disk device.

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