JP4696883B2 - Phase correction element and optical head device - Google Patents

Description

  The present invention relates to a phase correction element and an optical head apparatus that records and reproduces information with respect to an optical recording medium such as an optical disk, and more particularly to a phase correction element and an optical head apparatus that correct wavefront aberration.

  When recording / reproducing information to / from a multilayer optical recording medium having a plurality of information recording layers on one side of an optical recording medium such as a DVD, the wavefront aberration due to the separation of each layer is corrected to obtain desired information. A technique for condensing light on the recording layer is required. Here, the recording / reproducing means one or more operations of recording information on the optical recording medium and reproducing information recorded on the optical recording medium. The same applies hereinafter unless otherwise specified.

  As the above technique, there is a technique in which a phase correction element including a liquid crystal layer sandwiched between transparent substrates is installed in an optical path between a light source and an objective lens to correct wavefront aberration (for example, Patent Document 1). reference.). Specifically, when correcting spherical aberration as wavefront aberration, the transparent electrode for applying a voltage to the liquid crystal layer is divided into a plurality of divided electrodes, and each divided electrode is formed concentrically around the optical axis. There is a technique in which spherical aberration is removed by applying a voltage according to the aberration to be corrected between the transparent electrodes arranged and opposed to the respective divided electrodes. Since the phase correction element used in this technique has no mechanical movable part, it can be suitably used for a small optical head device.

  In addition, the above-mentioned divided electrode is a low resistance electrode having a low sheet resistance value, and a high resistance plate electrode having a sheet resistance value sufficiently higher than that of the low resistance electrode is further provided as a resistance film, and each low resistance electrode is electrically connected by a high resistance plate electrode. (For example, refer to Patent Document 2 and Patent Document 3). With this technique, the number of power supplies connected to the phase correction element can be reduced, so that the entire optical head device can be made smaller.

  Such a conventional phase correction element corrects the phase of light with respect to one polarization direction. In the Fabry-Perot etalon used for wavelength selection, a technique for removing the polarization dependence of a resonator having polarization dependence has been reported (for example, see Non-Patent Document 1). Specifically, a resonator having polarization dependency is realized by using TN liquid crystal whose orientation directions are orthogonal to each other in the vicinity of two substrate surfaces, and a voltage of a predetermined value or more is applied to the voltage application direction. The liquid crystal is aligned to remove the polarization dependency. The above-described technique described in Non-Patent Document 1 aims at selecting a wavelength and removing polarization dependency.

JP 2003-069766 A JP 2002-288866 A Japanese Patent Laid-Open No. 2001-084431 J.S Patel, Shindu Lee, “Liquid Crystal Fabry-Perot Etalon that is electrically variable and capable of removing polarization dependence”, Applied Physics Letters, Vol. 58, No. 22, p. 2491-2493 (J. S. Patel, Sin-Doo Lee, "Electrically Tunable and Polarization Insensitive Fabry-Perot Etalon with a Liquid-Crystal Film" Applied Physics Letters, Vol. 58, No. 22, p2491-2493)

  However, in such a conventional phase correction element, since the liquid crystal molecules are uniformly aligned in the liquid crystal layer of the phase correction element, polarized light that senses the extraordinary refractive index of liquid crystal and polarized light that senses the ordinary light refractive index. For the polarized light that senses the ordinary refractive index when incident, the transmitted wavefront cannot be changed according to the magnitude of the applied voltage.

  Specifically, since the optical head device generally performs recording and reproduction on an optical recording medium such as a DVD or a high-density optical disk using a polarization optical system, forward light (optical light whose polarization directions are orthogonal to each other) There is a problem that it functions as an element only with respect to either polarized light of light (light directed to the recording medium) or return light (light reflected from the optical recording medium). In order to correct the phase for any polarized light, it is necessary to arrange one phase correction element for each of the outward light and the backward light, which increases the size of the apparatus, the configuration, the cost, etc. Has been invited.

  The present invention has been made to solve such a problem, and provides a phase correction element and an optical head device capable of correcting the phase of any orthogonally polarized light using a single liquid crystal layer. Is.

In view of the above points, the invention according to claim 1 is formed on two opposing transparent substrates, one liquid crystal layer sandwiched between the transparent substrates, and a substrate surface of each transparent substrate. A phase correction element comprising a transparent electrode having a shape corresponding to a wavefront aberration to be corrected, wherein the liquid crystal layer is made of twisted nematic liquid crystal, and the liquid crystal molecules of the liquid crystal layer are positioned on the substrate surface of each of the transparent substrates. Thus, the liquid crystal layers are aligned substantially parallel to the substrate surface and at an angle of 90 ° + 180 ° × m (m = 0, 1, 2), and no voltage is applied to the liquid crystal layer through the transparent electrode. When the angle between all the liquid crystal molecules and the substrate surface is substantially constant, when a predetermined voltage is applied to the liquid crystal layer through the transparent electrode, it is close to an intermediate position between the substrate surfaces. The liquid crystal molecules at the position are larger than the substrate surface. Rises at an angle, it has a structure obtained substantially perpendicular to the substrate surface in the middle of the region between each said substrate surface.

  With this configuration, the liquid crystal molecules of the liquid crystal layer are aligned so as to be substantially parallel to the substrate surface and orthogonal to each other at the substrate surface position of each transparent substrate, and in a middle region between the substrate surfaces when a voltage is applied. Since it is substantially perpendicular to the substrate surface, it has a configuration having two liquid crystal layers with orientations that are effectively orthogonal, and can correct the phase of any linearly polarized light that is orthogonal using a single liquid crystal layer. A phase correction element can be realized.

The invention according to claim 2, in the phase correcting element according to claim 1, which phase plate that adds a phase difference of an odd multiple of [pi / 2 to incident light are arranged to overlap in either of the transparent substrate It has a configuration.

With this configuration, in addition to the effect of claim 1, since it is integrated with a phase plate such as a quarter wave plate, assembly adjustment such as optical axis alignment is easy and space saving can be achieved. A simple phase correction element can be realized.

According to a third aspect of the present invention, there is provided an optical head comprising a light source, an objective lens that condenses light emitted from the light source onto an optical recording medium, and a photodetector that detects return light from the optical recording medium. The apparatus has a configuration including the phase correction element according to any one of claims 1 and 2 in an optical path between the light source and the objective lens.

With this configuration, an optical head device having the effect of claim 1 or 2 can be realized.

According to a fourth aspect of the present invention, in the optical head device according to the third aspect, when the light emitted from the light source is incident on the light source side surface of the liquid crystal layer, the substrate surface position of the liquid crystal layer The liquid crystal molecules have a configuration in which the polarization direction is in the direction in which the ordinary light refractive index or the extraordinary light refractive index is felt.

  With this configuration, since the light emitted from the light source is polarized in a direction in which the ordinary refractive index or the extraordinary light refractive index of the liquid crystal molecules at the substrate surface position of the liquid crystal layer is felt, the light utilization efficiency can be improved. An optical head device can be realized.

  In the present invention, the liquid crystal molecules in the liquid crystal layer are aligned so that they are substantially parallel to the substrate surface and orthogonal to each other at the substrate surface position of each transparent substrate. Since it is substantially perpendicular to the substrate surface, it has a configuration having two liquid crystal layers with an orientation that is effectively orthogonal, and has the effect of being able to correct the phase of any linearly polarized light that is orthogonal using a single liquid crystal layer. A phase correction element can be provided.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
FIG. 1 is a sectional view conceptually showing the structure of the phase correction element according to the first embodiment of the present invention. In FIG. 1, a phase correction element 10 includes at least two opposing transparent substrates 1a and 1b and transparent electrodes that are formed on the opposing substrate surfaces of the transparent substrates 1a and 1b and apply a voltage to the liquid crystal layer 2. 3a and 3b, a seal 4 that seals the outer periphery so that liquid crystal does not leak from between the transparent substrates 1a and 1b, and a wiring 5 that is electrically connected to the transparent electrodes 3a and 3b. An external signal source 11 that applies a voltage to the liquid crystal layer 2 is connected to each of the transparent electrodes 3a and 3b.

  A plastic substrate made of PET, PC, or the like may be used as the transparent substrates 1a and 1b, but it is preferable to use a glass substrate from the viewpoint of durability and the like.

As the transparent electrodes 3a and 3b, it is preferable to use a thin film electrode made of an oxide such as ITO, SnO ₂ , or ZnO from the viewpoints of light transmittance and mechanical durability. Further, at least one of the transparent electrodes 3a and 3b may be constituted by a plurality of divided electrodes. Furthermore, you may have an electrode as a resistance film which electrically connects between division | segmentation electrodes. Hereinafter, the transparent electrode 3a is a composite electrode constituted by a low resistance electrode 3a11 to 3a14 having a low sheet resistance value and a high resistance flat electrode 3a15 as a resistance film for electrically connecting the low resistance electrodes 3a11 to 3a14. The transparent electrode 3b is an electrode made of a single flat plate.

  For the seal 4, it is preferable in terms of handling to use a resin adhesive such as epoxy or acrylic. Further, the resin adhesive may have thermosetting property, UV curable property, and the like. The seal 4 is formed, for example, by printing a sealing material on the outside of the optically effective area of any substrate surface on which the transparent electrodes 3a and 3b are formed. Moreover, it is preferable from the viewpoint of easy adjustment of the distance between the substrates to mix a spacer made of glass fiber at a predetermined ratio into the sealing material.

  As the liquid crystal of the liquid crystal layer 2, twisted nematic liquid crystal (hereinafter referred to as TN liquid crystal) is preferably used. The liquid crystal molecules of the liquid crystal layer 2 are each at an angle of 90 ° + 180 ° × m (m is an integer) when viewed from the direction perpendicular to the substrate surface, substantially parallel to the substrate surface at the substrate surface position of each transparent substrate 1a, 1b. It is twisted and oriented. In particular, the configuration in which the integer m is 0, that is, the twist angle is 90 ° is preferable from the viewpoint of being able to be driven at a low voltage and reducing the generation of scattered light.

  For alignment of liquid crystal, an alignment film (not shown) such as polyimide may be used, and alignment treatment may be performed by rubbing or the like. In addition, other techniques such as a technique of photo-alignment, a technique of aligning by obliquely depositing SiO or the like, and a technique of aligning by irradiating an ion beam to a diamond-like carbon film or the like may be used. As the liquid crystal, those having positive dielectric anisotropy (Δε) are preferable. The liquid crystal having positive dielectric anisotropy moves so that the longitudinal direction of the liquid crystal molecules faces the electric field direction when an electric field is applied.

  A flexible circuit board may be used as the wiring 5. The voltage applied between the wires 5 is preferably a rectangular AC voltage, and the frequency is preferably 10 Hz to 10 kHz. Further, it is more preferable that the rectangular AC voltage does not include a DC component. The voltage range to be applied may be an intermediate region excluding the vicinity of the substrate surface (hereinafter simply referred to as an intermediate region) as long as the liquid crystal molecules form a predetermined angle with respect to the substrate surface. By doing so, since the extraordinary refractive index in the intermediate region changes toward the ordinary refractive index, the light senses the extraordinary refractive index in the vicinity of both substrate surfaces relatively strongly.

  As a result, the phase of any polarized light having a polarization direction in the alignment direction in the vicinity of both substrate surfaces is corrected. Here, it is preferable to set the voltage so that the elevation angle formed by the plane parallel to the substrate surface and the longitudinal direction of the liquid crystal molecules is 80 ° or more at the maximum in the intermediate region. Further, it is preferable to set the voltage so that the change in the azimuth angle of the liquid crystal molecules when the angle in the plane parallel to the substrate surface is the azimuth angle becomes large in the intermediate region. Hereinafter, the azimuth angle in the longitudinal direction of the liquid crystal molecules is referred to as the azimuth angle of the liquid crystal molecules.

Here, it is preferable to provide an insulating film (not shown) between the opposing transparent electrodes 3a and 3b so as to prevent a short circuit. In addition, it is more preferable to adjust the film thickness and refractive index of the alignment film, the insulating film, and the transparent electrodes 3a and 3b to prevent interface reflection, which leads to improvement in light use efficiency. Further, as the insulating film, an inorganic material such as SiO ₂ , ZrO ₂ , or TiO ₂ may be formed using a sputtering method, a sol-gel method, or the like.

  Further, it is preferable to attach an antireflection film to the substrate surface opposite to the liquid crystal layer 2 side of the transparent substrates 1a and 1b, since the light use efficiency can be further improved. A dielectric multilayer film may be formed by using a vapor deposition method, a sputtering method, or the like to form an antireflection film, or a transparent thin film having a film thickness of a wavelength to be used may be formed to form an antireflection film.

  FIG. 2 is a diagram schematically showing an example of an electrode pattern of the transparent electrode 3a constituting the phase correction element 10 according to the embodiment of the present invention. Hereinafter, for convenience of explanation, the electrode pattern of the transparent electrode 3a shown in FIG. 2 will be described as an electrode pattern of a phase correction element used for spherical aberration correction. Here, the low resistance electrodes 3a11 to 3a14 have a circular or annular shape, the high resistance planar electrode 3a15 has a circular shape, and the electrodes 3a11 to 3a15 are arranged concentrically. Since the low resistance electrodes 3a11 to 3a14 have a sheet resistance value sufficiently lower than that of the high resistance planar electrode 3a15 and are arranged concentrically, a concentric and continuously changing potential distribution can be realized. Further, the potential distribution in the radial direction can be set by adjusting the voltage applied to the low resistance electrodes 3a11 to 3a14.

  The voltage from the external signal source 11 may be directly applied to the low resistance electrodes 3a11 to 3a14 via the wirings 51 to 54. However, in a predetermined case such as when the phase amount or phase distribution to be corrected is determined in advance. It is preferable to use a configuration in which a voltage from the external signal source 11 is divided and applied to the low resistance electrodes 3a11 to 3a14 using a predetermined thin film resistor because the number of power supplies to be connected can be reduced. The wiring 55 is connected to the transparent electrode 3b.

  The high-resistance planar electrode 3a15 has only to have a sheet resistance value sufficiently higher than that of the low-resistance electrodes 3a11 to 3a14 and is made of a transparent material, and is one or more elements such as zinc, lead, tin, and indium. It may be made of the oxide. The low resistance electrodes 3a11 to 3a14 may be made of one or more oxides of elements such as zinc, lead, tin, and indium, like the high resistance planar electrode 3a15. If present, it may be made of a metal material such as aluminum, gold, silver, or chromium.

  FIG. 3 is a diagram conceptually showing another example of the electrode pattern of the transparent electrode 3a constituting the phase correcting element 10 according to the embodiment of the present invention. Here, it is assumed that the transparent electrode 3a is composed of the low resistance electrodes 3a21 to 3a24 and the high resistance planar electrode 3a25, and the transparent electrode 3b is composed of a single electrode, as shown in FIG. Hereinafter, for convenience of explanation, the electrode pattern of the transparent electrode 3a shown in FIG. 3 will be described as an electrode pattern of a phase correction element used for coma aberration correction.

  The voltage from the external signal source 11 may be directly applied to the low resistance electrodes 3a21 to 3a24 via the wirings 51 to 54, but in a predetermined case such as when the phase amount or phase distribution to be corrected is determined in advance. It is preferable to use a configuration in which a voltage from the external signal source 11 is divided and applied to the low resistance electrodes 3a21 to 3a24 using a predetermined thin film resistor because the number of power supplies to be connected can be reduced. The wiring 55 is connected to the transparent electrode 3b.

  The operation of the phase correction element 10 according to the first embodiment of the present invention will be described below. 4 to 7 are explanatory diagrams for explaining the operation of the phase correction element 10 when the integer m is 0, that is, when the alignment direction of the liquid crystal molecules is twisted by 90 ° between the substrate surfaces. It is. FIG. 4 shows the orientation of the liquid crystal molecules when the liquid crystal layer 2 is formed using TN liquid crystal and no voltage is applied to the liquid crystal layer 2.

  When the applied voltage is 0 V, the liquid crystal molecules 20 have a uniform azimuth change with respect to the thickness direction of the liquid crystal layer 2 within the azimuth range of 90 ° as conceptually shown in FIG. Hereinafter, the distance in the thickness direction of the liquid crystal layer 2 required for the azimuth angle of the liquid crystal molecules 20 to change by a unit angle is referred to as a pitch. When the applied voltage is 0 V, the liquid crystal molecules 20 are twisted in the azimuth direction at a uniform pitch. If the angle between the liquid crystal molecules 20 and the substrate surfaces of the transparent substrates 1a and 1b is the polar angle (hereinafter referred to as the polar angle of the liquid crystal molecules), when the applied voltage is 0V, the polar angles of all the liquid crystal molecules are substantially It is constant.

  FIG. 5 is an explanatory diagram conceptually showing the state of alignment of liquid crystal molecules when a predetermined voltage Vo is applied. When the applied voltage is gradually increased, each liquid crystal molecule 20 rises in the electric field direction as shown in FIG. Here, the electric field direction is a direction perpendicular to the substrate surface. At this time, the electric field distribution is uniform, but the polar angle of the liquid crystal molecules varies depending on the distance from the substrate surface. Specifically, the polar angle of the liquid crystal molecules increases as the distance from the two substrate surfaces increases. As a result, in the intermediate region B, the liquid crystal molecules 20 become substantially orthogonal to the substrate surface.

  As the polar angle of the liquid crystal molecules increases, the pitch of twist in the azimuth direction of the liquid crystal molecules 20 becomes shorter in the portion where the polar angle is larger than in the portion where the polar angle is smaller. As a result, as shown in FIG. 5, in the regions A and C of the liquid crystal layer 2 near the substrate surface, the twist pitch in the azimuth direction of the liquid crystal molecules 20 becomes large, and the polar angle is relatively small. Conversely, in the intermediate region B, the twist pitch in the azimuth direction of the liquid crystal molecules 20 is shortened, and the polar angle is relatively large. As a result, the twist of the liquid crystal molecules 20 in the azimuth direction is mainly concentrated in the intermediate region B, so that the liquid crystal molecules in the region C have an azimuth angle of approximately 90 ° with respect to the liquid crystal molecules in the region A. It is twisted and oriented.

  When the azimuth angle of the liquid crystal molecules aligned parallel to the substrate surface changes greatly, in general, the light passing through the liquid crystal layer 2 is easily rotated. However, in the intermediate region B where the azimuth angle of the liquid crystal molecules changes greatly, the polar angle of the liquid crystal molecules is large, so that the light passes almost without being rotated, and in the regions A and C, the liquid crystal molecules are in the azimuth direction. The light passes through the light without being rotated.

  Further, at a predetermined applied voltage Vp higher than Vo, the polar angles of the liquid crystal molecules in the regions A and C increase according to the magnitude of the applied voltage. At this time, the azimuth angles of the liquid crystal molecules in the regions A and C are almost orthogonal and constant. As a result, phase modulation can be applied to the light beams in two polarization directions parallel to the substrate surface and orthogonal to each other in the regions A and C, respectively. Accordingly, it is possible to correct the phase of light having two polarization directions.

  Hereinafter, the voltage range in which the twist in the azimuth angle direction of the liquid crystal molecules 20 is mainly concentrated in the intermediate region B and the azimuth angles of the liquid crystal molecules are orthogonal in the regions A and C is referred to as a quasi-orthogonal alignment voltage range. As a result, by controlling the applied voltage, it is possible to produce a phase correction element that acts on polarized light orthogonal to each other with a single liquid crystal layer.

  As described above, the phase correction element according to the first embodiment of the present invention is configured so that the liquid crystal molecules of the liquid crystal layer are substantially parallel to the substrate surface and orthogonal to each other at the substrate surface position of each transparent substrate. Since it is aligned and substantially perpendicular to the substrate surface in the middle region between the substrate surfaces when a voltage is applied, it has a structure having two liquid crystal layers with orientations that are effectively orthogonal to each other. Can be used to correct the phase of any linearly polarized light that is orthogonal.

In addition, since the liquid crystal layer included in the phase correction element according to the first embodiment of the present invention includes a single layer, it can be easily manufactured and the manufacturing cost can be reduced. It should be noted that integrating the phase correction element with a phase plate such as a quarter-wave plate and other optical components is from the viewpoint of ease of adjustment such as optical axis alignment during assembly and space saving. preferable.
(Second Embodiment)

  The phase correction element according to the second embodiment of the present invention uses a liquid crystal that bends in a liquid crystal layer. 6 and 7 are explanatory views conceptually showing the state of alignment of liquid crystal molecules when bend-aligned liquid crystal is used for the liquid crystal layer. The configuration of the phase correction element according to the second embodiment of the present invention is the same as the configuration of the phase correction element according to the first embodiment of the present invention, except for the liquid crystal used in the liquid crystal layer.

  By using a liquid crystal that bends in the liquid crystal layer, the quasi-orthogonal alignment voltage range can be expanded as described below. As a result, the range of phases that can be corrected can be substantially expanded. FIG. 6 shows, as an example of bend alignment, the state of liquid crystal molecule alignment (spray alignment twisted by 90 °) when the applied voltage is 0V.

  In this case, the liquid crystal molecules 21 spread from a predetermined point and exhibit an alignment state almost twisted in the azimuth direction at a constant pitch. Here, the liquid crystal molecules 21 near the two substrate surfaces are aligned in directions orthogonal to each other. FIG. 7 is a diagram for explaining the state of alignment of each liquid crystal molecule when a predetermined voltage is applied to the liquid crystal having the alignment shown in FIG.

  When a predetermined voltage V1 is applied to the liquid crystal layer, the bend-aligned liquid crystal realizes the state shown in FIG. 7, that is, the state in which the polar angle of the liquid crystal molecules rapidly increases in the intermediate region B and is almost perpendicular to the substrate surface To do. This state occurs at an applied voltage lower than that of the liquid crystal aligned in TN (FIGS. 4 and 5) after transition to bend alignment. Therefore, it becomes possible to substantially widen the pseudo-orthogonal alignment voltage range.

  As the transparent electrode 3a, by using the composite electrodes 3a11 to 3a15 having the circular or annular shape shown in FIG. 2 and arranged concentrically, the phase correction element has a function of correcting spherical aberration and a focal point. It also has a function of adjusting the distance. Further, by using, for example, the composite electrodes 3a21 to 3a25 shown in FIG. 3 as the transparent electrode 3a, the phase correction element has a function of correcting wavefront aberration such as coma aberration. It should be noted that integrating the phase correction element 10 with optical components other than other optical components such as a phase plate such as a quarter-wave plate is easy to adjust during assembly and saves space. To preferred.

As described above, the phase correction element according to the second embodiment of the present invention is constituted by the liquid crystal in which the liquid crystal layer is bend-aligned in addition to the effects described in the first embodiment of the present invention. The thickness of the region where the liquid crystal molecules are aligned perpendicular to the substrate surface and do not contribute effectively to the phase correction can be reduced, and the thickness of the above region can be adjusted according to the voltage, with the result that the polarization directions are orthogonal. The voltage required for phase correction of the two linearly polarized light can be reduced and the phase correction range can be expanded.
(Third embodiment)

  FIG. 8 is a diagram conceptually illustrating an example of the configuration of the optical head device according to the third embodiment of the present invention. In FIG. 8, an optical head device 100 includes a semiconductor laser 101, a polarization beam splitter 102, a collimator lens 103, a phase correction element 10 according to the present invention, a quarter wavelength plate 104, and an output from the semiconductor laser 101. The objective lens 105 which condenses incident light on the information recording layers 201 and 202 of the optical disc 200 and the photodetector 106 which detects return light from the optical disc 200 are provided.

  The semiconductor laser 101 may be configured to switch the wavelength of the emitted light according to the type of the optical disc 200. Specifically, light of any wavelength in the 780 nm band is emitted for CD, light of any wavelength in the 660 nm band is emitted for DVD, and 405 nm for HD-DVD or the like. Light having any wavelength in the band may be emitted. Further, the semiconductor laser 101 may be constituted by, for example, a plurality of semiconductor lasers, and each semiconductor laser may be arranged at a different position. The components other than the semiconductor laser 101 and the phase correction element 10 are well known and will not be described. Here, the wavelength in the 780 nm band is a wavelength in the range of 780 ± 20 nm, that is, 760 to 800 nm. Others are similarly 660 ± 20 nm and 405 ± 15 nm.

  In addition to the optical components shown in FIG. 8, different optical components or mechanical components such as a diffraction grating, a hologram element, a polarization-dependent selection element, a wavelength-selective element, and a wavefront conversion means may be applied in appropriate combination. . Further, integrating optical components other than the optical components shown in FIG. 8 with the phase correction element 10 of the present invention is preferable from the viewpoints of ease of adjustment during assembly, space saving, and the like.

  Hereinafter, with respect to the operation of the optical head device 100 according to the third embodiment of the present invention, the operation when recording / reproducing information on / from the optical disc 200 having a plurality of information recording layers 201 and 202 on one side is taken as an example. ,explain. Such an optical disc 200 is called a double-layer disc, and includes a DVD, a high-density optical disc, and the like.

  The P-polarized light emitted from the semiconductor laser 101 and polarized in the X direction is transmitted through the polarization beam splitter 102, transmitted through the collimator lens 103, converted into a parallel beam, and transmitted through the phase correction element 10, thereby implementing the first embodiment of the present invention. As described in the second embodiment or the second embodiment of the present invention, the phase is corrected, and the light is converted into circularly polarized light by transmitting through the quarter-wave plate 104. The light is condensed on the information recording layer 201 or the second information recording layer 202.

  Next, the light reflected from any one of the information recording layers 201 and 202 of the optical disc 200 is returned as light, passes through the objective lens 105, and further passes through the quarter-wave plate 104 to be polarized in the Y direction. It is converted to S-polarized light, passes through the phase correction element 10 and the collimator lens 103 of the present invention, is reflected by the polarization beam splitter 102, and enters the photodetector 106.

  Next, a method for correcting spherical aberration will be described. Here, for convenience of explanation, in the initial state, a voltage is applied to the phase correction element 100 so that the phase correction amount is near the center of the phase variable range, and the objective lens 105 has two information recordings in this initial state. It is assumed that the aberration is minimized at the average position of the layers 201 and 202 (hereinafter referred to as a set position). Hereinafter, the distance from the protective layer surface to the set position is referred to as the set layer thickness.

  When the thickness of the protective layer is different from the set layer thickness, spherical aberration proportional to the thickness obtained by subtracting the set layer thickness from the protective layer thickness (hereinafter referred to as the different layer thickness) occurs, and information recording / reproduction is performed. It becomes difficult. The phase correction element performs concentric phase correction around the optical axis for the light incident on the objective lens 105 to correct the spherical aberration.

  When coma is a correction target, the coma can be corrected by applying a voltage to the phase correction element provided with the divided electrodes shown in FIG. 3 in the same manner as described above. A method for correcting coma aberration by adjusting the magnitude of the applied voltage is well known, and a description thereof will be omitted. For a dual-layer disc or the like that needs to correct coma aberration, the above-described phase correction element for correcting the power component and the phase correction element for correcting coma aberration may be integrated.

  As described above, in the optical head device according to the third embodiment of the present invention, the liquid crystal molecules of the liquid crystal layer constituting the phase correction element are substantially parallel to the substrate surface at the substrate surface position of each transparent substrate, and Since the liquid crystal layers are aligned perpendicularly to each other and are almost perpendicular to the substrate surface in the middle region between the substrate surfaces when a voltage is applied, the liquid crystal layer has two liquid crystal layers that are effectively orthogonally aligned. The phase of any linearly polarized light that is orthogonal can be corrected using a single liquid crystal layer.

  In addition, since the liquid crystal layer constituting the phase correction element is composed of bend-aligned liquid crystal, the thickness of the region where the liquid crystal molecules are aligned perpendicular to the substrate surface and do not effectively contribute to the phase correction can be reduced, and the voltage can be reduced. Accordingly, the thickness of the above region can be adjusted, and as a result, the voltage required for phase correction of two linearly polarized light whose polarization directions are orthogonal can be reduced and the correction range can be expanded.

  In addition, since it is integrated with a phase plate such as a quarter-wave plate, assembly adjustment such as optical axis alignment is easy and space saving can be achieved.

  In addition, since the light emitted from the light source is polarized in a direction in which the ordinary refractive index or extraordinary light refractive index of the liquid crystal layer is felt, the simple configuration and the light utilization efficiency can be improved.

  Specific examples based on the above-described embodiments of the present invention will be described below. The specific limitation of the phase correction element according to the present embodiment does not limit the gist of the present invention. Hereinafter, the phase correction element 10 according to the embodiment of the present invention will be described with reference to FIGS. 1 and 2.

  First, using a quartz glass substrate having a thickness of 0.5 mm as the transparent substrate 1a, an ITO transparent conductive film having a sheet resistance value of 40Ω / □ is formed on one surface of the transparent substrate 1a using a sputtering method. . Next, the transparent conductive film of the transparent substrate 1a is patterned into the low resistance electrode pattern shown in FIG. 2 to form the low resistance electrodes 3a11 to 3a14.

Next, a tin oxide film having a sheet resistance value of 10 ⁶ Ω / □ is formed on the surface of the transparent substrate 1a on which the low resistance electrodes 3a11 to 3a14 are formed, and patterned into the pattern of the high resistance flat plate electrode shown in FIG. Then, the high resistance planar electrode 3a15 is formed. Next, using a quartz glass substrate having a thickness of 0.5 mm as the transparent substrate 1b, an ITO transparent conductive film having a sheet resistance of 300Ω / □ is formed on one surface of the transparent substrate 1b by sputtering. To do.

  Next, the transparent conductive film of the transparent substrate 1b is patterned to form a transparent electrode 3b facing the divided electrodes 3a11 to 3a15. As shown in FIG. 2, the transparent electrode 3a includes a low resistance electrode 3a11 to 3a14 having a circular or annular shape, and a high resistance planar electrode 3a15 having a circular shape, and each divided electrode 3a11 to 3a15. Are arranged concentrically around the optical axis.

  In addition, a flexible circuit board is connected to the wirings 51 to 55 so that a voltage can be applied from the outside.

  Next, an alignment film made of polyimide is applied and baked on the substrate surface on which the divided electrodes 3a11 to 3a15 of the transparent substrate 1a are formed and on the substrate surface on which the transparent electrode 3b of the transparent substrate 1b is formed. An alignment treatment is performed by rubbing the surface of the alignment film in one direction. Next, a seal 4 is formed on the surface of the transparent substrate 1a on which the divided electrodes 3a11 to 3a15 are formed by patterning an adhesive mixed with a 10 μm diameter gap control material and conductive beads using a printing method. To do.

  Next, the substrate surface on which the seal 4 of the transparent substrate 1a is formed and the substrate surface of the transparent substrate 1b on which the transparent electrode 3b is formed are placed on top of each other and pressed to form a 10 μm gap between the transparent electrodes 3a and 3b. A cell is produced. At this time, the alignment treatment directions on the transparent substrate surfaces are orthogonal to each other. Next, nematic liquid crystal having a positive dielectric anisotropy having an ordinary light refractive index no of 1.52 and an extraordinary light refractive index ne of 1.70 is injected from an injection port of an empty cell (not shown), and the liquid crystal layer 2 and To do. In the liquid crystal, a chiral material is mixed so that the twist pitch becomes 200 μm.

  Next, the inlet of the empty cell is sealed with an ultraviolet curable resin and irradiated with ultraviolet rays to form the phase correction element 10 shown in FIG.

  Hereinafter, an optical head device 100 according to an embodiment of the present invention will be described with reference to FIG. In FIG. 8, an optical head device 100 includes a semiconductor laser 101 such as a semiconductor laser, a polarization beam splitter 102, a collimator lens 103, a phase correction element 10 according to the present invention, a quarter wavelength plate 104, a semiconductor The objective lens 105 which condenses the emitted light from the laser 101 on the information recording layers 201 and 202 of the optical disc 200 and the photodetector 106 which detects the return light from the optical disc 200 are provided.

  The semiconductor laser 101 is a semiconductor laser and emits laser light having a wavelength of 405 nm. The laser light emitted from the semiconductor laser 101 is linearly polarized light (hereinafter referred to as P-polarized light), and the polarization direction is an extraordinary refractive index axis (on the semiconductor laser 101 side) of the liquid crystal layer 2 constituting the phase correction element 10. The direction is parallel to the X direction in FIG. Therefore, the return light (hereinafter referred to as S-polarized light) from the optical disc 200 is in a direction parallel to the extraordinary refractive index axis (Y direction in FIG. 1) on the quarter wavelength plate 104 side of the liquid crystal layer 2. Yes.

  A voltage having a rectangular AC waveform and a frequency of 1000 Hz is applied to the phase correction element 10 constituting the optical head device 100. The phase correction element 10 exhibits a function of phase correction when a voltage of 4 Vrms or more is applied to the liquid crystal layer 2. The spherical aberration can be corrected by applying voltages of 5.5 Vrms, 7 Vrms, 8.5 Vrms, and 10 Vrms to the divided electrodes 3a11 to 3a14 from the optical axis to the outside.

  As described above, by applying a voltage to each of the divided electrodes 3a11 to 3a14, the light transmitted through the phase correction element 10 becomes linearly polarized light that oscillates in the same direction as the P-polarized light incident on the phase correction element 10 and is polarized. Is kept.

  The phase correction element and the optical head device according to the present invention have the effect of being able to correct the phase of any linearly polarized light that is orthogonal using a single liquid crystal layer, and optical recording with different protective layer thicknesses. Phase correction element and optical head device for recording / reproducing information on a medium, etc., and also for applications such as a phase correction element and optical head device for performing phase correction on light of two orthogonal polarization directions Applicable.

  Specifically, optical recording media with different protective layer thicknesses such as CD and DVD, optical recording media that perform recording and reproduction using a wavelength of 405 nm band, optical recording media with different standards such as BD and HD-DVD, etc. The present invention can also be applied to uses such as a phase correction element and an optical head device for recording and reproducing information. Further, it can also be used as a variable focus lens used for mobile phones with cameras, digital cameras, digital videos, endoscopes, and the like. In particular, it has features such as being small and thin and having no movable parts, and can be effectively used for mobile devices and small devices and devices that use light.

Sectional drawing which shows notionally the structure of the phase correction element based on the 1st Embodiment of this invention The figure which shows typically an example of the electrode pattern of the transparent electrode 3a which comprises the phase correction element 10 shown in FIG. The figure which shows typically an example of the electrode pattern different from the electrode pattern of the transparent electrode 3a shown in FIG. The figure which shows the mode of the orientation of the liquid crystal molecule of TN liquid crystal when the voltage is not applied to the liquid crystal layer 2 4 is a diagram showing the state of alignment of liquid crystal molecules of the TN liquid crystal when a voltage is applied to the liquid crystal layer 2 in the configuration shown in FIG. The figure which shows the mode of the orientation of the liquid crystal molecule | numerator of the liquid crystal which carries out a bend alignment when the voltage is not applied to the liquid crystal layer 2. FIG. 6 is a diagram showing the state of alignment of liquid crystal molecules of bend-aligned liquid crystal when a voltage is applied to the liquid crystal layer 2 in the configuration shown in FIG. The figure which shows notionally an example of a structure of the optical head apparatus based on the 3rd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1a, 1b Transparent substrate 2 Liquid crystal layer 3a, 3b Transparent electrode 3a11-3a14, 3a21-3a24 Low resistance electrode 3a15, 3a25 High resistance plane electrode 4 Seal 5, 51-55 Wiring 10 Phase correction element 11 External signal source 20, 21 Liquid crystal Molecule 100 Optical head device 101 Semiconductor laser 102 Polarizing beam splitter 103 Collimator lens 104 1/4 wavelength plate 105 Objective lens 106 Photo detector 200 Optical disc 201, 202 Information recording layer L Optical axis

Claims (4)

Two opposing transparent substrates, one liquid crystal layer sandwiched between the transparent substrates, and a transparent electrode formed on the substrate surface of each transparent substrate and having a shape corresponding to the wavefront aberration to be corrected In the phase correction element provided,
The liquid crystal layer is made of twisted nematic liquid crystal,
The liquid crystal molecules of the liquid crystal layer are aligned at an angle of 90 ° + 180 ° × m (m = 0, 1, 2) substantially parallel to the substrate surface at the substrate surface position of each transparent substrate,
When no voltage is applied to the liquid crystal layer through the transparent electrode, the angle formed by all liquid crystal molecules with the substrate surface is substantially constant,
When a predetermined voltage is applied to the liquid crystal layer via the transparent electrode, the liquid crystal molecules that are closer to the intermediate position between the substrate surfaces rise at a larger angle with respect to the substrate surface, and between the substrate surfaces. A phase correction element characterized by being substantially perpendicular to the substrate surface in an intermediate region. The phase correction element according to claim 1, wherein a phase plate that adds a phase difference of an odd multiple of π / 2 to incident light is disposed on one of the transparent substrates. In an optical head device comprising: a light source; an objective lens that condenses the light emitted from the light source onto an optical recording medium; and a photodetector that detects return light from the optical recording medium, the light source and the objective lens An optical head device comprising the phase correction element according to claim 1 in an optical path between the two. When the light emitted from the light source is incident on the light source side surface of the liquid crystal layer, the polarization direction is in a direction in which the normal light refractive index or the extraordinary light refractive index of the liquid crystal molecules at the substrate surface position of the liquid crystal layer is felt. Item 4. The optical head device according to Item 3.

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