JP4341474B2 - Phase correction element and optical head device - Google Patents

Description

  The present invention relates to a phase correction element and an optical head device.

  As an optical recording medium (hereinafter referred to as “optical disk”) in which an information recording layer is covered with a cover layer made of a transparent resin on the light incident side, an optical disk for CD and an optical disk for DVD are widely used. In recording and / or reproducing information on an optical disk for DVD in an optical head device, a semiconductor laser having a wavelength of 660 nm and an objective lens having an NA of 0.6 to 0.65 are used as a light source.

  In order to increase the amount of information per optical disc, read-only optical discs with two information recording layers are in widespread use. An optical disc with a single information recording layer (hereinafter referred to as “single layer optical disc”) has a cover thickness of 0.6 mm, whereas an optical disc with two information recording layers (hereinafter referred to as “double layer optical disc”) is light incident. An information recording layer is formed at positions where the side cover thickness is 0.56 mm and 0.63 mm.

  In an optical head device, when recording and / or reproducing a two-layer optical disk using an objective lens that is optimally designed to have zero aberration with respect to a single-layer optical disk, spherical aberration occurs due to the difference in cover thickness. Condensability of incident light on the information recording layer is deteriorated. In particular, in a recordable double-layer optical disc, the deterioration of the light condensing property causes a decrease in the light condensing power density at the time of recording, which causes a writing error.

  In order to improve the recording density of the optical disk, a semiconductor laser having a wavelength of 405 nm, an objective lens having an NA of 0.85, and an optical disk having a cover thickness of 0.1 mm have been proposed as light sources. In this case as well, in a recording type double-layer optical disc, spherical aberration caused by a difference in cover thickness causes a writing error, which is a problem.

  As a conventional technique for correcting the spherical aberration caused by the difference in the cover thickness of the two-layer optical disk as described above, a method using a movable lens group or a phase correction element in an optical head device is known.

  Patent Document 1 describes spherical aberration correction using a movable lens group. In the optical axis before the light emitted from the light source reaches the objective lens, a movable lens group including a concave lens and a convex lens is installed in the vicinity of the objective lens, and the convex lens is fixed to the actuator. By driving the actuator and moving the convex lens in the optical axis direction, the power of the convex lens movable lens group continuously changes from positive (convex lens) to negative (concave lens), and the focal length variable lens function is exhibited. By this driving, the incident light can be focused on the information recording layers having different cover thicknesses of the optical disc. However, when the movable lens group is used, there are problems that a concave and convex lens and an actuator are required, resulting in an increase in size of the optical head device and a complicated mechanism design for operation.

  Further, Patent Document 2 discloses a phase correction element using liquid crystal that corrects spherical aberration caused by a difference in the cover thickness of an optical disk.

  However, since the conventional phase correction element uses a liquid crystal, it has polarization dependency and acts only on linearly polarized light having a specific direction of polarization. Especially in writing optical head devices that require the light power of the light source, it is necessary to increase the light utilization efficiency. The polarization direction of the light emitted from the light source and the polarization direction of the reflected light from the optical disk to the detector by reflection Is usually used, but the phase correction element using a conventional liquid crystal works only for linearly polarized light in a specific direction. There has been a problem of acting only on one of the reflected light from the optical disk toward the detector.

  Patent Document 3 discloses that a phase correction element configured by using two liquid crystal layers is used to solve the problem of such a polarizing optical head device. The phase correction element includes a first liquid crystal layer and a second liquid crystal layer, and is mounted on the actuator in such a manner that the orientation directions of the liquid crystal molecules constituting the two liquid crystal layers are orthogonal to each other when no voltage is applied. . By adopting such a configuration, one of the two liquid crystal layers has a condensing characteristic on the optical recording medium by acting on the light emitted from the light source toward the optical recording medium and changing the wavefront. The light condensing property on the photodetector can be improved by changing the wavefront by acting on the reflected / returned light directed from the optical recording medium to the photodetector by the other liquid crystal layer.

JP 2003-115127 A JP 2003-36555 A JP 2002-319172 A

  However, the conventional phase correction element using two liquid crystal layers has a configuration in which the two liquid crystal layers are integrated to reduce the number of parts. There is a problem that it is difficult to mount, and ideal phase correction cannot be performed by driving integrally with the objective lens. Also, when the two liquid crystal layers are not integrated but used separately, the number of parts increases, so the optical system of the optical head device becomes complicated, leading to an increase in the adjustment process and a decrease in yield, resulting in a cost increase. There was a problem of increasing.

The present invention has been made to solve the above problems, a transparent conductive substrate and the inorganic electro-optic thin film made of an inorganic material formed on a transparent conductive substrate, the inorganic electro thin film a phase correction element and a transparent electrode formed by applying a voltage between the transparent electrode and the transparent conductive substrate, the inorganic electro-optic thin film, the refractive its plane direction rate is changed according to the size of the isotropically and applied voltage, and characterized in that the phase correcting element changes the polarization state to the isotropic independent of the incident light wave front of the incident light incident on the A phase correction element is provided.

Further, light source, an objective lens for focusing the optical recording medium the light emitted from the light source, which is a beam splitter and demultiplexes for demultiplexing the light reflected by the optical recording medium is converged by the objective lens in the optical head device including a photodetector for detecting light, and an optical head device, characterized in that said phase correction element is disposed in the optical path between the front Symbol light source and the objective lens provide.

  In the phase correction element according to the first aspect of the invention, the amount of change in the refractive index caused by the voltage applied to the inorganic material electro-optic thin film is substantially isotropic regardless of the polarization state incident on the inorganic material electro-optic thin film. The wavefront of transmitted light can be changed without depending on the state of incident polarized light.

  In addition, since the phase correction element of the present invention uses an inorganic material electro-optic thin film, it is possible to respond faster than a phase correction element using liquid crystal.

  Further, since the phase correction element of the present invention uses a solid inorganic material electro-optic element, liquid leakage caused by being a liquid like liquid crystal or liquid crystal specific resistance due to contaminants from the periphery to the liquid crystal layer. Since problems such as degradation do not occur, long-term environmental reliability such as durability and light resistance is also excellent.

Furthermore , since there is only one transparent electrode, the manufacturing process is further simplified and a phase correction element with high productivity is obtained.

In addition to the effect of the phase correcting element according to the first aspect of the invention, the phase correcting element according to the second aspect of the invention has a concentric feeding member so that the correction wavefront for correcting the spherical aberration has the optical axis. It has the effect of being uniform around. Furthermore, since the power supply member has a resistance value smaller than that of the transparent flat electrode, the potential (voltage) smoothly changes between adjacent power supply members in the transparent flat electrode having a large resistance value.

The invention of the optical head device according to claim 3 is equipped with a phase correction element made of an inorganic electro-optic thin film, so that even when a polarizing optical system is used, high light utilization efficiency can be obtained, and two liquid crystal layers are provided. The conventional phase correction element used can be easily mounted on the actuator, and the adjustment process is simplified, thereby reducing the cost of the apparatus.

  In addition, in a phase correction element using a liquid crystal layer, when applied to an optical head device having a light source wavelength of around 400 nm, the long-term laser irradiation gradually causes deterioration of the liquid crystal or the alignment film, which is an organic material. However, the phase correction element of the present invention has high laser resistance because it uses an inorganic electro-optic thin film. Further, material deterioration due to the use of a UV curable adhesive often used in the assembly adjustment process of the optical head device does not occur. Therefore, the reliability of the device is high.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
"First embodiment"
FIG. 1 shows a cross-sectional view of a configuration example of the phase correction element 100 according to the first embodiment of the present invention. An example of the configuration and manufacturing procedure of the phase correction element according to this embodiment will be described below.
The first transparent electrode 3 is formed on one flat surface of the transparent substrate 1. Further, an electro-optic thin film 4 (hereinafter referred to as “inorganic electro-optic thin film 4”) using an inorganic material is formed on the surface of the first transparent electrode 3. Further, the second transparent electrode 5 is formed on the surface of the inorganic electro-optic thin film 4. Further, antireflection films 6 and 7 are formed on the surface of the second transparent electrode 5 and the back surface of the transparent substrate 1 where the first transparent electrode 3 is not formed, respectively, to form the phase correction element 100.

First, the operation of the phase correction element 100 will be described.
By applying a voltage to the first transparent electrode 3 and the second transparent electrode 5, an electro-optic effect is generated in the inorganic electro-optic thin film 4, and the voltage application direction of the inorganic electro-optic thin film 4 and the direction orthogonal to the voltage application direction, That is, the refractive index in the in-plane direction changes. In the present invention, light is incident in the voltage application direction, and what is important in the refractive index change for phase correction is the refractive index change in the direction orthogonal to the voltage application direction.

The electro-optic effect includes a primary electro-optic effect (Pockels effect) in which the refractive index changes linearly with respect to the applied voltage, and a secondary electro-optic effect (Kerr effect) in which the refractive index is proportional to the square of the applied voltage. The amount of change in the refractive index is determined by the type and crystal structure of the inorganic electro-optic material. The first-order electro-optic coefficient (r _ij ) is proportional to the first-order voltage, and the second-order electro-optic coefficient is proportional to the second-order voltage. Along with (s _ij ), this also relates to the direction of the voltage applied to the inorganic electro-optic thin film material.
Among them, by appropriately selecting the inorganic electro-optic material and structure and the voltage application direction, the amount of change in the refractive index in the direction orthogonal to the voltage application direction can be made isotropic regardless of the incident polarization state.

  For example, when expressed by the Herman Morgan symbol having a secondary electro-optic effect, an m3m type crystal structure material has an electric field E in the X direction (100) and the Y direction perpendicular to the (100) direction. , The refractive index change in the Z direction, and the refractive index change in the Y direction and the Z direction are expressed by Equation 1, Equation 2, and Equation 3.

Here, s ₁₁ and s ₁₂ are the secondary electro-optic coefficients of the electro-optic crystal, g ₁₁ and g ₁₂ are the electro-optic coefficients of the crystal, ε ₀ is the dielectric constant of vacuum, and ε r is the relative dielectric constant. From Equations 2 and 3, the refractive index in two directions (Y direction and Z direction) orthogonal to the direction of voltage (or electric field) (X direction) and the refractive index change are equal, and the incident light propagating in the X direction It can be seen that it does not depend on the polarization state. By using such a material for the phase correction element 100, the wavefront of the transmitted light can be changed without depending on the state of incident polarized light. In the crystal having this structure, even when the voltage application direction is the (111) direction, the refractive index changes in the two directions orthogonal to the voltage application direction are equal without depending on the polarization state of the incident light.

  For example, when expressed by the Herman Morgan symbol having the primary electro-optic effect, an m3m type crystal structure material has an electric field E in the c-axis direction (Z direction). The refractive indexes in the X direction and the Y direction that are orthogonal to each other are described by Equations 4 and 5.

Here _{r 33, r 13} is the primary electro-optic coefficient of the electro-optic _crystal, n e, _{n o,} respectively, the extraordinary light refractive index, which is the ordinary refractive index. From Expressions 4 and 5, the refractive index changes in two directions (X direction and Y direction) orthogonal to the direction of voltage (ie, electric field) (Z direction) are equal, and incident light propagates in the Z direction (ie, c axis). It can be seen that it does not depend on the polarization state. By using such a material for the phase correction element 100, the wavefront of the transmitted light can be changed without depending on the incident polarization state.

Thus, the phase correction element 100 independent of the polarization state can be obtained by appropriately selecting the voltage application direction according to the crystal structure.
As the inorganic electro-optic thin film 4 described above, KTa _x Nb _1-x O ₃ (KTN), BaTiO ₃ , LiNbO ₃ , PLZT (Pb _(1-x) La _x (Zr _y Ti _{(1-y ) (1-x / 4)} O ₃ ) is preferable because of its large electro-optic coefficient.

Next, a transparent electrode for phase correction for correcting the generated spherical aberration will be described.
In order to correct the spherical aberration caused by the difference in the cover layer of the two-layer optical disk by the phase correction element 100, a wavefront that cancels the disturbance of the spatial wavefront of the spherical aberration is incident on the inorganic electro-optic thin film 4. To generate light. In order to generate a wavefront that cancels the disturbance of the spatial wavefront in the inorganic electro-optic thin film 4, it is obtained by providing a voltage distribution in the beam cross section of the portion where the incident light is transmitted through the inorganic electro-optic thin film 4. It is done.

  As an electrode shape for generating a spatial distribution change of the wavefront by the phase correction element 100, for example, an electrode divided concentrically as shown in FIG. 2 is used as the first transparent electrode 3 or the second transparent electrode of the phase correction element 100. 5 for correcting spherical aberration. Further, in order to generate a continuously and smoothly changing spherical aberration, a continuously and smoothly changing voltage distribution is obtained. For example, an electrode as shown in FIG. 3 is used as the first transparent electrode 3 of the phase correction element 100, Alternatively, there is a method of providing as the second transparent electrode 5.

  As a configuration for obtaining a voltage distribution that changes continuously and smoothly, for example, as shown in FIG. 3, first, a uniform transparent flat electrode 40 is formed as the second transparent electrode 5 of the inorganic electro-optic thin film 4. . A plurality of power supply members 41, 42, and 43 for supplying a voltage in accordance with the shape of the spherical aberration are provided on the transparent flat electrode 40 as a voltage application electrode shape that can correct the spherical aberration that occurs next.

  The first transparent electrode 3 facing the second transparent electrode 5 is provided with a uniform thin film electrode. Then, the signal voltage 51 is supplied between the power supply member 41 and the first transparent electrode 3 as an appropriate voltage having a desired voltage distribution for correcting the spherical aberration through the lead wire 44, and the power supply member 42 and the first transparent electrode. 3, and a signal voltage 53 is applied between the power supply member 43 and the first transparent electrode 3. At this time, the signal voltage 51, the signal voltage 52, and the signal voltage 53 are different.

  Here, the resistance value of the power feeding members 41 to 43 is made smaller than the resistance value of the transparent flat electrode 40. That is, when different voltages are applied to each of the power supply members 41, 42, and 43, the patterned electrodes of each of the power supply members 41, 42, and 43 become equipotential, and the transparent electrode 40 sandwiched between the different power supply members is applied to the transparent electrode 40. Select the resistances of the power supply members (41, 42, 43) and the transparent flat electrode 40 so that a potential difference (voltage distribution) between different power supply members is generated.

  With such a configuration, a continuously and smoothly changing voltage distribution can be generated in the cross section of the beam incident on the inorganic electro-optic thin film 4, and the phase correction element 100 made of the inorganic electro-optic thin film 4 can be generated. The refractive index according to the position in the cross section of the light beam changes continuously and smoothly, and the change of the transmitted wavefront can be made continuously and smoothly. This change in the transmitted wavefront is preferable because it is closer to the aberration that actually occurs than the discrete wavefront change obtained from the concentric segmented electrodes shown in FIG. In addition, such a continuous smooth wavefront change and a pattern that generates a discrete wavefront change by the divided electrodes may be combined, or the voltage to the liquid crystal layer may be corrected so that a plurality of aberrations other than the spherical aberration can be corrected. Application distributions may be combined.

  The inorganic electro-optic 4 can be produced by vapor deposition methods such as electron beam evaporation, ion plating, ion beam sputtering, RF magnetron sputtering, laser ablation, MBE, CVD, plasma CVD, MOCVD, sol-gel method, self-flux, etc. It is produced by a solid growth method produced by a wet process such as a method. In recent years, research on inorganic electro-optic thin films has been actively conducted as an optical waveguide device for optical communication. As a method for producing the inorganic electro-optic 4, a CVD method, a sol-gel method, and a self-flux method, in which a high-quality thin film as an optical waveguide thin film is obtained, are preferable.

Although the transparent substrate 1 depends on the material of the inorganic electro-optic thin film 4 and the manufacturing method thereof, in the above-described preferable process, the transparent substrate 1 is placed in a high-temperature environment such as film formation from about 400 ° C., depending on conditions, such as film formation near 1000 ° C. Therefore, an SiO ₂ substrate, a KTa _x Nb _1-x O ₃ substrate, an MgO substrate, an SrTiO ₃ substrate, or the like having a high softening point and melting point is preferable.

Similarly to the transparent substrate 1, the first transparent electrode 3 is placed in a high temperature environment. Therefore, a SnO ₂ thin film or an Au thin film that can be used at a high temperature is formed by electron beam evaporation, CVD, RF magnetron sputtering, or the like. use.

The second transparent electrode 5 has already been inorganic electro-optic thin film 4 is deposited, higher temperature resistance as the first transparent electrode 3 is not required. The film formation process of the antireflection film 6 in the subsequent process is about 300 ° C. even at high temperature film formation, and ITO or SnO ₂ can be used. In addition, when using the structure of FIG. 3 which can obtain the voltage distribution which changes continuously and smoothly as an electrode shape of the 2nd transparent electrode 5, as the transparent plane electrode 40 of uniform high resistance value, SnO ₂ doped with one or more elements such as Sb, In, and Ga, and a ZnO film doped with one or more elements such as Ga, Al, Si, Y, and In are preferable. ITO is used as the power supply members 41 to 43 that need to be smaller than the resistance value of 40.

  The antireflection films 6 and 7 are optical multilayer films so as to be antireflective at the wavelength used in consideration of the refractive index and film thickness of the transparent substrate 1, the first transparent electrode 3, the inorganic electro-optic thin film 4, and the second transparent electrode 5. It is only necessary to design and form by electron beam evaporation or RF sputtering.

  The first transparent electrode 3 and the second transparent electrode 5 are arranged in such a manner that a wiring for applying an external voltage can be drawn from an outer portion of an effective area (not shown) of the light beam incident on the phase correction element 100. The antireflection film 6, the second transparent electrode 5, and the inorganic electro-optic thin film 4 are applied to the transparent electrode 3, and the antireflection film 6 is applied to the second transparent electrode 5 by reactive ion etching or ion beam etching. Etching is performed, and further, chromium and gold electrodes (not shown) are formed on the first and second transparent electrodes, and connected to the voltage generator 8 by wiring such as metal wires and FPC cables.

"Second Embodiment"
FIG. 4 shows a cross-sectional view of the configuration example of the phase correction element 200 according to the second embodiment of the present invention. Components having the same functions as those in FIG. 1 are given the same reference numerals.
In the second embodiment, the transparent conductive substrate 2 is used as the transparent substrate, and the transparent conductive substrate 2 is the first embodiment without using the first transparent electrode 3 in the first embodiment. The functioning as the first transparent electrode 3 is different from the first embodiment. As a result, the inorganic electro-optic thin film 4 can be formed directly on the transparent conductive substrate 2, so that a thin film with high crystallinity of an electro-optic material having a lattice constant close to that of the transparent conductive substrate 2 can be obtained. it can.

The configuration and function of the phase correction element 200 are the same as those of the phase correction element 100 of the first embodiment except that the first transparent electrode 3 of the first embodiment is not used and the transparent conductive substrate 2 is used. is there. The transparent conductive substrate 2 is preferably made of a material having high transmittance and low resistivity. For example, SrTiO ₃ doped with a low concentration of Nb can be used.

"Third embodiment"
FIG. 5 shows a configuration example of an optical head device 300 used for recording and / or reproduction of a DVD optical disk on which the phase correction element 100 of the present invention having the sectional structure shown in FIG. 1 is mounted. For the phase correction element 100, the second transparent electrode 5 having a structure shown in FIG. 3 capable of generating a smooth wavefront change continuously is used. The outgoing light, which is linearly polarized light from the semiconductor laser 21 as the light source, becomes parallel light by the collimator lens 30, passes through the beam splitter 22 and the phase correction element 100, passes through the quarter wavelength plate 25, and becomes circularly polarized light, The light is condensed on the information recording layer of the optical disk 28 by the objective lens 26 installed in the actuator 27 for focus servo and tracking servo.

  Further, the circularly polarized reflected light reflected by the optical disk 28 passes through the objective lens 26 and the quarter wavelength plate 25 again, and becomes linearly polarized light whose polarization direction is substantially orthogonal to the light emitted from the light source. Then, the light passes again through the phase correction element 100, is reflected by the beam splitter 22, passes through the condenser lens 31, and is condensed on the photodetector 29.

  Operation for performing stable recording and / or reproduction on DVD single-layer and dual-layer recording and / or reproducing optical discs with different cover thicknesses using optical head device 300 on which phase correction element 100 is mounted Is described below.

  The objective lens 26 is designed to have zero aberration with respect to a single-layer optical disc for DVD having a working wavelength λ = 660 nm and a cover thickness of 0.60 mm, and has a numerical aperture NA = 0.65 and a focal length of 3. This is a 05 mm objective lens. For this reason, no voltage is applied to the phase correction element 100 during recording and / or reproduction of a single-layer optical disc.

  In recording and / or reproduction of a dual-layer optical disc, the cover thickness (distance to the information recording layer) is different from 0.60 mm, so that the aberration is zero for a DVD single-layer optical disc with a cover thickness of 0.60 mm. When the light is condensed by the objective lens 26 designed as described above, spherical aberration occurs. The spherical aberration occurring at this time, that is, the optical path length difference OPD is expressed as a third-order spherical aberration in the Zernike function with the center (r = 0) as a reference.

It becomes. r is the normalized radius, and a is the component ratio of the third-order spherical aberration to the spherical aberration. In Equation 6, OPD (0) = a at the center (r = 0), r = (1/2) ^0.5 , OPD (r) = − 0.5a (maximum or minimum), outer periphery (with respect to r) When r = 1), OPD (1) = a.

When recording and / or reproducing a layer having a cover thickness of 0.56 mm (a layer thinner than 0.60 mm), a transmitted wavefront that cancels the r-dependence of the optical path length difference OPD generated in Equation 6 is obtained. Through the lead wire 44 of the phase correction element 100, the signal voltage 51 is supplied between the power supply member 41 and the first transparent electrode 3, the signal voltage 52 is supplied between the power supply member 42 and the first transparent electrode 3, and the power supply member 43 and the first transparent electrode 3 are connected. A signal voltage 53 is applied between the transparent electrodes 3. At this time, the power supply member 42 is preferably disposed at a position of a normalized radius r = (1/2) ^0.5 where the OPD becomes an extreme value.

  Further, since the OPD in this case is minimized, the difference in cover thickness is achieved by applying the signal voltages 51, 52, and 53 so that the refractive index of the power supply member 42 is small and the refractive index of the power supply members 41 and 43 is large. Is corrected by the objective lens 26, and is efficiently condensed on a layer having a cover thickness of 0.56 mm.

  On the other hand, when recording and / or reproducing a layer having a cover thickness of 0.63 mm, a transmitted wavefront that cancels the r-dependence of the optical path length difference OPD generated in Equation 6 is obtained as in the case of the cover thickness of 0.056 mm. The signal voltage 51 is supplied between the power supply member 41 and the first transparent electrode 3 through the lead wire 44 of the phase correction element 100, the signal voltage 52 is supplied between the power supply member 42 and the first transparent electrode 3, and the power supply member 43. A signal voltage 53 is applied between the first transparent electrode 3 and the first transparent electrode 3. When the cover thickness is 0.63 mm, the OPD becomes a maximum, so that the signal voltage 51 is large so that the refractive index of the power supply member 42 is large and the refractive index of the power supply members 41 and 43 is small, contrary to the cover thickness of 0.56 mm. , 52 and 53 are applied to correct the spherical aberration caused by the difference in cover thickness, and the objective lens 26 efficiently collects the light on the layer having a cover thickness of 0.63 mm. Therefore, by switching the voltage applied to the phase correction element 100, stable recording and / or reproduction can be realized for a single-layer optical disc and a dual-layer optical disc for DVDs having different cover thicknesses.

  As described in the first embodiment, the phase correction element 100 can change the refractive index of the transmitted light without depending on the polarization state of the light transmitted through the phase correction element 100. The wavefront of the emitted light traveling toward the optical disk 28 is corrected for aberrations by the phase correction element 100, and a good light condensing characteristic is obtained on the optical disk. Further, even in the case of a polarization-type optical head device with high light use efficiency in which the polarization direction of reflected light from the optical disk 28 is orthogonal to the polarization direction of incident light, the phase correction element 100 depends on the incident polarization state. Therefore, the wavefront of the reflected light from the optical disk 28 toward the photodetector 29 is also corrected for aberrations by the phase correction element 100, and can exhibit a good condensing characteristic even on the photodetector. Deterioration can be prevented.

  When the phase correction element 100 and the quarter wavelength plate 25 are laminated as in the optical head device of the present invention shown in FIG. 5, the number of components can be reduced. The laminated surface to the phase correction element 100 may be either the side on which the inorganic electro-optic thin film 4 is formed or the side on which the inorganic electro-optic thin film 4 is not formed, but the working surface is laminated on the substrate side where there is no electrical wiring extraction portion from the transparent electrode. And preferable in terms of reliability.

  The phase correction element 100 is preferably mounted on an actuator 27 that holds the objective lens 26. Thereby, the objective lens 26 and the phase correction element 100 can be moved together during tracking, and therefore, the aberration correction performance is not deteriorated due to the deviation between the optical axes of the objective lens 26 and the phase correction element 100.

  In the example of the optical head device of the present invention shown in FIG. 5, the phase correction element 100 is arranged in the optical path on the light source side with respect to the quarter wavelength plate 25, and the emitted light from the semiconductor laser 21 is also The reflected light from the optical disk 28 is also linearly polarized light when passing through the phase correction element 100, but the phase correction element 100 may be arranged in the optical path on the optical disk side with respect to the quarter wavelength plate 25. In this case, the outgoing light, which is linearly polarized light from the semiconductor laser 21, passes through the quarter wavelength plate 25, becomes almost circularly polarized light, and passes through the phase correction element 100. At this time, since the phase correction element 100 generates a refractive index change independent of the polarization direction by applying a voltage, the wavefront can be changed even for incident light of circular polarization. Further, the reflected light from the optical disk 28 also becomes circularly polarized light and enters the phase correction element 100, but the wavefront can be changed for the same reason. Thereafter, the light passes through the quarter-wave plate 25 and is converted into a polarization direction orthogonal to the polarization direction of the light emitted from the semiconductor laser 21, and can be guided to the photodetector 29 with high efficiency by the beam splitter 22.

  That is, in the phase correction element 100, the amount of change in the refractive index caused by the voltage applied to the inorganic electro-optic thin film 4 is almost isotropic regardless of the polarization state incident on the electro-optic thin film. The wavefront of transmitted light can be changed without depending on the polarization state of incident light. With such an optical head device configuration, the phase correction element 100 composed of one inorganic electro-optic thin film 4 emits the light emitted from the light source toward the optical recording medium, and is reflected by the optical recording medium and is detected from the optical recording medium. The wavefront can be changed for both the outgoing light to the detector, the light collection characteristics on the optical recording medium, and the light collection characteristics on the photodetector can be improved. Applicable to the head device.

  The quarter wave plate may be a quarter wave plate or a 5/4 wave plate having a phase difference that is an integral multiple of 1/2 with respect to the quarter wave plate. A birefringent optical crystal such as that described above may be used, or an organic thin film such as a polymer liquid crystal or polycarbonate may be used.

  In the present embodiment, the optical head device mounted with a single layer for DVD using light with a wavelength of 660 nm and a phase correction element that operates with respect to a two-layer optical disk has been described. The same operation and effect can be obtained for an optical head device equipped with a phase correction element that operates on a layered optical disk.

"Example 1"
A reference example of the phase correction element 100 of the present invention shown in the first embodiment will be described below. FIG. 1 is a cross-sectional view of the phase correction element 100. In this phase correction element 100, SnO ₂ , which is a transparent conductive film, is formed as a first transparent electrode 3 on the SiO ₂ substrate of the transparent substrate 1 by a CVD method with a substrate temperature of 750 ° C. In addition, the inorganic electro-optic thin film 4 has a crystal composition of 900 ° C. while controlling the carrier gas under reduced pressure so that the crystal composition becomes a KTa _0.65 Nb _0.35 O ₃ film (hereinafter referred to as “KTN65”). A film is formed by a CVD method. The film thickness is 4.0 μm, and the crystal phase has a twist in the film surface, but faces a crystal growth (100) that is easy to grow. When used as an optical waveguide film, the influence of scattering due to the domain is large, but in the present invention, it can be used because light is incident on the thin film in the vertical direction. Further, after the second transparent electrode 5 is formed on the KTN 65, the antireflection films 6 and 7 are formed by electron beam evaporation.

As the second transparent electrode 5, the configuration shown in FIG. 3 capable of obtaining a voltage distribution that changes continuously and smoothly is used. As the transparent flat electrode 40 having a high resistance value, SnO ₂ doped with Sb is used as a transparent flat plate. ITO is used for the power supply members 41 to 43 which need to be smaller than the resistance value of the electrode 40. The resistance ratio between the transparent flat electrode 40 having a high resistance value and the power supply members 41 to 43 having a low resistance value is about 10,000.

The phase difference (Δnd) generated when a voltage is applied to the phase correction element 100 having the above configuration is expressed by Equation 3, g ₁₂ = −0.038 m ⁴ / C ² , and n = 2.32 (λ = 660 nm). ), Εr = 18760, and Δnd = 0.32 μm (when 14 V is applied). This is an objective with a numerical aperture NA = 0.65 and a focal length of 3.05 mm, which is designed so that the aberration is zero for a single-layer optical disk for DVD having a working wavelength λ = 660 nm and a cover thickness of 0.60 mm. Sufficient for the maximum optical path length difference of about 0.2λ (= 0.2 × 660 nm = 0.132 μm) that occurs when the lens is used for a double-layer optical disc for DVD with a cover thickness of 0.56 mm and 0.63 mm This is a correct value and can correct spherical aberration.

  Further, from Equation 2, since the refractive index increases with voltage, for example, the signal voltages 51, 52, and 53 are applied so that the applied voltage at the center of the phase correction element 100 is larger than the applied voltage at the peripheral part. Thus, a positive power, that is, a transmitted wavefront equivalent to a convex lens can be generated. Conversely, by applying the signal voltages 51, 52, and 53 so that the applied voltage at the center of the phase correction element 100 is smaller than the applied voltage at the peripheral part, a negative wave, that is, a transmitted wavefront corresponding to a concave lens can be generated. Further, there is no dependence on incident polarization in the generation of the transmitted wavefront.

"Example 2"
Examples of the phase correction element 200 of the present invention shown in the second embodiment will be described below. FIG. 4 is a cross-sectional view of the phase correction element 200. As the transparent conductive substrate 2, low-concentration Nb-doped SrTiO ₃ (100) having a resistivity of 1.5 × 10 ⁶ Ω · cm and a thickness of 1.0 mm is used. . On the transparent conductive substrate 2, a CVD epitaxial film is formed as the inorganic electro-optic thin film 4 at 1000 ° C. while controlling the carrier gas under reduced pressure so that the crystal composition becomes KTN65. The film thickness is 4.0 μm, and the crystal phase faces (100). Further, the second transparent electrode 5 is formed on the KTN 65, and the antireflection films 6 and 7 are formed by electron beam evaporation.

As the second transparent electrode 5, the configuration shown in FIG. 3 capable of obtaining a voltage distribution that changes continuously and smoothly is used. As the transparent flat electrode 40 having a high resistance value, SnO ₂ doped with Sb is used, and the transparent flat electrode 40 is used. ITO is used for the power feeding members 41 to 43 which need to be smaller than the resistance value of the above. The resistance ratio between the transparent flat electrode 40 having a high resistance value and the power supply members 41 to 43 having a low resistance value is about 10,000.

The phase difference (Δnd) generated when a voltage is applied to the phase correction element 200 having the above configuration is expressed by Expressions 2, 3 and g ₁₂ = −0.038 m ⁴ / C ² , n = 2.32 ( From λ = 660 nm) and εr = 27000, Δnd = 0.34 μm (when 10 V is applied). This is an objective with a numerical aperture NA = 0.65 and a focal length of 3.05 mm, which is designed so that the aberration is zero for a single-layer optical disk for DVD having a working wavelength λ = 660 nm and a cover thickness of 0.60 mm. Sufficient for the maximum optical path length difference of about 0.2λ (= 0.2 × 660 nm = 0.132 μm) that occurs when the lens is used for a double-layer optical disc for DVD with a cover thickness of 0.56 mm and 0.63 mm This is a correct value and can correct spherical aberration.

The reason why the relative dielectric constant is larger than that of Example 1 is that the lattice constant (0.39 nm) of SrTiO ₃ used as the substrate is close to the lattice constant (0.4 nm) of KTN (65) and lattice matching can be obtained. This is probably because a high quality epitaxially grown film can be formed. In the case of Example 2, since the dielectric constant is high, even with the same film thickness of 4.0 μm, it is possible to drive at a lower voltage than in Example 1. It is to be noted that, as in the case of Example 1, it is possible to generate a transmitted wavefront with a positive power and a transmitted wavefront with a negative power by switching the applied voltage.

"Example 3"
An example of the optical head device 300 of the present invention shown in the third embodiment will be described below. FIG. 5 is a configuration diagram of the optical head device 300. The phase correction element 100 is integrated with the quarter-wave plate 25 used in Example 1. For the integration, the phase correction element 100 does not form the antireflection film 7 of FIG. 1 and bonds the quarter wavelength plate 25 to the substrate surface using a UV curable resin. Since the configuration of the optical head device 300 has been described in the third embodiment, a description thereof will be omitted.

  In the optical head device 300, the objective lens is designed to have zero aberration with respect to a single-layer optical disc for DVD having a working wavelength λ = 660 nm and a cover thickness of 0.60 mm, and a numerical aperture NA = 0. 65 and an objective lens having a focal length of 3.05 mm. For this reason, when recording and / or reproducing a DVD single-layer optical disc having a cover thickness of 0.60 mm, incident light is focused on the information recording layer by the objective lens 26 without applying a voltage to the phase correction element 100. Is done.

In recording and / or reproduction of a dual-layer optical disc for DVD, the optical path length difference OPD generated has a minimum value r (standardized radius) dependency during recording and / or reproduction of a layer having a cover thickness of 0.56 mm. Therefore, so that the transmitted wavefront of the phase correction element 100 cancels the r dependence of the optical path length difference OPD, the power supply members 41 and 43 in the central part and the peripheral part have a voltage of approximately 10 V and the OPD has a minimum value. By applying a voltage of almost 0 V to the power supply member 42 arranged at the r (= 1/2 ^0.5 ) position, the maximum optical path length difference is about 0.2λ (= 0.2 × 660 nm = 0.132 μm). ) Is corrected. Thereby, the condensing property to the recording information layer can be improved, and the phase correction element 100 does not have the dependency on the incident polarization, and thus the condensing performance to the photodetector is improved at the same time.

When recording and / or reproducing a layer having a cover thickness of 0.63 mm, the generated optical path length difference OPD becomes r (standardized radius) dependence having a maximum value, and therefore the transmitted wavefront of the phase correction element 100 is the optical path length difference. In order to cancel the r dependency of OPD, the power supply members 41 and 43 in the central portion and the peripheral portion are arranged with a voltage of approximately 0 V at the r (= 1/2 ^0.5 ) position where the OPD has a minimum value. By applying a voltage of approximately 10 V to the power supply member 42, the maximum optical path length difference of about 0.2λ (= 0.2 × 660 nm = 0.132 μm) is corrected. Thereby, the condensing property to the recording information layer can be improved, and the phase correction element 100 does not have the dependency on the incident polarization, and thus the condensing performance to the photodetector is improved at the same time.

  As described above, in the phase correction element of the present invention, the amount of change in the refractive index caused by the voltage applied to the inorganic electro-optic thin film is isotropic regardless of the polarization state of the light incident on the inorganic electro-optic thin film. Therefore, the wavefront of the transmitted light can be changed without depending on the state of incident polarized light. For this reason, it is possible to improve the light collection characteristics on the optical disk and the photodetector, which are hindered by the spherical aberration generated by the difference in the cover layer of the two-layer optical disk, with a single phase correction element. The present invention can also be applied to a polarizing optical head device that requires light use efficiency.

Sectional drawing which shows an example of the phase correction element of this invention. The typical top view which shows the division | segmentation shape of the transparent electrode which applies a voltage to the inorganic electro-optic thin film using the inorganic material which correct | amends spherical aberration. The typical top view which shows the transparent electrode and electric power feeding member shape which apply a voltage to the inorganic electro-optic thin film using the inorganic material which correct | amends spherical aberration. Sectional drawing which shows another example of the phase correction element of this invention. The block diagram which shows the optical head apparatus carrying the phase correction element of this invention.

Explanation of symbols

1: transparent substrate 2: transparent conductive substrate 3: first transparent electrode 4: electro-optic thin film 5 of inorganic material: second transparent electrode 6, 7: antireflection film 8: voltage generator 21: semiconductor laser 22: beam split motor 25 1/4 wavelength plate 26: objective lens 27: actuator 28: the optical disc 29: light detector 30: collimating lens 31: condensing lens 40: high-resistance transparent electrodes 41, 42, 43: power supply member 44: lead wire 100, 200: Phase correction element 300: Optical head device

Claims (3)

A transparent conductive substrate;
An inorganic electro-optic thin film made of an inorganic material formed on the transparent conductive substrate;
A phase correction element and a transparent electrode formed on the inorganic electro thin film,
By applying a voltage between the transparent electrode and the transparent conductive substrate, the inorganic electro-optic thin film changes according to the magnitude of the refractive index is isotropic and the applied voltage in the plane direction Te, phase correction element for causing the wave front of the incident light is in a polarization state in isotropic independent change of the incident light incident on the phase correcting element. 2. The phase correction element according to claim 1, wherein the phase correction element has a structure in which two or more power feeding members that are concentric and have a resistance value smaller than that of the transparent electrode are formed on the transparent electrode. A light source;
An objective lens for focusing the light emitted from the light source to the optical recording medium,
In the optical head device and an optical detector for detecting light beam splitter and demultiplexes the light reflected demultiplexed by said optical recording medium is converged by the objective lens,
An optical head and wherein the phase correcting element is provided according to claim 1 or claim 2 in the optical path between the front Symbol light source and the objective lens.

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