JP2004296082A - Optical recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical recording/reproducing device in which correctional effect does not deteriorate even if an objective lens shifts, and an optical recording medium used for an optical recording and reproducing method. <P>SOLUTION: The multilayered optical recording medium for recording or reproducing information by using a light source for emitting light of a wavelength 390 to 420 nm and an optical head having the objective lens of a numerical aperture 0.7 to 0.9 has two or more recording layers for recording information. The distance from any one recording layer among the plurality of recording layers to the surface of the multilayered optical recording medium is approximately 100 μm and the management information of the multilayered optical recording medium is recorded in the position of approximately 100 μm from the surface of the multilayered optical recording medium. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical recording medium.

  Digital versatile disks (DVDs) have attracted attention as large-capacity optical recording media because they can record digital information at a recording density about six times that of compact disks (CDs). In order to reproduce a high-density DVD, it is necessary to shorten the wavelength of the laser beam and increase the numerical aperture (NA) of the objective lens as compared with the case of reproducing a CD. Therefore, a laser beam having a wavelength of 650 nm (780 nm for a CD) and an objective lens having an NA of 0.6 (0.45 for a CD) are used for DVD reproduction. Further, studies have been made to further shorten the wavelength of the laser beam and further increase the NA to increase the recording density. However, when the wavelength is shortened and the NA of the objective lens is increased, the recording or reproducing margin for the displacement of the recording layer in the thickness direction decreases. Therefore, in this case, it is necessary to correct spherical aberration.

  Further, an optical recording medium having a plurality of recording layers also has a high recording density, but in this case also, the distance from the surface of the optical recording medium to the recording layer (hereinafter, sometimes referred to as a substrate thickness) differs depending on the recording layer. As a result, spherical aberration occurs. In order to correct such spherical aberration, an optical head that corrects wavefront aberration (particularly, spherical aberration) using a liquid crystal element has been proposed (see Patent Document 1 below).

  An example of this conventional optical head will be described with reference to FIG. FIG. 13 is a diagram schematically illustrating a configuration of a conventional optical head 200 (also referred to as an optical pickup). As shown in FIG. 13, the optical head 200 includes a light source 201, a polarizing beam splitter 202, a liquid crystal panel 203, a quarter-wave plate 204, an objective lens 205, a condenser lens 206, a photodetector 207, and a substrate thickness sensor 208. , And an optical element drive circuit 209. A signal is recorded or reproduced on the optical disk 210 by the optical head 200.

  The light source 201 is composed of a semiconductor laser element, and outputs coherent light for recording and reproduction to the recording layer of the optical disk 210. The polarization beam splitter 202 is an element for splitting light. The liquid crystal panel 203 is provided with a plurality of concentric electrodes 203a to 203d as shown in FIG. 14, and changes the refractive index of the liquid crystal by applying different voltages to the electrodes to correct aberration. The quarter-wave plate 204 is made of a birefringent material, and converts linearly polarized light into circularly polarized light. The objective lens 205 focuses light on the recording layer of the optical disc 210. The condenser lens 206 condenses the light reflected by the recording layer of the optical disc 210 on the photodetector 207. The photodetector 207 receives light reflected by the recording layer of the optical disc 210 and converts the light into an electric signal.

  Next, the operation of the optical head will be described. The linearly polarized light emitted from the light source 201 passes through the polarization beam splitter 202 and enters the liquid crystal panel 203. Here, when the recording layer of the optical disk 210 is located at a position shifted from the design value, the base material thickness sensor 208 detects the amount of the shift and outputs the amount of the shift to the optical element drive circuit 209. The optical element driving circuit 209 drives the liquid crystal panel 203 based on the input shift amount so as to correct the wavefront aberration caused by the shift amount. Therefore, the light incident on the liquid crystal panel 203 is given a wavefront aberration that corrects the wavefront aberration (third-order spherical aberration) caused by the displacement of the base material thickness.

  Hereinafter, a method of correcting spherical aberration using the liquid crystal panel 203 will be described in detail. First, FIG. 15 shows a phase distribution when the base material thickness of the optical disc 210 deviates from a design value (optimal base material thickness). This phase distribution is a phase distribution when the wavelength of the laser beam is 405 nm, the NA of the objective lens is 0.85, the optimal substrate thickness of the optical disk 210 is 0.1 mm, and the deviation of the substrate thickness is 0.01 mm. 9 is a wavefront aberration distribution on the recording layer of the optical disc 210 at the best image point. If a phase that completely corrects this distribution is given to the laser beam, even if the substrate thickness of the optical disk 210 deviates from the optimum substrate thickness, the spot of the laser beam on the optical disk 210 is narrowed to the diffraction limit.

In order to correct the wavefront aberration shown in FIG. 15, a phase change that cancels the wavefront aberration shown in FIG. 15 may be given to the laser beam. That is, the optical path length may be partially changed. Since the refractive index of the liquid crystal changes according to the voltage applied from the outside, the optical path length can be partially changed by changing the applied voltage. Therefore, the spherical aberration shown in FIG. 15 can be corrected by applying an appropriate voltage to the plurality of electrodes 203a to 203d as shown in FIG.
JP-A-10-269611

  However, since the optical head 200 corrects the spherical aberration by generating the third-order spherical aberration, when the center of the objective lens 205 and the center of the electrodes 203a to 203d of the liquid crystal panel 203 are displaced, the spherical aberration is corrected. The correction effect of is deteriorated. That is, when the objective lens 205 and the liquid crystal panel 203 are separately disposed, the center of the objective lens 205 and the centers of the electrodes 203a to 203d are moved by moving the objective lens 205 in accordance with the eccentricity of the optical disk 210. The correction effect is degraded due to deviation.

  In the case where the wavelength of the laser beam is 400 nm, the NA is 0.85, and the substrate thickness of the optical disk 210 is 10 μm from the design value (0.1 mm), the deviation between the center of the objective lens 205 and the centers of the electrodes 203 a to 203 d. FIG. 16 shows the relationship between the amount and the aberration after the correction.

  As shown in FIG. 16, when the centers of the two are shifted, the correction effect deteriorates. This is because the spherical aberration generated in the liquid crystal panel 203 causes coma aberration when the centers of the two are shifted. In order to prevent the center from being shifted, the liquid crystal panel 203 and the objective lens 205 need to be integrated.

  However, when the liquid crystal panel 203 and the objective lens 205 are integrated, it is difficult to reduce the thickness of the optical head. Further, since it is necessary to move the liquid crystal panel 203 together with the objective lens 205, the f characteristic (sensitivity) of the actuator is reduced. In addition, wiring for driving the liquid crystal panel 203 complicates the structure of the actuator and makes it difficult to reduce the cost.

  As a method of correcting spherical aberration, a method of arranging two lenses on the optical axis and changing the distance between the lenses to correct spherical aberration has also been proposed (see JP-A-2000-131603). In this method, by changing the distance between the lenses, a phase change that changes parallel light into divergent light or convergent light is given to light transmitted through the lens, and as a result, spherical aberration is corrected. However, in the case of this method, a mechanical means for changing the distance between the lenses according to the deviation of the thickness of the base material is required, and it is difficult to reduce the size of the optical head. In addition, in order to prevent the occurrence of coma, it is necessary to precisely match the centers of the two lenses, and it has been difficult to manufacture an optical head at low cost. Further, in this method, since the distance between the lenses is changed in the optical axis direction, the optical system becomes an enlargement / reduction optical system. As a result, there is a problem that the efficiency of capturing light incident on the lens for correcting spherical aberration changes, and the RIM intensity (rim strength) of the light changes.

  The present invention has been made in view of such a situation, and provides an optical recording / reproducing apparatus and an optical recording medium used in an optical recording / reproducing method in which a correction effect is less deteriorated even when an objective lens moves.

  The first optical recording medium of the present invention records or reproduces information using a light source that emits light having a wavelength of 390 nm to 420 nm and an optical head having an objective lens having a numerical aperture of 0.7 to 0.9. A multi-layer optical recording medium having two or more recording layers on which information is recorded, wherein a surface of the multi-layer optical recording medium starts from any one of the plurality of recording layers. The management information of the multilayer optical recording medium is recorded at a position of about 100 μm from the surface of the multilayer optical recording medium.

  The second optical recording medium of the present invention records on each layer of a single-layer optical recording medium having only one recording layer on which information is recorded and a multilayer optical recording medium having two or more recording layers on which information is recorded. A multi-layer recording medium for use in an optical recording / reproducing apparatus having a spherical aberration correcting means for reproducing / managing, wherein management information of each optical recording medium is recorded on each of the single-layer optical recording medium and the multilayer optical recording medium. The distance from the surface of the multilayer optical recording medium to the management information recorded on the multilayer optical recording medium is from the surface of the single-layer optical recording medium to the management information recorded on the single-layer optical recording medium. It is characterized by being substantially equal to the distance.

  The present invention can provide an optical recording / reproducing apparatus and an optical recording medium used in an optical recording / reproducing method in which the correction effect is less deteriorated even when the objective lens moves.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the same components are denoted by the same reference numerals, and redundant description will be omitted.

(Embodiment 1)
In the first embodiment, an example of the optical element of the present invention will be described. FIG. 1 schematically shows a cross-sectional view of the optical element 10 according to the first embodiment.

  Referring to FIG. 1, an optical element 10 includes a first substrate 11, a second substrate 12 disposed substantially parallel to the first substrate 11, and a first substrate 11 and a second substrate 12. , A light-transmitting resin film 14, a liquid crystal (first phase change layer) 15, a light-transmitting resin film 16, and a counter electrode (first electrode). And a sealing resin 18. The liquid crystal 15 is sealed with a sealing resin 18.

  The first substrate 11 and the second substrate 12 are light-transmitting substrates, and are made of, for example, glass.

  The voltage application electrode 13 is an electrode for applying a desired voltage to the liquid crystal 15. The voltage application electrode 13 is formed on the main surface inside the first substrate 11 (the liquid crystal 15 side). The voltage application electrode 13 is made of a translucent and conductive material, for example, indium tin oxide (ITO). FIG. 2 shows a plan view of the voltage application electrode 13. The voltage applying electrode 13 includes a plurality of segment electrodes 13a to 13g arranged concentrically. FIG. 2 shows a case where the voltage application electrode 13 is composed of seven segment electrodes, but the number of segment electrodes is not particularly limited.

  The counter electrode 17 is formed on the second substrate 12 so as to face the voltage application electrode 13. The counter electrode is an electrode for applying a desired voltage to the liquid crystal 15 together with the voltage application electrode 13. The counter electrode 17 is made of a translucent and conductive material, for example, ITO. The counter electrode 17 is formed on at least the entire surface of the main surface inside the second substrate 12 (the liquid crystal 15 side) facing the segment electrodes 13a to 13g.

  The translucent resin films 14 and 16 are alignment films for aligning the liquid crystal 15 in a predetermined direction, and are made of, for example, a polyvinyl alcohol film. The rubbing treatment of the translucent resin film 14 or 16 allows the liquid crystal 15 to be oriented in a predetermined direction.

  The liquid crystal 15 functions as a phase change layer that changes the phase of incident light. The liquid crystal 15 is made of, for example, a nematic liquid crystal. By changing the voltage between the voltage application electrode 13 and the counter electrode 17, the refractive index of the liquid crystal 15 can be changed, and thereby the phase of the incident light can be changed.

  The sealing resin 18 is for sealing the liquid crystal 15, and is made of, for example, an epoxy resin.

  In the optical element 10, by changing the voltage between each of the segment electrodes 13 a to 13 g and the counter electrode 17, the refractive index of the liquid crystal 15 which is a phase change layer is partially changed. Gives a phase that converts a plane wave to a spherical wave. More specifically, for example, 0V is applied to the counter electrode, 0V is applied to the segment electrode 13a, 0.5V is applied to the segment electrode 13b, 1V is applied to the segment electrode 13c, 1.5V is applied to the segment electrode 13d, 2V is applied to the segment electrode 13e, and 13V is applied to the segment electrode 13f. It is sufficient to apply a voltage of 2.5 V and a voltage of 3 V to the segment electrode 13 g. As described above, by increasing the voltage applied to the segment electrode from the center of the voltage applying electrode 13 to the outside, the phase difference increases from the center of the optical element 10 to the outside, and the plane wave is converted into a spherical wave. It becomes possible. According to the optical element 10, as described in the second embodiment, it is possible to configure an optical head in which the correction effect does not decrease even when the objective lens moves.

  In addition, at least one electrode selected from the voltage application electrode 13 and the counter electrode 17 may be arranged on a curved surface (the same applies to the following embodiments). FIG. 3 schematically shows a cross-sectional view of the optical element 10a in which the counter electrode 17 is arranged on a curved surface. In the optical element 10a, the inner surface of the second substrate 12 is concave, and the counter electrode 17 is formed on the surface. In the optical element 10a, the voltage application electrode 13 is not divided into a plurality of segment electrodes, but is formed on one surface. Thus, as described in the second embodiment, it is possible to configure an optical head in which the correction effect is not particularly reduced even when the objective lens moves.

  Another example of an optical element that gives a smooth phase change is disclosed in JP-A-2001-143303. This element includes a voltage application electrode including a transparent electrode having a large sheet resistance and a voltage supply unit having a resistance smaller than that of the transparent electrode. When a voltage is externally applied to a voltage supply unit connected to a part of the transparent electrode, a voltage drop occurs due to a large sheet resistance of the transparent electrode, and the voltage decreases smoothly as the distance from the voltage supply unit increases. For this reason, the voltage between the voltage application electrode and the counter electrode changes smoothly, and accordingly, the refractive index of the phase change layer changes smoothly.

  The control of the alignment of the liquid crystal 15 may be performed by a method other than the rubbing method (the same applies to the following embodiments). For example, the film itself may have an orientation by oblique deposition or the like. Alternatively, a groove may be formed in the substrate to control the alignment of the liquid crystal.

  Further, the phase change layer may be formed using another material instead of the liquid crystal 15 (the same applies to the following embodiments). For the phase change layer, a material whose refractive index changes or a material whose volume changes depending on a voltage between the voltage application electrode 13 and the counter electrode 17 can be used. As a material whose volume changes with voltage, PLZT (a transparent crystal having a perovskite structure including lead oxide, lanthanum, zirconium oxide, and titanium oxide) may be used. Since PLZT is solid, unlike a liquid crystal, it does not require a substrate or a sealing resin. Therefore, by using PLZT, the optical element can be made thin.

(Embodiment 2)
In the second embodiment, an optical head of the present invention using the optical element described in the first embodiment will be described. FIG. 5 schematically shows a configuration of the optical head 50 according to the second embodiment.

  Referring to FIG. 5, an optical head 50 includes a light source 51, a diffraction grating 52, a collimator lens 53, an optical element 54, an objective lens 55, a substrate thickness sensor 56, and a first photodetector 58. And a second photodetector 59. The optical element 54 is driven by an optical element driving circuit 57. The collimator lens 53 and the objective lens 55 constitute a condensing optical system. The optical head 50 records or reproduces a signal on or from the optical recording medium 60. The optical element 54 is the optical element described in the first embodiment.

  As the light source 51, a semiconductor laser element can be used. The light source 51 outputs a recording / reproducing laser beam 61 (coherent light) to the recording layer of the optical recording medium 60. The diffraction grating 52 is a diffraction grating having a 50th order diffraction efficiency of about 50% and a ± 1st order diffraction efficiency of about 50%. The diffraction grating 52 can be formed by forming a predetermined resist pattern on the glass surface by photolithography and then etching the glass. The optical element 54 gives the light incident on the optical element 54 a phase for converting a plane wave into a spherical wave. A photodiode can be used for the first and second photodetectors 58 and 59.

  The objective lens 55 focuses the laser beam 61 on the recording layer of the optical recording medium 60. The first photodetector 58 receives the + 1st-order light diffracted by the diffraction grating 52 of the laser beam 61 reflected by the recording layer of the optical recording medium 60 and converts it into an electric signal. The second photodetector 59 receives the -1st-order light diffracted by the diffraction grating 52 of the laser beam 61 reflected by the recording layer of the optical recording medium 60 and converts it into an electric signal.

  The substrate thickness sensor 56 detects a deviation between a preset substrate thickness and an actual substrate thickness, and outputs a signal corresponding to the deviation. As the base material thickness sensor 56, for example, a sensor described in JP-A-10-334575 and US Pat. No. 6,115,336 can be used. Specifically, the base material thickness sensor 56 includes a light source, a first optical system that irradiates light emitted from the light source to an optical recording medium (measurement target), and a light receiving element that reflects light reflected from the optical recording medium. And a second optical system that leads to Here, the light source includes a laser, an LED, or a lamp. The first and second optical systems are configured by a convex lens or a combination of a convex lens and a concave lens. According to this configuration, the signal output from the light receiving element differs depending on the thickness of the base material. Another method for detecting the substrate thickness is described in Japanese Patent Application Laid-Open No. 2000-171346. In this method, the spherical position is determined based on the focal position of the first light beam on the side closer to the optical axis of the reflected light from the optical recording medium and the focal position of the second light beam outside the first light beam. Detect aberrations.

  The optical element driving circuit 57 drives the optical element 54 based on the deviation of the substrate thickness input from the substrate thickness sensor 56 so as to correct the deviation of the substrate thickness.

  The operation of the optical head 50 will be described with reference to FIG. Part of the linearly polarized light emitted from the light source 51 passes through the diffraction grating 52 and enters the collimator lens 53, where the collimator lens 53 converts the linearly polarized light into parallel light. This parallel light enters the optical element 54. Here, when the substrate thickness of the optical recording medium 60 deviates from the design value, the substrate thickness sensor 56 outputs a signal corresponding to the deviation amount, and the signal is input to the optical element driving circuit 57. You. The optical element driving circuit 57 outputs to the optical element 54 a signal necessary to correct the wavefront aberration generated when the thickness of the base material of the optical recording medium 60 is shifted based on the input signal. In this manner, the light incident on the optical element 54 has a wavefront aberration that gives a phase (power component) for converting parallel light into divergent light or a phase for converting parallel light into convergent light, due to the thickness of the base material. It is given according to the direction of displacement.

  The light transmitted through the optical element 54 enters the objective lens 55 in a state where the light is not a parallel light, so that spherical aberration occurs. Then, the spherical aberration caused by the deviation of the base material thickness of the optical recording medium 60 from the design value is corrected by the spherical aberration. That is, the optical element 54 gives the incident light a phase such that a desired spherical aberration occurs when the light enters the objective lens 55. Thus, a light spot having no aberration, that is, a light spot narrowed to the diffraction limit is formed on the optical recording medium 60.

  Next, the light reflected from the optical recording medium 60 becomes light having a wavefront aberration generated when the base material thickness of the optical recording medium 60 deviates from a design value. Is corrected. The light transmitted through the optical element 54 is transmitted through the collimator lens 53 and is diffracted by the diffraction grating 52. Then, the + 1st order diffracted light enters the first photodetector 58, and the −1st order diffracted light enters the second photodetector 59. The first photodetector 58 outputs a focus error signal indicating a focus state of light on the optical recording medium 60, and outputs a tracking error signal indicating a light irradiation position. Further, the second photodetector 59 outputs a signal for information recorded on the optical recording medium 60.

  The focus error signal output from the first photodetector 58 is input to a focus control circuit (not shown). The focus control circuit controls the position of the objective lens 55 in the optical axis direction based on the focus error signal so that the light is focused on the optical recording medium 60 in a focused state. The tracking error signal is input to a tracking control circuit (not shown). The tracking control circuit controls the position of the objective lens 55 based on the tracking error signal so that light is focused on a desired track on the optical recording medium 60. The position of the objective lens 55 is controlled using the actuator 62.

  Next, the operation of the optical element 54 will be described using an example in which the optical element 10 is used. In the optical element 10, by changing the voltage between each of the segment electrodes 13 a to 13 g and the counter electrode 17, the refractive index of the liquid crystal 15, which is a phase change layer, is changed. Gives the phase to convert to a wave. Here, in order to convert a plane wave into a spherical wave, a phase P satisfying the following equation (1) may be given to incident light.

(A-P) ² + r ² = a ² (1)
Here, a is a constant, and r is the distance from the center of the optical axis. As an example, when the wavelength of the laser beam is 405 nm, the NA is 0.85, the substrate thickness at the design value (that is, the optimal substrate thickness) is 0.1 mm, and the deviation of the substrate thickness from the design value is 10 μm. FIG. 4 shows a phase distribution satisfying the expression (1).

  Here, FIG. 6 shows the relationship between the base material thickness and the corrected aberration when the phase shown in FIG. 4 is generated by 40 concentric segment electrodes. FIG. 6 also shows, as a comparison, aberrations when no correction is performed. FIG. 7 shows the lens shift characteristics when the displacement of the substrate thickness of 10 μm is corrected. The horizontal axis in FIG. 7 represents the shift (lens shift) between the center of the optical axis and the center of the objective lens (here, the shift between the center of the 40 concentric segment electrodes and the center of the objective lens). Equivalent to).

  In the optical head 50, a phase (power component) for converting a plane wave into a spherical wave is given to the light incident on the optical element 54, and the aberration is corrected by generating a spherical aberration in combination with an objective lens. Therefore, unlike the case where the tertiary spherical aberration itself to be corrected is directly applied to the light incident on the optical element, the optical head 50 is extremely resistant to lens shift as shown in FIG.

  In the optical element 54, as described in Japanese Patent Application Laid-Open No. 10-360545, a thin-film resistor is formed on the optical element to divide a signal applied from the outside, and the divided voltages are respectively divided. The method of applying the voltage to the segment electrode can be used. According to this method, even if the number of the segment electrodes is as large as 40, the voltage applied from the outside can be three including the voltage applied to the counter electrode.

  When the optical element 10 is used, a required phase is approximately generated using a plurality of segment electrodes. On the other hand, by using the optical element 10a, a necessary phase can be generated according to its shape. In the optical element 10a, the liquid crystal 15 serving as the phase change layer has a convex shape, so that a continuously changed phase can be given to incident light without dividing the voltage application electrode. Therefore, the power component can be faithfully reproduced by using the optical element 10a. In this case, since no higher-order aberration is generated, the aberration after the correction can be made substantially zero. FIG. 6 shows the relationship between the base material thickness and the aberration after correction when the optical element 10a is used. FIG. 8 shows a lens shift characteristic when the deviation of the base material thickness of 10 μm is corrected using the optical element 10a. Also in this case, the lens shift characteristic is very good, as in the case of the correction approximated by the segment electrodes.

  As described above, the optical head 50 according to the second embodiment has good lens shift characteristics when correcting spherical aberration. In addition, since the objective lens and the optical element can be arranged without being integrated, the optical head 50 can reliably read out a signal recorded on an optical recording medium in which the thickness of the base material has shifted. Can be. Further, the optical head 50 can be further thinned. In addition, since it is not necessary to mount an optical element on the actuator, it is possible to prevent a decrease in the f characteristic (sensitivity) of the actuator, and to simplify the wiring and manufacture the actuator at low cost. Further, unlike a conventional optical head that includes two lenses and mechanically changes the distance between these lenses to generate a power component, the optical head 50 electrically generates a power component. For this reason, the optical head 50 is suitable for miniaturization. Further, in the optical head 50, since the efficiency of capturing the light incident on the optical element does not change, it is possible to prevent the RIM intensity of the light from changing.

(Embodiment 3)
In a third embodiment, another example of the optical element and the optical head of the present invention will be described. The optical head according to the second embodiment differs from the optical head according to the second embodiment in that a polarizing optical system including a polarization hologram and a quarter-wave plate is used and an optical element corresponding to the polarizing optical system is used. different. Note that portions denoted by the same reference numerals as the portions described in Embodiments 1 and 2 have the same functions as the portions described in Embodiments 1 and 2, unless otherwise specified.

  FIG. 9 schematically shows the configuration of an optical head 90 according to the third embodiment. Referring to FIG. 9, an optical head 90 includes a light source 51, a collimator lens 53, an objective lens 55, a substrate thickness sensor 56, a first photodetector 58, a second photodetector 59, a polarization hologram 91, an optical Includes element 100 and quarter wave plate 93. The optical element 100 is driven by an optical element drive circuit 57.

  The polarization hologram 91 has a function of separating polarized light. The polarization hologram 91 does not act on extraordinary rays (having a transmittance of almost 100%), and acts as a diffraction grating on ordinary rays. As the polarization hologram 91, the one disclosed in JP-A-6-27322 can be used. Such a polarization hologram can be formed by performing proton exchange on a predetermined part of a lithium niobate substrate having birefringence and etching the proton exchange part.

  The optical element 100 is an element that performs aberration correction by changing the refractive index of the liquid crystal. Details of the optical element 100 will be described later.

  The 波長 wavelength plate 93 is made of, for example, quartz. The 波長 wavelength plate 93 converts the linearly polarized light emitted from the light source 51 into a circularly polarized light and converts the light reflected by the recording layer of the optical recording medium 60 into a linearly polarized light in a direction different from that at the time of irradiation. It is an optical element. Note that an N / 4 wavelength plate (N is an odd number of 1 or more) may be used instead of the 1/4 wavelength plate.

  Next, the operation of the optical head 90 will be described with reference to FIG. The linearly polarized light (laser beam 61) emitted from the light source 51 passes through the polarization hologram 91 at a transmittance of almost 100%, enters the collimator lens 53, and is converted into parallel light by the collimator lens 53. This parallel light enters the optical element 100.

  Here, when the substrate thickness of the optical recording medium 60 is deviated from the design value, the substrate thickness sensor 56 outputs a signal corresponding to the deviation amount, and the signal is input to the optical element driving circuit 57. You. The optical element driving circuit 57 outputs to the optical element 100 a signal necessary for correcting the wavefront aberration generated when the base material thickness of the optical recording medium 60 is shifted based on the input signal. Light incident on the optical element 100 is given a wavefront aberration based on a signal output by the optical element drive circuit 57. Specifically, a wavefront aberration that gives a phase (power component) for converting parallel light into divergent light or a phase for converting parallel light into convergent light is given according to the direction of displacement of the base material thickness.

  Next, the light transmitted through the optical element 100 is incident on the quarter-wave plate 93, and is converted from linearly polarized light into circularly polarized light. The circularly polarized light is incident on the objective lens 55 in a non-parallel state, so that spherical aberration occurs in the objective lens 55, and the spherical aberration caused by the displacement of the base material of the optical recording medium 60 due to the spherical aberration is reduced. to correct. Therefore, a light spot having no aberration, that is, a light spot narrowed to the diffraction limit is formed on the optical recording medium 60.

  The light that has entered the optical recording medium 60 is reflected by the optical recording medium 60, and becomes light having a wavefront aberration that occurs when the base material thickness of the optical recording medium 60 shifts. This light passes through the objective lens 55 and enters the quarter-wave plate 93. Light incident on the １ ／ wavelength plate 93 is converted from circularly polarized light into linearly polarized light. The polarization direction of this linearly polarized light is orthogonal to the linearly polarized light emitted from the light source 51. This linearly polarized light having spherical aberration is incident on the optical element 100 of the present invention, and is given the same power component as the outward path, so that the spherical aberration is corrected.

  The linearly polarized light transmitted through the optical element 100 is diffracted almost 100% by the polarization hologram 91, the + 1st-order light of the diffraction is incident on the first photodetector 58, and the -1st-order light of the diffraction is converted into the second photodetector. It is incident on 59. The first photodetector 58 outputs a focus error signal indicating a focus state of light on the optical recording medium 60, and outputs a tracking error signal indicating a light irradiation position. The second light detector 59 outputs a signal for information recorded on the optical recording medium 60.

  The focus error signal output from the first photodetector 58 is input to a focus control circuit (not shown). The focus control circuit controls the position of the objective lens 55 in the optical axis direction based on the focus error signal so that the light is focused on the optical recording medium 60 in a focused state. The tracking error signal is input to a tracking control circuit (not shown). The tracking control circuit controls the position of the objective lens 55 based on the tracking error signal so that light is focused on a desired track on the optical recording medium 60. The position of the objective lens 55 is controlled using the actuator 62.

  Since the optical head 90 uses a polarization optical system, the efficiency of use of light emitted from the light source 51 is high, and recording / reproducing of a rewritable optical recording medium can be facilitated.

  Next, the optical element 100 of the present invention will be described. Since the liquid crystal is a uniaxial birefringent material, the phase change can be given to the incident light only when the rubbing direction of the liquid crystal and the polarization direction of the incident light are parallel. Therefore, when the polarization directions are orthogonal to each other in the forward path and the backward path as in the polarization optical system, the optical element of the first embodiment cannot provide a phase change in the backward path. In the optical head of the present invention, in which correction is performed by a combination of an optical element for defocusing incident light and an objective lens, if the same phase change as in the forward path cannot be given in the return path, the problem of defocusing in the photodetector is caused. There is. Therefore, in order to give the same phase change to the backward polarization, the optical head according to the third embodiment needs to use an optical element corresponding to the polarization optical system.

  Hereinafter, the optical element 100 will be described. A cross-sectional view of the optical element 100 is schematically shown in FIG. The optical element 100 includes a first substrate 101, a second substrate 102, a third substrate 103, a first voltage application electrode 104, a second voltage application electrode 105, a first counter electrode 106, and a second counter electrode 106. The electrode 107, the first light-transmitting resin film 108, the second light-transmitting resin film 109, the third light-transmitting resin film 110, the fourth light-transmitting resin film 111, the first liquid crystal 112, A second liquid crystal 113, a first sealing resin 114, and a second sealing resin 115.

  The first, second and third substrates 101, 102 and 103 are arranged substantially parallel to each other. These substrates are translucent and made of, for example, glass.

  The first voltage application electrode 104 is disposed between the first substrate 101 and the first liquid crystal 112. The first voltage application electrode 104 is an electrode for applying a desired voltage to the first liquid crystal 112. The first voltage applying electrode 104 is formed on the main surface inside the first substrate 101 (on the liquid crystal 112 side).

  The second voltage application electrode 105 is disposed between the third substrate 103 and the second liquid crystal 113. The second voltage applying electrode 105 is an electrode for applying a desired voltage to the second liquid crystal 113. The second voltage application electrode 105 is formed on the main surface inside the third substrate 103 (on the liquid crystal 113 side).

  The first and second voltage applying electrodes 104 and 105 include a plurality of segment electrodes, as shown in FIG. In addition, as shown in FIG. 3, when the voltage application electrode or the counter electrode is formed on a curved surface, it is formed continuously.

  The first counter electrode 106 is arranged substantially parallel to the first voltage application electrode 104. The first opposing electrode 106 is an electrode for applying a desired voltage to the first liquid crystal 112 together with the first voltage applying electrode 104. The first counter electrode 106 is formed substantially uniformly on at least a portion of the main surface of the second substrate 102 on the first liquid crystal 112 side facing the segment electrode.

  The second counter electrode 107 is arranged substantially parallel to the second voltage application electrode 105. The second opposing electrode 107 is an electrode for applying a desired voltage to the second liquid crystal 113 together with the second voltage applying electrode 105. The second counter electrode 107 is formed substantially uniformly on at least a portion of the main surface of the second substrate 102 on the second liquid crystal 113 side facing the segment electrode.

  The first and second translucent resin films 108 and 109 are formed so as to cover the first voltage application electrode 104 and the first counter electrode 106, respectively. The first and second translucent resin films 108 and 109 are alignment films for aligning the first liquid crystal 112 in a predetermined direction, and are made of, for example, a polyvinyl alcohol film. By rubbing the first light-transmitting resin film 108 or the second light-transmitting resin film 109, the first liquid crystal 112 can be oriented in a predetermined direction.

  The third and fourth translucent resin films 110 and 111 are formed so as to cover the second voltage application electrode 105 and the second counter electrode 107, respectively. The third and fourth translucent resin films 110 and 111 are alignment films for aligning the second liquid crystal 113, and are made of, for example, a polyvinyl alcohol film. By rubbing the third light-transmitting resin film 110 or the fourth light-transmitting resin film 111, the second liquid crystal 113 can be oriented in a predetermined direction. The alignment direction of the first liquid crystal 112 is orthogonal to the alignment direction of the second liquid crystal 113.

  The first liquid crystal 112 is disposed between the first and second translucent resin films 108 and 109 (between the first voltage application electrode 104 and the first counter electrode 106). The first liquid crystal 112 functions as a phase change layer that changes the phase of incident light. As the first liquid crystal 112, for example, a nematic liquid crystal can be used. By changing the voltage between the first voltage applying electrode 104 and the first counter electrode 106, the refractive index of the first liquid crystal 112 can be changed, thereby changing the phase of the incident light. Can be.

  The second liquid crystal 113 is disposed between the third and fourth translucent resin films 110 and 111 (between the second voltage applying electrode 105 and the second counter electrode 107). The second liquid crystal 113 functions as a phase change layer that changes the phase of incident light. As the second liquid crystal 113, for example, a nematic liquid crystal can be used. By changing the voltage between the second voltage applying electrode 105 and the second counter electrode 107, the refractive index of the second liquid crystal 113 can be changed, thereby changing the phase of the incident light. Can be.

  The first sealing resin 114 is for sealing the first liquid crystal 112, and is provided between the first and second light-transmitting resin films 108 and 109 so as to surround the first liquid crystal 112. Be placed. The second sealing resin 115 is for sealing the second liquid crystal 113, and is provided between the third and fourth translucent resin films 110 and 111 so as to surround the second liquid crystal 113. Be placed. For the first and second sealing resins 114 and 115, for example, an epoxy resin can be used.

  Next, the operation of the optical element 100 will be described. A control voltage is externally applied to the segment electrodes of the first and second voltage application electrodes 104 and 105. At this time, since the alignment direction of the first liquid crystal 112 is orthogonal to the alignment direction of the second liquid crystal 113, the linearly polarized light in the outward path is affected only by the change in the refractive index of the first liquid crystal 112, The phase of the power component is given by one liquid crystal 112. That is, the plane wave incident on the optical element 100 is converted into a spherical wave.

  On the other hand, the light reflected by the optical recording medium 60 becomes linearly polarized light that is orthogonal to the linearly polarized light on the outward path. For this reason, the light on the return path is affected only by the second liquid crystal 113, and the phase of the power component is given by the second liquid crystal 113. Here, when the patterns of the first and second voltage applying electrodes 104 and 105 and the applied voltage are the same, the same phase is given to the forward path and the return path, so that the spherical wave incident on the optical element 100 is obtained. Into a plane wave.

  As described above, by using two liquid crystal layers having different alignment directions, spherical aberration can be corrected even in a polarizing optical system. By performing the correction using the optical element 100, a good lens shift characteristic can be obtained as described in the second embodiment.

  In the third embodiment, the case where the optical element includes two liquid crystal layers has been described. However, the optical element described in the first embodiment may be arranged and used so that the orientation directions of the liquid crystals are orthogonal.

(Embodiment 4)
In a fourth embodiment, an example of the optical recording / reproducing apparatus of the present invention will be described. The optical recording / reproducing device according to the fourth embodiment is a device for recording or reproducing a signal on or from an optical recording medium, and may record and reproduce a signal.

  FIG. 11 schematically shows the configuration of the optical recording / reproducing apparatus 116 according to the fourth embodiment. Referring to FIG. 11, an optical recording / reproducing apparatus 116 includes an optical head 50, an optical element driving circuit 57, a motor 117, and a processing circuit 118.

  The optical head 50 has been described in the second embodiment and includes the optical element of the present invention. Note that an optical head 90 may be used instead of the optical head 50. A duplicate description of these optical heads will be omitted.

  In the optical recording / reproducing apparatus according to the fourth embodiment, a semiconductor laser device that emits a laser beam having a wavelength in a range of 390 nm to 420 nm is used as a light source. An objective lens having a numerical aperture in the range of 0.7 to 0.9 is used as the objective lens.

  Next, the operation of the optical recording / reproducing device 116 will be described. First, when the optical recording medium 60 is set in the optical recording / reproducing device 116, the processing circuit 118 outputs a signal for rotating the motor 117, and rotates the motor 117. Next, the processing circuit 118 drives the light source 51 to emit light. Light emitted from the light source 51 is reflected by the optical recording medium 60 and enters the first and second photodetectors 58 and 59.

  The first photodetector 58 outputs to the processing circuit 118 a focus error signal indicating a focus state of light on the optical recording medium 60 and a tracking error signal indicating a light irradiation position. Based on these signals, the processing circuit 118 outputs a signal for controlling the actuator 62, thereby condensing the light emitted from the light source 51 on a desired track on the optical recording medium 60. Further, the processing circuit 118 reproduces information recorded on the optical recording medium 60 based on a signal output from the second photodetector 59.

  Next, control when the base material thickness of the optical recording medium 60 is different from the design value will be described. In the optical recording / reproducing device 116, the optical head is configured such that when the base material thickness of the optical recording medium 60 is as designed, correction of spherical aberration is unnecessary. Therefore, when the substrate thickness deviates from the design value, it is necessary to correct spherical aberration.

  If the substrate thickness of the optical recording medium 60 is shifted, a signal corresponding to the shift of the substrate thickness of the optical recording medium 60 is output to the processing circuit 118 by the substrate thickness sensor 56. The processing circuit 118 controls the optical element driving circuit 57 according to the input signal, whereby the control signal necessary for correcting the spherical aberration caused by the displacement of the base material thickness of the optical recording medium 60 is converted into an optical signal. The light is output from the element driving circuit 57 to the optical element 54. Then, the spherical aberration is corrected by the optical element 54 (for details, see Embodiments 1 and 2). Thus, even if the base material thickness of the optical recording medium 60 deviates from the design value, the information signal recorded on the optical recording medium 60 can be correctly reproduced.

  In the fourth embodiment, an apparatus for detecting a deviation in the thickness of a substrate using a substrate thickness sensor has been described, but the optical recording / reproducing apparatus of the present invention is not limited to an apparatus using a substrate thickness sensor. For example, an optical recording / reproducing apparatus that learns the deviation of the base material thickness when the optical recording medium 60 is set on the motor and corrects the spherical aberration based on the learned deviation of the base material thickness may be used.

(Embodiment 5)
In a fifth embodiment, another example of the optical recording / reproducing apparatus of the present invention and an optical recording / reproducing method using the same will be described. The optical recording / reproducing apparatus and method according to the fifth embodiment record or reproduce a signal on / from a first optical recording medium including only one recording layer and a second optical recording medium including a plurality of recording layers.

  The optical recording / reproducing apparatus according to the fifth embodiment includes a light source and a spherical aberration correcting unit disposed between the optical recording medium and the light source. Specifically, an optical recording / reproducing apparatus including the optical head of the present invention can be used. In this case, the optical element and the objective lens of the present invention function as spherical aberration correction means. In the optical recording / reproducing apparatus of the fifth embodiment, a semiconductor laser element that emits a laser beam having a wavelength in a range of 390 nm to 420 nm is used as a light source. An objective lens having a numerical aperture in the range of 0.7 to 0.9 is used as the objective lens.

  Hereinafter, a case where the optical recording / reproducing device 116 described in the fourth embodiment is used as the optical recording / reproducing device of the fifth embodiment will be described.

  FIG. 12A schematically shows the substrate thickness of the first optical recording medium 121 and the second optical recording medium 122 recorded or reproduced by the optical recording / reproducing device 116. The optical recording medium 121 includes only the recording layer A as a recording layer. The optical recording medium 122 includes recording layers B and C as recording layers. Note that the optical recording medium 122 may include two or more recording layers.

  The substrate thickness (the distance from the surface 121s to the recording layer A) for the recording layer A is represented by a in FIG. The substrate thicknesses (distances from the surface 121s to the recording layers B and C) for the recording layers B and C are represented by b and c in FIG. 12A, respectively. In the optical recording / reproducing apparatus of the fifth embodiment, one of the substrate thicknesses (b or c) of the second optical recording medium 122 is equal to the substrate thickness a. FIG. 12A shows a case where the substrate thickness a and the substrate thickness b are equal. These substrate thicknesses are the total thickness of the substrate and a layer (for example, an ultraviolet curable resin) formed between the substrate and the recording layer. The recording layer is made of, for example, a phase change material whose refractive index changes by causing a phase change between a crystalline phase and an amorphous phase.

  It is preferable that the management information of the second optical recording medium 122 be recorded on the recording layer B having the substrate thickness equal to the substrate thickness a. According to this configuration, the management information of the optical recording medium 122 can be reproduced with the spherical aberration correction unit kept in the initial state.

  In the optical recording / reproducing apparatus of the fifth embodiment, the optical head 50 is designed according to the base thickness a of the first optical recording medium 121. That is, the optical head 50 is designed to have a margin that can be reproduced without correcting the spherical aberration with respect to the deviation of the base material thickness a of the first optical recording medium 121. In such a case, it is necessary to correct the spherical aberration for the deviation of the base material thickness of the second optical recording medium 122.

  Hereinafter, the operation of the optical recording / reproducing apparatus 116 according to the fifth embodiment will be described. First, when the optical recording medium 121 or 122 is set on the motor 117, the processing circuit 118 rotates the motor 117. Next, the processing circuit 118 determines the spherical aberration of the substrate thickness a without determining whether the set optical recording medium is the optical recording medium 121 or 122 in the initial state before performing recording or reproduction. Is driven to correct the spherical aberration. Specifically, no voltage is applied to the optical element of the present invention from the outside.

  Next, focus control is performed by focus control means. The focus control unit includes the actuator 62 and the processing circuit 118.

  The focus control method will be described below. First, the light source 51 is driven to emit light, and the light reflected by the optical recording medium 60 is detected by the first light detector 58. The first photodetector 58 outputs to the processing circuit 118 a focus error signal indicating a focus state of light on the optical recording medium 60 and a tracking error signal indicating a light irradiation position. Based on these signals, the processing circuit 118 outputs a signal for controlling the objective lens 55, and condenses the light emitted from the light source 51 on a desired track on the optical recording medium 60. Further, the processing circuit 118 reproduces information recorded on the optical recording medium 60 based on a signal output from the second photodetector 59.

  On the other hand, when recording or reproduction is performed on the recording layer C of the second optical recording medium 122, the spherical aberration correction unit is driven so as to correct the spherical aberration of the recording layer C. More specifically, almost simultaneously with the processing circuit 118 outputting a signal for moving the focal point from the recording layer B to the recording layer C, the processing circuit 118 outputs an optical element driving circuit so as to output a signal for correcting a deviation of the base material thickness. 57 is driven. According to this configuration, even when the recording layer for recording or reproduction is changed, a signal can be recorded or reproduced satisfactorily.

  As described above, in the optical recording / reproducing apparatus according to the fifth embodiment, the optical recording / reproducing method including the first step of driving the spherical aberration correction unit so as to correct the spherical aberration of the recording layer A before recording or reproducing. Record or reproduce signals. According to this optical recording / reproducing method, when recording or reproducing a signal on or from the recording layer C, the second step of driving the spherical aberration correcting means to correct the spherical aberration of the recording layer C after the first step. Including steps. That is, in the optical recording / reproducing apparatus of the fifth embodiment, a program for performing the above-described optical recording / reproducing method is stored in the processing circuit 118.

  In the optical recording / reproducing apparatus and method according to the fifth embodiment, since the spherical aberration can be corrected without determining whether the optical recording medium has one or a plurality of recording layers, the time until recording or reproducing can be reduced. . Even if it is known whether the optical recording medium has only one recording layer or a plurality of recording layers, the focus control is started with the spherical aberration correcting means adjusted to the substrate thickness a. Even in this case, since the spherical aberration correction unit is in the initial state, the time until focus control is shortened.

  When achieving high density by shortening the wavelength and increasing the NA, the margin for spherical aberration is reduced. Therefore, when the recording layers have different substrate thicknesses, it is necessary to correct spherical aberration for each recording layer. In such a case, assuming that the substrate thicknesses of the recording layers A, B, and C are different from each other, it is not necessary to correct the spherical aberration for the recording layer A, but it is necessary to correct the spherical aberration for the recording layers B and C. Correction is required. On the other hand, as described above, by making the substrate thickness of the recording layer A equal to the substrate thickness of the recording layer B, the spherical aberration needs to be corrected only in the case of the recording layer C. Therefore, in the optical recording / reproducing apparatus of the fifth embodiment, the circuit for correcting the spherical aberration can be simplified. Further, even when learning the substrate thickness of the recording layer, the learning time can be shortened because the substrate thickness of one layer is known.

  In the optical recording / reproducing apparatus according to the fifth embodiment, by using the optical element of the present invention, an optical recording / reproducing apparatus capable of reproducing information signals recorded on an optical recording medium with high reliability can be obtained. Further, by using the optical element of the present invention, the tolerance for the deviation of the base material thickness of the optical recording medium 60 is increased, so that an optical recording / reproducing apparatus which is inexpensive and easy to manufacture can be obtained.

(Embodiment 6)
In a sixth embodiment, another example of the optical recording / reproducing apparatus of the present invention and an optical recording / reproducing method using the same will be described. The optical recording / reproducing apparatus and method of the sixth embodiment record or reproduce a signal on / from a first optical recording medium having only one recording layer and a second optical recording medium having a plurality of recording layers.

  The optical recording / reproducing apparatus of the sixth embodiment includes a light source, a spherical aberration correction unit disposed between the optical recording medium and the light source, a focus error detection unit, and a focus control unit. Specifically, an optical recording / reproducing apparatus including the optical head of the present invention can be used.

  Hereinafter, a case where the optical recording / reproducing device 116 described in the fourth embodiment is used as the optical recording / reproducing device of the sixth embodiment will be described. In this case, the optical element 54 and the objective lens 55 of the present invention function as spherical aberration correction means, the first photodetector 58 functions as focus error detection means, and the actuator 62 and the processing circuit 118 function as focus control means. I do. In the optical recording / reproducing apparatus of the sixth embodiment, a semiconductor laser element that emits a laser beam having a wavelength in a range of 390 nm to 420 nm is used as a light source. An objective lens having a numerical aperture in the range of 0.7 to 0.9 is used as the objective lens.

  In the sixth embodiment, the substrate thicknesses of the first optical recording medium and the second optical recording medium recorded or reproduced by the optical recording / reproducing device 116 are different from each other. Assuming that the substrate thickness of the recording layer of the first optical recording medium is a and the substrate thickness of the recording layer of the second optical recording medium is b and c, a, b, and c are different from each other (FIG. 12). (B)).

  In the optical recording / reproducing apparatus according to the sixth embodiment, in an initial state before recording or reproducing a signal on or from an optical recording medium, spherical aberration correcting means is configured to correct the spherical aberration of the recording layer included in the first optical recording medium. Drive. Thereafter, the focus error is detected by the focus error detecting means, and focus control is performed by the focus control means based on the detected focus error. Recording or reproduction of a signal is performed after focus control. Note that the method described in the above embodiment can be used as a method for correcting spherical aberration, a method for detecting a focus error, and a method for focus control.

  As described above, in the optical recording / reproducing apparatus according to the sixth embodiment, before recording or reproduction, the first spherical aberration correction unit that drives the spherical aberration correction unit to correct the spherical aberration of the recording layer included in the first optical recording medium is performed. The signal is recorded or reproduced by the optical recording / reproducing method including the step of: In this optical recording / reproducing method, after a first step, a second step of detecting a focus error by a focus error detecting means, and a third step of performing focus control by a focus control means based on the detected focus error signal. Steps. Recording or reproduction of the signal is performed after this third step. Here, when the set optical recording medium is the second optical recording medium including a plurality of recording layers, based on the base material thickness error signal obtained by the base material thickness sensor after completion of the above-described steps. The spherical aberration correction means is driven, and thereafter, recording or reproduction is performed. The optical recording / reproducing apparatus of the sixth embodiment stores a program for implementing this optical recording / reproducing method in the processing circuit 118.

  In the optical recording / reproducing apparatus and the optical recording / reproducing method of Embodiment 6, regardless of whether the optical recording medium set in the optical recording / reproducing apparatus is the first optical recording medium or the second optical recording medium ( That is, whether the set optical recording medium is the first optical recording medium or the second optical recording medium is known or unknown), The spherical aberration correcting means is driven in accordance with the thickness of the recording layer to control the focus. According to this configuration, focus control can be performed without determining whether an optical recording medium to be recorded or reproduced is an optical recording medium including only one recording layer or an optical recording medium including a plurality of recording layers. Since it is performed, the time until focus control can be shortened.

  If it is known whether the optical recording medium to be recorded or reproduced is the first optical recording medium or the second optical recording medium, first, the standard of the target recording layer is set. The spherical aberration correcting means may be driven so as to correct the spherical aberration at an appropriate substrate thickness. After that, the focus error may be detected by the focus error detection means, and the focus control may be performed by the focus control means based on the detected focus error. Recording or reproduction is performed after focus control. In this method, before performing focus control, a spherical aberration correction unit is driven so as to correct spherical aberration at a standard substrate thickness. For this reason, the deviation between the standard substrate thickness and the actual substrate thickness is detected by the substrate thickness sensor, and the recording or reproduction can be performed after the spherical aberration is completely corrected based on the deviation. That is, in the sixth embodiment, the optical recording / reproducing includes the first step of acquiring information as to whether the optical recording medium to be recorded or reproduced is the first optical recording medium or the second optical recording medium. Recording or reproduction is performed according to the method. This method includes a second step of driving a spherical aberration correction unit to correct a spherical aberration at a standard substrate thickness of a recording layer to be recorded or reproduced based on the acquired information; The method includes a third step of detecting a focus error by the means, and a fourth step of performing focus control by the focus control means based on the detected focus error. Recording or reproduction is performed after focus control. In this case, the processing circuit 118 stores a program including the above four steps.

  In the first step, whether the optical recording medium to be recorded or reproduced is the first optical recording medium including only one recording layer or the second optical recording medium including a plurality of recording layers Is obtained as follows. A focus error signal indicating a focus error is detected corresponding to the recording layer. That is, if there are two recording layers, two focus error signals are detected. Therefore, if a circuit for detecting the number of detected signals is configured, information on the number of recording layers included in the optical recording medium can be obtained.

  According to the above method of first acquiring information on whether an optical recording medium to be recorded or reproduced is an optical recording medium including only one recording layer or an optical recording medium including a plurality of recording layers Since focus control is performed after spherical aberration correction corresponding to a standard substrate thickness corresponding to the recording layer, stable focus control can be performed.

  In the above embodiment, the case where the optical recording / reproducing apparatus including the optical head 50 is used has been described, but the optical head 90 may be used.

  Further, in the above embodiment, the case where the optical element of the present invention is disposed between the collimator lens and the objective lens (parallel light system) has been described, but it is disposed between the light source and the collimator lens (divergent light system). May be.

  In the above embodiment, the infinite optical head is described. However, the optical head of the present invention may be a finite optical head that does not use a collimator lens.

  In the above-described second and third embodiments, the case where the deviation of the substrate thickness is detected using the sensor has been described. However, the spherical aberration may be corrected using the deviation of the substrate thickness learned in advance.

  In the above embodiment, the case where the diffraction grating is used to separate the reflected light from the optical recording medium from the optical path from the light source has been described, but another optical element (for example, a half mirror) is used instead of the diffraction grating. May be.

  Further, in the above embodiment, the optical recording medium that records information only by light has been described, but the present invention can be applied to an optical recording medium that records information only by light and magnetism. Further, the present invention is not limited to a disk-shaped optical recording medium, but can be applied to a card-shaped optical recording medium and the like.

  Further, in the above-described embodiment, the optical element that changes the phase with respect to the transmitted light has been described. However, the optical element that changes the phase when the incident light is reflected may be used. For example, an optical element that changes the phase of reflected light by using a phase change layer made of a piezoelectric material and distorting the phase change layer may be used. More specifically, a total reflection mirror is formed on a glass substrate, a piezoelectric material is provided on the back surface, and the piezoelectric material is inflated according to an external voltage. For example, by inflating only a region of the piezoelectric material provided in the central portion of the total reflection mirror, the total reflection mirror becomes spherical and has a lens effect. Accordingly, the incident parallel light can be converted into divergent light, so that spherical aberration can be corrected.

  Further, in the optical recording / reproducing apparatus and the optical recording / reproducing method, the case where the optical element of the present invention and the objective lens are used as the spherical aberration correcting means has been described, but other spherical aberration correcting means may be used. For example, two lenses arranged along the optical axis may be used as the spherical aberration correcting means. The spherical aberration can be corrected by moving these two lenses in the optical axis direction.

  Further, in the optical recording / reproducing apparatus and the optical recording / reproducing method, the case where the photodetector is used as the focus error detecting means has been described, but other focus error detecting means may be used.

  Further, in the optical recording / reproducing apparatus and the optical recording / reproducing method, the case where the processing circuit and the actuator are used as the focus control means has been described. However, other focus control means may be used.

  In the above embodiment, the case where the semiconductor laser is used as the light source has been described. However, an SHG light source that generates the second harmonic may be used as the light source.

  Further, in the above-described embodiment, a description has been given using a phase-change type optical recording medium as the optical recording medium. However, another optical recording medium such as a magneto-optical recording medium may be used.

  As described above, the embodiments of the present invention have been described by way of examples. However, the present invention is not limited to the above embodiments, and can be applied to other embodiments based on the technical idea of the present invention.

It is sectional drawing which shows an example about the optical element of this invention. It is a top view which shows an example of a voltage application electrode about the optical element of this invention. It is sectional drawing which shows another example about the optical element of this invention. FIG. 6 is a diagram illustrating an example of a relationship between a distance from an optical axis center and a phase necessary for correcting spherical aberration. It is a figure which shows typically the structure of an example about the optical head of this invention. 5 is a graph illustrating an example of a relationship between a substrate thickness and aberration. 6 is a graph illustrating an example of a relationship between a lens shift and an aberration. 9 is a graph showing another example of the relationship between lens shift and aberration. FIG. 6 is a diagram schematically illustrating another example of the configuration of the optical head according to the invention. It is sectional drawing which shows another example about the optical element of this invention. FIG. 1 is a diagram schematically illustrating an example of a configuration of an optical recording / reproducing apparatus of the present invention. It is a schematic diagram which shows (a) an example of a base material thickness, and (b) another example of a 1st optical recording medium and a 2nd optical recording medium which are recorded or reproduced by the optical recording / reproducing apparatus of this invention. FIG. 9 is a diagram schematically illustrating an example of a configuration of a conventional optical head. It is a top view which shows an example about the electrode of the liquid crystal panel used for the conventional optical element. FIG. 4 is a diagram illustrating an example of a distribution of wavefront aberration when a substrate thickness deviates from a design value. 6 is a graph illustrating an example of a relationship between a lens shift and an aberration when a substrate thickness deviates from a design value.

Explanation of reference numerals

10, 10a, 54, 100 Optical element 11, 101 First substrate 12, 102 Second substrate 13, 104, 105 Voltage application electrode 13a-g Segment electrode 14, 16 Translucent resin film 15, 112, 113 Liquid crystal 17, 106, 107 Counter electrode 50, 90 Optical head 51 Light source 52 Diffraction grating 53 Collimator lens 55 Objective lens 56 Substrate thickness sensor 57 Optical element drive circuit 58 First light detector 59 Second light detector 60 Optical recording Medium 61 Laser beam 62 Actuator 91 Polarization hologram 93 Quarter-wave plate 103 Third substrate 116 Optical recording / reproducing device 117 Motor 118 Processing circuit 121 First substrate 121 s, 122 s Surface 122 Second substrate A, B, C recording Layers a, b, c Substrate thickness

Claims (2)

An optical recording medium for recording or reproducing information using a light source for emitting light having a wavelength of 390 nm to 420 nm and an optical head having an objective lens having a numerical aperture of 0.7 to 0.9. A multi-layer optical recording medium having two or more recording layers,
The distance from any one of the plurality of recording layers to the surface of the multilayer optical recording medium is approximately 100 μm, and the distance between the multilayer optical recording medium and the multilayer optical recording medium is approximately 100 μm. A multilayer optical recording medium characterized by recording the management information of the above. Optical recording / reproducing having spherical aberration correcting means for recording / reproducing on / from each layer of a single-layer optical recording medium having only one recording layer for recording information and a multilayer optical recording medium having two or more recording layers for recording information A multi-layer recording medium used for an apparatus,
Management information of each optical recording medium is recorded on each of the single-layer optical recording medium and the multilayer optical recording medium,
The distance from the surface of the multilayer optical recording medium to the management information recorded on the multilayer optical recording medium is the distance from the surface of the single-layer optical recording medium to the management information recorded on the single-layer optical recording medium. A multilayer optical recording medium characterized by being substantially equal to:

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