JP2007026512A - Optical head and disk playback device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical head capable of further improving the recording/reproducing characteristics of an optical recording medium by reducing the difference of a phase changing amount in an optical spot caused by a light incident angle. <P>SOLUTION: This device includes a liquid crystal element 27 disposed on the optical path of a return light between a beam splitter 24 and a photodetector 31 and divided into a plurality of areas, a voltage applying means 28 for applying voltages of the plurality of divided areas of the liquid crystal element 27 to change the refraction indexes of the divided areas, and a control means 29 for controlling the operation of the voltage applying means 28 to adjust a phase compensation amount provided to each divided area with respect to a light made incident on the divided area of the liquid crystal element 27 and a uniform phase change is given to the spot of a light passing through the liquid crystal element 27 in the spot. Thus, the difference of phase changing amounts in the optical spot caused by the incident angles of lights made incident on the liquid crystal element 27 is reduced, and the reproducing characteristics of an optical recording medium 32 are improved. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical head and a disk reproducing apparatus.

  Some disk reproducing apparatuses are capable of recording and / or reproducing a plurality of types of magneto-optical recording media having different physical formats, for example, MD (Mini Disk: registered trademark) and Hi-MD (registered trademark). An optical head provided in a disk reproducing apparatus that reproduces at least the plurality of types of magneto-optical recording media has a light source that emits laser light, and the laser light emitted from the light source is focused on the information recording surface of the magneto-optical recording medium. And an optical system that separates the laser light that is the return light reflected from the information recording surface of the magneto-optical recording medium, and a signal conversion means that converts the laser light separated by the optical system into an electrical signal.

  Magneto-optical recording media such as MD and Hi-MD are provided with guide grooves called grooves on the information recording surface. When reproducing a magneto-optical recording medium, the disk reproducing device irradiates the groove with laser light emitted from a light source, and reads information recorded in the groove by reflected light of the irradiated light. In recent years, the track pitch has been narrowed so that as many information signals as possible can be recorded on the magneto-optical recording medium, and the density of the magneto-optical recording medium has been increased.

The track pitch of MD used conventionally is 1.6 μm, and the track pitch of Hi-MD that can realize high-density recording developed in recent years is 1.25 μm. Further, EFM (Eight to Fourteen Modulation) modulated data is recorded in the MD groove, and RLL (1-7), which is a physical format for high-density recording, is used in the Hi-MD groove. PP (RLL: Run Length Limited, PP: Party preserve / Prohibit rmtr (
Repeated Minimum Transition Runlength)) Modulated data is recorded. Thus, as an optical head having compatibility between MD and Hi-MD having different physical formats, the wavelength of the laser light emitted from the light source is 780 nm, and the numerical aperture NA of the objective lens is 0.45. Things are used.

  When such an optical head is used, the spot diameter of the laser light emitted from the light source may be larger than the track pitch, and the spot diameter may protrude from the groove. The light that protrudes from the groove is reflected on the surface of the land adjacent to the groove, mixed into the light reflected by the groove, and converted into an electrical signal. Such a phenomenon is called crosstalk, which causes many errors in an electrical signal converted by mixing other light into the light reflected by the groove, for example, an information reproduction signal (RF signal), and degrades reproduction characteristics. Cause.

  Therefore, an optical head has been proposed in which a phase compensation element is inserted in the optical path of the reflected light from the magneto-optical recording medium to limit the crosstalk component from the land, thereby reducing errors and preventing deterioration in reproduction characteristics. (For example, refer to Patent Document 1).

  FIG. 10 is a side view schematically showing the configuration of the optical head 1 disclosed in Patent Document 1. As shown in FIG. An optical head 1 that is a discrete optical system includes a semiconductor laser element 2 that emits laser light, a grating 3 that separates light emitted from the semiconductor laser element 2, a beam splitter 4 that transmits or reflects incident light, and an incident light A collimating lens 5 that collimates the light, an objective lens 6 that focuses the laser light on the magneto-optical recording medium 11, a phase compensation element 7 that adjusts the phase of the incident light, and a Wollaston prism 8 that separates the incident light. And a cylindrical lens 9 that generates astigmatism in the incident light, and a photodetector 10 that is a light receiving element that converts the incident light into an electrical signal.

  For example, when the magneto-optical recording medium 11 is MD or Hi-MD, the semiconductor laser element 2 that is a light source that emits light emits laser light having a wavelength of 780 nm. The semiconductor laser element 2 is connected to an external circuit that supplies a drive current (not shown), and the intensity of the laser beam can be changed according to the amount of current from the external circuit.

  The grating 3 is a diffraction grating that separates light emitted from the semiconductor laser element 2 into 0th-order diffracted light, −1st-order diffracted light, and + 1st-order diffracted light. The beam splitter 4 transmits the forward light emitted from the semiconductor laser element 2 and directed to the magneto-optical recording medium 11, and reflects the return light reflected by the magneto-optical recording medium 11. The collimating lens 5 emits the diffused light emitted from the semiconductor laser element 2 and made incident as parallel light.

  The objective lens 6 has, for example, a numerical aperture NA of 0.45, and has an objective lens in a focus direction that is parallel to the optical axis of incident light and a tracking direction that is parallel to the radial direction of the magneto-optical recording medium 11. It is mounted on an actuator (not shown) that holds 6 in a movable manner. The objective lens 6 focuses the forward light emitted from the semiconductor laser element 2 on the information recording surface of the magneto-optical recording medium 11 to form a light spot.

  The phase compensation element 7 applies phase compensation to the return light from the magneto-optical recording medium 11 that is incident light, restricts the crosstalk component from the land, reduces errors, and obtains good reproduction characteristics. To do. Note that the phase compensation element 7 has a phase compensation amount that can provide good reproduction characteristics in both cases where the magneto-optical recording medium 11 is MD and Hi-MD, and MD, Hi-MD, To give the same phase compensation amount to the incident light.

  The Wollaston prism 8 separates the return light that is reflected by the magneto-optical recording medium 11 and reflected by the beam splitter 4, passes through the phase compensation element 7, and passes through the cylindrical lens 9. Then, the light is incident on a predetermined light receiving area of the photodetector 10 described later. The cylindrical lens 9 gives astigmatism to incident light so that the photodetector 10 can output a focus error signal (FE signal). The photodetector 10 is a signal conversion means in which a predetermined light receiving region is formed. The photodetector 10 converts the incident laser light into an electric signal and performs an operation, thereby performing the above-described FE signal, RF signal, and tracking error signal (TE signal). Is output.

  Laser light emitted from the semiconductor laser element 2 passes through the grating 3, the beam splitter 4 and the collimator lens 5, enters the objective lens 6, and is condensed on the information recording surface of the magneto-optical recording medium 11. The laser beam condensed on the information recording surface of the magneto-optical recording medium 11 is reflected on the reflecting surface of the magneto-optical recording medium 11, passes through the objective lens 6 and the collimating lens 5, and is reflected by the beam splitter 4. The light is transmitted through the compensation element 7, separated by the Wollaston prism 8, and further transmitted through the cylindrical lens 9 and received by the photodetector 10. In the photodetector 10, the received laser beam is converted into an electrical signal, and each signal is output.

  In the optical head 1 disclosed in Patent Document 1, the phase of the light reflected by the magneto-optical recording medium 11 is suitably adjusted by the phase compensation element 7 and the phase of the light reflected by the land is adjusted. It is supposed that the reproduction characteristics of MD and Hi-MD can be prevented for any magneto-optical recording medium 11.

  Here, the track pitch of each magneto-optical recording medium differs between when MD is reproduced and when Hi-MD is reproduced. Therefore, it is desirable that the phase compensation amount provided by the phase compensation element 7 is an optimum value for each according to the physical format of the magneto-optical recording medium 11 to be reproduced.

  However, as described above, the optical head 1 disclosed in Patent Document 1 uses the phase compensation element 7 that imparts the same phase compensation amount to incident light during MD reproduction and Hi-MD reproduction. For this reason, although the phase compensation amount that can be reproduced satisfactorily for both the MD and Hi-MD magneto-optical recording media 11 is selected and given by the phase compensation element 7, this phase compensation amount is different from each other. It is not always the optimum value according to the type of the magneto-optical recording medium 11.

  Therefore, when reproducing a plurality of types of magneto-optical recording media, an optimum phase compensation amount can be given to each of the types of magneto-optical recording media, and the reproduction characteristics of the magneto-optical recording medium can be further improved. There is a need for a head. As an optical head that meets such requirements, an optical head that uses a liquid crystal element as a phase compensation element has been proposed. The liquid crystal element changes the refractive index of the liquid crystal according to the applied voltage, and gives a phase change to the incident light. Since such a liquid crystal element can set the phase compensation amount to an optimum value according to the applied voltage, an optimum phase compensation amount is provided for each of MD reproduction and Hi-MD reproduction. can do.

  However, when a liquid crystal element is used as the phase compensation element, there are the following problems. A liquid crystal element has a property that when the incident angle of incident light changes due to the refractive index anisotropy of liquid crystal molecules constituting the liquid crystal element, the refractive index of the incident light changes. Here, if the light incident on the liquid crystal element is diffuse light or convergent light, the light near the optical axis is caused by the difference in the incident angle between the light near the center of the light spot and the light at the periphery of the light spot. The refractive index of the liquid crystal element with respect to the light near the center of the spot is different from the refractive index of the liquid crystal element with respect to the light at the periphery of the light spot away from the optical axis.

  In this way, when the refractive index is different for the light near the optical axis and the peripheral edge of the light spot away from the optical axis, when diffused light or convergent light is incident on the liquid crystal element, MD reproduction and Hi-MD reproduction are performed. Even if the optimum phase compensation amount for each magneto-optical recording medium at each time and the optimum phase compensation amount near the center of the light spot is given to the liquid crystal element, the actual phase change at the periphery of the light spot There is a problem that the amount becomes a value different from the optimum phase compensation amount, and the amount of phase change in the light spot varies.

JP 2003-296960 A (pages 14-15, FIG. 16)

  An object of the present invention is to provide an optical head and a disk reproducing apparatus that can reduce the difference in phase change amount in the light spot caused by the incident angle of light and further improve the reproducing characteristics of the optical recording medium. is there.

The present invention relates to an optical head for recording and / or reproducing information by irradiating light onto an optical recording medium.
A light source that emits light;
A light separating means for separating the outward light emitted from the light source and the backward light emitted from the light source and reflected by the optical recording medium;
A light receiving element for receiving the return light reflected from the optical recording medium;
A liquid crystal element provided on the optical path of the return path light between the light separating means and the light receiving element, the liquid crystal element having a plurality of divided regions;
Voltage application means for applying a voltage to each of the plurality of divided regions of the liquid crystal element to change the refractive index of each divided region;
The phase compensation amount given to each divided region with respect to the light incident on the divided region of the liquid crystal element is adjusted so that the spot of the light transmitted through the liquid crystal element is given a uniform phase change in the spot. An optical head comprising: control means for controlling operation of voltage applying means for applying a voltage to the divided region of the element.

In the present invention, the liquid crystal element is
It is provided with an inclination with respect to the optical axis of the return light between the light separating means and the light receiving element.

In the present invention, the liquid crystal element is
A transparent electrode is provided for each divided region.

  The present invention is also characterized in that the direction in which the plurality of divided regions of the liquid crystal element are arranged is parallel to the radial direction of the optical recording medium in a recording or reproducing state.

In the present invention, the direction in which the liquid crystal element is inclined with respect to the optical axis of the return light between the light separating means and the light receiving element is:
The optical recording medium is in a radial direction of a recording or reproducing state.

  According to the invention, the light source emits laser light having a wavelength of 780 nm, and the objective lens has a numerical aperture NA of 0.45.

The present invention also includes an objective lens that focuses light emitted from a light source onto an optical recording medium;
A diffusion angle adjusting element that adjusts the diffusion angle of light incident on the objective lens,
The diffusion angle adjustment element
It is arranged between the light source and the objective lens.

The present invention also includes an anisotropic element that separates the return light from the optical recording medium that is reflected by the optical recording medium and separated by the light separating means and enters the light receiving element.
Anisotropic elements are
It is arranged between the liquid crystal element and the light receiving element.

  According to another aspect of the present invention, there is provided a disc reproducing apparatus including the optical head according to any one of the above.

  According to the present invention, the optical head includes a light source that emits light, forward light emitted from the light source, and light separation means that separates the return light emitted from the light source and reflected by the optical recording medium; A light receiving element for receiving the return light reflected from the recording medium, and a liquid crystal element provided on the optical path of the return light between the light separating means and the light receiving element, the liquid crystal element having a plurality of divided regions Voltage applying means for changing the refractive index of each divided region by applying a voltage to each of the plurality of divided regions of the liquid crystal element, and a phase applied to each divided region with respect to light incident on the divided region of the liquid crystal element Control that adjusts the compensation amount and controls the operation of the voltage applying means for applying a voltage to the divided region of the liquid crystal element so that the spot of light transmitted through the liquid crystal element is given a uniform phase change in the spot. Means comprisingFor this reason, the difference in the amount of phase change in the light spot due to the incident angle of the light incident on the liquid crystal element can be reduced, and the reproduction characteristics of the optical recording medium can be improved.

  According to the present invention, since the liquid crystal element is provided to be inclined with respect to the optical axis of the return light between the light separating means and the light receiving element, astigmatism can be generated with respect to the incident light. Further, an optical component for generating astigmatism such as a cylindrical lens is not necessary. Therefore, it is possible to reduce the number of parts and reduce the size of the optical head.

  According to the present invention, since the transparent electrode provided for each divided region is used as an electrode for applying a voltage to the liquid crystal element, it is possible to prevent a decrease in light intensity caused by blocking light with the electrode.

  According to the invention, the direction in which the plurality of divided regions of the liquid crystal element are arranged is parallel to the radial direction of the optical recording medium in a recording or reproducing state. Of the light spots of light incident on the liquid crystal element, the information reproduction signal is not included in the light in the radial peripheral portion of the light spot. Accordingly, the incident angle of the critical portion between the portion including the information reproduction signal and the portion not including the information reproduction signal in the radial direction of the light spot is smaller than the incident angle of the peripheral portion in the tangential direction perpendicular to the radial direction. When phase compensation is applied in the radial direction, the difference in the amount of phase compensation to be applied in order to cause a uniform phase change in the light spot can be made smaller than when phase compensation is applied to. For this reason, it is preferable to provide phase compensation in the radial direction. By making the direction in which the divided regions are arranged parallel to the radial direction of the optical recording medium, the difference in the amount of phase compensation applied in the optical spot is reduced. In addition, the phase change in the light spot can be made uniform more easily.

  According to the invention, the direction in which the liquid crystal element is tilted with respect to the optical axis of the return light between the light separating means and the light receiving element is the radial direction of the optical recording medium in the recording or reproducing state. Using the liquid crystal element in which the divided regions are arranged in parallel in the radial direction as described above, the phase change in the light spot can be made uniform and astigmatism can be generated.

  According to the invention, when MD and Hi-MD are used as an optical recording medium, an optical head including a light source that emits laser light having a wavelength of 780 nm and an objective lens having a numerical aperture NA of 0.45. By using it, a good information reproduction signal can be obtained for both MD and Hi-MD.

  According to the invention, the objective lens that condenses the light emitted from the light source on the optical recording medium, and the coupling that is disposed between the light source and the objective lens and adjusts the diffusion angle of the light incident on the objective lens. Since a diffusion angle adjusting element such as a lens is provided, the optical head can be reduced in size and the coupling efficiency can be improved.

  Further, according to the present invention, Wollaston is arranged between the liquid crystal element and the light receiving element, separates the return light from the optical recording medium reflected by the optical recording medium and separated by the light separating means, and enters the light receiving element. Since an anisotropic element such as a prism is provided, light can be incident on a light receiving region provided in the light receiving element, and an information reproduction signal or the like can be obtained.

  According to the invention, since any one of the optical heads described above is provided, it is possible to further improve the reproduction characteristics of the optical recording medium by reducing the difference in the amount of phase change in the light spot due to the incident angle. And a disc reproducing apparatus capable of performing good reproduction is provided. Note that the disk reproducing device is not limited to only reproducing information recorded on the optical recording medium, and performs information recording on the optical recording medium and information recorded on the optical recording medium. Including.

  FIG. 1 is a side view showing a simplified configuration of an optical head 21 according to an embodiment of the present invention. The optical head 21 includes a semiconductor laser element 22, a grating 23, a beam splitter 24, a coupling lens 25, an objective lens 26, a liquid crystal element 27, a voltage applying means 28, a control means 29, a Wollaston prism. 30 and a photo detector 31.

  The optical head 21 of this embodiment is provided on the optical path of the return light between the beam splitter 24 that is a light separating means and the photodetector 31 that is a light receiving element, and a liquid crystal element 27 having a plurality of divided regions. A voltage applying means 28 for changing the refractive index of each divided region by applying a voltage to each of the plurality of divided regions of the liquid crystal element 27; and for each divided region with respect to light incident on the divided regions of the liquid crystal element 27 The voltage application means 28 for applying a voltage to the divided region of the liquid crystal element 27 so that the phase compensation amount to be applied is adjusted and the spot of light transmitted through the liquid crystal element 27 is given a uniform phase change in the spot. And control means 29 for controlling the operation.

  In the present invention, the “phase compensation amount” is the amount of phase change given to the incident light by the liquid crystal element 27, and the “phase change amount” is given by the liquid crystal element 27. This is the amount of change in the phase of the emitted light.

  The semiconductor laser element 22 is a light source that emits light. For example, when the optical recording medium 32 is a magneto-optical recording medium such as MD or Hi-MD, the semiconductor laser element 22 emits laser light having a wavelength of 780 nm. The semiconductor laser element 22 is connected to an external circuit that supplies a drive current (not shown), and the intensity of the laser beam can be changed according to the amount of current from the external circuit. The light emitted from the semiconductor laser element 22 enters the grating 23.

  The grating 23 is a diffraction grating that separates light emitted from the semiconductor laser element 22 into 0th-order diffracted light, −1st-order diffracted light, and + 1st-order diffracted light. The laser beam that has passed through the grating 23 enters the beam splitter 24.

  The beam splitter 24 is a light separating unit that separates the forward light emitted from the semiconductor laser element 22 and the return light emitted from the semiconductor laser element 22 and reflected by the optical recording medium 32. The beam splitter 24 is a light separating unit that separates the forward light emitted from the semiconductor laser element 22 and the return light emitted from the semiconductor laser element 22 and reflected by the optical recording medium 32. The beam splitter 24 transmits the forward light emitted from the semiconductor laser element 22 and guides it to the objective lens 26, and reflects the return light reflected by the optical recording medium 32 and guides it to the photodetector 31.

  The coupling lens 25 is a diffusion angle adjustment element that adjusts the diffusion angle of light incident on the objective lens 26. Between the semiconductor laser element 22 and the objective lens 26, and in this embodiment, the beam splitter 24 and the objective lens 26 are coupled. Between. The coupling lens 25 emits light with a divergence angle of light emitted from the semiconductor laser element 22 and transmitted through the grating 23 and the beam splitter 24, for example, to be small, and makes the incident light enter the objective lens 26. When such a coupling lens 25 is used, the optical head 21 can be downsized in the optical axis direction of the outgoing light and the objective lens 26 in the focus direction, and the intensity of outgoing light from the objective lens 26 can be improved. be able to.

  The objective lens 26 condenses the light emitted from the semiconductor laser element 22 on the optical recording medium 32. For example, when the optical recording medium 32 is a magneto-optical recording medium such as MD or Hi-MD, the objective lens 26 having a numerical aperture NA of 0.45 is used. The objective lens 26 is mounted on an actuator (not shown) that movably holds the objective lens 26 in a tracking direction that is parallel to the focus direction that is the optical axis direction of incident light and the radial direction of the optical recording medium 32. The forward light emitted from the semiconductor laser element 22 is condensed on the information recording surface of the optical recording medium 32 to form a light spot on the information recording surface. The light condensed on the optical recording medium 32 is reflected by the information recording surface of the optical recording medium 32 to become return light, passes through the coupling lens 25, is reflected by the beam splitter 24, and enters the liquid crystal element 27. . Here, the light condensed on the optical recording medium 32 by the objective lens 26 will be described.

  FIG. 2 is a schematic plan view showing how the light spot 41 is formed on the information recording surface of the optical recording medium 32. On the information recording surface of the optical recording medium 32, a guide groove-shaped groove 42 on which an information reproduction signal is recorded is formed. The three-dimensional XYZ axis shown in FIG. 2 is defined as follows. The direction perpendicular to the information recording surface (focus direction) of the optical recording medium 32 in the recording or reproduction state (focus direction) is the X-axis direction, the radial direction (tracking direction) of the optical recording medium 32 in the recording or reproduction state is the Y-axis direction, and light The direction in which the groove 42 of the recording medium 32 extends and is orthogonal to the tracking direction (tangential direction) is defined as the Z-axis direction. The definition of the XYZ triaxial direction is commonly used throughout this specification.

  When reproducing the information recorded on the optical recording medium 32, the optical head 21 irradiates the inside of the groove 42 with the light spot 41 of the laser light emitted from the semiconductor laser element 22, and the emitted laser light The information recorded in the groove 42 is read by the reflected light.

  When the optical recording medium 32 is MD, the track pitch 44 is 1.6 μm, and when the optical recording medium 32 is Hi-MD that realizes high-density recording, the track pitch 44 is 1.25 μm. When information is recorded on or reproduced from the optical recording medium 32 by the optical head 21, the light spot of the laser light emitted from the semiconductor laser element 22 and applied to the optical recording medium 32 is, for example, 1.6 μm in diameter. In this case, the light spot 41 protrudes from the groove 42.

  The light in the portion of the light spot area 41 a that protrudes from the groove 42 is reflected by the surface of the land 43 adjacent to the groove 42 and mixed into the light reflected by the groove 42. Such a phenomenon is called crosstalk. When other light is mixed into the light reflected by the groove 42, an electric signal converted by a photodetector 31 (to be described later) that receives the light, for example, an information reproduction signal (RF A large number of errors in the signal), leading to deterioration in reproduction characteristics.

  In the optical head 21 of the present embodiment, in order to prevent such deterioration of the reproduction characteristics, a liquid crystal element that limits the crosstalk component from the land 43 by applying a phase change to the return light and reduces errors. 27 is provided.

  FIG. 3 is a plan view showing the configuration of the liquid crystal element 27. The liquid crystal element 27 is provided on the optical path of the return light between the beam splitter 24 that is a light separating means and the photodetector 31 that is a light receiving element, and has divided areas 27a, 27b, and 27c that are divided into a plurality of parts. The liquid crystal element 27 provided in the optical head 21 of the present embodiment is a flat plate-like member having a rectangular shape when viewed from above, and a plurality of (three in the present embodiment) are formed by dividing lines 51 and 52 extending in the Z-axis direction. The direction in which the plurality of divided areas 27a, 27b, and 27c are arranged is parallel to the Y-axis direction that is the radial direction of the optical recording medium 32 in the recording or reproducing state. It is characterized by that.

  The liquid crystal element 27 includes a transparent electrode connected to the voltage applying unit 28 for each of the divided regions 27a, 27b, and 27c, and another transparent electrode arranged to face the transparent electrode connected to the voltage applying unit 28. And a liquid crystal layer disposed between the pair of transparent electrodes. Such a liquid crystal layer and a transparent electrode are sealed with a glass substrate.

  A voltage is applied to the liquid crystal element 27 from the voltage applying means 28 via a transparent electrode provided in each of the divided regions 27a, 27b, and 27c. Therefore, the voltage applying unit 28 is configured to apply different voltage values to the divided regions 27a, 27b, and 27c. By using a transparent electrode as an electrode provided in the plurality of divided regions of the liquid crystal element 27, light is not blocked by the electrode, so that a decrease in the intensity of light passing through the liquid crystal element 27 can be prevented. In the liquid crystal element 27, a voltage is applied between the pair of transparent electrodes to change the refractive index of the liquid crystal layer, and the change in refractive index gives a phase change to incident light.

  The liquid crystal element 27 to which a voltage is applied by the voltage applying means 28 gives phase compensation to the incident light and polarizes the incident light into substantially linearly polarized light. The voltage applying means for applying a voltage to each transparent electrode of the liquid crystal element 27 having the divided regions includes a power source (not shown) and PWM (Pulse Width Modulation). The operation of the voltage applying unit 28 is controlled by the control unit 29.

  The control unit 29 adjusts the amount of phase compensation given to each divided region with respect to the light incident on the divided regions 27a, 27b, and 27c of the liquid crystal element 27, and the spot of the light transmitted through the liquid crystal element 27 is within one spot. The operation of the voltage applying means 28 for applying a voltage to the divided regions 27a, 27b, 27c of the liquid crystal element 27 is controlled so that such a phase change is given. The reason for the difference in the phase change amount in the light spot of the diffused light incident on the liquid crystal element 27 and the method for adjusting the phase compensation amount so as to reduce the difference in the phase change amount will be described later.

  The control unit 29 not only controls the operation of the voltage application unit 28 so as to reduce the difference in the amount of phase change in the light spot, but also sets the phase compensation amount of the liquid crystal element 27 according to the type of the optical recording medium 32. The operation of the voltage applying means 28 is controlled so as to adjust. The control means 29 detects the type of the optical recording medium 32 and applies a predetermined voltage to each divided region of the liquid crystal element 27 according to the detected type of the optical recording medium 32. Depending on the type of the optical recording medium 32, the voltage to be applied to each divided region of the liquid crystal element 27, that is, the voltage with the refractive index that can give appropriate phase compensation to the incident light, is tested in advance. Thus, it can be obtained for each type of optical recording medium 32 and stored in the memory 29a attached to the control means 29 in the form of table data or the like. The memory 29a is configured by, for example, an LSI (Large Scale Integration).

  The control unit 29 determines the type of the optical recording medium 32 based on an electrical signal obtained by a photodetector 31 described later, for example, TOC (Table Of Contents) information recorded in advance on the optical recording medium 32.

  Furthermore, although the liquid crystal element 27 has a problem that various characteristics such as its optical characteristics change due to temperature change, the optical head 21 of this embodiment measures the surface temperature of the liquid crystal element 27 near the liquid crystal element 27. A temperature sensor (not shown) is provided, and changes in various characteristics accompanying changes in temperature of the liquid crystal element 27 are corrected using table data in which the temperature and voltage stored in advance in the memory 29a are associated.

  Next, phase compensation is performed so as to reduce the difference in the phase change amount by the reason why the phase change amount is generated in the light spot of the diffused light incident on the liquid crystal element 27 and the liquid crystal element 27 which is the most characteristic feature of the present invention. A method for adjusting the amount will be described.

  FIG. 4 is a cross-sectional view schematically showing how diffused light enters the liquid crystal element 27. FIG. 4A schematically shows a state in which light 61 a at one peripheral edge of a spot of incident light that is diffused light is incident on the liquid crystal element 27. 4B schematically shows a state in which the light 61b in the other peripheral edge opposite to the one peripheral edge shown in FIG. 4A of the incident light spot is incident on the liquid crystal element 27. FIG.

  The liquid crystal element 27 includes a pair of transparent electrodes (not shown) and a liquid crystal layer disposed between the pair of transparent electrodes. The liquid crystal 62 and the pair of transparent electrodes that form the liquid crystal layer are sealed with a glass substrate 63. The liquid crystal element 27 to which a voltage is applied by the voltage applying means 28 gives phase compensation to the incident light and polarizes the incident light into substantially linearly polarized light.

  When light enters such a liquid crystal element 27, the refractive index of the liquid crystal 62 with respect to the incident light changes when the incident angle of the incident light changes due to the refractive index anisotropy of the liquid crystal 62 constituting the liquid crystal element 27. In addition, when the incident angle of light incident on the liquid crystal element 27 changes, the voltage applied to the liquid crystal element 27 is the same, and the refraction characteristics of the liquid crystal element 27 itself are the same. Due to the change in the refractive index of the light, the amount of phase change of the light is different from the desired amount.

  Such a variation in the amount of phase change is not only a problem for a plurality of lights having different incident angles, but also when the incident light is diffuse light, the light near the optical axis of the light spot and the light spot There is also a problem with the light near the periphery on both sides with the optical axis as the center. In addition, if there is a difference between the refractive index of the liquid crystal near the center of the light spot and the refractive index of the liquid crystal near the periphery of the light spot due to the different incident angles, the phase near the center of the light spot is caused by the difference in refractive index. There is a difference between the amount of change and the amount of phase change near the periphery of the light spot.

Here, the difference between the phase change amount at the center of the light spot and the phase change amount at the peripheral portion of the light spot is expressed by the following equation (1). However, the “difference in refractive index” means the refractive index at the center of the light spot generated with the change in the incident angle even though the voltage applied to the liquid crystal element is the same and the refractive characteristics of the liquid crystal element itself are the same. And the refractive index of the periphery of the light spot.
(Difference in phase change)
= (Difference in refractive index) x (thickness of liquid crystal) x 360 / (wavelength of incident light) (1)

  As shown in the equation (1), if the variation in the phase change amount occurs in the same light spot, even if the optimum phase compensation amount near the center of the light spot is given to the entire light spot of the incident light that is the diffused light The phase change amount of the incident light at the periphery of the light spot becomes a phase change amount different from the optimum value.

  FIG. 5 shows the amount of phase change at each position in the radial direction of the light spot when the optimum phase compensation amount is uniformly applied to the entire incident light with respect to the diffused light incident on the non-divided liquid crystal element. FIG. The liquid crystal element imparts an optimum phase compensation amount to the entire light spot at the optical axis center (center of light spot) of incident light that is diffused light. As a result, the incident light can be substantially linearly polarized near the center of the light spot of the incident light.

  However, as described above, even when the optimum phase compensation amount similar to that at the center of the light spot is given to the entire light spot, the incident angle of the incident light is the central portion of the optical axis at the periphery of the light spot of the diffused light. Therefore, the value is out of the optimum phase change amount. As described above, when the amount of phase change at the periphery of the light spot deviates from the optimum value, the polarization state of the light near the periphery of the light spot changes from linearly polarized light to elliptically polarized light.

  In order to reduce the variation in the amount of phase change in the light spot caused by the difference in refractive index caused by the difference in the incident angle of light, the optical head 21 of the present embodiment has a configuration as shown in FIG. A liquid crystal element 27 having a plurality of divided regions 27a, 27b, and 27c is used. The liquid crystal element 27 having the divided regions 27a, 27b, and 27c as shown in FIG. 3 has different voltage values applied to the transparent electrodes provided in the divided regions 27a, 27b, and 27c. The refractive index with respect to the incident light can be set to a different value for each of the regions 27a, 27b, and 27c, and the amount of phase compensation applied to the light incident on the respective divided regions 27a, 27b, and 27c can be changed.

  FIG. 6 is a diagram showing the amount of phase compensation that the liquid crystal element 27 having a plurality of divided regions shown in FIG. 3 gives to incident light that is diffused light. In FIG. 6, the phase compensation amount imparted to the incident light in each of the divided regions 27a, 27b, and 27c is indicated by a solid line. In FIG. 6, a line 54 indicated by a two-dot chain line indicates a phase change amount when a uniform phase compensation amount is given in the light spot to the diffused light incident on the liquid crystal element that is not divided into a plurality of parts. .

  In the divided region 27a, a phase compensation amount larger than the phase compensation amount provided in the vicinity of the center of the light spot is given to one end side of the light spot peripheral portion where the actual phase change amount becomes smaller than the optimum value. In the divided region 27b, since the difference between the actual phase change amount and the optimum phase change amount is small, the phase compensation amount is not changed. In the divided region 27c, a phase compensation amount smaller than the phase compensation amount provided near the center of the light spot is given to the other end side of the light spot peripheral edge where the actual phase change amount becomes larger than the optimum value.

  As described above, the voltage value applied to the transparent electrodes provided in the divided regions 27a, 27b, and 27c in order to change the phase compensation amount in the divided regions 27a, 27b, and 27c of the liquid crystal element 27 is, for example, in advance A value obtained by a test or the like and stored in a memory 29a provided in the control means 29 can be used.

  FIG. 7 is a diagram showing a phase change amount 55 when different phase compensation amounts are given to the divided regions with respect to the diffused light incident on the liquid crystal element 27 having a plurality of divided regions. The liquid crystal element 27 is divided into a plurality of portions, and the phase compensation amount is changed by changing the applied voltage value from the voltage applying means 28 near the center and the peripheral portion of the light spot, thereby optimizing the phase change amount in the light spot. The difference from the value can be reduced, and the difference in the amount of phase change in the light spot can be reduced.

  FIG. 8 shows an error rate during reproduction of the optical recording medium 32 when phase compensation is given by a liquid crystal element that is not divided into a plurality of parts, and optical recording when phase compensation is given by a liquid crystal element 27 having a plurality of divided areas. It is a figure which shows the error rate at the time of medium 32 reproduction | regeneration. When the phase compensation is given by a liquid crystal element that is not divided into a plurality (conventional), the error rate is a white circle, and when the phase compensation is given by the liquid crystal element 27 having a plurality of divided regions provided in the optical head 21 of the present invention. The error rate in the present invention is indicated by black circles. When the liquid crystal element 27 having a plurality of divided regions is used, the phase compensation amount indicated by the horizontal axis indicates the phase compensation amount provided by the divided region 27b. The error rate is a measurement of the number of errors that occurred per unit time. An MD was used as the optical recording medium 32 for measuring the error rate.

  As shown in FIG. 8, the error rate when the phase change is applied by the liquid crystal element 27 having a plurality of divided regions in most of the phase compensation amount other than a certain range (about 90 ° to 120 °). This was much lower than the error rate when a phase change was applied by a liquid crystal element that was not divided into a plurality of parts. In this way, by dividing the liquid crystal element into a plurality of parts and appropriately selecting the voltage value to be applied to the liquid crystal element near the center and the periphery of the light spot, the amount of phase compensation in each divided region can be changed to change the inside of the light spot. It can be seen that the difference in the amount of phase change in can be reduced, and the reproduction characteristics of the optical recording medium 32 can be improved.

  The liquid crystal element 27 preferably gives a phase change so that the polarization axis of the light adjusted to linearly polarized light is parallel to the Y-axis direction that is the radial direction of the optical recording medium 32 in a recording or reproducing state. . The reason will be described below.

  As shown in FIG. 2 described above, the liquid crystal element 27 is used to limit a crosstalk component due to light reflected by the surface of the land 43 adjacent to the groove 42. Here, when the information reproduction signal of the groove 42 is obtained, looking at the light spot 41, the information reproduction signal recorded in the groove 42 is included up to the peripheral portion of the light spot 41 in the Z-axis direction. On the other hand, in the Y-axis direction, the light spot region 41a in the vicinity of the periphery of the light spot 41 is irradiated to the land 43, and no information reproduction signal is included. That is, when the light spot 41 is viewed in the Y-axis direction, the information reproduction signal exists only near the center of the light spot 41.

  Here, since the phase is a wave, the phase is divided into a P wave that is a wave in the Y-axis direction and an S wave that is a wave in the Z-axis direction. When the optical recording medium 32 is an MD, the phases of the reflected light from the optical recording medium 32 are in a state where both the P wave and the S wave are substantially aligned (SP = 0 °). However, when the optical recording medium 32 is Hi-MD, the reflected light from the optical recording medium 32 is in a state where the P wave is delayed by δ ° from the S wave (S−P = δ °).

  When δ = 0 can be achieved by applying phase compensation by the liquid crystal element 27, optimum reproduction characteristics can be obtained even when the optical recording medium 32 is Hi-MD. There are two methods for setting δ = 0 in this way, a method of delaying the S wave, which is a wave in the Z-axis direction, by δ °, and a method of advancing the P wave, which is a wave in the Y-axis direction, by δ °, that is, Y There is a method of delaying the P wave, which is an axial wave, by 2π−δ °.

  As described above, the information reproduction signal recorded in the groove 42 is included up to the periphery of the light spot 41 in the Z-axis direction. Therefore, in the method of delaying the S wave that is a wave in the Z-axis direction by δ °, it is necessary to give a phase change to the light at the peripheral edge that is incident on the liquid crystal element 27, and the optical head 21 of this embodiment is provided. Even if the liquid crystal element 27 is used, a slight difference in the amount of phase change in the light spot occurs.

  On the other hand, in the Y-axis direction, the information reproduction signal recorded in the groove 42 is not included in the peripheral portion of the light spot 41. Therefore, in the method in which the P wave, which is a wave in the Y-axis direction, is advanced by δ °, the information reproduction signal is not included in the peripheral portion of the light incident on the liquid crystal element 27, so the difference in phase change amount in the light spot The incident angle of the critical portion between the portion where the information reproduction signal is included and the portion where the information reproduction signal is not included in the Y-axis direction of the light spot is smaller than the incident angle of the peripheral portion of the light spot. As a signal, there is almost no difference in the amount of phase change.

  As described above, the incident angle of the critical portion between the portion including the information reproduction signal and the portion not including the information reproduction signal in the Y-axis direction of the light spot 41 is greater than the incident angle of the peripheral portion in the Z-axis direction perpendicular to the Y-axis direction. Is also small. Therefore, the difference in the amount of phase compensation to be applied in order to cause a uniform phase change in the light spot is smaller when the phase change is applied in the Y-axis direction than when the phase change is applied in the Z-axis direction. Therefore, it is preferable to give a phase change in the Y-axis direction of the optical recording medium 32, and the direction in which the divided regions 27a, 27b, and 27c of the liquid crystal element 27 are arranged is parallel to the Y-axis direction of the optical recording medium 32. By doing so, the difference in the amount of phase compensation applied in the light spot can be reduced, and the phase change of the light spot can be made uniform more easily.

  Further, in the present embodiment, the liquid crystal element 27 is provided to be inclined with respect to the optical axis of the return light between the beam splitter 24 and the photodetector 31, and the liquid crystal element 27 is provided between the beam splitter 24 and the photodetector 31. The direction inclined with respect to the optical axis of the return path light is the radial direction (Y-axis direction) of the optical recording medium 32 in the recording or reproducing state.

  By providing the liquid crystal element 27 so as to be inclined with respect to the optical axis of the return path light between the beam splitter 24 and the photodetector 31, astigmatism can be generated with respect to the incident light to the liquid crystal element 27. An optical component for generating astigmatism such as a cylindrical lens is not necessary. For this reason, the number of parts can be reduced and the optical head can be downsized.

  Further, the direction in which the liquid crystal element 27 is inclined with respect to the optical axis of the return light between the beam splitter 24 and the photodetector 31 is the Y-axis direction that is the radial direction of the optical recording medium 32 in the recording or reproducing state. As described above, by using the suitable liquid crystal element 27 in which the divided regions 27a, 27b, and 27c are divided in parallel to the Y-axis direction, the phase change in the light spot 41 is made uniform and the light spot 41 is not Point aberration can be imparted.

  The return light from the optical recording medium 32 transmitted through the liquid crystal element 27 as described above is given phase change and astigmatism by the liquid crystal element 27 and enters the Wollaston prism 30.

  The Wollaston prism 30 is an anisotropic element that separates the return light from the optical recording medium 32 reflected by the optical recording medium 32 and separated by the beam splitter 24 and makes it incident on the photodetector 31. The liquid crystal element 27 and the photodetector 31 are separated from each other. Between. The Wollaston prism 30 includes a main signal used in a servo system for detecting incident light, for example, a focus error signal (FE signal) and a tracking error signal (TE signal), and an MO (Magneto-Optical) signal. The signals are separated into I and J signals used for (RF signals) and are incident on the light receiving areas of the signals provided in the photodetector 31.

  The photodetector 31 is a light receiving element that receives the return light reflected from the optical recording medium 32. The photodetector 31 outputs the FE signal, the TE signal, and the RF signal by converting the incident laser light into an electric signal and performing an operation. The photodetector 31 is provided with a plurality of light receiving areas.

  FIG. 9 is a plan view schematically showing a configuration of a plurality of light receiving areas provided in the photodetector 31. The photodetector 31 includes, for example, light receiving areas A, B, C, and D in which light receiving areas divided into four equal-area rectangles are arranged in a matrix of 2 rows and 2 columns, and light receiving areas A to D. Two rectangular light receiving areas E and F arranged in the Y-axis direction on both sides and two rectangular light receiving areas I and J arranged in the Z-axis direction on both sides of the light receiving areas A to D are provided. . In the light receiving regions A to D, the zero-order diffracted light separated by the grating 23 is generated, and astigmatism is generated by the liquid crystal element 27 and light used for the main signal separated by the Wollaston prism 30 is received. The FE signal is output by the astigmatism method. In the light receiving areas E and F, light of −1st order diffracted light and + 1st order diffracted light separated by the grating 23 and used for the main signal separated by the Wollaston prism 30 is received, and the TE signal is detected. In the light receiving regions I and J, the 0th-order diffracted light separated by the grating 23 and used for the I and J signals separated by the Wollaston prism 30 is received and the RF signal is detected.

The photodetector 31 receives the return light in each of the light receiving areas A to J, and outputs each electrical signal as shown in the following equation. In the following formula, a value represented by a signal detected from each light receiving area is represented by adding “S” to the head of the alphabet representing each light receiving area.
FE signal = (SA + SC) − (SB + SD)
TE signal = SE-SF
RF signal = SI-SJ

  The operation of the optical head 21 will be described below. The laser light emitted from the semiconductor laser element 22 passes through the grating 23, is separated into 0th-order diffracted light, + 1st-order diffracted light, and −1st-order diffracted light, and passes through the beam splitter 24. The laser light transmitted through the beam splitter 24 spreads through the coupling lens 25, changes its angle, and is focused on the information recording surface of the optical recording medium 32 by the objective lens 26. The light condensed on the optical recording medium 32 is reflected by the optical recording medium 32, passes through the objective lens 26 and the coupling lens 25, is reflected by the beam splitter 24, and enters the liquid crystal element 27.

  When the return light from the optical recording medium 32 is incident on the liquid crystal element 27, a voltage determined in advance according to the type of the optical recording medium 32 is applied to the transparent electrode of the liquid crystal element 27 by the voltage applying unit 28. Adjust the phase compensation amount. Here, since the transparent electrode is divided into a plurality of liquid crystal elements 27 as shown in FIG. 3, the difference in the amount of phase change in the light spot due to the incident angle of light can be reduced. Further, the liquid crystal element 27 is provided to be inclined with respect to the optical axis of the return path light between the beam splitter 24 and the photodetector 31, so that astigmatism is given to the incident light to the liquid crystal element 27.

  The laser light to which the phase change and astigmatism are given by the liquid crystal element 27 is separated by the Wollaston prism 30 and received at a predetermined position of the photodetector 31. The photodetector 31 outputs each electric signal of the FE signal, the TE signal, and the RF signal using the received laser beam.

  As described above, the optical head 21 of the present embodiment is applied to the liquid crystal element 27 according to the position of the light spot in the Y-axis direction by dividing the liquid crystal element 27 into the plurality of regions 27a, 27b, and 27c. Since the phase compensation amount to be applied to the incident light can be adjusted by appropriately selecting the voltage value to be applied, the difference in phase change amount in the light spot can be reduced, and the reproduction characteristics of the optical recording medium 32 are improved. be able to. Furthermore, since the liquid crystal element 27 is provided with an inclination, an optical component such as a cylindrical lens for generating astigmatism is not necessary, and the optical head can be downsized.

Further, the optical recording medium 32 on which recording / reproduction is performed by the optical head 21 is not limited to the magneto-optical recording medium such as MD, Hi-MD, etc., but is not limited to CD (Compact Disk), DVD (
An optical recording medium such as a Digital Versatile Disk or a Blu-ray Disc (registered trademark) can also be used.

  Further, the disk reproducing apparatus including the optical head 21 of the present invention as described above can reduce the difference in phase change amount in the light spot caused by the incident angle of light, and can improve the reproducing characteristics of the optical recording medium 32. Further improvement can be achieved. Incidentally, the disk reproducing apparatus provided with the optical head 21 of the present invention is not limited to the one that only reproduces the information recorded on the optical recording medium 32, and the information recording and optical recording on the optical recording medium 32 are possible. The information recorded on the medium 32 may be both reproduced.

It is a side view which simplifies and shows the structure of the optical head 21 which is one Embodiment of this invention. 3 is a schematic plan view showing a state where a light spot 41 is formed on an information recording surface of an optical recording medium 32. FIG. 3 is a plan view showing a configuration of a liquid crystal element 27. FIG. 4 is a cross-sectional view schematically showing how diffused light enters a liquid crystal element 27. FIG. FIG. 5 is a diagram showing the amount of phase change at each position in the radial direction of the light spot when an optimum phase compensation amount is uniformly applied to the entire incident light with respect to diffused light incident on a liquid crystal element that is not divided. is there. It is a figure which shows the phase compensation amount provided with respect to the incident light which is a diffused light by the liquid crystal element 27 which has several division area shown in FIG. It is a figure which shows the phase change amount 55 when a different phase compensation amount is provided for every division area with respect to the diffused light which injects into the liquid crystal element 27 which has the division area divided | segmented into plurality. When reproducing the optical recording medium 32 when phase compensation is applied by a liquid crystal element that is not divided into a plurality, and when reproducing optical recording medium 32 when phase compensation is applied by a liquid crystal element 27 having a plurality of divided regions. It is a figure which shows these error rates. 3 is a plan view schematically showing a configuration of a plurality of light receiving regions provided in the photodetector 31. FIG. It is a side view which shows roughly the structure of the optical head 1 disclosed by patent document 1. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 21 Optical head 22 Semiconductor laser element 23 Grating 24 Beam splitter 25 Coupling lens 26 Objective lens 27 Liquid crystal element 28 Voltage application means 29 Control means 29a Memory 30 Wollaston prism 31 Photo detector 32 Optical recording medium

Claims (9)

In an optical head for recording and / or reproducing information by irradiating light onto an optical recording medium,
A light source that emits light;
A light separating means for separating the outward light emitted from the light source and the backward light emitted from the light source and reflected by the optical recording medium;
A light receiving element for receiving the return light reflected from the optical recording medium;
A liquid crystal element provided on the optical path of the return light between the light separating means and the light receiving element, the liquid crystal element having a plurality of divided regions;
Voltage application means for applying a voltage to each of the plurality of divided regions of the liquid crystal element to change the refractive index of each divided region;
The phase compensation amount given to each divided region with respect to the light incident on the divided region of the liquid crystal element is adjusted so that the spot of the light transmitted through the liquid crystal element is given a uniform phase change in the spot. An optical head comprising: control means for controlling operation of voltage applying means for applying a voltage to the divided region of the element. The liquid crystal element
2. The optical head according to claim 1, wherein the optical head is provided to be inclined with respect to the optical axis of the return light between the light separating means and the light receiving element. The liquid crystal element
3. The optical head according to claim 1, further comprising a transparent electrode for each divided region.   4. The optical head according to claim 1, wherein a direction in which the plurality of divided regions of the liquid crystal element are arranged is parallel to a radial direction of the optical recording medium in a recording or reproducing state. . The direction in which the liquid crystal element is inclined with respect to the optical axis of the return light between the light separating means and the light receiving element is:
5. The optical head according to claim 4, wherein the optical head is in a radial direction of an optical recording medium in a recording or reproducing state.   6. The optical head according to claim 1, wherein the light source emits laser light having a wavelength of 780 nm, and the objective lens has a numerical aperture NA of 0.45. An objective lens for condensing the light emitted from the light source onto the optical recording medium;
A diffusion angle adjusting element that adjusts the diffusion angle of light incident on the objective lens,
The diffusion angle adjustment element
The optical head according to claim 1, wherein the optical head is disposed between the light source and the objective lens. Including an anisotropic element that separates the return light from the optical recording medium that is reflected by the optical recording medium and separated by the light separating means and enters the light receiving element;
Anisotropic elements are
The optical head according to claim 1, wherein the optical head is disposed between the liquid crystal element and the light receiving element.   A disk reproducing apparatus comprising the optical head according to claim 1.

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