JP2000306278A - Optical information reproducing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical information reproducing device capable of unnecessitating the mechanical displacement and being miniaturized by using a liquid crystal wavelength element as a polarization phase compensating element. SOLUTION: A liquid crystal wavelength plate is arranged as the polarization phase compensating element 8 on an optical path of the light beam for reproduction through a magneto-optical disk 6, and also a driving circuit 26 for applying the voltage to this liquid crystal wavelength plate is provided, and the voltage to be applied to the liquid crystal wavelength plate is changed for a land part and a groove part. By this arrangement, the polarization phase difference given to the light beam for reproduction by the liquid crystal wavelength plate is changed over at the land part and the groove part so that the amplitude of a reproduced signal becomes maximum and the crosstalk from adjacent tracks becomes minimum.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical information reproducing apparatus for reproducing information recorded on both lands and grooves of a recording medium, and more particularly to an optical information reproducing apparatus for reproducing recorded information using a magneto-optical recording medium. Things.

[0002]

2. Description of the Related Art In recent years, so-called land / groove recording, in which information is recorded and reproduced on both lands and grooves of an optical disc, has been developed as a technique for achieving a large capacity of an optical disc. Etc. In land / groove recording, further development of a narrower track pitch is being pursued. By the way, regarding such land / groove recording, for example, IS
O'98 (INTERNATIONAL SYNPOS
IUM ON OPTICAL MEMORY199
8) TECHNICAL DIGEST TU-F-0
4, when land / groove recording is applied to a magneto-optical disk, by appropriately selecting a phase difference applied between orthogonal polarization components of a reproducing light beam in a reproducing optical system including a groove and a medium, It is described that the reproduction signal level becomes maximum and crosstalk from an adjacent track becomes minimum.

[0003] Japanese Patent Application Laid-Open No. 10-255347 discloses the same. According to the above-mentioned document based on ISO '98, specifically, the groove depth of the disk is set to 3λ / 8 (λ is the wavelength), and the land is around 40 °, and the groove is −4.
It is described that when a phase difference near 0 ° is given, the reproduced signal level becomes almost maximum and crosstalk from an adjacent track becomes minimum. Further, as a specific means for realizing this, a rotatable λ / 2 wavelength is interposed between λ / 4 wavelength plates having a fixed azimuth, and a phase difference is obtained by rotating the λ / 2 wavelength plate. Provided optics are disclosed. Japanese Patent Application Laid-Open No. Hei 10-255347 discloses that an electro-optic crystal is used as a means for giving a phase difference.

[0004]

In the prior art, a method of rotating a λ / 2 wave plate to provide a phase difference requires a driving device for creating a mechanical displacement. System) has a problem of being enlarged. In addition, in the method using an electro-optic crystal, the electro-optic crystal generally has a high driving voltage, which is not suitable for peripheral equipment of a computer or a consumer device, and also has a long device in the optical axis direction, so that the apparatus is also increased in size. There was a problem of doing.

[0005] In view of the above-mentioned conventional problems, the present invention provides a liquid crystal wavelength element as a polarization phase compensating element.
It is an object of the present invention to provide an optical information reproducing apparatus that does not require mechanical displacement and can be downsized.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to irradiate a reproducing light beam to a land portion and a groove portion of a magneto-optical recording medium, and to detect the reproducing light beam passing through the recording medium by an optical sensor. In an optical information reproducing apparatus for reproducing recorded information in the land portion and the groove portion, a liquid crystal wavelength element for giving a polarization phase difference to the reproducing light beam is disposed in an optical path of the reproducing light beam through the recording medium. Means for applying a voltage to the liquid crystal wavelength element, and changing the voltage to be applied to the liquid crystal wavelength element between when a land portion of the recording medium is reproduced and when a groove portion is reproduced. This is achieved by an optical information reproducing apparatus characterized in that a polarization phase difference given to a reproducing light beam by a wavelength element is switched between a land portion and a groove portion.

[0007]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a diagram showing a configuration of an embodiment of the present invention. In FIG. 1, reference numeral 1 denotes a semiconductor laser serving as a recording / reproducing light source, which is driven by a laser driving circuit 21. A part of the laser beam emitted from the semiconductor laser 1 is transmitted through the polarization beam splitter 2, and the transmitted beam is collimated by a collimator lens 4, then narrowed down to a minute light spot by an objective lens 5, and is focused on a magneto-optical disk 6. Irradiated. Lands and grooves are formed at equal intervals on the magneto-optical disk 6, and the magneto-optical disk 6 is a recording medium for land / groove recording. The magneto-optical disk 6 is rotated at a predetermined speed by driving a spindle motor (not shown).

On the other hand, a light beam emitted from the semiconductor laser 1 is partially reflected by the polarization beam splitter 2 and detected by the monitor sensor 3. The detection signal of the monitor sensor 3 is detected as an electric signal by the detection circuit 22 and sent to the optical head controller 25. The optical head controller 25 is a control circuit that controls the optical head based on the control of the magneto-optical disk controller 28 that controls the entire apparatus. The optical head controller 25 controls the laser drive circuit 21 based on the detection signal of the detection circuit 22, and controls the laser output of the semiconductor laser 1. Further, it performs focus control, tracking control, control of the polarization phase compensation element driving circuit 26, and the like, as described later.

The laser light applied to the magneto-optical disk 6 is partially reflected on the disk surface, and the reflected light passes through the objective lens 5 and the collimator lens 4 again to enter the polarization beam splitter 2. This incident light beam is partially reflected by the polarization beam splitter 2 and guided to the reproduction sensor 11 via the polarization phase compensation element 8, the Wollaston prism 9, and the detection lens 10. The polarization phase compensating element 8 is an element for giving a polarization phase difference to a light beam. In this embodiment, a combination of a fixed polarization phase difference element and a liquid crystal wave plate is used.

The polarization phase compensating element 8 is driven by a polarization phase compensating element driving circuit 26, and an optical head controller 25
A different polarization phase difference between the land and the groove is applied to the light beam based on the information indicating whether the currently reproduced track from the land is the land or the groove. In this case, since the reproducing circuit 27 reproduces the track address where the light spot is currently scanning, the optical head controller 25 recognizes the land portion or the groove portion based on the information. The luminous flux given the polarization phase difference is polarized and separated by the Wollaston prism 9, collected by the detection lens 10, and detected by the reproduction sensor 11.

Here, when information is recorded on the magneto-optical disk 6, the laser drive circuit 21 drives the semiconductor laser 1 to irradiate the magneto-optical disk 6 with a recording light spot having a constant intensity. During recording and reproduction, the optical head controller 25 detects a focus error and a tracking error of the light spot based on the output of the reproduction sensor 11, and controls the actuator drive circuit 23 based on the detected error to perform focus control and tracking control. That is, by driving a focus actuator and a tracking actuator (not shown) and displacing the objective lens 5 in the focusing direction and the tracking direction, the light spot is focused on a land track or a groove track of the rotating magneto-optical disk 6. Also, focus control and tracking control are performed so that scanning is performed following land tracks and groove tracks. A modulation magnetic field is applied from the magnetic head 7 to the magneto-optical disk 6 simultaneously with the irradiation of the recording light spot. That is, the magnetic head 7 is driven by the magnetic head drive circuit 24 in accordance with the recording signal.
To apply a magnetic field modulated according to recording information. As described above, information is recorded by applying a modulation magnetic field while irradiating a light spot with a constant intensity. Further, information can be recorded not only by the magnetic field modulation recording method but also by an optical modulation method.

On the other hand, when reproducing the information recorded on the magneto-optical disk 6, the semiconductor laser 1 is driven by the laser drive circuit 21.
Is driven, and a reproducing light spot having a constant intensity is scanned on the magneto-optical disk 6. The reflected light from the magneto-optical disk 6 passes through the objective lens 5, the collimator lens 4, the polarization beam splitter 2, the polarization phase compensator 8, the Wollaston prism 9, and the detection lens 11 as described above, and the reproduction sensor 11
Is detected by As described above, the polarization phase compensator 8 applies a reproducing light beam having a different polarization phase difference between the land portion and the groove portion. The reproduction circuit 27 processes the output signal of the reproduction sensor 11, generates a reproduction signal, and performs predetermined signal processing such as waveform equalization, binarization, and demodulation of the reproduction signal to reproduce the recorded information.

FIG. 2 is a diagram showing an example of the polarization phase compensator 8. In the example of FIG. 2, a fixed polarization phase difference element 31 and a liquid crystal wave plate 32 arranged in a direction substantially perpendicular to the incident light beam are used. These two elements are integrated but may be separated. As the fixed polarization phase difference element 31, an optical crystal is used, and in FIG. 2, a crystal whose thickness is controlled so that the polarization phase difference is 40 ° is used. The optical crystal axis (optically anisotropic axis) is perpendicular to the optical axis in the plane of FIG.

On the other hand, the groove depth of the magneto-optical disk 6 is 3λ /
8 This is known because the push-pull tracking error signal has the maximum amplitude. Also, the polarization phase difference of the element in the reproduction optical system before the polarization phase compensating element 8 is controlled to be substantially an integral multiple of 180 °. That is, the polarization phase difference is substantially zero. Therefore, the polarization phase difference given to the reproducing light beam may be about ± 40 °. That is, when the groove depth of the disk is set to 3λ / 8 and a phase difference of 40 ° at the land portion and -40 ° at the groove portion is given as described in the background art, the amplitude of the reproduction signal becomes maximum. Phase difference to be applied to the light beam
° degree. Therefore, as described above, the polarization phase difference of the fixed polarization phase difference element 31 is set to −40 °.
Although the polarity of the polarization phase difference is determined by the coordinate definition, in the present embodiment, the phase difference applied to the light beam in the arrangement of FIG. 2 is negative.

The polarization phase difference caused by the liquid crystal wave plate 32 is about 80 °. When an electric potential is applied to the liquid crystal wave plate 32, an optically anisotropic axis is generated in a plane perpendicular to the optical axis as described later, and the direction of this optical anisotropy is orthogonal to the optical crystal axis of the fixed polarization retardation element 31. I am trying to do it. By applying the potential in this manner, the polarization phase difference by the liquid crystal wave plate 32 can be made 80 °. Therefore, as the polarization phase compensating element 8 obtained by combining the fixed polarization phase difference element 31 and the liquid crystal wave plate 32, −40 ° (−40 + 0) when no potential is applied to the liquid crystal wave plate 32, and 40 ° when a potential is applied.
By controlling the potential applied to the liquid crystal wave plate 32 at the time of reproducing the land portion and the time of reproducing the groove portion, the polarization phase difference given to the reproducing light beam can be changed to (−40 + 80). To switch to ± 40 °.

FIG. 3 is a view showing a cross-sectional structure of the liquid crystal wave plate 32. FIG. 3A shows a state where no voltage is applied, and FIG.
(B) shows a state where a voltage is applied. First, the liquid crystal wave plate 32 is provided between the two glass substrates 42 and 51.
Are enclosed, and the gap of the liquid crystal is determined by the spacers 46 and 47. The electrodes 44 and 45 and the electrodes 48 and 4 are sandwiched between the two glass substrates 42 and 51 so as to sandwich a portion through which light passes.
9 are provided. Between the glass substrate 42 and the liquid crystal 41,
Between the glass substrate 51 and the liquid crystal 41, the alignment film 4
3,50 are provided. As the liquid crystal 41, a nematic liquid crystal is used. As shown in FIG. 3A, when no voltage is applied between the electrodes 44 and 45 and between the electrodes 48 and 49, the liquid crystal molecules 61 are oriented in a direction parallel to the incident light, and are viewed from the light incident side. And isotropic.

On the other hand, as shown in FIG.
When a voltage of + V ₀ is applied to 49 and a voltage of −V ₀ is applied to the electrodes 44 and 48, an electric field E is generated in a direction perpendicular to the direction of the incident light, and the liquid crystal molecules 61 start to be oriented in the direction of the electric field. Then, an optically anisotropic axis is generated in the alignment direction of the liquid crystal molecules 61, and a polarization phase shift occurs in the incident light. When the electric field intensity becomes a sufficient value, the maximum polarization phase shift amount determined by the difference between the ordinary light refractive index and the extraordinary light refractive index of the liquid crystal 41 and the gap of the liquid crystal is obtained.

FIG. 4 is a diagram showing a qualitative relationship between the voltage applied to the liquid crystal wave plate and the retardation (polarization phase shift amount). In FIG. 4, when the applied voltage is increased, the retardation sharply increases at a certain threshold voltage, and the retardation becomes a saturation retardation at a voltage of about twice the threshold voltage (the maximum polarization phase shift determined by the gap of the liquid crystal). About 80%. In this region, the change of the retardation with respect to the voltage is gentle.

When the liquid crystal wave plate 32 is driven by the polarization phase compensating element driving circuit 26 shown in FIG.
It is desirable to apply a voltage of about twice the threshold voltage of FIG. Although the voltage at which the retardation saturates depends on the gap of the liquid crystal, the liquid crystal material, the element size, and the electrode shape, it is usually about several volts to several tens of volts. And a sufficiently low voltage. Further, a liquid crystal having a difference Δn between the ordinary light refractive index and the extraordinary light refractive index of 0.10 is used, and the saturated retardation is set to 100 ° so that about 80% of the saturated retardation corresponds to 80 °.

The wavelength of the semiconductor laser 1 is 650 nm. Therefore, the gap of the liquid crystal is (100 ° / 360) ×
(0.65 / 0.1) ≒ 1.8 μm. In the present embodiment, when the land portion of the magneto-optical disk 6 is reproduced by using the optical head having the polarization phase compensation element shown in FIG. 2, a voltage is applied to the liquid crystal wave plate 32 from the polarization phase compensation element driving circuit 26, When reproducing the groove, control is performed so that no voltage is applied. Therefore, when reproducing the land portion, a polarization phase difference of 40 ° can be given to the reproduction light beam, and when reproducing the groove portion, a polarization phase difference of −40 ° can be provided, so that the reproduction signal level can be maximized and the adjacent track Crosstalk from the user can be minimized.

FIG. 5 is a plan view showing another example of the polarization phase compensator 8. In the example of FIG. 5, only the liquid crystal wave plate 71 is used, and the optical head has a simpler configuration. FIG. 5 is a plan view when viewed from the incident light side. First, FIG.
As in the case of the above, electrodes 44 and 45 are provided so as to sandwich a portion through which light passes on the upper glass substrate of the two glass substrates, and the electrodes 48 and 45 are provided on the lower glass substrate in opposition to the electrodes 44 and 45. 49 are provided. The electrodes 81 and 82 are provided on the upper glass substrate so as to be orthogonal to these electrode pairs.
Electrodes 83 and 84 are provided on the lower glass substrate.
The gap of the liquid crystal is half that in the case of FIG. 3, and when a voltage is applied, a polarization phase shift of about 40 ° can be obtained.

When the liquid crystal wave plate 71 shown in FIG. 5 is used, a voltage of -V ₀ is applied to the electrodes 44 and 48 and a voltage of + V ₀ is applied to the electrodes 45 and 49 from the drive circuit 26 when reproducing the land of the magneto-optical disk 6. To play. No voltage is applied to the electrodes 81-84. At this time, a polarization phase difference of 40 ° is given to the reproduction light beam by the liquid crystal wave plate 71. On the other hand, when reproducing the groove portion, the drive circuit 26 applies V
₀ , and a voltage of −V ₀ is applied to the electrodes 82 and 84. Electrode 4
No voltage is applied to the electrodes 4, 48 and the electrodes 45, 49. At this time, the liquid crystal wave plate 71 gives a polarization phase difference of −40 ° to the light beam for reproduction.

That is, as shown in FIG. 5, when reproducing the land portion, the optical anisotropy axis is in the direction of arrow A. On the other hand, when reproducing the groove portion, the optical anisotropy in the direction of arrow B is perpendicular. It becomes a sex axis. Therefore, since the optically anisotropic axis changes in a direction orthogonal to the optical axis, the polarization phase difference becomes −40 °. In this manner, by providing two pairs of electrodes perpendicular to the optical axis and perpendicular to each other on the pair of upper and lower glass substrates and controlling the voltage applied to the two pairs of electrodes, the land portion is reproduced exactly in the same manner as in FIG. In this case, a light beam for reproduction can be given a polarization phase difference of 40 °, and when reproducing a groove portion, a polarization phase difference of −40 ° can be given, so that the reproduction signal level can be maximized.

Here, the apparatus of the embodiment shown in FIG. 1 does not require any mechanical displacement and requires only a low driving voltage, so that it can be suitably used particularly for portable equipment. FIG. 6 is a block diagram in the case where the device of FIG. 1 is used in an electronic camera device as an example. In FIG. 6, first, a camera section 97 is constituted by a lens 91 and a CCD element 92. The light condensed by the lens 91 forms an image on the CCD element 92, and the signal of the CCD element 92 is processed by the signal processing circuit 94 using the memory 93. The controller 96 controls input and output of signals of the CCD element 92 and the signal processing circuit 94. The signal processed by the signal processing circuit 94 is sent to the magneto-optical disk device 95 under the control of the controller 96, and stored in the magneto-optical disk. The magneto-optical disk device 95 corresponds to the device shown in FIG. The image data stored in the magneto-optical disk device 95 is reproduced under the control of the controller 96,
The image is sent to the monitor 98 and the printer 99 to display and print an image.

By using the apparatus shown in FIG. 1 for an electronic camera apparatus as described above, it is possible to increase the capacity by land / groove recording, so that moving images can be recorded. Also,
As described above, since the optical head has no mechanical drive unit and can drive the polarization phase compensation element at a low voltage, it is suitable for a portable device such as an electronic camera device.

[0026]

As described above, according to the present invention, by controlling the voltage applied to the liquid crystal wavelength element when reproducing the land and the groove using the liquid crystal wavelength element,
Since the optical head can be driven at a low voltage without requiring any mechanical displacement, the size of the optical head can be reduced. Therefore, while the optical head can be downsized, the amplitude of the reproduced signal can be maximized, and crosstalk from an adjacent track can be minimized. Further, the size of the optical head can be further reduced by using a liquid crystal wavelength element that controls the polarization phase difference given to the reproducing light beam by changing the optically anisotropic axis in a direction perpendicular to the optical head.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of a polarization phase compensation element of the embodiment of FIG.

FIG. 3 is a diagram for explaining an operation of a liquid crystal wave plate of the polarization phase compensation element of FIG. 2;

FIG. 4 is a diagram illustrating a relationship between an applied voltage of a liquid crystal wave plate and retardation.

FIG. 5 is a plan view showing another example of the polarization phase compensation element.

FIG. 6 is a block diagram when the device of the embodiment of FIG. 1 is used in an electronic camera device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor laser 5 Objective lens 6 Magneto-optical disk 7 Magnetic head 8 Polarization phase compensation element 9 Wollaston prism 11 Reproduction sensor 21 Laser drive circuit 25 Optical head controller 31 Fixed polarization phase difference element 32, 71 Liquid crystal wave plate 41 Liquid crystal 42, 51 Glass substrate 44, 45, 48, 49 Electrode 46, 47 Spacer 61 Liquid crystal molecule 81-84 Electrode

Claims (3)

[Claims] 1. A method of irradiating a land and a groove of a magneto-optical recording medium with a light beam for reproduction and detecting a light beam for reproduction via the recording medium by an optical sensor, thereby recording information on the land and the groove. In the optical information reproducing apparatus for reproducing, a liquid crystal wavelength element for giving a polarization phase difference to the reproduction light flux is arranged in an optical path of the reproduction light flux through the recording medium, and a voltage is applied to the liquid crystal wavelength element. Means for applying the liquid crystal wavelength element to change the voltage applied to the liquid crystal wavelength element between when the land portion of the recording medium is reproduced and when the groove portion is reproduced, so that the polarization position given to the reproduction light beam by the liquid crystal wavelength element is changed. An optical information reproducing apparatus characterized in that a phase difference is switched between a land portion and a groove portion. 2. The method according to claim 1, wherein the liquid crystal wavelength element is combined with a fixed polarization phase difference element, and a polarization phase difference resulting from the combination of the liquid crystal wavelength element and the fixed polarization phase difference element is applied to the light beam for reproduction. An optical information reproducing apparatus according to claim 1. 3. The liquid crystal wavelength element changes a polarization phase difference given to a reproduction light beam between a land portion and a groove portion by changing directions of optically anisotropic axes to directions orthogonal to each other. The optical information reproducing device according to claim 1.