JPH05144073A - Signal recording and reproducing device for optical recording medium - Google Patents

Abstract

PURPOSE:To record information with a high density at the time of recording and to prevent a bad influence of phase rotation, crosstalk, or the like at the time of reproducing. CONSTITUTION:A variable phase plate 20 is arranged in a position between a semiconductor laser 11 and a magnetooptical disk 15, for example, between a collimator lens 12 and a beam splitter 13. The transparent electrode on one surface of an electrostrictive element plate 21 of the variable phase plate 20 is divided into a center electrode 23 and a peripheral electrode 24, and a potential difference is given between the center electrode 23 and the peripheral electrode 24 to generate a phase difference in the center part of a pupil by a control circuit 30 at the time of recording, and the control voltage which eliminates this potential difference to eliminate the phase difference is applied at the time of reproducing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal recording / reproducing apparatus for an optical recording medium, and more particularly to an optical recording medium for recording / reproducing a signal by irradiating an optical disc with a laser beam from a laser light source. The present invention relates to a signal recording / reproducing device.

[0002]

2. Description of the Related Art As an optical recording medium for recording and reproducing a signal while irradiating a laser beam from a laser light source, a magneto-optical recording medium and a phase change type optical recording medium are known. For example, a magneto-optical recording medium has a magnetic layer (for example, a rare earth element) having an excellent magneto-optical effect, which has an easy axis of magnetization in the direction perpendicular to the film surface on one main surface of a transparent substrate or a light transmissive substrate made of, for example, polycarbonate. -Transition metal alloy thin film) is laminated together with a dielectric layer, a surface protection layer and the like, and signals are recorded and reproduced by irradiating a laser beam or the like from the transparent substrate side.

It is desired to further increase the recording density of such an optical recording medium. This is because when a digital video signal is considered as a signal to be recorded, a data amount which is several times to ten and several times as large as that in the case of audio is required. This is because there is a demand to make the size of the recording medium smaller and further downsize products such as players. Also, when recording general computer data, notebook-sized or notebook-sized computer devices are becoming widespread, and there is an increasing demand for downsizing and large-capacity recording media.

The recording density of information on the optical recording medium as described above is determined by the wavelength λ of laser light and the numerical aperture NA of the objective lens.
In order to increase the density, it is necessary to shorten the wavelength λ of light and increase the numerical aperture NA.

[0005]

By the way, when considering an optical recording medium recording / reproducing system such as an optical disc system, for example, by giving a phase difference to a part of a light beam incident on a pupil of an objective lens (apposition), The shape of the image spot can be changed, and it is considered that high density recording is performed by utilizing the change in the shape of the image spot. This is to record a smaller pit by narrowing the main lobe of the image forming spot. However, during signal reproduction, it is desirable that the pupil function (amplitude distribution on the pupil) of the objective lens has a phase as uniform as possible, that is, no aberration. That is, at the time of reproduction, the spot shape having no aberration is the best from the viewpoint of the phase of the transfer function and the crosstalk.

The present invention has been made in view of such a situation, and can realize optimum states for recording and reproducing signals, improving recording frequency characteristics, and
It is an object of the present invention to provide a signal recording / reproducing apparatus for an optical recording medium that can also improve reproducing characteristics.

[0007]

A signal recording / reproducing apparatus for an optical recording medium according to the present invention is an optical recording for recording / reproducing a signal by irradiating a light beam from a light source onto the optical recording medium via an optical system. In a medium signal recording / reproducing apparatus, an electrostrictive element provided with a transparent electrode having a central portion and a peripheral portion (or an outer peripheral portion) inserted between the light source and the optical recording medium, and the electrostrictive element. A voltage is applied to the transparent electrode of the element (or piezoelectric element), a potential difference is generated between the central portion and the peripheral portion during recording to generate a phase difference in the central portion of the pupil of the objective lens, and during reproduction, the potential difference is 0 By having a control means for eliminating the above-mentioned phase difference as
The above problems are solved.

Here, as the electrostrictive element or piezoelectric element, for example, a so-called PLZT device can be used. The phase difference at the time of recording may be a phase difference that can obtain a light intensity distribution in which the main lobe can be narrowed in the range where the peak value of the side lobe does not exceed the recording threshold in the light intensity distribution characteristic of the imaging spot. Since the light intensity distribution characteristic of the image forming spot also changes depending on the radius of the center portion with respect to the pupil radius of the objective lens, the peak value of the side lobe and the thinness of the main lobe in the light intensity distribution characteristic of the image forming spot are as described above. It is determined by the phase difference and the radius of the central portion with respect to the radius of the objective lens pupil, and the phase difference and the radius of the central portion may be set so that the peak value of the side lobe does not exceed the recording threshold and the main lobe becomes thin.

[0009]

When recording, a phase difference is generated between the central portion and the peripheral portion of the electrostrictive element, so that the main lobe of the image forming spot becomes thin and minute dots and pits are recorded to realize high density recording. At the time of reproduction, by eliminating the phase difference between the central portion and the peripheral portion of the electrostrictive element, the pupil function of the objective lens is made into an aberration-free spot shape with a uniform phase, in terms of the phase around the transfer characteristic and crosstalk. Get the best characteristics.

[0010]

FIG. 1 is a diagram showing a schematic configuration of an optical disk recording / reproducing apparatus as an embodiment of a signal recording / reproducing apparatus for an optical recording medium according to the present invention. In FIG. 1, a laser light source such as a semiconductor laser 11 is used as a light source. The laser light emitted from this semiconductor laser 11 is
The collimator lens 12 collimates the beam, and the beam splitter 13 is used to irradiate the objective lens 14 to a recordable optical recording medium such as a magneto-optical disk 15. The laser beam applied to the magneto-optical disk 15 is reflected by the recording layer of the magneto-optical disk 15, enters the beam splitter 13 through the objective lens 14, is reflected, and is condensed by the condenser lens 16. The light is incident on the photodetector 17 such as a photodiode.

Here, the semiconductor laser 11 which is the above-mentioned light source.
The variable phase plate 20 is arranged between the optical disk 15 and the magneto-optical disk 15 which is an optical recording medium. As shown in FIG. 2, the variable phase plate 20 has a transparent electrode (entire electrode) 22 on one entire surface of a transparent electrostrictive element plate 21 such as a so-called PLZT.
And transparent electrodes 23 and 24 divided into a central portion and a peripheral portion are provided on the other surface. The so-called PLZT is a zircon / lead titanate-based ceramic,
That is, a so-called PZT, which is a solid solution of PbZrO ₃ and PbTiO ₃ , to which La (lanthanum) is added, is a transparent ceramic electrostrictive element. When electrodes are formed on both surfaces of the transparent electrostrictive element plate 21 and a voltage is applied, the thickness of the electrostrictive element plate 21 changes.

FIG. 3 shows the so-called PLZT described above.
FIG. 4 is a characteristic diagram showing a change in thickness with respect to a voltage applied between the double-sided electrodes of the device, and as is apparent from FIG. 3, an electrostrictive effect of changing a thickness represented by a quadratic function with respect to an applied voltage is obtained. Has been. Therefore, the voltage applied to the central transparent electrode (central electrode) 23 of the electrostrictive element plate 21 of the variable phase plate 20 and the voltage applied to the peripheral transparent electrode (peripheral electrode) 24 are made to have a difference (potential difference). As a result, it is possible to give a difference in plate thickness at each of these portions, and it is possible to give a phase difference optically.

In the example of FIG. 1, the entire surface electrode 22 and the peripheral electrode 2
4 is connected and grounded via a terminal 31 to have a ground potential (GND), and a predetermined voltage V is applied to only the center electrode 23 from the control circuit 30 via the terminal 32. The control voltage V from the control circuit 30 causes the above-mentioned optical phase difference as a non-zero predetermined value at the time of recording a signal, and sets it to 0 at the time of reproduction to make a difference in plate thickness from the peripheral portion (that is, an optical phase difference). Is lost.

Now, when the phase difference between the central portion (the portion of the central electrode 23) and the peripheral portion (the portion of the peripheral electrode 24) of the variable phase plate 20 is φ, the pupil function f (r) is Is normalized to 1 and expressed as a function of the radius r as follows. f (r) = 1 (0 <r <a) = exp (-iφ) (a <r <1) (1) where a in the formula (1) is the pupil radius set to 1 It is the radius of the center part when normalized, and when the pupil radius is r ₁ and the radius of the center electrode 23 is r _a as in the example of FIG.
Therefore, r ₁ : r _a = 1: a. Generally, changing the phase or the amplitude of a part of the light beam incident on the pupil of the objective lens is called “positioning”, and the shape of the image-forming spot can be changed by this positioning. The light intensity distribution I (v) of the imaging spot can be expressed by the Fourier transform of the pupil function f (r), and when calculated from the equation (1), it becomes the following equation (2).

[0015]

[Equation 1]

When there is no phase difference (φ = 0),
It is only the first term of the equation (2), which is well known as the Airy equation. When there is a phase difference (φ ≠ 0),
The light intensity distribution of the imaging spot changes variously depending on the phase difference φ and the radius a of the central portion. In FIG. 4, the radius a of the central portion when the pupil radius is normalized is 0, 0.35, 0.43,
The change of the light intensity distribution when changing to 0.53 and 0.57 is shown, and the phase difference φ is fixed at π (= 180 °) at this time. On the other hand, FIG. 5 shows changes in the light intensity distribution when the phase difference φ is changed to 0, 0.45π, 0.63π, 0.77π, 0.89π, π, and the radius a is 0.57.
It is fixed at. The vertical axis of these figures represents the light intensity obtained by normalizing the maximum intensity to 1, and the horizontal axis represents the coordinate axis on the focal plane in units of λ / NA.

As is apparent from FIGS. 4 and 5, the central main lobe on the focal plane can be made thin by selecting a large radius a and a large phase difference φ.
It can be seen that smaller marks or pits can be recorded and high density recording is possible. However, if the radius a is increased, the phase difference φ is increased, and the main lobe is thinned, the side lobes in the periphery become high at the same time. Therefore, it is necessary to prevent the side lobe from recording at the time of recording. It is said that.

That is, in the recording process of a recordable optical recording medium, for example, a magneto-optical recording medium, a portion where the light intensity is equal to or higher than a certain threshold becomes the Curie temperature or higher, and the magnetization is reversed at this portion to perform recording. .. If such a threshold value is smaller than the peak value of the side lobe, recording by this side lobe is not performed. Therefore, at the time of recording, the main lobe can be thinned within a range in which the peak value of the side lobe does not exceed the threshold value. Therefore, the radius a and the phase difference φ need to be set in advance so that the main lobe can be thinned within a range in which the peak value of the side lobe does not exceed the threshold value at the time of recording. As a specific example, the value a of the center radius with respect to the pupil radius is 0.
The radius r _a of the center electrode 23 is set so as to be about 5 and the control circuit 3 is set so that the phase difference φ becomes approximately π.
It is conceivable to set the control voltage V by 0. In addition to this, it goes without saying that it may be appropriately set according to the above-mentioned threshold value for recording on the optical recording medium.

On the other hand, at the time of reproduction, as described above, the control voltage V from the control circuit 30 is set to 0 (the same ground level as the voltage applied to the peripheral electrode 24), and the central part and the peripheral part of the electrostrictive element plate 21 are connected. The difference in plate thickness is eliminated and the phase difference φ is set to 0. This is because when reproducing, there is no distinction by the threshold value as in the above recording, and the reflected light from a portion with low light intensity is also detected according to the light intensity, and the side lobe as described above increases (pupil). This is in consideration of the fact that the adverse effect of causing waveform distortion in a signal read from a recording medium such as a disk due to the occurrence of wavefront aberration) becomes larger.

Therefore, the variable phase plate 20 such as the so-called PLZT device described above is controlled by the control circuit 30, and at the time of recording, a difference in plate thickness is provided between the central portion and the peripheral portion so that the phase difference φ is a non-zero predetermined value. When reproducing, by eliminating the difference in the plate thickness and setting the phase difference φ to 0, the main lobe is thinned during recording to enhance the recording frequency characteristic, and during reproduction, it is optimal in terms of the phase rotation and crosstalk of the transfer characteristic. A spot shape without aberration can be realized. In a magneto-optical disk or the like, switching between recording and reproduction needs to be performed in several tens of microseconds, but the so-called PLZT or other electrostrictive element plate 21 is used.
The variable phase plate 20 using is generally responsive to a response speed of several tens of kHz, so that it can be sufficiently dealt with.

More preferably, at the time of erasing a recordable optical recording medium such as a magneto-optical disk, the control voltage V applied from the control circuit 30 to the central electrode 23 is applied to the peripheral electrode 2.
It is preferable that the same ground level as that of the voltage applied to 4 is set to 0 to eliminate the difference in plate thickness between the central portion and the peripheral portion of the electrostrictive element plate 21 and set the phase difference φ to 0.

The present invention is not limited to the above embodiment, and for example, the variable phase plate 20 using the electrostrictive element plate 21 is arranged at the semiconductor laser 11 which is a light source.
To the magneto-optical disk 15 which is an optical recording medium may be located at any position, and specific examples include the space between the beam splitter 13 and the objective lens 14, the space between the objective lens 14 and the magneto-optical disk 15, and the like. Be done. As the recordable optical recording medium, not only the magneto-optical disk 15 but also
It may be a phase change type optical disc or the like, and may be a card or the like other than the disc. Further, various control voltages from the control circuit 30 can be considered. For example, a predetermined voltage V ₀ is applied between the entire surface electrode 22 and the peripheral electrode 24,
It is also possible to apply V ₀ + ΔV to the center electrode 23 to give a potential difference of ΔV, and control the potential difference ΔV to be a non-zero predetermined value at the time of recording and 0 at the time of reproducing. Furthermore, the electrostrictive element plate 21 of the variable phase plate 20 is a so-called PL.
In addition to ZT, a transparent material having an electrostrictive effect (or piezoelectric effect) can be used, and the shape of the transparent electrode 23 is not limited to a circular shape, and may be, for example, a rectangular shape or a polygonal shape. The whole surface electrode 22 may be divided into a central portion and a peripheral portion.

[0023]

As is apparent from the above description, according to the signal recording / reproducing apparatus for the optical recording medium of the present invention, the transparent structure having the central portion and the peripheral portion is provided between the light source and the optical recording medium. An electrostrictive element provided with an electrode is inserted and arranged, and a potential difference is generated between the central portion and the peripheral portion at the time of recording to generate a phase difference at the central portion of the pupil of the objective lens. Since the control to eliminate the phase difference is performed, the main lobe of the image forming spot becomes thin during recording, and minute dots and pits are recorded to achieve high-density recording.
At the time of reproduction, it becomes possible to obtain the best characteristics in terms of the phase shift of the transfer characteristics and crosstalk by making the pupil function of the objective lens a spot shape with uniform phase and no aberration.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a schematic configuration of an embodiment of a signal recording / reproducing apparatus for an optical recording medium according to the present invention.

FIG. 2 is a partially cutaway perspective view of a variable phase plate used in the embodiment.

FIG. 3 is an applied voltage of a variable phase plate used in the embodiment.
It is a characteristic view which shows the characteristic of thickness change.

FIG. 4 is a characteristic diagram showing a change in the light intensity distribution of the imaging spot when the value a of the center radius with respect to the pupil radius of the variable phase plate used in the example is changed.

FIG. 5 is a characteristic diagram showing a change in the light intensity distribution of the imaging spot when the phase difference φ between the central portion and the peripheral portion of the variable phase plate used in the example is changed.

[Explanation of symbols]

 11 ... Semiconductor laser (laser light source) 12 ... Collimator lens 13 ... Beam splitter 14 ... Objective lens 15 ... Magneto-optical disk (optical recording medium) 20 ... Variable phase plate 21 ... Electrostrictive element (piezoelectric element) 22 ... Full surface electrode 23 ... Center electrode (transparent electrode at the center) 24 ...・ Peripheral electrode (transparent electrode in the peripheral part) 30 ... Control circuit

Claims (1)

[Claims] 1. A signal recording / reproducing apparatus for an optical recording medium, which records and reproduces a signal by irradiating an optical recording medium with a light beam from a light source through an optical system, wherein a signal is recorded between the light source and the optical recording medium. An electrostrictive element provided with a transparent electrode having a central portion and a peripheral portion that are inserted and arranged, and a voltage is applied to the transparent electrode of the electrostrictive element, and a potential difference is generated between the central portion and the peripheral portion during recording. A signal recording / reproducing apparatus for an optical recording medium, comprising: a control means for generating a phase difference in the center of the pupil of the objective lens and controlling the potential difference to be 0 during reproduction so as to eliminate the phase difference.