JP3965843B2 - Optical head device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical head device for writing / reading information on / from an optical information recording medium such as an optical disk.
[0002]
[Prior art]
A DVD, which is an optical disk, records digital information at a higher density than a CD, which is also an optical disk, and an optical head device for reproducing a DVD has a light source (semiconductor laser) wavelength of 650 nm, which is shorter than the 780 nm of the CD. Alternatively, the beam spot diameter of the laser beam is reduced to about half of the CD by setting it to 635 nm, or setting the numerical aperture (NA) of the objective lens to 0.6 which is larger than 0.45 of CD.
[0003]
Furthermore, in the next generation optical recording, a higher recording density can be obtained by setting the wavelength of the light source to about 400 nm and the NA to 0.6 or more. However, due to the shorter wavelength of the light source and higher NA of the objective lens, the optical disc tilt tilt tolerance (tilt margin) and the optical disc thickness variation tolerance (thickness variation margin) where the optical disc surface tilts from the right angle with respect to the optical axis. Becomes smaller. These margins are reduced because coma aberration occurs in the case of optical disc tilt, and spherical aberration occurs in the case of unevenness in the thickness of the optical disc. This is because it becomes difficult.
[0004]
In high-density recording, several methods have been proposed in order to increase the allowable amount of the optical head device with respect to optical disc tilt and optical disc thickness unevenness.
One is a method of adding a movable axis so that the objective lens is tilted according to the detected tilt angle with respect to the objective lens actuator that is normally driven by two axes. However, this additional method has problems that spherical aberration cannot be corrected and that the structure of the actuator is complicated.
[0005]
The other is a method in which wavefront aberration is corrected by a phase correction element provided between the objective lens and the light source. With this correction method, it is possible to widen the tilt margin and the uneven thickness margin only by incorporating the element into the optical head device without significantly modifying the actuator. An example of this correction method is Japanese Patent Laid-Open No. 10-20263 for correcting optical disc tilt. This is by applying a voltage according to the optical disc tilt angle to the partial electrode formed by dividing the electrode of the phase correction element to change the effective refractive index of the birefringent material such as liquid crystal inside the phase correction element, The coma aberration due to the optical disc tilt is corrected by the generated phase shift distribution.
[0006]
[Problems to be solved by the invention]
However, in the conventional phase correction element using liquid crystal, the response speed of the liquid crystal is slower than the access speed of the optical head device, the rotation speed of the optical disk, etc. Was limited. In particular, the magnitude of wavefront aberration caused by changes in optical disk thickness unevenness and optical disk tilt angle in the tangential direction (optical disk rotation direction) varies depending on the rotation speed of the optical disk, and thus cannot be corrected by a conventional phase correction element.
[0007]
[Means for Solving the Problems]
The present invention has been made to solve the above-described problem, and includes a light source, an objective lens for condensing light emitted from the light source on an optical recording medium, and an optical path between the light source and the objective lens. A phase correction element that is installed in the optical recording medium to correct the wavefront aberration on the optical recording medium by changing the phase distribution of the wavefront of the outgoing light, and a control voltage generator that supplies the phase correction element with a voltage for changing the phase distribution A pair of transparent substrates each having an electrode formed thereon, so that the phase correction element can correct wavefront aberration generated by the optical recording medium warped in a saddle shape, And a bend-aligned liquid crystal layer sandwiched between the substrates, and the applied voltage of the phase correction element changes so that the response speed is about four times the rotational speed of the optical recording medium rotating. Control voltage generator That have been made to provide an optical head apparatus according to claim.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Phase correcting element provided in the optical head apparatus of the present invention, as can be corrected wavefront aberration optical recording medium warped saddle occurs, a pair of transparent substrates on which electrodes are formed respectively, a pair of substrates And a bend-aligned liquid crystal sandwiched therebetween. This bend-aligned liquid crystal is optimal for correcting wavefront aberrations that change at a relatively high speed because its response speed is an order of magnitude faster than that of normal nematic alignment.
[0009]
The optical head device of the present invention includes a phase correction element that corrects wavefront aberration (mainly coma aberration) generated when reproducing an optical disc that is an optical recording medium warped in a saddle shape as shown in FIG. . In the optical disk as shown in FIG. 2, the optical disk tilt angle caused by warpage changes at a cycle twice the rotation speed of the optical disk. Therefore, at least the rotation speed of the optical disk is required to correct wavefront aberration following the rotation of the optical disk. 4 times the response speed is required.
[0010]
The optical head device shown in FIG. 1 is for reproducing information recorded on an optical disk 8 such as a CD or a DVD. Light emitted from a semiconductor laser 1 as a light source is, for example, a hologram type polarization beam splitter 2. Then, the collimating lens 3 converts the light into a parallel light beam, and after passing through the phase correction element 4 and the quarter-wave plate 5 having the bend-aligned liquid crystal according to the present invention, the objective lens 6 installed in the actuator 7 is used. Thus, the light is condensed on the optical disk 8. The condensed light is reflected by the optical disk 8 and sequentially passes through the objective lens 6, the quarter-wave plate 5, the phase correction element 4, and the collimating lens 3 in the reverse order, and then diffracted by the polarizing beam splitter 2. The light enters the detector 9. A phase correction element including a bend-aligned liquid crystal according to the present invention will be described later.
[0011]
When light emitted from the semiconductor laser over THE 1 is reflected by the optical disc 8, it is amplitude modulated by the information recorded on the optical disc 8, the recording information is read as an optical intensity signal by the photodetector 9. The polarization beam splitter 2 includes, for example, a polarization hologram, and strongly diffracts light having a polarization component in the direction of optical anisotropy of the polarization hologram and guides this diffracted light to the photodetector 9.
[0012]
The phase correction element 4 is driven by applying a voltage by a phase correction element control circuit 10 which is a control voltage generator so that the reproduction signal from the optical disk 8 obtained from the photodetector 9 is the best. The voltage output from the phase correction element control circuit 10 corresponds to the tilt amount of the optical disk 8 and the shift amount of the objective lens, and is an effective voltage applied to the phase correction element 4.
[0013]
Next, the configuration of the phase correction element in the present invention will be described with reference to the drawings. In the phase correction element shown in FIG. 3, glass substrates 21a and 21b using, for example, non-alkali glass having a thickness of 0.5 mm are bonded by a sealing material 22 containing an epoxy compound as a main component to form a liquid crystal cell. . The sealing material 22 contains, for example, a glass spacer and a conductive spacer having a resin surface coated with gold, for example.
[0014]
On the inner surface of the glass substrate 21a constituting the liquid crystal cell, an electrode 24a, an insulating film 25a mainly composed of silica, and an alignment film 26a are sequentially coated from the glass substrate side. Similarly, an electrode 24b, an insulating film 25b mainly composed of silica, and an alignment film 26b are coated in this order on the inner surface of the glass substrate 21b constituting the liquid crystal cell. Further, an antireflection film may be coated on the outer surface of the liquid crystal cell.
Here, the electrodes 24a and 24b are, for example, ITO films, and the alignment films 26a and 26b are formed by, for example, applying polyimide on a glass substrate by a spin coat method and rubbing a polyimide film formed by baking.
[0015]
The electrode 24 a is pattern-wired so that it can be connected to the phase correction element control circuit 10 at the electrode lead-out portion 27. The electrode 24b is in electrical contact with the electrode formed on the glass substrate 21a through the conductive spacer included in the sealing material 22, and the electrode 24b is phase-corrected by the electrode lead-out portion 27 in the same manner as the electrode 24a. It can be connected to the element control circuit 10. The liquid crystal cell is filled with liquid crystal 23, and for example, a non-twisted nematic liquid crystal having a refractive index anisotropy (difference between ordinary light refractive index and extraordinary light refractive index) of 0.15 is used. The interval (liquid crystal cell interval) between the glass substrate 21a and the glass substrate 21b is usually about 6 μm.
[0016]
The alignment films 26a and 26b are rubbed from the left to the right in FIG. 3, and the liquid crystal molecules 28 are splayed as shown in FIG. 3 when no voltage is applied. When actually correcting the wavefront aberration by operating the phase correction element, a bend alignment with a fast response speed is used. In order to transfer the splay alignment to the bend alignment, it is necessary to apply a high voltage for a certain period of time.
[0017]
For example, by applying 10 V to the splay alignment liquid crystal for 10 seconds to confirm that all the liquid crystal regions have transitioned to the bend alignment, the voltage is reduced to, for example, 2.5 V that is equal to or higher than the bias voltage V _c . In this example, the actual bias voltage is 2.3 V, but is stopped at 2.5 V, which is slightly higher than this voltage in order to maintain the bend alignment.
[0018]
Here, the bias voltage V _c is the transition voltage of the alignment state at the bias voltage below are stable state splay alignment, the bend alignment is a stable state in the bias voltage or more. The bias voltage has a different value depending on conditions such as the liquid crystal material used, the alignment film material, and the pretilt angle.
In the present invention, the bend alignment is realized by applying a voltage. However, the bend alignment may be realized by performing an alignment process by rubbing on one substrate and applying a vertical alignment film material to the other substrate.
[0019]
Next, the principle of correcting the wavefront aberration caused by the tilt of the optical disc will be described. When the voltage applied to the phase correction element is increased, the liquid crystal molecules are aligned in the direction of the electric field. Therefore, the alignment direction component of the liquid crystal molecules in the light incident on the phase correction element, that is, the light component polarized in the alignment direction of the alignment film is effective. The refractive index changes with the applied voltage and approaches the ordinary refractive index of the liquid crystal. Therefore, the phase of the light of this component changes with the voltage value.
[0020]
On the other hand, the phase of the component light in the direction orthogonal to the alignment direction of the alignment film does not change because it always senses the ordinary refractive index regardless of the applied voltage value. Therefore, it is preferable that the light incident on the phase correction element is linearly polarized light that is polarized in the alignment direction of the alignment film because the phase change due to the change in applied voltage can be maximized.
[0021]
The state (a) in FIG. 4 is splay alignment, and the states (b) and (c) are bend alignments. The phase correction element in the present invention utilizes the effective refractive index change δn based on voltage application in the bend alignment state shown in FIGS. Here, when d is a liquid crystal cell interval and V is an applied voltage, the phase shift of the light generated by the phase correction element is δn (V) · d. As shown in FIG. 5, with respect to δn (2.5) = 0, the phase shift amount decreases as the applied voltage increases.
[0022]
On the other hand, the response speed of the liquid crystal in the bend alignment is about an order of magnitude faster than that of the normal nematic alignment, which is due to the extremely short liquid crystal rearrangement time during driving. The response speed of the phase correction element in the present invention is 20 msec or less for both the rise time and the fall time at each voltage between 2.5 V and 6 V, and it was found that the response speed can sufficiently follow the rotation of the optical disk. .
[0023]
Here, the rise time is defined as the time required to reach a phase difference 10 nm smaller than the finally reached phase difference when the 0.1 V voltage is increased starting from an arbitrary voltage within the voltage interval. On the other hand, the fall time was the time required to reach a phase difference 10 nm larger than the finally reached phase difference when the 0.1 V voltage was decreased.
[0024]
In the present invention, the phase correction element has a function of correcting, for example, the wavefront aberration (coma aberration) shown in FIG. 6 caused by the tilt of the optical disk in the radial direction (radial direction of the optical disk). A number in the figure, for example, 100 nm to 200 nm represents the magnitude of the wavefront aberration, and positive means a wavefront, that is, a state in which the phase is advanced, and negative means a state in which the phase is delayed.
[0025]
The electrode pattern of the phase correction element for correcting this phase shift is shown in FIG. 7, and the electrode 24a in FIG. 3 is divided into five partial electrodes 31 to 35 using a photolithography technique. The partial electrodes 31 to 35 are formed of an ITO film, and the electrode interval corresponding to the solid line in the figure is set to a different voltage because the ITO film is removed by an etching method.
[0026]
From the dependence of the phase shift amount on the applied voltage in FIG. 5, if an appropriate voltage is applied to each of the partial electrodes 31 to 35, a phase shift distribution that cancels the wavefront aberration in FIG. 6 is generated for each liquid crystal region of each partial electrode. Can be made. When an optical head device incorporating the phase correction element having the electrode pattern of FIG. 7 is used to reproduce a warped DVD as shown in FIG. 2, a better reproduction signal can be obtained compared to the case without the phase correction element. It was.
[0027]
Also, the optical head device of the present invention dynamically corrects the wavefront aberration caused by the disc tilt amount and disc thickness unevenness distributed in the rotation direction of the optical disc by controlling the phase correction element following the disc rotation. It is characterized by. That is, the phase correction element control circuit 10 changes the voltage with respect to the phase correction element at a speed that is an integral multiple (m times) of the disk rotation frequency f. Here, m = 4.
[0028]
When the optical disk is warped as shown in FIG. 2, by setting m = 4, it is possible to cope with a change in the disk tilt amount accompanying the rotation of the optical disk. The frequency band required for the phase correction element control circuit 10 is represented by the product of the disk rotation frequency f and the m, and usually only about 1 KHz. Therefore, the control voltage generator as in synchronization with the rotation period of the optical recording medium is the applied voltage of the phase correcting element changes is configured, Ru can respond to changes in the disc tilt amount due to the rotation of the optical disk.
[0029]
In the above, the method for correcting the coma aberration generated by the optical disc tilt in the radial direction has been described. For reference, by changing the electrode pattern according to the type of wavefront aberration to be corrected, The spherical aberration caused by the uneven thickness of the optical disk can be corrected.
[0030]
An example of an electrode pattern for correcting coma aberration is the one shown in FIG. For reference, in the case of spherical aberration, an electrode having a shape in which a planar electrode is divided by concentric lines is preferable. In the case of astigmatism, a flat electrode is preferably divided by a radial straight line passing through one point.
[0031]
【The invention's effect】
As described above, in the optical head device of the present invention, the phase shift distribution of the transmitted light can be changed at high speed by using the liquid crystal in the bend alignment state for the phase correction element, and the wavefront changing at the rotational speed of the optical disc. Aberrations, especially coma, can be corrected at any time.
[Brief description of the drawings]
FIG. 1 is a principle configuration diagram of an optical head device of the present invention.
FIG. 2 is a schematic diagram showing a warped shape of an optical disc.
FIG. 3 is a schematic cross-sectional view showing an example of a phase correction element in the present invention.
FIG. 4 is a schematic diagram showing the alignment state of liquid crystal molecules depending on the magnitude of an applied voltage, (a) splay alignment, (b) bend alignment, and (c) bend alignment.
FIG. 5 is a graph showing an example of the relationship between the phase shift amount of the phase correction element and the applied voltage in the present invention.
FIG. 6 is a conceptual diagram showing a coma aberration distribution when an optical disc tilt occurs.
FIG. 7 is a schematic diagram of an electrode pattern of a phase correction element according to the present invention for correcting coma.
[Explanation of symbols]
1: Semiconductor laser 2: Polarizing beam splitter 3: Collimating lens 4: Phase correction element 5: Quarter wavelength plate 6: Objective lens 7: Actuator 8: Optical disk 9: Photo detector 10: Phase correction element control circuit 21a, 21b: Glass substrate 22: Sealing material 23: Liquid crystal 24a, 24b: Electrode 25a, 25b: Insulating film 26a, 26b: Alignment film 27: Electrode extraction part 28: Liquid crystal molecules 31-35: Partial electrode

Claims (2)

Optical recording with a light source, an objective lens for condensing the light emitted from the light source on the optical recording medium, and an optical path between the light source and the objective lens to change the phase distribution of the wavefront of the emitted light a phase correction element for correcting the wavefront aberration on the medium, an optical head apparatus and a control voltage generator for supplying a voltage for changing the phase distribution in the phase correction device, the phase correcting element saddle as wavefront aberration can be corrected for optical recording medium warped mold occurs comprises a pair of transparent substrates on which electrodes are formed respectively, and a liquid crystal layer is sandwiched bend alignment between the substrates, wherein An optical head device characterized in that the control voltage generator is configured such that the applied voltage of the phase correction element changes at a response speed of about four times the rotational speed at which the optical recording medium rotates . The alignment direction of an alignment film formed between the electrode and the liquid crystal layer of at least one of the pair of substrates matches the direction of linearly polarized light incident on the phase correction element. 2. An optical head device according to 1 .

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