JP2001249315A - Optical device for correction of aberration - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-performance optical device for correction of aberrations which can correct aberration with high accuracy and to provide a pickup device. SOLUTION: The device has a liquid crystal device, which generates phase change for the transmitting light by applying a voltage, electrode layers facing other to apply a voltage on the liquid crystal device, and an insulating layer disposed between the liquid crystal device and the electrode layer. At least one electrode layer, of the electrode layers facing each other, consists of a plurality of divided electrodes electrically separated from one another by a gap in one plane. A part of the insulating layer, held between at least one electrode layer and the liquid crystal device, has sufficient layer thickness to produce a higher electric field strength than the specified strength in the part of the liquid crystal device facing the gap, when a prescribed voltage is applied on the liquid crystal element.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aberration correction optical element for correcting aberration when information is recorded or reproduced on an information recording medium such as an optical disk, and a pickup device provided with the aberration correction optical element. .

[0002]

2. Description of the Related Art CDs (Compact disks) and DVDs are used as information recording media for optically recording or reproducing information.
(Digital Video Disk or Digital Versatile Disk)
Optical disks such as a read-only optical disk, a write-once optical disk capable of additionally recording information,
Different types of optical disks, such as rewritable optical disks capable of erasing and re-recording information, have been developed.

[0003] Further, research and development of higher density optical discs and pickup devices and information recording / reproducing apparatuses corresponding to the higher densities have been promoted, and there is a so-called compatibility in which different types of optical discs can be used. Research and development of the pickup device and the information recording / reproducing device are also progressing. In order to cope with the high density of the optical disc, the numerical aperture (nu) of the objective lens provided in the pickup device is set.
It is considered that the optical disk is irradiated with a light beam having a small irradiation diameter by increasing the merical aperture (NA). In addition, it has been considered that the use of a short-wavelength light beam corresponds to high density.

However, if the numerical aperture NA of the objective lens is increased or a short-wavelength light beam is used, the influence of aberration on the light beam by the optical disk increases, and the accuracy of information recording and information reproduction can be improved. The problem that it becomes difficult arises. For example, when the numerical aperture NA of the objective lens is increased, the range of the incident angle of the light beam to the optical disk is widened, so that the distribution width of the amount of birefringence on the pupil plane of the optical disk, which is an amount dependent on the incident angle, is also increased. For this reason,
There is a problem that the influence of the spherical aberration caused by the birefringence increases. Also, if the numerical aperture NA of the objective lens is increased and a short-wavelength light beam is used, the optical disk tilts during information recording or information reproduction, and the incident angle (tilt angle) of the light beam with respect to the normal direction of the optical disk is reduced. When tilted, the influence of coma cannot be ignored. Further, in a pickup device provided with a plurality of light sources such as a two-wavelength laser,
Slight differences in the optical paths of these light beams cause astigmatism.

[0005] As described above, an optical disk is composed of a CD and a D.
Since the structure and the recording density are different depending on the type such as VD, the effects of aberrations such as spherical aberration, coma and astigmatism are different depending on the type of the optical disk. Equipment development becomes difficult. In order to reduce the influence of such aberrations, conventionally, a pickup device having a liquid crystal unit for aberration correction has been proposed. (JP-A-10-20263).

As shown schematically in FIG. 1, this liquid crystal unit has a liquid crystal element C between transparent electrodes A and B facing each other.
The orientation of the liquid crystal element C is changed by adjusting the applied voltage between the transparent electrodes A and B, and light incident on one transparent electrode A (or B) side is When passing through the inside, the light undergoes a change in birefringence according to the orientation state, and is emitted to the other transparent electrode B (or A) side.

Further, at least one of the transparent electrodes A and B is, for example, a plurality of transparent electrodes a1, a2, a3 and b1,
b2, b3, and the transparent electrodes a1, a
2 and a3 are electrically separated from each other and the transparent electrode b
1, b2 and b3 are also electrically isolated from each other. For this reason, when different voltages are respectively applied between the transparent electrodes facing each other, for example, between the transparent electrodes a1 and b1, between the transparent electrodes a2 and b2, and between the transparent electrodes a3 and b3,
The liquid crystal element C can be adjusted to a plurality of different alignment states, and simultaneously changes the birefringence according to the respective alignment states for incident light.

[0008] The liquid crystal unit is arranged in an optical path between a light source for emitting laser light and an objective lens. When the laser light is emitted from the light source, the liquid crystal unit gives the laser light a birefringence change corresponding to the plurality of alignment states described above and transmits the laser light to the objective lens side, and the objective lens converges the transmitted laser light. To generate a light beam to irradiate the optical disk. When the reflected light generated by the light beam irradiation on the optical disk enters the liquid crystal unit via the objective lens, the reflected light is transmitted by giving a birefringence change corresponding to the plurality of alignment states. The reflected light is detected by a photodetector. By properly adjusting the plurality of alignment states of the liquid crystal unit, the influence of aberrations such as spherical aberration and coma is reduced.

[0009]

However, the above-mentioned conventional liquid crystal unit has a problem that the plurality of transparent electrodes a1, a
In order to electrically separate 2, a3 and b1, b2, b3 from each other, a gap SP for electrical insulation is provided between the respective transparent electrodes as shown in FIG. That is, the gap SP is provided along the boundary between the transparent electrodes a1, a2, and a3, and the transparent electrodes b1,
A gap SP is provided along each boundary between b2 and b3.

Therefore, no voltage is applied to these gaps SP, and therefore, these gaps S in the liquid crystal element C are not applied.
Since no electric field was applied to the portion corresponding to P, sharp discontinuity occurred in the alignment in the liquid crystal element C. That is,
Aberration correction can be performed on a light beam or reflected light passing through the transparent electrodes a1, a2, a3 and b1, b2, b3, whereas aberration can be corrected on a light beam or reflected light passing through the gap SP. Correction cannot be performed, and therefore, there has been a problem that it is not possible to perform highly accurate aberration correction on the light beam or the reflected light as a whole.

SUMMARY OF THE INVENTION The present invention has been made to overcome such problems of the prior art, and an object of the present invention is to provide a high-performance aberration correction device capable of accurately correcting the effects of aberrations occurring in an optical path. A correction optical element and a pickup device are provided.

[0012]

An aberration correcting optical device according to the present invention is arranged in an optical path of an optical system for irradiating a recording medium with a light beam emitted from a light source and guiding a reflected light beam reflected by the recording medium. An optical system for correcting an aberration generated in an optical path, wherein the liquid crystal element causes a phase change with respect to light passing therethrough by applying a voltage, and the liquid crystal element faces each other for applying a voltage to the liquid crystal element. An electrode layer, an insulating layer disposed between the liquid crystal element and the electrode layer,
And at least one of the electrode layers facing each other
Is composed of a plurality of divided electrodes that are electrically separated from each other by gaps in the same plane, and a portion of the insulating layer sandwiched between at least one electrode layer and the liquid crystal element is a predetermined part of the liquid crystal element. It is characterized in that it has a layer thickness that generates an electric field larger than a predetermined intensity in a portion facing the gap of the liquid crystal element when a voltage is applied.

An optical pickup device according to the present invention is an optical pickup device including the above-described aberration correcting optical device, wherein the light source emits a light beam and the recording medium is irradiated with the light beam emitted from the light source. It is characterized by having an optical system for guiding a light beam reflected by the recording medium and a photodetector for detecting the reflected light beam.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described in detail with reference to the drawings. FIG. 2 is a diagram schematically illustrating a configuration of a pickup device provided in the information recording / reproducing device.
In FIG. 2, the present pickup device PU includes a laser beam H
1, a polarizing beam splitter 3, an aberration correcting optical element 4, an objective lens 5, a condensing lens 6, and a photodetector 7. These components 1 to 7 are arranged on the optical axis OA. Are arranged along. Further, a control circuit 8 for controlling the aberration correcting optical element 4 is provided in the present pickup device PU or the information recording / reproducing device.

The aberration correcting optical element 4 has an electro-optical element that generates an electro-optical effect by an electric field. More specifically, it has a liquid crystal optical element that changes the birefringence according to the magnitude of the control voltage Vi applied by the control circuit 8. That is, as shown schematically in FIG. 3, the optical element 4 has a structure in which a liquid crystal (liquid crystal) element 14 is sealed between insulating substrates 10 and 11 such as two transparent glass substrates. The electrode portions 12 and 13, insulating layers 23 and 24, and liquid crystal alignment films 21 and 22 are formed on opposing surfaces of the glass substrates 10 and 11 that oppose each other.

When a control voltage Vi is applied between the electrode portions 12 and 13, an electric field Ei generated by the control voltage Vi is generated.
Accordingly, the alignment of the liquid crystal molecules in the liquid crystal element 14 changes.
As a result, the phase of light passing through the liquid crystal element 14 changes due to birefringence of the liquid crystal element 14. That is, the polarization state (phase) is controlled by the control voltage Vi applied to the liquid crystal element 14.
Can be controlled by

Further, the aberration correcting optical element 4 has bidirectional optical transparency, so that either side of the insulating substrates 10 and 11 may be arranged to face the objective lens 5 side. I have. When the aberration correction optical element 4 is viewed from the optical axis OA side, as shown in the plan view of FIG. 4, a plurality of aberration correction areas AR1, AR2 to ARi determined in association with the distribution of aberrations generated in the optical disk 9. These aberration correction areas AR1, AR2 to ARi are divided into electrode sections 12,
13 is realized by a transparent electrode (ITO: indium tin oxide) layer. FIG. 4 shows a typical example of the aberration correction areas AR1, AR2 to ARi for correcting the spherical aberration caused by the optical disk 9. Actually, various types of aberration correction areas AR1, AR2 to ARi are provided in association with the aberration distribution of the optical disk 9. It is divided into shapes. For example, when correcting coma caused by tilting the optical disk 9 during information recording or information reproduction, aberration correction regions BR1 to BRi having the shape as shown in FIG. 5 are provided. Further, the number of sections of these aberration correction areas is also determined in association with the distribution of aberration of the optical disc 9.

An example of the aberration correcting optical element 4 provided with concentric aberration correcting areas AR1 to ARi as shown in FIG. 4 will be described below. FIG. 6 is a cross-sectional view showing a cross-sectional structure along line XX in FIG. As shown in the drawing, the electrode portion 12 has a structure including transparent electrode layers A1 to Ai formed electrically separated from each other and a plurality of gaps W1 to Wi existing between the transparent electrode layers A1 to Ai. Have.

The transparent electrode layer A1 has an aberration correction area AR
The transparent electrode layer A2 is formed in a shape (circular in the case of FIG. 4) corresponding to the aberration correction region AR2, and the remaining transparent electrode layer A2 is formed in a shape (circular in the case of FIG. 4). Similarly, the electrode layers A3 to Ai also include aberration correction areas AR3 to AR3.
It is formed in a shape conforming to Ri. The gaps W1 to Wi separating the transparent electrode layers A1 to Ai are formed in an annular shape.

On the other hand, the electrode portion 13 also has a structure including transparent electrode layers C1 to Ci formed electrically separated from each other and a plurality of gaps W1 to Wi between the transparent electrode layers C1 to Ci. Have. Note that if one electrode portion 12 is formed as a separated electrode layer, the electrode portion 13 does not need to be separated. For example, the electrodes may be formed over the entire surface, or may be formed in a required shape according to the characteristic of the aberration to be corrected, or may be formed separately in a required number.

Japanese Patent Laid-Open No. 10-289465 describes that spherical aberration and coma are corrected by one liquid crystal unit. That is, in the present invention, spherical aberration and coma are corrected by one liquid crystal unit by forming the upper electrode in an electrode shape for correcting spherical aberration and the lower electrode in an electrode shape for correcting coma. It is possible. At this time, the insulating layer between the upper electrode and the liquid crystal element and the insulating layer between the lower electrode electrode and the liquid crystal element only need to have a thickness capable of correcting each aberration.

With reference to FIG. 7, a description will be given of aberration correction when a voltage is applied to each transparent electrode. For convenience of explanation, FIG. 7 shows an element cross section in the radial direction from the center of the liquid crystal element. As schematically shown in this figure, each of the transparent electrodes (A1, C1) to (Ak, Ck) of the aberration correction regions AR1 to ARk, which are directly opposite each other, is controlled by the control voltage Vi from the control circuit 8. When different predetermined voltages V1 to Vk are applied to the
The electric fields (E1 to E1) caused by the applied voltages V1 to Vk
A plurality of alignment states according to k) occur. The applied voltages V1 to Vk are determined so that the alignment state of the liquid crystal element 14 in each of the aberration correction areas AR1 to ARk has a characteristic opposite to the characteristic of the aberration generated by the optical disc 9, that is, is corrected.

However, the gaps W1 to W1 between the transparent electrodes
Electric fields (EW1 to EW) generated in the liquid crystal element portion facing Wk
k) changes according to the thickness of the insulating layers 23 and 24. FIG.
Shows the electric field strength at the interface between the insulating layer and the liquid crystal element portion when the voltage is applied to each electrode (here, A1 to A6), using the layer thickness (THi) of the insulating layer as a parameter. In this figure, the layer thickness of the liquid crystal element is 5
Micrometer (μm), width of gaps W1 to Wk is 5 μm
It is assumed that three insulating layer thicknesses THi = 1, 6, 15 μm. Also, each electrode has A1 = 10
(V), A2 = 12 (V),..., A6 = 20 (V) and 2V
The case where the voltage is applied in the step of FIG.

As shown in FIG. 8, when the thickness THi of the insulating layer is small (THi = 1 μm), the electric field drops sharply in the liquid crystal element portion facing the gap. Therefore, in this portion, the orientation of the liquid crystal cannot be changed sufficiently. That is, a phase difference cannot be generated in the passing light, and it is difficult to correct the aberration. On the other hand, when the thickness of the insulating layer is THi = 6 μm, the decrease in the electric field is small, and when the thickness is THi = 15 μm, the electric field intensity profile becomes almost flat, and the aberration can be sufficiently corrected by the liquid crystal element even in the gap.

FIG. 9 is a view showing an example of the wavefront aberration (spherical aberration) caused by the optical disk 9. In FIG. Liquid crystal element 14
Are plotted in the radial direction with reference to the value at the center of the liquid crystal element 14 in the light incident plane (nm). That is, the light beam incident on the liquid crystal element 14 has a phase difference of the profile shown in FIG. 9 in the radial direction of the liquid crystal element 14, and a predetermined voltage is applied to the liquid crystal element 14 so as to cancel this phase difference. By causing an in-plane phase change in the incident light, the aberration can be corrected.

FIG. 10 shows a case where the insulating layer 23 sandwiched between the liquid crystal element 14 and the electrode layer 12 having a gap is smaller than the thickness of the liquid crystal element 14 (for example, the thickness 5 of the liquid crystal element 14).
When the thickness of the insulating layer 23 is about 1 μm or less with respect to μm, the phase difference (nm) generated in the light passing through the liquid crystal element 14 by applying the predetermined voltage to each of the electrodes (A1 to Ak, C1 to Ck). ). The phase difference is plotted on the basis of the value in the gap. In the aberration correction area (AR1 to ARk) of the liquid crystal element 14 corresponding to each electrode, a desired phase difference can be obtained by applying a voltage. On the other hand, in the region of the liquid crystal element 14 corresponding to the gaps W1 to Wk, the electric field intensity is small, so that there is almost no phase change. Therefore, a steep discontinuity in the phase difference occurs, which significantly hinders the correction of aberration.

FIG. 11 shows a phase difference generated in light passing through the liquid crystal element 14 by applying a predetermined voltage when the insulating layer 23 of the liquid crystal element 14 is equal to or more than the thickness of the liquid crystal element 14. As shown in the figure, the phase difference in the region of the liquid crystal element 14 corresponding to the gaps W1 to Wk is a value between the phase differences generated by two adjacent electrodes, and the aberration can be corrected well. Note that it is optimal to set the phase difference amount to match the phase difference curve shown in FIG.

FIG. 12 shows a case where the insulating layer 23 is smaller than the thickness of the liquid crystal element 14 (for example, the thickness of the liquid crystal element 14 is 5 μm).
m, the thickness of the insulating layer 23 is about 3 μm)
It shows a phase difference generated in light passing through the liquid crystal element 14 by applying a predetermined voltage. As shown in the figure, the phase difference in the gap region is larger than in the conventional case shown in FIG. That is, the phase difference is improved even in the gap region, and the aberration which has been a problem in the related art can be corrected. It should be noted that the value of the phase difference required in the gap region differs depending on various conditions and parameters such as the type of optical system used, the type of disc, and the control circuit. Therefore, the thickness of the insulating layer may be determined so that a desired phase difference is obtained according to these conditions. That is, the electric field applied to the portion of the liquid crystal element corresponding to the gap region may be determined so as to be equal to or higher than a predetermined intensity.

It is sufficient that at least one of the insulating layers sandwiched between the liquid crystal element 14 and the electrode layer having a gap is sufficiently thick, for example, thicker than the liquid crystal element. It is not necessary that both of the sandwiched insulating layers are formed thick. As described above, a sufficient phase change can be obtained even in the gap region, and it is possible to accurately correct the influence of aberration occurring in the optical path.

In the above embodiment, description has been made from the viewpoint of correcting aberration of reflected light generated by the optical disk.
The aberration correcting optical element according to the present invention may be provided at any position in the optical path from the light source to the light detector. Further, the aberration correcting optical element according to the present invention can be applied to correction of various aberrations such as astigmatism as well as spherical aberration and coma. Further, the present invention provides a plurality of light sources,
Alternatively, the present invention can be applied to a pickup device having a plurality of optical paths. For example, the present invention can be applied to a pickup device provided with a light source such as a two-wavelength laser having different wavelengths for recording and reproducing each of CD and DVD.

[0031]

As is apparent from the above, according to the present invention, it is possible to realize a high-performance aberration correcting optical element and a pickup device capable of accurately correcting aberrations occurring in an optical path.

[Brief description of the drawings]

FIG. 1 is a view schematically showing a structure of an aberration correction liquid crystal unit.

FIG. 2 is a block diagram schematically illustrating a configuration of a pickup device provided in the information recording / reproducing device.

FIG. 3 is a perspective view schematically showing a structure of an aberration correction liquid crystal unit and a change in alignment of liquid crystal molecules.

FIG. 4 is a plan view of an aberration correction optical element for correcting spherical aberration.

FIG. 5 is a plan view of an aberration correction optical element for correcting coma aberration.

6 is a cross-sectional view showing a structure of the aberration correction optical element shown in FIG. 4 along a line XX.

FIG. 7 is a cross-sectional view illustrating an electric field of a liquid crystal element in an aberration correction region and a gap region when a voltage is applied to each transparent electrode.

FIG. 8 shows that when a voltage is applied to each electrode, the electric field strength at the interface between the insulating layer and the liquid crystal element portion is determined by the thickness of the insulating layer (T
It is a figure which shows Hi) as a parameter.

FIG. 9 is a diagram illustrating an example of wavefront aberration (spherical aberration) generated in a light beam by an optical disc.

FIG. 10 is a diagram illustrating a phase difference (nm) generated in a light beam passing through a liquid crystal element when a predetermined voltage is applied to the liquid crystal element when the thickness of the insulating layer is smaller than the thickness of the liquid crystal element.

FIG. 11 is a diagram illustrating a phase difference (nm) generated in a light beam passing through a liquid crystal element when a predetermined voltage is applied to the liquid crystal element when the thickness of the insulating layer is equal to or greater than the thickness of the liquid crystal element.

FIG. 12 is a diagram illustrating a phase difference (nm) generated in a light beam passing through a liquid crystal element when the thickness of the insulating layer is smaller than the thickness of the liquid crystal element.

[Explanation of Signs of Main Parts]

 DESCRIPTION OF SYMBOLS 1 Light source 3 Polarization beam splitter 4 Aberration correction optical element 5 Objective lens 6 Condensing lens 7 Photodetector 8 Control circuit 10,11 Insulating substrate 12,13 Electrode part 14 Liquid crystal element 21,22 Liquid crystal aligning film 23,24 Insulating layer PU Pickup device

Claims (4)

[Claims] 1. An optical system for irradiating a recording medium with a light beam emitted from a light source and arranged in an optical path of an optical system for guiding a reflected light beam reflected by the recording medium, and correcting an aberration generated in the optical path. An aberration correction optical device, comprising: a liquid crystal element that causes a phase change with respect to light passing therethrough by applying a voltage; electrode layers facing each other for applying a voltage to the liquid crystal element; the liquid crystal element and the electrode An insulating layer disposed between the layers;
At least one electrode layer among the opposing electrode layers is composed of a plurality of divided electrodes that are electrically separated from each other by a gap in the same plane, and the at least one electrode of the insulating layer is A portion sandwiched between the layer and the liquid crystal element has a layer thickness that, when a predetermined voltage is applied to the liquid crystal element, generates an electric field larger than a predetermined intensity in a portion of the liquid crystal element facing the gap. Aberration correcting optical device. 2. The method according to claim 1, wherein the insulating layer has a layer thickness for causing a phase change of a magnitude between phase changes generated by two adjacent divided electrodes with respect to light passing through the gap. 2. The aberration correcting optical device according to claim 1, wherein 3. The aberration correction optical device according to claim 1, wherein the insulating layer has a layer thickness larger than a thickness of the liquid crystal element. 4. An optical pickup device including the aberration correction optical device according to claim 1, wherein a light source for emitting a light beam, and a light beam emitted from the light source are applied to a recording medium, and the recording medium is An optical pickup device comprising: an optical system that guides a reflected light beam; and a photodetector that detects the reflected light beam.