JPH09197363A - Optical modulator and optical head device - Google Patents

Abstract

(57) An optical modulator and an optical head device having a dual focus lens effect, which has high light utilization efficiency and can be manufactured at low cost. SOLUTION: A solid electrode is formed of an ITO film on the surfaces of two glass substrates, and a polyimide film is formed on the solid electrode.
The mask rubbing method is used to perform rubbing treatment so that a periodic distribution of orientation direction angles of 0 °, 45 °, and 90 ° is obtained from the central region of the glass substrate toward the peripheral region, and two glass substrates are stacked to form a liquid crystal. Injected in vacuum.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is for writing or reading optical information on an optical recording medium such as an optical disk such as a CD (compact disk), a CD-ROM, a video disk and a magneto-optical disk. And an optical modulator suitable for the optical head device.

[0002]

2. Description of the Related Art Conventionally, in an optical head device for writing optical information on an optical disc or for reading the optical information, like an CD, a CD-ROM and a DVD disc,
It is often necessary to read and write signals with respect to optical disks having different thicknesses with one optical head device.

In order to realize an optical head device for such a purpose, it has been conventionally known that a Fresnel lens type blazed hologram is formed on the surface of a lens. In this case, about half of the light incident on the lens from the semiconductor laser is diffracted by the blaze hologram in the direction in which the light beam spreads, the other half is transmitted as it is, and then the lens body is used to converge each of the two focal points. Is generated by one optical head device.

It has also been attempted to form the lens in the same shape as the conventional one and separately install a Fresnel hologram lens plate having the same function as described above.

However, these methods have a major drawback that the amount of light is halved by passing through the hologram once. Therefore, the amount of light is about half in each of the going direction (direction from the light source side to the optical recording medium side) and the returning direction (direction from the optical recording medium side to the light source side), so that the amount of light is 1 / reciprocal. There is a problem of becoming 4. Therefore, in the case of an optical head device using a red semiconductor laser in which it is particularly difficult to obtain a large output, the load on the light source increases, resulting in an increase in cost and a decrease in reliability.

[0006]

SUMMARY OF THE INVENTION The present invention provides a light modulating element having a double focus lens effect which can solve the above-mentioned problems and improve the light utilization efficiency and can be manufactured at low cost, and an optical head device using the same. The purpose is to provide.

[0007]

The present invention has a plurality of regions in which liquid crystals are concentrically distributed and arranged, and the alignment state of liquid crystals in each region changes periodically from the central region to the peripheral region. Further, there is provided a light modulation element comprising a liquid crystal cell in which the cycle of the change is changed from the center toward the periphery.

A preferred aspect of the present invention is that the period of change in the alignment state of the liquid crystal becomes smaller from the central region to the peripheral region.

Another preferred embodiment of the present invention is that the alignment state is asymmetric in the direction from the central region to the peripheral region within one cycle of the change in the alignment state of the liquid crystal.

Another preferred embodiment of the present invention is that the alignment state of the liquid crystal is controlled by the lattice-shaped concave portions formed on the surface of the transparent substrate.

In another preferred embodiment of the present invention, the liquid crystal cell includes two transparent substrates which are substantially parallel to each other and a liquid crystal which is sandwiched between the transparent substrates, and the alignment direction of the liquid crystal is substantially parallel to the two transparent substrates. That is, the alignment direction angle of the liquid crystal in a plane parallel to the substrate periodically changes from the central region to the peripheral region.

Another preferred embodiment of the present invention is that within one cycle of the change of the alignment direction angle of the liquid crystal, there are two regions having different alignment direction angles of 90 °.

Another preferred embodiment of the present invention is that electrodes are formed on the liquid crystal side surfaces of the two transparent substrates of the liquid crystal cell.

Further, according to the present invention, in the optical head device for reading and / or writing information by irradiating the optical recording medium with the light from the light source through the diffractive element, the diffractive element is the above light modulating element. Provided is an optical head device.

According to the present invention, a hologram plate which is separated from an aspherical lens (objective lens) and is made of liquid crystal is provided, and two focal points can be switched by applying a voltage to an electrode provided on the hologram plate. . Therefore, it is possible to realize an optical modulator and an optical head device with high light utilization efficiency.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION When the light modulator of the present invention is viewed from above, the alignment state (alignment direction angle) of liquid crystal is as shown in FIG. However, in FIG. 1, only two cycles of the change of the alignment state of the liquid crystal are shown, and the cycle is omitted.

In order for the light modulation element to have the same function as that of the Fresnel lens, the period of change in the alignment state of the liquid crystal is made smaller from the central region to the peripheral region.

In FIG. 1, the straight lines in the concentric circles indicate the alignment direction of the liquid crystal, and in this case, three layers (three regions) form one cycle.
Although one cycle may include three or more layers, the alignment state of the liquid crystal is preferably asymmetric in the direction from the central region to the peripheral region within one cycle.

It is preferable to provide three or more regions in one period, and the greater the number of regions into which one period is divided, the finer the change in the orientation state (orientation direction angle) between adjacent regions becomes. It is preferable because they are equivalent. However, it is difficult to provide a large number of regions within one period of several tens to several hundreds of μm, and about 10 regions is the upper limit. Therefore, it is better that the change in the orientation direction angle between adjacent regions is smaller, and it is preferably 45 ° or less.

In the case of FIG. 1, the orientation angle is from the central region to the peripheral region, and one cycle is 0 ° (region 1) and 45 °.
(Region 2) and 90 ° (region 3) are repeated, and 0
The distribution of orientation angle in one cycle of °, 45 °, and 90 ° is asymmetric. However, for example, 0 °, 30 °, 60
When repeated at 0, 30 and 0 degrees, the symmetry is centered around the 60 degree region. In this case, it is not equivalent to what is called a blazed diffraction grating or a pseudo blazed diffraction grating, and both + 1st-order diffracted light and −1st-order diffracted light are generated with the same intensity, and the efficiency is reduced.

In one cycle, it is preferable that there are two regions having different orientation direction angles, such as 0 ° and 90 °, in which case the optical anisotropy of the liquid crystal (refractive index anisotropy). Is the largest in the molecular axis direction (long axis direction) and in the direction perpendicular to it (short axis direction), so that a diffraction grating with high diffraction efficiency can be obtained. Furthermore, it is preferable to match the polarization direction of the incident light to either the minor axis direction or the major axis direction because the diffraction efficiency can be maximized.

The periodic control of the alignment state can be realized by a mask rubbing method using a photolithography method and a rubbing treatment together.

FIG. 2 is a side sectional view of the device of FIG. 1 taken along a plane including the line A1-A2 passing through the central region. Liquid crystal cell 2
An electrode 6 is formed on the surface of one transparent substrate on the liquid crystal side, and an alignment state of the liquid crystal 7 can be controlled by applying an electric field to the liquid crystal 7 by the electrode 6. Liquid crystal 7 when no electric field is applied
The state of orientation is as shown in FIG. 3, and the element functions as a Fresnel lens depending on the distribution of the orientation angle of the liquid crystal 7. Light can be diffracted with high efficiency by appropriately selecting the value (Δn) of the refractive index anisotropy of the liquid crystal 7, the cell gap and the orientation angle. For example, 68% when there are three areas in one cycle
In principle, when there are four regions, a diffraction efficiency of about 81% can be obtained.

When an electric field is applied, the alignment state of the liquid crystal 7 is as shown in FIG. 4, the liquid crystal 7 is vertically aligned, the liquid crystal 7 does not function as an optical diffraction grating, and all light is transmitted.

FIG. 5 shows an example in which such an optical modulator is used in an optical head device. When an electric field is applied, almost all of the incident light (incident from below in FIG. 4) is directly transmitted and focused on the recording surface on the optical disk 10. The reflected light returns through the same optical path and passes through almost 100%. When no electric field is applied, most of the light that has passed through the central liquid crystal hologram portion is diffracted and spread, and then is condensed by the lens 12 at a slightly distant place. The reflected light also passes through the same path, and most of it is also diffracted by the liquid crystal hologram portion and returns to the original optical path. At this time, the incident light is assumed to be polarized in the A1-A2 direction.

The alignment state of the liquid crystal can be controlled by forming a polyimide film on the surface of the transparent substrate and rubbing it. It is also possible to provide relatively shallow grid-shaped grooves on the surface of the transparent substrate and align the liquid crystal by the alignment force of the grid-shaped recesses.

As the light source of the present invention, a semiconductor laser, YA
Various solid and gas lasers such as solid-state lasers such as G lasers and gas lasers such as He-Ne can be used, and semiconductor lasers are preferable in terms of downsizing and weight reduction, continuous oscillation, maintenance and inspection. Further, by using a harmonic generator (SHG) in which a semiconductor laser or the like and a non-linear optical element are incorporated in a light source unit and a short wavelength laser such as a blue laser is used, high density optical recording and reading can be performed.

The optical recording medium of the present invention is a medium in which information can be written and / or read by light. Examples are CD (Compact Disc) and CD-R
Examples thereof include optical disks such as OM, video disk, DVD (digital video disk), magneto-optical disk, and phase change optical disk.

[0029]

Embodiments of the present invention will be described below with reference to FIGS. A solid electrode 6 made of an ITO film was formed on the surface of a glass substrate 4 having a thickness of 0.5 mm, a size of 10 × 10 mm, and a refractive index of 1.52. After that, the polyimide film 5 is formed on the solid electrode 6, and the orientation direction angles of 0 °, 45 °, and 90 ° from the central region to the peripheral region of the glass substrate 4 are formed by the mask rubbing method using the photolithography method. The rubbing process was performed so that a periodic distribution of Here, the Fresnel hologram structure 11 formed by the periodic distribution of the orientation angle is a circular portion having a diameter of 2.5 mm at the center of the glass substrate 4.

The central region of the Fresnel hologram structure has a period of 285 μm and the peripheral region has a period of 33 μm, and the period is shortened from the central region to the peripheral region. The orientation direction angle in one cycle is 0 °, 45
And 90 °, and each region has a width of 1/3 of one cycle.

Two glass substrates 4 are prepared for the upper and lower sides,
A seal portion 8 is printed on the periphery of them to reduce the cell gap to 1
The two glass substrates 4 were superposed on each other to have a thickness of μm. After that, liquid crystal (nematic liquid crystal, product name PL-0 manufactured by Merck & Co., Inc.
08, ordinary light refractive index 1.525, extraordinary light refractive index 1.77
1) 7 was vacuum-injected. After that, the injection port was sealed with an epoxy resin to manufacture a light modulation element.

When no voltage is applied to the light modulation element, the light modulation element functions as a concave lens and the light is diffracted. The forward diffraction efficiency was about 50% and the backward diffraction efficiency was about 50% with respect to the semiconductor laser light having the wavelength of 780 nm. Therefore, the round trip efficiency is about 25 with respect to the optical path with a long focal length.
%Met.

On the other hand, when a voltage of 5 V is applied, the light is not diffracted and the optical path has a short focal length. At this time, the forward light transmittance was about 90%, the returning light transmittance was also about 90%, and a reciprocating efficiency of about 81% was obtained for the reciprocating.

As shown in FIG. 5, an optical head device was manufactured by using the light modulation element, a semiconductor laser (not shown, which is located below in the figure), a lens 12 and an optical disk 10. In this case, two focal points were obtained by one light modulation element, and two types of optical disks 10 having different thicknesses could be used at the same time.

[0035]

According to the present invention, a light modulation element having a dual focus lens effect which has high light utilization efficiency and can be manufactured at low cost, and an optical head device using the same are obtained. CD, CD-ROM and DV as it has a double focus lens effect
A single optical head device can read and write signals with respect to optical disks having different thicknesses such as a D disk.

[Brief description of drawings]

FIG. 1 is a plan view of a central portion of a light modulation element according to an embodiment of the present invention.

FIG. 2 is a side sectional view of the light modulation element taken along the line A1-A2 of FIG. 1, showing an embodiment of the present invention.

FIG. 3 is a partial side cross-sectional view of a light modulation element illustrating an alignment state of liquid crystal when an electric field is not applied, showing an embodiment of the present invention.

FIG. 4 is a partial side cross-sectional view of a light modulation element illustrating an alignment state of liquid crystal when an electric field is applied according to an embodiment of the present invention.

FIG. 5 is a side view of the optical head device according to the embodiment of the present invention.

[Explanation of symbols]

 1: area 2: area 3: area 4: glass substrate 5: polyimide film 6: solid electrode 7: liquid crystal 10: optical disk 11: Fresnel hologram structure

Claims (8)

[Claims] 1. A plurality of regions in which liquid crystals are concentrically distributed and arranged, the alignment state of the liquid crystals in each region periodically changes from a central region to a peripheral region, and the period of the change is An optical modulator comprising a liquid crystal cell changing from the center toward the periphery. 2. The light modulation element according to claim 1, wherein the cycle of change in the alignment state of the liquid crystal becomes smaller from the central region to the peripheral region. 3. The light modulation element according to claim 1, wherein the alignment state is asymmetric in the direction from the central region to the peripheral region within one cycle of the change in the alignment state of the liquid crystal. 4. The light modulation element according to claim 1, wherein the alignment state of the liquid crystal is controlled by a lattice-shaped concave portion formed on the surface of the transparent substrate. 5. A liquid crystal cell comprises two transparent substrates which are substantially parallel to each other and a liquid crystal which is sandwiched between the transparent substrates. The alignment direction of the liquid crystal is substantially parallel to the two transparent substrates and in a plane parallel to the transparent substrates. The light modulation element according to claim 1, wherein the alignment direction angle of the liquid crystal periodically changes from the central region toward the peripheral region. 6. The two regions in which the alignment direction angles differ by 90 ° are present in one cycle of the change in the alignment direction angle of the liquid crystal.
The light modulation element according to any one of the preceding claims. 7. The light modulation element according to claim 1, wherein electrodes are formed on the liquid crystal side surfaces of the two transparent substrates of the liquid crystal cell. 8. An optical head device for reading and / or writing information by irradiating an optical recording medium with light from a light source through the diffractive element, wherein the diffractive element is used.
An optical head device comprising the light modulation element according to any one of claims 1.