JP3648581B2 - Polarization diffraction grating, aperture control device using the same, aperture control method, and optical head device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is used in an optical head device for writing optical information on an optical recording medium such as a CD (compact disc), a CD-ROM, a CD-E, a CD-R, and a video disc, and for reading the optical information. The present invention relates to a polarization diffraction grating using a polymer liquid crystal and an optical head device using the same.
[0002]
[Prior art]
In an optical head device for writing optical information on an optical recording medium such as an optical disk and a magneto-optical disk or reading optical information, it is desired to read both DVD and CD. Thick with DVD and CDTheThe thickness is differentTheAttempts have been made to read different disks with one optical head device. It is difficult to read both discs with only one condenser lens.
[0003]
Therefore, specifically, two condensing lenses are used, which are mechanically switched, or a TN (twisted nematic) type liquid crystal element is combined with the condensing lens so that the aperture diameter is electrically controlled by the liquid crystal element. It has been proposed to read two types of discs with reduced aberrations by switching between them.
[0004]
[Problems to be solved by the invention]
However, mechanically switching the condenser lens increases the size of the optical head device, makes it difficult to obtain a compact optical head device, and tends to cause problems in reliability, switching time, and the like. In addition, the use of a TN liquid crystal element for aperture control has a problem in that the light transmittance is greatly reduced because a polarizing film is essential for the TN liquid crystal element.
For this reason, there has been a demand for an optical head device that can be easily switched electrically, has a low light utilization efficiency, and has high reliability.
[0005]
[Means for Solving the Problems]
  The present invention has been made to solve the above-mentioned problems, and in a polarization diffraction grating in which a polymer liquid crystal is sandwiched between substrates,The polarization diffraction grating is a polarization diffraction grating formed by polymerizing the polymer liquid crystal in a partially different orientation state,Diffractive efficiencies for at least one polarized light are different between the central part and the peripheral part.Of the two orthogonally polarized lights, one of the polarized lights is almost totally transmitted in the central part and is diffracted in the peripheral part, and the other polarized light is almost entirely in the central part. Transmits and transmits almost completely at the peripheryA polarizing diffraction grating is provided.
  Further, the polarization diffraction grating formed in the peripheral portion may be a concentric circle whose grating period changes, a concentric circle whose grating period increases from the inside to the outside, or a linear diffraction whose grating period changes. Provide a grid.
[0006]
Of the two orthogonally polarized lights, one of the polarized lights is almost totally transmitted in the central part and diffracted in the peripheral part, while the other polarized light is almost totally transmitted in the central part. In addition, the present invention provides a polarization diffraction grating that transmits almost all of the light in the peripheral portion, and a polarization diffraction grating in which the central portion is a lens-shaped unevenness or a Fresnel lens-shaped unevenness.
[0007]
  further,The polarizing diffraction grating includes a liquid crystal compound that is polymerized by polymerization between substrates with electrodes formed so that the peripheral portion of the opposing electrode has a desired grating pattern in the peripheral portion, and the opposing electrode It is formed by polymerizing a liquid crystalline compound while applying a voltage in between, and the central part is formed by polymerizing while applying a voltage by a transparent electrode independent of the peripheral part. Polarized diffraction grating consisting of a polymer liquid crystal in the intermediate state between the aligned horizontal and vertical alignments and with reduced phase difference from the peripheral partI will provide a.
[0008]
  Moreover,A polarization direction control element that changes the polarization direction of incident polarized light by an external signal and transmits the polarization direction control element, and the polarization diffraction element, and the incident light is polarized by an external signal applied to the polarization direction control element. An aperture control apparatus and an aperture control method are provided, wherein the direction is changed and the polarized light is guided to a polarization diffraction element having a different aperture by the polarized light.
  Also,The polarization diffraction grating is incorporated in an optical head device including a light source, a beam splitter, a photodetector, and a condenser lens, and the polarization direction incident on the polarization diffraction grating is switched by a polarization direction control element. An optical head device is provided.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, it is a polarization diffraction grating in which a polymer liquid crystal is sandwiched between substrates, and a polarization diffraction grating in which the diffraction efficiency is different for at least one of the polarized light in the central portion and the peripheral portion. This at least one polarized light includes those having different diffraction efficiencies in the central part and the peripheral part in all polarization directions, but the diffraction efficiency differs in the central part and the peripheral part only for polarized light in a specific direction. Including things.
[0010]
In the latter example, one of the two orthogonally polarized lights is almost totally transmitted in the central part and diffracted in the peripheral part, and the other polarized light is almost the same in the central part. Permeate completely, and almost completely permeate the surrounding areaRumoThere is. Specifically, for P-polarized light, it is totally transmitted at the center and diffracted at the peripheral part, but for S-polarized light, it is completely transmitted at both the central part and the peripheral part.RumoThere is.
[0011]
FIG. 1 is a cross-sectional view of a polarization diffraction grating according to the present invention, where (A) is S-polarized light (light having a polarization direction perpendicular to the drawing sheet), and (B) is P-polarized light (polarization direction parallel to the drawing sheet). The light transmission state in the case of the light having () has been described. FIG. 2 is a plan view of the polarization diffraction grating of FIG.
[0012]
1 and 2, 1 is a polarization diffraction grating, 2 is its central part, 3 is its peripheral part, 4 is S-polarized light, 5 is P-polarized light, 6A to 6F are incident light, 7A to 7F are outgoing light, 8 Denotes a first orientation part, and 9 denotes a second orientation part. W₁Is the width of the center, W₂And W₃Indicates the width of the periphery.
[0013]
In the present invention, the central portion 2 and the peripheral portion 3 of the polarization diffraction grating 1 have different diffraction efficiencies for at least one of the polarized lights. In the example of FIGS. 1 and 2, the orientation of the polymer liquid crystal is different between the central portion and the peripheral portion, and the diffraction grating is formed in an axially symmetrical manner in the peripheral portion.
[0014]
In this example, both the upper and lower substrates are aligned in the horizontal direction in FIG.CornerIs 0 °), and the incident lights 6A, 6B, and 6C are transmitted as they are with respect to the S-polarized light and become the emitted lights 7A, 7B, and 7C. For P-polarized light, the incident lights 6D, 6E, and 6F are transmitted as they are at the central portion to become outgoing light 7D, and diffracted at the peripheral portion to become outgoing lights 7E and 7F.
[0015]
However, even in FIG. 1A, there is a diffracted portion other than the cross-sectional portion. In other words, in the peripheral part farther from the cross section, the longitudinal direction of the grating coincides with the polarization direction, and diffraction occurs. This corresponds to the case of viewing in a cross section orthogonal to the cross section of FIG. 1A, which is the same as FIG. That is, when the grating is axisymmetric in this way, diffraction occurs somewhere in the periphery.
[0016]
In this case, a transparent substrate such as normal glass or plastic can be used as the substrate, but a glass substrate is preferable from the viewpoint of reliability. When forming irregularities on the substrate, the substrate itself is shaved by etching or mechanical cutting method, press-molded, or a film is formed on the substrate surface, and this is etched by etching or mechanical cutting method, It may be formed by press molding.
[0017]
The unevenness may be a straight line or a ring shape in the case of a lattice in the peripheral part. Further, when the central portion is formed into a lens shape as in the example described later, it may be a simple concave portion or convex portion, or may be formed into a Fresnel lens shape. The relationship between the refractive index of the liquid crystal to be used and the refractive index of the substrate or the film on the surface of the substrate produces a lattice function or a lens function. Therefore, it is easier to adjust the refractive index by forming a film on the substrate surface and making it uneven. It is preferable.
[0018]
Specifically, an inorganic compound film is preferable as this film, but Si film is particularly preferable._xO_yIs preferable. The refractive index of this film can be changed in a fairly wide range depending on the values of x and y, and the refractive index between the ordinary light refractive index and the extraordinary light refractive index that the liquid crystal normally has can be selected almost freely.
[0019]
When an electrode is formed on a substrate, ITO (In₂O₃-SnO₂), But other transparent electrodes can also be used. This electrode is used after being patterned into a desired pattern, but may be used as a solid electrode. In this example, the electrodes are patterned on both substrates, and a portion where no electrode is provided is provided in the central portion 2 and a part of the peripheral portion 3.
[0020]
An alignment film is formed on this electrode as necessary. As this alignment film, an alignment film used in a normal liquid crystal display element can be used. As a typical horizontal alignment film, there is a film obtained by rubbing a resin film such as polyimide or polyamide, or an oblique deposition film of SiO. A typical vertical alignment film is a film using a vertical alignment agent such as aminosilane.
[0021]
In addition, a sealing technique, a liquid crystal injection technique, an inlet sealing technique, a multi-cavity manufacturing technique, and the like may be techniques used in a normal liquid crystal display element process. The polarizing diffraction grating of the present invention is preferably used with an antireflection film formed on the outer surface.
[0022]
There are several methods for manufacturing such a polarization diffraction grating, but the following manufacturing method is preferable.
[0023]
The most suitable first method is to use a pair of substrates on which electrodes are formed so that the facing portions of the facing electrodes have a desired lattice pattern, while applying a voltage between the facing electrodes, This is a method of polymerizing to polymerize.
[0024]
That is, a pair of substrates on which electrodes are formed so that opposing portions of the opposing electrodes have a desired lattice pattern is prepared. Specifically, an electrode may be formed on one substrate so as to have a desired lattice pattern, and a solid electrode may be left on the other substrate. However, both may be patterned in the same manner, and a combination of both patterns may form a desired lattice pattern.
[0025]
Next, a liquid crystalline compound that is polymerized by polymerization is sandwiched between the substrates with electrodes. Specifically, these substrates with electrodes may be overlapped to form a cell, and a liquid crystalline compound that is polymerized by polymerization may be injected into the empty cell. In the case of a highly viscous liquid crystalline compound, a method of sealing at the same time as injection or a method of sealing after a subsequent polymerizing step is also possible.
[0026]
Next, the liquid crystalline compound is polymerized to be polymerized while applying a voltage between the opposing electrodes. Specifically, it may be cured by light irradiation using a photocurable liquid crystalline compound. In the case of the thermosetting type, it is cured by heating. Thereby, in the part pinched between the electrodes to which the voltage is applied, the polymer is polymerized in a state where the alignment state is partially different.
[0027]
When nematic liquid crystal having positive dielectric anisotropy is used and the substrate surface is horizontally aligned by rubbing or the like, the liquid crystal molecules are horizontally aligned in the alignment processing direction of the substrate in a portion where no voltage is applied. On the other hand, the liquid crystal molecules are vertically aligned in a portion where a voltage is applied between the electrodes. Thereby, when it polymerizes, it will be in two types of orientation states.
[0028]
In this case, when considering the state of parallel alignment between the upper and lower substrates, the ordinary refractive index (n_o) To extraordinary light refractive index (n_e). In a portion vertically aligned by voltage application, the ordinary refractive index (n_o).
[0029]
As a second method, unevenness is formed on at least one substrate itself or a film provided on the surface of the substrate so as to have a desired lattice pattern. Using this uneven substrate and another substrate, a cell is formed, and a liquid crystal compound that is polymerized by polymerization is injected into the empty cell.
[0030]
Next, the injected liquid crystal compound is cured. The liquid crystal molecules are aligned along the recess grooves or the alignment treatment direction. In this case, diffraction may or may not occur depending on the refractive index of the substrate or the film on the surface of the substrate and the refractive index when the liquid crystal is aligned.
[0031]
The third method is similar to the first method, but there is a method in which the electrode is a solid electrode that is not patterned, a light-shielding mask having a desired pattern is disposed on the surface, and photocuring is performed while applying a voltage to the entire surface. .
[0032]
In this case, there are two types of methods. One is to dispose a light-shielding mask, photocuring while applying voltage across the entire surface, and then removing the light-shielding mask and curing without applying voltage across the entire surface. The other is to dispose a light shielding mask, photocure without applying voltage across the entire surface, then remove the light shielding mask and apply voltage across the entire surface to cure.
[0033]
Furthermore, these may be combined, and a polarization diffraction grating having different diffraction efficiencies with respect to at least one polarized light may be formed at the central portion and the peripheral portion.
[0034]
In the case where the lattices are formed symmetrically as shown in FIGS. 1 and 2, the central portion 2 in the above description is horizontally oriented, and light is always totally transmitted regardless of the polarization direction. On the other hand, in the peripheral portion 3, the liquid crystal molecules (side chain portions after polymerization) are horizontally aligned in the first alignment portion 8, and the liquid crystal molecules are vertically aligned in the second alignment portion 9. And a diffraction grating is formed.
For this reason, in the peripheral part 3, since the diffraction grating is formed in a ring shape, a part of the light in any polarization direction is diffracted.
[0035]
Further, in this example, the influence of stray light is avoided by gradually changing (chirping) the grating period, but this can also be omitted when the grating pitch is fine. In this figure, the lattice period is exaggerated in order to make it easy to understand that it is changing gradually, but the rate of change is actually a change of about several tens of percent. Much less than. The same applies to the description of other examples below.
[0036]
As a result, the inner emitted light 7E having a narrow grating pitch is diffracted more than the outer emitted light 7F having a larger grating pitch at the periphery.
[0037]
In addition, in the polarization diffraction grating having this configuration, it is preferable to implement a phase correction coat in order to correct the transmitted wavefront aberration generated in the central portion and the peripheral portion. In order to reduce the transmitted wavefront aberration at the central portion with respect to the polarized light to be diffracted by omitting the phase correction coating process and using a thin ITO film of 50 nm or less, In order to reduce the transmitted wavefront aberration, it is preferable to perform patterning so that the area ratio of the portion with and without the ITO film on the two opposite sides is equal.
[0038]
In order to suppress the transmitted wavefront aberration within the effective diameter for polarized light in all directions or circularly polarized light, an ITO electrode with a thickness of 50 nm or less, which is less affected by the film itself, is provided in the central part independently of the peripheral part. It is preferable that a voltage lower than that in the peripheral portion is applied at the same time so that the liquid crystal is solidified in an intermediate state between horizontal alignment and vertical alignment to reduce the phase difference from the peripheral portion.
[0039]
In the polarization diffraction grating having this configuration, the entire peripheral portion of one side is made of an ITO film, so that alignment is not required and the manufacturing process is simplified. However, the transmittance for the diffracted polarized light in the peripheral portion is increased. For this reason, when a large extinction ratio (transmittance for transmitted polarized light / transmittance for diffracted polarized light) is required in the peripheral portion, it is preferable to pattern ITO electrodes having the same shape on both sides.
[0040]
In this example, since the diffracted light in the diffractive part may become stray light and become optical noise, the diffractive part is made axially symmetric to suppress it. A straight grid may be used.
[0041]
FIG. 3 is a cross-sectional view of another example of the polarization diffraction grating of the present invention, where (A) is S-polarized light (light having a polarization direction perpendicular to the paper surface of the figure), and (B) is P-polarized light (on the paper surface of the figure). The light transmission state in the case of light having a parallel polarization direction is described. FIG. 4 is a plan view of the polarization diffraction grating of FIG.
[0042]
3 and 4, 11 is a polarization diffraction grating, 12 is its central part, 13 is its peripheral part, 14 is S-polarized light, 15 is P-polarized light, 16A to 16F are incident light, 17A to 17F are outgoing light, 18 Denotes a first orientation part, and 19 denotes a second orientation part. W₁₁Is the width of the center, W₁₂And W₁₃Indicates the width of the periphery.
[0043]
In this example, a linear lattice is formed in the peripheral portion 3 except for the central portion 2. Moreover, the pitch of the grating is changed as in Example 1. The lattice may be formed by the same manufacturing method as that described with reference to FIGS.
[0044]
In this example as well, both the upper and lower substrates are aligned in the horizontal direction in the figure (a twist of liquid crystal molecules).CornerIs 0 °), and the incident lights 16A, 16B, and 16C are transmitted as they are with respect to the S-polarized light, and become the emitted lights 17A, 17B, and 17C. For P-polarized light, incident light 16D, 16E, and 16F are transmitted as they are at the central portion to become outgoing light 17D, and diffracted at the peripheral portion to become outgoing light 17E and 17F. In this case, the left emitted light 17E having a narrow grating pitch in the peripheral portion is diffracted more than the right emitted light 17F having a larger grating pitch.
[0045]
FIG. 5 is a cross-sectional view of still another example of the polarization diffraction grating of the present invention, where (A) is S-polarized light (light having a polarization direction perpendicular to the paper surface of the figure), and (B) is P-polarized light (paper surface of the figure). In the case of light having a polarization direction parallel to the light transmission state, the light transmission state is described. FIG. 6 is a plan view of the polarization diffraction grating of FIG.
[0046]
5 and 6, 21 is a polarization diffraction grating, 22 is its central part, 23 is its peripheral part, 24 is S-polarized light, 25 is P-polarized light, 26A to 26F are incident light, 27A to 27F are outgoing light, 28 Denotes a first orientation part, and 29 denotes a second orientation part. W₂₁Is the width of the center, W₂₂And W₂₃Indicates the width of the periphery.
[0047]
In this example, a linear lattice is formed in the peripheral portion 3 as in the examples of FIGS. 3 and 4, and a concave portion is formed in the central portion 2 in the substrate itself. The pitch of the lattice of the linear lattice in the peripheral portion 3 is changed as in the examples of FIGS. The lattice may be formed by the same manufacturing method as that described with reference to FIGS.
[0048]
In this example, by forming a lens-like curved surface at the center of the substrate, the optical wavefront aberration is further improved by controlling not only the aperture diameter but also the phase. For this purpose, irregularities are formed in the central portion 2 as long as the central portion is formed in a desired shape so as to function as a lens. The unevenness may be a simple recess or protrusion, or may be formed in a Fresnel lens shape.
[0049]
In this example as well, both the upper and lower substrates are aligned in the horizontal direction in the figure (a twist of liquid crystal molecules).CornerIs 0 °), and incident light 26A, 26B, and 26C are transmitted as they are in this cross section to become outgoing light 27A, 27B, and 27C with respect to S-polarized light. For P-polarized light, the incident lights 26D, 26E, and 26F are transmitted as they are at the central portion to become outgoing light 27D, and are diffracted at the peripheral portions to become outgoing lights 27E and 27F. In this case, the left outgoing light 27E having a narrow grating pitch in the peripheral portion is diffracted more than the right outgoing light 27F having a wide grating pitch.
[0050]
Further, in this example, the liquid crystal functions as a convex lens only for P-polarized light by the substrate provided with the concave portion in the central portion 22. This is because the liquid crystal is horizontally aligned in parallel (not twisted), so that the liquid crystal exhibits an extraordinary refractive index in the direction of P-polarized light, and the refractive index difference is maintained by keeping the substrate near the ordinary refractive index. And functions as a convex lens. On the other hand, for S-polarized light, when the liquid crystal exhibits a normal light refractive index and the substrate is in the vicinity of the normal light refractive index, no difference in refractive index occurs and the lens does not function as a convex lens.
In this case, the refractive index of the substrate may be matched with the extraordinary light refractive index, the central portion may be vertically aligned, or a convex portion may be formed at the central portion of the substrate.
[0051]
Accordingly, in the present invention, the light aperture diameter is controlled for light in a certain polarization direction, and the light aperture diameter is not limited for light in a certain polarization direction.
[0052]
When the polarization diffraction grating of the present invention and a polarization direction control element using TN liquid crystal are combined, the aperture can be easily switched electrically. In this case, the polarization direction control element using the TN liquid crystal may be a normal TN liquid crystal cell having a solid electrode in which 90 ° twisted liquid crystal is enclosed. Since the polarization direction control element using the TN liquid crystal is used for the purpose of switching between P-polarized light and S-polarized light, it is not necessary to use a polarizing film. For this reason, there is no loss of light quantity due to the polarizing film.
[0053]
The polarization diffraction grating thus manufactured is used by being incorporated in an optical head device.
FIG. 7 is a front view showing a basic configuration of the optical head device of the present invention.
In FIG. 7, 31 is a light source such as a laser diode, 32 is a beam splitter, 33 is a retardation plate such as a quarter-wave plate, 34 is a polarization direction control element, 35 is a polarization diffraction grating, 36 is a condenser lens, 37 Denotes a first optical recording medium, 38 denotes a second optical recording medium, and 39 denotes a photodetector.
[0054]
In this optical head device, the light emitted from the light source 31 passes through the beam splitter 32, the phase difference plate 33, the polarization direction control element 34, the polarization diffraction grating 35, and the condenser lens 36, and enters the first optical recording medium 37. The light reaches the light detector 39 by being reflected by the beam splitter 32 and diffracted by the beam splitter 32. When the polarization direction is changed by the polarization direction control element 34, the light transmission state of the polarization diffraction grating 35 is changed, and the light reaches the second optical recording medium 38.
[0055]
That is, the incident polarization direction to the polarization diffraction grating 35 is switched between P-polarized light and S-polarized light according to the change in the voltage application state to the polarization direction control element 34. As a result, the light emitted from the polarization diffraction grating 35 changes, and the focus is switched to the first optical recording medium 37 and the second optical recording medium 38.
[0056]
The configuration of this optical head device is merely a representative configuration. A polarizing diffraction grating using liquid crystal is used for the beam splitter, the beam splitter is divided into a plurality of parts and diffracted by a plurality of photodetectors, or an SHG (SecondApplications that are applied to known optical head devices, such as using a harmonic generation device) or using a diffraction grating that divides light directed to an optical recording medium into three beams, are possible within the range that does not impair the effects of the present invention. It is.
[0057]
【Example】
Example 1 (Example)
An ITO transparent conductive film having a thickness of 30 nm was formed on a glass substrate having a thickness of 0.5 mm by a sputtering method. The ITO film is patterned by photolithography and wet etching, and a radius of 1.25 mmφ (w₁= 2.5 mm) Axisymmetric ITO grid-like electrodes were formed on the outer periphery.
[0058]
However, an ITO electrode having a width of 20 μm was left in the 90 ° and 270 ° directions in order to ensure energization. The grating period was gradually increased from a pitch of 16 μm at a radius of 1.25 mm to a pitch of 20 μm at a radius of 2.5 mm. The same patterning was performed on the ITO electrode on the counter substrate side.
[0059]
A polyimide alignment film having a thickness of 60 nm was formed on the opposing surfaces (ITO electrode formation surfaces) of both substrates, and rubbing with a rubbing cloth was performed. The rubbing direction is parallel to the paper surface of FIG. 1 and both substrates are opposed to each other so that the direction is 180 ° between the upper and lower substrates. Was made.
[0060]
An unpolymerized liquid crystal monomer containing 1% by weight of a polymerization initiator is injected into the empty cell, and 5V is applied to the ITO electrode._rmsA rectangular voltage of 100 Hz was applied. In this state, power density 10 mW / cm² UV was irradiated for 300 seconds, and the vertical / horizontal periodic repetition of the alignment direction caused by the periodically applied voltage was immobilized by polymerisation. Thereafter, the outer side was cut to produce a 5.0 mm square polarization diffraction grating.
[0061]
The produced polarization diffraction grating has a radius of 1.25 mm to 2 even at the center of the radius of 1.25 mm for linearly polarized light (S-polarized light of FIG. 1) perpendicular to the rubbing direction (parallel to the paper surface of FIG. 1). The transmittance of 90% or more was exhibited even in the peripheral portion where a .5 mm grid was formed.
[0062]
On the other hand, for linearly polarized light parallel to the rubbing direction (P-polarized light in FIG. 1), a transmittance of 90% or more was shown at the center of the radius of 1.25 mm. Incident light was diffracted at the periphery where the 5 mm grating was formed, and only transmitted about 20%.
[0063]
Using this polarization diffraction grating and a 90 ° twisted TN liquid crystal cell (without a polarizing film) as a polarization direction control element, the beam diameter of transmitted light is electrically reduced to about 5 mmφ and about 2.5 mmφ. I was able to switch.
[0064]
A polarizing diffraction grating (Example 1A) was manufactured in the same manner as in Example 1 except that the entire surface of the polarizing diffraction grating of Example 1 was a solid ITO film. By making the electrode of one substrate a solid electrode, the alignment of the two substrates is not required and the manufacturing process is simplified, but the transmittance for the diffracted polarized light in the peripheral portion has increased to about 45%.
[0065]
An empty cell was prepared in the same manner as in Example 1 except that an electrode was provided in the center portion separately from the peripheral lattice. During polymerization of the liquid crystal monomer, the same voltage as in Example 1 was applied to the peripheral portion while applying a lower voltage to the central electrode than to vertically align the liquid crystal. This polarization diffraction grating (Example 1B) had lower transmission wavefront aberration than the polarization diffraction grating of Example 1.
[0066]
Example 2 (Example)
An ITO transparent conductive film having a thickness of 30 nm was formed on a glass substrate having a thickness of 0.5 mm by a sputtering method. The ITO film is patterned by photolithography and wet etching, and a radius of 1.25 mmφ (w ₁₁ = 2.5 mm), an ITO lattice-like electrode having a linear periodic lattice was formed on the periphery.
[0067]
The grating period was gradually increased from a pitch of 16 μm at a position 2.5 mm to the left of the polarization diffraction grating center to a pitch of 20 μm at a position of 2.5 mm to the right from the center. The same patterning was performed on the ITO electrode on the counter substrate side. Rubbing was performed in a direction perpendicular to the grating, and a polarization diffraction grating was produced in the same manner as in Example 1 using both substrates.
[0068]
The produced polarization grating has a radius of 1.25 mm to 2 even in the central portion of the radius of 1.25 mm for linearly polarized light (S-polarized light of FIG. 3) perpendicular to the rubbing direction (parallel to the paper surface of FIG. 3). The transmittance of 90% or more was exhibited even in the peripheral portion where a .5 mm grid was formed.
[0069]
On the other hand, for linearly polarized light parallel to the rubbing direction (P-polarized light in FIG. 3), a transmittance of 90% or more was shown at the center of the radius of 1.25 mm. Incident light was diffracted at the periphery where the 5 mm grating was formed, and only transmitted about 15%.
[0070]
Using this polarization diffraction grating and an electrically drivable 90 ° twisted TN liquid crystal cell (without a polarizing film) as a polarization direction control element, the beam diameter of transmitted light is about 5 mmφ and about 2.5 mmφ. It was possible to switch electrically.
[0071]
A polarizing diffraction grating (Example 2A) was manufactured in the same manner as in Example 1 except that the entire surface of the polarizing diffraction grating of Example 2 was a solid ITO film. By making the electrode of one substrate a solid electrode, the alignment of the two substrates becomes unnecessary and the manufacturing process is simplified, but the transmittance for the diffracted polarized light in the peripheral portion has increased to about 40%.
[0072]
An empty cell was prepared in the same manner as in Example 2 except that an electrode was provided in the central portion separately from the peripheral lattice. During polymerization of the liquid crystal monomer, the same voltage as in Example 2 was applied to the peripheral portion while applying a lower voltage to the central electrode than to vertically align the liquid crystal. This polarization diffraction grating (Example 2B) had lower transmission wavefront aberration than the polarization diffraction grating of Example 2.
[0073]
Example 3 (Example)
An aspherical concave shape having a diameter of 2.5 mm and a depth of 5 μm was produced on a 1.0 mm-thick glass substrate having a refractive index of 1.52 by a pressing method. On this glass substrate, an ITO transparent conductive film having a thickness of 30 nm was formed by sputtering, and patterned into an ITO film by photolithography and wet etching. As shown in FIG. 6, a peripheral portion having a radius of 1.25 mmφ or more was formed. Then, an ITO lattice electrode having a linear periodic lattice was formed.
[0074]
The grating period was gradually increased from a pitch of 16 μm at a position 2.5 mm to the left of the polarization diffraction grating center to a pitch of 20 μm at a position of 2.5 mm to the right from the center. The same patterning was performed on the ITO electrode on the counter substrate side.
[0075]
An empty cell was prepared in the same manner as in Example 2, and an unpolymerized acrylic liquid crystal monomer containing 1% by weight of a polymerization initiator in the cell (n after polymerization)_o= 1.5, n_e= 1.6) was injected, and a polarization diffraction grating was produced in the same manner as in Example 2.
[0076]
The produced polarization grating has a radius of 1.25 mm to 2.5 mm even in the central portion of the radius 1.25 mm for linearly polarized light (S-polarized light) perpendicular to the rubbing direction (parallel to the paper surface of FIG. 5). A transmittance of 90% or more was exhibited even in the peripheral portion where the grating was formed. The central portion did not function as a lens because there was almost no refractive index difference with respect to this linearly polarized light.
[0077]
On the other hand, for linearly polarized light (P-polarized light) parallel to the rubbing direction, a transmittance of 90% or more was shown in the central part with a radius of 1.25 mm, but a grating with a radius of 1.25 mm to 2.5 mm. The incident light was diffracted at the periphery where the sapphire was formed and was transmitted only about 15%. Further, the central portion functions as a phase correction convex lens having a refractive index difference of 0.1 and a thickness of 5 μm, and the phase change of the transmitted wavefront was confirmed.
[0078]
Using this polarization diffraction grating and an electrically drivable 90 ° twisted TN liquid crystal cell (without a polarizing film) as a polarization direction control element, the beam diameter of transmitted light is about 5 mmφ and about 2.5 mmφ. It was possible to switch electrically.
[0079]
【The invention's effect】
The present invention is a polarization diffraction grating in which a polymer liquid crystal is sandwiched between substrates, and the diffraction efficiency for at least one of polarized light differs between the central portion and the peripheral portion. Thereby, electrical aperture control can be performed without using a polarizing film, and a compact optical head device with high light utilization efficiency and high reliability can be easily obtained.
[0080]
Since the aperture control can be electrically performed, a mechanism such as mechanical lens switching is unnecessary, which is suitable for reducing the size and weight of the optical head device and increasing the reliability. Further, since it is not necessary to use a polarizing film, the light utilization efficiency is increased, the light source can be reduced in output and size, and there are also advantages such as low power consumption.
The present invention can be applied in various ways as long as the effects of the present invention are not impaired.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a polarization diffraction grating of the present invention. (A) illustrates S-polarized light, and (B) illustrates light transmission in the case of P-polarized light.
FIG. 2 is a plan view of the polarization diffraction grating of FIG.
FIG. 3 is a cross-sectional view of another example of the polarization diffraction grating of the present invention. (A) illustrates S-polarized light, and (B) illustrates light transmission in the case of P-polarized light.
4 is a plan view of the polarization diffraction grating of FIG. 3;
FIG. 5 is a cross-sectional view of still another example of the polarization diffraction grating of the present invention. (A) illustrates S-polarized light, and (B) illustrates light transmission in the case of P-polarized light.
6 is a plan view of the polarization diffraction grating of FIG.
FIG. 7 is a front view showing a basic configuration of an optical head device of the present invention.
[Explanation of symbols]
Polarization diffraction grating: 1
Center: 2
Peripheral part: 3
S-polarized light: 4
P-polarized light: 5
Incident light: 6A-6F
Outgoing light: 7A-7F
First orientation part: 8
Second orientation part: 9

Claims (9)

In the polarization diffraction grating in which the polymer liquid crystal is sandwiched between the substrates, the polarization diffraction grating is a polarization diffraction grating including a polymer liquid crystal layer formed by polymerizing a liquid crystalline compound in a partially different orientation state. Te, the diffraction efficiency for at least one of the polarization between the central portion and the peripheral portion is Te different for Ttei, among two orthogonal polarization, for one polarization, substantially total transmission in the central portion, the peripheral portion while diffraction for the other polarization, and also almost all the transmission is in the center, polarization gratings, characterized that you almost total transmission in the peripheral portion. The polarization diffraction grating according to claim 1 , wherein the polarization diffraction grating formed in the peripheral portion has a concentric shape with a varying grating period . 3. The polarization diffraction grating according to claim 2, wherein a grating period of the concentric polarization diffraction grating increases from the inside toward the outside. The polarization diffraction grating according to claim 1 , wherein the polarization diffraction grating formed in the peripheral portion is a linear polarization diffraction grating having a grating period changing . The polarizing diffraction grating according to any one of claims 1 to 4, wherein the central portion is a lens-shaped unevenness or a Fresnel lens-shaped unevenness. The polarizing diffraction grating has a liquid crystal compound that is polymerized by polymerization between substrates with electrodes formed so that the peripheral portion of the opposing portion of the opposing electrode has a desired grating pattern, and the opposing electrode It is formed by polymerizing a liquid crystalline compound while applying a voltage in between, and the central part is formed by polymerizing while applying a voltage with a transparent electrode independent of the peripheral part. The polarization diffraction grating according to any one of claims 1 to 5, wherein the polarization diffraction grating is made of a polymer liquid crystal in an intermediate state between the horizontal alignment and the vertical alignment, and has a reduced phase difference with a peripheral portion. A polarization direction control element that changes the polarization direction of incident polarized light by an external signal and transmits the polarization direction control element, and the polarization diffraction element according to any one of claims 1 to 6, wherein the incident light is converted into the polarization direction. An aperture control apparatus, wherein the polarization direction is changed by an external signal applied to the control element and is guided to the polarization diffraction element. An aperture control method in which incident polarized light is transmitted through a polarization direction control element and the polarization diffraction grating in this order, and is emitted by changing the beam diameter by an external signal, wherein the polarization direction control element A polarization direction control element that changes the polarization direction according to an external signal and transmits the polarization direction control element, wherein the aperture control method changes the polarization direction by applying an external signal to the polarization direction control element and changes the polarization direction. 7. An aperture control method, wherein the aperture is guided to the polarization diffraction grating according to claim 1 having a different aperture. Source polarization diffraction grating child according to any one of claims 1 to 6, the beam splitter, a photodetector, built in an optical head device including a condenser lens, a polarization direction enters the polarization diffraction grating by the polarization direction control element An optical head device characterized by switching between the two.

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