JPH06258656A - Liquid crystal spatial optical modulator and its driving method - Google Patents

Abstract

PURPOSE:To provide the liquid crystal spatial optical modulator which can make continuous gradation display and has high reliability and the driving method for the modulator. CONSTITUTION:Transparent electrode layers 12a, 12b are formed on the surfaces of a pair of substrates 11a, 11b for holding a liquid crystal. A photoconductive layer 15 and a multilayered dielectric substance film mirror 16 are formed on the one transparent electrode 12a. Further the surfaces of both substrates 11a, 11b are provided with oriented film layers 13a, 13b. A sealing material prepd. by mixing and dispersing spacer materials into an outer peripheral sealing material is applied on a pair of these substrates 11a, 11b and thereafter, two sheets of the substrates 11a, 11b are adhered to form a spacing to hold the liquid crystal 14. A nematic liquid crystal compsn. having 180 to 360 deg. rotating angle is held in the spacing. An AC electric field is used as the driving method thereof. As a result, the continuous gradation expression is realized more easily and inexpensively than the use of a ferroelectric liquid crystal with high reliability with the spatial optical modulator to be used for systems, such as projection displays, optical information processing systems and optical neural networks.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photo-writing type liquid crystal spatial light modulator applied to a projection display, an image processing device, a spatial light modulator for optical information processing and the like and a driving method thereof.

[0002]

2. Description of the Related Art Conventionally, a spatial light modulator using a liquid crystal has been used as an element for intensity-modulating and outputting input image information in real time. Generally, a binarization device using a surface-stabilized ferroelectric liquid crystal as a modulation material is known. The authors have also shown, in Japanese Patent Application No. 02-239594, a driving method for obtaining an output having continuous gradation in the above device.

[0003]

However, depending on the conventional spatial light modulator using the surface-stabilized ferroelectric liquid crystal and its driving method, the method as disclosed in Japanese Patent Application No. 02-239594 is used. Although it is possible to express gradation in the continuous driving state by using it, it is necessary to apply a DC bias to the element, which causes a problem in reliability.

[0004]

In order to solve the above problems, the present invention provides a spatial light modulator using a liquid crystal and a method for driving the same, wherein a liquid crystal composition used as a modulation material is 180 ° to A composition having a nematic phase at a room temperature having a rotation angle of 360 ° and being driven by an alternating electric field makes it possible to express continuous gradation in a liquid crystal spatial light modulator and further improve the reliability of the device. It is a creaking.

[0005]

By using the above method, there is a possibility that the liquid crystal spatial light modulator can be used in applications such as projection displays that require continuous gradation. Furthermore, when a ferroelectric liquid crystal is used, The seizure due to the deformation of the layer structure that occurs does not occur, and the impact resistance is also improved, so that the range of application thereof can be increased.

[0006]

The present invention will be described in detail below with reference to the drawings. FIG. 1 is a schematic view showing the structure of a photo-writing type liquid crystal spatial light modulator according to the present invention.

As substrates 11a and 11b for sandwiching liquid crystal molecules, both surfaces have parallel flatness λ / 5 at He-Ne laser wavelength.
A transparent glass substrate having a thickness of 5 mm, which was polished below, was used. ITO transparent electrode layers 12a and 12b were provided on the surfaces of both substrates. 3.0 μm thick hydrogenated amorphous silicon (a-Si: H) photoconductive layer is formed on the transparent electrode layer 12a on the light writing side.
Formed 15. Furthermore, a dielectric multilayer mirror 16 was formed as a mirror layer on the photoconductive layer. However, this mirror layer may not be necessary depending on the application of the spatial light modulator, and a spatial light modulator without this mirror layer was also manufactured and evaluated. Here, there is no problem even if the mirror layer has a structure in which metal films insulated from each other are arranged.

The total thickness of the mirror layer is determined by the requirement for the reflectance at the wavelength used, but an increase in the total thickness affects the resolution of the device and cannot be excessively increased. Therefore, the total thickness of the dielectric multilayer mirror formed in this example is set to 2.6 μm.

It should be noted that the photoconductive layer 15 can be made of other materials such as cadmium sulfide without any problem, and the optimum value of the layer thickness varies depending on the thicknesses of the mirror layer and the liquid crystal layer. It is set in the range of 5 to 10 μm in consideration of the electrical characteristics and film thickness of each of the above films.

Furthermore, alignment film layers 13a and 13b made of polyimide are provided on the surfaces of both substrates. The alignment film layers 13a and 13b include
The rotation angle of liquid crystal molecules is 240 when both substrates are combined.
The rubbing treatment was performed in the direction such that the angle became °. here,
There is no problem even if a polymer film other than polyimide is used as the alignment film layer. Alternatively, a film obtained by obliquely depositing silicon monoxide or the like (in this case, rubbing treatment is not necessary) may be used.

Next, silica spheres having an average particle diameter of 2.8 μm are mixed and dispersed in the outer peripheral sealing material, the sealing material is applied by printing using a letterpress printing method, and then two substrates are adhered to each other to form a liquid crystal. Formed a nipping space. Here, in this embodiment, since the hydrogenated amorphous silicon photoconductive layer having a thickness of 3.0 μm was used, the liquid crystal layer thickness was set to 2.8 μm. Since the optimum value fluctuates depending on the value, it is set within the range of 1 to 10 μm.

As the liquid crystal composition 14, RDP21125 is used.
(Rodick Co., Ltd.) containing 2.6% of a left-handed chiral agent S-
After using 811 added, it was vacuum-injected and then once heated to an isotropic phase and gradually cooled to obtain a uniform alignment.
Refractive index anisotropy Δn of the used liquid crystal RDP21125
It is 0.254. Here, the optimum value of the refractive index anisotropy of the liquid crystal varies depending on the wavelength used, the liquid crystal layer thickness, the rotation angle, and the polarization direction of the incident light.
The liquid crystal was used because 3 nm was assumed. Of course, the liquid crystal material may be changed depending on the setting of each of the above conditions. However, since an increase in the liquid crystal layer thickness leads to a deterioration in resolution, it is desirable to use a liquid crystal layer thickness that is as thin as possible. Therefore, a liquid crystal having a refractive index anisotropy of 0.05 or more is used.

Here, a nematic liquid crystal of a vertical alignment or a hybrid alignment, which is a spatial light modulator having a structure similar to that of the present invention and which is practically used as a light valve for a high-resolution projector, or a so-called TN of 90 ° twist. The advantages of the present invention over spatial light modulators using modes are noted. The liquid crystal spatial light modulator using these modes requires a larger liquid crystal layer thickness than the spatial light modulator according to the present invention in order to maintain the modulation efficiency. The voltage divided by the liquid crystal layer with respect to the drive voltage is determined by the layer thickness and resistivity of the liquid crystal layer and the layer thickness and resistivity of the photoconductive layer (bright and dark). Spatial light modulators inevitably require the use of thick photoconductive layers. Therefore, both the liquid crystal layer and the photoconductive layer become thicker, and the resolution is significantly inferior to that of the spatial light modulator according to the present invention. Further, the liquid crystal layer of these modes has a sharpness of the threshold value with respect to the applied voltage which is significantly inferior to that of the liquid crystal layer used in the spatial light modulator according to the present invention, so that a large contrast cannot be obtained. In the present invention, the rotation angle of the liquid crystal layer is set to 18
The range is from 0 degree to 360 degrees, but when the rotation angle is 180 degrees or less, sufficient threshold steepness is not obtained, and when the rotation angle is 360 degrees or more, hysteresis occurs near the threshold value. Drive becomes difficult. Within the scope of the present invention, the threshold value shows sufficient steepness.

Including the conventional spatial light modulator using a nematic liquid crystal as described above, the present inventors have applied for a liquid crystal spatial light modulator that shows satisfactory characteristics at the present time in terms of resolution and contrast. Only the liquid crystal using the ferroelectric liquid crystal shown in Japanese Patent Laid-Open No. 02-239594 is used, and the comparison in the present invention is made between the spatial light modulator using the ferroelectric liquid crystal.

The spatial light modulator of the present invention uses a nematic liquid crystal as a modulation material. Therefore, unlike the surface-stabilized ferroelectric liquid crystal layer that has been conventionally used in a liquid crystal spatial light modulator, it does not have a layer structure, and the allowable range for the accuracy of the gap is expanded. Therefore, the substrate is a cheaper material such as soda glass, which is used in ordinary liquid crystal cells,
There is no problem even if it is made of plastic or the like, and the required specifications of surface accuracy are gentler than the values shown in the above embodiment. Further, the gap of the liquid crystal layer is larger than that of the ferroelectric liquid crystal, and the demand for the precision of the gap distribution is remarkably small. Therefore, the yield is remarkably improved, and the manufacturing cost is remarkably reduced.

Furthermore, since there is no layered structure as described above, reliability, especially impact resistance, is remarkably improved. Figure 2
[Fig. 3] is a system diagram of an optical system in which writing and reading experiments were performed. A halogen lamp is used as the writing light source 21,
The wavelength distribution was controlled through the interference filter 22a. The writing light is applied to the liquid crystal spatial light modulator 24 according to the present invention. When writing an image, a mask was placed at 25 in the figure, and an image was formed on the input surface of the spatial light modulator 24 by the lens 26. The readout light source 27 uses a halogen lamp, a diaphragm 28 for adjusting the amount of light, and an interference filter 22 for controlling the wavelength distribution.
b, through the polarizer 29, the beam splitter 30 to the output surface of the liquid crystal spatial light modulator 24 according to the present invention, at the interface between the liquid crystal layer and the hydrogenated amorphous silicon photoconductive layer while being modulated by the liquid crystal layer. It is reflected and again enters the beam splitter 30. Here, the light reflected by the beam splitter surface is detected by the analyzer 31 and observed by the CCD camera 32. Alternatively, a photograph camera is installed at the position of the CCD camera 32 to photograph the readout image. The polarizer 29 and the analyzer 31 are arranged in crossed Nicols, and the reading is set to be in the dark state when there is no writing light at the wavelength used. For observation, the image captured by the CCD 32 was displayed on the CRT. In addition, the photodetector 33 was installed and the optical response was also measured. The driving was performed using a self-made driver 34.

Here, the polarizer 29 and the analyzer 31 are arranged in a crossed nicols, in which a polarizing beam splitter is used instead of the beam splitter 30, and the optical system is simplified except for the polarizer 29 and the analyzer 31. This is so that it can be done. However, if the polarizer 29 and the analyzer 31 are used as in the present embodiment, the contrast can be further improved by optimizing the respective angles.

The light source used for writing is a halogen lamp, He-Ne laser, Ar laser, semiconductor laser, LED, etc., and the light source used for reading light is a halogen lamp, He-Ne laser, Ar laser, semiconductor laser. , LEDs, etc.

It is possible to adjust sensitivity and gamma characteristics by providing a bias light source separately from the writing and reading light sources and applying bias light to the device. Writing experiments were performed using the above system.

When a rectangular wave of 1 kHz and 10 Vp-p is applied as a driving voltage and an input image (slide film) having gradation is placed on the mask 25 for writing and reading, reproduction with sufficient gradation is performed. The image was obtained. The frequency of the applied electric field is 10 Hz depending on the settings such as the thickness of each layer of the element.
It is set in the range of up to 100 kHz.

Here, in the driving method of this embodiment, since the applied voltage having no DC component is used, there is no fear that the liquid crystal is deteriorated by the DC electric field, and the ferroelectric liquid crystal which requires the application of the DC bias electric field. The reliability of the device is improved compared to the gradation expression using. Further, there is no inconvenience such as image sticking due to the deformation of the layer structure which is seen in the spatial light modulator using the ferroelectric liquid crystal, and the reliability is improved also in this respect.

[0022]

As described above, according to the method of the present invention, in the spatial light modulator using liquid crystal and the driving method thereof, the liquid crystal composition used as the modulating material is 180 °
A composition that takes a nematic phase at a room temperature having a rotation angle of 360 ° and is driven by an alternating electric field makes it possible to express continuous gradation in a liquid crystal spatial light modulator.
Liquid crystal spatial light modulators may be used in applications that require continuous gradation such as projection displays, and furthermore, image sticking due to deformation of the layer structure that occurs when ferroelectric liquid crystals are used does not occur. The impact resistance can also be improved and its application range can be increased.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the structure of a photo-writing type liquid crystal spatial light modulator according to the present invention.

FIG. 2 is a system diagram of an optical system in which writing and reading experiments are performed.

[Explanation of symbols]

 11a, 11b Bright substrate 12a, 12b Bright electrode 13a, 13b Directive layer 14 Crystal layer 15 Conductive film 16 Electroelectric multilayer film mirror 21 Writing light source 22a, 22b Interference filter 24 Liquid crystal spatial light modulator 25 Mask 26 Lens 27 Reading light source 28 Aperture 29 Polarizer 30 Beam splitter 31 Analyzer 32 CCD camera 33 Photodetector 34 Driver

Claims (9)

[Claims] 1. A glass substrate on which a photoconductive film is formed on a transparent electrode and a glass substrate on which a transparent electrode is formed, each of which is provided with a light writing device, a light reading device, and a voltage applying device. In a liquid crystal spatial light modulator in which a pair of glass substrates having a liquid crystal alignment film formed on the surface thereof are arranged to face each other, and a liquid crystal composition is sealed in the gap,
The photoconductive film has a thickness of 0.5 to 10 μm, the liquid crystal layer has a thickness of 1 to 10 μm, and the liquid crystal composition is a liquid crystal composition exhibiting a nematic phase at room temperature. A liquid crystal spatial light modulator, characterized in that the orientation directions of the surfaces of the substrates facing each other in the sandwiched space have an angle in the range of 180 ° to 360 °. 2. The liquid crystal spatial light modulator according to claim 1, wherein the liquid crystal composition has a refractive index anisotropy at room temperature of more than 0.05. 3. The photoconductive film has a total film thickness of 1 to 1.
The liquid crystal spatial light modulator according to claim 1, wherein the spatial light modulator is in a range of 0 μm. 4. The liquid crystal spatial light modulator according to claim 1, wherein the photoconductive film is a hydrogenated amorphous silicon film. 5. The liquid crystal spatial light modulator according to claim 1, wherein a mirror layer is formed between the photoconductive film and the liquid crystal alignment layer. 6. The applied drive voltage is 10 Hz to 10 Hz.
2. The method of driving a liquid crystal spatial light modulator according to claim 1, wherein the alternating electric field has a frequency range of 0 kHz. 7. The method of driving a liquid crystal spatial light modulator according to claim 6, wherein the applied drive voltage is a rectangular wave or a sine wave. 8. The method of driving a liquid crystal spatial light modulator according to claim 6, wherein the liquid crystal spatial light modulator is arranged in a reading optical system set in a crossed Nicols. 9. The liquid crystal spatial light according to claim 6, wherein bias light is irradiated from a writing side or a reading side in addition to a light source used for writing and reading light, and the intensity of the bias light is controlled. Driving method of modulator.