JPH0792484A - Driving method for liquid crystal spatial optical modulation element - Google Patents

Abstract

PURPOSE:To vary the range of the intensity of a write light beam capable of effectively modulating or the range of the contrast of the intensity of the write light beam by varying a frequency of an AC voltage applied between a pair of electrodes as necessary. CONSTITUTION:When the AC voltage is applied to transparent electrodes 2a, 2b beforehand, a specifc resistance of a photoconductive film 4 is reduced in a part irradiated with the write light beam 7, and an applying voltage to a nematic liquid crystal layer 5 is increased, and when a threshold value of a liquid crystal is exceeded, the liquid crystal oriented along the surfaces of a glass substrate 1b and a dielectric mirror layer 10 is faced in the direction of the applying voltage. On the other hand, in the part irradiated with no write beam 7, since the liquid crystal is oriented along the surfaces of the glass substrate 1b and the dielectric mirror layer 10, no read beam 8a is phase-modulated, and becomes a reflection read light beam 8b. Then, in such a case, the frequency applied to the transparent electrodes 2a, 2b is varied for corresponding to the range of the intensity of the objective write light beam as necessary, and the liquid crystal spatial optical modulation element is driven.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of driving a liquid crystal spatial light modulator applied to an image display device, an image processing device, an optical information processing system, and the like, and particularly to a writing light sensitivity or an intermediate output light. The present invention relates to a driving method that makes it possible to change the gradation display characteristics as needed.

[0002]

2. Description of the Related Art The function of a liquid crystal spatial light modulation element is to write a two-dimensional pattern such as an image into a liquid crystal spatial light modulation element with writing light and read the written two-dimensional pattern with reading light. is there. Thereby, processing such as amplification of image light, threshold value processing, inversion, incoherent / coherent conversion between reading light and writing light, wavelength conversion, and the like can be performed.

In the conventional driving of the liquid crystal spatial light modulator, the difference between the voltage V applied to the liquid crystal spatial light modulator and the voltage VLC applied to the liquid crystal layer, VLC / V, is the same as when the writing light is not irradiated. It was performed with an AC voltage of a specific frequency that becomes the largest between time and time (Applied Optic
s, vol. 26, p. 241, 1987).

When the liquid crystal spatial light modulating element is driven by an AC voltage having a fixed frequency, a change in writing light intensity in the range of 1 to 2 digits is applied to the liquid crystal spatial light modulating element by the voltage V.
The ratio of the voltage VLC applied to the liquid crystal layer can be effectively changed, and the change of VLC modulates the read light. The modulation characteristic at this time is determined from the modulation characteristic of the liquid crystal layer with respect to the change of VLC.

In order to effectively modulate the intermediate state in the liquid crystal layer in the target writing light intensity range, the ratio of the voltage at which the modulation saturates to the voltage at which the modulation starts and the writing light intensity are It is desirable that the ratio of VLC / V at the maximum and VLC / V at the minimum writing light intensity be close to each other. The ratio of VLC / V when the writing light intensity is maximum and VLC / V when the writing light intensity is minimum can be changed by changing the thickness of the photoconductive layer relative to the liquid crystal layer. .

For this reason, VLC / V when the writing light intensity is maximum and VLC / V when the writing light intensity is minimum.
The thickness of the photoconductive layer is set so that the ratio with V matches the modulation characteristics of the liquid crystal layer with respect to changes in VLC.

[0007]

However, the liquid crystal spatial light modulator according to the above conventional technique has the following problems.

(1) When the liquid crystal spatial light modulator is driven with a fixed frequency, a change in write light intensity in the range of about two digits can effectively change VLC / V. The change causes the read light to be modulated, but the write light weaker than the intensity range is not modulated at all, and the write light stronger than the intensity range is modulated, but there is no difference in modulation. Does not happen.

For example, a liquid crystal spatial light modulator is used as an infrared light
When applied to an IR scope that performs visible light conversion, in order to increase infrared sensitivity, it is necessary to set a driving frequency that corresponds to a dark infrared image so as to correspond to a bright infrared image. In order to attenuate the light, it is necessary to install a variable filter or variable squeezer and adjust it as necessary.

(2) In order to effectively modulate the intermediate state in this liquid crystal layer in the target writing light intensity range, VLC / V and writing light intensity when the writing light intensity is maximum. The thickness of the photoconductive layer can be set so that the ratio of VLC / V at the minimum can be adjusted to the modulation characteristic of the liquid crystal layer with respect to the change of VLC, but it cannot be changed after the device is manufactured. This limits the ratio of writing light intensity.

When the liquid crystal spatial light modulator is driven by an AC voltage of a single frequency, the modulation characteristic of the liquid crystal layer with respect to the writing light intensity is changed even if the thickness of the photoconductive layer is set to the optimum value. I can't do that. Therefore, in order to control the halftone of the output light, the write light intensity is corrected or the output light from the liquid crystal spatial light modulator is corrected in order to correct the modulation characteristic of the liquid crystal layer with respect to the write light intensity. Will be needed.

[0012]

In order to solve the above-mentioned problems, a photoconductor layer and a liquid crystal layer are held between a pair of electrodes, and the information written by the light irradiation to the photoconductor layer is used. In the method of driving a liquid crystal spatial light modulator that modulates the intensity, phase, or plane of polarization of light applied to the liquid crystal layer, the frequency of the AC voltage applied between the pair of electrodes is changed as necessary. Alternatively, the voltage applied between the pair of electrodes is an AC voltage in which a plurality of different frequencies are superimposed. Alternatively, the voltage applied between the pair of electrodes is an AC voltage in which a plurality of different frequencies are superposed, and the frequency or peak value is changed as needed.

[0013]

According to the present invention, a photoconductor layer and a liquid crystal layer are held between a pair of electrodes, and the light irradiated to the liquid crystal layer by the information written by the light irradiation to the photoconductor layer. Strength of
In a method of driving a liquid crystal spatial light modulator that modulates a phase or a plane of polarization, the frequency of an AC voltage applied between a pair of electrodes is changed as necessary. Alternatively, the voltage applied between the pair of electrodes is an AC voltage having a plurality of different frequencies superimposed, or the voltage applied between the pair of electrodes is an AC voltage having a plurality of different frequencies superimposed, and the frequency is It is possible to change the writing light intensity range or the writing light intensity ratio that can be effectively modulated by changing each as needed or by changing the frequency of the AC voltage applied between the pair of electrodes. Alternatively, the halftone characteristic can be changed.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

First, FIG. 1 shows an example of the structure of a liquid crystal spatial light modulator to which the driving method of the present invention is applied. FIG. 1 is a sectional view showing the structure of a liquid crystal spatial light modulator. In FIG. 1, a liquid crystal spatial light modulation element 9 is composed of two glass substrates 1a and 1b facing each other, transparent electrodes 2a and 2b made of ITO or the like formed on the glass substrates 1a and 1b, and a transparent electrode 2a, respectively. On the hydrogenated amorphous silicon (a-Si: H) film 4 which is a photoconductive film formed above, the dielectric mirror 10 formed on the photoconductive film 4, the dielectric mirror 10 and the transparent electrode 2b. The liquid crystal alignment films 3a and 3b for aligning the formed nematic liquid crystals, the nematic liquid crystal layer 5 sandwiched therebetween, and the spacer 6 for keeping the layer thickness of the liquid crystal layer 5 constant.
Incidentally, 7 is a writing light and 8 is a reading light.

The nematic liquid crystal 5 is made of E44 (Merck) and has a homogeneous orientation, and a crossed polarizer or a polarization beam splitter is used as a filter for reflected reading light, and the molecular axes of the liquid crystal are the polarization direction of the polarizer and the polarization direction of the analyzer, respectively. A HeNe laser with a wavelength of 633 nm was used as the writing light source and the reading light source. The thickness of the liquid crystal layer is such that when the voltage applied to the liquid crystal layer is less than or equal to the threshold value, the amount of light passing through the analyzer is
In order to obtain (dark state), the polarization component in the alignment direction of the liquid crystal,
The phase difference with the polarization component in the direction orthogonal thereto is set to be an even multiple of π. In this embodiment, the wavelength is 633 nm.
Since the reading light is used, the liquid crystal layer thickness is 2.5 μm.
And the phase difference is 4π.

In the liquid crystal spatial light modulator having the above structure, when an alternating voltage is applied to the transparent electrode 1a and the transparent electrode 1b in advance, in the portion irradiated with the writing light 7,
The specific resistance of the photoconductive film 10 decreases, and the voltage applied to the nematic liquid crystal layer 5 increases accordingly. When this exceeds the liquid crystal threshold value, the liquid crystal oriented along the surfaces of the glass substrate 1b and the dielectric mirror layer 10 is oriented in the direction of the applied voltage. As a result, in the portion irradiated with the writing light 7, the reading light 8a undergoes phase modulation, the above-mentioned phase difference of the reflected reading light 8b becomes smaller, and when this phase difference becomes an odd multiple of π, The amount of transmitted light of the analyzer becomes maximum.

On the other hand, in the portion where the writing light 7 is not irradiated, the liquid crystal is oriented along the surfaces of the glass substrate 1b and the dielectric mirror layer 10, so that the reading light 8a is not subjected to phase modulation, and the reading light 8a is reflected. 8b. Therefore, the reflected read light cannot pass through the analyzer. That is, a pattern corresponding to the writing pattern can be read by reading through the analyzer.

FIG. 2 shows changes in the output light intensity with respect to changes in the writing light intensity when the frequencies applied to the transparent electrodes 2a and 2b are 100 Hz, 1 kHz and 10 kHz. 2 (a) to 2 (c) respectively show cases of applied frequencies of 100 Hz, 1 kHz and 10 kHz. As shown in FIG. 2, as the applied frequency is increased, the output light intensity rises and the writing light intensity increases. As a result, the liquid crystal spatial light modulation element is driven by changing the applied frequency as necessary in order to correspond to the intensity range of the target writing light, so that a wide range can be obtained without using a variable filter or variable aperture. It can be seen that the intermediate state of the modulation of the liquid crystal layer can be controlled for the writing light in the intensity range.

The modulation characteristics of the liquid crystal layer have the modulation characteristics shown in FIG. 3 due to the threshold characteristics of the change in the orientation of the liquid crystal. Here, the maximum value of the output light intensity between 3 and 4 V corresponds to the above-mentioned phase difference of 3π, and the maximum value of 4 to 4
The minimum value between 5V corresponds to this phase difference of 2π. In order to effectively modulate the intermediate state in the liquid crystal layer with the writing light in the target intensity range, the ratio of the voltage at which the modulation saturates to the voltage at which the modulation starts and the writing light intensity is maximum. It is desirable that the ratio between VLC / V of VLC / V and VLC / V when the writing light intensity is minimum be close to each other. The ratio of VLC / V when the writing light intensity is maximum and VLC / V when the writing light intensity is minimum is changed by changing the thickness of the photoconductive layer relative to the liquid crystal layer and the mirror layer. In this embodiment, the liquid crystal layer has a thickness of 2.5 μm, the mirror layer has a thickness of 0.4 μm, and the photoconductive layer has a thickness of 4.5 μm.

100 Hz and 1 k for the transparent electrodes 2a and 2b
When an AC voltage having frequencies of 100 Hz, 100 Hz, 1 kHz and 10 kHz superimposed is applied, the output light intensity changes as shown in FIG. 4 with respect to the change of the writing light intensity. FIG. 4 (a) shows a case where an AC voltage in which frequencies of 100 Hz and 1 kHz are superimposed is applied, and FIG. 4 (b) shows 100 H.
The case where an alternating voltage in which the frequencies z, 1 kHz and 10 kHz are superimposed is applied is shown. As shown in FIG. 4, it can be seen that the intensity ratio of the writing light that causes a change in the output light intensity is increased by applying the AC voltage in which a plurality of different frequencies are superimposed. The intensity ratio of the writing light caused by the change of the output light intensity can be arbitrarily widened by selecting the frequency to be applied in a superimposed manner.

Further, FIG. 5 shows 100 Hz, 1 kHz and 1
The change in output light intensity due to the change in write light intensity when the crest value of the applied voltage of 0 kHz is equal and when the crest value ratio is 1: 2: 4 is shown. FIG. 5A shows the case where the peak values are equal, and FIG. 5B shows the case where the peak value ratio is 1: 2: 4. As shown in FIG. 5, the modulation characteristics of the liquid crystal layer can be changed by changing the ratio of the respective crest values of the alternating voltage applied in superposition. Like this
It is possible to change the modulation characteristic of the liquid crystal layer with respect to the writing light intensity by changing the ratio of the respective crest values of the AC voltage that is superimposed and applied.

[0023]

As described above, according to the present invention, the photoconductor layer and the liquid crystal layer are held between the pair of electrodes, and the information written by the light irradiation to the photoconductor layer is used to store the photoconductor layer and the liquid crystal layer. In the method of driving a liquid crystal spatial light modulator that modulates the intensity, phase, or plane of polarization of light applied to the liquid crystal layer, the frequency of the AC voltage applied between the pair of electrodes is changed as necessary, or A plurality of different frequencies of the voltage applied between the pair of electrodes is an alternating voltage superposed, or the voltage applied between the pair of electrodes is an alternating voltage of a plurality of different frequencies, the frequency or peak value The range of the intensity of the writing light or the range of the contrast of the writing light intensity that can be effectively modulated can be changed by changing each as necessary, or the halftone reproducibility It is possible to improve.

[Brief description of drawings]

FIG. 1 is a sectional view showing a structure of a liquid crystal spatial light modulator to which the present invention is applied.

FIG. 2 is a graph showing changes in output light intensity with respect to writing light intensity when the driving frequency is changed.

FIG. 3 is a graph showing a modulation characteristic of liquid crystal.

FIG. 4 is a graph showing changes in output light intensity with respect to writing light intensity when frequencies are superimposed and applied.

FIG. 5 is a graph showing changes in output light intensity with respect to writing light intensity when the frequencies of different peak values of voltages are superimposed and applied.

[Explanation of symbols]

 1a, 1b Glass substrates 2a, 2b Transparent electrodes 3a, 3b Liquid crystal alignment film 4 Hydrogenated amorphous silicon (a-Si: H) film 5 Nematic liquid crystal layer 6 Spacer 7 Write light 8a, 8b Read light 9 Liquid crystal spatial light modulator 10 Dielectric mirror

Claims (3)

[Claims] 1. A photoconductor layer and a liquid crystal layer are sandwiched between a pair of electrodes, and the intensity, phase, or phase of light radiated to the liquid crystal layer according to information written by irradiating the photoconductor layer with light. A method of driving a liquid crystal spatial light modulator that modulates a plane of polarization, wherein the frequency of an AC voltage applied between the pair of electrodes is changed. 2. A photoconductor layer and a liquid crystal layer are sandwiched between a pair of electrodes, and the intensity, phase, or phase of light radiated to the liquid crystal layer according to information written by the light irradiation to the photoconductor layer. A method of driving a liquid crystal spatial light modulator for modulating a plane of polarization, wherein the voltage applied between the pair of electrodes is an AC voltage obtained by superposing a plurality of different frequencies. . 3. A photoconductor layer and a liquid crystal layer are sandwiched between a pair of electrodes, and the intensity, phase, or phase of light irradiated to the liquid crystal layer according to information written by the light irradiation to the photoconductor layer. In a method of driving a liquid crystal spatial light modulator that modulates a plane of polarization, the voltage applied between the pair of electrodes is an AC voltage in which a plurality of different frequencies are superimposed, and the frequency or the peak value is changed. And a method for driving a liquid crystal spatial light modulator.