JP3643658B2 - Optical writing reflective spatial light modulator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical writing reflection type spatial light modulator used for a projection type liquid crystal display device or the like.
[0002]
[Prior art]
The optical writing reflection type spatial light modulator is widely used as a display device of a projection type liquid crystal display device such as a projector. Such a spatial light modulator has a laminated structure in which a photoconductive layer, a light shielding layer, a dielectric mirror, and a light modulation layer are sequentially stacked between two transparent electrodes. By projecting an image onto the photoconductive layer with writing light, a voltage corresponding to the contrast of the image is applied to the light modulation layer, and the readout light is modulated and projected to the outside to obtain an enlarged image.
[0003]
Here, the dielectric mirror and the light shielding layer have a role of preventing the reading light from entering the photoconductive layer side and preventing the resolution of the enlarged image to be projected from deteriorating. Furthermore, the dielectric mirror also has a role of reflecting the projection light obtained by modulating the readout light by the light modulation layer and projecting it outside. As such a dielectric mirror, one formed by laminating TiO ₂ and SiO ₂ in a multilayer film is widely used.
[0004]
However, it has been difficult to completely reflect broadband light such as that used as a light source for readout light of a single-plate color projector, with these multilayer dielectric mirrors. For example, as shown in FIG. 8, a mirror having a reflectance of nearly 100% could be produced in a narrow wavelength range of about 500 to 600 nm (hereinafter referred to as Conventional Example 1). However, when trying to achieve a high reflectance over a wide wavelength range of about 400 to 700 nm, only a reflectance of about 95% was obtained as shown in FIG. 9 (hereinafter referred to as Conventional Example 2). For this reason, the readout light leaks to the photoconductive layer side, which has been a factor of sensitivity and resolution degradation. In particular, single-plate color projectors use a high-output light source for readout light, so even if the percentage of light leaking to the photoconductive layer is small, the amount of light leaking is greater than that of writing light, so measures against leakage light are important. It was a difficult task.
[0005]
On the other hand, if the number of layers of the multilayer film is increased, the reflectance is improved. However, the electric resistance of the dielectric mirror increases, the voltage of the light modulation layer decreases, and the sensitivity decreases. Therefore, the number of layers and the film thickness of the multilayer film cannot be significantly increased. For the same reason, it is also difficult to increase the thickness of the light shielding layer and absorb the reading light transmitted through the dielectric mirror to block the leaked light to the photoconductive layer side.
[0006]
As an apparatus for solving such a problem, the invention of Japanese Patent Laid-Open No. 7-294958 is known. This realizes a high reflectance without changing the thickness of the layer by changing the laminated material to ZnS and MgF2.
[0007]
[Problems to be solved by the invention]
However, with this device, the internal stress of MgF2 is large, so distortion and peeling are likely to occur during film formation, and high-quality multilayer mirrors can be generated as in the case of using conventional TiO ₂ or SiO ₂ as a laminated material. There was a problem that it was difficult to do.
[0008]
Accordingly, an object of the present invention is to provide a high-sensitivity optically writing reflective spatial light modulator having a high-quality mirror that is easy to manufacture and has no distortion.
[0009]
[Means for Solving the Problems]
The optical writing reflective spatial light modulator according to the present invention includes (1) a first transparent electrode layer on which writing light is incident, and (2) a layer laminated on the transparent electrode layer, and impedance depending on the writing light. (3) a light-shielding layer that is laminated on the light-shielding layer and blocks light that has arrived, and ( 4 ) a light-shielding layer that is laminated on the light-conducting layer and has a low reflectivity for wavelength regions with a low light-shielding rate . A dielectric mirror having a reflection characteristic higher than the reflectance for the wavelength region and reflecting the reading light incident from the opposite surface to the writing light in the incident direction; and ( 5 ) a liquid crystal layered thereon to modulate the reading light. And ( 6 ) a second transparent electrode layer that is stacked on the light modulation layer and on which reading light is incident.
[0010]
The dielectric mirror used in the present invention may be any dielectric mirror that selectively reflects readout light in a wavelength region where the light shielding layer has a low light shielding rate. Therefore, a mirror having high reflectance and no distortion can be easily applied. And reading light in this wavelength region can be prevented from entering the photoconductive layer.
[0016]
As described above, since the wavelength region where the reflectance is high is limited, the film thickness and the number of films of the dielectric mirror can be reduced as compared with the case where a dielectric mirror having a high reflectance comparable to that in the entire region is created. Can be thinned. Accordingly, the leakage of the readout light to the photoconductive layer can be effectively suppressed by the thin dielectric mirror having a small electric resistance. If TiO2 or SiO2 is used as the laminated material, a high-quality multilayer mirror can be easily produced as in the conventional case.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail. FIG. 1 is a cross-sectional structural view of the present embodiment, and FIG. 2 is a block diagram of a single-plate color projector using the present embodiment.
[0018]
First, the configuration of a single-plate color projector will be described with reference to FIG. The writing light that is the basis of the image displayed on the screen 113 is generated by the image writing CRT 104. The optical path of the writing light and the optical path of the projection light for projecting the image on the screen 113 are arranged on the same straight line in the opposite direction. On the other hand, the optical path of the readout light serving as the light source of the projection light is orthogonal to this, and the beam splitter 110 is placed at a position where the two optical paths intersect. A 250 W metal halide lamp is used as the light source 104 for reading light. A collimating lens 105, a UV filter 106, and an IR filter 107 are arranged in the optical path of the readout light from the light source 104 side. Here, the wavelength of the readout light after passing through the IR filter is 400 to 700 nm and has the spectral distribution shown in FIG. As is clear from FIG. 3, the intensity around the wavelength of 555 nm is extremely strong. In the optical path of the readout light, a color filter 108 that temporally decomposes the readout light into three primary colors of red, blue, and green and a polarizing plate 109 that converts the readout light into linearly polarized light are provided between the beam splitter 110 and the optical path. Yes.
[0019]
On the other hand, the light source of the writing light is the triple scan type image writing CRT 103, which outputs an image corresponding to the primary color of the separated reading light. The image writing CRT 103 is connected to the spatial light modulator 101 via the optical fiber plate 102. On the optical path of the projection light emitted from the spatial light modulator 101, a beam splitter 110, a polarizing plate 111 that extracts image information in the projection light, and a projection lens 112 that enlarges and projects an image are provided. Is connected to a screen 113 for use.
[0020]
Next, a detailed configuration of the spatial light modulator 101 will be described with reference to FIG.
[0021]
An antireflection film 22 for the writing light is formed on the incident surface of the glass substrate 20 on which the writing light is incident, and a transparent electrode layer 10 that transmits the writing light is provided on the surface opposite to the incident surface. ing. A photoconductive layer 1 is further formed on the transparent electrode layer. The photoconductive layer 1 is preferably an amorphous silicon layer. The internal impedance of the photoconductive layer 1 changes according to the incident writing light. In this embodiment, the amount of change in impedance of amorphous silicon constituting the photoconductive layer 1 varies depending on the wavelength of incident light, and the impedance is minimized when light having a wavelength of about 700 nm is incident even if the amount of light is the same. Therefore, the light amount of writing light (π modulation required light amount) necessary for maximizing the light amount of projection light, which will be described in detail later, becomes smaller on the long wavelength side as shown in FIG. That is, the sensitivity of the output of the spatial light modulator 101 to the writing light is increased on the long wavelength side.
[0022]
On the photoconductive layer 1, there is formed a light shielding layer 2 that blocks light incident on the photoconductive layer 1 from a surface opposite to the writing light. The light shielding layer 2 is preferably CdTe having a high light shielding rate, that is, a low transmittance. Here, FIG. 5 is a diagram showing the transmission characteristics of the light shielding layer 2, and it can be seen that light on the long wavelength side is easily transmitted. A dielectric mirror layer 3 made of a multilayer film is formed on the light shielding layer 2. The dielectric mirror layer 3 is preferably a laminate of TiO ₂ and SiO ₂ because a good quality multilayer film can be easily formed. A preferred example of the dielectric mirror layer 3 is a multilayer mirror having the reflection characteristics and configuration shown in FIG. 6 (hereinafter referred to as Example 1). On the other hand, FIG. 9 shows the reflection characteristics and configuration of the dielectric mirror layer of Conventional Example 2 in which the broadband reflectance is increased. In Conventional Example 2, the reflectance for light having a wavelength of 400 to 700 nm is substantially uniform. However, in Example 1 shown in FIG. 6, in the vicinity of 550 nm where the reading light intensity is strong, the transmittance of the light-shielding layer 2 is high, and the spatial light. The characteristic is that the reflectivity for a long wavelength region where the sensitivity of the modulator is high is selectively increased to about 98% compared to about 95% of the conventional example.
[0023]
Here, a multilayer laminated mirror having desired reflection characteristics can be obtained by laminating films whose optical length (thickness × refractive index) is equal to a quarter wavelength of light to be reflected. Therefore, the thicker the dielectric mirror layer 3 is, the higher the reflectivity can be. However, when the thickness is increased, the impedance is increased and the sensitivity of the spatial light modulator 101 is deteriorated. Therefore, the thickness is preferably 3 μm or less. In addition, as the number of stacked films increases, the wavelength region with high reflectivity can be expanded. However, increasing the number of films increases the thickness and complicates the process. preferable.
[0024]
On the other hand, a glass substrate 21 having an antireflection film 23 for the reading light is provided on the surface on which the reading light of the spatial light modulator 101 is incident, and the reading light is transmitted to the surface opposite to the incident surface of the glass substrate. A second transparent electrode layer 11 is formed.
[0025]
On the dielectric mirror layer 3 and the second transparent electrode layer 11, a liquid crystal alignment layer 4C that keeps the liquid crystal alignment direction constant is formed, and each liquid crystal alignment layer 4C is interposed via a frame-shaped spacer 4B. The liquid crystal layer 4A is formed by filling the inside with liquid crystal, and the light modulation layer 4 is formed. The liquid crystal layer 4A in this embodiment uses parallel alignment liquid crystal. The modulation method of the light modulation layer 4 is preferably a birefringence control type (ECB mode) that can easily perform multicolor display. In this ECB mode, when a voltage is applied to the light modulation layer, the alignment of the liquid crystal is tilted according to the voltage, and as a result, the optical path length of the light incident on the light modulation layer changes. The polarization of the incident light changes according to the change in the optical path length. Then, when the optical path length is shifted by a half wavelength, the polarization direction of the linearly polarized incident light is rotated by 90 ° (π modulation). At this time, the amount of projection light passing through the polarizing plate 111 is maximized.
[0026]
Next, the operation of this embodiment will be described with reference to FIGS. First, an outline will be described with reference to FIG. By the triple scan type CRT 103, the bright and dark images corresponding to the three primary colors are guided as writing light to the spatial light modulator 101 of the present embodiment via the optical fiber plate.
[0027]
On the other hand, the white readout light emitted from the light source 104 is adjusted to parallel rays by the collimator lens 105, and then the UV filter 106 and the IR filter 107 are used to remove light having wavelengths in the ultraviolet region and infrared region. The visible light is adjusted to 400 to 700 nm. Thereafter, the color shutter 108 splits the three primary color lights of red, blue and green in terms of time. Further, the reading light linearly polarized by the polarizing plate 109 is bent at a right angle by the beam splitter 110 and enters the spatial light modulator 101.
[0028]
In the spatial light modulator 101, read light, that is, projection light in which image information is written by an operation described in detail later is output from the spatial light modulator 101, passes straight through the beam splitter 110, and travels straight. Only the image information included in the projection light is extracted by 111 and projected onto the screen 113 through the projection lens 112 to display an enlarged image.
[0029]
Next, the internal operation of the spatial light modulator 101 will be described in detail with reference to FIG. A constant bias voltage is always applied between the transparent electrode layers 10 and 11 of the spatial light modulator 101. When an image is projected into the photoconductive layer 1 by writing light incident from the transparent electrode layer 10 side, the impedance of the photoconductive layer 1 locally changes according to the brightness of the image. An electric field is applied to the liquid crystal layer 4A corresponding to the portion where the impedance is reduced. As a result, the inclination of the alignment of the liquid crystal changes, and an image is written in the liquid crystal layer 4A according to the image information included in the writing light. On the other hand, most of the reading light incident from the transparent electrode layer 11 side is reflected by the dielectric mirror layer 3 through the liquid crystal layer 4A, passes through the liquid crystal layer 4A again, and corresponds to the image written in the liquid crystal layer 4A. Then, it is polarized and goes out of the spatial light modulator 101 as projection light in which image information is written.
[0030]
Here, since light absorption is almost negligible in the dielectric mirror layer 3, the amount of light that is not reflected among the reached read light passes through the dielectric mirror layer 3. A part of the transmitted light is mostly absorbed by the light shielding layer 2 ahead, but a part of the light is transmitted through the light shielding layer 2 and leaks to the photoconductive layer 1 side to impede the impedance of the photoconductive layer 1. Reduce. This becomes noise and causes the sensitivity and resolution degradation of the spatial light modulator.
[0031]
FIG. 10 shows the intensity of leakage light when the dielectric mirror 3 of Example 1 having the reflection characteristics shown in FIG. 6 and the dielectric mirror 3 of the conventional example 2 having the reflection characteristics shown in FIG. 9 are used. The comparison results are shown. The configuration of the spatial light modulator 101 excluding the dielectric mirror 3 is the same for both. In the conventional example 2 shown by the solid line in FIG. 10, the reflectivity of the dielectric mirror layer 3 is almost uniform, so that the read light intensity is high near 550 nm and the transmittance of the light shielding layer 3 is high in a long wavelength region of about 650 nm or more. The intensity of leaked light has increased. As is clear from the figure, the amount of leakage light in the long wavelength region where the sensitivity of the photoconductive layer 1 is high is large. On the other hand, in Example 1 indicated by the broken line, the reflectance of the dielectric mirror in the vicinity of 550 nm and in the long wavelength region is improved as described above, so that the leakage light intensity is low overall. The leakage light in the long wavelength region having a high sensitivity of 3 was suppressed to about half compared to the conventional example 2. For this reason, the impedance change of the photoconductive layer 1 caused by the leaked light and the noise of the spatial light modulator resulting therefrom are also suppressed to be lower in the first embodiment than in the conventional example.
[0032]
The dielectric mirror 3 may be a multilayer mirror having reflection characteristics and configuration as shown in FIG. 7 (hereinafter referred to as Example 2). In the dielectric mirror 3 of Example 2, the reflectance with respect to light on the long wavelength side of 650 to 700 nm reaches about 98% as compared with about 95% of Conventional Example 2. FIG. 11 shows a result of comparing the wavelength distribution of the leakage light intensity when the dielectric mirror according to Example 2 is used and when the dielectric mirror according to Conventional Example 2 is used. From the figure, in the case of Example 2 indicated by a broken line, the leakage light intensity was suppressed to half or less in a region having a wavelength longer than 570 nm as compared with Conventional Example 2 indicated by a solid line. As a result, the impedance change of the photoconductive layer 1 caused by the leaked light and the noise of the spatial light modulator output resulting therefrom can be suppressed to be lower in the second embodiment as compared with the conventional example.
[0033]
If (1) the wavelength region where the photoconductive portion has a high sensitivity, (2) the wavelength region where the reading light intensity is high, and (3) the wavelength region where the light-absorbing layer has a high absorptance are different, all of these It is most desirable to use a dielectric mirror 3 having a high reflectivity. This is because the light in the wavelength region (1) has a large impedance change compared to other wavelength regions even if the amount of leakage light is small, and noise of the spatial light modulator is likely to occur. Further, the light in the wavelength region (2) has a larger amount of leakage light than the other wavelength regions even if the ratio of the leakage light is small. In addition, when the light in the wavelength region (3) is transmitted through the dielectric mirror 3 compared to the other wavelength regions, the proportion absorbed by the light shielding layer is low, and the proportion reaching the photoconductive layer 1 is low. Because it is big. However, depending on the characteristics of the photoconductive portion, the light shielding layer, and the readout light, when a multilayer mirror having a desired reflectance is formed, the film thickness increases, and the impedance of the light shielding layer and the multilayer mirror layer increases, resulting in a space Since the sensitivity of the optical modulator may be deteriorated or the number of films may be increased, which may cause a problem in processing steps, it is preferable to employ a combination of the reflection characteristics (1) to (3) as appropriate. Alternatively, when the leakage light to the photoconductive layer can be reduced only by the multilayer mirror layer, the light shielding layer may be omitted.
[0034]
【The invention's effect】
As described above, according to the present invention, in the optical writing reflection type spatial light modulator, the reflection characteristics of the dielectric mirror and the reflectance for the wavelength region where the light-shielding layer has a high transmittance are higher than those of other wavelength regions. The reflection characteristics are adjusted. By selectively reflecting an arbitrary wavelength region of the reading light, the amount of the reading light that passes through the dielectric mirror and the light shielding layer and leaks to the photoconductive layer is effectively suppressed. As a result, unnecessary fluctuations in the impedance of the photoconductive layer caused by unwanted light incident on the photoconductive layer can be prevented, noise generated in the spatial light modulator can be prevented, and sensitivity and resolution can be improved. There is.
[0035]
Furthermore, since the wavelength range of the high reflectivity is limited compared to the case of producing a broadband, high reflectivity dielectric mirror, the thickness of the dielectric mirror is reduced and the electrical resistance of the dielectric mirror itself is reduced. it can. Further, the light shielding layer itself can be eliminated or made thin, and the electric resistance of the light shielding layer portion can be reduced. As a result, the voltage drop of the optical modulator can be prevented and high resolution can be ensured.
[Brief description of the drawings]
FIG. 1 is a cross-sectional structure diagram of an optical writing reflective spatial light modulator according to an embodiment of the present invention.
2 is a configuration diagram of a single-plate color projector using the spatial light modulator according to FIG. 1. FIG.
FIG. 3 is a spectrum distribution diagram of readout light used in the single-plate color projector according to FIG.
FIG. 4 is a π-modulation necessary light amount distribution diagram of the spatial light modulator according to FIG. 1;
FIG. 5 is a transmission characteristic diagram of the light shielding layer according to FIG. 1;
6 is a reflection characteristic diagram and a configuration of the dielectric mirror of Example 1. FIG.
7 is a reflection characteristic diagram and a configuration of a dielectric mirror of Example 2. FIG.
8 is a reflection characteristic diagram and a configuration of a dielectric mirror of Conventional Example 1. FIG.
9 is a reflection characteristic diagram and a configuration of a dielectric mirror of Conventional Example 2. FIG.
10 is a diagram comparing the intensity of readout light incident on a photoconductive layer in a spatial light modulator using the dielectric mirrors of Conventional Example 1 according to FIG. 9 and Example 1 according to FIG.
11 is a diagram comparing the intensity of readout light incident on a photoconductive layer in a spatial light modulator using the dielectric mirrors of Conventional Example 1 according to FIG. 9 and Example 2 according to FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Photoconductive layer, 2 ... Light-shielding layer, 3 ... Dielectric mirror, 4 ... Light modulation layer, 4A ... Liquid crystal layer, 4B ... Spacer, 4C ... Liquid crystal alignment layer, 10 * 11 ... Transparent electrode layer, 20 * 21 ... Glass substrate, 22.23 ... Antireflection film, 101 ... Spatial light modulator, 102 ... Optical fiber plate, 103 ... Triple scan type CRT, 104 ... Light source, 105 ... Collimator lens, 106 ... UV filter, 107 ... IR filter, 108, a color shutter, 109, 111, a polarizing plate, 110, a beam splitter, 112, a projection lens, 113, a screen.

Claims (1)

A first transparent electrode layer on which writing light is incident;
A photoconductive layer laminated on the first transparent electrode layer, the impedance of which changes according to the writing light;
A light-shielding layer that is laminated on the photoconductive layer and blocks light that has arrived;
It is laminated on the light shielding layer, and the reflectance for the wavelength region where the light shielding rate of the light shielding layer is low is higher than the reflectance for other wavelength regions, and is incident from the surface opposite to the writing light. A dielectric mirror that reflects the readout light in the direction of incidence;
A light modulation layer using a liquid crystal layered on the dielectric mirror and modulating the readout light;
A second transparent electrode layer that is stacked on the light modulation layer and on which the reading light is incident;
Optical writing reflective spatial light modulator comprising

Images