JPH0230494B2 - - Google Patents

Description

Detailed Description of the Invention (Description of the Technical Field) The present invention includes an electro-optic crystal plate enclosed in a vacuum container having an electron source, and a refractive index distribution corresponding to a charge image formed by the electron source. This invention relates to a spatial light modulation device using a spatial light modulation tube formed in an electro-optic crystal.

According to this spatial light modulation device, an image formed by incoherent light can be converted into an image by coherent light.

(Description of Prior Art) There are various types of electro-optic crystals, but wafers with a relatively large area are easy to obtain, the half-wave voltage is low, there is no photoconductivity, and the temperature at 350°C when creating the photocathode is low.
Items that can withstand a certain degree of baking include:
LiNbO ₃ crystals appear to be optimal.
A LiNbO ₃ crystal cut at 55° as shown in Figure 1 has the lowest half-wave voltage and is practical.

However, because it has natural birefringence, when a crystal is irradiated with laser light (coherent light) even when no charge is applied, the polarization state of the reflected light (output) changes, causing a bias (direct current) to the brightness of the output. ingredients) were given.

(Object of the Invention) An object of the present invention is to provide a spatial light modulation device that can solve the above-mentioned problems.

(Description of Structure of the Invention) In order to achieve the above object, the spatial light modulation device according to the present invention includes an electron source formed in a vacuum-tight container, an electric current having natural birefringence disposed opposite to the electron source, and an electron source formed in a vacuum-tight container. a spatial light modulation tube having a charge image forming means for forming a charge image on the surface of the electro-optic crystal using electrons emitted from an optical crystal and the electron source; A laser light source device that injects linearly polarized laser light into the back surface of the crystal, a polarizer that passes the laser light reflected from the electro-optic crystal and converts it into intensity information, and a crystal of the same type as the electro-optic crystal. an electro-optic crystal for bias level correction, with transparent electrodes formed between both surfaces thereof, and an electro-optic crystal for bias level correction inserted into the optical path of the laser light from the laser light source device to the polarizer; It consists of a power source that applies a voltage between the transparent electrodes of the electro-optic crystal.

(Description of Examples) Hereinafter, the present invention will be described in more detail with reference to the drawings and the like.

FIG. 2 is a block diagram showing an embodiment of the spatial light modulation device according to the present invention.

An input image 1 is illuminated with incoherent light (incandescent light) and is reflected by a lens 2 to a photocathode 3 of a spatial light modulation tube 3.
Projected to 1.

The input optical image is converted into a photoelectron image at the photocathode 31,
The converted electron image forms an electron image on the input surface of the microchannel plate 33 by the lens action of the accelerating and focusing electrode 32 .

The electron image is the microchannel plate 33
The charge image is multiplied by , and a charge image is formed on the surface of the LiNbO ₃ single crystal plate 34 cut at 55°.

Due to this surface charge, the refractive index of the crystal in the x and y' directions is given by the following equations.

In this specification, the index is indicated as A of B.
This is based on the convention that the power is expressed as AΛB.

nx≒n ₀ − (n ₀ Λ3)/2 ・Δ[1/(nxΛ2)] …(1) ny′≒ne′−(ne′Λ3)/2 ・Δ[1/(ny′Λ2)] ……(2) However, Δ[1/(nxΛ2)] =r ₂₂ Esinθ+r ₁₃ Ecosθ Δ[1/(ny′Λ2)] =−r ₂₂ Esinθcos ² θ+r ₁₃ Ecos ³ θ +r ₃₃ Esin ² θcosθ−2r ₄₂ Esin ² θcosθ where θ: 55゜n ₀ , ne′: x direction when no charge exists, respectively;
Refractive index E in the y′ direction: Electric field generated in the crystal due to the presence of charges r ₁₃ , r ₂₂ , r ₃₃ , r ₄₂ : Represents components of the electro-optic tensor.

Here, the light from the laser 4 is magnified by the objective lens 5, and after removing extra diffracted light by the pinhole 6,
Polarizer 7 sets the polarization direction to the x-axis (or y'-axis) of the crystal.
The direction is fixed at 45° from (linear polarization).

Then, the collimating lens 8 converts the light into parallel light, which passes through the half mirror 9 and enters the crystal 34 from the back side.

Since the refractive index in the x and y' directions within the crystal 34 is different, the speeds of the x-direction component and the y'-direction component of the incident light are different.

As a result, the x-direction component and the y'-direction component of the light reflected by the surface A of the crystal 34 and the y' direction component have a phase difference as shown in the following equation (3), and are generally output as elliptically polarized light.

Γ=(2π/λ)・{(ne′−n ₀ ) +(ne′Λ3)/2・Δ[1/ny′Λ2)] −(n ₀ ′Λ3)/2・Δ[1/(nxΛ2) )〕｝×2l
...(3) Here, λ: Wavelength of light output from laser 4 l: Thickness of crystal 34 If this output light passes through polarizer 11, only one polarization direction component is extracted, and screen 1
2, a coherent optical image modulated by the input image 1 is obtained.

At this time, as can be seen from equation (3), even if the surface charge of the crystal 34 is 0, Γ ₀ = (2π/λ)・(ne′−n ₀ )×2l ……(4) A phase difference occurs.

FIG. 3 is a graph showing the relationship between electric charge and brightness I of transmitted light.

This measurement was performed on screen 12.

As shown by the solid line in FIG. 3, the brightness I of the output image appears to be a brightness I' even when the charge σ is 0.

Therefore, when operating, it is necessary to start from a state in which a uniform charge σ ₀ (charge bias) is applied.

Furthermore, the level of I' also depends on the crystal thickness, as seen from equation (4), and therefore varies depending on the crystal. Therefore, it is necessary to set the charge σ ₀ for each crystal so that the brightness I is 0.

In this invention, in order to make the output 0 when the charge is 0, the path of the light reflected by the half mirror 9,
An electro-optic crystal 10 for bias correction is inserted into the optical path of the coherent optical system.

This electro-optic crystal 10 for bias correction is
It is an electro-optic crystal of LiNbO ₃ cut at 55°, and transparent conductive films 10a and 10b of In[ _1-X ]SnxO ₃ are uniformly deposited on both sides by high-frequency sputtering in order to apply a voltage.

A DC voltage is applied by a power source 20 between the transparent electrical films 10a and 10b.

Further, the axial direction of the crystal 10 is arranged in the same manner as the crystal 34.

Then, the voltage of the power source 20 is adjusted so that the output light level becomes the darkest level when no charge is present on the surface of the electro-optic crystal 34 in the spatial light modulation tube 3.

In FIG. 3, the combined characteristics of the electro-optic crystal 34 of the spatial light modulation tube 3 and the electro-optic crystal 10 for bias correction are shown by dotted lines.

The bias-corrected light containing image information that has passed through the electro-optic crystal 10 for bias correction is converted by the polarizer 11 into light containing information about the intensity distribution of charges on the surface of the electro-optic crystal 34 of the spatial light modulation tube 3. and is projected onto the screen 12.

When performing optical calculations, the screen 12 is not used, and coherent optical processing is performed by an optical system (not shown).

In the embodiment described above, the electro-optic crystal 10 for bias level correction is placed immediately before the polarizer 11, but the same effect can be obtained even if it is placed at another position.

As shown in FIG. 4, an electro-optic crystal 10 for bias level correction may be placed within the spatial light modulation tube 3 between the electro-optic crystal 34 and the reading laser beam entrance surface 35 of the spatial light modulation tube. .

Further, as shown in FIG. 5, a crystal 10 can also be used as the incident surface of the reading laser beam of the spatial light modulation tube 3.

In the device of the embodiment, the polarizer 7 is arranged in the laser light source device to obtain linearly polarized light, but the polarizer 7 is unnecessary when the laser oscillation device itself can output linearly polarized light. It is. In the above embodiment, a photocathode is used as the electron source, but the present invention can be similarly applied to a type of writing using an electron gun as the electron source.

(Effects of the Invention) As described above, according to the present invention, when a crystal having natural birefringence is used in a spatial light modulation tube, the DC light bias due to the natural birefringence can be applied to the crystal by arranging the same type of crystal. This can be minimized by simply applying a voltage.

Therefore, it is possible to obtain a coherent optical image that accurately corresponds to the charge on the surface of the electro-optic crystal within the spatial light modulation tube.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram for explaining a cut mode of an electro-optic crystal used in a spatial light modulator according to the present invention. FIG. 2 is a block diagram showing an embodiment of the spatial light modulation device according to the present invention. FIG. 3 is a graph for explaining the characteristics of an electro-optic crystal. FIGS. 4 and 5 are cross-sectional views each illustrating a modification of the arrangement of the electro-optic crystal for bias correction. 1... Input image, 2... Laser light, 3... Spatial light modulation tube, 31... Photocathode, 32... Accelerating and focusing electrode, 33... Microchannel plate, 34
...Electro-optic crystal, 35... Reading laser beam incidence surface, 4... Laser oscillator, 5... Lens, 6...
...Pinhole plate, 7...Polarizer for linearly polarized light, 8
... Collimating lens, 9 ... Half mirror, 10 ... Electro-optic crystal for bias correction, 10
a, 10b...Transparent conductive film, 11...Polarizer for obtaining intensity modulation, 12...Screen (output surface), 20...Power source for electro-optic crystal for bias correction.

Claims (1)

[Scope of Claims] 1. An electron source formed in a vacuum-tight container, an electro-optic crystal with natural birefringence placed opposite to the electron source, and electrons emitted from the electron source to convert the charge image into the electron beam. a spatial light modulation tube having a charge image forming means for forming a charge image on the surface of the optical crystal; and a laser light source device for inputting linearly polarized laser light onto the back surface of the electro-optic crystal from outside the vacuum-tight container of the spatial light modulation tube. , a polarizer that passes laser light reflected from the electro-optic crystal and converts it into intensity information; and a transparent electrode made of the same type of crystal as the electro-optic crystal, and a transparent electrode is formed between both surfaces thereof, and the laser light source device an electro-optic crystal for bias level correction inserted into an optical path through which a laser beam from the laser beam reaches the polarizer; and a power source for applying a voltage between the transparent electrodes of the electro-optic crystal for bias level correction. Spatial light modulator. 2. The spatial light modulator according to claim 1, wherein the electro-optic crystal is a 55° cut LiNbO ₃ crystal. 3. The spatial light according to claim 1, wherein the electro-optic crystal for bias level correction is disposed immediately before a polarizer that passes laser light reflected from the electro-optic crystal and converts it into intensity information. Modulator. 4. The spatial light modulation device according to claim 1, wherein the electro-optic crystal for bias level correction is disposed inside the spatial light modulation tube. 5. The spatial light modulation device according to claim 1, wherein the electro-optic crystal for bias level correction is arranged as a window into which a laser beam of a laser light source device of the spatial light modulation tube is incident.