JPH0422483B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention provides an electro-optic crystal that is arranged opposite to an electron source formed in a vacuum container, and that emits electrons from the electron source onto the surface of the crystal. accumulate and cause the crystal to undergo a change in refractive index corresponding to the accumulated charge;
The present invention relates to a spatial light modulation tube whose refractive index changes are read out using a laser, and particularly relates to an improvement of an electro-optic crystal section for improving the resolution of such a spatial light modulation tube.

(Prior Art) First, the basic configuration of a spatial light modulation tube will be briefly explained along with its operation.

FIG. 4 is a schematic diagram showing the basic configuration of the spatial light modulation tube.

Photocathode 4 on the inner surface of the glass container 3 of the spatial light modulation tube
An image from an input pattern 1 illuminated with incoherent light is made incident through a lens 2.
At this time, the photocathode 4 emits photoelectrons corresponding to the incident image. The photoelectrons are made incident on a microchannel plate 6 via an accelerating/focusing electron lens system 5, and are multiplied several thousand times.

These multiplied electrons are accumulated on the surface of an electro-optic crystal 8 made of LiNbO ₃ or the like on which a transparent electrode 8a is formed, and change the refractive index of this crystal 8 in accordance with the charge image.

When the electro-optic crystal 8 is irradiated with laser light from the laser light source 10 via the half mirror 9, a laser light image 11 (coherent image) is obtained. This laser beam image 11 can be used to perform coherent parallel optical calculations.

FIG. 5 is a diagram for explaining the relationship between the thickness of the electro-optic crystal plate and the electric field.

In a spatial light modulation tube having such a configuration, when a minute point charge P is charged on the crystal surface, if the thickness of the electro-optic crystal 8 is relatively thick, as shown in FIG. The electric field spreads widely as shown by δ ₁ .

On the other hand, when the thickness of the crystal 8 is relatively thin, as shown in FIG. 5B, the electric field due to the charges has a small spread as shown by δ ₂ .

Since this electric field changes the refractive index of the electro-optic crystal 8 and modulates the light from the laser 10, it is easy to understand that the thinner the crystal 8, the better the resolution of the extracted coherent image.

Here, the resolution will be determined when the electro-optic crystal 8 is polished as thin as possible while maintaining the flatness of λ/10 and the parallelism of 5 seconds or less, and is incorporated into a spatial light modulation tube.

For example, a 55° cut with a diameter of 25 mm and a thickness of 0.3 mm.
Using a LiNbO ₃ single crystal plate, the resolution of the spatial light modulation tube was 3 line pairs/mm (50% modulation depth).

The electro-optic crystal 8 used in such a spatial light modulation tube is a crystal that is easy to obtain on a wafer with a relatively large area, has a low half-wave voltage, is not photoconductive, and is difficult to form a photocathode. In particular, it is desirable that the crystals satisfy the condition that they do not deteriorate even when baked at relatively high temperatures. Based on this condition, 55° cut LiNbO ₃ single crystal plates are often used.

(Problems to be Solved by the Invention) The resolution of conventional spatial light modulation tubes is insufficient to perform coherent parallel optical operations, and in order to improve this resolution, it is necessary to make the electro-optic crystal 8 even thinner. There is.

However, if the LiNbO ₃ crystal was made thinner than the above-mentioned thickness, the crystal would warp, making it unusable.

An object of the present invention is to provide a spatial light modulation tube with excellent resolution by using only the electro-optic characteristics of an extremely thin electro-optic crystal plate provided on a crystal substrate.

(Means for Solving the Problems) In order to achieve the above object, a spatial light modulation tube according to the present invention includes an electron source formed in a vacuum container;
In a spatial light modulation tube comprising an electro-optic crystal part that accumulates electrons emitted from the electron source and causes an optical change, the electro-optic crystal part is composed of two electro-optic crystals of the same type and different thicknesses. The plates are bonded and integrated through a transparent conductive film, and the thin crystal plate surface is arranged to face the electron source, and an electro-optic crystal plate having substantially the same thickness is placed opposite to the thick crystal plate surface and has a refractive index. It is constructed by symmetrically arranging ellipsoids.

According to the above structure, the object of the present invention can be completely achieved.

(Example) Hereinafter, the present invention will be described in detail with reference to the drawings and the like.

FIG. 1 is a schematic diagram showing an embodiment of an electro-optic crystal section used in a spatial light modulation tube according to the present invention, in which FIG. 1A shows the first embodiment, and FIG. 1B shows the second embodiment. FIG.

In the first embodiment, in order to obtain an extremely thin electro-optic crystal plate, a relatively thick single-crystal plate 33 is bonded to the crystal substrates 31 and 32 closely bonded together, and then the crystal plate 33 is polished thin. An example is shown.

The crystal substrates 31 and 32 have a diameter of 25 mm and a thickness of 5 mm.
It is a 55° cut LiNbO ₃ crystal plate.

First, the parallelism of one surface 31a and the other surface 31b of the crystal substrate 31 and the one surface 32a and the other surface 32b of the crystal substrate 32 are kept within 5 seconds, and the flatness is maintained at about λ/10. Mirror polish. Thereafter, a transparent conductive film ITO is uniformly deposited on the other surface 31b of the crystal substrate 31.

On the other hand, the single crystal plate 33 has a diameter of 20 mm and a thickness of 2 mm.
This single crystal plate 33 is a 55° cut LiNbO ₃ crystal plate, and one surface 33a of the single crystal plate 33 is polished to a flatness of λ/10.

Then, the crystal substrates 31 and 32 are adhesively fixed with a transparent adhesive 34 so that the refractive index ellipsoid is symmetrical with respect to the bonding surface. As the transparent adhesive 34,
Epoxy adhesives, acrylic adhesives, etc. can be used. The reason why two crystal substrates are used in this manner will be described later.

Next, one surface 33a of the single crystal plate 33 is adhesively fixed to the surface of the crystal substrate 31 on which the transparent conductive film ITO is deposited.

Further, one surface 32a of the crystal substrate 32 is fixed to a polishing jig, and the other surface 33b of the single crystal plate 33 is polished. Then, the thickness d of the single crystal plate 33 is
The other surface 33 is thinned to about 50 μm.
The flatness of b is about λ/10, and the single crystal plate 33 is polished so that the parallelism between one surface 33a and the other surface 33b is within several seconds.

The thus obtained electro-optic crystal part is measured using a flat prototype and an autocollimator, and after it is confirmed that the desired flatness and parallelism have been obtained, the other surface 33b of the single crystal plate is A dielectric mirror is formed and incorporated into the spatial light modulation tube. The transparent conductive film ITO is used as an electrode as in the conventional case.

When we measured the resolution of the spatial light modulation tube with the above configuration, we obtained 15 line pairs/mm (50% modulation depth), which is about five times the resolution of conventional methods and is suitable for coherent optical information processing. It has become a value that can be used effectively.

Additionally, spatial light modulation tubes are baked at a high temperature of 200°C or higher for degassing and formation of a photocathode, but the electro-optic crystal used in the present invention is made of the same material bonded together, so it has a coefficient of thermal expansion. There were no problems with cracking or peeling due to the difference in color.

Furthermore, in the case of a 55° cut LiNbO ₃ crystal plate having natural birefringence, a transparent conductive film is also deposited on one surface 32a of the crystal substrate 32, and an appropriate voltage is applied between the surface 31b and the surface 32a. By providing this, it is possible to compensate for the phase change that occurs when there is no charge on the single crystal plate 33.

In the first embodiment described above, an example was shown in which the crystal 33 was polished thin after being bonded, but in the second embodiment, as shown in FIG. 1B, the following method can also be used.

That is, a 55° cut LiNbO ₃ single crystal plate 35 with a diameter of 20 mm and a thickness of 50 to 100 μm is placed on its two surfaces 35.
A and 35b are polished so that they are parallel and the flatness is about λ/10.

At this time, if such a thin crystal is removed from the polishing jig, it will warp, but by carefully adhering it to the single crystal plate 31 serving as the substrate, the flatness and parallelism immediately after polishing can be reproduced.

Next, the reason why the crystal substrates 31 and 32 are arranged so that the index ellipsoids are symmetrical will be explained.

In the 55° cut LiNbO ₃ wafer, as shown in FIG. 2A, the index ellipsoid is inclined with respect to the normal N to the wafer surface. Therefore, when the light I is incident as shown in FIG. 2B, the extraordinary ray travels in a direction tilted by the angle θ.

For a 55° cut LiNbO ₃ crystal plate, θ=2.2°, so
If the substrate crystal thickness is 10 mm, the ordinary ray o and the extraordinary ray e will deviate by 0.4 mm at the time of emission. Therefore, the two light waves (o and e) are modulated at different locations within the thin crystal, making it impossible to expect an improvement in resolution. Conventionally, the crystal thickness was about 300 μm, and the deviation between the ordinary ray o and the extraordinary ray e was about 10 μm and had no effect.

Therefore, as shown in FIG.
By arranging and 32 so that the index ellipsoids are symmetrical, the ordinary ray o and the extraordinary ray e become crystal 33.
The incident time and the exit time (surface 32a) coincide, and the above-mentioned problem can be avoided.

In practice, the bonding may be performed by rotating the crystal by 180 degrees around the crystal normal, or by rotating it by 180 degrees around an axis perpendicular to the x-axis within the wafer plane. Note that the two crystal plates do not necessarily need to be bonded together, as it is sufficient that the refractive index ellipsoids are arranged symmetrically.

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 explained in detail above, according to the present invention,
Since the electro-optical properties of only the thin polished portion can be used, the resolution of the spatial light modulation tube can be greatly improved.

Furthermore, since the electro-optic crystal part is made of the same material bonded together, problems such as crystal cracking, peeling, and warping are completely eliminated. Furthermore, the disadvantages of having natural birefringence can also be eliminated.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing an embodiment of an electro-optic crystal section used in a spatial light modulation tube according to the present invention, in which FIG. 1A shows the first embodiment, and FIG. 1B shows the second embodiment. FIG. Figure 2 shows a 55° cut.
FIG. 3 is a diagram for explaining the separation state of the refractive index ellipsoid, ordinary ray, and extraordinary ray in a LiNbO ₃ wafer. FIG. 3 is a diagram showing optical paths of ordinary rays and extraordinary rays of the electro-optic crystal section used in the spatial light modulation tube according to the present invention. FIG. 4 is a schematic diagram showing the basic configuration of the spatial light modulation tube. FIG. 5 is a diagram for explaining the relationship between the thickness of the electro-optic crystal plate and the electric field. 1... Input pattern, 2... Lens, 3... Glass container, 4... Photocathode, 5... Accelerating/focusing electron lens system, 6... Microchannel plate,
8... Electro-optic crystal, 9... Half mirror, 10
... Laser light source, 11 ... Laser light image, 31,
32... Crystal substrate, 33, 35... Single crystal plate, 3
4...Transparent adhesive.

Claims (1)

[Scope of Claims] 1. In a spatial light modulation tube consisting of an electron source formed in a vacuum container and an electro-optic crystal section that accumulates electrons emitted from the electron source and causes an optical change, The optical crystal unit is formed by bonding and integrating two electro-optic crystal plates of the same type and having different thicknesses through a transparent conductive film, and arranging the thin crystal plate surface to face the electron source and the thick crystal plate. 1. A spatial light modulation tube characterized in that an electro-optic crystal plate having substantially the same thickness is arranged facing the plate surface so that the refractive index ellipsoid is symmetrical. 2. The spatial light modulation tube according to claim 1, wherein the thick crystal plate and the electro-optic crystal plate in which the crystal and the refractive index ellipsoid are arranged symmetrically are bonded together. 3. The spatial light modulation tube according to claim 1, wherein the thin crystal plate is formed by bonding a relatively thick crystal plate to the thick crystal plate and then polishing the thin crystal plate. 4. The spatial light modulation tube according to claim 1, wherein the thin crystal plate is thinly polished and adhered to the thick crystal plate.