JP2895598B2 - Optical memory device and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to LiNbO ₃ or LiTaO ₃ manufactured by a special production method.
The present invention relates to an optical memory device using a fiber-like single crystal as a crystal having a photoinduced refractive index change and a method of manufacturing the same.

[Problems to be Solved by Conventional Techniques and Inventions] An optical memory device is a device that stores image information by hologram recording in a photorefractive waveguide made of a crystal having a photoinduced refractive index change. That is, when a single crystal such as LiNbO ₃ is irradiated with a strong laser beam, a non-uniform region of the refractive index is formed in the beam path and the beam is scattered (that is, a photorefractive effect; a photo-induced refractive index change phenomenon). The hologram recording has been conventionally known and has advantageous features as an optical memory device, such as free rewriting, multiple recording, and no need for development and fixing.

The light-induced refractive index change crystal for hologram recording is one of the most promising optical parallel processing substrates because of its ability to record holograms in real time and its capacity for phase conjugation calculations. Its application to hologram and phase conjugation operation mirrors is possible for associative memories, optical interconnects, real-time interferometers, image-haze recovery devices, image amplifiers and data recorders. Further, it can be an important device as a unit constituting the optical neural network.

Conventionally, when performing volume hologram recording on a light refraction path by utilizing the photorefractive effect, a bulk single crystal has been used as shown in FIG. This volume hologram recording has been viewed as promising, because it has a large recording capacity and can be repeatedly rewritten in real time using a photorefractive crystal.

In recent years, L.Hesselink and S.Redfiel
d. Opt. Lett. 13, 877 (1988)) reported that hologram recording was performed on a cesium-doped strontium barium niobate (SBN) fiber waveguide. This is because the fiber geometry provides a high energy density and long effective interference length for laser waves and a flexible assembly structure. That is, a high angular accuracy is obtained for angle-multiplex recording due to the length of the fiber, and all the characteristics due to this fiber shape are advantageous when used as a hologram recording medium. That is, by using the fiber-shaped photorefractive crystal, there is a possibility that recording can be performed with higher density and higher resolution than a device using a conventional bulk crystal.

This is because light can be confined at a high density by using a fiber-shaped light-induced refraction changing crystal. This is because the signal light and the reference light can be simultaneously confined.

However, the above-mentioned cesium-doped strontium barium niobate fiber crystal has a short recording time and is insufficient for practical use.

The present invention has been made to solve these problems, and utilizes an iron-doped LiNbO ₃ crystal fiber, that is, a fiber-shaped crystal (or a crystal in which a memory beam confinement phenomenon occurs) as an optical memory. Provided is an optical memory device using a photorefractive index crystal having a large photorefractive effect, and a method for manufacturing the same. Such a LiNbO ₃ crystal is conventionally in a bulk state, and there is no fiber state. In addition, the reference light and the signal light must be incident at an angle θ. However, if the light is incident on a fiber having a small cross-sectional area, the light leaks to the mount portion of the fiber, causing difficulty in inputting and reproducing an image.

The present invention further order to solve the problems described above of fiber shapes, using iron-doped LiNbO ₃ or LiTaO ₃ crystal fiber-like crystals were mounted visible light absorption filter surrounding the fiber crystal as an optical memory It is another object of the present invention to provide an optical memory device using a photorefractive index crystal having a large photorefractive effect and a method for manufacturing the same.

[Means for Solving the Problems] In order to solve the above technical problems, the present invention is to melt a crystal base material mainly composed of iron-doped LiNbO ₃ or LiTaO ₃ in a heating furnace. A bottomless conical container was provided, the crystal base material was placed and heated in the container, and the melt of the crystal base material was drawn out from the lower part of the container in a fiber form and drawn without using a nozzle. A single crystal optical fiber of the crystal base material grown by a pull-down method of crystallizing while pulling down a single crystal fiber, or a thinned single crystal fiber cut out from a single crystal bulk of the base material grown by a pull-up method of the base material In the optical memory device used for the photorefractive hologram crystal member, a single crystal optical fiber or a thinned single crystal fiber made of the base material is mounted on a visible light absorption filter member. To provide an optical memory device. In addition, iron doping in the heating furnace
Providing a bottomless conical container for melting a crystal base material containing LiNbO ₃ or LiTaO ₃ as a main component, installing and heating the crystal base material in the container, and heating the crystal base material from the lower part of the container The melt is drawn out into a fiber form, and the single crystal optical fiber of the crystal base material grown by a pulling-down method of crystallizing while pulling down a single crystal fiber drawn without using a nozzle, or the base material is grown by a pulling-up method. In the optical memory device using a single crystal optical fiber of the crystal base material, or a thinned single crystal fiber cut out from a single crystal bulk of the base material grown by pulling up the base material, as a photorefractive hologram crystal member, A method for manufacturing an optical memory device, comprising mounting a single crystal optical fiber or a thinned single crystal fiber made of a base material on a visible light absorbing filter member. To provide.

First, the light-induced refractive index change effect used in the present invention is as follows.
When laser light is incident on the optical crystal, the refractive index change Δ (n ₀ −
n _e ) occurs and reaches saturation after a few seconds. Second, the refractive index change of saturation depends on the incident light intensity, that value is ^10-3 to about ^-5, the third, refractive index change Δ (n ₀ -n _e)
Be due to the change in the refractive index n _e for the most extraordinary light,
Fourth, the change in the refractive index decreases as the wavelength of the laser beam decreases (however,
(Above the absorption edge wavelength). Also, the sensitivity of the refractive index change is proportional to the magnitude of the light absorption, which is also almost proportional to the sensitivity of the photocurrent. Further, the spatial distribution of the refractive index change Δ (n ₀ -n _e), when the light incident on the a-axis direction, but not sign change in the refractive index change in the b-axis direction, c-axis direction (polarization axis) code occurs an abrupt change in refractive index in the optical beam end near Δ (n ₀ -n _e) is reversed to.

When a voltage is applied in the direction opposite to the spontaneous polarization, the change in the refractive index is promoted. When the entire surface of the crystal where the local refractive index change occurs is irradiated with laser light, the entire refractive index changes. Will be erased.

A photorefractive crystal hologram (or a photorefractive memory device) utilizing the above-mentioned change in the refractive index induced by light uses a conventional bulk crystal, in which signal light and reference light are made incident and recorded. Then, these beams are recorded only in the optical path incident on the single crystal. (Thus, the angle of incidence must be shallow.) On the other hand, if a fiber-like single crystal is used,
Each beam is easily confined therein (therefore, the confinement effect is large) and guided, so that not only the incident angle can be increased, but also the formation density of interference fringes in recording increases, and the power density increases. Therefore, when different images are recorded at different angles, the number of images that can be recorded during a change angle of, for example, 1 ° can be increased. Therefore, in the optical memory device of the present invention, by using a fiber as a crystal shape to be used, high multiplexing,
High energy density can be expected.

Therefore, a crystal in which iron-doped LiNbO ₃ (LN) is made into a fiber is capable of real-time multiplex recording of two-dimensional information.

Thus, the present invention provides an iron-doped LiNbO ₃ crystal fiber with improved readout beam output, extinction (extinguishing) time constant, diffraction efficiency as a function of incident reference angle, and the effect of angle multiplexing. Therefore, a method of manufacturing an optical memory device using a photorefractive index crystal having a large photorefractive effect, a high operation efficiency, and a high quality reproduced image can be obtained.

According to the present invention, in the lowering method developed by the present applicant, a bottomless conical container for melting a crystal base material is provided in a heating furnace, and a crystal base material melt is drawn out from a lower portion of the container.
A single crystal fiber is grown while pulling down the single crystal fiber without using a nozzle to grow the single crystal fiber. That is, a melt of the crystal base material is placed in a heating furnace, the melt is drawn out, and the single crystal fiber is grown while pulling down the single crystal fiber without using a nozzle to grow the single crystal fiber.

That is, when growing a crystal fiber, in order for the melt composition and the composition of the grown crystal fiber to be the same, that is, in order to grow an extremely fine fiber having a diameter of 50 μm or less, the composition can be extremely uniform. The following three points must be noted. That is, (1) to create a temperature gradient to release the heat of crystallization generated as a crystal from a melt as smoothly as possible, and (2) to prevent the evaporation from the melt from causing a shift in the composition of the crystal. (3) When there is no congruent composition or when there is some evaporation, the use of a nozzle will emphasize the shift in crystal composition. That is.

In view of the above, in the production method of the present invention, as a heat treatment method, means utilizing the advantages of resistance heating and laser heating was considered. That is, this method is a method of pulling and growing a single crystal fiber directly from a melt without using a nozzle by using resistance heating. Furthermore, laser heating can be used together.

In this method, the single crystal fiber is pulled down from the melt and grown, so that it is easy to take an appropriate temperature gradient due to the rising airflow from below, and it is possible to adopt a structure that has good heat retention for resistance heating and can prevent evaporation. Since no nozzle or the like is used, there is little deviation in composition.

Under the same conditions as in the conventional method, a single crystal fiber cannot be produced by the pulling method by arbitrarily controlling the diameter.
For example, when the diameter is 80 μmφ or less, it becomes extremely difficult to control the diameter.
When a fiber of 5 cm or more is grown, the grown fiber itself expands and contracts repeatedly, and the pulling speed becomes uneven.
On the other hand, in the method for growing a pulled single crystal fiber according to the present invention, even if the diameter is 20 to 30 μmφ, the diameter can be controlled, and there is no unevenness in the pulling speed due to the thermal expansion of the fiber itself.

In this single crystal fiber growing method, since the temperature does not locally rise and an appropriate temperature gradient is obtained, there is no deviation in composition due to evaporation, and a single crystal fiber of several tens of μm has a uniform composition and a uniform diameter. It can be brought up. That is, instead of the resistance heating method, a high-frequency induction heating method or the like that can improve heat retention can be applied.

Furthermore, in the creation method of the present invention, the LiNbO ₃ bulk crystal created by ordinary bulk crystal creating, also LiNbO ₃ single crystal fiber was cut into fiber-like can be utilized.

The LiNbO ₃ single crystal fiber made and reduced in diameter by the method as described above is used for a method of manufacturing an optical memory device.

In the pulling-down method, a single crystal fiber having a diameter of 10 to several hundred μmΦ can be grown, and can be grown to a length of several tens cm or more.

This has an advantage that a length that cannot be obtained with a LiNbO ₃ bulk crystal can be grown in a single time. In addition, a fiber having a diameter of 1 mm or more can be easily cut out from a LiNbO ₃ bulk crystal, and is used as a fiber having a reduced diameter.

When recording an image on the LiNbO ₃ fiber, an optical system as shown in FIG. 3 is prepared. The signal light causes the reference light to enter at a constant angle θ. As the signal light and the reference light, an argon laser is used. An image is input to the signal light.

When these two lights are incident at an angle of 1 to 20 degrees, interference occurs in the LiNbO ₃ fiber, and a diffraction grating (phase grating) is formed. The diffraction grating portion is recorded as a change in the refractive index in the single crystal LiNbO ₃ fiber. This is a single crystal LiNbO ₃
This is because the optical refractive index crystal is used in the fiber, so that when a strong laser beam is incident, the refractive index changes due to the photorefractive effect. In the apparatus of FIG. 3, an Fe-doped single crystal LiNbO ₃ fiber is used,
The irradiation of the Fe ions with the laser changes the divalent state from trivalent to trivalent, thereby generating free electrons and generating an electric field in the crystal. The electric field changes the refractive index due to the photoelectric effect.

Thereafter, when only the reference light is incident at the same angle, the laser light is diffracted by the diffraction grating generated inside the single crystal LiNbO ₃ fiber, and the image is reproduced.

The leakage of light from the mount can be minimized by using a filter that absorbs visible light wavelengths in the mount.

This makes it possible to relatively easily put an image in the single crystal fiber.

The material of the single crystal optical fiber used in the present invention is made of a single crystal having LiNbO ₃ or LiTaO ₃ as a main component.
Those doped with a small amount of iron are preferable.

LiNbO ₃ or LiTaO ₃ single crystal fibers can be used in either multimode or single mode.

The optical memory device of the present invention is useful not only as a memory using light but also as a device that can be used for optical calculation.

Next, the optical memory device according to the present invention and its structure will be specifically described with reference to Examples, but the present invention is not limited thereto.

[Examples] Hereinafter, description will be made with reference to the accompanying drawings.

FIG. 1 illustrates one embodiment of the photorefractive memory device of the present invention.

First, a visible light absorbing filter 2 with a thickness of 2.5 mm is 5 mm x 10 mm.
Process to the size of.

On the top surface of the first filter plate (5 mm x 10 mm) 2, 200
A groove 4 having a width and depth of μm is formed.

In the groove 4, LiNbO doped with 0.005% by weight of iron
₃ Put a ₃ fiber crystal 3 (LiNbO ₃ fiber single crystal grown by pulling-down method with outer diameter 200μmΦ, length 10mm), flow epoxy-based adhesive there, and make a second filter plate (5mm × 10mm) Paste with 5.

Thereafter, both end surfaces 6 and 7 are polished. The unevenness accuracy of the surface should be within λ / 10. The degree of parallelism should be within one minute. The filter used was a visible light absorbing filter that absorbs a wavelength in the range of 420 to 660 μm.

The LiNbO ₃ fiber crystal 1 thus configured was used for the hologram memory device shown in FIG.

FIG. 4 shows a photograph of the end face of the fiber crystal 3 configured as described above. This is a LiNbO ₃ fiber crystal doped with 0.005% by weight of iron. Multiplex recording was performed on this fiber crystal with a reflection hologram as follows.

This LiNbO ₃ fiber crystal was grown in the z-axis direction by resistance heating by a pull-down method, and the dopant was about 0.
005%. Since no cladding is provided, the transmission loss of the fiber is relatively high, at 1.4 dB. Third
In the figure, an input image 15 is
Is Fourier-transformed by the lens 16 and is incident on the axis of the fiber crystal 19, while the reference beam inclined with respect to the axis
22 enters from the opposite direction. Multiple recordings are made by changing the angle of incidence of the reference beam for each image. Therefore, the hologram recording was performed in a reflection type.

That is, the hologram is of a reflection type, and the input image is Fourier-transformed and recorded on the fiber crystal 19. An argon laser (λ = 0.515 μm) 11 was used for recording and reading.

LN fiber crystal 19, 15 degrees to the fiber axis, 17
Characters U and J are recorded with two reference beams having different degrees and incident angles, and a reproduced image 25 is formed when the characters U and J are read out with the respective reference beams.

An actual photograph of this reconstructed image is shown in FIG.
B. It is considered that the deterioration of an image is mainly caused by a defect of a high-order spatial frequency component. That is, an experiment was performed on a U-shape, and the reproduced image was as shown in FIG. 5A.
This is _{one in which} the angle θ ₁ of the reference light with respect to the fiber axis is 15 degrees.

Phase deformation occurs because the fiber crystal end face is not flat,
Due to the small diameter of the test fiber crystal, the spatial frequency component is high, which degrades the image performance.

The resolution time constant of the index grating was measured as a function of the reference beam power and is shown in FIG. The extinction time constant is defined as the time for the index grating depth to decrease to half of the maximum value. This constant is inversely proportional to the reference beam output and is approximately the same as that of a bulk photorefractive index crystal. FIG. 7 shows that the diffraction efficiency depends on the incident angle θ ₁ of the reference beam. The fluctuation (fluctuation) of the diffraction efficiency is as shown in the error in FIG.
For each of theta _1, and at about 50% ±, in the apparatus of the experiment is due to unknown perturbations. Using Snell's equation, the critical angle for total internal reflection is calculated to be 27 °. In the fiber crystal, with respect to the fiber axis, its refractive index is 2.2. Thus, all reference beams have an angle of incidence in free space of up to 90 °,
The shape of the waveguide is ideal and, without perturbation, propagates as a mode introduced into the fiber.

Therefore, the diffraction efficiency, when θ ₁ is less than 10 °,
It turns out that it becomes large rapidly. This assumes that a small θ ₁ relies on the fact that the effective interference length between the on-axis signal and the off-axis reference beam increases as the two waves propagate. Is done. In other words, as the reference beam energy decreases as θ ₁ decreases, the integer number of overlaps in the electric field intensity distribution between these two beams increases, as they are confined around the core center. This trend will be revealed by the numerical analysis used.

The angle multiplex recording capability is a key for obtaining a high-density recording medium. Therefore, Reference lance beam two angular multiplexing, each beam of the incident angle to the fiber axis of 15 ° and 17 °, with R ₁ and R _2, and experiments. U at R ₁
Text, images were reproduced in J-by R ₂ and shows an image obtained by inserting the U on J obtained by irradiating simultaneously R ₁ and R ₂ each FIG. 5 A, B and C. From FIG. 5, it is clear that multiplex recording did not degrade image quality. This fact is due to the fact that a bottomless conical container for melting the iron-doped LiNbO ₃ crystal base material is provided, the iron-doped LiNbO ₃ crystal base material melt is drawn out from the lower part of the container, and without using a nozzle, Devices using an iron-doped LiNbO ₃ single crystal optical fiber grown by a pull-down method, in which the single crystal fiber is crystallized while being pulled down, need to be further studied so that the medium can be recorded at a high density. To maximize angular sensitivity and improve image quality, it is necessary to experiment with fiber parameters such as core diameter, length and iron dopant concentration.

As described above, an optical memory device can be configured according to the present invention. At the same time, the optical memory device of the present invention has a structure as shown in FIG. 1, and can be used with high performance as a hologram recording device.

Although the optical memory device of the present invention has the above-described structure, the image deterioration is considered to be mainly caused by the lack of higher-order spatial frequency components. Better results would be obtained.

[Effects of the Invention] The following remarkable technical effects were obtained by the method of manufacturing an optical memory device using the iron-doped LiNbO ₃ single crystal fiber of the present invention.

First, an optical memory device that can be expected to have high multiplexing and high energy density has been provided.

Second, therefore, an optical memory device with improved diffraction efficiency, angle multiplex recording, and diffraction grating resistance can be provided.

Third, further, the iron-doped LiNbO ₃ single crystal optical fiber has a long lifetime as a refractive index grating and has about twice the capacity as a refractive index modulator.

Fourth, there is provided an optical memory device capable of increasing the diffraction efficiency as a function of the incident reference angle.

Fifth, the fibrous iron-doped LiNbO ₃ of the present invention
The single crystal can be sufficiently used by replacing the conventional bulk medium for hologram recording.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view showing the structure of the optical memory device of the present invention. FIG. 2 is a schematic sectional view showing a conventional optical memory device. FIG. 3 shows a configuration of an optical memory device incorporating the optical memory device of the present invention. FIG. 4 is a photograph showing a (crystal structure) of an end face of a fiber crystal having a change in a refractive index used in the present invention. FIGS. 5A, 5B, 5C and 5D show characters U and J respectively recorded with separate reference lights in the optical memory device of the present invention, and reproduced images U and J read with each reference light and read with two reference lights. 5 is a photograph showing a reproduced image in which U and J overlap each other and a morphology (crystal structure) of the reproduced image when character U is recorded. FIG. 6 is a graph showing a resolution time constant plotted against a reference beam output by the optical memory device of the present invention. FIG. 7 is a graph of the diffraction efficiency plotted against the incident reference beam angle θ ₁ . [Description of Signs of Main Parts] 1... Photorefractive effect memory medium 2, 5... Visible light absorption filter 3.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Naoyoshi Yoshinaga Nippon Telegraph and Telephone Corporation, 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo (56) References Optics Letters Vol. 13, No. 10 (1988) p. 877-P.879 (58) Field surveyed (Int.Cl. ⁶ , DB name) G03H 1/02 G02F 1/35 G02B 6/00

Claims (2)

(57) [Claims] An iron-doped LiNbO ₃ or LiTa in a heating furnace
A bottomless conical container for melting a crystal base material containing O ₃ as a main component is provided, and the crystal base material is placed and heated in the container, and a melt of the crystal base material is heated from the lower part of the container. A single-crystal optical fiber of the crystal base material grown by a pull-down method of pulling out a single crystal fiber drawn without using a nozzle and crystallizing while pulling down, or the base material grown by a pull-up method of the base material In an optical memory device using a reduced diameter single crystal fiber cut out of a single crystal bulk as a photorefractive hologram crystal member, the single crystal optical fiber or the reduced diameter single crystal fiber of the base material is placed on a visible light absorbing filter member. An optical memory device mounted on a device. 2. An iron-doped LiNbO ₃ or LiTa in a heating furnace.
A bottomless conical container for melting a crystal base material containing O ₃ as a main component is provided, and the crystal base material is placed and heated in the container, and a melt of the crystal base material is melted from a lower portion of the container. A single-crystal optical fiber of the crystal base material grown by a pull-down method of pulling out a single crystal fiber drawn without using a nozzle and crystallizing while pulling down, or the base material grown by a pull-up method of the base material In an optical memory device using a reduced diameter single crystal fiber cut out of a single crystal bulk for a photorefractive hologram crystal member, the single crystal optical fiber or the reduced diameter single crystal fiber of the base material is placed on a visible light absorption filter member. A method for manufacturing an optical memory device, wherein the optical memory device is mounted.