JP3493427B2 - Holographic memory made of lithium niobate single crystal with improved photo-induced refractive index characteristics, method of manufacturing the same, and optical amplifying device using the memory - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to lithium niobate (LiNb) used in fields such as optical information processing using laser light, optical processing technology, photochemical reaction technology, and optical measurement control.
O ₃ ) (hereinafter abbreviated as LN) single crystal. More specifically, the present invention relates to an LN single crystal memory for an optical element, which has improved control of light-induced refractive index change, that is, hologram diffraction efficiency, and an optical amplifier including the LN single crystal memory.

[0002]

2. Description of the Related Art The phase diagram of LN single crystal has been known for a long time. Conventionally, in order to produce LN single crystal having a high composition homogeneity, the congruent melting (equal coexistence of crystal and melt with the same composition) Li ₂ O / (Nb ₂ O ₅ +) having a congruent composition
It was grown by a rotary pull-up method from a melt having a Li ₂ O) mole fraction of 0.485.

Since the grown as-grown LN single crystal is in a multi-domain state, a voltage is applied in the Z-axis direction of the crystal while the grown crystal is heated to a Curie temperature of about 1150 ° C. or higher. It was subjected to a process called poling for cooling the crystal after it was divided into one area. The single-domain-processed crystal was processed into a predetermined size and then used for various purposes.

When an LN single crystal is used for optical applications, when a strong laser beam is applied, a local change in the refractive index (light-induced change in the refractive index, commonly called "optical damage") appears, and an optical modulator or While this is an obstacle to the application of wavelength conversion elements, on the other hand, on the contrary, there is growing interest in applying this photo-induced refractive index change positively to a phase hologram recording element as a high-sensitivity optical memory. There is.

Regarding the photo-induced refractive index change (photorefractive effect), the mechanism has been described so far. That is, the photo-induced refractive index change is a phenomenon that occurs when light is irradiated to an electro-optic crystal having a deep trap level (photorefractive center) due to impurities and defects, and the photorefractive center is photoionized, The free carriers generated during the diffusion
After moving due to an external electric field or a photovoltaic effect, recombination occurs in a portion not irradiated with light, resulting in a space charge distribution corresponding to the light intensity distribution. This space charge distribution causes a change in the refractive index due to the electro-optic effect. This is the light-induced refractive index change.

In order for the above mechanism to work, there must be an ionized trap level in the initial state. For example, in LN single crystal, Fe ²⁺ forms a donor level, and this is ionized to become Fe ³⁺ to form a trap level. Therefore, especially the transition metal impurities contained in the crystal play an important role, and until now, the light-induced change in the refractive index of the LN single crystal, which is an electro-optic crystal, is
It has been known that the photosensitivity and diffraction efficiency are further improved by adding a transition metal such as Mn or Cu.

Further, the LN single crystal of the congruent melting composition to which no transition metal is added is not suitable for these applications because the hologram diffraction efficiency is extremely low. Conventionally, several hundred ppm of Fe is added to the LN single crystal of the congruent melting composition. Was added to produce crystals.
It was used in the light application technology.

As a single crystal used in an optical laser device for writing a three-dimensional hologram in a single crystal using a visible light laser beam, an LN single crystal having a congruent melting composition to which a transition metal is not added is relatively transparent, Since the hologram diffraction efficiency is extremely low, there is a problem that it is difficult to write a hologram in application.

On the other hand, although the LN single crystal obtained by adding several hundred ppm of Fe to the ordinary congruent composition has a high hologram diffraction efficiency, it has a large absorption in the visible light region of about 400 to 600 nm and is colored brown. Also, from the viewpoint of crystal production, the transition metal segregates in the crystal, so it is difficult to grow the crystal uniformly added, and the LN single crystal with Fe added causes macroscopic crystal defects that cause light scattering. However, sufficient quality was not obtained as an optical element.

The coupling coefficient is used as a figure of merit for comparing crystal materials exhibiting such a photorefractive effect. However, in the case of the Fe-added LN single crystal which has been known so far, it is about 4 to 4. It is said that it is 12 cm ^-1 (for example, "Optical Crystal", page 200, Shintaro Miyazawa, Baifukan Co.), and there is a problem that it cannot be said that it has sufficient performance for optical applications.

By the way, in general, the optical element material absorbs light,
It is desirable that light scattering be as small as possible, but an increase in absorption of laser light in an optical crystal often deteriorates the characteristics of the crystal, and even in a phase-type three-dimensional hologram element, if the characteristics are the same, absorption of It can be said that less is desirable.

Also, from the viewpoint of crystal production, since transition metals segregate in the crystals, it is difficult to grow crystals with uniform addition thereof, and LN single crystals with addition of Fe have a macroscopic factor that causes light scattering. Since many crystal defects are included, an optical element of sufficient quality has not been obtained.

Further, in the conventional crystal, it is necessary to perform poling, which requires a process time of several tens of hours, such as poling in order to form a single domain, but Fe added in the LN single crystal is required.
Has a problem that it easily moves in the crystal due to the polarization treatment, so that a remarkable concentration gradient occurs in the crystal and the optical characteristics of the crystal become non-uniform. Further, it has been said that noise caused by random light scattering due to the non-uniformity of the Fe-added LN single crystal is a problem that hinders the improvement of the recording density of the hologram.

[0014]

DISCLOSURE OF THE INVENTION The present inventors have improved the above-mentioned drawbacks and controlled the light-induced refractive index change without adding a transition metal in LN single crystal, and have high diffraction efficiency and high optical efficiency. As a result of various studies to provide an LN single crystal having excellent transmission characteristics without scattering, it was found that the object can be achieved by controlling the crystal composition of the LN single crystal, and the present invention has been completed. An object of the present invention is to use an LN single crystal memory in which a light-induced refractive index change is controlled, and an LN single crystal memory in which a light-induced refractive index change is controlled when applied to fields such as optical information processing using laser light, optical processing technology, photochemical reaction technology, and optical measurement control. The present invention is to provide a conventional optical amplifier.

[0015]

That is, the present invention is
(1) The molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.495 to 0.50 and 400 to 600 nm.
With a light absorption coefficient of 1 cm-1 or less in the visible light range of
Coupling coefficient is 20 ~ 27cm without adding Fe
-1, which is 900 to 10 in an atmosphere in which the oxygen concentration is controlled after the growth of the lithium niobate (LN) single crystal.
It is a lithium niobate single crystal hologram memory characterized by being subjected to heat treatment at 00 ° C.

The present invention also provides (2) lithium niobate.
A method of manufacturing a holographic memory comprising
A process for producing lithium niobate using the double crucible method.
However, the lithium niobate is Li2O / (N
b2O5 + Li2O) has a molar fraction of 0.495 to 0.5.
0, the light absorption coefficient in the visible light region of 400 to 600 nm
The number is 1 cm-1 or less, and the transition metal Fe is added.
The coupling coefficient is 20 to 27 cm −1,
And the oxygen concentration of the manufactured lithium niobate is controlled
Of heat treatment at 900 to 1000 ° C. in a controlled atmosphere
It is a method that includes and.

That is, the hologram memory of the present invention is used.
The single crystal has a composition closer to the stoichiometric ratio (stoichiometry) than a normal congruent melting composition (hereinafter referred to as congruent composition), has a low defect density, and is free of transition metal Fe and the like. excellent light transmission characteristics in the light scattering is small visible light region because it is added, the hologram diffraction efficiency is not high.

[0018] The LN single crystal, after a further development, is obtained by adding a heat treatment for controlling the writing speed and the hologram recording time in the hologram.

The present invention also provides (3) a visible light laser.
Write a hologram diffraction grating in a single crystal using light
In a device for amplification, the above-mentioned niobium is used as the single crystal.
A special feature is that it uses a lithium crystal monolithic hologram memory.
This is a characteristic optical amplification device. This optical amplifier does not contain a transition metal, has a single domain, and has a stoichiometric composition close to that of the above L.
For example , a phase hologram memory utilizing a light-induced change in refractive index generated by irradiating N single crystal with visible light.

[0020]

First, the LN single crystal according to the present invention is made of Li ₂ O /
(Nb ₂ O ₅ + Li ₂ O) is a single crystal having a mole fraction of 0.495 to 0.50, which is closer to the stoichiometric ratio than a normal congruent composition having a mole fraction of 0.485. Therefore, crystal perfection is high and defect density is low. Furthermore, since transition metals such as Fe are not added, light scattering is small and the light absorption coefficient in the visible light region of 400 to 600 nm is 1 c.
It has an excellent light transmission property of m ^-1 or less, and has a high hologram diffraction efficiency.

The LN single crystal for holographic memory of the present invention has the same composition as the conventional crystal and the melt when the LN single crystal is grown from the melt when growing the LN single crystal by the pulling method. Is not the same melting composition that equilibrium coexists with the same composition, but Li ₂ O / (Nb ₂ O ₅ +
It is obtained by growing a crystal by a double crucible method from a melt in which the molar fraction of Li ₂ O) is kept in a specific range of 0.56 to 0.60.

If grown from a melt having the above composition, the Curie temperature of the crystal composition to be crystallized is about 1200 ° C., which is higher than the normal congruent composition crystal of 1150 ° C. and close to the crystal growth temperature. Most of the crystals are in the single domain state. Therefore, there is an advantage that there is no need to perform the post-growth poling treatment which is necessary for the multi-domain crystal, which is significantly different from the conventional multi-domain crystal grown from the congruent composition melt.

Further, by adopting a double crucible method as the crystal growing method, the composition of the melt in which the crystal is grown is always kept constant, and the temperature fluctuation of the melt in the inner crucible is extremely small. Therefore, it is possible to produce an LN single crystal having an extremely uniform composition and good optical homogeneity.

Furthermore, the laser device according to the present invention is an optical amplifier device for writing a three-dimensional hologram in a single crystal by using visible laser light, and the three-dimensional hologram has a high speed and a potentially large storage capacity. Therefore, as a new recording method related to multimedia, future development is expected. Although various materials such as silver salt photographs are used for holograms, photo-induced refraction crystals are considered to be the most effective for memory applications. It is the material with the longest retention time among the crystalline crystals. Until now, most of the memory experiments have used brown-colored LN single crystals whose diffraction efficiency is increased by adding Fe, which is a transition metal. There is.

However, the Fe-added LN single crystal has a large absorption and scattering in the visible light region of about 400 to 600 nm, and has a large variation in the characteristics depending on the sample, so that it cannot be put to practical use as a hologram recording material. Didn't. The present inventors have found that LN
For the first time, we have found a phenomenon that by controlling the composition of a single crystal, it is possible to obtain a crystalline homogeneity and high quality, and to obtain a diffraction efficiency sufficient for a laser device. The optical amplifying device for writing a three-dimensional hologram using the LN single crystal according to the present invention was first discovered by the present inventor.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail below. As a raw material used in the method for producing the LN single crystal of the hologram memory according to the present invention, the Li component and the Nb component are oxides of these or compounds such as Li ₂ O and Li ₂ CO which become oxides by heating. ₃ , Nb ₂ O ₅ or the like is used. Then, these components are added to Li ₂ O / (Nb ₂
A single crystal having a composition with a mole fraction of 0.495 to 0.50, which is closer to the stoichiometric ratio than the usual congruent composition (congruent composition) in which the mole fraction of O ₅ + Li ₂ O) is 0.485, is obtained. This single crystal is 400-60
It has a characteristic that the light absorption coefficient in the visible region of 0 nm is 1 cm ^-1 or less, and the hologram diffraction efficiency is high without adding a transition metal.

The high hologram diffraction efficiency means that a larger refractive index change is written in the crystal by irradiating the laser beam. Further, by applying heat treatment to this lithium niobate, it is possible to control the hologram writing speed and the hologram recording time. The heat treatment condition at this time is 900 to 1000 ° C. in an atmosphere in which the oxygen concentration is controlled.

A preferable method for producing the LN single crystal of the hologram memory according to the present invention is a crystal growing method in the double crucible method. The principle of the crystal growing method in the double crucible method will be described with reference to FIGS. 1 and 2.

FIG. 1 is a phase diagram of LN, and FIG. 2 is an explanatory view of a single crystal growth apparatus in the double crucible method. As shown in FIG. 1, the congruent composition of the LN single crystal was Li ₂
Since the molar fraction of O / (Nb ₂ O ₅ + Li ₂ O) is 0.485, the LN single crystal obtained from the congruent composition melt by the usual pulling method has an excess of Nb component. Is markedly excessive in Li component (for example, Li ₂ O / (Nb ₂
When the mole fraction of (O ₅ + Li ₂ O) is 0.56 to 0.60, preferably 0.58), a single crystal having a stoichiometric composition close to that of the nonstoichiometric defect concentration can be obtained. it can.

However, if the composition of the crystal to be grown is different from that of the melt, the composition of the melt and the crystal becomes more distant as the growth proceeds by the normal pulling method, and thus the crystal becomes difficult. Therefore, in order to precisely control the density and structure of the non-stoichiometric defects, the single crystal growth apparatus by the double crucible method shown in FIG. 2 was developed.

In the double crucible method, the crucible has a double structure, and the bottom of the inner crucible is provided with a hole leading from the outer crucible to the inner crucible. Furthermore, Li ₂ O / (Nb ₂ O ₅ + Li ₂
The weight of crystal growth grown from the melt of the inner crucible in which the molar fraction of O) is 0.56 to 0.60 and the excess of the Li component is measured by a load cell, and an amount of Li commensurate with the crystallized growth amount is obtained.
A raw material having a stoichiometric composition ratio with a molar fraction of ₂ O / (Nb ₂ O ₅ + Li ₂ O) of 0.50 was automatically supplied to the outer crucible. By this method, by flowing the raw material from the outside to the inside, it is possible to grow a large single crystal of homogeneous composition for the first time because the crystal can be grown from the melt of the inner crucible that always maintains a constant composition at a constant depth. became.

Next, the LN of the hologram memory according to the present invention
A method for producing a single crystal will be shown by a specific example. Specific Example 1 Commercially available high-purity Li ₂ CO ₃ and Nb ₂ O ₅ (purity 9 each
9.999%) raw material powder is prepared, and the ratio of Li ₂ CO ₃ : Nb ₂ O ₅ is 0.56 to 0.6 as the Li component excess raw material.
Mixing in the ratio of 0: 0.44 to 0.40, and Li ₂ CO ₃ : Nb ₂ O ₅ = 0.50: 0.5 as a stoichiometric composition raw material
Mixed at a ratio of 0. Next, rubber press molding was performed under a hydrostatic pressure of 1000 kg / cm ² , and each was sintered in oxygen at about 1050 ° C. to prepare a raw material rod. In addition, about 1050 the stoichiometric composition raw material that has already been mixed as the powder raw material for continuous supply is used.
A stoichiometric composition raw material was also prepared by sintering in oxygen at 0 ° C.

Next, when growing a single crystal by the double crucible method, an Li-rich material is charged in the inner crucible and a stoichiometric composition raw material is charged in the outer crucible in advance, and then the crucible is heated to form a melt. did. Here, the crucible used for growth was made of platinum, and the seed crystal used was an LN single crystal in a single domain state of 5 mm × 5 mm × length 70 mm cut out in the Z-axis direction.
The growth conditions were a crystal rotation speed of 5 rpm, a pulling speed of 0.5 to 2 mm / h, and an atmosphere of air.

Further, in order to make the melt composition uniform, the crucible was slowly rotated at a speed of 0.2 rpm in the direction opposite to the seed crystal. By growing for about 1.5 weeks, a colorless and transparent LN crystal body having a diameter of 40 mm and a length of 70 mm and having no crack was obtained. The obtained as-grown crystal is cut into various orientations,
Observation of the internal domain states revealed that the inside was uniformly in a single domain state, except for a small portion near the surface of the crystal.

SPECIFIC EXAMPLE 2 The LN single crystal obtained in Specific Example 1 was analyzed by chemical analysis to have a composition close to that of a stoichiometric ratio, Li ₂ O / (Nb ₂ O ₅ + Li).
It was confirmed that the mole fraction of ₂ O) was 0.495 to 0.500 and that the single crystal had a nonstoichiometric defect concentration suppressed as much as possible. Next, the Curie temperature of the grown crystal was measured by the suggestive thermal analysis method. As a result, the Curie temperature of each of the samples cut out from each portion of the crystal was 1198 to 1200.0 ° C.
It was confirmed that the Curie temperature of the sample cut out from one crystal was constant irrespective of the cutting position of the sample, and the homogeneity of the crystal composition was extremely good.

From the further grown as-grown single crystal, Y of 10 mm × 10 mm and 2 mm and 10 mm in thickness was obtained.
The cut sample was cut out and surface-polished by mechanochemical polishing. The sample was annealed under various conditions to control its redox state. The light transmittance of the sample after the annealing treatment was measured with a spectrophotometer. As shown in FIG. 3a, L of stoichiometry composition in as-grown state
The light absorption coefficient of the N single crystal at a wavelength of 400 to 600 nm was as high as 1 cm ^-1 or less, and its characteristics were not significantly decreased even by annealing.

On the other hand, it was confirmed that, in the conventional as-grown crystal with added congruent composition containing iron and the crystal subjected to the heat treatment, all the samples show a low transmission property of 60% or less at the wavelength of 400 to 600 nm. Was done.
Further, Fig. 3b shows the light absorption coefficient characteristics near the basic absorption edge. In the additive-free stoichiometric composition crystal, the basic absorption edge extends to a shorter wavelength side than the congruent composition crystal, and the light absorption It can be seen that is small and good.

Further, when the optical homogeneity of the sample was observed with a laser interference device, defects such as striations and voids caused by crystal defects were not observed, and the variation was high in the whole sample of 1 × 10 ⁻⁵ or less. It was confirmed that there was a refractive index homogeneity. Furthermore, the scattering contained in the crystal was evaluated by the laser microprobe method.

FIG. 4 shows that a 10 mm square sample has a wavelength of 633 n.
3 schematically shows the state in the sample when a He-Ne laser of m is incident. In the LN crystal according to the present invention, no laser scattering is observed, and Fe is added to a commercially available congruent composition. It was confirmed that the crystal quality was remarkably superior to that of the LN single crystal added by several 100 ppm.

[0040]

Examples and Comparative Examples Example 1 The optical amplification factor of the grown LN crystal was evaluated by a two-wave mixing experiment. The experimental optical system is schematically shown in FIG. As shown in FIG. 5, two coherent light waves, which are respectively called pump light and signal light, were crossed in the LN single crystal, which is a photoinduced refractive crystal, to form a plurality of interference fringes. A spatial electric field corresponding to the spatial intensity change of the interference fringes was formed, and as a result, a refractive index grating was formed in the crystal.

The phase of the refractive index grating is π / 2 with respect to the interference fringes.
Therefore, the transmitted signal light that has passed through the photo-induced refractive crystal undergoes amplification of the light intensity, and the pump transmitted light undergoes attenuation of the light intensity. As a result, the energy transfer from the pump light to the signal light due to the two-wave mixing shown in FIG. 6 was observed on the oscilloscope, and the amplification factor was obtained from the ratio of the signal-light intensities before and after the two-wave mixing. Here, as the pump light and the signal light, the wavelength 5 which is the second harmonic of the Nd: YAG laser is used.
32 nm green light was used. Beam diameter is 1m each
m, crossing angle 20 °, light intensity ratio of pump light and signal light is 1
It was set to 00: 1.

FIG. 7 shows the congruent composition LN without addition.
It is the figure which compared the result which calculated | required each diffraction efficiency, the amplification factor, and the coupling coefficient about the single crystal, the Fe added congruent composition LN single crystal, and the additive-free stoichiometric composition LN crystal (sample thickness 2 mm). However, no antireflection coating was applied to the surface of each sample.

In the LN single crystal of the congruent composition without addition, both the diffraction efficiency and the amplification factor are as small as 1% or less.
It can be seen that in the eN-added congruent composition LN single crystal, the addition of Fe increased the diffraction efficiency to about 34%, the amplification factor to 15%, and the coupling coefficient to 15 cm ⁻¹ .

On the other hand, in the additive-free stoichiometric composition LN single crystal, the diffraction efficiency, the amplification factor, and the coupling coefficient were higher than those of the other crystals by 60%, respectively, even though Fe was not added. , 42%, 20 to 27 cm ^{−1, which} is a large value.

Example 2 Next, using a Y-cut sample having a thickness of 2 mm and a size of 10 mm × 10 mm heat-treated under various conditions, the diffraction efficiency, the writing time and the hologram recording time in the two-wave mixing experiment were measured. The heat treatment was performed as follows. As-Grown L with a curie temperature of 1200 ° C and a stoichiometric composition
Enclose the N single crystal in a heat treatment furnace capable of controlling the atmosphere,
The temperature was raised to 0 ° C. in 10 hours, and the temperature was maintained at 950 ° C. for about 12 hours, then cooled to room temperature in about 10 hours, and the sample was taken out. Three atmospheres were used for the atmosphere of the furnace: 100% oxygen in a flow rate of 1 liter / minute, dry 100% nitrogen gas in a flow rate of 1 liter / minute, and nitrogen gas containing water vapor in a flow rate of 1 liter / minute.

Table 1 shows the results of comparing the two-wave mixing measurement results of the respective samples after the heat treatment with those of the as-grown crystals. Oxygen treatment makes writing time about 3 times longer,
On the other hand, the nitrogen treatment reduced the writing time to 1/5 to 1/3. That is, writing at higher speed has become possible. In addition, the retention time of hologram recording was largely dependent on the temperature of the crystal, and could be retained for several months or longer at room temperature. As an acceleration test, when the crystal temperature was heated to about 60 ° C. and the retention time of hologram recording was measured, as shown in Table 1, the recording time of the as-grown crystal was about 240 hours, but the treated crystal in oxygen was observed. It was found that can be retained for about 710 hours, which is about three times as long.

On the other hand, each crystal treated in nitrogen has 8
It was shortened to 0 hours and 55 hours. From the above results, it was clarified that the writing of the hologram and the recording retention time can be arbitrarily controlled by the heat treatment of the stoichiometric LN crystal.

[0048]

[Table 1]

Example 3 Next, a prototype of a device for writing a three-dimensional hologram in a single crystal using visible laser light was manufactured. A schematic configuration of the device is shown in FIG. This device is a volume hologram memory device of an angle multiplexing system using an LN single crystal having a high hologram diffraction efficiency of the present invention.

Digital image input data is developed as a graphic on the spatial light modulator. Next, this was read out with a laser beam and used as a hologram object wave. A reference wave was incident on this at a substantially right angle, and interference fringes were written in an LN single crystal as a recording medium. Here, the LN crystal was placed so that the c-axis of the crystal was orthogonal to the direction of the interference fringes, and was placed on a stage that can be rotated with high precision. Crystal size is 1 ×
It is 1 × 1 cm ³ . While changing the crystal little by little, the data of about 200 sheets were recorded in multiplex by utilizing the selectivity of black diffraction.

These data are reproduced by the reference wave,
It was converted into an electric signal by a two-dimensional photodetector. The feature of hologram recording here is that it is a phase-type hologram whose refractive index changes, so high diffraction efficiency is expected, and it is possible to write a diffraction grating simply by irradiating interference fringes without requiring development processing. Moreover, this once written hologram can be held for a long time.

Although the hologram holding time depends on environmental conditions such as temperature, the LN single crystal having a stoichiometric composition was able to hold data for a longer period of time than several months and for more than several months. The data recording density in this configuration is determined by the diffraction efficiency and the magnitude of noise.

Conventionally, noise caused by random light scattering due to the non-uniformity of Fe-doped LN single crystal has been a problem. However, the LN single crystal according to the present invention has high diffraction efficiency and the crystal is homogeneous and scattered. It was confirmed that the noise is significantly reduced and the recording density can be improved because it does not exist.

[0054]

As described above in detail, according to the present invention, the composition of LN single crystal can be changed to Li ₂ without adding a transition metal.
_{O / (Nb 2 O 5 +} Li 2 O) molar fraction of 0.495～
By controlling to 0.50, it is possible to obtain an LN single crystal that is crystalline and homogeneous and has high quality, high transmission characteristics, and sufficient diffraction efficiency required for a laser device. By utilizing this characteristic, it is possible to provide a high-speed three-dimensional hologram optical amplification device having a large storage capacity and a long holding time by using an LN single crystal. For these reasons, the stoichiometric LN single crystal in which the light-induced change in the refractive index is controlled can be widely used for optical application technology.

[Brief description of drawings]

FIG. 1 is a phase diagram of LN.

FIG. 2 is a diagram for explaining a single crystal growth apparatus by a double crucible method for growing an LN single crystal having a stoichiometric composition.

FIG. 3 shows the light transmittance in the visible light region of the stoichiometric composition and the iron-containing congruent melt composition LN single crystal, (a), and the light absorption coefficient of the stoichiometric composition and the congruent melt composition LN single crystal ( It is the figure which showed b).

FIG. 4 shows a wavelength of 633 nm for an LN single crystal having a stoichiometric composition and an LN single crystal having a congruent composition containing Fe.
FIG. 3 is a diagram schematically showing the state of light scattering in the sample when the He-Ne laser of FIG.

FIG. 5 is a diagram schematically showing a state of an experiment in which a hologram is written in an LN single crystal by two-wave mixing to obtain diffraction efficiency.

FIG. 6 is a diagram for obtaining an amplification factor from measurement of energy transfer from pump light to signal light by two-wave mixing.

FIG. 7 is a diagram comparing the results of obtaining the diffraction efficiency, amplification factor, and coupling coefficient in two-wave mixing of various LN single crystals. The conditions are: wavelength 532 nm, sample thickness 2 mm, intensity ratio of pump light and signal light 100: 1.

FIG. 8 is a diagram schematically showing the configuration of a hologram memory device for writing a three-dimensional hologram in a single crystal using visible laser light.

Continuation of front page (56) Reference JP-A-1-320294 (JP, A) JP-A-4-134479 (JP, A) JP-A-4-300281 (JP, A) JP-A-4-300293 (JP , A) JP 4-300295 (JP, A) JP 5-97591 (JP, A) JP 6-293597 (JP, A) Yasunori FURUKAWA et al. , Growth and characterisation of off-congruent LiNbO3 single crystal als grown by the ・ ・ ・ method, Journal of Crystal Grow, 1993. 128, pp. 909-914 K. KITAMURA et al. , Stoichiometric LiNbO3 single crystal by grow by double czochra lski methususin g ... system, Journal of Crystal, 1992, Gross. 116, pp. 327-332 Shin-ji KAN et al. , LiNbO3 single crystal growth by the continuous charging Czochralski method with Li / Nbr atio control, Journal of the Vistor, 1992. 119, pp. 215-220 (58) Fields investigated (Int. Cl. ⁷ , DB name) C30B 1/00-35/00 G02B 5/32 G02F 1/35 G03H 1/02 CA (STN) JSTPlus (JOIS )

Claims (3)

(57) [Claims] 1. The molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.495 to 0.50 and 400 to 60.
The light absorption coefficient in the visible region of 0 nm is 1 cm ^-1 or less, and the coupling coefficient is 20 to 2 without adding the transition metal Fe.
7 cm ⁻¹ , and after growing a lithium niobate single crystal
Furthermore, 900 to 100 in an atmosphere with controlled oxygen concentration.
A hologram memory made of a lithium niobate single crystal, which is characterized by being subjected to heat treatment at 0 ° C. 2. A hologram memo composed of lithium niobate.
A method for producing a lithium niobate containing the lithium niobate
A process for manufacturing using a crucible method, comprising:
Lithium oxide is Li2O / (Nb2O5 + Li2O)
The molar fraction is 0.495 to 0.50, 400 to 60
Light absorption coefficient in the visible region of 0 nm is 1 cm-1 or less
And the coupling coefficient is 2 without adding the transition metal Fe.
A process of 0 to 27 cm-1, and the manufactured nio
Lithium boroate 900 in an atmosphere with controlled oxygen concentration
Heat treating at ~ 1000 ° C. 3. An apparatus for writing and amplifying a hologram diffraction grating in a single crystal by using visible laser light, wherein the lithium niobate single crystal memory according to claim 1 is used as the single crystal. And an optical amplifier.