JP3298766B2 - Method of manufacturing hologram recording medium - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a real-time holography technique using a photorefractive crystal which forms a refractive index distribution similar to interference fringes of light, and records and reproduces a plurality of holograms in the same element. It is used as a recording medium of a volume multiplex hologram apparatus.

[0002]

2. Description of the Related Art Using a light source such as a laser, light scattered by an object (object light) interferes with non-diffused light (reference light) from the same light source, and the interference fringes are stored in a photographic dry plate or the like capable of optical recording. At the time of recording on a medium and at the time of reproduction, a technique of irradiating the recorded interference fringes with only reference light to reproduce scattered light by an object is called holography. Here, when the depth of the storage medium is sufficiently longer than the wavelength of the recording light, a plurality of holograms can be recorded in the same medium. This technique is called volume multiplex holography. Certain dielectrics, called photorefractive materials, are used as recording media for volume multiplex holography because they change the refractive index when irradiated with light.

In a photorefractive material, electrons or holes are excited in the material by light irradiation, and subsequently move through the material to form a charge distribution in the same pattern as light interference fringes. The charge distribution results in an electric field distribution and, due to the electro-optic effect, forms a refractive index distribution in the material. Since the refractive index distribution has the same pattern as the interference fringes, a hologram has been recorded.

[0004] In volume multiplex holography, the pitch and direction of interference fringes are changed by changing the incident direction and wavelength of light, and different pages are recorded. The (N-1) th page is partially erased from the recorded first page. Therefore, the amount of exposure when recording the Nth page is made smaller than that of the (N-1) th page, and adjustment is performed so that the diffraction efficiencies of all pages are equal. This is called a schedule recording method.

At this time, the hologram to be recorded is M
Assuming a page, the diffraction efficiency (η) per page is
It is represented by equation (1). η = γ / M ² (1)

As expressed by this equation, the diffraction efficiency is M
Is inversely proportional to the square of, and the diffraction efficiency decreases rapidly as the number of holograms increases. That is, when the proportionality constant γ is small, signal detection cannot be performed due to noise due to the noise of the optical detector before the storage capacity is limited to noise due to crosstalk between pages of the hologram or image distortion of one page. This has the disadvantage that the data storage capacity is reduced.

Further, lithium niobate (LiNbO ₃ )
In some cases, the proportional constant γ was large and the reduction in diffraction efficiency was not a drawback, but there were drawbacks such as low optical sensitivity for hologram generation and a long recording time, which made it impractical as a real-time hologram. There is.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of manufacturing a hologram recording medium in which a decrease in diffraction efficiency due to multiplex recording is reduced and the optical recording sensitivity for producing a hologram is improved. is there.

The method for producing a hologram recording medium according to the present invention is a method for producing a hologram recording medium used in a volume multiplex hologram recording / reproducing apparatus, wherein strontium barium niobate (chemical formula: Sr _x Ba _{1 -x} Nb ₂ O ₆ ; 0 < x ≦
0.75) the growth of the crystal,

At the time of growth, cerium dioxide (chemical formula: CeO
₂ ) is added in a weight ratio of 0.1% or less of the whole,

The partial pressure of oxygen is not less than 1/12 and not more than 1/6
Performing in a mixed gas atmosphere of argon and oxygen,

The strike to which the cerium dioxide is added
800 ° C. after the growth of the rontium barium niobate crystal
A step of performing a heat treatment in an oxygen atmosphere of 1200 ° C. or less.
It is characterized by having.

[0013]

Embodiments of the present invention will be described below. In the present invention, strontium barium niobate (SBN) is used as a recording medium to achieve high light sensitivity. In particular, strontium barium niobate doped with cerium is used . This ensures, in comparison with the lithium niobate, can be obtained 1000x photosensitivity to 100 times.

The crystal of strontium barium niobate is grown by a Czochralski method using a platinum crucible.
At that time, a growth atmosphere is an oxygen / argon atmosphere of 1 atm, and an oxygen partial pressure is set to 1/6 or less and 1/12 or more. The reason for reducing the oxygen partial pressure to 1/6 or less is that the growth temperature is 1500 ° C.
This is to prevent the platinum crucible from being deteriorated by oxygen. The reason why the oxygen partial pressure is set to 1/12 or more is to facilitate crystal growth.

In the case of doping with cerium, if the weight of cerium oxide exceeds 0.1% of the weight of strontium barium niobate, crystal growth becomes difficult.

The grown cerium-doped strontium barium niobate (Ce: SBN) is sealed in a quartz tube together with oxygen, and is heat-treated at a temperature of 800 ° C. to 1200 ° C. The heat treatment time varies depending on the size and temperature of the crystal. For example, when a 5 mm square crystal is heat-treated at 800 ° C., it takes about 12 hours. The heat treatment is usually performed in a pure oxygen atmosphere, but is not necessarily limited to a pure oxygen atmosphere, and may be any atmosphere including, for example, about 90% oxygen.

After the heat-treated cerium-doped strontium barium niobate (Ce: SBN) is cooled to room temperature, a poling operation is performed. In the poling operation, the crystal is immersed in silicon oil, and 500 V /
cm or more. After raising the temperature of the silicon oil containing the crystal to 100 ° C., a voltage is applied to the crystal. The temperature is cooled to room temperature (25 ° C.) over about 30 minutes to 1 hour while applying a voltage, and then the voltage is reduced to zero. Thus, the manufacture of the hologram recording medium is completed.

As described above, by using Ce: SBN, the optical sensitivity of hologram formation is improved by about two or three orders as compared with the case of using LiNbO ₃ . Regarding crystal growth,
By growing in an atmosphere with relatively little oxygen, it is possible to grow without damaging the platinum crucible.

Further, by performing a heat treatment in oxygen, SBN
The ratio of Ce ions in the material can be controlled. Ce can take two kinds of states, Ce ³⁺ and Ce ⁴⁺ , in the crystal. Among them, as the ratio of photoactive Ce ³⁺ to Ce ⁴⁺ approaches 1, the photosensitivity increases. Is high, and the proportionality coefficient γ in equation (1) is
Becomes larger. By heat treatment in an oxygen atmosphere at a temperature sufficiently lower than the melting point of the crystal, Ce ³⁺ that was excessively present due to the reduction atmosphere during the crystal growth due to the decrease in the amount of oxygen defects without the movement of Ce ions. And can be converted to Ce ⁴⁺ .

As described above, the Ce ³⁺ / Ce ⁴⁺ ratio in the crystal is controlled by heat-treating Ce: SBN grown in a reducing atmosphere in an oxidizing atmosphere at a relatively low temperature.
It is possible to reduce the reduction in diffraction efficiency due to multiplex recording.

[0021]

EXAMPLE In this example, the effect of heat treatment after crystal growth was examined as follows. Strontium barium niobate (Sr _0.6 Ba _0.4 Nb ₂ O ₆ ) was doped with cerium-doped crystal (Ce: SBN) in an argon / oxygen atmosphere (gas flow rate: Ar = 1000 cc / min, O ₂ = 2).
(00 cc / min) by the Czochralski method. SrCO ₃ , BaCO ₃ , Nb ₂ O ₅
The powder was mixed in a molar ratio of 0.6: 0.4: 1.0, and a mixture obtained by adding 0.02% by weight of CeO ₂ powder and sintering the powder was used. After the crystal was grown, it was kept at 1200 ° C. in the air for one day.

The crystal orientation of the pulled crystal is determined by X-ray diffraction, and along the crystal axis (a = 3.847 mm, a
= 6.075 mm, c = 3.809 mm) A rectangular parallelepiped was cut out, and all six surfaces were optically polished.

In the poling, a silver paste was applied to both c-planes, sandwiched and fixed between two copper blocks, immersed in silicon oil, and the temperature was raised to 100 ° C. In this state, a voltage of 300 V was applied to the copper block, and the copper block was gradually cooled over 1 hour until it reached room temperature, and finally the voltage was reduced to zero.

As described above, various characteristics of the manufactured Ce: SBN crystal were measured. Next, this Ce: SB
After performing a heat treatment of holding the N crystal in pure oxygen at 800 ° C. for 12 hours, the same poling was performed and the characteristics were measured in the same manner.

FIG. 1 shows an example of the result of measurement of the number of effective carriers, which plays an important role in suppressing the reduction in diffraction efficiency during multiple writing and is one of the parameters of a photorefractive crystal. FIG. 1 shows the change in the number of effective carriers before and after the heat treatment. White circles before heat treatment,
The filled circles are the data after heat treatment. The horizontal axis is the square of the wave number of the grating vector, and the vertical axis is the function of the coupling strength of the two-wave mixing and the grating vector. The number of effective carriers is obtained according to equation (2).

[0026] Here, ε is the dielectric constant of the crystal, K is the wave number of the grating vector, e is the electron charge, N _eff is the number of effective carriers,
k _B is the Boltzmann constant, T is the temperature, λ is the wavelength of light in vacuum, and n is the refractive index.

A is a measuring method described below, and is a vibration amplitude when the detector signal (S) vibrates as S = ζ (1 + AcosF (ω, t)) (3) .

The measuring method will be described. Using light having a wavelength of 514.5 nm of an argon ion laser, the light was set to an equal intensity of 2
After the division, the light was incident on the a-plane of the crystal. Here, the direction of the grating vector points in the c-axis direction of the crystal, and the direction of oscillation of the electric field of light points in the a-axis direction of the crystal. Before the light split into two is incident on the crystal, it is vibrated at a frequency ω by a piezo mirror that moves in parallel, and the phase of the grating is changed. Here, the vibration amplitude of the phase is larger than the grating pitch. The detector is placed behind a crystal of light that does not pass through the piezo mirror, and its time change is examined. The description of the behavior of the vibration is omitted, but what is needed here is only the vibration amplitude of Expression (3).

In FIG. 1, the point where the extension of the straight line intersects the horizontal axis was determined, and N _eff was determined from the intercept. The relative dielectric constant is 8
00. The obtained number N _eff of effective carriers is 2.4 × 10 ¹⁶ cm ⁻³ before the heat treatment, and 5.2 × 10 ¹⁶ after the heat treatment.
cm ^-3 , and it was found that the number of effective carriers increased more than twice by the heat treatment.

Further, assuming that the amount of carriers required to record one page of hologram is ΔN, by recording the M-th page, the amount (erasing) from the first page to the (M−1) th page ( Δη) is expressed by −Δη = c (ΔN / N _eff ) (4) (c is a proportional constant), and is proportional to the ratio between ΔN and N _eff . That is,
It can be seen that γ in equation (1) was improved about twice by the heat treatment.

[0031]

As described above, by using strontium barium niobate and further using strontium barium niobate doped with cerium, the photosensitivity of hologram formation can be greatly improved as compared with the case where lithium niobate is used. The improvement and the heat treatment in an oxygen atmosphere can increase the number of effective carriers, thereby suppressing the drawback of reducing the diffraction efficiency due to multiplex recording.

[Brief description of the drawings]

FIG. 1 is a graph showing a change in the number of effective carriers before and after heat treatment.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-6-56594 (JP, A) Tsuyoshi Yagi, Optimization and digital recording of SBN crystal for holographic recording, Technical Report of the Institute of Image Information and Television Engineers, Japan, ( ) The Institute of Image Information and Television Engineers, Vol. 22 No. 14, p. 45-p. 52 Takeshi Yagi et al., Holographic memory using ferroelectrics, rare earth, Japan, November 3, 1998, 33, p. 59-p. 70 Physica Status Solidi (a), vol. 147, p. 585-p. 589 Applied Optics, vol. 33 No. 16, p. 3348-p. 3351 (58) Field surveyed (Int.Cl. ⁷ , DB name) G03H 1/04-1/34 G02B 5/32 G02F 1/03 G02F 1/35 JICST file (JOIS) Web of Science (Maruzen)

Claims (1)

(57) [Claims] 1. A method of manufacturing a hologram recording medium for use in a volume multiplex hologram recording / reproducing apparatus, comprising the steps of: strontium barium niobate (chemical formula: Sr _x Ba _1-x Nb)
₂ O ₆ ; 0 <x ≦ 0.75) The crystal was grown by adding cerium dioxide (chemical formula: CeO ₂ ) at a weight ratio of 0.1% or less of the whole during the growth, and the partial pressure of oxygen was reduced to 12 minutes. Performing at least one-sixth and less than 1/6 in a mixed gas atmosphere of argon and oxygen; and after growing the strontium barium niobate crystal to which the cerium dioxide is added, at 800 ° C. or more and 1200 ° C.
A method for producing a hologram recording medium, comprising a step of performing a heat treatment in an oxygen atmosphere at a temperature of not more than ° C.