JPH0456887A - Volume multiplexed hologram element - Google Patents

Abstract

PURPOSE:To obtain a volume multiplexed hologram element having high diffraction efficiency by using photorefractive crystal whose refractive index is changed by light irradiation as a recording material and applying stress from the external to the crystal to distort its crystal grating. CONSTITUTION:The photorefractive crystal for changing the refractive index by light irradiation is used as the recording material and constituted so that the crystal grating is distorted by applying stress from the external to the crystal. When a prism 24 is cut out from Sr0.6BA0.4Nb2O6(0.05% Cedope) monocrystal e.g. used as the photorefractive crystal material, two C faces on the surfaces of the prism are held between copper blocks 23, 23 and pressure is applied in the C axis direction, the diffraction efficiency is improved. Consequently, an electro-optical effect is increased by applying pressure to the prism and high diffraction efficiency can be obtained in the volume multiplexed hologram element.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a volumetric multiplexing hologram element that uses a photorefractive crystal to record and reproduce a plurality of holograms in the same element.

[Conventional technology and its issues]

Scattered light by an object (object light) using a light source such as a laser
Interferes with unscattered light (reference target) from the same light source, records the interference fringes (hologram) on a recording medium such as an optically recordable photographic plate, and when reproducing, the recorded interference fringes are Holography is a technique that reproduces the light scattered by an object by irradiating only the reference target.

Furthermore, in the case of a recording medium with depth, it is possible to record a plurality of holograms in the same medium by changing the incident directions of the object beam and the reference destination. This technique is called volumetric multiplex holography.

As a volume hologram recording medium, a photorefractive crystal (LiN bo 3
．． B aT io 3. S rxB al-xN
bos, Bt, tst○,. , B1゜Ge0to, etc.)
is being considered.

However, these materials alone are not suitable for practical use as hologram recording media because their photosensitivity and refractive index change are small and their diffraction effects are low. The photorefractive phenomenon is a phenomenon in which electrons that are photo-excited from impurity atoms or color centers scattered in a crystal to the conduction band move spatially due to diffusion, 1-lift, or photovoltaic effect, and respond to the light intensity distribution. It originates from the fact that an electric field distribution is formed in the crystal, and the electric field causes a spatial change in the refractive index due to the electro-optic effect.

Therefore, in order to increase photosensitivity and diffraction efficiency, efforts have been made to adjust the concentration and type of impurity atoms and to increase the amount of drift by applying an external electric field. but,
This does not mean that sensitivity or diffraction efficiency can be increased no matter how much. For example, if the concentration of impurity atoms is increased too much, the transparency of the medium deteriorates, making it impossible to perform volume multiplexing, and if the external electric field is strengthened, the crystals may be destroyed due to dielectric breakdown.

In addition, the recording medium is made into a fiber (diameter〉〉wavelength; multi-mode fiber) to increase the interaction distance between the medium and light.
A method of increasing diffraction efficiency with a small change in refractive index has been proposed and studied. However, even if a plane wave is incident, the wave number vector of the light reflected from the side surface will spread between the inverted characters depending on the curvature of the side surface, and the wave number vector of the holographic grating to be recorded will also spread in the inverted space. 1
On the other hand, it is not desirable to make the fiber too long because it increases the number of reflections and increases the crosstalk between images during triple angle recording.

The present invention was made in view of the above-described drawbacks of the photorefractive medium for volume holograms, and an object of the present invention is to provide a volume multiplex hologram element with high diffraction efficiency. Here, the electro-optic effect is increased and the diffraction efficiency is increased.

[Means to solve the problem]

In order to achieve the above object, the present invention provides a multiplex hologram element in which a plurality of interference fringes formed by interference of light beams are recorded in the same element volume, wherein the multiplex hologram element changes its refractive index by light irradiation. It is characterized by using a photorefractive crystal as a recording material and increasing the electro-optic effect by applying external stress to distort the crystal lattice.

The configuration of the present invention will be described below.

First, we will explain how the crystal lattice, when distorted, affects the electro-optic constant of the crystal.

Here, as an example, S rxB a, -1h bo 3
(square tungsten bronze structure), Table 1 shows the lattice constant and electro-optical constant when the composition ratio of strontium and norium is changed.

As the amount of strontium increases, the size of the unit cell (a
, c) are small, and conversely, a/c becomes large, and it can be seen that the electro-optic constant becomes large accordingly. on the other hand,
In the case of this crystal system, the valence pant is mainly made of the 2p orbital of oxygen, and the conduction pant is mainly made of the 4d orbital of niobium, and the optical properties in the visible light region are determined by these two bands. There is. Therefore, the change in the electro-optic constant is directly derived from the change in the lattice constant, and it can be interpreted that the strontium/barium ratio brings about the effect of changing the lattice constant. Therefore, in addition to changing the composition ratio, by reducing the lattice constant by applying pressure or other methods, it is possible to increase electro-optics.

(Table-1) Composition ratio Electro-optical constant Lattice constant Sr/Bar rzp(XIO-12m/V) a(
A) c(A) a/c75/25 1400
12.440 3.924 3.17060/4
0 420 12.467 3.937 3.
16750150 180 12.475 3
．． 952 3.157 Next, the configuration of a volumetric multiplex hologram element to which pressure is applied from the outside will be described. Figure 1 (a
) to FIG. 1(e) show examples of applying pressure according to the present invention, and FIG. 1(a) shows a photorefractive crystal 1 which is mechanically sandwiched between blocks 2 and 2 from the upper and lower surfaces. This is the addition of compressive force in the direction.

Figures 1(b) to 1(e) show a transparent material (cladding) surrounding the photorefractive crystals (core) 3 to 8.
9 to 12, each having a covering structure, and the cladding is made of a material having a larger coefficient of thermal expansion than the core material.

In this case, cores 3 to 8 are compressed by claddings 9 to 12, respectively, during the process of cooling from the temperature at which the device is manufactured (the melting point of the core material or cladding material) to room temperature. Compression does not necessarily have to be isotropic, and may be changed depending on the properties of the photorefractive crystal used, and generally it may be strained in a direction that reduces the symmetry of the crystal.

Therefore, variations such as those shown in FIG. 1(b) to FIG. 1(e) are possible. 1(b) is a type that applies hydrostatic pressure to the core 3, and FIG. 1(C) is a type that has a hollow part 13 on the side surface and applies compressive force to the core 4 only in one direction.
Fig. 1(d) shows a type in which compressive forces are applied to the core 5 in two directions, and Fig. 1(e) shows a type in which compressive forces of different magnitudes are applied in two directions to the cores 6 to 8.

〔Example〕

FIG. 2 shows an embodiment of the present invention.

As a photorefractive crystal material, SrO, llB
ao, 4N bto e(o, 05%Cedop
e) 5 [mm) x 3 [mm+] x 1 from single crystal
, 6 mm) was cut out, and the two 0 faces of the prism surfaces were sandwiched between copper blocks 23, 23, and the shape shown in FIG.
Pressure was applied in the C-axis direction using method a). Here, as a method for measuring changes in electro-optic constants, diffraction efficiency using interference fringes was measured. As a light source, an Alfon ion laser (wavelength 514.5 [nrr+)) 21 is used, and a half mirror 22 is used to generate two light beams 2 of equal intensity.
After dividing into 5 and 26 beams, the two beams were made to enter the crystal 24 at an angle of 20 degrees, and the exposure amount was 10 [mW/m m '
] for 2 [m5ec). After that, the second beam 26 is cut by the shutter 27, and the first beam 26 is cut off by the shutter 27.
5 was irradiated and the diffraction efficiency was measured. At this time, the half-wave plate 28 is set so that the electric field vector of the incident light points in the C-axis direction.
adjusted. When no pressure is applied, the diffraction efficiency is 10
%, whereas when the same measurement was performed while applying a force of 20 CN, the diffraction efficiency improved to 25%.

〔Effect of the invention〕

As described above in detail, according to the present invention, the electro-optic effect is increased by applying pressure, and high diffraction efficiency can be obtained in the volume multiplexed hologram element.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of a method for applying pressure in a volume multiplexed holographic element according to the present invention, and FIG. 2 is a diagram showing an embodiment of the present invention, in which an SBN crystal is used to apply pressure or increase diffraction efficiency. FIG. 2 is a diagram illustrating an experimental example to demonstrate that the method is effective. 1.3-8...Photorefractive crystal 2nd
figure

Claims (1)

[Claims] A multiple hologram element that records a plurality of interference fringes formed by interference of light beams in the same element volume, wherein the multiple hologram element records a photorefractive crystal whose refractive index changes depending on light irradiation. A volumetric multiplex hologram element characterized by having a material whose crystal lattice is distorted by applying stress from the outside.