JPH07261642A - Hologram recording and reproducing method - Google Patents

Abstract

PURPOSE:To obviate dissipation of a hologram in spite of preservation in a dark place and to prevent limitation on diffraction efficiency by raising the temp. of a recording medium to the extent of not exceeding the phase transition temp. of the ferrodielectric and paradielectric substance of the material of the recording medium at the time of writing the hologram. CONSTITUTION:The temp. of the recording medium 12 is heated by a heater 13 to the temp. of not exceeding the phase transition temp. of the ferrodielectric and paradielectric substance of the material of the recording medium and is further held to maintain the specified temp. Next, a shutter 3 and a shutter 5 are opened and desired information is imparted to object light 16 by an air modulator 9, by which the recording medium 12 is simultaneously exposed with the object light 16 and reference light 17. The shutter 3 and the shutter 5 are thereafter closed and in succession, the heater 13 is turned off to lower the temp. down to room temp. The shutter 3 is then opened and the hologram is reconstructed where the temp. settles down. The temp. at the time of the reproduction is kept in a temp. range below the temp. at the time of recording and above the absolute zero degree.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hologram recording / reproducing method. More specifically, it relates to a real-time holography technique using a photorefractive crystal that forms a refractive index distribution similar to the interference fringes of light, and is applicable to a volume multiplexing hologram device that records and reproduces multiple holograms in the same element. Is.

[0002]

2. Description of the Related Art A light source such as a laser is used to cause light scattered by an object (object light or signal light) to interfere with non-scattered light (reference light or pump light) from the same light source, and the interference fringes can be optically recorded. A technique of recording on a recording medium such as a photographic dry plate, and irradiating only the reference light on the recorded interference fringes at the time of reproduction to reproduce scattered light by an object is called a holography technique.

Here, when the depth of the recording medium is sufficiently longer than the wavelength of the recording light, it is possible to record a plurality of holograms in the same medium. This technique is called volume multiplex holography, and a certain kind of dielectric material called a photorefractive material changes the refractive index by irradiating light, and is therefore used as a recording medium for volume multiplex holography.

In this photorefractive material, electrons or holes are excited in the material by light irradiation and subsequently move in the material, so that a charge distribution having the same pattern as the interference fringes of light is formed. The charge distribution results in an electric field distribution and the electro-optic effect forms a refractive index distribution in the material.

[0005]

In the photorefractive material described above, electrons or holes are not only excited by light but also slightly excited by temperature. Therefore, even if stored in a dark place, There is a drawback that the charge distribution gradually disappears, that is, the recorded hologram disappears.

Further, in writing a hologram, the diffraction efficiency is highest when the intensity ratio of the object light and the reference light is equal. However, in a normal photorefractive material, the interference fringes of light and the fringes due to the refractive index distribution are in phase. There is a shift, and energy is exchanged between the object beam and the reference beam during writing of the hologram. Therefore, there is a drawback that the intensity ratio of the object light / reference light changes, and the diffraction effect decreases.

Further, the magnitude of change in the refractive index that can be realized is
It was limited by the electro-optical constant of the material, the impurity concentration, the angle formed by the object light and the reference light, and the like. The present invention has been made in view of the above-described conventional art, and an object of the present invention is to prevent the hologram from disappearing even when stored in a dark place and to prevent the diffraction efficiency from being limited by various factors.

[0008]

According to the present invention to achieve such an object, in a hologram recording / reproducing method using a ferroelectric material, of which a refractive index is changed by light irradiation, as a recording medium, recording is performed when a hologram is written. It is characterized in that the temperature of the medium is raised to a level not exceeding the ferroelectric / paraelectric phase transition temperature of the material. Here, the temperature at which the hologram is reproduced is set to a temperature equal to or lower than the recorded temperature, and when the thermal expansion coefficient of the recording medium is positive, a pressure is applied during recording so that the thermal expansion coefficient of the recording medium is When it is negative, it is characterized in that pressure is applied during regeneration.

Further, it is characterized in that the wavelength of the irradiation light at the time of recording is different from that at the time of reproducing, and the light at the time of reproducing is irradiated at an incident angle different from that at the time of recording. Further, as the recording medium, for example, strontium / barium niobate (Sr _x Ba _1-x
It is characterized by using Nb ₂ O ₆ , 0 <x <1).

[0010]

In some ferroelectric photorefractive crystals, when a hologram is recorded at a high temperature and reproduced at a low temperature, the diffraction efficiency is increased. The reason for this is unclear, but when holograms are recorded near the phase transition temperature, spontaneous polarization is inverted at the same pitch as the interference fringes to form domains, and when the temperature is lowered, domains grow and diffraction efficiency increases. I suspect that it is.

This spontaneous polarization inversion domain is not caused by the charge transfer found in the usual photorefractive effect, but its origin is thermodynamically stable spontaneous polarization. Therefore, if it is left in the dark, the hologram disappears. Does not happen. Here, since the temperature of the photorefractive material changes during recording and during reproduction, thermal contraction or thermal expansion occurs,
Bragg conditions change.

The Bragg condition mentioned here means that the wave number vector of the reference light, the wave vector of the object light (diffracted light at the time of reproduction) and the wave vector of the gradient index grating in the recording material are expressed by the following equation. To meet. If this Bragg condition is not satisfied, the diffraction efficiency will decrease or become zero.

[0013]

[Equation 1]

The refractive index distribution recorded at a high temperature changes the pitch at the same magnification as the expansion and contraction of crystals as the temperature is lowered during reproduction. Therefore, the wave number vector of the refractive index distribution grating at the time of reproduction when the linear expansion coefficient in the direction along the refractive index distribution grating is α is expressed by the following equation.

[0015]

[Equation 2]

Therefore, the Bragg condition at the time of recording is not satisfied at the time of reproducing. The inventions of claims 3, 4 and 5 aim to correct this Bragg mismatch. The invention described in claim 3 is a method of compensating thermal expansion and contraction by elastic strain by applying pressure to the photorefractive material.

The invention described in claim 4 is a method for compensating for a Bragg mismatch by changing the wavelength of the irradiation light at the time of recording and at the time of reproducing, and the wave number is expressed by the following equation by the wavelength change of Δλ = αλΔT. It changes as shown. (Δk / k) = αΔT Therefore, K → K (1 + αΔT), and a new Bragg condition is satisfied.

The present invention described in claim 5 is a method of changing the incident direction of the reference light, and if the angle formed by the new reproduction light and the diffracted light is 2θ + 2Δθ and Δθ = αΔT tanθ, the following equation holds. But a new Bragg condition holds. K = k ₀ −k _r = 2k sin (θ + Δθ) = 2k (sin θcos Δ
θ + cos θsin Δθ) ≈ 2 ksin θ (1 + αΔT) = K (1 + αΔT)

[0019]

Embodiments will be described in detail below with reference to the embodiments shown in the drawings. FIG. 1 shows an embodiment of the basic configuration of the present invention. In this example, a hologram is recorded in a state where the temperature of the photorefractive material, which is a recording medium at the time of hologram recording, is raised to near the ferroelectric / paraelectric phase transition temperature of the material, and the temperature is lowered and reproduced. It is a thing.

As shown in FIG. 1A, the laser light 6 emitted from the laser light source 1 is spread by the beam expander 2 to the same size as the spatial light modulator 9, and after passing through the shutter 3, The half mirror 4 splits the object light 16 and the reference light 17. The object light 16 and the reference light 17 are respectively reflected by the mirrors 7 and 8 and adjusted so that the photorefractive material, which is the recording medium 12, can be irradiated. Furthermore,
The object light 16 passes through the shutter 5 before being reflected by the mirror 7, and is given information when passing through the spatial modulator 9 after being reflected by the mirror 7.

The recording medium 12 is installed so that its temperature can be controlled, as shown in FIG. 1 (b). That is, the heater 13 is arranged on the rotary stage 14, the recording medium 12 is arranged on the heater 13, and the heater 1
A thermocouple 15 is attached to 3. Therefore, by energizing the heater 13, the recording medium 12 can be heated to a predetermined temperature, and the temperature at that time is the thermocouple 1.
It is configured to be monitored at 5.

Further, by rotating the rotary stage 14, it is possible to irradiate the recording medium 12 with light for reproduction at an incident angle different from that for recording. A lead wire 11 is connected to the recording medium 12 for polling. As the recording medium 12, a photorefractive material which has been conventionally used, for example, BaTiO ₃ or LiNb is used.
In addition to O ₃ , strontium barium niobate (Sr _x Ba
_1-x Nb ₂ O ₆ , 0 <x <1) can also be used.

The lead wire 11 and the rotary stage 14 are related to the poling of the photorefractive material and the invention of claim 5, and are not necessarily required for the inventions of claims 1, 2, 3, and 4.

In the above-described embodiment, recording / reproduction of hologram is performed as follows. First, the temperature of the recording medium 12 is raised by the heater 13 to a temperature which does not exceed the ferroelectric / paraelectric phase transition temperature of the material, and is further maintained so as to be constant. Next, the shutter 3 and the shutter 5 are opened, and desired information is added to the object light 16 by the spatial modulator 9, and the object light 16 and the reference light 17 are simultaneously exposed on the recording medium 12.

After exposure for a sufficient time, the shutter 3 and the shutter 5 are closed, the heater 13 is subsequently cut off, and the temperature is lowered to room temperature. After that, when the temperature has settled down, the shutter 3 is opened and the hologram is reproduced. If the temperature during reproduction is lower than the temperature during recording, the lower limit is not recognized. That is, the temperature at the time of reproduction can be within the temperature range at the time of recording and at the absolute zero degree or more.

However, depending on the type of crystal, there is one that exhibits a new phase transition as it is lowered to a lower temperature (for example, BaTiO ₃ , KNb _x , Ta _1-x O ₃ ), and in such a case. , Limited to the temperature range exhibiting the same phase at the recorded temperature.

Since the temperature is different between the time of recording and the time of reproduction as described above, there is a concern that the pitch of the recorded interference fringes may change during reproduction due to thermal expansion or contraction, and optimum diffraction efficiency may not be obtained. . In particular, the greater the temperature difference between the recording temperature and the reproducing temperature, the greater the tendency. Therefore, in order to correct the Bragg mismatch, for example, the measures shown in the following embodiments can be adopted.

FIGS. 2A and 2B show another embodiment of the present invention. In this embodiment, a pressure is applied to the recording medium 25, which is a recording medium, in the direction of the wave vector of the object light 21 and the reference light 22, that is, in the direction along the grating vector of the interference fringes. It is intended to reduce the thermal contraction or thermal expansion due to the temperature difference. That is, stage 2
A heater 24 is arranged on the heater 3, a recording medium 25 is arranged on the heater 24, and a thermocouple 26 is attached to the heater 24.

A vise 27 for sandwiching the recording medium 25 is arranged on the heater 24 so that pressure can be applied to the recording medium 25 in the direction along the grating vector. Here, when the thermal expansion coefficient in the direction along the grating vector of the recording medium 25 is positive,
The pressure is applied by the vise 27 at the time of recording, and the pressure is released at the time of reproducing. On the contrary, when the thermal expansion coefficient of the recording medium 25 in the direction along the grating vector is negative, no pressure is applied during recording, and a vise 27 is applied during reproduction.

The pressure is not necessarily applied along the grating vector, and for example, the recording medium 25 may be pressed against the heater 24. At this time, if the coefficient of thermal expansion of the recording medium 25 in the direction along the grating vector is positive, no pressure is applied during recording and pressure is applied during reproduction, and conversely, recording medium 2
When the coefficient of thermal expansion in the direction along the grating vector of 5 is negative, pressure is applied during recording and pressure is released during reproduction.

Further, the magnitude of the pressure to be applied to the recording medium 25 depends on the thermal expansion coefficient α, elastic modulus ε and Poisson's ratio σ of the recording medium 25. It is preferable to minimize the expansion and contraction of the recording medium in the direction along the grating vector.

For example, when the pressure is applied in the direction along the grating vector as the temperature difference ΔT, the pressure P should be applied so as to satisfy the following equation. | ΑΔT | = | εp | When pressure is applied in the direction orthogonal to the grating vector, the pressure P should be applied so as to satisfy the following equation. | ΑΔT | = | σεp |

In the above embodiment, pressure was applied to the recording medium, but instead of this, the wavelength at the time of reproduction may be changed from that at the time of recording so as to satisfy the Bragg condition. That is, this is a method of using a wavelength tunable laser as the laser light source 1 shown in FIG. When the coefficient of thermal expansion of the recording medium 12 in the direction along the grating vector is positive, the wavelength at the time of reproduction is made longer than the wavelength at the time of recording, and conversely, the coefficient of thermal expansion in the direction along the grating vector of the recording medium 25. If is negative, the wavelength for reproduction is set shorter than the wavelength for recording. That is, as the wavelength λ at the time of recording, the amount of wavelength change Δλ
Changes the wavelength so that satisfies the formula below. Δλ = αλΔT

Further, instead of the pressure on the recording medium and the wavelength change at the time of recording and at the time of reproducing, the incident angle of the reference beam on the recording medium at the time of reproducing may be changed. That is, when the angle formed by the object beam 16 and the reference beam 17 during recording is 2θ,
It is preferable to rotate the rotating stage 14 shown in FIG. 1 by αΔTtanθ (radian) so as to satisfy the Bragg condition. When the coefficient of thermal expansion in the direction along the grating vector of the recording medium 12 is positive, the rotation direction is as shown in FIG.
In the counterclockwise direction when viewed from the paper, on the contrary, the recording medium 1
When the coefficient of thermal expansion in the direction along the grating vector of 2 is negative, it is preferable to make it clockwise when viewed from the paper surface in FIG. It has been confirmed that the method of the present invention can be applied, not all photorefractive materials,
These are some crystals that bring about an increase in diffraction efficiency at low temperatures.

Next, specific examples of the present invention will be described. [Embodiment 1] As shown in FIG. 3A, the laser light emitted from the Ar ion laser 30 has a special filter 31.
After passing through the
And the reference beam 38. Object light 39 and reference light 38
After passing through the shutters 33 and 34, respectively, the mirror 3
It is reflected by 5, 36 and adjusted so that the photorefractive material that is the recording medium 42 can be irradiated. Furthermore,
After the reference light 38 is reflected by the mirror 36, the half-wave plate 3
Pass 7.

The recording medium 42 is installed so that the temperature can be controlled, as shown in FIG. 3 (b). That is, the Peltier element 45 is arranged on the rotary stage 44, the recording medium 42 is arranged on the Peltier element 45, and the Peltier element 45 further includes a copper constantan thermocouple 45.
Is attached. Therefore, by energizing the Peltier element 45, the recording medium 42 can be heated to a predetermined temperature, and the temperature at that time is monitored by the copper constantan thermocouple 45.

By rotating the rotary stage 44, it is possible to irradiate the recording medium 42 with light at the time of reproduction at an incident angle different from that at the time of recording. A lead wire 41 is connected to the recording medium 42 for polling. In this embodiment, strontium barium niobate as a recording medium _{42 (Sr 0 .6 Ba 0.4 Nb} 2 O 6, hereinafter simply abbreviated as SBN) using single crystal. This single crystal was prepared by doping 1 mol of SBN with cerium dioxide (CeO ₂ ) at a weight ratio of 0.05% when pulling the crystal. The size of this single crystal is 3.94 mm × 2.65 mm × 2.
93 mm, 2.93 mm is the c-axis direction of the crystal.

In this embodiment having the above-mentioned structure,
Recording and reproduction of holograms are performed as follows. First, in order to poling the single crystal that is the recording medium 42, 20 nm of gold is vapor-deposited on the c-plane, that is, both sides of 3.94 mm × 2.65 mm, and the temperature is raised to 95 degrees Celsius. Next, 1.5 kv is applied to both surfaces of the recording medium 42 on which gold has been vapor deposited, and the temperature is gradually cooled to room temperature over 30 minutes. After that, the voltage is reduced to 0v and the polling is completed.

Subsequently, the temperature is raised to 60 degrees Celsius and stabilized until the temperature fluctuation becomes 0.1 degrees or less, and the reference light 3
8 is 128 μm and object light 39 is 82 μmW, 3.94 mm × 2.6
Irradiation was performed for 3 minutes so that the 5 mm incident surface was just covered. Here, the angle formed by the reference light 38 and the object light 39 is 6.3 degrees,
The wavelength is 514.5 nm by the argon ion laser 30, and the polarization of the object light 38 and the object light 39 (the vibration direction of the electric field of the light) includes the reference light 38 and the object light 39 by using the polarization beam splitter 32 and the half-wave plate 37. It was set to be in-plane (in-plane).

FIG. 4 shows the temperature change of the diffraction efficiency when the temperature is gradually lowered. In the figure, the ∘ mark indicates the diffraction efficiency actually measured in this example, and the ● mark indicates the theoretical prediction of the diffraction efficiency when the Bragg mismatch due to thermal expansion is corrected. In the figure, the X mark indicates 20 degrees Celsius for comparison.
It is a conventional example which shows the diffraction efficiency when a hologram is recorded with a degree.

As shown in FIG. 4, the present embodiment in which a hologram is recorded by holding a Ce-doped SBN single crystal as a recording medium at a high temperature has a higher diffraction efficiency than the conventional example recorded at a low temperature. It can be confirmed that the diffraction efficiency is further increased by correcting the Bragg mismatch. Furthermore, the hologram thus written showed no phenomenon of diffraction efficiency even when kept at 60 degrees Celsius for several hours in the dark. This means a semi-permanent shelf life at room temperature.

[Embodiment 2] In this embodiment, a hologram is written at 20 ° C. in the same manner as in Embodiment 1 shown in FIG. 3, and after obtaining a diffraction efficiency of 6% at the temperature, it is cooled. The temperature was kept at 20 degrees. After that, the stage 44 was rotated counterclockwise by 0.002 ° when viewed from above the paper surface of FIG. 3A, and thereby the diffraction efficiency was increased from 11% to 18%.

This hologram showed no change in diffraction efficiency even after being stored at room temperature in the dark for several days. With the conventional method, SBN: Ce is 20 degrees Celsius.
In terms of the degree, a significant decrease is observed even for one day, which confirms the effect of extending the shelf life of the hologram according to the present invention.

[0044]

As described above in detail with reference to the embodiments, the present invention has the effect of increasing the diffraction efficiency by recording a hologram at a high temperature and reproducing it at a low temperature. Furthermore, by correcting the Bragg mismatch due to thermal expansion, the diffraction efficiency can be further increased. In addition, the shelf life of holograms can be extended almost semi-permanently.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an embodiment according to the basic configuration of the present invention.

FIG. 2 is an explanatory diagram showing another embodiment of the present invention.

FIG. 3 is an explanatory diagram showing a first embodiment according to the specific configuration of the present invention.

FIG. 4 is a graph showing the diffraction efficiency corresponding to the temperature change according to the first embodiment.

[Explanation of symbols]

 1 laser light source 2 beam splitter 3, 5, 33, 34 shutter 4 half mirror 6 laser light 7, 8, 35, 36 mirror 9 spatial light modulator 11, 41 lead wire 12, 25 recording medium (photorefractive material) 13, 24 Heater 14,23,44 Rotating Stage 15,26 Thermocouple 16,21,39 Object Light 17,22,38 Reference Light 270,000 30 Ar Ion Laser 31 Special Filter 32 Polarizing Beam Splitter 37 Half Wave Plate 42 Ce Doped SBN 45 Peltier element

Claims (6)

[Claims] 1. In a hologram recording / reproducing method using a material of a ferroelectric material whose refractive index changes by irradiation with light as a recording medium, when the hologram is recorded, the temperature of the recording medium is set to the ferroelectric material of the material. A hologram recording / reproducing method characterized by raising the paraelectric phase transition temperature to a level not exceeding the temperature. 2. The hologram recording / reproducing method according to claim 1, wherein the recorded hologram is reproduced at a temperature equal to or lower than the recorded temperature. 3. The pressure according to claim 1 or 2, wherein the pressure is applied during recording when the coefficient of thermal expansion of the recording medium is positive, and the pressure is applied during reproduction when the coefficient of thermal expansion of the recording medium is negative. A hologram recording / reproducing method, comprising: 4. The hologram recording / reproducing method according to claim 1, wherein the wavelength of the irradiation light at the time of recording is different from that at the time of reproducing. 5. The hologram recording / reproducing method according to claim 1, wherein the reproducing light is emitted at an incident angle different from that at the recording time. 6. The method according to claim 1, 2, 3, 4, or 5,
A hologram recording / reproducing method, wherein strontium / barium niobate (Sr _x Ba _1-x Nb ₂ O ₆ , 0 <x <1) is used as the recording medium.