JPH07160181A - Volume multiplex holography element and volume multiplex holography recording method - Google Patents

Abstract

PURPOSE:To provide a real time holography system, i.e., element or device and method improved in the degree of multiplicity and diffraction efficiency. CONSTITUTION:Light (object light) 20 including image information is made incident from the one optically polished surface of at least two sets of photorefractive crystals 22 having optically polished parallel surfaces and is condensed to nearly the central part of the photorefractive crystals 22. Reference light 21 which is parallel rays is made incident on the surface on which the object light 20 is made incident. This reference light 21 reflects once within the photorefractive crystals and passes nearly the central part; thereafter, the reference light 21 is further made incident on the photorefractive crystals 22 in such a manner that the light reflects once within the photorefractive crystals. The object light 20 and the reference light 21 intensify each other in interaction regions 23, 25 and offset each other in an interaction region 24.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a real-time holographic technique using a photorefractive crystal that forms a refractive index distribution similar to interference fringes of light, and records and reproduces a plurality of holograms in the same element. A volume multiplex holographic element and a volume multiplex holographic recording method applicable to a volume multiplex hologram device.

[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) and non-scattered light (reference light or pump light) from the same light source to interfere with each other to produce an interference fringe. Record on a recordable recording medium such as a photo plate,
Also, during reproduction, a technique of irradiating the recorded interference fringes with only the reference light and reproducing scattered light by an object is called holography. 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. Different holograms in the same medium can be recorded and reproduced separately by changing the incident directions of the object light and reference light (angle multiplexing), changing the recording wavelength (wavelength multiplexing), and changing the electric field applied to the recording medium (electric field multiplexing). ), And changing the phase of the reference light (face code multiplexing), and the like.

Regarding volume multiplex holography, since one reproduced image contains two-dimensional or three-dimensional information, it is expected to be applied to an information recording medium having a high density and a high transfer rate. It has been. FIG. 1 shows an example of a conventional holographic recording device. In FIG. 1, 10
Is a laser beam, 11 is a laser, 12 is a half mirror, 13 is a mirror, 14 is an acousto-optic device, 15 is a lens having a focal length f, 16 is an optical spatial modulator, 17 is a photorefractive crystal as a recording medium, and 18 is A camera, 19 is a shutter, 100 is object light, and 101 is reference light.
By changing the light transmission pattern of the spatial light modulator and changing the incident angle of the reference light by the acoustooptic device, it is possible to record a plurality of different holography.
With this method, 5000 hologram recordings have been realized in a 2 cm × 1.5 cm × 1 cm n lithium niobate (LiNbO ₃ ) crystal.

Here, the diffraction efficiency (diffracted light intensity / incident reference light intensity) of the hologram is proportional to the square of the interaction length of the reference light and the object light, and the angular resolution is the interaction length of the reference light and the object light. Proportional. Thus, stretching the interaction length gives very favorable results. However, even if the depth of the crystal is increased to increase the interaction length in order to improve the multiplicity and diffraction efficiency, the reference light and the object light intersect at a finite angle, so the interaction length is limited by the light intersection angle. Will be done. Therefore, instead of intersecting the object light and the reference light only once, it is possible to reflect the reference light on the side surface of the crystal and again interfere with the object light.

However, in a general photorefractive crystal, since the phase of the interference pattern of light and the phase of the refractive index distribution formed are different, the phase of the object light at the time of recording and the phase of the diffracted light at the time of reproducing are different. Here, the direction of the phase shift is determined by the crystal orientation, the direction of the phase shift and the normal line of the surface on which the reference light is reflected face the same direction, and the magnitude of the phase shift is close to π / 2. Under the condition, the diffracted light from the reference light before reflection on the side surface and the diffracted light after reflection differ in phase by π and cancel each other out. Thus, in this case, the side folds reduce the diffraction efficiency and
There is a drawback that the angular resolution is reduced.

Here, strontium barium niobate (Sr), which is a well-known photorefractive crystal, is used.
_x Ba _1-x Nb ₂ O ₆ ), barium titanate (BaTi
O ₃ ), lithium niobate (LiNbO ₃ ), the crystal orientation when trying to obtain a large diffraction efficiency, and the incident direction of the light satisfy the above conditions, and the diffraction efficiency and the angle due to the folding of the reference light No improvement in resolution has been realized.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the drawbacks of the side reflection type hologram multiplex recording system resulting from the phase inversion of the diffracted light described above. It is an object of the present invention to provide a real-time holographic system, that is, an element or a device and a method, in which the adverse effects of light reversal are minimized and the effect of interaction length stretching due to reflection is maximized. In practice, the aim is to improve multiplicity and diffraction efficiency.

[0008]

A volume multiplex holographic element of the present invention is a volume multiplex holographic element in which a plurality of interference fringes produced by interference of a plurality of coherent light beams are recorded in a photorefractive crystal. The photorefractive crystal is characterized by having at least two sets of optically polished parallel surfaces.

On the other hand, in the volume multiplex holographic recording method of the present invention, light containing image information is incident from one optically polished surface of a photorefractive crystal having at least two pairs of optically polished parallel surfaces. After that, the light is condensed in substantially the central portion of the photorefractive crystal, and the reference light that is a parallel light ray is incident on the surface on which the light including the image information is incident, and the reference light is generated in the photorefractive crystal. After being reflected once and passing through the substantially central portion, it is further reflected once in the photorefractive crystal,
It is characterized in that the reference light is incident on the photorefractive crystal.

The basic configuration of the present invention will be described with reference to FIG. In FIG. 2, 20 is the object light, 21 is the reference light, and 2
Reference numeral 2 denotes a photorefractive crystal which is a recording medium.
The object light is focused so that it is focused on the center of the crystal. On the other hand, the reference light is a parallel light beam, which is
Reflects twice. In the photorefractive crystal 22,
The incident surface and the emission surface of the object light / reference light are parallel to each other and are optically polished. Further, a pair of surfaces, which are perpendicular to the incident surface, on which the reference light is reflected are also parallel and optically polished.

[0011]

As shown in FIG. 2, the interaction areas of the object light and the reference light are three areas 23, 24 and 25. The light diffracted by the area 23 and the light diffracted by the area 25 have the same phase, and reinforce each other. On the other hand, the light diffracted by the area 24 has a phase difference of 180 degrees from the light diffracted by the other two areas, and cancels each other.

Now, when the light intensity of the reference light 21 is I _R and the light intensity of the object light 20 is I _O , the amplitude of the formed refractive index distribution is proportional to the modulation depth m. here,
m is

[0013]

[Expression 1] m = 2√ (I _R I _O ) / (I _R + I _O ), which takes a maximum value of 1 when the light intensities of the object light and the reference light are equal, and the light intensities of both are significantly different. When it approaches zero.

Here, in order to make the light intensity of the object light 20 and the light intensity of the reference light 21 in the region 23 substantially equal,
When the incident light intensity of the reference light is set, the light intensity of the object light is condensed and becomes strong in the region 24, so that the amplitude of the refractive index distribution is significantly smaller than that in the regions 23 and 25. On the other hand, the diffraction efficiency is proportional to the square of the amplitude of the refractive index distribution (when the efficiency is low). Therefore, since the diffracted light from the region 24 is significantly weaker than the diffracted light from the regions 23 and 25, the adverse effect of the presence of the region 24 is negligible. Therefore, when the reference light is folded back twice on the side surface of the medium, the intensity of the diffracted light is increased by about four times as compared with the case where there is no reflection. Further, since the regions 23 and 24 are spatially separated, the angle selectivity / wavelength selectivity at the time of multiple recording is improved.

[0015]

The present invention will be described in more detail with reference to the following examples, but it goes without saying that the present invention is not limited thereto.

(Example) As a recording medium, a cerium-doped strontium barium niobate 60 (Ce:
The case where Sr _0.6 Ba _0.4 Nb ₂ O ₆ ) single crystal fiber is used is shown. The end face (a face) of the pulled-up fiber and the four side faces are optically polished so that the c face becomes a flat face. At this time, ⅛ is scraped from both sides so that the distance between the polished c-planes becomes ¾ of the original fiber diameter.
FIG. 3 (A) shows the crystal orientation of the Ce: SNB60 single crystal whose four faces were polished. FIG. 3B shows incident directions of the reference light 31 and the object light 30 on the single crystal 32. The object light 30 is condensed by a lens (not shown) and adjusted so that the focal point is almost at the center of the crystal. The reference light 31 is adjusted so that the center of the beam passes through the center at the incident surface, the center portion, and the exit surface of the crystal. The polarization planes of both the reference light and the object light are within the plane of FIG. 3 (B). Also, the direction of the reference light is
Similarly, it is in the plane of FIG. 3 (B). Since the crystal has a refractive index of 2.4, the reference light can be scanned within ± 24.6 ° within the crystal. Therefore, when the multiplicity is obtained using the crystal length and diameter (distance between the two polished c-planes) as parameters, the result is as shown in FIG. That is, the contour line showing the multiplicity when the reference light is reflected twice on the side surface of the fiber is 1300 multiplexes per line.

(Comparative Example) For comparison, FIG. 5 shows the multiplicity in a configuration in which the reference light is not reflected on the side surface. In this case 1
There are 188 multiplexes per line.

As can be seen by comparing FIGS. 4 and 5, it can be seen that the side reflection method of the present invention dramatically increases the multiplicity. Regarding the diffraction efficiency, when the crystals of the same shape have the same incident angle of the reference light, the diffraction efficiency is improved four times as compared with the case of non-reflection.

[0019]

As described above, by adopting the structure in which the reference light is reflected by the crystal side surface, the effect of extending the interaction length between the object light and the reference light is brought about. Further, by condensing the object light, it is possible to remove the adverse effect caused by the reversal of the reference light and the phase shift peculiar to the photorefractive crystal, which is caused by the phase inversion of the diffracted light. The above effect has an effect of improving the multiplicity and the diffraction efficiency in the volume multiplex holography device.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a conventional angular multi-volume holography technique.

FIG. 2 is a diagram showing a configuration of the present invention.

FIG. 3 is a view showing a volume multiplex holography element of an embodiment of the present invention, (A) is a sectional view showing a crystal orientation,
(B) is a side view showing the incident directions of the object light and the reference light.

FIG. 4 is a diagram showing multiplicity by the method shown in the embodiment.

FIG. 5 is a diagram showing multiplicity according to a conventional method as a comparative example.

[Explanation of symbols]

 10 Laser Beam 11 Laser 12 Half Mirror 13 Mirror 14 Acousto-Optical Element 15 Lens with Focal Length f, 16 Optical Spatial Modulator 17 Photorefractive Crystal 18 Camera 19 Shutter 20 Object Light 21 Reference Light 22 Photorefractive Crystal 23 First Interaction Area 24 2nd interaction area 25 3rd interaction area 30 Object light 31 Reference light 32 Ce: SNB60 single crystal 100 whose 4 planes were polished 100 Object light 101 Reference light

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Iwaha Hatakeyama 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (2)

[Claims] 1. A volume multiplex holographic element in which a plurality of interference fringes produced by interference of a plurality of coherent light beams are recorded in a photorefractive crystal, wherein the photorefractive crystal has at least two sets of optical polishing. Volume multiplex holographic element, characterized in that it has parallel planes formed. 2. After injecting light containing image information from one optically polished surface of a photorefractive crystal having at least two sets of optically polished parallel surfaces, the photorefractive crystal is irradiated with substantially the same. The reference light which is a parallel light ray is made incident on the surface on which the light containing the image information is incident, and the reference light is reflected once in the photorefractive crystal, and the substantially central portion After passing through the photorefractive crystal, the reference light is incident on the photorefractive crystal so that the reference light is reflected once in the photorefractive crystal.