A simple and compact rewritable holographic memory system using a fieldstone of Ulexite is proposed. The role of the fieldstone is to impose random patterns on the reference beam to record plural images with the random-reference multiplexing scheme. The operations for writing and reading holograms are carried out by simply rotating the fieldstone in one direction. One of the features of this approach is found in a way to generate random patterns without computer drawings. The experimental study confirms that our system enables the smooth readout of the stored images one after another so that the series of reproduced images are projected as an animation.
© 2007 Optical Society of America
A holographic memory is regarded as a potentially promising data storage device . Several recording techniques have been developed in order to increase the memory storage capacity. Angle multiplexing, wavelength multiplexing, and shift multiplexing exemplify these efforts. Although they have contributed to improvement in performance, more accurate control of illumination angle of the reference beam, the wavelength, or the use of larger recording mediums are required [2-4].
To relieve such requirements, random reference scheme is used. We change reference beam’s wavefront into random patterns by diffusers or liquid-crystal spatial light modulators (SLM). By using diffusion light, precision of optical axis control is mitigated and therefore, this approach is suitable for a simple and compact system [5-13]. Moreover, multiple recording can be performed by changing random patterns on the reference side using the SLM. An animation is made possible by continuous reproduction of recorded images. But those devices with computer control generating random patterns are large-scale and costly.
Our research team has adopted a fiber bundle for generating random patterns and demonstrated the hologram multiplexing. One of the features in our system is that multiple images can be recorded only by rotating the fiber bundle. We have experimented on 100 images recording [14, 15]. Another important aspect for such application is cost for building the system. For constructing an inexpensive setup, a fieldstone is found attractive as an alternative to the fiber bundle due to its similar structure which could produce random reference patterns. As an example, we adopted Ulexite (chemistry:NaCaB5O9 8H2O) for our demonstrations. The advantages are six-fold: first, this method needs no strict optical axis control of the reference beam because of diffusion light. Second, the system can reproduce images by one directional continuous rotation of a device generating random patterns. Third, the fieldstone becomes a key to read out recorded information because it is irreproducible as a natural mineral resource. Fourth, Ulexite is commercially available and cost efficient. For the safety of the recorded information, the safety of the key to reproduce the recorded information is also important. Thus, fifth, the key may take less attention when we design such a field stone to be an accessory and put it on to bring with us. Sixth, the elaborate appearance as a personal adornment provides an additional advantage to restrict the amount of freedom for installation of the key after detaching so that both the locational accuracy and the angular accuracy to put the key back in the optical system to reproduce the recorded information are much improved. Furthermore, the approach is accompanied by additional possibilities that the system can be combined with other multiple recording methods to increase storage capacity and automatically adopt encryption effects on the stored images.
In this paper, first we demonstrate the multiple-recording by rotating Ulexite with LiNbO3 crystal. Next we perform the animation reproduction using the stored images.
2. Ulexite-based multiplexing recording system by random reference
2.1 Optical setup
The optical setup for multiple-recording with the random reference scheme is presented in Fig. 1. The Ar laser (λ=514nm) is applied as a light source, and a 45 degree-cut LiNbO3:Fe photorefractive crystal(1cm3) is used as the recording medium. In the figure, BE means a beam expander, PBS means a polarizing beam splitter. M signifies a mirror, and SLM stands for a spatial light modulator. Although extraordinary polarization shows larger diffraction efficiency than the ordinary polarization, the ordinary polarization allows us to set the propagation directions of the object and the reference beams to be orthogonal on a plane containing the crystal’s optical axis. This layout brings an advantage in reading holograms that the reference beam hardly overlaps with the reproduced images. Therefore, we apply the ordinary polarization for hologram multiplexing throughout our experiments.
For use of a rotary movement, the Ulexite is held at a motorized rotary stage. Inside LiNbO3 crystal, the object beam interferes with the reference beam. When the recorded images are reproduced, only the reference beam is turned on. The readout images are captured by CCD camera.
2.2 Ulexite to produce random reference patterns
We have already carried out experiments on random multiplexing scheme using a fiber bundle and found out excellent readout images. Ulexite shown in Fig. 2(a) has a structure similar to the fiber bundle. The form of the Ulexite in the setup is trapezoidal. The dimensions are that the upper base is 1.0cm, the lower base is 2.0cm, the height is 1.3cm and the thickness is 1.0cm. The random patterns of the Ulexite are confirmed to be more granular than those of the fiber bundle by observing speckle patterns. In this case, Ulexite is considered to be more profitable to produce random reference patterns.
3.1 100images multiplex recording
With respect to the size of optical setups, reduction of the dimensions serving a function for hologram multiplexing makes a strong contribution to the compact systems. In our approach, the part is provided by using Ulexite mounted on a rotary stage. Since the Ulexite is rotated around an axis parallel to the plane where optical elements are arranged, it is not necessary to keep an excessive space for the rotary movement.
In the hologram multiplexing experiment, Arabic numerals, from 1 to 100, are used for recorded images. Images are projected onto a liquid-crystal SLM. The object beam power is 0.55mW, while the reference beam power is 5.55mW. Recording time is 120seconds per figure. We write an Arabic numeral 1 at first, turn off the object beam, and rotate the Ulexite 0.2 degrees. We then turn on the object beam again on which the numeral 2 is imposed and write it for 120seconds. This procedure is repeated until the figure 100 is written. In the process of reading holograms, the recorded holograms are read out in numerical order. Figure 3 shows the images just as monitored by the CCD camera .
3.2 Readout animation images
In this chapter, for comprehensive visualization we use images of a running person as shown in Fig. 4. The record condition is identical to that of the experiment on the multiple recording of the still pictures. We rotate Ulexite at the speed of 1.2deg/seconds, while the frame rate is set at 6 frame /seconds. Since the one directional rotation enables smooth readout of the stored images one after another, series of reproduced images are projected as an animation. Animation is carried out with small gaps, which denote the degree of synchronization between one image-capturing and another.
Figure 5 shows the result of readout animation images. Because this method rotates Ulexite continuously, the intermediate patterns illuminate the medium in some periods. That causes the alteration in brightness of the reproduces images. In order to avoid undesirable intermediate patterns, a rotation corner of the random pattern irradiating the recording media should be controlled.
We experimented with a simple and compact rewritable holographic memory system using a type of fieldstone called Ulexite which was employed to impose random patterns on the reference beam. We experimented 100 hologram recording and found the possibility of the animation readout by continuously reproducing a series of images. As expected, the system succeeded in reading out images as an animation by rotating the Ulexite in one direction. As a result, we came to consider that a simple and compact rewritable all-optical animation projection system could be developed using a commercially available low-priced material such as Ulexite.
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