Y. Kawata

Three-dimensional optical memory with a photorefractive crystal

We propose a three-dimensional optical-memory device in which refractive dot data are recorded directly into a photorefractive crystal. To record a single bit of datum, one focuses a laser beam with an objective lens onto a specific spot in a crystal, thereby changing its refractive index locally as a result of photorefraction. To record in three dimensions, one keeps the objective lens stationary while the crystal is translated. The beam-spot intensity is modulated with a beam shutter according to the logic state of the data point. The recorded data points are read with a phase-contrast microscope objective lens. We present experimental results of three-dimensional recording and reading with a LiNbO(3) crystal. The distribution of the refractive index formed by a focused beam is also analyzed with the charge-transport model.

Three-dimensional optical bit-memory recording and reading with a photorefractive crystal: analysis and experiment

We analyze the three-dimensional refractive-index distribution that is induced locally when a laser beam is focused onto a very small region in a photorefractive crystal. The formation of the index distribution is deduced from the temporal behavior of the electron density distribution in the crystal under non-steady-state conditions. The density distribution is computed by the use of a set of the recurrence relations that was derived from Kukhtarevs equations, which describe the transport of electrons in time. In particular, we calculated the index distribution formed in Fe-doped LiNbO(3) crystals. To verify the validity of our analysis, we read, by using a phase-contrast microscope, refractive-index dots that were recorded in Fe-doped LiNbO(3) crystals. A good agreement was obtained between experimental results and the calculated phase-contrast images when the characteristics of the imaging system are taken into account. We also found that the induced index change is largest when the c axis of the LiNbO(3) crystal is oriented parallel to the polarization direction of the reading beam. Under this optimal condition, we succeeded in recording up to 10 layers of readable data in a LiNbO(3) crystal.

Readout of three-dimensional optical memories

The method of recording data in a volume memory determines the spatial frequency content. We use a threedimensional transfer function approach to show that this has serious implications for the choice of readout system. In particular, one cannot use a ref lection confocal system to read a photopolymer-based memory. A solution to this problem is proposed.

Differential phase-contrast microscope with a split detector for the readout system of a multilayered optical memory

A transmission differential phase-contrast microscope with a split detector is used as a readout system for a multilayered three-dimensional optical memory. The system is applicable to data recorded as refractive-index changes. The system is compact and easy to use. The three-dimensional optical transfer function for the system is derived. This shows that the spatial bandwidth of the system is the same as that of a conventional microscope with incoherent illumination but with much improved contrast. Six layers of information are recorded in a photopolymer and are read out with little cross talk and high contrast.

Three-dimensional confocal optical memory using photorefractive materials

Research on 3-D optical memory with photorefractive materials is described for ultra-high density/capacity memory exceeding the classical limit of conventional optical memory. In this system bit data are recorded by a focused laser beam as refractive digital bits in 3-D volume. A confocal laser-scan microscope is used for reading the recorded bit-data. In this paper, we show the system and the methods for writing and reading data, the materials we used, and some devices for practically realizing this type of memory. Some experimental examples obtained through the preliminary experiments are shown and the problems to be solved in future research and development are discussed.

Randomly accessible, multilayered optical memory with a Bi(12)SiO(20) crystal.

A read-and-write, randomly accessible, multilayered optical memory with a Bi(12) SiO(20) crystal as the medium is demonstrated. Data are recorded in the crystal as an absorption change that is due to the photochromic effect. These data are successfully recorded, read, and selectively erased in five layers in the crystal. The axial-separation distance between neighboring layers is 30 μm, and the lateral distance between bits is 5 μm. Selective bit erasure of the data is accomplished by illumination of the recorded bit datum with He-Ne laser light. To our knowledge, this is the first successful demonstration of the selective optical erasure of the photochromic effect in a BSO crystal.

4Pi confocal optical system with phase conjugation

We propose a 4Pi confocal system that incorporates a phase-conjugate mirror into the system to solve the alignment problem and automatically correct the spherical aberration associated with the conventional 4Pi confocal system when observing a thick sample. The proposed system is experimentally verified by use of cerium-doped barium titanate crystal as the phase-conjugate mirror. The full width of the central peak is ~0.15 microm. We also theoretically verify the advantage of automatic correction of the spherical aberration by calculating the intensity distribution at the focal spot.

Image amplification with local addressing by two-wave coupling in a Bi 12 SiO 20 crystal by application of direct-current voltage

We present a proposal and an analysis of local image amplification by two-wave coupling in a Bi12SiO20 crystal by applying external dc voltage. The analogy of two-wave coupling to an optical transistor array is discussed. It is found through the experiments that the local gain in amplification is given not by the reference-beam pattern alone but also by the electric field that is formed by the local photoconductance of the Bi12SiO20 crystal. A resistance-network model is used for analysis of the electric-field distribution. A variety of experiments verify the theoretical and calculated analyses. A sample reference pattern for amplifying segments of a signal image is designed through the resistance-network analysis of the electric-field distribution.

Gain enhancement by signal beam chopping for two-wave coupling with a BSO crystal

Gain enhancement via signal beam chopping for two-wave coupling with a Bi(12)SiO(20) crystal is demonstrated. In this method the gain of a two-wave coupling scheme is increased using a transient effect. This effect is created during the grating formation due to the phase difference between the interference fringe of the light intensity incident on the crystal and the photoinduced refractive index fringe in the crystal. The maximum gain of 11.7 was achieved with an applied electric field of 6.0 kV/cm, a fringe spacing of 29.5 microm, a beam intensity ratio of 1240, and a chopping frequency of 6.0 Hz. This gain is nearly as high as that obtained with the moving interference-fringe method proposed by Huignard and Marrakchi. The gain enhancement using the transient effect caused by the polarization rotation of the signal beam (using a rotating halfwave plate) is also described. Preliminary experimental results are shown.

Speckle-free image amplification by two-wave coupling in a photorefractive crystal

A technique of speckle-noise reduction for an image amplified by two-wave coupling with a photorefractive crystal is proposed. The spatial coherence for the axis along the fringe formed in the photorefractive crystal is reduced to remove the speckle, but the spatial coherence for the axis vertical to the fringe is unaltered. This is obtained by rotating a mirror to deflect the illumination beams one dimensionally for both the signal and reference patterns during the exposure of the hologram. Because of the imageholography configuration, the images of the signal and the reference do not move on the photorefractive crystal as the two beams deflect. The beam deflection can be faster than the response of the photorefractive crystal. Some experimental results for the image amplification with the technique developed are presented, and they are compared with results obtained without speckle-noise reduction. Fringe deformation caused by deflection of the signal and reference beams is analyzed.