Hisatoshi Funakoshi

Long-term reading experiment on a photorefractive holographic memory with the hologram sustainment technique by optical feedback

We demonstrate a long-term continuous readout of a two-dimensional image in a photorefractive holographic memory with a BaTiO3 crystal. A considerable extension of reading time is achieved by use of a hologram sustainment technique with an optical feedback circuit. Hologram rewritings by the simultaneous illumination of the reading beam and the feedback beam, which is incident on a crystal from the opposite direction to the reading beam, contribute to all-optical hologram sustainment without any fixing technique. In this paper, the effectiveness of the hologram sustainment technique is explained by the analysis of the temporal property of the amplitude of the index grating in a crystal. By calculating the temporal properties of the reconstructed beam intensity, we reveal the feedback rate and the coupling strength for high output efficiency. We perform an experiment on two-dimensional image reading and writing with a 45° cut BaTiO3 crystal and show that a recorded image can be reconstructed over 20 min without critical image degradation whereas a reconstructed image fades away within 20 s in the conventional readout technique.

Image reading and writing experiment on photorefractive nondestructive memory

In this report, we experiment on the two-dimensional image reading and writing in the photorefractive nondestructive memory with BaTiO3 crystal. The great extension of the reading time is archived by use of the grating maintenance technique with optical feedback. By calculating the temporal property of the output beam intensity and the index grating amplitude under the incident conditions of the conventional readout technique and our readout technique, we estimate the feedback rate and the coupling strength for high output generating efficiency. We experiment on the two-dimensional image reading and writing with BaTiO3 crystal, and succeed at over 20 minutes reading without image degradation.

Effect of random phase mask on input plane in photorefractive authentic memory with two-wave encryption method

We have proposed an all-optical authentic memory with the two-wave encryption method. In the recording process, the image data are encrypted to a white noise by the random phase masks added on the input beam with the image data and the reference beam. Only reading beam with the phase-conjugated distribution of the reference beam can decrypt the encrypted data. If the encrypted data are read out with an incorrect phase distribution, the output data are transformed into a white noise. Moreover, during read out, reconstructions of the encrypted data interfere destructively resulting in zero intensity. Therefore our memory has a merit that we can detect unlawful accesses easily by measuring the output beam intensity. In our encryption method, the random phase mask on the input plane plays important roles in transforming the input image into a white noise and prohibiting to decrypt a white noise to the input image by the blind deconvolution method. Without this mask, when unauthorized users observe the output beam by using CCD in the readout with the plane wave, the completely same intensity distribution as that of Fourier transform of the input image is obtained. Therefore the encrypted image will be decrypted easily by using the blind deconvolution method. However in using this mask, even if unauthorized users observe the output beam using the same method, the encrypted image cannot be decrypted because the observed intensity distribution is dispersed at random by this mask. Thus it can be said the robustness is increased by this mask. In this report, we compare two correlation coefficients, which represents the degree of a white noise of the output image, between the output image and the input image in using this mask or not. We show that the robustness of this encryption method is increased as the correlation coefficient is improved from 0.3 to 0.1 by using this mask.

Noise reduction in photorefractive memory by double mutually pumped phase conjugate mirrors

In this report, we demonstrate a noise reduction recording in a photorefractive memory with a dynamic refreshing technique by mutually pumped phase conjugate mirrors (MPPCMs). The dynamic refreshing, that means the continuous rewriting of an original hologram with the optical feedback in synchronism with the readout, largely contributes to the maintenance of the recorded hologram by all-optical process. Two MPPCMs are used in the dynamic refreshing technique; one is built for the optical feedback, the other is used as the storage. The nondestructive readout without any fixing technique can be achieved by the continuous rewriting with the beam resonance between the phase conjugate mirrors. Our technique additionally offers the noise reduction of an input image in the recording process without any external image processing systems. The noise reduction is caused by the characteristics of the MPPCM that the incident intensity ratio required for the MPPCM generation is determined by the coupling strength of the crystal. Moreover, the application of the MPPCM to the photorefractive memory brings great advantages of the high quality image retrieving and the simple optical configuration because the illumination of two incoherent beams into a PR crystal can generate the mutual phase conjugate beams in the MPPCM. We show the noise reduction effect is controllable by the appropriate adjustment of the incident beam intensities. We also experiment on the photorefractive memory with this technique using barium titanate crystals and show the high-quality noise-reduced image can be read out over 10 times longer than the conventional readout technique.

Security Improvement with Multiplexing for Optical Encryption Based on Cross-Correlation of Random Phase Masks

Optical random phase encryption methods have the vulnerability of decoding by readout with only a part of correct key. To overcome this vulnerability, we propose a new method by multiplexing original data and dummy data.

Optical security switch based on spatially phase matching in photorefractive media

In photorefractive four-wave mixing, when two counter-propagating wave fronts of pump beams match, a phase conjugate beam is generated on the probe beam. Here we propose an optical switch or optical hub with security functions as an application example of this phase matching switching to optical communication devices. To confirm the basic property of the security function, two-wave mixing configuration was used as the substitution for the four-wave mixing configuration.

Photorefractive bidirectional connection module with mutual-pumped phase conjugate mirror

The purpose of this study is to apply a free-space optical interconnection to a reconfigurable board-to-board connection where the wiring patterns connecting boards are optically formed without electrical-optical conversion. We regard a photorefractive bi-directional connection module (PBCM) based on a mutually pumped phase conjugate mirror as a key device to construct such a connection network and employ PBCMs at input/output interfaces of each board. Although optical behaviors of PBCM are influenced by the exposure conditions, we especially focus on the diameter of beams illuminating photorefractive media placed inside PBCM so as to find some geometrical restrictions in a design of networking system. Through numerical analyses, we show a sample configuration of PBCM for the board-to-board interconnection and present a conceptual design of input/output interface.

Optical-controlled matrix switch and free-space optical interconnection by photorefractive polymers

By irradiating alternative external light into the system with four-wave mixing in photorefractive polymers, the restriction on the generation of a phase conjugate beam or diffracted light will be demonstrated in this experiment. In addition, in order to verify the use of a two-dimensional array for simultaneous switching, the elimination of only part of diffraction grating recorded will be experimentally conducted.

Long-term retrieving experiment on rewritable holographic memory with dynamic refreshment by mutual-pumped phase conjugator

In this paper, we demonstrate long-term readout in a photorefractive memory with a dynamic refreshment technique by mutual-pumped phase conjugate mirror (MPPCM). The dynamic refreshment means that a volatile hologram is sustained in parallel with its reconstruction by only all-optical process. A simple optical feedback circuit for hologram sustainment by MPPCM is added on to a photorefractive memory in our dynamic refreshment technique. By continuously rewriting with feedback of reconstructed image, long-term continuous readout of a volatile hologram can be realized. This technique with MPPCM also offers grayscale-to-binary conversion by the beam resonance and intensity thresholding of MPPCM that it cannot be built up where the intensity is lower than the threshold level. By adjusting the proper threshold value, therefore, a noise-reduced binary image can be retrieved without any external processing systems. Moreover, high quality output data can be obtained because phase distortion on the image can be automatically removed by optical phase conjugation. This technology is applicable to recording technique such as angular and phase-code multiplexing and allows long-term readout of multiple holograms by the appropriate refreshing schedules. We perform the experiment on long-term readout using barium titanate (BaTiO3) crystals and demonstrate a recorded image is reconstructed over 1 hour with our technique whereas a reconstructed image fades away within 5 minutes with conventional readout technique.

Experiment on two-dimensional data restoring in fault-tolerant holographic memory

We propose a fault-tolerant holographic memory (FTHM) composed of a pair of photorefractive crystals. This memory offers not only non-destructive readout but also data restoring function by only pure optical operations without any electrical controls. In writing process, the same holographic data are simultaneously recorded as index gratings to the crystals laid out in series. In reading process, a reading beam is diffracted by the index gratings in each crystal. Here, some of the diffraction beams are detected as an output beam, and the others are used as a feedback beam. The hologram in each crystal is continuously refreshed by the feedback beam from the other crystal since the feedback beam has the same information as the original holographic data. When the data refreshing effect by the feedback beams sufficiently exceeds the erasure effect by the exposure of the reading beam, the stored data are always maintained. Furthermore, even if a certain fault such as vibration and stray beam incidence happens, the lost data in one crystal are all-optically restored as long as the corresponding holographic data remain in the other crystal. The experiment with a two-dimensional image is carried out for the purpose of checking the data restoring function in FTHM. The two-dimensional image divided in quarters is recorded as into a pair of 45°-cut BaTiO3 crystals, and the original holographic data is successfully restored by the refreshing effect in the case that a quarter of the image in the one crystal is partially lost.