Jiro Minabe

Holographic recording and retrieval of polarized light by use of polyester containing cyanoazobenzene units in the side chain.

We examined the holographic recording properties of polyester containing cyanoazobenzene units in the side chain. It was revealed that, when a linearly polarized object beam with an arbitrary polarization direction was recorded, the retrieved beam had the same polarization state as that of the object beam, with a constant diffraction efficiency of 0.1. Moreover, two object beams with mutually orthogonal linear polarization could be recorded and retrieved separately from the same area. This is, to our knowledge, the first example of holographic polarization multiplexing. The retrieved images were found to resolve details of ~40line pairs/mm and a 40-mum -pitched checkerboard pattern. The resolution of the system is limited by the CCD cameras that are used. Polarization encoding to increase the storage capacity and (or) the signal-to-noise ratio of holographic data recording is also discussed.

Design of Dye Concentrations in Azobenzene-Containing Polymer films for Volume Holographic Storage

Diffraction efficiencies of holograms in thick azo polymer films were improved by designing optical densities of the films. A nonphotosensitive polymer was used as a binder for controlling the dye concentrations. We fabricated 250-µm-thick films by changing the concentrations. While the optically induced birefringence decreased with decreasing concentration, reduction of absorption loss of the reading beam increased the diffraction efficiency. Thus, owing to the competition between the decrease in birefringence and the increase in efficiency, there exist appropriate concentrations for improving both sensitivity and magnitude of the diffraction efficiency. Consequently, the diffraction efficiency reached 25% without degradation of the sensitivity. We also demonstrated holographic multiplexing using the film. A set of 20 data pages (66 kbits per page) was successfully multiplexed and retrieved with a low bit-error rate of 1.3×10-3. The use of an error correction coding would allow error-free recovery of the data pages.

Thermally assisted recording of holographic gratings in semicrystalline azobenzene-containing polymers

We examine optically induced birefringence in semicrystalline azopolymer films that are held at glass-transition temperature Tg. The birefringence increases markedly after interception of the pump beam; the saturation value depends on exposure time. In addition, the induced birefringence is completely erased by irradiation with a circularly polarized beam at Tg. Using this thermally assisted method, we demonstrate the holographic recording of a test image. The intensity of the diffracted beam also increases after interception of the writing beams. Furthermore, the retrieved image is found to have a resolution of approximately 30 lp/mm. This resolution is comparable with that of the optical setup that is used. Accordingly, the thermally assisted recording by use of semicrystalline azopolymers is a promising method for reversible holographic storage.

Optical noise reduction by reconstructing positive and negative images from Fourier holograms in coaxial holographic storage systems.

A technique for reconstructing positive and negative images from an identical intensity-modulated hologram is demonstrated theoretically and experimentally by use of a coaxial holographic storage system. Negative images are obtained by adding a phase-modulated dc component of signal beam on reading. By comparing positive and negative images, the bit error rate (BER) is improved by two orders of magnitude. This technique can reduce optical noise of reconstructed images to attain low BERs.

Coaxial holographic data storage without recording the dc components

A technique of recovering the data pages from Fourier holograms recorded without the dc components is demonstrated theoretically and experimentally by use of a coaxial holographic storage system. A reconstructed image is obtained by adding a phase-modulated dc component of the signal beam on reading. The bit error rate of the reconstructed image is comparable with that for the hologram recorded with the dc component as well. Since high intensities of the dc components are not recorded in this technique, the dynamic range of the recording media can be saved, which potentially contributes to increasing the number of multiplexed holograms.

Optical noise reduction for dc-removed coaxial holographic data storage.

A method of reconstructing positive and negative images from Fourier holograms recorded without the dc components is demonstrated by use of a coaxial holographic storage system. Reconstructed images are obtained by adding a phase-modulated dc component of the signal beam on reading. Contrast reversal of the reconstructed images can be achieved by reversing the readout reference pattern. This method can realize not only optical noise reduction but also less consumption of the dynamic range of the recording medium, potentially contributing to increasing the number of multiplexed holograms.

Digital Data Storage and Computing by a Vector Holographic Memory System

We have applied polarization multiplexing to digital data storage and logic operation. Two digital data pages with mutually orthogonal polarization directions were recorded with a reference beam within the same volume of polyester containing cyanoazobenzene units in the side chain. Illuminating the polyester with the reference beam simultaneously yielded the two data pages. The polarization states of the two retrieved images were mutually orthogonal, because the polyester film is capable of recording arbitrary polarization states. Thus, there was no significant cross talk between the two reconstructed data pages. The bit error rates of the two retrieved pages were estimated to be 10-9 and 10-6. Furthermore, by detecting an appropriate polarization component of the diffracted beams, we have successfully obtained subtraction and addition signals between the two multiplexed pages. Since this system is also applicable to logic operation, it would take advantage of the high-density storage, fast transfer rates, and fast processing speeds.

Homodyne readout on dc-removed coaxial holographic data storage

Multiplexing characteristics of a dc-removed coaxial holographic storage system were evaluated for what is believed to be the first time. Our dc-removed coaxial system achieved 3.5 times higher raw data density than a conventional coaxial system that involved dc recording. The increase of the data density was due not only to less M/# consumption but also to the effects of signal amplification and noise reduction by use of the positive and negative images reconstructed from the same holograms.

Multilayer Holographic Storage Using Coaxial Optical Systems

We propose the multilayer recording of shift-multiplexed holograms with a coaxial transmission-type configuration, which can effectively utilize the recording medium in the thickness direction. In this method, two shift-multiplexing layers are formed in a single recording layer, in which the focal point of the Fourier transform lens lies in two interfaces between the recording material and the substrates. We confirmed experimentally that this dual-focusing method improved the error rate of the data by one order of magnitude. Furthermore, four-layer recording in a double-layered medium was successfully demonstrated, which enabled the areal data density to increase by a factor of four.

Polarization Encoding for Digital Holographic Storage

We have proposed polarization encoding for digital holographic storage, in which each pixel or bit encodes two orthogonal polarization states, 0 and 90 degrees, corresponding to binary 1 and 0, respectively. We recorded the polarization-encoded page within an azobenzene-containing polyester film that is capable of recording arbitrary polarization states. After retrieval from the hologram, the polarization-encoded page was divided into mutually orthogonal polarization components. The two polarization components had an intrapage variation, and their bit error rates (BERs) were estimated to be 10-3 and 10-5. Subsequently, to remove the intrapage variation, we calculated the difference between the two polarization components. The intrapage variation successfully decreased on the subtraction image, and its BER was improved to 10-7. Therefore, this method is of great use for eliminating intrapage variations on a reconstructed data page.