Katsunori Kawano

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.

Excited-state absorption and pump propagation effects on optical phase conjugation in a saturable absorber

Phase conjugation by degenerate four-wave mixing in a saturable absorbing medium with a relatively long-lived excited state is analyzed; the analytical results are compared with experimental results. We propose a four-level model and take into account the effects of both excited-state absorption and pump propagation to explain the experimental results, which could not be explained by the models proposed previously. The numerical result predicts that the value of the phase-conjugate reflectivity as a function of the pump intensity is maximum at a certain absorbance (as absorbance increases) that is dependent on the cross section of the excited-state absorption. The present analysis explains most observations in the experiment, which was performed with a saturable-dye-doped polymer film as the saturable absorbing medium.

Contribution of population or thermal grating to phase conjugation in saturable xanthene-dye-doped polymer films

The time constant of a decaying phase-conjugate (PC) signal was measured as a function of a grating period in polyvinyl alcohol films doped with xanthene dyes such as erythrosin B, eosin Y, and uranin. The time constant was independent of the grating period and was of the same order as the time required for the population grating of each dye to decay. These experimental results indicated that there was a population grating contribution but not a thermal grating contribution in the generation of PC signals by these saturable xanthene-dye films.

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.

High-Pass Filtering in Coaxial Holographic Data Storage

To reduce the loss of the dynamic range M/# of medium, we have proposed and demonstrated a high-pass filtering method for coaxial holographic data storage. Total exposure power decreased to 1/2 by removing the DC components of the Fourier-transformed signal and reference beams with a high-pass filter. Our proposed method can reduce the wasted dynamic range of medium. We evaluated the bit error rate of a high-pass-filtered signal beam to be 5.0×10-4 and that of an unfiltered signal beam to be 1.3×10-3. Thus, we confirmed that the quality of the signal beams without the DC component is comparable to that with the DC component. Next, we successfully recorded and reconstructed the filtered signal beam with the coaxial holographic configuration. The bit error rate of the retrieved signal beam was 1.4×10-3, showing the absence of degradation without the DC component of the retrieved signal beam. We also evaluated the shift selectivity improved from 5 to 2 µm by high-pass filtering.

Reversible Control of Thermally Enhanced Birefringence in Semicrystalline Azo Polymer Films with Polarized Light

When a semicrystalline azobenzene-containing polymer film was irradiated with a linearly polarized beam at glass-transition temperature Tg, the induced birefringence grew further even after the interception of the pump beam. The resulting birefringence reached 146% of the value at the interception. Because the saturation value of birefringence was proportional to the value induced optically, it can easily be controlled by exposure energy. In addition, the enhanced birefringence showed a high recording stability. In other words, the recovery of birefringence was performed by heat treatment after an erasing process in which the film was irradiated with a circularly polarized beam at room temperature. In contrast, the circularly polarized beam irradiation at Tg eliminated birefringence completely. Hence, the thermally enhanced birefringence was reversed by the irradiations at Tg. These properties enable the photocontrol of thermally enhanced birefringence, which is highly useful for reversible holographic storage.

Photofabrication of Kinoforms as Multilevel Relief Structures on Azobenzene-Containing Polymer Films.

We proposed a new method of fabricating kinoforms using azobenzene-containing polymers. The kinoform was designed as a phase pattern by applying the Gerchberg-Saxton algorithm. We converted the phase pattern into a gray-tone image, and used it as an amplitude mask. The azopolymer film was irradiated through the mask with a circularly polarized beam. In this way, the film was inscribed with a multilevel relief structure. By irradiating the film with a probe beam, we obtained the desired image in the diffracted beam. Thus, we successfully fabricated the kinoform on the azopolymer film by only one exposure. The method offers the advantage of the fabrication of multilevel relief structures with single-step processing, without the necessity for any subsequent processing steps.

Azopolymer/liquid crystal complex for polarization holograms

We have proposed and demonstrated a new holographic medium that can act as a write once read many (WORM) polarization-recordable medium. The new medium, composed of azobenzene-containing polymer and photocurable liquid crystal, exhibited birefringence when irradiated by polarized light. Additionally, the diffraction efficiency and the birefringence of this recorded complex increased to several times those of the simple rewritable azopolymer. This occurred because the orientation of photocurable liquid crystals was as good as that of the azopolymer because of the interaction with the mesogens. By measuring the birefringence, we confirmed that unlike the azopolymer, the orientation of the liquid crystal polymer was maintained even after circularly polarized beam irradiation. In this medium, only the photocurable liquid crystals were cured and became polymers with fixed orientation. Therefore, we propose that the new medium, which is polarization-recordable, WORM and highly sensitive, is very promising and practical for digital holographic storage.

Additional phase change between saturable absorption and holographic components in phase-conjugate interferometry

Abstract Measured are the unwanted additional phase change between two components of phase-conjugate (PC) signals, the saturable absorption and holographic components, simultaneously generated in a real-time PC interferometry with an erythrosin B-doped polyvinyl alcohol film used as a PC mirror. For the pump and probe intensities of 1 W/cm 2 and 0.3 W/cm 2 and optical density of 1.8, the unwanted additional phase change was less than about π/4 within 5 min for the hologram-recording times of 15 s, 30 s and 60 s, and less than about π/8 for the hologram-recording time of 120 s. The temperature rise of the irradiated dye-doped film was mainly responsible for the unwanted additional phase change.