In angle-multiplexed holographic memory, the full width at half maximum of the Bragg selectivity curves is dependent on the angle formed between the medium and incident laser beams. This indicates the possibility of high density and high multiplexing number by varying the angular intervals between adjacent holograms. We propose an angular interval scheduling for closely stacking holograms into medium even when the angle range is limited. We obtained bit error rates of the order of 10 À4 under the following conditions: medium thickness of 1 mm, laser beam wavelength of 532 nm, and angular multiplexing number of 300.

Photopolymer materials shrink because of photopolymerization. This shrinkage distorts the recorded interference fringes in a medium made of such material, which in turn degrades the reconstructed image quality. Adaptive optics controlled by a genetic algorithm was developed to optimize the wavefront of the reference beam while reproducing in order to compensate for the interference fringe distortion. We defined a fitness measure for this genetic algorithm that involves the mean brightness and coefficients of the variations of bit data "1" and "0". In an experiment, the adaptive optics improved the reconstructed image to the extent that data could be reproduced from the entire area of the image, and the signal to noise ratio of the reproduced data could be improved.

Photopolymer materials are feasible for holographic recording media. However, these materials shrink owing to photopolymerization and interference fringes recorded in them distort. In addition, temperature variation causes shrinkage and expansion of these materials and thus distorts recorded interference fringes. This distortion degrades reconstructed image quality and decreases the signal-to-noise ratio of the reproduced data. We applied adaptive optics controlled by a genetic algorithm to compensate for the distortion and improved the reconstructed image quality at 25 and 30 °C ambient temperature. Under these conditions, the signal-to-noise ratio of reproduced data was more than 4 dB. Furthermore, we evaluated the distortion due to the temperature variation by using a medium angle and the wavefront of the reference beam. We found that the distortion caused by anisotropic shrinkage is slight; consequently, an optimised wavefront at 25 °C can compensate for the interference fringe distortion and increase the signal-to-noise ratio by adjusting only the medium angle even if a temperature variation occurs. Adaptive optics can thus be used to compensate for interference fringe distortion caused by shrinkage and expansion due to temperature variation.

We are studying a form of holographic data storage with phase conjugation, and we compensated for hologram distortion due to shrinkage of photopolymer materials in the holographic medium by controlling the wavefront of the reference beam. When a high NA lens and narrow angle interval of angle multiplexing are employed to obtain a high data recording density, some wavefronts cause interpage crosstalk on the reconstructed image. We tried to determine the moving range of actuators in a deformable mirror for controlling the wavefront. As a result, we found that the distortion in the hologram could be compensated while avoiding interpage crosstalk and that the bit error rates of the reproduced data could be decreased. We also found that the optimized wavefront could compensate for distortions in several neighboring data pages. This method can ensure a high data recording density in holographic data storage.

No abstract