Compensation of hologram distortion by controlling defocus component in reference beam wavefront for angle multiplexed holograms

Abstract

Holographic memory has the potential to function as a recording system with a large capacity and high data-transfer-rate. Photopolymer materials are typically used as a write-once recording medium. When holograms are recorded on this medium, they can distort due to shrinkage or expansion of the materials, which degrades the reconstructed image and causes a higher bit error rate (bER) of the reproduced data. We propose optically compensating for hologram distortion by controlling aberration components in the reference beam wavefront while reproducing data, thereby improving the reproduced data quality. First, we investigated the relation between each aberration component of the reference beam and the signal to noise ratio (SNR) of the reproduced data using numerical simulation and found that horizontal tilt and the defocus component affect the SNR. Next, we experimentally evaluated the reproduced data by controlling the defocus component in the reference beam and found that the bER of the reproduced data could be decreased by controlling the defocus center with respect to the hologram position and phase modulation depth of the defocus component. Then, we investigated a practical control method of the defocus component using an evaluation value similar to the definition of the SNR for actual data reproduction from holograms. Using a defocus controlled wavefront enabled us to decrease the bER from 3.54 × 10 −3 with a plane wave to 3.14 × 10 −4 . We also investigated how to reduce the bERs of reproduced data in angle multiplexed holograms. By using a defocus controlled wavefront to compensate for hologram distortion on the 40th data page in 80-page angle multiplexed holograms, the bERs of all pages could be decreased to less than 1 ×10 −3 . We showed that controlling the defocus component is an effective way to compensate for hologram distortion and to decrease the bER of reproduced data in holographic memory.

Data demodulation using convolutional neural networks for holographic data storage

“…To improve the performance of the HDS, the signal-tonoise ratio (SNR), which affects demodulation errors in reproduction, must be improved. However, there are various types of optical noise, 11) which are difficult to completely remove from the optical path of recording and reproduction in the HDS (there have been a number of unsuccessful proposed solutions [12][13][14][15] ). For this reason, an efficient method with correct demodulation is required even when the SNR is low.…”

Applying digital filter to data pages before recording to increase signal-to-noise ratio in holographic memory

“…19) Furthermore, a compensation method of increasing the SNR by optically compensating for hologram distortion caused by temperature variation and medium shrinkage has been reported. [20][21][22] Another factor is optical elements in an optical setup. Kinoshita et al eliminated moiré fringes added to the reproduced data pages caused by optical elements.…”