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— Presentation at ISOM 2015 —
November 4, 2015
Fig. 1 Schematic diagram of holographic memory
ISOM (International Symposium on Optical Memory) 2015 was held at Toyama international conference center in Toyama city from 4th to 8th in October 2015. ISOM is the largest international conference on optical memory, and a lot of techniques about next generation optical memory are presented in this conference. In ISOM 2015, there were 127 participants and 72 presentations, and Hitachi made 6 presentations regarding holographic memory, 1 presentation regarding wave front coding and 1 presentation regarding semi-permanent data storage using fused silica.
Hitachi is researching next generation optical memory systems including holographic memory. The holographic memory system records 2-dimensional data at one exposure of shorter than a millisecond and angular-multiplexes hundreds of the data at same location in a medium (Fig. 1). And then it has more than 10 times larger data capacity and faster transfer rate than Blu-ray Disc. We have confirmed the feasibility of 2.4Tbit/inch2 holographic memory system by using RLL (Run-Length Limited) high density recording method (Fig. 2), and we are focusing on higher recording density by using phase information.
I presented "High Density Recording in Holographic Data Storage System by Dual 2-Level RLL Modulation" regarding to phase information recording. In this presentation we showed an RLL modulation method in phase 4-level recording. The holographic memory system recording phase information enables multi-level recording more easily than that recording amplitude information. However, the RLL modulation method for 4-level recording requires not only complicated calculation, but also a large circuit.
Therefore I suggested the RLL modulation method processing in the following steps,
- separate 4-level signal into binary real part signal and binary imaginary part signal,
- modulate each signal by using conventional 2-level RLL modulation, and
- combine two 2-level RLL modulated signals to be 4-level modulated signal.
And I presented this method requires only the same RLL modulation as conventional (Fig. 3) and effects the same densification. I expect this method can achieve to double the recording density without the complicated calculation.
In ISOM 2015, we could show the cutting-edge techniques contributing toward the holographic memory system with large data capacity.
(By TAJIMA Kazuyuki)