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— Presentation at International Symposium on Optical Memory 2016 —
November 11, 2016
The International Symposium on Optical Memory 2016 (ISOM'16) was held in Kyoto Research Park in Kyoto, Japan from 16th to 20th in October, 2016. Various technologies related to optical memory systems, computational imaging, and sensing were discussed at ISOM'16. Researchers from various countries such as Japan, USA, Korea, and China attended this conference. This year, Hitachi Ltd. made a presentation entitled "Relaxation of Tolerance for Holographic Disc Positioning Based on Triaxial Position Control of Nyquist Aperture".
Total amount of worldwide archival data has been increasing due to recent rapid growth in the industry of information technology. For data archiving, optical disc raise expectations for a green archival storage with a large capacity in addition to low running cost and long-term retention. Holographic data storage system (HDSS) is one of promising candidates for future optical data storage system because of its abilities of high recording density and a fast data transfer rate.
To realize HDSS having capacity of TB class and a transfer rate of Gbit/s class, it is necessary to control a relative position between a recorded hologram and an optical unit of the HDSS. In conventional HDSS, the optical unit has no moving mechanism and a disc mounted on a disc stage (disc unit) moves to a position that maximizes signal-to-noise ratio (SNR). It is difficult to control the disc position with high precision and to shorten a lead-in time of the positioning because the holographic disc has no groove structure which is necessary for traditional position control system, as shown in Fig. 1. Therefore, we developed new position control system to compensate for the relative position.
A recorded hologram and the optical unit are successfully aligned as shown in Fig. 2 (left), readout beam diffracted from the hologram is focused by a relay lens, and the beam passes through a Nyquist aperture (NA). However, when the hologram is not aligned the expected position as shown in Fig. 2 (right), a part of the readout beam is interrupted by the NA and does not reach a camera. As a result, SNR of the hologram decreases. In our developed system, amount of the relative position of a disc position from the expected one is acquired from intensity of a readout beam blocked by the NA. To achieve high precision and high-speed performance, the system controls the position of the NA with a triaxial actuator because the weight of the NA is exceptionally lighter than the disc unit.
Additionally, the system controls position of the NA with the triaxial actuator by using non-linear position error signal (PES) because the PES of the focus direction shows non-linear function of the NA position. In the case of the non-linear PES, the NA position cannot be determined uniquely from the PES. This is because the PES gives the same value for several different NA positions. Hence, we intentionally shift an operating position of the NA from the expected position. As a result, even though the PES is the non-linear function, the direction to move the NA can be uniquely determined (Fig. 3). Figure 4 shows the focus-axis position control with the non-linear PES could compensate for the relative position by experiment. As a result, the positing precision and the lead-in time which satisfy the specifications for the focus direction were observed. Herewith realization of HDSS having capacity of TB class and transfer rate of Gbit/s class is highly expected.
(By TANAKA Yukinobu)