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— Presentation at ISOM 2015 —
October 29, 2015
The International Symposium on Optical Memory (ISOM) 2015 was held in Toyama, Japan from 4th to 8th in October, 2015. The topic of ISOM is techniques about optical memory as represented by DVD or Blu-ray Disc™, and their applied techniques. Researchers from various areas such as Japan, USA, Korea, and China attended this conference and had an active discussion.
Hitachi, Ltd made seven presentations including oral and poster session. The present writer presented an oral presentation entitled "Highly Accurate Servo Control of Reference Beam Angle in Holographic Memory with Polarized Servo Beam".
Holographic memory is one of the promising candidates for next generation optical disc, and has feasibility in high speed and high capacity system compared with conventional optical disc system. This presentation is related in high speed technique, especially newly developed high speed and high accurate control technique of reference beam angle in holographic memory (Fig. 1).
In this technique, a servo beam is introduced using Wollaston prism. The polarization of servo beam is perpendicular to reference beam, and the angle of servo beam is a little different to reference beam. The reproduced lights with servo beam and reference beam are detected with two of high speed photo detector independently (Fig. 2). This detection unit doesn't affair or disturb the reproduced light detected with camera for user data, and enables the generation of control signal for high speed control of reference beam angle.
Control signal of reference beam angle can be calculated with difference between reproduced signals with servo beam and reference beam. The reference beam angle can be controlled with 2 step sequence using the control signal. In first step, we control the reference beam angle to zero cross position of control signal as watching the control signal. In second step, the reference beam angle is brought forward by half of split angle between servo beam and reference beam. With this 2 step sequence, the reference beam angle can be controlled as reproduced signal become maximum condition (Fig. 3).
As the result of verification of control signal, the assumed ‘s’ character shaped control signals were obtained with simulation and experiment. We also investigated the angle error of control signal and confirmed the high-accurate enough for system margin design (Fig. 4). In addition, we presented the tolerance investigation results, and showed wavelength error is to be care. We also showed that the wavelength error tolerance can be improved using special filter to practical quality.
(By HOSAKA Makoto)