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— Presentation at ODF'16 —
March 25, 2016
The 10th International Conference on Optics-photonics Design and Fabrication, "ODF'16", was held on February 28th to March 2nd, 2016 at Hochschule Ravensburg-Weingarten, Weingarten, Germany. The conference consisted of the session for optical design/simulation, optical components, optical devices, and optical technology. These sessions covered the many fields of optical technique. Hitachi made a presentation entitled "Holographic Data Storage System for Realizing Angular Multiplexing employing Ultra-narrow Angular Interval".
Fig. 1 Schematic diagram of our designed optical
system for AES detection
We are surrounded by digital products and digital technology. The necessity for large-capacity storage system has increased. Holographic data storage system employing angularly multiplexing recording, which volumetrically records two-dimensional page data in a recording medium, is a promising breakthrough for advanced optical storage systems for realizing large storage capacity due to their narrow Bragg selectivity. In Hitachi, we are studying high density recording technique, reliable recording technique, robust recording/reproducing technique, high speed recording/reproducing technique for holographic data storage system. This presentation is related to high speed reproducing technique, especially newly developed high speed and high accurate control technique of reference beam angle for holographic data storage system.
In order to realize high capacity system, the information should be recorded in the narrow angle. Therefore, highly precise control for reference beam angle is required during readout. In this study, we set an angle error within 5 [mdeg] from Bragg matched angle. This made us newly develop angle error signal (AES) detection technique.
The schematic diagram of our designed optical system is shown in Fig. 1. We newly placed HWP, Wollaston prism and detection module in the optical system for reference beam in order to obtain AES.
In the record, the reference beam that has transmitted through the HWP is only S-polarized beam. And the reference beam enters a recording medium via Wollaston prism, galvanometer mirror and scanner lens. At this time, the signal beam and reference beam enter the recording medium while being overlapped with each other so that the interference pattern is formed in the recording medium. The interference pattern is recorded in the recording medium as the hologram.
Fig. 2 Result of verification experiment
In the readout, The HWP of reference optical path converts the incident beam S-polarized beam into polarization of S and P. The reference beam that has transmitted through the HWP enters the Wollaston prism. The Wollaston prism splits the incident reference beam into two different beams in different propagation directions by split angle θ with its polarization of S (reference beam) and P (servo beam). Therefore the polarizations of two beams transmitting through the Wollaston prism are orthogonal each other, and the propagation directions are different. Then, two beams enter the recording medium via galvanometer mirror and scanner lens. At this time, two diffracted beams in accordance with the incident angle are generated in direction of the detection module from the recorded region of the recording medium. These diffracted beams enter the PBS. Because the PBS splits incident two beams with polarization, the diffraction beams of reference beam and servo beam are detected by photodiode 1 (PD1) and photodiode 2 (PD2) respectively. Here, the angular error signal of galvanometer mirror is generated by difference between the two signals. This detection method doesn't affect the reproduced light detected with the camera for data signal, and enables the generation of AES for high speed control of reference beam angle.
In order to realize exact and stable servo control, we have to take AES amplitude, detection range, and sensitivity into consideration because they depend on the split angle. After designing optimal split angle, we actually made experiment of AES. As a result of the experiment, we confirmed that experiment AES is same as designed AES (Fig. 2). In addition, we achieved our target («5 [mdeg]).
(By YAMAZAKI Kazuyoshi)