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Corporate InformationResearch & Development

December 4, 2014

Report from Presenter

The International Symposium on Optical Memory 2014 (ISOM'14) was held in Hsinchu, Taiwan over October 20th through 23rd. Various technologies related to optical memory systems were discussed at ISOM. The 1st Symposium was held as a domestic conference in 1985 and launched as an international conference in 1987. This year, there were about 100 presentations and Yokohama Research Laboratory, Hitachi Ltd., made a presentation entitled "Decoupling Direct Tracking Control for Multilayer Optical Disk with a Separated Guide Layer" [1].

Fig. 1 Serious Problem (recording)

Total amount of worldwide archival data has been increasing due to recent rapid growth in the industry of information technology. For data archiving, expectations for optical disk as a green archival storage with a large capacity in addition to low running cost and longevity are raised further. A multilayer optical disk composed of a separated guide layer with groove structure and several recording layers without the groove structure, so-called "groove-less disk" is one of solutions for the archival storage because cost of disk fabrication can be reduced due to simplicity of its disk structure [2].

In a recording system employing such disk, data marks are recorded on the recording layer right above of the guide layer by using two spatially separated spots with different wavelengths. One spot is focused on the recording layer for recording and the other spot is focused on the guide layer for tracking servo. It is important to keep relative position between each spot on the recording layer and the guide layer so that data marks can be recorded at an accurate point of the recording layer. However, some disturbances such as a disk tilt or a lens shift cause relative displacement Δ between the two laser spots. For example, under the condition that a disk tilt θ is 0.1 degree and distance between the recording layer and the guide layer is 150µm, Δ reaches to 0.16µm. The displacement induces overwriting previous recorded data marks on the recording layer as shown in Fig. 1. Additionally, due to scratches or fingerprints on the disk surface, a coupling action between the two laser spots destabilizes the tracking control. Therefore, we developed a new tracking control method named "decoupling direct tracking control" to solve these problems.

Fig. 2 Position control of recording spot

Fig. 3 Effectiveness confirmation

In this new control method, adjacent recorded data track previously recorded on the same recording layer is utilized as a virtual track and at least 2 more spots are newly added as shown in Fig. 2. One is original main spot for recording data to the optical disk, and the others are sub spots for following the virtual track. Furthermore, we have added decoupling path to solve the coupling action between the two laser spots.

Fig. 3 shows experimental result of readout signal from the groove-less disk. Stable readout signals without coupling action were obtained through the developed system and the validity of our new control method was verified.

(By TANAKA Yukinobu)


  • [1] Y. Tanaka et al.: Tech. Dig. Int. Symp. Optical Memory 2014, Tu-B-02
  • [2] M. Nakano et al.: Tech. Dig. Int. Symp. Optical Memory 2009, Th-I-03
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