Analysis of servomechanisms for high-density optical disks with the vectorial and scalar diffraction theory

Abstract

ABSTRACT Cu rrently, next generation optical disks with high capacity and data transfer rates are being developed. These optical disks include multilayer Blu -ray disk (BD), holographic data storage (HDS), microholographic data storage and near -field optical disks. In these disks, some sort of structures on the disk is used for a servomechanism. The size of these structures is almost the same as the wavelength. We analyzed these servomechanisms based on the vectorial and scalar diffraction theory. Keywords: FDTD , angul ar spectrum , optical disk 1. INTRODUCTION Currently, next generation optical disks with high capacity and data transfer rates are being developed. These optical disks include multilayer Blu -ray disk (BD), holographic data storage (HDS) [1,2], microholograph ic data storage [3] and near -field optical disks [4]. In these disks, some sort of structures on the disk is used for a servomechanism. The size of these structures is almost the same as the wavelength. We analyzed these servomechanisms based on the vector ial and scalar diffraction theory . Heretofore, land/groove structures are widely used for a servomechanism of optical disks. In optical disks with the land/groove structure, digital data is recorded on the groove, and readout beam sweeps along the groove. The width of the land/groove is almost the same as the spot size of the beam. For the analysis of these structures of the wavelength order, the finite -difference time -domain (FDTD) [5 ] method is popularly used. The FDTD method is a numerical algorithm for VROYLQJ0D[ZHOO¶VHTXDWLRQVDQGLWLVHDV\WRLPSOHPHQWWKHDOJRULWKP,QWKH)'7'PHWKRG$PSqUHVODZDQG)DUDGD\VODZLQ0D[ZHOO¶VHTXDWLRQVDUHGLVFUHWL]HGXVLQJDVWDJJHUHGJULGFDOOHGWKH<HHFHOO7KH)'7'method can handle subwavelength structure s, and polarization. However, in order to suppress numerical dispersion, the grid size should be maintained at less than 1/10 of the wavelength, so the FDTD method needs high computational costs. In general, it is hard to handle a domain more than 1,000 ti mes of the wavelength. In contrast, the scalar diffraction theory does not have these phenomena, but it can handle large domain compared with the FDTD method. Therefore, we present a combined method that is the combination of vectorial and scalar diffracti on theory. In this method, we use the FDTD method in the domain include structures of the wavelength or subwavelength order, and the scalar diffraction theory is used in other area. By using this method, they can analyze servomechanisms of optical disks in clude subwavelength structures such as the land/groove. In this study, we analyzed servomechanisms for high density optical disks as a practical matter. In this analysis, we assumed following optical system as an analysis model. An analysis object is multi layer optical disk that mainly consists of protecting layer, recording layer, and reflection layer. It is assumed that the reflection layer has a groove and its depth and width is tens - hundreds of nm. The material of protection layer is polycarbonate (PC ), and it behaves as a dielectric in the electromagnetic field. The reflection layer is constructed from aluminum. Since aluminum has large conductivity, electromagnetic waves are reflected on the reflection layer. We assumed that the material of the recor ding layer is a photopolymer. Photopolymers are suitable for recording thick phase holograms and stable media with high diffraction efficiency; therefore, they are widely used as holographic recording media. We calculated diffraction of the wave, and obtai ned the distribution of the energy density of reflected beam in the far field. We show that the declination of the disk can be detected by measuring the bias of the energy density distribution.