Servo Technologies in a Near-Field Optical Disk Drive System

A near-field optical disk drive (NFDD) system has been predicted as a storage system with a capacity higher than 100 Gbytes in a 12-cm-sized disk and with a data transfer rate higher than 100 Mbps. The major advantage of an NFDD system with a solid immersion lens is that it is possible to utilize the technologies accumulated through conventional optical drive systems. However, the working distance in an NFDD system is only 25 nm or less, which is much smaller than that in a conventional optical disk drive system. The nano-working distance must be successfully maintained without regard to any tilts over the entire disk radius. In this study, we discuss the servo technologies in the NFDD system by focusing on the tilt servo technology which contributes to the stable gap servo performance over the entire disk radius. First, we evaluate the tilt in the NFDD system by utilizing the optically detected tilt error. Second, we propose specifications for an NFDD system on the basis of the results. We also report a tilt servo system that satisfies the specifications and discuss its effect from the viewpoint of drive performance over the entire disk radius. In addition, we suggest a novel gap servo technology utilizing the tangential tilt information and discuss its performance.