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Gap Servo System for a Biaxial Device Using an Optical Gap Signal in a Near Field Readout System

A readout method using a near field optical head has been proposed as a technique to read out over 30 Gbytes on a 12 cm disc. For the purpose of reading out the RF signal in the system, a gap servo is required to maintain a constant air gap between the solid immersion lens (SIL) and the disc. In this paper, we propose a practical near field readout system with a similar system to an ordinary optical disc, where a biaxial device is used as an optical head actuator and the gap error is detected optically. First, we explain the gap servo system by which it is possible to control the air gap from a far field initial position to a near field target position at a relatively high speed without a head colliding with a disc. Second, we report the result of applying this gap servo system to a 30 Gbyte and a 50 Gbyte ROM disc. Finally, its performance is confirmed by comparing simulated signal with an observed signal.