Gap servo system for a biaxial device using an optical gap signal in a near field readout system

An optical head with a biaxial device is considered to be the most promising device for a practical optical disc system. However, since it has a long working range of hundreds of micrometers and high voltage sensitivity, a new technique is required for applying a gap servo in a near field system. In particular, the overshoot upon starting a gap servo is a severe problem. An overshoot of less than one micron is considered a trivial problem for a far field optical system, but it could cause a collision of the optical head in a near field system. In this paper, we report on a gap servo technique with a biaxial device in a near field readout system that suppresses the overshoot at the beginning of the gap servo.