Advanced focus capture control for high NA optical storage system

High-density optical storage systems, utilizing a violet laser diode and high NA (numerical aperture) objective lens, are being developed by various companies. Consequently, a need has arisen for improved focus servo capturing, i.e. transition from seeking to servo lock. This is due to the improvement in the sensitivity of focus sensor optics at the expense of its linear range. Also, the small working distance of the high NA objective lens has increased the risk of collision. A DVD focus servo system is shown in Fig. 1. A focus error signal is detected to be S-shaped as shown in Fig.2, and has limited linear range that can be used to servo. When the relative deviation exceeds a certain limit, some form of insurance measure must be adopted, such as brakes, to put the lens back into the linear region. The relative velocity at which the servo loop can settle decreases according to the decrease in linear range. So high-speed servo capturing is more difficult in high-density optical storage systems than in conventional DVD. In this paper, we propose two approaches to improve the performance of servo capturing. One approach is a scheme to optimize the initial states of the controller, and another is a scheme to estimate the relative position between the objective lens and disk by introducing a model of the system.