Development of dynamic tilt control method and servo algorithm in gap servo NFR

In the NFR system, since the gap between SIL and disc is under 100 nm, the traditional servo method can not be applied, and its modification is necessary for the stable servo performance. The gap error servo method is well-known as a typical servo in the NFR system (Verschuren et al. in JJAP 44:3554–3558, 2005) for focusing direction, and pull-in procedure (Zijp et al. in IEEE Trans Magn 41(2):1042–1046, 2005) and hand-over (Lee et al. in JJAP 44(5B):3423–3426, 2005) methods are also good approaches for the stable servo in the near field. We can obtain a good servo performance using the gap error servo method, and its experimental result shows that it is consistent with the previous researches. In case of the track servo, 1-beam push pull was generally used for the traditional ODD. However, it contains a DC offset components with lens movement, and thus, we can not have a good RF signal and track error signal simultaneously. Therefore, in this paper we introduce the modified servo method, which uses gap error lens position and the successful servo result for surface disc and 1.7 μm cover layer disc. Test was performed with a 4-axis actuator, and it gives a good performance not only for gap and track servo, but also for radial and tangential tilt. Finally, we suggested the decoupling method between track-shift and tilt components using the gap error differential signal and the simple slit structure. Based on the experimental result, it was verified that slit structure can eliminate the track-shift component effectively.