Tracking servo method for the gap servo near field optical storage systems

ABSTRACT Gap servo systems based on one of near field technologies are generally using the 1-beam push pull method to generate servo signal and this causes the optical DC offset problem in servo signal. In this paper, we presented new method of track error generation using the proposed GELP signal for the gap servo near field optical storage systems. By using the GELP signal and 2-divided photo-detector for the gap error signal detection, test result shows the optical DC offset in track error signal generated from lens movement could be eliminated. Keywords : NF(near field), SIL(solid immersion lens), tracking servo, GELP(Gap Error Lens Position), GES, gap error, gap servo 1. INTRODUCTION To increase capacity of the optical disc, generally we choose the method of decreasing beam spot size or changing media structures for reading smaller pits or marks. Multilevel, super-RENS are the research areas changing media structure in far-field optics. On the other hand, NFR (Near Field Recording) is one of technologies to reduce beam spot size by increasing NA (Numerical Aperture) of the OL (Objective lens). To achieve higher NA, SIL (Solid Immersion Lens) is used. Applying SIL component, current maximum effective NA of OL and diamond SIL with super-hemisphere type assembly is nearby 2.3[1]. The near field effect can be expected under the range of O /4 gap distance where O is the wavelength used. In case of using a blue LD (405 nm) as the light source the gap distance should be controlled under 100 nm with much tighter margin. Recently, to control the distance between SIL and disc, some research groups used actuator system of conventional DVD pick-up as the active actuating tools for gap servo control and reported successful results. A few years ago, small gap controlling method was introduced by using evanescent effect from foregoing research groups [2,3]. In addition to gap servo method, to see the RF signals from th e disc, optical beam spot has to follow data track. In case of far field systems, 3-beam method is used in CD ROM disc, and DPD (Differential Phase Detection) method is used in DVD ROM disc normally. However above TE (tracking error) methods can not be available to the recordable systems because of DC offset in 2 sub beams and no pit in formation in a blank media. To solve this problem, push-pull method, which is comparing signals between left and right side of track was introduced, and DPP (Differential Push-Pull) is the most popular method for reading and writi ng systems. If disc and spindle motor are well made, then 1-beam push-pull method can be a solution for tracking servo. But normal discs and motors have considerable eccentricities. In the case of discs with large eccentricity, OL moves same amount of disc eccentricity to follow track. By lens movement, reflected beam at the TE detection photo-detector is moved in same direction. Consequently this causes DC offset in TE signal and making de-track effect. On the contrary, DPP method use one main beam and two sub beams. Two sub beams have 180 deg. phase difference from main push-pull signal and same amount of beam movement exist in photo-detector. Therefore final TE signal is achieved by followi ng numerical calculation. TE = MPP (Main Push Pull signal)  k  SPP (Sub Push Pull signal) k : servo gain coefficient