In this paper, error analysis and tolerance allocation methods for the optical head of NFR (Near Field Recording) system are presented. We fabricate the NFR system and test the reading & writing performance of the NFR system. The test results show that the performance is not good enough. In order to find the cause of the performance drop in the NFR system, assembly and manufacturing tolerances in the optical head of NFR system are simulated. The tolerances analysis result shows that it needs to allocate the tolerances of the optical head of NFR system. So we proposed optimal compound tolerance allocation method using WOW (worst on worst) method and Monte-Carlo method base on sensitive analysis of the optical system. We used two tolerance allocation methods to allocate the compound tolerance in the optical head of NFR system. The results show that WOW method is an over-design method and the Monte-Carlo method is the optimal method of tolerance allocation in optical system.
Near field recording (NFR) has been introduced as a new optical data storage method to realize higher data density beyond the diffraction limit. As the data density increases, the track pitch is remarkably reduced to about 400nm. Thus, more precise actuator is required and we propose a dual servo actuator to improve the accuracy of actuator.
The proposed dual servo actuator consists of a coarse actuator and a fine actuator, multisegmented magnet array (MSMA) voice coil motor (VCM) and PMN-PT actuator. In design of VCM actuator, a novel magnetic circuit of VCM with MSMA is proposed. It can generate higher air gap flux density than the magnetic circuit of VCM with the conventional magnet array. In design of fine actuator, the fine actuator including PMN-PT single crystal instead of the conventional PZT is proposed. The displacement gain of PMN-PT fine actuator is 26 nm/V and that of PZT fine actuator is 17 nm/V. The displacement gain is increased by 53 %.
To evaluate tracking performance of the manufactured dual servo actuator and to assign the proper role to each actuator, the PQ method is selected. From experiment results, the total bandwidth of the dual servo actuator is increased to 2.5kHz and the resolution is 25 nm. Comparing with the resolution of one servo actuator, 70 nm, we can find that the accuracy of actuator is remarkably improved. And the proposed dual servo actuator shows satisfactory performances to be applied to NFR and it can be applied to other future disk drives.
Evaluation results and an autoalignment method for an optical head of a near-field recoding (NFR) system are presented. The focusing unit is an optical head of a NFR system and is composed of a solid immersion lens (SIL) and an objective lens (OL). Generally, the size of the focusing unit is smaller than that of the conventional optical recording head. Hence there are difficulties in assembling the small focusing unit precisely and a novel method for an effective assembly is required. We compose an evaluation system with an interferometer and evaluate some focusing unit samples aligned and assembled manually and present the obtained results. We also propose a conceptual method of autoalignment to assemble the focusing unit well by a pattern recognition using a neural network. Using the conventional optical tool, Code V, a tolerance analysis of the assembly error between the SIL and the objective lens and an interference pattern analysis for the assembly error are executed. Then, through an analysis of the simulation results, the autoalignment methodology using a neural network approach is proposed.