Tolerance analysis and compensation for focusing unit of near field recording system

Recently, the NFR (near field recording) system has been introduced as a new optical data storage method to realize higher density recording beyond the diffraction limit (H.K. Yoon et al., KSPE, vol. 17, no. 6, pp. 46-52, 2000; E. Betzig et al., Appl. Phys. Lett., vol. 61, pp. 142-144, 1992; B.D. Terris et al, ibid., vol. 68, no. 2, pp. 141-143, 1996; B.D. Terris et al., ibid., vol. 65, no. 4, pp. 388-390, 1994). It is based on focusing propagating light with an objective lens (OL) onto the bottom of the solid immersion lens (SIL), which is made of a high refractive index (>1.0) to make the numerical aperture (NA) larger than one. However, using the OL and SIL as a focusing unit (FU), there are serious problems, such as alignment error between SIL and OL, and manufacturing errors of the OL and SIL. These errors can cause aberrations and influence the performance of whole recording system. Tight assembling and manufacturing tolerance in NFR is one of the major barriers to be overcome. In this paper, tolerance analysis for the focusing unit of a near field recording system is presented. The assembling and manufacturing tolerances of SIL and OL are simulated. In addition, we proposed to move the collimating lens (CL) back and forth to compensate and control these tolerances, especially in the optical axis direction.