Three-zone phase-only filter increasing the focal depth of optical storage systems with a solid immersion lens
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The field distribution of a Weierstrass solid immersion lens (SIL) is presented, which shows that its focal depth is only in nanoscale. A novel filter design based on a three-zone binary phase pupil function is introduced for increasing the focal depth of a near-field Weierstrass SIL system and a procedure for designing a rotationally symmetric binary phase filter is presented. Numerical results show that an appropriate three-zone phase-only filter not only can evidently increase the focal depth of the SIL system, but also can effectively lower the spreading of the spot size with distance and suppress the side-lobe intensity.
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