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6 July 2000 Diffractive phase element for reducing the diameter of the main lobe of a focal spot
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A diffractive phase element (DPE) reducing the diameter of main-lobe of a focal light spot has been developed. A light spot focused by an optical system spreads due to the light diffraction from a limited aperture of lens. The developed DPE reduces the main-lobe diameter by modulating the incident wave-front of the focusing lens. Although the DPE increases the side-lobe intensity of the focal spot, appropriate design can reduce its magnitude enough not to affect the photoresist in lithography process. An iterative method based on the Gerchberg-Saxton algorithm combined with new constraints was applied to design a DPE. A rotation symmetrical binary DPE was designed, which reduces the main- lobe diameter to 74% and makes the side-lobe intensity under 2.7% of the main-lobe. The designed DPE was fabricated with the laser beam lithography system developed by the authors, and its performance was measured by mounting it on this system. The minimum line width obtained with the DPE becomes 1.0 micrometers while it is 1.2 micrometers without the DPE. It is also shown by a computer simulation that the focal depth of the focusing system with the DPE becomes wider than that without the DPE when both systems produce the same focal spot size.
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Yusuke Ogura, Jun Tanida, Yoshiki Ichioka, Yoshiaki Mokuno, and Katsunori Matsuoka "Diffractive phase element for reducing the diameter of the main lobe of a focal spot", Proc. SPIE 4081, Optical Storage and Optical Information Processing, (6 July 2000);

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