Abstract

In focus detection of non-imaging systems, the common image-based methods are not available. Also, interference techniques are seldom used because only the degree with hardly any direction of defocus can be derived from the fringe spacing. In this paper, we propose a vortex-beam-based shearing interference system to do focus detection for a focused laser direct-writing system, where a vortex beam is already involved. Both simulated and experimental results show that fork-like features are added in the interference patterns due to the existence of an optical vortex, which makes it possible to distinguish the degree and direction of defocus simultaneously. The theoretical fringe spacing and resolution of this method are derived. A resolution of 0.79 μm can be achieved under the experimental combination of parameters, and it can be further improved with the help of the image processing algorithm and closed-loop controlling in the future. Finally, the influence of incomplete collimation and the wedge angle of the shear plate is discussed. This focus detection approach is extremely appropriate for those non-imaging systems containing one or more focused vortex beams.

© 2018 Optical Society of America

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Supplementary Material (4)

NameDescription
» Code 1       Simulation of the interference patterns of the shearing interference of vortex beams for focus detection of non-imaging systems, corresponding to Figs. 3, 6, 7, 8 in the manuscript.
» Code 2       The relationship between the wedge angle of the shear plate and the defocus at the infinite fringe spacing, corresponding to Fig. 4 in the manuscript.
» Code 3       Resolution of the focus detection method as a function of three parameters, corresponding to Fig. 5 in the manuscript.
» Code 4       The relationship between the defocus and the fringe tilt when the wedge angle of the shear plate varies, corresponding to Fig. 9 in the manuscript.

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