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Nonlinear optical vortex coronagraph
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DOI：10.1364/OL.383311.png)
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Abstract
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A nonlinear optical vortex coronagraph (n-OVC) based on sum-frequency generation (SFG) in a periodically poled lithium niobate (PPLN) crystal is presented. We demonstrate an n-OVC by mixing the image of an on-axis point source (lambda(g) = 1.6 mu m) inside the PPLN crystal with a pump beam (lambda(p) = 1064 nm) imprinted with a helical phase profile from a vector vortex mask (topological charge l = 2). Due to quasi-phase matching and orbital angular momentum conservation, a coronagraphic image is produced at the SFG wavelength (lambda(up) similar to 630 nm). We validate that the n-OVC is tunable to signal wavelength but only requires a vortex mask operating at the pump wavelength. The acceptance bandwidth of the SFG process provides the n-OVC a degree of achromaticity even with a monochromatic vortex mask. The n-OVC exhibits an inner working angle of similar to lambda(s)/ D and an experimental contrast of 10( -4) at 3 lambda(s)/ D. (C) 2020 Optical Society of America
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