Nonlinear optical vortex coronagraph

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 (λs = 1.6 µm) inside the PPLN crystal with a pump beam (λp = 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 (λup∼ 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 ∼λs/D and an experimental contrast of 10−4 at 3λs/D.