Compact, low-cost, and low-crosstalk orbital angular momentum sorter based on binary grating

This paper demonstrates the design, fabrication, and testing of a compact, low-cost, low-crosstalk orbital angular momentum (OAM) sorter. The sorter is based on optical geometric transformation (OGT). This sorter can realize the separation of the OAM states as the traditional sorter, but uses a compact design that utilizes only two phase modulations to realize the transformation and focus. The phase modulations are introduced by distorting the binary gratings by curving the geometrical structure rather than optical path difference. The fabrication is simple and only needs single-step lithography and single-step dry etching on a fused quartz plate. The etching depth error does not affect the sorting result but only the diffraction efficiency. In the experiment, the two binary gratings are fabricated on the same substrate to lower the degrees of freedom of alignment and ensure uniform etching depth. The sorter is further improved by incorporating fan-out diffraction, resulting in a higher separation and lower cross-talk. This sorter can provide a compact, low-cost, low-crosstalk device to realize OAM demultiplexing.