Abstract

In this Letter, we demonstrate, to the best of our knowledge, a novel method to generate an orbital angular momentum (OAM) based on the principle of the modal interference in a two-mode fiber. At the interference dips, the left- or right-handed circular polarized ${\mathrm{HE}}_{11}$ modes can be ideally converted into the $\pm 1$-order OAM beam. To verify this concept, we employed the femtosecond laser micro-processing technology to write micro-waveguides in the two-mode fiber and hence realized the in-line modal interferometer. To enhance the mode conversion efficiency at the dips, we optimized the waveguide parameters both theoretically and experimentally. The interference spectrum and spiral/fork patterns confirm the OAM beam generation with an efficiency as high as 99%.

© 2019 Optical Society of America

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