Resonant Coupling between Orbital-Angular-Momentum Modes in Femtosecond Laser Written Helical Bragg Waveguides

Helical gratings in waveguide systems are usually considered from the point of view of converting fiber modes with zero orbital angular momentum (OAM) to OAM modes or OAM modes to each other [1] . In this case, the fiber is located inside the helical grating and the presence of the grating leads to a resonant coupling between these modes. In this work we combine the optical properties of a helical grating and a waveguide in one microstructure and demonstrate resonant mode coupling between OAM modes in femtosecond laser written helical Bragg waveguide (HBW) inscribed in YAG crystall [2] .