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Excitation of Orbital Angular Momentum Modes in Helical Bragg Waveguide Inscribed by Femtosecond Laser Beam in YAG Crystal

Optical vortices, also known as beams with phase singularity or beams possessing orbital angular momentum (OAM), open new opportunities for research of light interaction with matter and for space division multiplexing in optical communication systems [1] . Periodic refractive index structures formed in optical fibers or in cylindrical channel waveguides are an effective tool for selective excitation and control of OAM modes. The helical Bragg gratings have the highest spectral selectivity among these structures. Effective excitation of OAM modes in waveguides with helical Bragg structure has been predicted in several theoretical studies, but such structure has not been realized yet. Direct femtosecond laser writing (DLW) possesses unique capability for 3D structure manufacture in the volume of transparent dielectrics. This technique allowed us to create a helical Bragg waveguide (HBW) for the first time to the best of our knowledge. At present the fabricated HBWs convert Gaussian beam to OAM modes with topological charge of up to 6.