Helical Bragg Gratings: Experimental Verification of Light Orbital Angular Momentum Conversion

Structured light, in particular light possessing orbital angular momentum (OAM), has been actively studied recent decades. Helical Bragg grating (HBG) is a reflecting optical element, which predicted to be able convert the OAM of waveguides and fibers. However, HBG not demonstrated experimentally yet. Here we report first experimental manifestation created form waveguide with depressed-index cladding. Few-mode channel helical-depressed-cladding (HDCWs) have written YAG crystal by femtosecond laser beam. The HDCW pitches were comparable near-IR wavelengths. spectral polarization characteristics transmitted reflected studied. It was shown that HDCWs behave like HBGs providing narrowband resonance coupling counter propagating modes, modes differing order or, what same, topological charge helical cladding structure. Adjusting parameters allows one excite wave at predetermined wavelength change respect coupled waveguide. Thus, for time, suitable generation filtration vortex demonstrated. results obtained agree well mode theory calculations.