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