  Helical gratings (HGs) have an achievement of flexible mode conversion for
fibre guided orbital angular momentum (OAM) modes. Sampled reflection HGs can
realise the generation and conversion of OAM mode with comb spectra. They can
be used to simultaneously filter wavelengths and convert modes, and thus may be
applied to the hybrid multiplexing technology with wavelength division
multiplexing and mode division multiplexing.
