Helical Long-Period Fiber Grating-Based Band- Selectable and Bandwidth-Enhanced Flat-Top Filter and Its Application to Wideband OAM Mode Converter

A band-selectable and bandwidth-enhanced flat-top rejection filter is proposed and demonstrated both theoretically and experimentally, which is realized by using single-mode fiber (SMF)-based helical long-period fiber gratings (HLPGs) precisely operated at dispersion turning points (DTPs) of the same azimuthal order cladding mode but with different radial indices. As an important application of such HLPGs, the orbital angular momentum (OAM) mode converters enabling to have either a flat-top bandwidth of ~116 nm@-16 dB but centered at wavelength of ~1530 nm, or a flat-top bandwidth ~130 nm @-16 dB but centered at wavelength of ~1650 nm, have been successfully demonstrated. This is the first report for the experimental demonstration of such high-performance band-rejection filters with bands fully matching the C-L bands of the fiber communication. The proposed HLPGs may find potential applications to OAM mode converters, OAM tweezers, and OAM sensors.