Vortex chirality-dependent filtering in helically twisted single-ring photonic crystal fibers

Helically twisted microstructured optical fibers have a wide application prospect in the field of optical vortex communications. In this paper, a helically twisted single-ring photonic crystal fiber (HS-PCF) is proposed for orbital angular momentum (OAM) vortex modes selective filtering. And the theoretical framework of OAMs filtering is also constructed. Positive and negative OAM vortexes have different transmission losses in HS-PCF, and the loss difference between them increases significantly after the twist rate reaches a certain value. Such fibers can filter out OAMs with a certain sign of the topological charge (depending on the handedness of the thread), while dissipating oppositely charged OAMs. In addition to the general OAMs, i.e., zero-order radial vortex modes, the helically twisted fiber also performs a good selective filtering for the first-order radial vortex modes. Remarkably, the filtering bandwidth of HS-PCF is very broad, covering four communication bands from O- to C-band. This kind of fiber can be used as a broadband OAMs filter.