Spin-entangled orbital angular momentum filter of photons based on a chiral long-period fiber grating

The orbital angular momentum (OAM) of light is particularly promising for multi-channel data transmission in optical fiber communications. In the implementation, one of the impediments is the lack of an effective all-fiber method to demultiplex and filter OAM modes. To solve the problem, by utilizing the inherent spiral characteristics of a chiral long-period fiber grating (CLPG), we propose and demonstrate experimentally a CLPG-based scheme for filtering spin-entangled OAM of photons. We unveil theoretically and verify experimentally that the co-handed OAM with the same chirality of helical phase wavefront as CLPG couples to higher-order cladding modes and suffers loss, while the cross-handed OAM with the opposite chirality freely passes in CLPG. Meanwhile, by combining its grating characteristics, CLPG can realize the filtering and detection of a spin-entangled OAM with arbitrary order and chirality without exerting extra loss for other OAM. Our work has great potential in analyzing and manipulating spin-entangled OAM, paving the way for the development of all-fiber OAM applications.