Mode manipulation in a ring-core fiber for OAM monitoring and conversion.

The monitoring and conversion of photonic orbital angular momentum (OAM) play fundamental and important roles for both classic and quantum technologies, especially in low-loss transmission media such as ring-core fibers (RCFs), which make many OAM applications practical or vastly more flexible. However, in a RCF, the modes associated with different OAM states are highly overlapping due to the circular refractive index distribution structure, which makes it difficult to distinguish and monitor the OAM modes and in turn limits its inline conversion. Here, we report the first experimental realization of mode monitoring in a RCF using mode filters (MFs), which takes advantage of the difference in the mode adiabatic evolution and the higher-order mode cutoff conditions in tapered RCFs. Different-order OAM can be filtered using MFs with different geometric parameters, as demonstrated by the linearly polarized mode intensity. Combined the mode manipulations in RCF and single-mode fiber, the fundamental mode coupling efficiency can reach 90%, the RCF mode conversion monitoring through inline transmission spectrum evolution can be realized, and the inline fabrication of RCF grating, which couples one mode to a desired mode, can be demonstrated by the fabricating process of three long-period fiber gratings. The mode conversion efficiency between 0-order and 1, 2- or 3-order OAM modes exceeds 96%. Our work provides an efficient approach to monitor and convert OAM modes in higher-order mode supporting RCFs and even other special fibers and further promotes the improvement of the capacity of OAM transmission in RCFs.