All-in-Fiber Dynamically Reconfigurable Orbital Angular Momentum Mode Sorting

The orbital angular momentum (OAM) spatial degree of freedom of light has been widely explored in many applications, including telecommunications, quantum information, and light-based micromanipulation. The ability to separate and distinguish between the different transverse spatial modes is called mode sorting or mode demultiplexing, and it is essential to recover the encoded information in such applications. An ideal d mode sorter should be able to faithfully distinguish between the different d spatial modes, with minimal losses, and have d outputs and fast response times. All previous mode sorters rely on bulk optical elements, such as spatial light modulators, which cannot be quickly tuned and have additional losses if they are to be integrated with optical fiber systems. Here, we propose and experimentally demonstrate, to the best of our knowledge, the first all-in-fiber method for OAM mode sorting with ultrafast dynamic reconfigurability. Our scheme first decomposes the OAM mode in-fiber-optical linearly polarized (LP) modes and then interferometrically recombines them to determine the topological charge, thus correctly sorting the OAM mode. In addition, our setup can also be used to perform ultrafast routing of the OAM modes. These results show a novel and fiber-integrated form of optical spatial mode sorting that can be readily used for many new applications in classical and quantum information processing.