Perfect Vortex Modes for Nondestructive Characterization of Mode Dependent Loss in Ring Core Fibers

Abstract

Ring core fibers (RCF) enable high-performance modal multiplexing with low crosstalk and can support orbital angular momentum (OAM) modes. RCFs are challenging to characterize due to the lack of commercial multiplexers, especially for high OAM orders. For fibers supporting large numbers of modes, typical cutback techniques for characterization are extremely wasteful of fiber, especially as one cutback is required for each mode. We show the differential modal loss across modes 3 to 10 was significantly underestimated using an OTDR when exciting modes individually or when exciting all modes indiscriminately. We exploit perfect vortex beams to achieve reliable and nondestructive characterization of mode-dependent loss (MDL) for OAM modes. Perfect vortex beams allow us to maximize the coupling efficiency at each mode launch, increasing the accuracy of MDL estimate. We fabricated fiber with a refractive index difference between the ring core and the cladding of a 5.1 × 10 −2 . For this fiber, mode orders 3 to 10 are the most suitable for data transmission and were the focus of our work (the fiber support up to OAM order 13). Such a high index difference can lead to MDL. We demonstrate that the modal loss spans from 2.14 to 4.38 dB/km for orders 3 to 10. Index Terms-Orbital angular momentum (OAM), ring core fiber (RCF), mode-dependent loss (MDL), perfect vortex (PV), mode-division-multiplexing (MDM), time-of-flight (ToF) I. INTRODUCTIONO RBITAL angular momentum (OAM) modes have attracted significant interest for mode-divisionmultiplexing (MDM) systems as they reduce multiple-input multiple-output (MIMO) digital signal processing (DSP) complexity [1]. A large modal index separation makes MIMO-free operation more robust to perturbations [2] but increases loss. Ring core fibers (RCFs) for OAM MIMO-free operation are effective over kilometer lengths (suitable within data centers) [1], [3], [4]. In addition to large index separation, the higher the OAM order, the more resistant it is to fiber perturbations [2], [5], [6].An RCF with a ring diameter of the same order of magnitude as the ring thickness is a good candidate for MIMO-free data transmission. Such a fiber was demonstrated in [3], where

Dense Space-Division Multiplexing Exploiting Multi-Ring Perfect Vortex

“…Perfect vortex (PV), which has an annular ring with a radius and thickness independent of the TC, is well known as the Fourier transform of the Bessel beam [35]. Although PVs are mostly used in optical fiber communications to improve the coupling efficiency when launching OAM beams into fibers [36], they can be collimated and transmitted in free space with the assistance of a microscope objective and a simple lens [37]. Since then, researchers have studied its transmission characteristics and developed several wireless communication links both in free-space and underwater turbulent environments [38][39][40][41][42][43][44].…”