Propagation stability in optical fibers: role of path memory and angular momentum

Abstract

With growing interest in the spatial dimension of light, multimode fibers, which support eigenmodes with unique spatial and polarization attributes, have experienced resurgent attention. Exploiting this spatial diversity often requires robust modes during propagation, which, in realistic fibers, experience perturbations such as bends and path redirections. By isolating the effects of different perturbations an optical fiber experiences, we study the fundamental characteristics that distinguish the propagation stability of different spatial modes. Fiber perturbations can be cast in terms of the angular momentum they impart on light. Hence, the angular momentum content of eigenmodes (including their polarization states) plays a crucial role in how different modes are affected by fiber perturbations. We show that, accounting for common fiber-deployment conditions, including the more subtle effect of light’s path memory arising from geometric Pancharatnam–Berry phases, circularly polarized orbital angular momentum modes are the most stable eigenbasis for light propagation in suitably designed fibers. Aided by this stability, we show a controllable, wavelength-agnostic means of tailoring light’s phase due to its geometric phase arising from path memory effects. We expect that these findings will help inform the optimal modal basis to use in the variety of applications that envisage using higher-order modes of optical fibers.

Angular momentum redirection phase of vector beams in a non-planar geometry

“…Due to their robustness against aplanarity, radial and azimuthal vector modes constitute ideal candidates for optical communication based on optical fibres, inherently subject to twisting and bending constraints [12], and for microscopy applications, where these modes can also be tightly focused [31]. Our findings are also relevant for twisted optical cavities and mirror-based resonators [32][33][34] and for quantum metrology [35].…”

“…This has been confirmed using optical fibres curled into a helix [7][8][9] or by using a succession of mirror reflections [10,11]. Propagation along a non-planar trajectory also causes a rotation of the beam intensity profile [12][13][14], which can be seen from simple ray tracing.…”

Spatiotemporal Helicon Wavepackets

“…Recall that 𝑙 and 𝑝 refer here to azimuthal and radial indices, where 𝑙(0, ±1, ±2, ±3, … ) is the topological charge, related to the phase front of the OAM mode. OAM modes are higherorder modes defined on a different basis as compared to more conventional modes in fiber, such as linearly polarized (LP) modes and vector modes, so that they can be also regarded as the linear combination of the latter [36][37]. For example, we can here write 𝐴 ± , = (LP ± 𝑖LP ) √2 ⁄ , as well as 𝐴 , corresponds to the fundamental mode LP , of the fiber.…”

Full-Vectorial Fiber Mode Solver Based on a Discrete Hankel Transform

“…In CW operation, intra-modal dispersion of specialty fiber designs allows establishing phase-matching for efficient nonlinear frequency generation via four-wave mixing, e.g., in the short-wavelength UV regime [6]. Another crucial example are few-mode or multimode fibers for spatial multiplexing in next-generation telecommunication networks [7] or lens-less endoscopy [8] as the differential modal dispersion is relevant (besides other effects) for the propagation robustness of the individual modes [9]. Recently, machine learning algorithms were used for the first time to optimize a fiber design with respect to the propagation robustness of the guided modes [10].…”