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

Zelin Ma,Siddharth Ramachandran

doi:10.1515/nanoph-2020-0404

Abstract

AbstractWith 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.

Highlights

Multimode fibers (MMFs) and their spatially diverse higher-order modes (HOMs) have experienced alternating levels of interest ever since the invention of optical fibers
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
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

Summary

Introduction

Multimode fibers (MMFs) and their spatially diverse higher-order modes (HOMs) have experienced alternating levels of interest ever since the invention of optical fibers. Perhaps most significantly over the last few years, there has been an emerging realization that individual modes, especially those carrying OAM, can enable signal propagation with low or limited mode mixing [44], as a means of increasing the capacity of classical communications networks [45,46,47,48] or for enhancing the security of quantum links [49] All these applications have two critical requirements: (1) the ability to accurately control mode transformations with, for instance, fiber gratings [50], diffractive optics [51], Pancharatnam–Berry optical elements (PBOE) [52], spatial light modulators [53], or metasurfaces [54]; and (2) crucially, the need for linearly, stably propagating desired modes in fibers. We show that, accounting for typical perturbations an optical fiber encounters, the circularly polarized OAM eigenbasis represents the most stable set of modes for light transmission

Mode classifications

Fiber perturbations

Propagation stability experiments

Geometric phase control

Findings

Discussion, summary, and conclusions