Frenet–Serret analysis of helical Bloch modes in N-fold rotationally symmetric rings of coupled spiraling optical waveguides

Chen Y, Russell PSJ (2021)

Publication Type: Journal article

Publication year: 2021

Journal

Journal of the Optical Society of America B-Optical Physics Optical Society of America

Book Volume: 38

Pages Range: 1173-1183

Journal Issue: 4

DOI: 10.1364/JOSAB.412186

Abstract

The behavior of electromagnetic waves in chirally twisted structures is a topic of enduring interest, dating back at least to the 1940s invention of the microwave travelling-wave-tube amplifier and culminating in contemporary studies of chiral metamaterials, metasurfaces, and photonic crystal fibers (PCFs). Optical fibers with chiral microstructures, drawn from a spinning preform, have many useful properties, exhibiting, for example, circular birefringence and circular dichroism. It has recently been shown that chiral fibers with N-fold rotationally symmetric (symmetry group CN) transverse microstructures support families of helical Bloch modes (HBMs), each of which consists of a superposition of azimuthal Bloch harmonics (or optical vortices). An example is a fiber with N coupled cores arranged in a ring around its central axis (N-core single-ring fiber). Although this type of fiber can be readily modeled using scalar coupled-mode theory, a full description of its optical properties requires a vectorial analysis that takes account of the polarization state of the light, which is particularly important in studies of circular and vortical birefringence. In this paper, we develop, using an orthogonal 2D helicoidal coordinate system embedded in a cylindrical surface at constant radius, a rigorous vector coupled-mode description of the fields using local Frenet–Serret frames that rotate and twist with each of the N cores. The analysis places on a firm theoretical footing a previous HBM theory in which a heuristic approach was taken, based on physical intuition of the properties of Bloch waves. After a detailed review of the polarization evolution in a single spiraling core, analysis of the N-core single-ring system is carefully developed step by step. Accuracy limits of the analysis are assessed by comparison with the results of finite element modeling, focusing in particular on the dispersion, polarization states, and transverse field profiles of the HBMs. We believe this study provides clarity into what can sometimes be a rather difficult field and will facilitate further exploration of real-world applications of these fascinating waveguiding systems.

Authors with CRIS profile

Philip St. John Russell Lehrstuhl für Experimentalphysik (Alfried Krupp von Bohlen und Halbach-Professur)

Involved external institutions

Max-Planck-Institut für die Physik des Lichts (MPL) / Max Planck Institute for the Science of Light DE Germany (DE)

How to cite

APA:

Chen, Y., & Russell, P.S.J. (2021). Frenet–Serret analysis of helical Bloch modes in N-fold rotationally symmetric rings of coupled spiraling optical waveguides. Journal of the Optical Society of America B-Optical Physics, 38(4), 1173-1183. https://doi.org/10.1364/JOSAB.412186

MLA:

Chen, Y., and Philip St. John Russell. "Frenet–Serret analysis of helical Bloch modes in N-fold rotationally symmetric rings of coupled spiraling optical waveguides." Journal of the Optical Society of America B-Optical Physics 38.4 (2021): 1173-1183.

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