Propagation-invariant space-time supermodes in a multimode waveguide

TL;DR

This paper demonstrates experimentally that specific superpositions of multiple modes in a multimode waveguide, called space-time supermodes, can propagate invariantly, maintaining their localized transverse profile over long distances.

Contribution

It provides proof-of-principle experimental confirmation of space-time supermodes, a new class of guided fields with invariant propagation in multimode waveguides.

Findings

Successfully constructed space-time supermodes with up to 21 modes

Achieved propagation invariance over 257 Rayleigh lengths

Generated diverse transverse profiles including single-peak, multi-peak, and flat profiles

Abstract

When an optical pulse is spatially localized in a highly multimoded waveguide, its energy is typically distributed among a multiplicity of modes, thus giving rise to a speckled transverse spatial profile that undergoes erratic changes with propagation. It has been suggested theoretically that pulsed multimode fields in which each wavelength is locked to an individual mode at a prescribed axial wave number will propagate invariantly along the waveguide at a tunable group velocity. In this conception, an initially localized field remains localized along the waveguide. Here, we provide proof-of-principle experimental confirmation for the existence of this new class of pulsed guided fields, which we denote space-time supermodes, and verify their propagation invariance in a planar waveguide. By superposing up to 21 modes, each assigned to a prescribed wavelength, we construct space-time supermodes in a 170-micron-thick planar glass waveguide with group indices extending from 1 to 2. The initial transverse width of the field is 6 microns, and the waveguide length is 9.1 mm, which is 257x the associated Rayleigh range. A variety of axially invariant transverse spatial profiles are produced by judicious selection of the modes contributing to the ST supermode, including single-peak and multi-peak fields, dark fields (containing a spatial dip), and even flat uniform intensity profiles.