  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.
