Observation of Eisenbud-Wigner-Smith states as principal modes in multimode fibre

Generally, light becomes scattered in space and time as it is coupled among multiple spatial paths during propagation through disordered media or multimode waveguides. This limits the pulse duration and spatial coherence that can be obtained after light exits such a medium. Eisenbud-Wigner-Smith eigenstates, originally proposed in nuclear scattering, are a unique set of input/output states that, despite spatiotemporal scattering during propagation, arrive at the output temporally unscattered. In fibre optics, these states manifest as principal modes that allow pulses and spatial coherence to be maintained despite propagation through a medium that would otherwise have destroyed these properties. These states generalize the phenomena of orthogonal fast/slow axes in a birefringent object to a basis with N axes, where N is the total number of spatial/polarization modes in the scattering medium. We experimentally demonstrate the existence of principal modes using a 100 m length of multimode fibre as the propagation medium.