Black hole analogue lightring observed in optical fiber

Scientists have achieved the complete observation of a black-hole lightring resonance in an optical analogue system. This breakthrough generalizes the concept of the photon sphere to non-stationary systems, showing that the lightring is associated with a spatiotemporal caustic. The complex frequency of this caustic, determined by its propagation and divergence, is key to understanding the observed properties. The experiment, conducted in a 5-km-long dispersion-shifted optical fiber, used an optical soliton to act as an analogue black-hole potential. A characteristic spectral plateau was predicted, with its boundaries set by the real part of the lightring frequency and its spectral decay governed by the imaginary part. These spectral signatures were observed for two distinct pump wavelengths using a pump-probe experiment. The measured spectra align with numerical simulations and the predicted lightring-mode profile, providing direct access to both the oscillatory and decay properties of the resonance. These results establish a direct experimental connection between black-hole lightrings, spacetime caustics, and quasinormal-mode spectroscopy in a dispersive analogue system, opening new avenues for studying extreme gravitational phenomena in laboratory settings.