We report what are to our knowledge the first continuous observations of optical fiber solitons and nonsoliton pulses in the spectral domain. Using a novel optical time-resolved spectral reflectometer that measures the backscattered light spectrum, we directly observed (with 200-m spatial resolution) the propagation of ∼8.5-ps solitons and nonsolitons traveling down 5-km fibers. Spectral breathing and the recovery of the original spectral widths are clearly seen for higher-order solitons. Possible applications include in situ measurements of soliton parameters, fiber dispersions, and nonlinearities, experimental verifications of pulse propagation theories, and optical sensing.
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