Fundamental nonlinear-optical interactions in photonic fibers: Time-spectral visualization

The nonlinear dynamics of optical signals propagating in fibers or waveguides can be quite complex. Many nonlinear regimes manifest themselves in spectral transformations observed at the output of the fiber. Such time-integrated measurements are severely limiting, however, and, thus, a number of time spectrally resolved techniques have been developed in the past. One of the simplest and most versatile appears to be the cross-correlation frequency-resolved optical gating (X-FROG), because it offers high sensitivity, broad bandwidth, and produces very intuitive two-dimensional spectrograms showing relative temporal positions of various frequency components comprising the output signal. Indeed, certain experiments described in this article can only be performed with X-FROG. For others, X-FROG offers better insight into the fundamental physics of nonlinear interactions; yet others yield beautiful and visually stunning images. Some of the fundamental non-linear-optical interactions in waveguides, such as soliton formation and propagation, soliton stabilization and the emission of Cherenkov continuum, resonant scattering of continuous waves on solitons, and the supercontinuum generation have been visualized with X-FROG and are summarized in this article.