  Paraxial propagation of a quasi-monochromatic light wave with two circular
polarizations in a defocusing Kerr medium with anomalous dispersion inside a
waveguide of annular cross-section is considered. In the phase-separated
regime, the dynamics is similar to a flow of immiscible fluids. For some
initial conditions with relative gliding of the fluids along the interface, the
Kelvin-Helmholtz instability in its ``quantum'' variant is developed. Numerical
simulations of the corresponding coupled nonlinear Schroedinger equations have
shown formation of specific structures on the nonlinear stage of the
instability. Similar structures have been known in the theory of binary
Bose-Einstein condensates, but for optics they are presented here for the first
time.

Optics;Quantum Gases;Pattern Formation and Solitons