Within a narrow spectral region around the wavelength of zero-group-velocity dispersion of a nonlinear-optical waveguide, phase-matched four-wave mixing couples the Stokes Raman sideband of a pump field to its anti-Stokes counterpart. This wave coupling suggests a sensitive probe for linear and nonlinear-optical parameters of the waveguide, enabling the detection of nanoscopic size variations of microchannel waveguides in photonic-crystal fibers, and facilitates the generation of broadband supercontinuum radiation.