Group velocity matching has been observed in an optical fiber. The matching occurred in a fiber Raman oscillator pumped by mode-locked laser pulses. The group velocity difference between pump and Stokes pulses due to material dispersion was compensated for by the group delay between two different waveguide modes.
Chinlon Lin, R. H. Stolen, and R. K. Jain, "Group velocity matching in optical fibers," Opt. Lett. 1, 205-207 (1977)