A method to achieve an extremely wide and flexible external control of the group velocity of signals as they propagate along an optical fibre will be presented. This control is achieved by means of the gain and loss mechanisms of stimulated Brillouin scattering in the fibre itself. Our experiments show that group velocities below 71,000 km/s on one hand, well exceeding the speed of light in vacuum on the other hand and even negative group velocities can readily be obtained with a simple benchtop experimental setup.
The experimental result shows tuneable optical delay as much as 152 ns with a 40 ns pulse. Stimulated Brillouin scattering offers the key advantage of the generation of synthesized gain spectra, so that innovative slow light schemes can be realized, ranging from broadband tunable delays to a zero-gain situation identical to an ideal electromagnetically-induced transparency.