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Light Controlling Light in an Optical Fibre: From Very Slow to Faster-Than-Light Speed

We demonstrate a method to achieve an extremely wide and flexible external control of the group velocity of signals as they propagate along an optical fibre. 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. Advanced schemes can be realized thanks to the extremely flexible possibility to shape the gain spectrum to make it optimized for applications. Ultra wide bandwidth, delaying with flat amplitude response and lower distortion were successfully demonstrated this way.