Optically-controlled delays in optical fibres using Brillouin-generated slow and fast light

Recent experiments have demonstrated the possibility of optically changing the group velocity of an optical pulse as it travels through a material [1-4]. These experiments have shown that it is possible to slow the speed of light up to nearly stopping it [2] and also faster-than-light propagation of the pulse envelope (although not its information) has been demonstrated [3]. The possibility of slowing or advancing light in optical fibres is very interesting since it can be potentially used for the development of fast-access memories and optically-controlled delay lines which would be compatible with optical signal processing and fibre-optic communication systems. To date, there is no known method of having such an optical delay line in optical fibres, although they are believed necessary for the development of the future all-optical packet routers. It was recently suggested [5] that it is possible to introduce a delay in a pulse propagating in a single-mode fibre by placing it in the stimulated Brillouin gain region of a moderately powerful, counter propagating pump. In this paper we present the basic theory behind this phenomenon and we demonstrate this effect experimentally. We also demonstrate fast light propagation in the spectral region of Brillouin loss of the spectrum. To our knowledge, this is the first experimental demonstration of optically-controlled pulse advancement and delaying in optical fibres. For a given fibre the optical delay experienced by the signal turns out to depend only on the total gain (loss) experienced by the signal, so that large delays can be obtained in long fibres using a few mW pump power. But the same gain using high power in short fibres results in a drastic change of the group velocity and group indices ranging from 2.7 down to 0.7 were actually observed, well above the vacuum light velocity in the latter case.