GETTY

Slow light — the propagation of an optical pulse at a low group velocity — could find a use in all-optical routing and signal processing as it permits information storage in the optical domain without conversion to an electrical signal. Up to now, however, engineers of such systems have faced a problem: delays to an optical pulse can be made longer but the intrinsic dispersion in the optical fibre leads to increasing spectral broadening and distortion, thus limiting the bandwidth for communication.

J. T. Mok and colleagues from the University of Sydney have demonstrated that this trade-off is not inevitable. They use fibre Bragg gratings — an optical fibre with periodic variations in the refractive index written into the core, which create a band of frequencies where the group velocity is zero. The team work at a wavelength on the edge of this band where the group velocity is non-zero, but still much lower than that in bulk glass. The large dispersion can be balanced by the nonlinear optical properties of the fibre. This so-called soliton effect has been used to improve the performance of optical fibres in communication applications, but this is the first time it has been used in the context of slow light. Mok et al. show that fibre Bragg gratings enable subnanosecond pulses to travel at 16% of the speed of light without broadening. They hope that they will soon extend this work to chalcogenide waveguides, which offer higher nonlinearities and can be readily integrated into a device.