Slow and fast light in optical fibres

The development of high-quality silica optical fibres with excellent transmission capabilities has directly contributed to the tremendous expansion of the global communication network. The success is due to their unrivalled low-loss performance (typically 50% of light is still present after a 15-km transmission distance), their wide bandwidth, which allows terabits per second of information to be transmitted over more than 1,000 km, and also their extremely low production cost. Realizing tunable delays for optical data signals directly in fibres, as well as integrating such devices into existing communication networks, is certainly a very attractive approach for optimizing the flow of data traffic in future networks. The approach may also enhance the capabilities of optical and microwave-on-optical-carrier signal processing. Given the limited possibilities for engineering the dispersion curve of standard single-mode fibres without creating complex photonic-crystal structures (see pages 448 and 465 of this issue1,2), much research on fibre-based delays has been directed towards solutions based on spectral resonances to generate slow and fast light. The initial pioneering demonstrations of slow and fast light in various media all exploited narrow spectral resonances, typically created by electromagnetically induced transparency3 or coherent population oscillation4. Narrow spectral resonances have a dramatic effect on optical propagation, as any sharp spectral change in the medium’s transmission curve results in a steep quasi-linear variation of the effective refractive index with wavelength in the narrow spectral region near the resonance. This in turn results in a strong change in the group velocity at the exact centre of the resonance5. The velocity change is strongest for narrower spectral resonances (as explained in detail below) and for this reason, the early experiments were all carried out in special media, such as ultracold atomic gases or atomic transitions in a crystalline solid at a well-defined wavelength. Within the resonance, the relationship between the change in the optical transmission and the delay can be determined as follows. The refractive-index change, Δn, as a function of the optical frequency, ν, is calculated using the Kramers–Kronig transformation, and the index change associated with the group velocity, Δng, is then found using the relationship Δng = Δn + ν(dΔn/dν) (ref. 5). Finally, the delay, ΔT, added to the normal transit time in the medium is deduced from Δng. Most spectral resonances can be described by a Lorentzian-shaped distribution, and in this important case the additional delay ΔT experienced by a signal placed at the centre of the resonance and spanning over a bandwidth well contained within the resonance is expressed as: