A crucial part of optical signal processing is buffering and storage — sometimes a signal has to wait a while until the system is ready for it. Although this is readily accomplished electronically, it is more of a challenge optically. One approach being explored in many laboratories around the world is to reduce the speed of light; that is, to slow it down until the system is ready to process it.Figure 1. In stimulated Brillouin scattering (SBS), stimulating light creates a sound wave via electrostriction, and part of the light is reflected backward from the sound wave. The reflected light is Doppler-shifted, and a probe beam in resonance with the reflected light will experience gain.The group velocity of a light pulse is reduced in a highly dispersive medium, one in which the refractive index is steeply dependent on the light’s frequency. Such a dependence occurs in the vicinity of an optical resonance, such as that of stimulated Brillouin scattering (SBS). SBS occurs when light is intense enough to create a sound wave in the medium, and part of the light is subsequently reflected from the traveling sound wave. The resonance occurs when a counterpropagating probe beam matches the Doppler-shifted frequency of the reflected light (Figure 1). If the probe light is a short pulse, its velocity will be reduced — it becomes slow light.Figure 2. Probe pulses traveling upward in the photonic crystal fiber experience Brillouin gain when they resonate with the Doppler-shifted pump light reflected from the sound wave in the fiber. Because the Brillouin resonance creates a steep variation in the fiber’s refractive index at the resonant frequency, the group velocity of the probe pulses is reduced. The scientists enhanced this speed reduction by tuning the probe to match a longitudinal mode of the Fabry-Perot created between the two splices.This effect has been well explored, and investigators have seen significant SBS-induced slowing of light in optical fibers when the fibers were long enough (~kilometers) or the pump (stimulating) light was strong enough (~watts). Recently, however, Sigang Yang and his colleagues at Tsinghua University in Beijing, together with scientists at FiberHome Telecommunications Technologies Co. Ltd. in Wuhan, China, created very significant slowing of light in short (~50 m) sections of fiber at low pump powers (~180 mW). This development makes slow light much more attractive as a practical buffering mechanism in optical signal processing.Figure 3. With 180 mW (22.5 dBm) of pump power (P), a 40-ns probe pulse was delayed by 21 ns (a). The pulse delay scaled exponentially with Brillouin gain, rather than linearly as it would in the absence of the Fabry-Perot (b). Reprinted with permission of Optics Letters.There are two secrets of the scientists’ success. First, the fiber they used was a photonic crystal fiber, in which the index contrast between the air cladding and the tiny glass core was greater than the contrast of a conventional fiber. This allowed tighter confinement of both the sound and the light, to enhance the SBS. Second, they increased the delay by narrowing the resonance of the SBS process.The physics of slow light predicts that the magnitude of the delay is inversely proportional to the SBS gain bandwidth. To reduce that bandwidth, the scientists took advantage of the splices at either end of the photonic crystal fiber, whose small reflectivities created a low-finesse Fabry-Perot interferometer between them (Figure 2). By aligning the probe-light frequency with a Fabry-Perot resonance, the investigators provided feedback to the SBS process at that frequency only, effectively narrowing the gain bandwidth. As a result, they delayed a 40-ns probe pulse by 21 ns in a 50-m fiber, with only 180 mW (22.5 dBm) of pump power (Figure 3a). Although the delay in conventional SBS-induced slow light scales linearly with the SBS gain, the addition of the Fabry-Perot caused the delay to scale exponentially with the gain (Figure 3b).Optics Letters, Jan.15, 2008, pp. 95-97.