Stored light is achieved by coherently transferring the information content of optical pulses to an acoustic excitation in a room-temperature optical telecommunication fiber through their interaction with an additional optical “write” pulse. After a controllable storage time, the acoustic excitation is converted back to the optical domain by interaction with a “read” pulse. The information conversion process is based on stimulated Brillouin scattering (SBS)  and works at any wavelength where the fiber is transparent, including the important telecommunication band in the near-infrared spectral region. Figure 1 shows a piece of standard telecomuunication optical fiber.
Fig.1: A piece of standard optical fiber held in a hand.
Bits of information, represented by pulses of light, pass through the fiber while, simultaneously, a write pulse passes through the fiber in the opposite direction (Fig. 2(a)). Through the process of SBS, essentially all the data-pulse energy is depleted and a coherent acoustic excitation is left behind in the fiber, which contains the information content of the data pulses (Fig. 2(b)). Only a small fraction of the data-pulse energy is converted to the acoustic excitation; most of it is transferred to the write pulse.
Fig. 2: Schematic of encoding information onto an induced acoustic excitation.
At a later time, a read pulse passes through the fiber in the same direction as the write pulse (Fig. 3(a)). It depletes the acoustic excitation and the data pulses are released from the fiber, propagating in the same direction as the original data pulses (Fig. 3(b)). Energy from the read pulse is transferred to the released data pulses in this process.
Fig. 3: Schematic of retrieving information from the induced acoustic excitation.
A simulated movie (AVI, needs XVID codec, size 220 KB) that illustrates the storage and retriveal processes is available here. In this example, the data pulse is 2-ns long and enters a short piece of highly nonlinear optical fiber (HNLF) from the left-end, while the control pulses (write and read pulses) are both 1.5-ns long and enter the HNLF from the opposite end (center panel). The data pulse exiting the right-end is shown in the right panel, while the control pulses exiting the left-end are shown in the left panel. For more informaiont about simulations, please refer to the Supporting Online Material.
Stimuated Brillouin scattering is a well-known nonlinear process in which two optical waves interact through an acoustic wave. The acoustic wave is generated through the process of electrostriction . The acoustic wave modulates the refractive index of the medium, which induces both amplifying and absorbing resonances in the vicinity of the applied optical frequencies. SBS is the strongest when the frequency difference between the two optical waves is tuned to the frequency of the acoustic wave (also called the Brillouin frequency shift ΩB) that is proportional to the speed of sound in the material. The high-frequcy optical wave is attenuated due to the energy transfer to the low-frequency optical wave which experience amplification. The amplifiying resonance experienced by the low-frequency optical wave (i.e., Stokes wave) has been recently used to achieve slow light [2,3], where the group index for the the Stokes wave is reduced at the resonance.
The SBS stored light relies on the process of anti-Stokes absorption, where the carrier frequency (central frequency) of the incident data pulses is higher than the carrier frequency of the applied write and read pulses by the Brillouin frequency shift ΩB, which is approximately 9.6 GHz for our fiber at wavelengths near 1.55 μm. The SBS process works for data-pulses at any carrier frequency so long as the write- and read-pulse frequencies are lower than the data-pulse frequency by ΩB; this condition can be achieved easily using standard tunable-laser technology.
Electrostriction is a mechanism that leads to the nonlinear coupling of optical waves in stimulated Brillouin scattering. Electrostriction is the tendency of materials to become compressed in the presence of an electric field. Through electrostriction, an applied electic field will change the density and therefore refractive index of the material. In stimulated Brillouin scattering, the beating of the electric fields of the two optical waves genearates an temporal-spatial density change in the material, i.e., an acoustic wave. Thus, the optical waves are scattered by the moving index grating, leading to the nonlinear coupling between optical waves.
 R. W. Boyd, Nonlinear Optics, 2nd Ed. (Academic Press, San Diego, 2003), Ch. 9.
 K. Y. Song, M. G. Herráez, and L. Thévenaz, "Observation of pulse delaying and advancement in optical fibers using stimulated Brillouin scattering," Opt. Express 13, 82 (2005).
 Y. Okawachi, M. S. Bigelow, J. E. Sharping, Z. Zhu, A. Schweinsberg, D. J. Gauthier, R. W. Boyd, and A. L. Gaeta, "Tunable all-optical delays via Brillouin slow light in an optical fiber," Phys. Rev. Lett. 94, 153902 (2005).