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A Simpler Way to Store Light

Photonics Spectra
Jan 2008
Rather than slowing lightwaves, new technique converts light to slow-moving sound -- and then back again.

Stimulated Brillouin scattering (SBS) is the bane of fiber laser researchers because it is a parasitic effect that saps the energy out of high-power fiber lasers. But now Zhaoming Zhu and Daniel J. Gauthier of Duke University in Durham, N.C., working with Robert W. Boyd of the University of Rochester in New York, have demonstrated a way to use the effect to store pulses of light for as long as 12 ns in a short length of optical fiber.

PTSimple_Illustration.jpg

Figure 1. A Brillouin interaction between incoming data pulses and a write pulse (a) converts the data pulses to an acoustic signal (b) that can be stored in the optical fiber for many nanoseconds. Subsequently, a read pulse (c) converts the acoustic signal back into optical pulses (d).


SBS occurs when a pulse of light generates a sound wave in a material through electrostriction, and part of the light in the pulse is reflected back from the traveling sound wave. Because the wave is traveling at the speed of sound, the reflected light is Doppler shifted down in frequency by the characteristic Brillouin frequency.

In the Duke/Rochester experiment, the data pulses traveling left to right through an optical fiber were not intense enough to trigger SBS, but the scientists introduced a counterpropagating “write” pulse whose carrier frequency was lower than the data-pulse frequency by exactly the Brillouin frequency (Figure 1a). The electric fields of the counterpropagating pulses interfered, generating sound waves that traveled to the right, and virtually all the energy in the data pulses was subsequently coupled via SBS into either the sound waves or the write pulse (Figure 1b).

Many nanoseconds (as many as 12) later, the scientists propagated a “read” pulse right to left through the fiber (Figure 1c). The read pulse’s wavelength was identical to the write pulse’s, and because it was traveling in the opposite direction from the sound waves, it was Doppler shifted up in frequency when it reflected from them. These reflected pulses recreated the original data pulses. In essentially a reversal of the previous process, the energy in the sound waves, along with some of the energy in the read pulse, was coupled via SBS into the recreated data pulses (Figure 1d).

The practical application of this work is the same as the application of slow light: the ability to buffer information traveling on optical data links. The SBS technique described here has an advantage over slow-light techniques that involve spin coherence in an atomic gas: It is accomplished using only commercially available components at room temperature, making it likely to be more robust than other approaches to optical data storage. Moreover, the technique does not depend on any atomic resonances and can be invoked with any wavelength. (These experiments were conducted at the communication wavelength around 1550 nm.)

However, more work remains. The researchers want to increase the storage time from 12 ns, and they want to whittle down the write- and read-pulse power from the 100 W that was necessary in these experiments, which were described in the Dec. 14 issue of Science.


GLOSSARY
fiber laser
A laser in which the lasing medium is an optical fiber doped with low levels of rare-earth halides to make it capable of amplifying light. Output is tunable over a broad range and can be broadband. Laser diodes can be used for pumping because of the fiber laser's low threshold power, eliminating the need for cooling.
optical fiber
A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index material to promote total internal reflection (TIR). It may be used singly to transmit pulsed optical signals (communications fiber) or in bundles to transmit light or images.  
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