Brillouin scattering is an effect caused by the χ(3) nonlinearity of a medium, specifically by that part of the nonlinearity which is related to acoustic phonons. An incident photon can be converted into a scattered photon of slightly lower energy, usually propagating in the backward direction, and a phonon.
The coupling of optical fields and acoustic waves occurs via electrostriction. The effect can occur spontaneously even at low optical powers, then reflecting the thermally generated phonon field. For higher optical powers, there can be a stimulated effect, where the optical fields substantially contribute to the phonon population.
Above a certain threshold power of a light beam in a medium, stimulated Brillouin scattering can reflect most of the power of an incident beam. This process involves a strong nonlinear optical gain for the back-reflected wave: an originally weak counterpropagating wave at the suitable optical frequency can be strongly amplified. The frequency of the reflected beam is slightly lower than that of the incident beam; the frequency difference νB corresponds to the frequency of emitted phonons.
The Brillouin gain can be used for operating a Brillouin fiber laser. Such devices are often made as fiber ring lasers. Due to low resonator loss, they can have a relatively low pump threshold and a very small linewidth.
In optical fibers, Brillouin scattering occurs essentially only in backward direction. However, rather weak forward Brillouin scattering is also possible due to effects of the acoustic waveguide.
The Brillouin frequency shift depends on the material composition and to some extent the temperature and pressure of the medium. The temperature dependence of the Brillouin shift can be used for temperature and pressure sensing in fiber-optic sensors.
Stimulated Brillouin scattering describes a way to excite coherent acoustic phonons solely by optically forces in integrated waveguides. It was shown recently that one can use these acoustic phonons to store and delay optical signals
Researchers at the University of Sydney in Australia, have figured out how to turn a light wave into a sound wave, creating an acoustic memory that they say will help data centers save energy by eliminating some electrical connections between processors. “Our vision is to replace the electronic interconnects between different processors and computing machines with photonic ‘wires,’’’ said Birgit Stiller, a postdoctoral researcher who led the project. “So light transmission will be used instead of electronic connections.”
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