Brillouin Scattering | previous | next | feedback |
Definition: a nonlinear scattering effect involving acoustic phonons
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. This can occur spontaneously even at low optical powers, and can become very strong as a stimulated effect which occurs above a certain threshold power of a light beam in a medium. Above the threshold, 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 corresponds to the frequency of emitted phonons. This so-called Brillouin frequency shift is set by a phase-matching requirement. It depends on the material composition, the optical frequency, and to some extent the temperature and pressure of the medium.
An important application of stimulated Brillouin scattering is optical phase conjugation. There are for example phase-conjugate mirrors for high-power Q-switched lasers which make it possible that the thermal distortions occurring in forward and backward direction in the laser crystal compensate each other.
Stimulated Brillouin Scattering in Optical Fibers
Stimulated Brillouin scattering (SBS) is frequently encountered when narrow-band optical signals (e.g. from a single-frequency laser) are amplified in a fiber amplifier, or just propagated through a passive fiber. While the material nonlinearity of e.g. silica is actually not very high, the typically small effective mode area and long propagation length strongly favor nonlinear effects. The Brillouin gain in fibers has a bandwidth of typically 50-100 MHz for silica fibers. The Brillouin threshold of optical fibers for narrow-band continuous-wave light typically corresponds to a Brillouin gain of the order of 90 dB. (With additional laser gain in an active fiber, the threshold can be lower.) For trains of ultrashort pulses, the SBS threshold is determined not by a peak power, but rather by a power spectral density, as explained in a Spotlight article. The Brillouin frequency shift is typically in the gigahertz range (e.g. of the order of 10-20 GHz for silica fibers).
SBS introduces the most stringent power limit for the amplification and the passive propagation of narrow-band optical signals in fibers. In order to raise the Brillouin threshold, it is possible to increase the bandwidth of the light beyond the Brillouin gain bandwidth, reduce the fiber length, concatenate fibers with slightly different Brillouin shift, or (in high-power active fiber devices) exploit the longitudinally varying temperature [4]. There are also attempts to reduce the overlap of guided optical and acoustic waves, or to introduce significant propagation losses for the acoustic wave. To some extent, SBS problems can be reduced via basic amplifier design modifications, concerning e.g. the doping concentration, effective mode area and pump propagation direction.
On the other hand, 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.
The temperature dependence of the Brillouin shift can be used for temperature and pressure sensing (→ fiber-optic sensors). Another application of Brillouin scattering is for optical phase conjugation.
Bibliography
| [1] | R. G. Smith, "Optical power handling capacity of low loss optical fibers as determined by stimulated Raman and Brillouin scattering", Appl. Opt. 11 (11), 2489 (1972) |
| [2] | R. W. Boyd et al., "Noise initiation of stimulated Brillouin scattering", Phys. Rev. A 42 (9), 5514 (1990) |
| [3] | G. P. Agrawal, Nonlinear Fiber Optics (Academic, San Diego, Calif., 1995). |
| [4] | V. I. Kovalev and R. G. Harrison, "Suppression of stimulated Brillouin scattering in high-power single-frequency fiber amplifiers", Opt. Lett. 31 (2), 161 (2006) |
See also: Raman scattering, Kerr effect, nonlinearities, fibers, Spotlight article 2007-09-01
Categories: fibers and other waveguides, nonlinear optics
