|Stimulated Brillouin Scattering (SBS) in optical fibers|
Brillouin scattering arises from the interaction of light with acoustic waves that are always present in any material. Acoustic waves modulate the refractive index (photo-elastic effect); thus a low power laser traversing the medium is scattered with a different frequency (which depends on the angle of observation; in the backward direction the frequency shift is larger and of the order of 10 GHz in silica).
In general, the optical field also induces strains (via electrostriction) that reinforce the acoustic waves, but this a small effect at low laser powers. At high laser intensities, however, this leads to a stimulated Brillouin scattering regime. SBS is manifested as a strong reflection of light in the medium (back-scattering). In typical telecom fibers the SBS threshold is about 1 mW and is, therefore, a limitation for optical communications.
SBS is of great concern in photonic devices using nonlinear optical effects, such as parametric amplifiers and wavelength converters. SBS is usually avoided by broadening the laser spectrum above the width of the Brillouin line, which in silica fibers is typically 20-40 MHz. For wavelength converters, however, this solution is inconvenient, since it also broadens the spectrum of the converted signal.
In this project we study a different method to avoid SBS in fibers that is particularly useful for applications were one needs to use high power narrow-line lasers. Examples are fiber-optic parametric devices, high power fiber lasers and booster amplifiers.
In our method we broaden the Brillouin spectrum (rather than the laser spectrum) by applying a tensile-strain distribution along the length of the fiber. In this way the sound velocity (and consequently the Brillouin frequency shift) is different in different parts of the fiber. Therefore, light scattered in a given portion of the fiber does not experience gain in the other portions.
With this method we demonstrated recently an increase of 11 dB in the SBS threshold.
1. J.M. Chávez Boggio, J.D. Marconi, and H.L.Fragnito, "Experimental and Numerical Investigation of the SBS Threshold Increase in an Optical Fiber by Applying Strain Distributions", Journal of Lightwave Technology, vol. 13, n. 11, pp. 3808 -3814 (2005). [full-paper].