Light backscattered from an optical fiber, apart from discrete reflections, has two components: Rayleigh scattering is the largest, caused by density fluctuations in the material. This light is at the same frequency as the transmitted beam. Brillouin scattering in contrast involves a frequency shift of the scattered beam, ~ 11 GHz for fused silica at 1.5 μm. The magnitude of this frequency shift depends on the velocity of sound and the wavelength of the light in the glass, and the sound velocity in turn depends on Young’s modulus and density of the glass. Thus the Brillouin frequency shift can provide information on the material composition of the fiber.1,2

© 1987 Optical Society of America

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