Nonlinear Effects in Optical Fiber

International Zone (English) General Knowledge
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What are Brillouin and Raman scattering in high-power laser sensing?

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Brillouin scattering and Raman scattering are both nonlinear optical effects that occur when high-power laser light propagates through an optical fiber. These effects are particularly relevant in high-power laser sensing, especially for distributed fiber optic sensing (DFOS).

  1. Brillouin Scattering: This is an interaction between light and acoustic phonons (sound waves) within the fiber. When a high-power laser beam travels through the fiber, it generates acoustic waves due to electrostriction. The laser light then scatters off these acoustic waves, resulting in a frequency shift (either Stokes or anti-Stokes) in the scattered light. The magnitude of this frequency shift is sensitive to local changes in temperature and strain along the fiber. By analyzing the frequency shift of the backscattered Brillouin light, distributed temperature and strain measurements can be achieved along the entire length of the fiber. This phenomenon forms the basis of Brillouin Optical Time Domain Reflectometry (BOTDR) and Brillouin Optical Time Domain Analysis (BOTDA).

  2. Raman Scattering: This is an inelastic scattering process where photons interact with molecular vibrations in the fiber material. When a high-power laser photon interacts with a silica molecule, it can either lose energy to excite a molecular vibration (Stokes scattering, resulting in lower frequency scattered light) or gain energy from a pre-excited vibration (anti-Stokes scattering, resulting in higher frequency scattered light). The intensity ratio of the Stokes and anti-Stokes components is highly dependent on temperature. This characteristic makes Raman scattering ideal for distributed temperature sensing (DTS), where temperature profiles along the fiber can be mapped by analyzing the backscattered Raman signal.

In high-power laser sensing, these effects are exploited for their ability to provide spatially resolved measurements over long distances. For instance, OFSCN offers various seamless steel tube fiber cables that are designed to be used as distributed fiber optic sensing cables, compatible with sensing equipment based on Raman scattering, Rayleigh scattering, and Brillouin scattering.

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