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Physical measurement dimensions, such as temperature or pressure and tensile forces, can affect glass fibres and locally change the characteristics of light transmission in the fibre. As a result of the dampening of the light in the quartz glass fibres through scattering, the location of an external physical effect can be determined so that the optical fibre can be employed as a linear sensor.
Optical fibres are made from doped quartz glass. Quartz glass is a form of silicon dioxide (SiO2) with amorphous solid structure. Thermal effects induce lattice oscillations within the solid. When light falls onto these thermally excited molecular oscillations, an interaction occurs between the light particles (photons) and the electrons of the molecule. Light scattering, also known as Raman scattering, occurs in the optical fibre. Unlike incident light, this scattered light undergoes a spectral shift by an amount equivalent to the resonance frequency of the lattice oscillation.
The light scattered back from the fibre optic therefore contains three different spectral shares:
- the Rayleigh scattering with the wavelength of the laser source used,
- the Stokes components with the higher wavelength in which photons are generated, and
- the Anti-Stokes components with a lower wavelength than the Rayleigh scattering, in which photons are destroyed.
The intensity of the so-called Anti-Stokes band is temperature-dependent, while the so-called Stokes band is practically independent of temperature. The local temperature of the optical fibre is derived from the ratio of the Anti-Stokes and Stokes light intensities.
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