I'm a bit confused about the meaning of parameters that are used in Rayleigh scattering formula (in Fiber Optics). Can someone please explain in a simple way, what are:
1. photoelastic coefficient
2. isothermal compressibility
3. fictive temperature
I've already read a lot of explanation about them, but I still don't get the meaning, maybe because the explanation was too difficult to understand..
Thanks.
Photoelastic coefficient defines the stress distribution which sometimes occurs during manufacturing and cause scattering.
In optical fibres there is intrinsic Rayleigh scattering due to the molecular structure, and that caused by maufacturing imperfections. Rayleigh scattering losses are minimised by operating at longer wavelengths (benefitting from the inverse fourth power dependance on wavelength, and the Rayleigh scattering losses due to manufacturing are reduced by careful and slower drawing of the fibre from the preform rod during manufacture.
In Silica based glasses, there is little benefit from operating at wavelengths greater than 1.55 microns, as "infra-red" losses begin to dominate to overall loss. http://www.fiberoptics4sale.com/wordpress/optical-fiber-loss-and-attenuation/
Extensive work went into developing other glass materials for fibres, that would operate at longer wavelengths due to lower phonon losses, but they all failed to achieve sufficiently low losses due to ther mechanisms such as absorbtion. Fibres made from Silica (and doped with Ge, P etc.) now are exclusively used for long distance fibre communication
To Stevan: The Rayleigh scattering is due to use of Germanium dopant in the core of silica based optical fiber. As you know, the Germanium, which is in the form of GeO2, is used to increase the refractive index of the fiber core. Unfortunately, the addition of GeO2 would cause in-homogeneous structure in the fiber core as far as the molecular bonding is concerned. So, this form of structural in-homogeniety comparable with the propagating light wavelength causes Rayleigh scattering which usually develops optical losses via back-scattering towards the launching OTDR. For this practical reason, addition of excess GeO2 in the fiber core is avoided. On the other hand, to increase the refractive index of the fiber core to an optimum level without much increase of the Ge dopant, depressed clad refractive index profile is proposed. This type of profile with different versions are used for the commercial optical fibers.
The photoelastic coefficient describes how a material's refractive index changes when the material is stressed or deformed. It is used to describe strain-induced birefringence in photoelastic experiments, but in the context of Rayleigh scattering it could be some other combination of components of the strain-optic tensor or stress-optic tensor.
The isothermal compressibility describes how the volume of a material changes when external pressure is applied.
The fictive temperature is a measure of how disordered the glass is.
Your formula probably uses the Boltzmann relation to estimate the strain energy locked into the glass, based on the fictive temperature. The compressibility and photoelastic coefficients are used to predict fluctuations in refractive index from the strain disorder.
As Richard points out, glasses are intrinsically disordered. Rayleigh scattering is caused by fluctuations in refractive index. Dopants such as Germania increase the amount of scattering, but even pure silica glass shows Rayleigh scattering. Fibres drawn at low temperature have a lower fictive temperature and smaller refractive index fluctuations.
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