How Does Birefringence Influence FBG measurements?

In general, inscription of FBGs can result in non-isotropic changes of the refractive index in the fiber core, resulting in Birefringence effects. Due to this Birefringence effect, the reflected wavelength becomes dependent on the polarization state of the incident light. This dependency of the wavelength of the FBG can be quantified using the Polarization Dependent Frequency Shift (PDFS). This unwanted effect is most pronounced for interrogators with a polarized light source, which impacts the stability of the FBG sensors and ultimately their accuracy. These effects are worse manifested in FBGs inscribed via Point-by-Point approach, which exhibit a PFDS of about (20-40) pm [Ref. 1] [Ref. 2]. In comparison, the PFDS from an FBGS’s FSG® is about 5-8pm, which is very close to the typical 5pm value one gets from a DTG®. The exhibited lower value of PFDS offers system designers more leeway to configure the optimal design and to lower the measurement error for the more demanding applications.

Note: To reduce these PFDS effects, it is recommended to work with interrogators, which make use of depolarized light for a more accurate FBG measurement. 

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