Optical responses of FBG sensors under deformations

Most of the sensing applications of FBG sensors focus on its reflection spectra, which depend on the relationship between the Bragg wavelength of FBG and its physical quantities. The key detection issue is the determination 

The coupled mode theory is often used to describe the polarization characteristics, or modes coupling in optical fibre, and the reflection spectra of FBG sensors. When the optical response of an optical fibre is analysed based on the coupled mode theory, the electric field is normalized as j j E ( E (dxdy = 1. Based on the perturbation approach, the slowly varying amplitudes c are determined by the following coupled mode equations  

When the polarization behaviour of FBGs under various deformations is discussed, one has to consider three types of perturbation, that is, those induced by the fibre making process (intrinsic), UV side exposure and strain caused by deformation. 

An evolution velocity Q is usually introduced to describe qualitatively the polarization characteristics of specific polarization behaviour. in a generalized Poincare sphere can be expressed by the coupled coefficients as  

A fibre Bragg grating (FBG) consists of a periodic modulation of refractive index in the core of a single mode fibre. The electric fields in FBGs are represented by the superposition of the ideal modes travelling in both forward and backward directions  

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This chapter has given the optical responses of FBG sensors under different modes of deformation. Both the polarization behaviour and optical spectrum of FBGs have been used to analyse the deformation perturbations. In the case of an FBG under tension, the wavelength sensitivity to tension is significant. In the case of torsion, the wavelength sensitivity is very small, and the polarization behaviour is sensitive to it. In the case of lateral compression, the wavelength sensitivity is much smaller than that under tension, but the deformation induced birefringence can broaden and split the reflection peaks. 

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