Optical responses under torsion

Unlike the optical responses of an FBG sensor under tension, the sensitivity of the Bragg wavelength shift of an FBG sensor under torsion is very small. However, the polarization response to torsion is significant. The coupled mode theory can be used to analyse the polarization behaviour of an FBG sensor under torsion.  

The torsion introduces shearing stress in the cross-section of the fibre. If the twisted length of a fibre is L, and the angle of twist is = zL, where t is torsion ratio that is the angle of twist per unit length along the axis of the fibre, the matrix for the strain due to this torsion is  

The relationship between the dielectric permittivity perturbation and the optical impermeability perturbation can be expressed as  

The induced polarization behaviour can be analysed by using Eq. (9.26) and Subsection 9.2.2. The induced circular birefringence in a single mode optical fibre is given by  

The polarization of pulse UV beam and asymmetric geometry associated with the side-exposure of UV light during the FBG fabrication process - will 

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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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