Optical responses under tension

No significant polarization signals can be observed for FBG sensors under tension. But an FBG sensor has good linear characteristics when it is applied 

Hence the right side of Eq. (9.18) can be written as — Peff z-If it can be assumed that the strain-induced change in the period of the FBG is only dependent on the axial strain, the second term on the right side of Eq. (9.17) will be equal to Equation (9.17) is then expressed as  

If the fibre is extended at both ends and no body force applied, the fibre core and cladding can be regarded as homogeneous and isotropic, the strain response of the germanosilicate FBG sensor at constant temperature under tension is then given by  

In a silica fibre there are three transmission wavelength windows in which the losses are very low, such as 0.15dB/km at 1550 run. According to Eq. (9.23), typical values for the sensitivity to an applied axial strain in these windows are 1.2 nm/milli-strain at 1550 nm, 1 nm/milli-strain at 1300 run, and 0.66 nm/miUi-strain at 850 nm. The resolutions of FBG are 8.3 micro-strain at 1550 nm, 9.8 micro-strain at 1300 nm and 15 micro-strain at 850 mn by an optical spectrum analyser with a wavelength resolution of 10 pm. 

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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. ... 

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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