Optical responses under bending

Another characteristic of the silica fibre is the non-linear stress-strain relationship. It is known that the neutral axis is also shifted toward the outer side of the fibre under bending. Its direct effect on the FBG is that the grating period will be compressed, so these effects should also be considered, together with the change of effective refractive index of the FBG under bending. 

Other fibre gratings can also be used in bend sensing, such as long-period fibre gratingand multicore fibre Bragg gratings.  

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. 

Mode of deformation Bragg wavelength shift UV induced linear birefringence 8, Deformation induced linear/circular birefringence 8,/8, 

The lateral compression can be sensed by the spectrum behaviour in some sensing schemes. Both the polarization behaviour and the optical spectrum of FBGs can be used to analyse lateral eompression. Bending of FBGs can be analysed for both the polarization behaviour and the optical spectrum, but it is difficult to measure the polarization evolution of fibre under bending. The optical responses of normal germanosilicate FBG under different individual modes of deformations are summarized in Table 9.1. 

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