Practical spatial resolution limits of high-resolution fibre Bragg grating sensors using layer peeling

Inverse scattering methods such as layer peeling have been shown to reconstruct the longitudinal refractive index of fibre Bragg gratings with very high spatial resolution. These high spatial resolution measurements can be used to detect strain fields and thermal variations through the strain-optic and thermo-optic effects. However, physical constraints do not allow the refractive index to change on scales as short as these methods are capable of resolving. We demonstrate that this imposes a practical limitation of resolution on sensing applications. We use finite element modelling and compare this with experimental results. Longitudinal and transverse strains, as well as high resolution temperature sensing are considered. We show that the practical resolution limits are 40 mum for transverse stress sensing, 100 mum for longitudinal strain and 800 mum for thermal sensing.