Modelling of optical fibre long-period grafting sensors.

In this thesis a mathematical model of the long-period fibre grating (LPG) sensor has been developed. Numerical simulation of the LPG transmission spectrum behavior is studied when changing the refractive index of the ambient. The attenuation bands are seen to shift to shorter wavelengths as the surrounding index is increased from 1 (air) to 1.45 (near fibre cladding index). The results show that the long-period grating is very sensitive to changes in the surrounding index, especially when this index value approaches that of the cladding. This model shows for the first time that the coupling wavelength will shift even when the surrounding index is higher than the cladding index. Numerical results for cladding mode propagation constants, transmission spectra and resonance wavelengths are presented and compared with published data whenever appropriate. The proposed mathematical model can be used as a simulation tool to investigate LPG as a sensor. The LPG fibre when coated with a material and exposed to external analytes will change its refractive index (n[3]), which in turn causes a shift in the transmission spectrum. This model can be applied to assist in the development of chemical sensors based on long-period fibre gratings.