Effects of fibre grating parameters on the spectral characteristics of cascaded long- and short-period gratings

Using coupled-mode theory combined with the transfer matrix method, this paper presents the impact of fibre grating parameters(grating period, grating length and average index modulation) and the separation between long-period fibre grating and fibre Bragg grating (FBG) on the reflection spectra of cascaded long-and short-period grating (CLBG). Two grating periods and two grating lengths have only impact on the two reflection peaks. The two resonance wavelengths shift towards the longer wavelength when the period of FBG decreases. Further, we study the sensing characteristics of coated CLBG. By analysing the relationship between the refractive index sensitivity and the thin film parameters (film refractive index and film thickness), the optimized parameters for the coated CLBG used as film sensor were obtained. Data simulation shows that the resolution of the refractive index of this coated CLBG film sensor is predicted to be 10^?6. This implies that the coated CLBG has an important application value in sensing and measurement field.