Numerical analysis of reflection characteristics of cascaded non-uniform fiber Bragg gratings

Abstract

The paper is devoted to the simulation and reflectance performance of the cascaded non-uniform fiber Bragg gratings (FBG). In a FBG, periodically spaced regions in the fiber core are varied. FBGs are key enabling technologies for fiber optical sensing for their high sensitivity and potentially low cost. Uniform, chirped, phase-shifted and sampled FBGs can be used in the sensing systems. The cascaded FBG configuration is created by several sections where each section has a specific number of segments. In our simulations the transfer matrix method has been used assuming the entire grating made up of sections. Using this method the FBG reflectance spectra have been simulated in MATLAB. The simulated reflectances of FBGs with several sections alternating along the length of the FBG have been described. The simulation results show that the FGB reflectance at the Bragg wavelength and the reflectance bandwidth depend on the Bragg grating periodicity that means on the periodicity in the sections. The bandwidth and the amplitude reflectance at the Bragg wavelength for the length grating of 10 mm with several sections have been studied. The effects of sections of different Bragg grating periods and refractive index profiles (the Gaussian profile or the sinc profile) have been numerically investigated. From the simulations one can see that the maxima of the energy of FBGs reflected by several apodized sections depend on the grating length. Moreover, it has been found out that the undesirable sidelobes between main reflectance maxima can be partly suppressed by using an appropriately designed cascaded grating.