The calculation of a two-cascade long-period fiber grating of the refractive index

A method is proposed for calculating the spectral characteristics of cascade long-period fiber gratings of the refractive index of the core based on the method of partial regions and decomposition. According to the method, the structure is represented as a cascade connection of basic inhomogeneities, each of which is a junction of several optical fibers with different dielectric and geometric parameters. Approbation of the method was carried out by reducing the structure to a single long-period fiber grating and comparing the results with the results obtained using other calculation methods. The results matched graphical accuracy. The advantage of the method is the possibility to theoretically study the effect of dielectric and geometric parameters on the spectral properties of the structure, as well as significantly save time resources. The transmission spectra of gratings with different parameters of period T, grating length L, and distance between gratings Lf are presented.