On the base of simplified three layered model, this paper researched the theory of the cladding mode in single mode optical fiber, the cladding mode field distribution and the coupling coefficients of the cladding modes coupled with the core mode in long period fiber grating (LPG).

A new refractive index sensing scheme based on the cladding modes of fiber Bragg grating(FBG) is presented.

The rules of the shift of resonant wavelengths were obtained conveniently by the curve of the efficient refractive index′s difference between core mode and cladding modes or the curve of resonant wavelengths versus period.

In order to know well the characteristics of the metal-coated long-period fiber gratings, it is necessary to investigate the cladding modes of the metal-coated fibers.

The spectral characteristics of the long period fiber gratings based on that the core mode is coupled to various cladding modes were studied by use of the coupled-mode theory and the approximately linear relationship between the propagation constants and the wavelength.

By use of scalar approximation of three-layer model, we performed the numerical computation of effective refractive index of cladding modes of long period fiber gratings, analyzed the relationship between resonant wavelengths and grating period, and drew a conclusion on the dependence of resonant wavelengths with respect to the varying refractive indices of environment.

Structural design and analysis of dual-cooled lithium-lead blanket module for the fusion power reactor FDS-Ⅱ;