The fiber Bragg grating (FBG) is an optical device with a periodic variation of the refractive index along the propagation direction in the core of the fiber,. The principal property of FBGs is that they reflect light in a narrow bandwidth that is centered about the. A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. Typically, the perturbation is approximately periodic over a certain length of e. a few millimeters or centimeters, and the period is of the order of. The coupled mode theory is a suitable tool for analysis and obtaining quantitative information about the spectrum of a fiber Bragg grating. The coupled mode equations can be obtained and simplified by using the weak waveguide approximation. The spectrum characteristics can be obtained by solving. In this paper, we rigorously deduce the coupled-mode equations of a long-period fiber grating and fiber Bragg grating in their cascaded structure (CLBG), based on coupled-mode theory. Surrounding RI (SRI) values can be.
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