Fiber Bragg Gratings (FBGs) attract great attention due to their present and prospective applications in fiber-optical communication systems and modern opto-electronics. FBGs are components having a wide range of applications in the fields of communication, lasers and sensors. This study is a theoretical study in the field of FBG. Our aim is, by using coupled-mode theory and changing-of-index variation function, to model the FBG and to elucidate the appropriate parameters of FBG. Diffraction spectra of a FBG and its first and second degrees were obtained. Maximum reflection of FBGs versus saturation coefficient was also examined.

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The prototropic, tautomeric, and ionization equilibria of all of the structures of 2-hydroxy-3-(3-oxo-1-phenylbutyl)chromen-4-one (warfarin) were studied experimentally and theoretically. The structure elucidation and acidity constant determination along with the tautomeric equilibrium constants were carried out experimentally by a UV−visible spectroscopic method and theoretically investigated at the HF/6-31G(d) and B3LYP/6-31G(d) levels of theory. The obtained K a values were evaluated by structure elucidation and a protonation mechanism. Using the gauge-including atomic-orbital (GIAO) method for calculating 1H and 13C nuclear magnetic shielding tensors, the Hartree−Fock (HF) and density functional (DFT) levels of theory were applied to warfarin. Geometry optimizations were also performed on each species, and the absence of negative frequencies verified that all structures were true minima. Theoretical values were compared to the experimental data. Satisfactory agreement between the experimental chemical shifts and the theoretical values of shielding constants (calculated by GIAO/DFT and GIAO/HF methods) was obtained.