Scaling symmetries and optimization of the refractive index profile in optical fibers

Resum

The optical fiber refractive index profile has a significant role in the optical devices fabrication like fiber Bragg gratings (FBG) and acousto-optic devices and the way guided light interacts in the presence of external factors like strain, stress or even bends. The analysis of the fiber profile gets to be feasible throughout understanding the properties of the optical guided modes such as the effective index, group index, and the chromatic dispersion. Taking a broader look at previous publications that use standard commercial optical fibers, it becomes noticeable that the ideal core-cladding step index profile gives incompatible results when it comes to the experimental versus the simulated dispersion curves. This difference is usually caused by various factors that changes the nominal geometrical and material parameters of the fiber. The stress resulted during the fabrication process, due to thermal and drawing effects is an example of many effects that causes the alteration of these parameters. In general, any irregularity in the fiber, geometrically speaking or material-wise, will produce a coupling of the energy of one mode to the others. Hence, studying coupled modes, whether they are induced by an inscribed FBG or an acousto-optic wave, provides useful information to test the effective refractive index profile required for an accurate theoretical simulation of fiber modes. This research analyzes the characterization of the fibers refractive index profile and the effect of scaling transformation on the dispersion curves. A fiber scaling through two degrees of freedom, geometrical scaling and refractive index difference scaling, gives a significant improvement on fitting the simulated dispersion curves with the experimental ones. However, in many cases, an additional cladding alteration is also needed and shows effectiveness, especially when we analyze a wide wavelength range. According to our final results, a scaling in the geometrical properties of the core and a perturbation of linearly decreasing refractive index in the cladding both describe the fiber profile correctly. As a consequence of our modified step index model, a perfect matching between the experimental and the theoretical dispersion curves is achieved. This process of fiber profile optimization, through the fiber scaling and cladding alteration, have also proven to be efficient, comprehensive and applicable for a wide range of commercial standard fibers.