Improved evaluation model for macro-bending loss and power variation in single-mode fiber

It is challenging to calculate the macro-bending loss of optical fiber when the bending radius is reduced to sub-millimeter levels, based on Marcuse's theory and its derivative theories. Furthermore, these theories are limited in their ability to assess the distribution and changes of the optical field within the fiber, focusing solely on the analysis of loss. To overcome these limitations, we proposed an evaluation model which can simultaneously provide results for both macro-bending loss and power variation in a bent single-mode fiber. Our model achieves this by conformally transforming the fiber's refractive index profile of the axis section according to the bending radius. The ratio of output power to input power exhibits oscillation with changes in the bending radius, thereby resolving the issue of inability of Marcuse's theory to obtain an oscillation loss curve for sub-millimeter bending radii. The proposed model was validated by finite element method. We also obtained the internal power variation along the axial distance of the bent fiber through this model. This research improves the current fiber macro-bending loss evaluation model and lays a foundation for further studies on bend-insensitive fibers and the design of sub-millimeter bending sensors.