Theory Of Scattering Loss Of Micro- Or Nanofiber Caused By Surface Roughness

Surface roughness induced scattering loss is one of the most important properties of an optical waveguide, especially when the width or diameter of the waveguide is close to or smaller than the wavelength of the guided light, in which sidewall scattering is usually the dominant loss mechanism. Recently, optical micro- or nanofibers have been fabricated by taper drawing of molten glasses. Previous research has shown that, during the solidification of the glass, the thermally excited surface capillary waves froze onto the surface at the glass transition temperature, which will inevitably contribute to surface roughness of a molten-drawn glass fiber. This paper mainly talks about three method of calculating the scattering loss caused by surface roughness of the perturbed fiber: induced current, mode coupling and solving the Maxwell function.The fundamental theory of the induced current method is that, the perturbed fiber is assumed as a perfect fiber with current at the surface of the fiber. In this paper, we mainly study about the connection of the loss and the current. According to mode coupling method, the coupling of the propagating mode and the scattering mode are studied. The method of solving Maxwell function also uses an induced current representation of perturbation of the micro- or nanofiber, solve Maxwell function with current to get scattering mode of the fiber.At last, loss coefficients of different fibers with sinusoidal deformations on their surfaces are numerically calculated with respect to typical parameters (such as perturbation period and roughness amplitude) of the guiding system. Interesting phenomena are observed, such as the loss coefficient shows oscillation dependence with the increasing of roughness perturbation period, the loss coefficient increases monotonously with roughness amplitude, the loss coefficient has a maximum when the micro- of nanofiber gets certain diameter, and the loss coefficient gets a maximum at certain wavelength. We also give the difference between these methods, we find that the difference is very little. Results presented in this work may be generalized to all kinds of surface deformation and may provide useful guidelines for both estimating and tailoring waveguiding properties of micro- or nanofibers.