Selection of Radial Modes in Optical Fiber Using Thin Absorbing Cladding

Abstract— We propose a new method for transverse mode selection of an optical cylindrical fiber using thin absorbing cladding. The results of calculations using the COMSOL\({}^{\circledR}\) Multiphysics software demonstrate that the \(HE_{12}\) mode can be selected by losses in a multimode fiber at a wavelength of 1550 nm, while the thin absorbing cladding (with a thickness of \(2.5{-}10\) nm) is placed near the minima of the \(|E_{r,\varphi}|\) field components. The cladding radius is optimized to minimize the value of loss per unit length \(\alpha\) for a given mode, which are relatively small: from \({-}0.02\) to \({-}0.06\) dB/cm depending on the parameters of the cladding, while for other modes this value is tens of times greater. For an analytical description, an estimate formula for loss per unit length is given. We present the search for optimal absorbing cladding radii from the point of view of minimizing the losses of the selected mode for various absorption coefficients and cladding thickness. A weak spectral dependence of the loss per unit length coefficient was demonstrated for a given cladding thickness and absorption coefficient.