Abstract: An in-line, distributed optical fiber filter comprises a core region with a raised refractive index (with respect to the surrounding cladding material) so as to allow for total internal reflection (TIR) of the desired transmission wavelength(s). One or more raised index features are formed within the cladding region and are configured so as to result in mode mixing between the cladding mode and core mode at determined wavelength(s) to be removed by filtering. The parameters associated with determining the proper core specifications and cladding specifications can be separately determined to provide for enhanced performance in terms of both filtering unwanted signals and propagation of desired communication signals.