All-fiber spectral filters have the advantages of providing low coupling loss and being readily integrated into fiber-optic networks. Spectral filters made of single-mode identical or dissimilar core parallel fibers provide 3-dB spectral widths on the order of 1 to 10 nm. A spectral filter made of single-mode and dual-mode fibers and operating based on coupling of power between LP01 and LP11 modes is proposed for applications as narrowband demultiplexers and as broadband mode converters with spectral widths of fraction of 1 nm to few 10 nm, respectively.

With appropriate choice of parameters, filters can be designed such that the LP01 mode of the single-mode fiber is phase matched with the LP11 mode of the dual-mode fiber at a desired wavelength. Thus, significant exchange of power between these two modes can occur. The coupled-mode theory of parallel dielectric waveguides is used to analyze the proposed filter. Transmission expressions are derived from the governing coupled-mode equations and evaluated numerically for example cases.

Two cases corresponding to maximum power coupling at 1.33 mm and 1.55 mm wavelengths are examined. Design data and transmission characteristics versus wavelength for these two cases are presented. The influence of the distance between fiber cores on peak transmission wavelength, spectral width, and coupling length is investigated. The application of the proposed filter as mode converter, which is required in the implementation of dispersion compensation using LP11 mode, is elucidated.