All-fiber spectral filter based on LP01-LP11 mode coupling

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 wideband mode converters with spectral widths on the order 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 analyse the proposed filter. Design data and transmission characteristics for maximum power coupling at 1.33 μm and 1.55 μm are presented