Spectrally Selective Optical Loss in Fibers With High-Index Rods Embedded Into Silica Cladding

In this work, we perform a detailed study of a novel type of optical fiber for spectral filtering, which is based on a step-index core with high-index rods embedded into silica cladding. Recently, it was theoretically predicted that such a structure could demonstrate a very sharp stopband due to the resonant core mode interaction with the eigen high-index rod mode. In the current paper, we realize the proposed structure and experimentally confirm its applicability for spectral filtering. In our experiments, we show the possibility of creating a very sharp shortpass fiber spectral filter. In particular, we demonstrate a change in the optical loss of the core mode by two orders of magnitude (from less than 0.1 dB/m to more than 10 dB/m) for a 12 nm wavelength shift (from 1008 nm to 1020 nm) and by more than 3 orders of magnitude (from 0.02 dB/m to 60 dB/m) for a 70 nm wavelength shift (from 980 to 1050 nm). Possible applications of the realized fiber design are discussed. Mechanisms of optical loss in the fabricated structure are studied, and the presence of an additional mechanism, which is not related to absorption in the high-index rods, is revealed.