Anomalous spectral behaviour of weakly-fused optical fibre couplers with sub-micron diameters

Weakly-fused optical fibre couplers can be simplified as a system of two optical fibre tapers, which are often called nanofibers when having sub-micron diameters. While conventional optical fibre couplers exhibit a wide spectral region of modal interference where two modes provide a continuous oscillatory behaviour associated to the large difference between phase constants of interfering supermodes, nanofiber couplers (NFC) exhibit an anomalous behaviour (Fig. 1) in close proximity to the supermodes cut-off wavelengths, where the high extinction ratio oscillations can exhibit a very slow oscillation. This region of interest exhibits flat transmission over nearly 20nm and results from the variation of the supermodes dispersion near their cut-off. This hybrid behaviour, where a flat transmission region is proximal to rapid spectral oscillations, allows the NFC to operate at high sensitivity as well as large dynamic range for application related to optical detection. &more..