Transmission properties of tapered air-core photonic bandgap fibers

Abstract

Fiber tapers have found a wide range of important applications in communication and sensing, including narrow-band filters, mode-matching between waveguides, evanescent mode-coupling and fused couplers. Applying these taper-based technologies to air-core photonic bandgap fibers (PBFs) is very appealing because it would enable creating these same components directly in air-core fibers. Although there have been several studies of tapers in solid-core microstructured fibers, the transmission properties of tapered air-core photonic-bandgap fibers have not yet been studied. In this work, we report on the fabrication and testing of tapered air-core photonic-bandgap fibers. Our motivation in this work was to study the basic transmission properties of PBF bitapers in the bandgap region, and in particular, to see how the overall transmission was impacted by the taper, e.g., whether the taper induced resonant coupling to one or more cladding modes. Our experimental results indicate that air-core PBFs are highly sensitive to tapering, and unlike conventional single-mode telecommunication fibers, even a small tapering ratio results in significant modal interference in the transmission spectrum. Furthermore, we found out that the mechanical silica support surrounding the holey region of the PBF contributes as a lossy Fabry-Perot resonator to the observed transmission properties.