Robust low-loss optical fiber joints are a prerequisite in all-fiber devices. Joining structurally dissimilar fibers, such as microstructured optical fibers, is a timely challenge, as they are becoming important building blocks in photonics. In this paper we have revealed the mechanism of robust mode matching and demonstrated ultralow loss and high strength fusion splices between ultralarge mode-area photonic crystal fibers (ULMA-PCFs) and standard single mode fibers (SMFs) without using any intermediate bridging elements. To provide precise matching in both the mode field distribution and the cladding size between an SMF and an ULMA-PCF, we develop a two-step reverse tapering approach, involving reverse tapering an SMF and thermally expanding its core, where the fundamental mode can be adiabatically transferred and reach the optimized mode shape in the tailored reverse-taper. Using our new tapering approach, we achieve a record-low splice loss of 0.23 dB, regardless of the transmission direction, together with a mechanical strength an order of magnitude higher than that of a conventional sharp-edge joint. Our approach provides an effective way to overcome the splicing challenge for ultralarge mode specialty fibers and thus greatly facilitates the development of components and devices with such fibers.

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Negative curvature hollow-core fibers (NC-HCFs) can boost the excellent performance of HCFs in terms of propagation loss, nonlinearity, and latency, while retaining large core and delicate cladding structures, which makes them distinctly different from conventional fibers. Construction of low-loss all-fiber NC-HCF architecture with conventional single-mode fibers (SMFs) is important for various applications. Here we demonstrate an efficient and reliable fusion splicing method to achieve low-loss connection between a NC-HCF and a conventional SMF. By controlling the mode-field profile of the SMF with a two-step reverse-tapering method, we realize a record-low insertion loss of 0.88 dB for a SMF/NC-HCF/SMF chain at 1310 nm. Our method is simple, effective, and reliable, compared with those methods that rely on intermediate bridging elements, such as graded-index fibers, and can greatly facilitate the integration of NC-HCFs and promote more advanced applications with such fibers.

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