To understand the loss limitations of a splice between a hollow-core fiber and a conventional fiber, we use a numerical model to calculate the expected coupling loss between the NKT Photonics' HC-1550-02 fiber and a single-mode fiber (SMF) of arbitrary step-index profile. When the SMF parameters are optimized, the splice loss is predicted to be as low as~0.6 dB. This minimum is believed to be largely due to mode-shape mismatch. These predictions are confirmed experimentally by optimizing the splice loss between this photonic-bandgap fiber and five SMFs with different mode-field diameters (MFDs) and V numbers. With the SMF-28 fiber, the measured loss is 1:3 dB, in excellent agreement with theory. Using a SMF with parameters close to the optimum values (MFD ¼ 7:2 μm and V ¼ 2:16), this loss was reduced to a new record value of 0:79 dB. . A lower splice loss of 1 dB has also been demonstrated by applying longitudinal pressure to these two fibers prior to applying the arc [4]. In contrast, a simple overlap calculation using a Gaussian mode approximation based on the mode-field diameter of the two fibers (∼7:5 and ∼10:4 μm, respectively) predicts that the buttcoupling loss should be 0:46 dB [3]. Although this 0.5 to 1 dB difference has been tentatively explained by differences in the mode shapes, there is still a need to conduct thorough simulations to explain this measured loss. It is also important to explore the possibility of achieving a lower splice loss by selecting a solid-core fiber with a mode-field diameter (MFD) better matched to that of the hollow-core fiber.In this Letter, we use a numerical model previously reported [5] to calculate the exact fields of the HC-1550-02 fiber's fundamental mode, and with them we compute the expected coupling loss between this fiber and an arbitrary step-index SMF. This analysis predicts that, when the SMF parameters are optimized (MFD ≈ 7:25 μm and V ¼ 2:405), the splice loss should be as low as ∼0:6 dB. We confirm these predictions by optimizing the splice loss between this PBF and five SMFs with different MFDs and V numbers. With the SMF-28 fiber, we measured a loss of 1:3 dB, in good agreement with theory. Using a SMF with parameters close to the optimum values (MFD ¼ 7:2 μm and V ¼ 2:16), we reduced the splice loss to 0:79 dB. This is the lowest value reported to date for a splice between a hollow-core and a solid-core fiber using an arc splicer.The field transmission (t) and reflection (r) coefficients at a butt-coupled junction between a SMF and a PBF can be described in terms of the normalized vector electric and magnetic fields E i and H i of the HE 11 mode of the SMF, and the corresponding fields E t and H t of the PBF's fundamental mode. These modes are normalized to carry 1 W power, orwhere A is the cross-sectional area of the fiber. A similar expression applies for the SMF mode. Neglecting coupling into higher-order modes of the PBF, the continuity of the transverse fields at the interface between the fibers imposeswhere the subscript T represents the transvers...

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