Low-loss and low-back-reflection hollow-core to standard fiber interconnection

We present a new approach to permanently interconnect hollow-core fiber (HCF) to solid-core fiber, which does not involve fusion splicing. Our approach is based on a modification of the glue-based fiber-array technology routinely used for fiber pigtailing of planar lightwave circuits. The resulting interconnection provides for a low insertion loss due to the fact that the HCF microstructure is not deformed during the gluing (low temperature) process that is almost impossible to achieve with the standard (high temperature) fusion splicing method. Furthermore, this low temperature technique enables the reposition and preservation of thin films deposited at the solid-to-hollow core fiber interface, allowing for additional functionality without the introduction of extra losses or any increase in complexity. To demonstrate this, we have applied an anti-reflection (AR) coating. A further feature of our approach is the ability to control very precisely the length of the graded-index (GRIN) fiber mode field (MF) adapter inserted in between the standard single-mode fiber (SMF-28) and the HCF. We show experimentally how the length of the GRIN fiber MF adapter influences the coupling between the SMF-28 and the fundamental as well as higher order modes of the HCF. We coupled between SMF-28 (10µm Mode Field Diameter, (MFD)) and the fundamental mode of a 19-cell hollow-core photonic bandgap fiber (HC-PBGF, 21.1µm MFD) with the lowest-ever reported insertion loss of 0.30dB per interface