Low loss interconnection between single-mode fiber and hollow-core fiber using silica micro-ball lens with large fabrication tolerance

We propose what we believe to be a novel interconnection technique between single-mode fiber (SMF) and nested anti-resonant node-less fiber (NANF), using a coreless fiber (CLF) melted micro-ball lens for mode field matching. The ball lens is fabricated by melting a section of the CLF, while the remaining portion provides an air gap for beam expansion and collimation. Theoretical analysis indicates that this structure exhibits significant dimensional tolerance, enabling efficient mode field matching for NANF with varying mode field diameters (MFDs). Experimental results validate the large fabrication tolerance of interconnection, with coupling loss fluctuations within 0.2 dB across a diffraction length variation of approximately 78 µm. The fabricated SMF–NANF–SMF sample achieves an insertion loss of less than 0.24 dB per interconnection and a return loss below −36 dB attributed to the anti-reflection coating applied to the ball lens. Furthermore, higher-order mode excitation is suppressed to below −45 dB, indicating good mode purity. This interconnection method offers a reliable and versatile solution for NANF-to-SMF integration across various applications.