End-capping Hollow-core Fibers with Suppressed Coupling into Higher-order Modes

We propose a hollow-core fiber (HCF) end-cap that incorporates a short segment of a single-mode fiber (SMF) that serves as a modal filter. To adapt the end-cap input and output beam to the desired size, the SMF was fusion spliced with short segments of mode-field adapting graded index (GRIN) fibers on both sides. The end-cap is anti-reflective coated to minimize insertion loss and parasitic reflections. The presented proof-of-concept experiments show its ability to suppress coupling into HCFs' higher-order modes. For example, without any end-cap, the extinction ratio between the LP $_{11}$ and the fundamental mode was found to be as low as 9 dB when coupling light from a free-space beam that was misaligned by as little as 1.1 $^{\circ }$ . This was improved to 23 dB when inserting the developed end-cap. Such small angle misalignment often exists when aligning the input beam with 3-axis (x,y,z) stages only (rather than 5-axis that also include pitch and yaw). Finally, we glued the end-cap with the HCF, providing hermetic sealing to the HCF input/output. This is of interest for stable operation in applications that use free-space light launch or require HCF output into free space.