Coupling light from laser to NANF with a hyperbolic profile fiber microlens

In this article, a mode-field adaption is presented for an interconnection between a hollow-core fiber (HCF) and laser diode (LD), thanks to the addition of a microlens with a hyperbolic profile. This solution enhances the coupling efficiency by 9 dB if compared to a butt-coupling configuration without mode-field adaptation. Our proposal maintains a practical, flexible working distance (>30 µm) and mitigates most of the Fresnel reflections (return loss (RL) >50 dB) without requiring any anti-reflective coating. At the same time, its compact size makes it suitable for integration.