Direct VCSEL interconnection with a hollow core fiber

The direct interconnection of a hollow-core fiber (HCF) to a laser diode or photodiode is essential to full exploitation of the unique characteristics of HCFs. In this paper, we investigate two specific methods for HCF interconnection (i.e., butt coupling and simple two-lens imaging) for coupling light from a single-mode vertical -cavity surface-emitting laser (VCSEL) operating at 850 nm. Our research findings indicate that direct (lensless) butt coupling leads to poor coupling efficiency (-10 %) due to the significant mode field diameter (MFD) mismatch between the waveguide and the HCF. However, the efficiency can be significantly improved (up to approximately 69 %) by using a graded-index fiber based all-fiber MFD converter. On the other hand, a two-lens system enables an extremely high coupling efficiency of -96 %.