Robust lensed fibers with high focusing power even in fluid

Two robust lensed fiber structures with high focusing effects (even in fluid with refractive-index of 1.501) were fabricated and characterized. The plano-convex lensed fiber (PC-LF) has a high focusing power that can contract the 10.4-μm mode-field diameter (MFD) of a standard single-mode fiber to a diameter as small as 1.0 μm at the wavelength of 1.55 μm. The high-index-layer-coated lensed fiber (HILC-LF) focuses the MFD to around 3 μm and has a specific feature that a fabrication process is extremely simple. Each lensed fiber employs a graded-index fiber with eigen-beam diameter to provide ideal lens-profile and to improve reproducibility. Both lensed fibers were mechanically reinforced by ceramic ferrules. An application to coupling between the single-mode fiber and a silicon-wire waveguide with a spot-size converter that has MFD of around 3 μm is demonstrated, showing that a coupling loss as low as 1.1 dB has been achieved.