High Focusing Power Lensed Fibers Employing Graded-Index Fiber With Eigen-Beam Diameter

Two advanced structures of lensed fiber each employing a graded-index fiber with eigen-beam diameter (EB-GIF) to provide superior focusing power are reported. The high-index-layer (a-Si:H)-coated lensed fiber and the plano-convex lensed fiber focus the 10.4-μm mode-field diameter of a standard single-mode fiber to 2.7 and 1.1 μm , respectively, even in oil with refractive index of 1.501 at the wavelength of 1.55 μm with the corresponding transmission losses as low as 0.78 and 0.08 dB, respectively. Thus, the new lensed-fiber structures would facilitate efficient coupling between the standard single-mode fibers and the high-index-contrast silicon-oxynitride or the super high-index-contrast silicon-rib waveguides. Employment of the EB-GIF not only significantly simplifies fabrication of the lensed fibers, but also greatly improves their reproducibility.