Efficient Fiber-to-Waveguide Edge Coupling Using an Optical Fiber Axicon Lens Fabricated by Focused Ion Beam

Abstract

Efficient edge coupling into a silicon waveguide is demonstrated using an optical fiber axicon lens operating at 1550 nm. The axicon was fabricated on the cleaved endfacet of an SMF-28e optical fiber using a focused ion beam milling process. The lens converts the guided mode of the optical fiber (mode field diameter of 10.4 $\mu$ m) into a Bessel-type beam with an extended depth of focus, which was verified both experimentally and by three-dimensional finite-difference time-domain simulations. Axicon lenses with a diameter of 15 $\mu$ m and heights of 3.5 and 5.0 $\mu$ m were fabricated, and their focusing effect optically characterized through high-resolution confocal imaging, showing a focal spot size of 3.57 and 2.34 $\mu$ m, respectively. The coupling efficiency from the axicon into a silicon-on-insulator photonic integrated circuit is comparable to that of a commercial tapered lensed fiber, and in some cases slightly higher. Due to the extended depth of focus, the axicon lens also offers a larger alignment tolerance in the longitudinal direction.