Mode-Field Diameter and “Spot Size” Measurements of Lensed and Tapered Specialty Fibers

Lensed and tapered specialty fibers are designed to optimize coupling between the fiber and various optical components such as edge-emitting laser diodes and AWGs. The light from these fibers forms a focus at some finite distance away from the fiber end. The Mode-Field Diameter (MFD) and corresponding spot size of these fibers is typically in the range of 5 microns or less. These small spot sizes and corresponding high divergence present challenges to near-field and far-field measurements. Typically the near-field measurements are limited by the optical performance, and the far-field measures must extend to very large angles. Results of measurements of 6 commercially available fibers are presented here, including different type lensed/tapered fibers (4 axially symmetric and 1 elliptical) and a standard single-mode fiber for reference. Also, since the focused beams are in free space, there are some fundamental questions as to the applicability of the MFD, a measure originally formulated to describe the field distribution within a fiber.