Improving fiber to waveguide coupling efficiency by use of a highly germanium-doped thermally expanded core fiber

Possessing a gradient refractive index (RI), thermally expanded core (TEC) fibers have been studied to be able to improve coupling efficiency of waveguide-fiber or LD-fiber according to the qualitative analysis of geometrical optics. The experiments show that the mode field radius (MFR) expansion of single-mode fiber with highly Germanium-doped (HGD) is more effective than that of conventional single-mode fiber (SMF). The coupling efficiency of a fabricated TEC fiber with HGD can enhance 5%–6% compared with that of a conventional-SMF. So the TEC fiber with HGD is a good candidate for the coupling of waveguide-fiber or LD-fiber.