A novel coupler measurement station and technique for manufacturing fused biconical tapered multi-port multimode couplers with improved uniformity among ports is presented. Improvement in the uniformity of the couplers is achieved by diffusion of the minimum taper region of the coupler. The phenomenon of dopant transport from the core to the cladding (or from the cladding to the core) at high temperatures is known as diffusion. Diffusion of a germanium-doped core results in the germanium dopants migrating from the core into the cladding thus increasing the effective diameter of the core which is accompanied by a decrease in the refractive index. The cores of individual fibers in the minimum taper region are a few micrometers apart. Diffusion thus results in a minimum taper region that has approximately uniform refractive index leading to better uniformity among ports.

The experimental setup and results of diffusion tests on multimode fiber with different core/cladding ratios are presented. A coupler station that is capable of making bi- directional measurements of the coupler is demonstrated successfully. The coupler station also enables a user to dynamically monitor the ports of the coupler during manufacture, and can be adapted to manufacture star couplers ranging in size from two to sixteen fibers.