SPIE Photonics Europe
The evanescent field coupling between fibers plays an important role in many different applications such as novel sensors to detect variations of the environmental refractive index  or broad fiber-based band-pass filters . We are particularly interested in fused fiber couplers that allow for coupling light into higher order modes via evanescent field coupling . Such devices are very attractive for increasing the number of channels in large-scale telecommunication networks by an implementation of spatial multiplexing. For the fabrication of fused fiber couplers, the diameter of the fiber cores is reduced constantly during the fusion process. As a consequence, the effective refractive indices of the guided modes are decreasing as well. If the core diameters become too small the modes break out of the fiber core before any evanescent field coupling occurs, in particular when fibers with standard 125 µm diameter are used. In this case, the coupling between the fiber cores is due to the super-mode coupling phenomena, which is unwanted for our application. Thus, if evanescent field coupling shall be obtained, the diameter of the optical fibers has to be reduced before the fusion process. We developed an easy-to-use and reliable etching process based on hydrofluoric acid, which results in a uniform and reproducible reduction of the fiber diameter on a length of around 50 mm. Within the etched fiber section the diameter of the fiber can be as small as 40 µm and the deviation of the diameter is below one micron. A uniform diameter is critical during the fabrication process of the fused fiber couplers since it guarantees a sufficiently good contact and fusion of the outer fiber surfaces. We also developed a sophisticated numerical beam-propagation-method (BPM) simulation which allows us to predict, for a given etched fiber diameter, the extension at which the evanescent field coupling starts. Since the evanescent field coupling is difficult to detect during the fabrication step, the numerical simulations are very crucial. We also optimized the experimental parameters of the fusion process, such as the processing temperature, the fiber tension and the extension speed, for the etched fibers. Currently, we are focusing on two different types of fused fiber couplers: (i) symmetric couplers consisting of two identical single mode fibers (SMF) and (ii) asymmetric couplers consisting of a SMF and a few-mode fiber. Experimental results recorded during and after the fabrication processes, in particular for the symmetric couplers, match perfectly with the predictions of our simulations. In conclusion, we have developed an etching-process and a numerical simulation, which allows us to fabricate evanescent field coupled fused fiber couplers with a reliable, reproducible and controllable process. The asymmetric couplers are going to be used in a joint project with the Hannover Center for Optical Technologies (HOT) to fabricate mode selective fused fiber couplers.