Simulation of bending effects of planar waveguides for optical bus-couplers

The well-known advantages of optical data communication are the main driver for the introduction of optics on PCB- and module-level in order to meet the increasing bandwidth demands. In these cases conventional copper wires reach their limits. Nevertheless, optical short range connections are not yet competitive enough to replace standard electrical connections because of the lack of robust and SMT-compatible schemes for the optical in- and out-coupling from integrated waveguides. The integration of optics into structural elements such as in aircraft or car components using flexible planar optical waveguides opens new possibilities for applications such as Internet of Things or the structural health monitoring of CFK-materials. These optical bus systems require a new coupling method, which enables optical termination on arbitrary positions and field assembly. Most state-of-the-art coupling methods for board-level optical interconnects use an interruption of the waveguides at the coupling position, where transmitting or receiving elements are assembled. Hence, a coupling in the field is difficult and, if feasible, expensive. Furthermore it is impossible to connect two or more modules to one single waveguide to realize a bus system.

In our work an optical bus-coupler is proposed, which enables easy bidirectional connection between two waveguides without interrupting the bus using a core-to-core coupling principle. With bended waveguides the coupling ratio can be tuned by adjusting the overlap area of the two cores. In order to ensure large overlap areas at short coupling lengths, the waveguides have rectangular cross sections. To examine the feasibility of this coupling concept a simulation was performed, which is presented in this paper. Due to the highly multimode waveguides used in short range data communication, a non-sequential raytracing simulation is reasonable.

Important for an adequate simulation of bended waveguides a mathematically accurate description of the core without any approximations such as interpolation with short straight sections is necessary. In the ZEMAX® simulation we used, four “Cylinder Volumes” fitted one into the other were the basic optical structure for a 180 degree waveguide bending. The model is completed by two straight waveguides at the beginning and the end of the curve as well as by a straight coupling waveguide connected to the core of the bended structure. Furthermore the maximum number of intersections and segments per ray were used to obtain the highest possible accuracy.

Due to the simulation of the coupling ratio between the bended and the straight waveguide it was discovered that the ratio not only depends on the size of the overlap area but also on the bending radius. With a constant overlap area the coupling efficiency is significantly lower at higher bend radii. Further simulations showed that the bending of the waveguide causes a redistribution of the energy within the core. Small radii push the main energy to the outer region of the core increasing the coupling efficiency. On the other hand at excessive lowered bend radii additional losses occur (due to a coupling into the cladding), which is why an optimum has to be found. Based on the simulation results it is possible to derive requirements and design rules for the coupling element.



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