Analysis of bending effects 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 such applications 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.


The goal of the presented work is the development of an optical bus-coupler, which enables easy connection between two waveguides without interrupting the bus. Hence, an enormous reduction of costs and effort can be obtained. It is possible to realize optical bus systems by connecting several modules onto one waveguide with a core-core-coupler. In this paper we present the concept of a bus-coupler, which is suitable for a bidirectional coupling with a definable ratio. In order to tune the coupling efficiency bended flexible planar waveguides are used, which are pressed onto the bus with a defined force adjusting the overlap area of the two coupling waveguides. We will introduce the manufacturing of the multimode waveguides on flexible substrates by photolithographic structuring. Using the hybrid polymers OrmoClad® and OrmoCore® on flexible PEN-foil, nearly rectangular waveguides (50 x 50) µm can be manufactured. Furthermore we present results of our investigations on the behavior of planar waveguides under bending in terms of optical losses and power distribution within the waveguide core. High bend radii push the main energy on the outer region of the waveguide core, which influences the coupling efficiency significantly. Based on the experimental results we will derive the requirements and design rules for the coupling element.



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Lukas Lorenz

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