Flexographic and Inkjet Printing of Multimodal Optical Waveguides

Optical phenomena and their application have always been amongst the most favoured ways to design sensors, as such devices typically allow fast, precise, and contact-free measurements. Today, optical sensors are employed in many fields to measure a wide range of physical and chemical properties. Typically, such sensors are discrete elements, positioned at critical positions. In many cases, however, it would be favourable to be able to measure not at one point, but rather over a whole area.

To develop a technical solution for this demand, the research project PlanOS was initiated. The vision of this project is to create a flexible, planar foil which can monitor several physical quantities like pressure, humidity or elongation over the extent of the whole foil by a network of optical sensors, without the need for any electrical components. To form this network, light sources that emit an optical signal, passive sensor structures which alter this signal in a way that allows to deduct the measurand, and light detectors which pick up this signal are created on the foil and connected by optical waveguides. One possible application for such sensor foils would be stress monitoring in heavily loaded structures like airplane wings or wind turbine blades.

A key requirement of PlanOS is that the sensor foils can be manufactured cost-effectively with high throughput technologies. Because of this the bulk of the waveguide is to be manufactured by flexographic printing. This established printing process is adapted to allow for the manufacturing of multimodal optical waveguides with a minimum lateral dimension of approx. 50 µm. The waveguide cladding and core material are printed sequentially which allows the additive manufacturing of many material systems, depending on the specific needs of the application. Current results show a parabolic cross section with a minimum lateral dimension of 167 µm and an aspect ratio of 0.11.

To allow a finer resolution and the deposition of specific functional inks, inkjet printing is employed as a complementary technique. Here, picoliter amounts of specifically tuned polymers are deposited on the prestructured foil at specific positions. This way, a flexible individualisation while still maintaining an efficient overall process is achieved. Both commercial and experimental inks were used to print optical waveguides and OLED light sources. To control the behaviour of the material on the substrate , chemical methods, like plasma treatment and use of surfactants, and physical tools, like temperature control or UV-radiation, are employed. In first tests, a minimal width of 40 µm could be achieved, and an aspect ratio of 0.15 was possible by heating the substrate to 70° celsius. These early results promise that printing techniques are suitable for a cost-efficient production of planar optical networks.

[315]