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8 October 2008


IMEC embeds first functional optical links in flexible substrate

Nanoelectronics research center IMEC of Leuven, Belgium says that INTEC, its associated laboratory at Ghent University, has made the first functional optical links embedded in a flexible substrate. The links include optical waveguides, light sources, and detectors. The technique makes it becomes possible to fabricate foils that sense changes in pressure. Such skin-like foils could be used for monitoring irregular or moving surfaces, e.g. in robots, pliable machinery, or as an artificial skin.

Integrated optical interconnections have the advantage that they are insensitive to electromagnetic interference, applicable in harsh environments, and highly sensitive. Already, last year, IMEC reported embedded optical links on rigid surfaces. The current research takes optoelectronics a step further.

Standard, commercially available gallium arsenide photodetectors and VCSELs (vertical-cavity surface-emitting laser) are thinned down to 30 microns. Next, they are embedded into a flexible foil of optically transparent material and optically coupled with embedded waveguides and out-of-plane micro-mirrors. The resulting structure shows good adhesion and flexible behavior, IMEC claims.

Using this technology, IMEC is working on two types of sensors: array waveguide sensors and optical-fiber sensors. Both can be used for sensor foils.

Figure 1. Optical foil, bent with radius of curvature
of 1cm.

Array waveguide sensors rely on the change in coupling between arrays of crossing waveguides. Two layers of polymer waveguides are separated by a thin layer of soft silicone. When no pressure is applied, no crosstalk is detected. But when pressure is applied to the foil, the distance between the waveguides in the separated layers decreases and light is transmitted from one layer to the other. This low-cost sensor is suited to high-density pressure sensors on small areas, says IMEC.

Figure 2. Crossing waveguides: principle of the array waveguide sensor.

Optical sensing foils combine two technologies that have lately seen growing interest: integrated optical interconnections and flexible, stretchable electronics. The aim of researchers is to create a flexible and stretchable skin-like foil sensitive to touch, pressure, or deformation. Such artificial skin could be used in medical and industrial environments.

To this end, a group of European research institutes, including IMEC, are collaborating in the three-year 7th Framework project PHOSFOS (Photonic Skins For Optical Sensing), which started in April and aims to develop photonic foils based on optical-fiber sensors. These foils are targeted at applications in civil engineering and medicine. They will, for example, continuously monitor the integrity and behavior of buildings, dams, bridges, roads, or tunnels. Other uses include monitoring aircraft wings, helicopter blades, or windmill blades. They will enable early warning of failure or anomaly. Skin-like PHOSFOS membranes can also be used in long-term monitoring of respiration and cardiac activity, as well as the detection of pressure points under bed-ridden patients, says IMEC.

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