First stretchable optical link demonstrated for bendy waveguides

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A team of Belgian researchers has reported the first optical circuit that uses interconnections that are stretchable. These new interconnections, made of a rubbery transparent material called poly-dimethylsiloxane (PDMS), guide light along their path even when stretched up to 30 per cent and could be used for building networks of wearable body sensors, moving machine parts such as robotic limbs, and deformable consumer electronics.

For applications such as wearable body sensors and robotic skin, researchers need to ferry information along flexible routes. Electronics that bend and stretch have become possible in recent years, but similar work in the field of optics has lagged behind. Optics that stretch and lengthen when someone wearing body sensors bends to tie their shoe, or when a robotic arm twists through a full range of motion, have been particularly difficult to engineer.

By integrating the stretchy interconnections into a circuit – with a light source on one end and a detector on the other – the researchers created a miniature stretchable, bendable ‘link’ that could be incorporated into optical communications systems. This work was performed at the Centre for Microsystems Technology (CMST), a laboratory associated with Imec and Ghent University, and was published 18 February in The Optical Society’s (OSA) open-access journal Optics Express.

Previously, researchers had created optical interconnections – also called lightguides or waveguides – from other similar rubbery materials. But until now, the researchers say, no one had discovered a way to enable these materials to carry light while stretched. ‘To our knowledge, this is indeed the first truly bendable, stretchable optical link with these miniature dimensions,’ said lead author Jeroen Missinne of Ghent University and Imec.

The new connector consists of two materials, both made of PDMS: a transparent core through which the light travels, surrounded by another transparent layer of PDMS with a lower refractive index, a characteristic of the material that describes how light moves through it. This configuration traps light in the guide’s core, causing it to propagate along its length.

Bending a waveguide beyond a certain point causes some of the light trapped in the core to escape, a process called optical loss. The Belgian team tested how far they could bend and stretch their new optical connector before too much light escaped. ‘We were surprised that stretching had so little influence on the waveguides and also that their mechanical performance was so good,’ Missinne said.

But, Missinne said: ‘Waveguides are useless if you cannot launch light into them and collect light on the other end. If you want to obtain a truly stretchable optical link, the light sources and detectors need to be integrated together with the stretchable waveguide.’ In this case, a vertical-cavity surface-emitting laser (VCSEL) served as the light source, and a photodiode was the detector. This configuration allowed the team to create the first truly stretchable optical interconnector.

The team now plans to make their waveguide smaller, down from 50µm to just a few micrometres in diameter, which will also require a redesign of the parts of the waveguide where light enters and exits.