First stretchable optical link created

While electronics that bend and stretch have become possible in recent years similar work in the field of optics – communicating with light instead of electrons – has lagged behind, limiting their use in applications like wearable body sensors and robotic skin where information needs to be ferried along flexible routes.

Engineering optics that stretch has been of particular difficulty, but now a team of Belgian researchers have created interconnections, made of a rubbery transparent material called PDMS (poly-dimethylsiloxane), that guide light along their path even when stretched by up to 30 per cent and when bent around an object the diameter of a human finger.

"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.

In a paper published in open-access journal Optics Express, the team describe how, by integrating these stretchy interconnections into a circuit with a light source on one end and a detector on the other, they created a miniature stretchable, bendable "link" that could be incorporated into optical communications systems.

Previously, researchers had created optical interconnections – also called lightguides or waveguides – from other similar rubbery materials, but according to the Ghent team, until now no one had discovered a way to enable these materials to carry light while stretched.

Past efforts also included embedding waveguides made of semi-rigid glass fibres into a stretchable substance, but in the new method the stretchable substance itself is the waveguide.

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, which traps light in the guide's core causing it to propagate along its length.

While bending the waveguide beyond a certain point causes some of the light trapped in the core to escape, a process called optical loss, the team were impressed with their guide's performance.

"We were surprised that stretching had so little influence on the waveguides and also that their mechanical performance was so good," Missinne said.

The guide's reliability was also "remarkable," he said – the researchers did not see degradation in the material even after mechanically stretching it to a 10 per cent elongation 80,000 times.

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 VCSEL (vertical-cavity surface-emitting laser), commonly used for fibre optic communications, served as the light source, and a photodiode was the detector. This configuration allowed the team to create the first truly stretchable optical interconnector.

Future uses for the new optical link might include building networks of wearable body sensors, moving machine parts such as robotic limbs, and deformable consumer electronics.

The team plan to focus future research on making their waveguide smaller, down from 50 micrometres to just a few micrometres in diameter, which will also require a redesign of the parts of the waveguide where light enters and exits.