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Belgian team develops 'stretchable' optics

27 Feb 2014

Flexible polymer waveguide extends 30%, to enable new bio and robotic applications.

For applications like wearable body sensors, researchers need to send information through flexible media - including optical fibers. Electronics that bend and stretch have been developed in recent years, but similar work in the field of optics has lagged. A significant problem has been optics that can stretch to accommodate extension movements, whether by the human body or by robotic arms in manufacturing.

Now a team of Belgian researchers at Ghent University have reported progress with what they say are the first optical circuits to use interconnections that are not only bendable, but also stretchable. The interconnections are made of the rubbery transparent material PDMS (poly-dimethylsiloxane), which transports light effectively even when stretched up to 30% and when bent around an object the diameter of a human finger.

The work of the team led by Jeroen Missinne, of Ghent University and imec, has been published this week in the journal Optics Express. Missinne says that by integrating the stretchy PDMS interconnections into an optical circuit – with a light source at one end and a detector on the other – the researchers have created a miniature stretchable, bendable “link” that could be incorporated into optical communications systems.

“To our knowledge, this is indeed the first truly bendable, stretchable optical link with these miniature dimensions,” he said.
Previously, researchers had created optical interconnections from other similar rubbery materials. But until now, no one had discovered a way to enable these materials to carry light while stretched.

Previous efforts included embedding waveguides made of semi-rigid glass fibers into a stretchable substance. 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, 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, by "optical loss". The Belgian team tested how far they could bend and stretch their new optical connector before too much light escaped. He added, “We were surprised that stretching had so little influence on the waveguides and also that their mechanical performance was so good. The guide’s reliability is also remarkable." The researchers did not see a degradation in the material even after mechanically stretching it to a 10 percent elongation 80,000 times.

"But waveguides are useless if you cannot launch light into them and collect it at the other end. If you want to obtain a truly stretchable optical link, the light sources and detectors need to be integrated with the stretchable waveguide.” In this case, a VCSEL, commonly used for fiber optic communications, serves as the light source, and a photodiode as the detector.

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. Meanwhile, the team plans to make their waveguide smaller, down from 50µm to just a few micrometers in diameter, which will also require a redesign of the parts of the waveguide where light enters and exits.
The work has been performed at the Centre for Microsystems Technology (CMST), a laboratory associated with imec and Ghent University.

About the Author

Matthew Peach is a contributing editor to optics.org.

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