Power/Performance Bits: June 30

Up-converting lasers
Researchers at the University of Pennsylvania developed a filter chip that can convert the output from low-cost lasers to have the same frequency noise as big, expensive lasers, making them suitable for applications such as LiDAR.

The noise in a laser’s frequency is an important indicator of quality. Low-quality, noisy lasers have more random variations, making them useless for systems that are meant to return accurate measurements or convey densely packed information.

The researcher’s three-square-millimeter ‘phase noise filter’ chip operates independently of the laser chip and thus can work with many different types of lasers. It reduces noise in a low-cost laser’s frequency by forming a loop around the laser, feeding back the laser noise to itself.

The laser’s frequency is also reflected in its color. “The highest-quality red laser, for example, would generate a single, pure red color at its output. That means its frequency can be represented with a thin line on the color spectrum corresponding to that exact tone of red,” said Firooz Aflatouni, Assistant Professor in Electrical and Systems Engineering at Penn. “In practice however, due to noise and other factors, lasers may generate multiple closely packed tones, resulting in a thicker line on the spectrum. The width of this line, also known as the laser linewidth, is therefore a way of measuring laser performance; the thinner the line, the closer it is to an ideal single-color laser.”

Narrow linewidth lasers are preferred for LiDAR and communications. “For example, in an advanced type of LiDAR, so-called ‘coherent’ LiDAR, the achievable range is inversely proportional to the laser linewidth; the lower the linewidth, the higher the range,” Aflatouni said.

“Our implemented chip measures the noise that broadens the linewidth, amplifies it and subtracts it from the laser output light in a loop, ultimately narrowing its linewidth,” said Mohamad Hossein Idjadi, a postdoctoral researcher at Penn.

The researchers say their filters could be incorporated into existing manufacturing processes for the laser chips found in fiber-optic modems, making these laser systems more cost-effective for mass production than their larger counterparts. Other applications could include low-cost, compact LiDARs and hand-held diagnostic systems as well as smaller high-data-rate optical communications systems.

Breathable electronics
Researchers at Nanjing University and North Carolina State University developed a gas-permeable, stretchable electronic material. Intended for use in biomedical or wearable applications, the breathable material allows sweat and volatile organic compounds to evaporate away from the skin.

“The gas permeability is the big advance over earlier stretchable electronics,” said Yong Zhu, a professor of mechanical and aerospace engineering at North Carolina State University. “But the method we used for creating the material is also important because it’s a simple process that would be easy to scale up.”

The team used the ‘breath figure method’ to create a stretchable polymer film featuring an even distribution of holes. The film is coated by dipping it in a solution that contains silver nanowires. The researchers then heat-press the material to seal the nanowires in place.

“The resulting film shows an excellent combination of electric conductivity, optical transmittance and water-vapor permeability,” Zhu said. “And because the silver nanowires are embedded just below the surface of the polymer, the material also exhibits excellent stability in the presence of sweat and after long-term wear.”

The researchers incorporated the material into two applications as proofs of concept.

The first prototype consisted of skin-mountable, dry electrodes for use as electrophysiologic sensors. These have multiple potential applications, such as measuring electrocardiography (ECG) and electromyography (EMG) signals. “These sensors were able to record signals with excellent quality, on par with commercially available electrodes,” Zhu said.

The second prototype demonstrated textile-integrated touch sensing for human-machine interfaces. The authors used a wearable textile sleeve integrated with the porous electrodes to play computer games such as Tetris.

This sleeve incorporates the new electronic material, allowing it to function as a video game controller. (Credit: Yong Zhu, NC State University)

“The end result is extremely thin – only a few micrometers thick,” said Shanshan Yao, a former postdoctoral researcher at NC State who is now on faculty at Stony Brook University. “This allows for better contact with the skin, giving the electronics a better signal-to-noise ratio. And gas permeability of wearable electronics is important for more than just comfort. If a wearable device is not gas permeable, it can also cause skin irritation.”