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System Bits: May 6

Laser radio transmitter; artificial synapse; reading vital signs with radar.

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Transmitting data with a semiconductor laser
Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences demonstrated a laser that can emit microwaves wirelessly, modulate them, and receive external radio frequency signals.

“The research opens the door to new types of hybrid electronic-photonic devices and is the first step toward ultra-high-speed Wi-Fi,” said Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, at SEAS and senior author of the study.

This research builds off previous work from the Capasso Lab. In 2017, the researchers discovered that an infrared frequency comb in a quantum cascade laser could be used to generate terahertz frequencies, the submillimeter wavelengths of the electromagnetic spectrum that could move data hundreds of times faster than today’s wireless. In 2018, the team found that quantum cascade laser frequency combs could also act as integrated transmitters or receivers to efficiently encode information.

Now, the researchers have figured out a way to extract and transmit wireless signals from laser frequency combs.

Unlike conventional lasers, which emit a single frequency of light, laser frequency combs emit multiple frequencies simultaneously, evenly spaced to resemble the teeth of a comb. In 2018, the researchers discovered that inside the laser, the different frequencies of light beat together to generate microwave radiation. The light inside the cavity of the laser caused electrons to oscillate at microwave frequencies — which are within the communications spectrum.

“If you want to use this device for Wi-Fi, you need to be able to put useful information in the microwave signals and extract that information from the device,” said Marco Piccardo, a postdoctoral fellow at SEAS and first author of the paper.

The first thing the new device needed to transmit microwave signals was an antenna. So, the researchers etched a gap into the top electrode of the device, creating a dipole antenna (like the rabbit ears on the top of an old TV). Next, they modulated the frequency comb to encode information on the microwave radiation created by the beating light of the comb. Then, using the antenna, the microwaves are radiated out from the device, containing the encoded information. The radio signal is received by a horn antenna, filtered and sent to a computer.

The researchers also demonstrated that the laser radio could receive signals. The team was able to remote control the behavior of the laser using microwave signals from another device.

“This all-in-one, integrated device, holds great promise for wireless communication,” said Piccardo. “While the dream of terahertz wireless communication is still a ways away, this research provides a clear roadmap showing how to get there.”

Brain-like computing in a battery-like device
The field of neuromorphic computing is of great interest to the semiconductor industry, leading to system-level implementations that aim to emulate how the human brain operates.

Researchers at Stanford University and Sandia National Laboratories previously developed one portion of such a computer: a device that acts as an artificial synapse, mimicking the way neurons communicate in the brain.

In a paper published online by the journal Science on April 25, the team reports that a prototype array of nine of these devices performed even better than expected in processing speed, energy efficiency, reproducibility and durability.

Looking forward, the team members want to combine their artificial synapse with traditional electronics, which they hope could be a step toward supporting artificially intelligent learning on small devices.

“If you have a memory system that can learn with the energy efficiency and speed that we’ve presented, then you can put that in a smartphone or laptop,” said Scott Keane, co-author of the paper and a graduate student in the lab of Alberto Salleo, professor of materials science and engineering at Stanford who is co-senior author. “That would open up access to the ability to train our own networks and solve problems locally on our own devices without relying on data transfer to do so.”

The team’s artificial synapse is similar to a battery, modified so that the researchers can dial up or down the flow of electricity between the two terminals. That flow of electricity emulates how learning is wired in the brain. This is an especially efficient design because data processing and memory storage happen in one action, rather than a more traditional computer system where the data is processed first and then later moved to storage.

Seeing how these devices perform in an array is a crucial step because it allows the researchers to program several artificial synapses simultaneously. This is far less time-consuming than having to program each synapse one-by-one and is comparable to how the brain actually works.

In previous tests of an earlier version of this device, the researchers found their processing and memory action requires about one-tenth as much energy as a state-of-the-art computing system needs in order to carry out specific tasks. Still, the researchers worried that the sum of all these devices working together in larger arrays could risk drawing too much power. So, they retooled each device to conduct less electrical current – making them much worse batteries but making the array even more energy efficient.

The 3-by-3 array relied on a second type of device – developed by Joshua Yang at the University of Massachusetts, Amherst, who is co-author of the paper – that acts as a switch for programming synapses within the array.

During testing, the array outperformed the researchers’ expectations. It performed with such speed that the team predicts the next version of these devices will need to be tested with special high-speed electronics. After measuring high energy efficiency in the 3-by-3 array, the researchers ran computer simulations of a larger 1024-by-1024 synapse array and estimated that it could be powered by the same batteries currently used in smartphones or small drones. The researchers were also able to switch the devices over a billion times – another testament to its speed – without seeing any degradation in its behavior.

“It turns out that polymer devices, if you treat them well, can be as resilient as traditional counterparts made of silicon. That was maybe the most surprising aspect from my point of view,” Salleo said. “For me, it changes how I think about these polymer devices in terms of reliability and how we might be able to use them.”

Monitoring vital signs with radar
The University of Waterloo reports its development of a radar system capable of wirelessly monitoring the vital signs of patients.

Housed in a device smaller than a cellphone, the new technology records heart and breathing rates using sensitive radar waves that are analyzed by sophisticated algorithms embedded in an onboard digital signal processing unit.

Researchers developed the system to monitor sleep apnea patients by detecting subtle chest movements instead of connecting them to equipment in labs via numerous cumbersome wires.

“We take the whole complex process and make it completely wireless,” said George Shaker, an engineering professor at Waterloo. “And instead of a clinic, it could be done in the comfort of your own bed and run daily for continuous monitoring.”

In the study, the radar unit was mounted to the ceiling over the bed of more than 50 volunteers as they slept normally in a model long-term care apartment.

The system, which collects and analyzes data from radar waves that are reflected back to the unit from the bodies of patients, achieved results more than 90% as accurate as standard hard-wired equipment.

“This is the first time radar has been used for heart sensing with this degree of accuracy and in such an uncontrolled environment,” said Mostafa Alizadeh, a research associate who led the study. “Our subjects slept unobstructed, in any position, for up to eight hours.”

Researchers are also exploring use of the technology to monitor activity levels and falls by residents of long-term care homes, and in hospitals for routine monitoring of heart and breathing rates of all kinds of patients.

Advantages of the system for apnea monitoring include complete privacy since no cameras are used, much improved comfort and potential use in homes rather than special sleep clinics.

“With traditional systems involving wires and appointments booked weeks in advance, you can’t sleep as you normally do in your own bed at home, making the common sleep study an unpleasant experience,” said Shaker, a cross-appointed professor of electrical and computer engineering, and mechanical and mechatronics engineering.

In addition to sleep apnea, which involves breathing that repeatedly stops and starts, the system can monitor conditions such as periodic limb movement disorder, restless leg syndrome and seizures.

Alizadeh and Shaker collaborated with Waterloo professors Plinio Pelegrini Morita and Safeddin Safavi-Naeini, and Joao Carlos Martins de Almeida, a professor at the University of Campinas in Brazil.



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