Research Bits: March 22

Securing wireless communications; on-chip frequency shifters; modulating visible light.


Securing wireless communications without encryption
Researchers from Princeton University, University of Michigan–Shanghai Jiao Tong University Joint Institute, and Xi’an Jiaotong University developed a millimeter-wave wireless chip that allows secure wireless transmissions and makes it challenging to eavesdrop on high-frequency wireless transmissions, even with multiple colluding bad actors.

The chip, built using standard foundry processes, can prevent interception without reducing latency, efficiency, and speed of the 5G network.

“We are in a new era of wireless—the networks of the future are going to be increasingly complex while serving a large set of different applications that demand very different features,” said Kaushik Sengupta, an associate professor of electrical and computer engineering at Princeton. “Think low-power smart sensors in your home or in an industry, high-bandwidth augmented reality or virtual reality, and self-driving cars. To serve this and serve this well, we need to think about security holistically and at every level.”

Instead of using encryption, the method shapes the transmission itself to prevent eavesdropping. It uses multiple antennas working as an array to generate radio waves that interfere with each other. An array of antennas is able to use this interference to direct a transmission along a defined path. But besides the main transmission, there are secondary paths. These secondary paths are weaker than the main transmission, but in a typical system they contain the exact same signal as the main path. By tapping these paths, potential eavesdroppers can compromise the transmission.

The team was able to make the signal at the eavesdroppers’ location appear similar to noise. To do this, they chop up the message randomly and assign different parts of the message to subsets of antennas in the array. The researchers were able to coordinate the transmission so that only a receiver in the intended direction would be able to assemble the signal in the correct order. Everywhere else, the chopped up signals arrive in a manner that appear noise-like.

The researchers created the system in a chip that can be manufactured in a standard chip foundry. (Image by Princeton University)

Sengupta compared the technique to chopping up a piece of music in a concert hall. “Imagine in a concert hall, while playing Beethoven’s symphony no.9, every instrument, instead of playing all the notes of the piece, decides to play randomly selected notes. They play these notes at correct times, and remain silent between them, such that each note in the original piece gets played by at least some instrument. As the sound waves carrying these notes from all the instruments travel through the hall, at a certain location, they can be made to arrive precisely in the correct fashion. The listener sitting there would enjoy the original piece as if nothing has changed. Everyone else would hear a cacophony of missing notes arriving at random times, almost like noise. This is, in principle, the secret sauce behind the transmission security —enabled by precise spatial and temporal modulation of these high-frequency electromagnetic fields.”

If the eavesdropper tried to interfere with the main transmission, it would cause problems detectable to the intended receiver.

While multiple eavesdroppers could work together to collect the noise-like signals and attempt to reassemble them into a coherent transmission, the number of receivers needed to do that would be “extraordinarily large,” Sengupta said. “We showed for the first time that it is possible to stitch several noise-like signatures into the original signal by colluding eavesdroppers applying AI, but it is very challenging. And we also showed techniques how the transmitter can fool them. It is a cat-and-mouse game.”

Sengupta said it also would be possible to use encryption along with the new system for additional security. “You can still encrypt on top of it but you can reduce the burden on encryption with an additional layer of security. It is a complimentary approach.”

On-chip frequency shifters
Researchers from Harvard University developed on-chip frequency shifters that can convert light in the gigahertz frequency range. The frequency shifters are controlled using continuous and single-tone microwaves.

“Our frequency shifters could become a fundamental building block for high-speed, large-scale classical communication systems as well as emerging photonic quantum computers,” said Marko Lončar, professor of electrical engineering at Harvard SEAS.

The team built two types of on-chip frequency shifter on a lithium niobate platform. The researchers had previously demonstrated a technique to fabricate high-performance lithium niobate microstructures using standard plasma etching to physically sculpt microresonators in thin lithium niobate films.

In the latest work, they etched coupled ring-resonators and waveguides on thin-film lithium niobate. In the first device, two coupled resonators form a figure eight-like structure. Input light travels from the waveguide through the resonators in a figure eight pattern, entering as one color and emerging as another. This device provides frequency shifts as high as 28 gigahertz with about 90% efficiency. It can also be reconfigured as tunable frequency-domain beam splitters, where a beam of one frequency gets split into two beams of another frequency.

The second device uses three coupled resonators: a small ring resonator, a long oval resonator called a racetrack resonator, and a rectangular-shaped resonator. As light speeds around the racetrack resonator, it cascades into higher and higher frequencies, resulting in a shift as high as 120 gigahertz.

In the top device, two coupled resonators form a figure eight-like structure. Input light travels from the waveguide through the resonators, entering as one color and emerging as another. The bottom device uses three coupled resonators: a small ring resonator, a long oval resonator called a racetrack resonator, and a rectangular-shaped resonator. As light speeds around the racetrack resonator, it cascades into higher and higher frequencies, resulting in a shift as high as 120 gigahertz. (Credit: Second Bay Studios/Harvard SEAS)

“We are able to achieve this magnitude of frequency shift using only a single, 30-gigahertz microwave signal,” said Yaowen Hu, a research assistant at Harvard SEAS. “This is a completely new type of photonic device. Previous attempts to shift frequencies by amounts larger than 100 gigahertz have been very hard and expensive, requiring an equally large microwave signal.”

“This work is made possible by all of our previous developments in integrated lithium niobate photonics,” said Lončar. “The ability to process information in the frequency domain in an efficient, compact, and scalable fashion has the potential to significantly reduce the expense and resource requirements for large-scale photonic circuits, including quantum computing, telecommunications, radar, optical signal processing and spectroscopy.”

Modulating visible light
Researchers at Columbia University developed an optical phase modulator for visible wavelength light that can manipulate it without dimming.

Optical phase modulators control the phase of a light wave and are used in on-chip optical switches that channel light into different waveguide ports. However, visible range phase modulators are difficult to make due to a lack of materials that offer both transparency and a high degree of tunability. Two of the most suitable materials are silicon nitride and lithium niobate, which while being highly transparent to visible light, lack much tunability, meaning devices based on them are large and power-hungry.

The researchers’ approach uses micro-ring resonators to reduce both the size and the power consumption of a visible-spectrum phase modulator, from one millimeter to 10 microns, and from tens of milliwatts for π phase tuning to below one milliwatt.

“Usually the bigger something is, the better. But integrated devices are a notable exception,” said Nanfang Yu, associate professor of applied physics at Columbia. “It’s really hard to confine light to a spot and manipulate it without losing much of its power. We are excited that in this work we’ve made a breakthrough that will greatly expand the horizon of large-scale visible-spectrum integrated photonics.”

A visible-spectrum phase modulator (the ring at the center of a radius of 10 microns) is tinier than a butterfly wing scale. (Photo credit: Heqing Huang and Cheng-Chia Tsai/Columbia Engineering)

“The key to our solution was to use an optical resonator and to operate it in the so-called ‘strongly over-coupled’ regime,” said Michal Lipson, professor of electrical engineering and professor of applied physics at Columbia.

In the “strongly over-coupled” regime, a condition in which the coupling strength between the micro-ring and the “bus” waveguide that feeds light into the ring is at least 10 times stronger than the loss of the micro-ring. “The latter is primarily due to optical scattering at the nanoscale roughness on the device sidewalls,” Lipson said. “You can never fabricate photonic devices with perfectly smooth surfaces.”

“Our best phase modulators operating at the blue and green colors, which are the most difficult portion of the visible spectrum, have a radius of only five microns, consume power of 0.8 mW for π phase tuning, and introduce an amplitude variation of less than 10%,” said Heqing Huang, a graduate student at Columbia. “No prior work has demonstrated such compact, power-efficient, and low-loss phase modulators at visible wavelengths.”

The researchers note that while they are nowhere near the degree of integration of electronics, their work shrinks the gap between photonic and electronic switches substantially. “If previous modulator technologies only allow for integration of 100 waveguide phase modulators given a certain chip footprint and power budget, now we can do that 100 times better and integrate 10,000 phase shifters on chip to realize much more sophisticated functions,” said Yu.

The team is working to demonstrate visible-spectrum LIDAR consisting of large 2D arrays of phase shifters based on adiabatic micro-rings. They also note the design strategies can be applied to electro-optical modulators to reduce their footprints and drive voltages and can be adapted in other spectral ranges such as ultraviolet, telecom, mid-infrared, and terahertz, as well as in other resonator designs beyond micro-rings.

“Thus, our work can inspire future effort where people can implement strong over-coupling in a wide range of resonator-based devices to enhance light-matter interactions, for example, for enhancing optical nonlinearity, for making novel lasers, for observing novel quantum optical effects, while suppressing optical losses at the same time,” Lipson said.

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