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Information Ultra-Highway
Transmitting data via terahertz waves shows promise in unclogging the data logjam.
A simulation of radiation emerging from a terahertz multiplexer. Terahertz could enable the next generation of ultra-high-bandwidth networks to handle more data.
When Alexander Graham Bell transformed communications with the telephone in the 1870s, he had an assist from two Brown professors—Eli Whitney Blake and John Peirce—whose work on a phone receiver was ultimately adopted by Bell.
With the revolution in communications now going at speeds that Bell could never have imagined, another Brown professor—Daniel Mittleman in the School of Engineering—is working with colleagues to find a way to solve a critical logjam on the modern-day information superhighway.
Today’s cellular networks and Wi-Fi systems rely on microwave radiation to carry data, but the demand for more and more bandwidth is rapidly becoming more than microwaves can handle. That has researchers thinking about transmitting data on higher-frequency terahertz waves, which have as much as 100 times the data-carrying capacity of microwaves.
Mittleman is at the forefront of those exploring the field of terahertz technology. Though terahertz transmission remains in an early stage, with much basic research to be done and plenty of challenges to overcome, Mittleman is leading many key avenues of investigation. He and his colleagues are working to develop the basic components and techniques needed to make terahertz communications a reality.
There are also tremendous opportunities for new science, and new technologies.
– Daniel Mittleman
Multiplexing, the ability to send multiple signals through a single channel, is a fundamental feature of any voice or data communication system. An international research team led by Mittleman has demonstrated for the first time a method for multiplexing data carried on terahertz waves, which may enable the next generation of ultra-high-bandwidth wireless networks.
“The terahertz range is often called the ‘last frontier’ of the electromagnetic spectrum, since it is the least well explored range of the spectrum,” Mittleman said. “There’s a good reason for this: everything is more challenging in this range, including generating the radiation, manipulating it, and detecting it. But, with these challenges, there are also tremendous opportunities for new science and new technologies.”
In the journal Nature Communications, Mittleman and his team reported the transmission of two real-time video signals through a terahertz multiplexer at an aggregate data rate of 50 gigabits per second, approximately 100 times the optimal data rate of today’s fastest cellular network. “We showed that we can transmit separate data streams on terahertz waves at very high speeds and with very low error rates,” Mittleman said.
Mittleman and his team have even made the streets of Providence near their Barus & Holley offices a literal living laboratory. They have conducted measurements under the first license from the Federal Communications Commission to perform outdoor tests of data transmission in several frequency bands in the terahertz range. “These kinds of outdoor tests will be important for understanding what’s possible in terahertz communication,” Mittleman said.
The first outdoor tests have proven promising, in some cases easing concerns about the versatility of terahertz links. For example, it’s long been assumed that terahertz links would require a direct line of sight between receiver and transmitter. But Mittleman and his team showed that non-line-of-sight terahertz data links are possible because the waves can bounce off of walls and other obstacles without losing too much data. Mittleman and his colleagues bounced terahertz waves at four different frequencies off of a variety of objects—mirrors, metal doors, cinderblock walls, and others—and measured the error rate of the data on the wave after the bounces. They showed that acceptable error rates were achievable with modest increases in signal power.
Brown graduate student Rabi Shrestha works on terahertz testing equipment.
The researchers also looked at what’s known as multipath interference. When a signal is transmitted over long distances, the waves fan out, forming an ever-widening cone. As a result of that fanning out, a portion of waves will bounce off the ground before reaching the receiver. That reflected radiation can interfere with the main signal unless a decoder compensates for it. It’s a well-understood phenomenon in microwave transmission, and Mittleman and his colleagues wanted to test it in the terahertz range.
They showed that this kind of interference occurs in terahertz waves but to a lesser degree over grass compared to concrete. That’s likely because grass contains a lot of water, which tends to absorb terahertz waves. Over grass, the reflected beam is absorbed to a greater degree than over concrete, leaving less of it to interfere with the main beam. That means that terahertz links over grass can be longer than those over concrete because there’s less interference to deal with, Mittleman said.
There’s also an upside to that kind of interference with the ground. “You can imagine that if your line-of-site path is blocked,” Mittleman said, “you could think about bouncing it off the ground to get there.”
In other terahertz work, Mittleman and others, including Masaya Nagai, an academic colleague in Japan, have developed a new method of manipulating the polarization of light at terahertz frequencies.
The technique, outlined in a paper in the journal Scientific Reports, uses stacks of carefully spaced metal plates to make a polarizing beamsplitter, a device that splits a beam of light by its differing polarization states, sending vertically polarized light in one direction and horizontally polarized light in another. Such a beamsplitter could be useful in a wide variety of systems that make use of terahertz radiation, from imaging systems to future communications networks.
Terahertz radiation is a hot area of study, and the work isn’t limited to data transmission. Mittleman and Professor Vicki Colvin from Brown’s chemistry department are heading a team that has improved the resolution of terahertz emission spectroscopy—a technique used to study a wide variety of materials— by a thousandfold, making the technique useful at the nanoscale. Laser terahertz emission microscopy is a burgeoning means of characterizing the performance of solar cells, integrated circuits, and other systems and materials.
The researchers believe their new technique could be broadly useful in characterizing the electrical properties of materials in unprecedented detail.
“Terahertz emission has been used to study different materials—semiconductors, superconductors, wide-band- gap insulators, integrated circuits, and others,” Mittleman said. “Being able to do this down to the level of individual nanostructures is a big deal.”
