Solitons are a special kind of wave. First noticed in water in the 19th century, they are a single pulse that moves at a constant velocity without losing its shape. They also occur in light, and researchers at Purdue University in Indiana are investigating their generation and use.
One barrier to the use of solitons has been an inability to harness them within devices small enough to fit onto a microchip. The Purdue team has devised a method which uses rings of silicon nitride with a radius of around 100 µm. Known as microrings, these are compatible with silicon electronics and act as a device called a microresonator (a structure that can store moving light waves), and the team claims in the journal Optica that this has allowed them for the first time to generate single stable solitons without the need for any active control of the system to tune it.
Key to the technique is the harnessing of a phenomenon that was previously thought to be a nuisance in such systems: Cherenkov radiation, which is emitted when charged particles exceed the speed of light within a medium.
Although it is impossible to exceed lightspeed in a vacuum, light slows down within transparent media like water and can be outpaced – astronauts have reported seeing flashes of light while in orbit, which has been ascribed to Cherenkov radiation from cosmic rays passing through the liquids and gels inside their eyes. “The important novelty of this work is that this Cherenkov interaction isn’t just harmful, as it is usually regarded, but actually can in some cases be harnessed to guide you to this nice clean single soliton,” said electrical and computer engineer Andrew Weiner, who directed the research. “So, we can use Cherenkov radiation to our advantage.”
The team used a moderately weak source of Cherenkov radiation to generate single solitons in the microring, a feat previously only possible by precise tuning of a specific type of laser equipment. Each time the soliton completed a circuit, “a small portion of the soliton’s power couples out of the ring where it is available for use in measurements and applications,” Weiner said.
As a circuit only takes a few trillionths of a second, this results in a periodic sequence of optical pulses known as a "frequency comb", a phenomenon of light containing a large number of equally spaced optical frequencies. The advantage of using a single soliton to generate a frequency comb rather than several of them is that the power of the pulses emitted is always the same, Weiner said.
Frequency combs are beginning to be used in precise optical sensors that detect airborne pollutants, and also have applications in ultraprecise "optical clocks" for navigation or timekeeping. But until now, the only way to generate frequency combs has been to use very large, expensive devices called mode-locked lasers which are mostly confined to specialist laboratories. “Environmental monitoring is really starting to happen with larger frequency combs based on lasers, but can we do that with chip-scale sources at lower cost for widespread use?” Weiner said. “We’re not there yet, but the potential is promising.”
Future work is likely to focus on increasing the control of the effect by coupling together two closely spaced microrings whose properties can be tuned by controlling their temperature. Other applications of single-soliton frequency combs could include transmission of hundreds of independent indications channels within optical fibres.
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