The University of Texas Arlington and the University of Vermont researchers have discovered a new method that could lead to a reduction in the cost and energy consumption of high-speed internet connections.
Nonlinear-optical effects, like intensity-dependent refractive index, can be used to process data thousands of times faster than what can be achieved electronically. In the past, this type of processing has worked only for one optical beam at a time because the nonlinear-optical effects can cause an unwanted inter-beam interaction, called crosstalk when multiple light beams are present.
The research group was led by Michael Vasilyev, an electrical professor at UTA, in collaboration with Taras I. Lakoba, a mathematics professor at UVM. They ran and experimental demonstration of an optical medium where multiple beams of light can autocorrect their own shapes without affecting each other.
Their work allows for simultaneous nonlinear-optical processing of multiple light beams by a single device, without converting them to electrical form. This opens the way for the technology to reach the full, multi-Terabit per second potential, resulting in cheaper and more energy-efficient high-speed internet communications.
In order to eliminate the noise accumulated during light propagation in optical communication links, right now, telecom carriers must resort to frequent optoelectronic regeneration. This requires converting optical signals to electrical signals through fast photodetectors, processing them with silicon-based circuitry and converting the electrical signals back to optical with lasers followed by electro-optic modulators. Because each optical fiber can carry over a hundred different signals at various wavelengths, known as wavelength-division multiplexing (WDM), this optoelectronic regeneration needs to be done separately for each wavelength. This makes regenerators large, expensive and inefficient consumers of power.
The researchers have developed an alternative to processing the optical signal directly, without converting it to electrical and back. The speed of light propagating in a transparent medium can be modified by a change in the light intensity. This is a manifestation of a nonlinear optical effect called “self-phase modulation” or SPM. If light contains signal and noise, SPM can help clean the signal from noise by scattering the noise energy into frequencies outside of the signal band, where the noise can be easily removed by a filter. When it is applied to light containing useful data, the SPM-enabled noise removal operation is called “all-optical regeneration,” which can result in optical auto-correction of the signals that carry hundred-times-faster data rates than what is currently electronically processed.
But the adoption of the all-optical regeneration in communication systems has been hindered by its inability to work with the WDM signals. This is because, in the presence of multiple signal beams (WDM channels), the desired SPM is always accompanied by two undesirable effects. These effects are cross-phase modulation, where a channel’s intensity modifies propagation speed of other channels, and four-wave mixing, where the interaction of a few channels leads to interface with other channels.
Vasilyev and colleagues report an experimental demonstration of a novel group-delay-managed nonlinear-optical medium. In this demonstration, strong SP effect is achieved without inner-channel interference. Splitting a conventional nonlinear medium, like an optical fiber, into several short sections separated by special periodic-group-delay filters harvests a medium where all frequency components of the same WDM channel travel with the same speed, which ensures a strong SPM. Different WDM channels travel with different speed, which dramatically suppresses any inter-channel interaction.
"Our new nonlinear medium has allowed us to demonstrate simultaneous all-optical regeneration of 16 WDM channels by a single device, and this number has only been limited by the logistical constraints of our laboratory," Vasilyev said. "This experiment opens the opportunities to scale the number of channels to over a hundred without increasing the cost, all in a book-sized device."
The multi-channel regenerator could potentially shrink to the size of a matchbox if the nonlinear-optical medium could be implemented onto a microchip.
"This breakthrough is an example of how UTA researchers can positively impact the physical and economic well-being of society in the area of data-driven discovery and global environmental impact, themes in UTA's Strategic Plan 2020 Bold Solutions | Global Impact," said Jonathan Bredow, professor and chair of the Department of Electrical Engineering in UTA's College of Engineering.
"Previous efforts to implement nonlinear-optical processing, such as regeneration, failed to make an impact because there was no advantage to employing them over electrical signals due to the inability to use more than one channel. Now that Dr. Vasilyev's group has overcome that obstacle, there are tremendous new possibilities for faster, more efficient transmission of messages," Bredow said.
The paper on this research was published in Nature Communications.