In a groundbreaking achievement, scientists from ETH Zurich, in collaboration with European partners, have demonstrated the remarkable capability of optical data communications lasers to transmit several tens of terabits per second, even in the face of disruptive air turbulence. The successful test, conducted between the majestic mountain peak of Jungfraujoch and the vibrant city of Bern in Switzerland, holds promising implications for the future of internet connectivity, potentially rendering expensive deep-sea cables obsolete.
The internet’s backbone relies heavily on a dense network of fiber-optic cables, each capable of transporting over 100 terabits of data per second between network nodes. Intercontinental connections are predominantly facilitated through costly deep-sea networks, with a single transatlantic cable demanding investments in the hundreds of millions of dollars. TeleGeography, a specialized consulting firm, has reported a growing number of 530 active undersea cables, underscoring the increasing demand for global connectivity.
However, a significant reduction in costs may soon be on the horizon. Scientists from ETH Zurich, in collaboration with partners from the space industry, have successfully demonstrated terabit optical data transmission through the air as part of a European Horizon 2020 project. This breakthrough could potentially pave the way for more cost-effective and significantly faster backbone connections utilizing near-earth satellite constellations. Their groundbreaking work has been published in the esteemed journal Light: Science & Applications.
Conquering Challenging Conditions from Jungfraujoch to Bern
To achieve this significant milestone, the project partners embarked on an ambitious endeavor to establish a satellite optical communication link. The test involved transmitting high-volume data across a free-space distance of 53 kilometers (33 miles) between the High Altitude Research Station situated on the awe-inspiring Jungfraujoch and the Zimmerwald Observatory at the University of Bern. Notably, the laser system was not directly tested with an orbiting satellite, yet it successfully accomplished high-data transmission under arduous conditions.
“Our test route between the High Altitude Research Station on the Jungfraujoch and the Zimmerwald Observatory at the University of Bern presents far greater challenges for optical data transmission than the link between a satellite and a ground station,” elucidated Yannik Horst, the lead author of the study and a distinguished researcher at ETH Zurich’s Institute of Electromagnetic Fields, headed by Professor Jürg Leuthold.
During the transmission, the laser beam traversed the dense atmosphere near the ground, encountering a multitude of influential factors such as turbulent air over snow-covered mountains, the vast water surface of Lake Thun, the densely populated Thun metropolitan area, and the presence of the Aare plane. These elements significantly affected the movement of the light waves and consequently impacted data transmission. The shimmering of the air, caused by thermal phenomena, disrupted the smooth propagation of light, perceptible even to the naked eye on scorching summer days.
Laser Precision Outperforms Microwave Alternatives
While satellite internet connections have long been established, typically utilizing radio technologies, such as Elon Musk’s renowned Starlink network comprising more than 2,000 satellites in close Earth orbit, these technologies have their limitations. Radio technologies operate within the microwave range of the spectrum, characterized by longer wavelengths measuring several centimeters. Conversely, laser optical systems operate in the near-infrared range, boasting wavelengths that are approximately 10,000 times shorter, thereby enabling the transportation of greater information per unit of time.
To ensure a robust signal upon reaching the receiver, the laser’s parallel light waves are channeled through a telescope, typically measuring several dozen centimeters in diameter. The wide beam of light must be meticulously aimed at a receiving telescope with a similar diameter as the transmitted light beam upon arrival.
Turbulence Mitigated for Uninterrupted Transmission
To achieve the highest possible data rates, the laser’s light wave is modulated in a manner that allows the receiver to detect various states encoded within a single symbol. This means that each symbol carries more than one bit of information. Practical implementation involves manipulating the amplitude and phase angles of the light wave, with each combination forming a distinct information symbol encoded into a transmitted symbol. Consequently, employing a scheme comprising 16 states (16 QAM) enables the transmission of 4 bits per oscillation, while a scheme comprising 64 states (64 QAM) facilitates the transmission of 6 bits.
However, the fluctuating turbulence of air particles introduces variability in the speed of light waves within and at the edges of the light cone. Consequently, when the light waves reach the detector at the receiving station, the amplitudes and phase angles either add up constructively or cancel each other out, leading to erroneous values.
To rectify these errors, the project’s French partner, ONERA, incorporated a microelectromechanical system (MEMS) chip containing a matrix of 97 minute adjustable mirrors. These mirrors deform to correct the phase shift of the light beam on its intersection surface, adjusting an astounding 1,500 times per second. This iterative process improves the signal quality by a factor of approximately 500.
Crucial advancements such as these enabled the team to achieve an impressive bandwidth of 1 terabit per second over a distance of 53 kilometers. “The results of our experiment are truly remarkable. We have demonstrated, for the first time, the feasibility of transmitting data at such high rates over long distances using this approach,” highlighted Horst.
New Modulation Formats Pave the Way for Enhanced Bandwidth
The groundbreaking experiment showcased the introduction of robust light modulation formats, revolutionizing detection sensitivity and enabling high data rates even under adverse weather conditions or with low laser power. This achievement was made possible by ingeniously encoding information bits into properties of the light wave, including amplitude, phase, and polarization. Horst elucidated, “With our newly developed 4D binary phase-shift keying, or BPSK, modulation format, an information bit can be accurately detected at the receiver, even with a minimal number—approximately four—of light particles.”
The successful outcome of this project required the collaboration and expertise of three partners. Thales Alenia Space, a prominent French space company renowned for its precision laser targeting capabilities over vast distances in space, contributed their expertise to the endeavor. ONERA, an esteemed French aerospace research institute specializing in MEMS-based adaptive optics, played a pivotal role in mitigating the effects of air shimmering. Finally, the ETH Zurich research group, led by Professor Leuthold, contributed their extensive knowledge in signal modulation, a crucial component for achieving high data rates.
Scalability and Future Prospects
The remarkable test results, unveiled for the first time at the prestigious European Conference on Optical Communication (ECOC) held in Basel, have garnered significant attention worldwide. Professor Leuthold expressed his enthusiasm, stating, “Our system represents a groundbreaking achievement. Up until now, there were only two possibilities: either connecting vast distances with limited bandwidths of a few gigabits or covering short distances of a few meters with large bandwidths utilizing free-space lasers.”
Moreover, the team achieved the impressive data rate of 1 terabit per second using a single wavelength. In future practical applications, the system can be easily expanded to accommodate 40 channels, ultimately reaching an astonishing 40 terabits per second using standard technologies.
While the practical implementation of this concept into a marketable product will be carried out by industry partners, ETH Zurich scientists remain committed to further exploring the potential of the newly developed modulation format. It holds the promise of enhancing bandwidths in various other data transmission methods, particularly those constrained by the energy of the light beam.
The successful transmission of terabits of data through turbulent air marks a significant milestone in the quest for affordable and high-speed internet connectivity. With the potential to replace expensive deep-sea cables, this achievement brings us closer to a future where seamless global connectivity becomes a reality for all.
