Researchers from ETH Zurich, in collaboration with European partners, have achieved groundbreaking results in optical data communications. Despite challenging air turbulence, their lasers successfully transmitted several tens of terabits per second between the Jungfraujoch mountain peak and the city of Bern in Switzerland. This remarkable feat paves the way for eliminating the need for expensive deep-sea cables in the near future.
The internet’s infrastructure heavily relies on a dense network of fiber-optic cables, each capable of transporting over 100 terabits of data per second between network nodes. Intercontinental connections currently rely on costly deep-sea networks, with a single transatlantic cable requiring a substantial investment in the hundreds of millions of dollars. As per TeleGeography, a specialized consulting firm, there are presently 530 active undersea cables, and this number continues to grow.
However, the scenario is poised to change dramatically. Through a European Horizon 2020 project, ETH Zurich scientists, in conjunction with space industry partners, have demonstrated terabit optical data transmission through the air. This breakthrough will facilitate more cost-effective and significantly faster backbone connections using near-earth satellite constellations. The research findings have been published in the esteemed journal Light: Science & Applications.
Challenging conditions between the Jungfraujoch and Bern
The project partners achieved a remarkable feat by successfully establishing a satellite optical communication link between the majestic mountain peak of Jungfraujoch and the vibrant city of Bern in Switzerland. Despite not conducting a direct test with an orbiting satellite, they accomplished a significant breakthrough by transmitting high volumes of data over a challenging free-space distance of 53km (33 miles).
Yannik Horst, the lead author of the study and a researcher at ETH Zurich’s Institute of Electromagnetic Fields under the guidance of Professor Jürg Leuthold, highlights the rigorous nature of their test route between the High Altitude Research Station on Jungfraujoch and the Zimmerwald Observatory at the University of Bern. This ground-based route poses greater challenges compared to a satellite-to-ground station link in optical data transmission.
The laser beam’s journey traverses the dense atmosphere near the ground, encountering numerous factors that affect the movement of light waves and data transmission. The diverse air turbulence over the snow-covered peaks, the reflective surface of Lake Thun, the densely populated Thun metropolitan area, and the Aare plane all play a role in influencing the behavior of light waves. Notably, the shimmering of the air, caused by thermal phenomena, disrupts the smooth movement of light and can be observed by the naked eye on hot summer days.
Satellite internet uses slow microwave transmission
While satellite internet connections are not a new concept, one prominent example being Elon Musk’s Starlink, which consists of over 2,000 satellites orbiting close to the Earth, providing global internet access, the method of transmitting data between satellites and ground stations typically relies on less powerful radio technologies. These technologies, similar to wireless local area networks (WLAN) or mobile communications, operate in the microwave range of the spectrum, with wavelengths spanning several centimeters.
In contrast, laser optical systems utilize the near-infrared range, where wavelengths measure just a few micrometers, making them approximately 10,000 times shorter. This characteristic allows laser systems to transport significantly more information within a given timeframe.
To ensure a robust signal upon reaching a distant receiver, the laser’s parallel light waves pass through a telescope with a diameter of several dozen centimeters. This wide beam of light must be precisely directed towards a receiving telescope of similar size, matching the width of the transmitted light beam upon arrival.
Turbulence cancels out modulated signals
To achieve the highest data rates, the laser’s light wave undergoes modulation that enables the receiver to detect multiple encoded states within a single symbol. This means that each symbol can convey more than one bit of information. This modulation technique involves manipulating the amplitude and phase angles of the light wave, where each combination of amplitude and phase represents a distinct information symbol. For instance, a 16-state scheme (16 QAM) allows each oscillation to transmit 4 bits, while a 64-state scheme (64 QAM) can transmit 6 bits.
However, the fluctuating turbulence of air particles introduces challenges. It leads to variations in the speed of light waves within the light cone and its edges. Consequently, when these light waves reach the detector at the receiving station, the amplitudes and phase angles can either combine constructively or cancel each other out, resulting in erroneous values and data corruption.
Mirrors correct wave phase 1,500 times per second
The project benefitted from the involvement of three key partners who contributed their specialized skills, leading to its successful outcome. Thales Alenia Space, a French space company, brought expertise in precise laser targeting across vast distances in space, ensuring centimeter-level accuracy. ONERA, a French aerospace research institute, contributed their knowledge in MEMS-based adaptive optics, employing a matrix of 97 small adjustable mirrors on a microelectromechanical system (MEMS) chip. These mirrors effectively corrected the phase shift of the laser beam, dynamically adapting 1,500 times per second. This correction significantly improved signal quality, enhancing it by a factor of around 500.
Notably, the achievement of a remarkable bandwidth of 1 terabit per second over a distance of 53 kilometers was made possible by leveraging robust light modulation formats. These formats enabled increased detection sensitivity, allowing for high data rates even under challenging weather conditions or with low laser power. The encoding of information bits in various properties of the light wave, such as amplitude, phase, and polarization, played a crucial role. The novel 4D binary phase-shift keying (BPSK) modulation format, developed by the research group at ETH Zurich led by Professor Leuthold, enabled accurate detection of information bits even with a minimal number of light particles, typically around four.
The combination of Thales Alenia Space’s expertise in laser targeting, ONERA’s advancements in MEMS-based adaptive optics, and ETH Zurich’s specialized knowledge in effective signal modulation formed the foundation for the project’s success.
Easily expandable to 40 terabits per second
The groundbreaking results of the experiment, unveiled at the European Conference on Optical Communication (ECOC) in Basel, have generated worldwide excitement. Professor Leuthold describes the system as a significant breakthrough, as it offers a previously unattainable option. Traditionally, only two choices were available: either connecting large distances with limited bandwidths in the gigabit range or establishing short-distance connections using free-space lasers with high bandwidths.
Impressively, the achieved performance of 1 terabit per second was accomplished using a single wavelength. Looking ahead, the system can be readily expanded to accommodate 40 channels, translating to a remarkable 40 terabits per second, utilizing standard technologies.
While the scaling-up aspect will be handled by industry partners for practical implementation in marketable products, the ETH Zurich scientists will continue to focus on a specific area of their work. The newly developed modulation format they pioneered holds the potential to enhance bandwidths in other data transmission methods, particularly in scenarios where the energy of the beam may impose limitations.
The promising outcomes of this research hold great potential for revolutionizing high-speed data transmission and pave the way for further advancements in the field.
Source: ETH Zurich