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Optical data communications lasers can transmit several tens of terabits per second, despite an enormous amount of disruptive air turbulence. Scientists from ETH Zurich and their European partners demonstrated this capability with lasers between the mountain peak, Jungfraujoch, and the city of Bern in Switzerland. This will soon eliminate the need for expensive deep water cables.
The backbone of the Internet is formed by a dense network of fiber optic cables, each carrying up to more than 100 terabits of data per second (1 terabit = 1012 digital signals 1/0) between network nodes. Connections between continents are via deep-sea networks, which is a huge expense: a single cable across the Atlantic requires an investment of hundreds of millions of dollars. TeleGeography, a specialist consultancy, has announced that there are currently 530 active submarine cables and that the number is rising.
Soon, however, this expense could drop significantly. Scientists from ETH Zurich, in collaboration with partners from the space industry, have demonstrated the optical transmission of terabit data through the air in a European project Horizon 2020. In the future, this will enable much cheaper and much faster backbone connections via constellations of near-earth satellites. Their work is published in the magazine Light: science and applications.
Difficult conditions between the Jungfraujoch and Bern
To reach this milestone, project partners made a significant leap forward in establishing an optical satellite communication link through a successful test conducted between the Alpine mountain peak, Jungfraujoch, and the Swiss city of Bern. Although the laser system has not been tested directly with an orbiting satellite, it has been able to transmit a large amount of data over a free-space distance of 53 km (33 miles).
“For optical data transmission, our test route between the High Altitude Research Station on the Jungfraujoch and the Zimmerwald Observatory at the University of Bern is much more demanding than between a satellite and a ground station,” explains Yannik Horst, lead author of the study and a researcher at the ETH Zurich’s Institute of Electromagnetic Fields headed by Professor Jrg Leuthold.
The laser beam travels through the dense atmosphere near the ground. In the process, many factorsdifferent turbulences in the air above the high snow-capped mountains, the water surface of Lake Thun, the densely built-up metropolitan area of Thun and the Aare plaininfluence the movement of the light waves and consequently also the transmission some data . The shimmer in the air, triggered by thermal phenomena, disturbs the uniform movement of light and is visible to the naked eye on hot summer days.
Satellite Internet uses slow microwave transmission
Satellite Internet connections are nothing new. The best-known example today is Elon Musk’s Starlink, a network of more than 2,000 satellites in near-Earth orbit that provides Internet access to virtually every corner of the world. However, data transmission between satellites and earth stations uses radio technologies, which are significantly less powerful. Like a wireless local area network (WLAN) or mobile communications, such technologies operate in the microwave range of the spectrum and therefore have wavelengths measuring several centimeters.
Laser optical systems, by contrast, operate in the near infrared range with wavelengths of a few micrometres, which are about 10,000 times shorter. As a result, they can carry more information per unit of time.
To ensure a strong enough signal by the time it reaches a distant receiver, parallel light waves from the laser are sent through a telescope that can measure several tens of centimeters in diameter. This broad beam of light must be aimed precisely at a receiving telescope with a diameter of the same order of magnitude as the width of the transmitted light beam upon arrival.
Turbulence cancels the modulated signals
To achieve the highest possible data rate, the laser light wave is modulated in such a way that a receiver can detect different states encoded on a single symbol. This means that each symbol conveys more than one bit of information. In practice, this results in different amplitudes and phase angles of the light wave. Each combination of phase angle and amplitude thus forms a different information symbol which can be encoded into a transmitted symbol. Thus, with a 16-state (16 QAM) scheme, each oscillation can transmit 4 bits, and with a 64-state (64 QAM) scheme, 6 bits.
The fluctuating turbulence of the air particles results in varying speeds of the light waves both within and at the edges of the light cone. Consequently, as the light waves arrive at the receiving station’s detector, the amplitudes and phase angles add or cancel each other, producing false values.
Mirrors the correct wave phase 1,500 times per second
To avoid these errors, Paris-based project partner ONERA used a microelectromechanical system (MEMS) chip with an array of 97 tiny tunable mirrors. The deformations of the mirrors correct the phase shift of the beam at its intersecting surface along the currently measured gradient 1,500 times per second, ultimately improving the signals by a factor of about 500.
This improvement was essential in achieving a bandwidth of 1 terabit per second over a distance of 53 kilometers, Horst points out.
New robust light modulation formats were demonstrated for the first time. This enabled a huge increase in detection sensitivity and therefore a high data rate, even in the worst weather conditions or with low laser power. This enhancement is achieved by intelligently encoding the bits of information in light wave properties such as amplitude, phase and polarization. “With our new 4D binary phase shift, or BPSK, modulation format, one bit of information can still be detected correctly in the receiver even with a very small number, about four, of light particles,” Horst explains.
All in all, the specific expertise of three partners was required for the success of the project. French space company Thales Alenia Space is an expert at pointing lasers with centimeter accuracy over thousands of kilometers in space. ONERA, also French, is an aerospace research institute with expertise in MEMS-based adaptive optics, which has largely eliminated the effects of shimmer in the air. The most effective method of signal modulation, which is essential for high data rates, is a specialty of the ETH Zurich research group in Leuthold.
Easily expandable up to 40 terabits per second
The results of the experiment, presented for the first time at the European Conference on Optical Communication (ECOC) in Basel, are causing a worldwide sensation. Leuthold says: “Our system represents a breakthrough. Until now, only two options were possible: bridging large distances with small bandwidths of a few gigabits or short distances of a few meters with large bandwidths using free-space lasers.”
Also, performance of 1 terabit per second has been achieved with a single wavelength. In future practical applications, the system can easily be scaled up to 40 channels and then to 40 terabits per second using standard technologies.
However, scaling isn’t something Leuthold and his team will be dealing with; practical implementation of the concept into a marketable product will be carried out by industry partners. However, there is one piece of work that the scientists at ETH Zurich will continue to do: In the future, the new modulation format they have developed will likely increase bandwidths in other data transmission methods where energy of radius can become a limiting factor.
Yannik Horst et al, Tbit/s line rate satellite feed links enabled by coherent modulation and fully adaptive optics, Light: science and applications (2023). DOI: 10.1038/s41377-023-01201-7
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Light: science and applications
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