The internet’s foundation is composed of a dense network of fiber-optic cables. Each of these cables has the capability to transport over 100 terabits of data per second (1 terabit = 10^12 digital 1/0 signals) between network nodes. Connections between continents however, are made via deep sea networks, which come with an enormous cost. For instance, a single cable across the Atlantic requires an investment of hundreds of millions of dollars. According to TeleGeography, a specialized consulting firm, there are currently 530 active undersea cables, and this number is increasing.
Fortunately, this expense may decrease in the near future. Scientists from ETH Zurich, in collaboration with partners from the space industry, have demonstrated terabit optical data transmission through the air in a European Horizon 2020 project. This development will allow much more cost-effective and faster backbone connections through near-earth satellite constellations via laser-based network channels.
Satellite Communication Test in Challenging Conditions between Jungfraujoch and Bern
To reach this significant milestone, the project partners successfully tested a satellite optical communication link between the alpine mountain peak of Jungfraujoch and the Swiss city of Bern. Even though the laser system was not tested directly with an orbiting satellite, it achieved high-data transmission over a free-space distance of 53km (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 challenging than between a satellite and a ground station,” explains Yannik Horst, the study’s lead author and a researcher at ETH Zurich’s Institute of Electromagnetic Fields headed by Professor Jürg Leuthold.
The laser beam travels through the dense atmosphere near the ground, and many factors affect the movement of the light waves, including diverse turbulence in the air over the high snow-covered mountains, the water surface of Lake Thun, the densely built-up Thun metropolitan area, and the Aare plane. These factors influence the transmission of data. The air shimmering, triggered by thermal phenomena, disturbs the uniform movement of light, and it can be seen by the naked eye on hot summer days.
Satellite Internet Uses Slow Microwave Transmission
Satellite internet connections are not new. Elon Musk’s Starlink, a network of over 2,000 satellites orbiting close to the Earth, provides internet access to virtually every corner of the world. However, transmitting data between satellites and ground stations uses radio technologies, which are less powerful. Like a wireless local area network (WLAN) or mobile communications, such technologies operate in the microwave range of the spectrum and have wavelengths measuring several centimeters.
In contrast, laser optical systems operate in the near-infrared range with wavelengths of a few micrometers, which are about 10,000 times shorter. As a result, they can transport more information per unit of time.
To ensure a strong enough signal at a distant receiver, the laser’s parallel light waves are sent through a telescope measuring several dozen centimeters in diameter. The light beam must be precisely aimed at a receiving telescope with a diameter of the same order of magnitude as the width of the transmitted light beam on arrival.
Turbulence Affects Modulated Signals
To achieve high data rates, a laser’s light wave is modified so that a receiver can detect different states encoded onto a single symbol. This means each symbol transmits more than one bit of information. In practice, this involves different amplitudes and phase angles of the light wave. Each combination of phase angle and amplitude forms a different information symbol that may be encoded into a transmitted symbol. Thus, with a scheme comprising 16 states (16 QAM), each oscillation can transmit 4 bits, and with a scheme comprising 64 states (64 QAM), 6 bits.
The fluctuating turbulence of the air particles results in varying speeds of light waves both within and at the edges of the light cone. As a result, when the light waves arrive at the detector of the receiving station, the amplitudes and phase angles either add together or cancel each other out, producing false values.
Mirrors adjust wave phase 1,500 times per second
To avoid mistakes, ONERA, a Paris-based project partner, used a microelectromechanical system (MEMS) chip with a matrix of 97 small adjustable mirrors. The mirrors’ deformations fix the phase shift of the beam on its intersection surface along the currently measured gradient 1,500 times per second, leading to better signals by about 500 times.
This improvement was crucial in achieving a bandwidth of 1 terabit per second over a distance of 53 kilometers, according to Horst.
For the first time, new light modulation formats were demonstrated, resulting in a significant increase in detection sensitivity and high data rates, even under severe weather conditions or at low laser power. This is achieved by encoding information bits in light wave properties such as amplitude, phase, and polarization. “With our new 4D binary phase-shift keying (BPSK) modulation format, an information bit can still be correctly detected at the receiver even with a very small number (about four) of light particles,” Horst explains.
Overall, the project’s success required the specific skills of three partners. Thales Alenia Space, a French space company, is an expert in targeting 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 shimmering in the air. Leuthold’s ETH Zurich research group specializes in the most effective method of signal modulation, which is essential for high data rates.
Easily expandable to 40 terabits per second
The results of the experiment were presented for the first time at the European Conference on Optical Communication (ECOC) in Basel and have caused a sensation worldwide. Leuthold says, “Our system represents a breakthrough. Until now, only two options were possible: connecting large distances with small bandwidths of a few gigabits or short distances of a few meters with large bandwidths using free-space lasers.”
Moreover, the system achieved a performance of 1 terabit per second with a single wavelength. In future practical applications, the system can be easily scaled up to 40 channels and thus to 40 terabits per second using standard technologies.
Additional potential for the new modulation format
However, scaling up is not something Leuthold and his team will be concerned with; the industry partners will carry out the practical implementation of the concept in a marketable product. Nevertheless, there is one part of the work that the ETH Zurich scientists will continue to pursue: In the future, the new modulation format they developed is likely to increase bandwidths in other data transmission methods where the energy of the beam can become a limiting factor.
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