Free-space optical communication: High data rate connectivity from the ground up

Until recently, atmospheric conditions were a limiting factor for distance and bandwidth capabilities in the FSOC. However, techniques are now being used to mitigate these problems, such as adaptive optics, which result in improved data rates for a given bit error rate (BER).

As part of a collaboration between Airbus Defense and Space and the DLR (German Aerospace Center) Institute for Communication and Navigation, the first high-performance space-to-ground laser communication system was installed on the Bartolomeo platform of the International Space Station (ISS) and Tesat-Spacecom GmbH & Co. KG. The 2018 project, called OSIRIS, was designed to provide direct-to-earth technology (DTE) with a data rate of 10 Gbit/s over a range of about 1,500 km.

In 2024, a joint European initiative to improve Earth-to-FSOC technology was launched. This project, supported by the European Space Agency (ESA), brought together a specialist sensor manufacturer, Phlux Technology, Airbus Defense and Space and the University of Sheffield (UK). The main goal of the ongoing work is to develop more efficient FSOC satellite terminals. The medium-range goal is reliable 2.5 Gbit/s communications links operating at an IR wavelength of 1550 nm with satellites in low Earth orbit (LEO). The satellites typically orbit at altitudes of up to 2,000 km above the Earth’s surface. Looking to the future, the team aims to create systems that enable consistent transfer rates of 10 Gbit/s. A radiation-proof, integrated IR sensor and amplifier is being developed for the system.

How does sensor sensitivity affect FSOC performance?

One of the major technical challenges in achieving Earth-to-satellite and terrestrial FSOC is that the IR signals used to transmit data are diffracted as they pass through the troposphere, the atmospheric layer closest to Earth. Fluctuations in air temperature, humidity, and turbulence in our atmosphere cause variations in the intensity and angle of incidence of IR signals. This causes the beam to travel across the signal detector area, limiting performance. The problem is solved by using large-area receptors consisting of several IR sensors.

These IR sensors are critical components in FSOC receivers. Better sensors detect weaker signals, enabling the development of faster, higher bandwidth, lower latency connections. In Earth-satellite communications, they also improve performance because higher sensitivity allows link integrity to be maintained over a wider angle as satellites fly overhead – resulting in longer operating times.

1550 nm is a generally preferred wavelength for FSOC. It has a sufficiently longer wavelength than visible light and is therefore “eye-safe” when people come into contact with the signal. Avalanche photodiodes (APDs) – based on indium gallium arsenide APDs (InGaAs) – have maximum sensitivity to IR light at this wavelength. The Fraunhofer Heinrich Hertz Institute states that 1550 nm rays are 50 times safer than those at 850 nm, which have also been proposed for FSOC.

Until recently, the sensitivity of 1550 nm APDs was limited by the internal noise generated in the devices, limiting the range and data rates achievable in FSOC systems. But in early 2024, Phlux Technology announced silent InGaAs APDs. These sensors, which add antimony alloy to the compound semiconductor manufacturing process, can detect exceptionally low levels of light down to single photons, helping to maintain signal integrity over long distances and under changing atmospheric conditions. They increase the performance of FSOC systems and provide 12 times the sensitivity of traditional InGaAs APDs, representing a potential 10.79 dB improvement in interconnect efficiency before accounting for other noise sources such as amplifiers (see Figure 4).