It is little wonder that the concept of transmitting information across open air — free of wires, channels, or any other incumbent medium — is a tantalizing one. No longer confined to the realm of science fiction, the reality of today has constellations of satellites encircling the planet, pinging streams of data of all kinds to almost anywhere. While ground-based communications systems and low-flying UAVs continue to evolve, it’s when these optical systems rise above the troublesome atmosphere that they come into their own. Before the telephone, early forays into free-space optical communications (FSOC) included a demonstration by Alexander Graham Bell of what he dubbed the “photophone.” Bell used beams of light to transmit voice conversations through the air, and although the photophone never translated into a commercial device, the principle was established. Satellites form constellations of effective coverage for optical communications. Courtesy of Mynaric. Then the revolution came: The invention of the laser in the 1960s transformed FSOC. With the technology’s genesis in military applications, the ability to communicate over unknown terrain without fear of an enemy severing contact was quickly recognized. Since then, steady improvements in light sources, detectors, and optical components continue to extend the reach of FSOC networks. Today’s FSOC systems have benefited from advancements in the global commercial fiber optic fixed-plant infrastructure. According to Rick Sanford, vice president of strategy and business development at BridgeComm Inc., based in Denver, Colo., rapid progress has been achieved in the past few years, led by developments in amplifiers, lasers, and pointing and acquisition tracking technologies. “In addition to the billions of dollars invested in technology by the terrestrial fiber optic communication industry, these efforts have been augmented in the free-space domain, in part, by commercial investment from the traditional defense and aerospace companies in the U.S., Japan, and Europe, as well as a wave of smaller companies looking to commercialize these advancements,” Sanford said. Commercial telecommunications is a significant driver for LAN-to-LAN, 5G, mobile backhaul, and “last mile” applications. According to Malcolm Watson, a principal research engineer at AVoptics Ltd. (an optical technology SME based in Somerset, England, and Cwmbran, Wales), the major factors driving growth are faster and more secure data transfer, radio frequency (RF) spectrum crunch, and reduced energy consumption. A medium-range optical link using a small UAV and a modulated retroflector. This type of optical system benefits from the proposed Cardiff/AVoptics collaboration project. Courtesy of Airbus/AVoptics/Cardiff University. “Other applications, such as building-to-building high-speed links, commercial aerospace, and within-room communications, are already using or starting to use FSO. FSO can also be used for disaster recovery or temporary building installations, as it can be quick and easy to set up a point-to-point communication link in an environment that is not suitable for laying fiber or using radio [communications],” Watson said. A growing area for innovation in FSOC is unmanned aircraft systems (UASs) or UAVs operating at very low altitude, typically less than 200 m. Advanced UASs with high-bandwidth data transfer over long ranges are highly desirable. Low-flying drones and satellite constellations are good examples of technological endeavors that will drive growth by bringing broadband connectivity to the forgotten regions of the planet, according to Markus Knapek of Mynaric. Courtesy of Mynaric. “Breakthrough low size, weight, and power (SWaP) FSO technology can open up innovation in precision agriculture, Earth observation, satellite uplink/downlink, infrastructure monitoring (rail, shipping, and energy), and a range of other terrestrial and maritime applications,” Watson said. But ground-based and low-altitude operation proves challenging because various atmospheric conditions such as dust, fog, and rain can scatter the transmitted signal and limit performance. High-sensitivity detectors can improve the transmission rate and distance of the link. Adaptive optics and other spatial techniques such as multiple transmit-or-receive apertures are also helping to overcome the problem. AVoptics, a major long-term supplier to the Airbus FSOC group and others in the aerospace and defense sectors, is currently collaborating with scientists at Cardiff University in Wales to develop new receivers for potential use on a UAV. For low-Earth-orbit networks, each satellite will host four laser communication terminals: one facing the front, one facing the back, and one on each side. These will connect the satellite to both an intra-and an intersatellite plane. Courtesy of Mynaric. “We think this is an emerging field in FSOC and have submitted a research proposal together to UK EPSRC [the Engineering and Physical Sciences Research Council] for funding support,” said Shiyu Xie, Sêr Cymru Research Fellow of Advanced Materials and Devices in professor Diana Huffaker’s group at Cardiff. “The prime advantages of FSO are license-free bands, high data-rate transmission, and negligible signal interference. FSO is also compatible with quantum-based optical techniques (using single-photon sources and detectors) that can be used to encrypt the data transfer securely. Its ability to transmit and receive multiple signals at once is relatively simple thanks to wavelength division multiplexing techniques.” Connecting rural regions of Earth The trend toward reductions in the SWaP of optical communications systems has opened up the skies. Compact, lightweight designs can now be fitted into microsatellites, also known as CubeSats, to deliver internet connectivity to remote and rural regions without the need for expensive and difficult-to-lay cabling. Previous hardware relied on large, complex steering mechanisms on heavy stable optical benches, in addition to power-inefficient multistage optical amplifiers. But as BridgeComm’s Sanford explains, current designs use fast-steering mirrors (FSMs) and optical closed-loop beacon tracking to provide a tighter optical communications beam in a much smaller package. “In turn, this reduces the required power output for the optical amplifiers, resulting in a smaller electronics package,” Sanford said. “These advancements have significantly reduced SWaP, with commercially available 10- to 100-plus-Gb/s designs that are less than 5 kg with very low power consumption in form factors that can even fit into CubeSats.” A still frame from a video on data relay laser communications, showing Tesat’s Laser Communications Terminal LCT 135. Courtesy of Tesat-Spacecom GmbH & Co. KG. These compact commercial designs also meet the requirements of mobile commercial and military platforms, as well as terrestrial point-to-point and airborne systems. Many companies are undertaking networks of satellites in the sky — called constellations — and most look to use laser communications for intersatellite linking to create global canopies of internet coverage that can then be directed down to Earth where needed. One such example is Mynaric, a German company specializing in laser communications for satellite networks, airborne platforms, and their respective ground terminals. Mynaric, which has its U.S. headquarters in Los Angeles, was founded by former employees of the German Aerospace Center (DLR). “From a strictly commercial point of view, the main drivers are the several companies who recognize the importance of laser communications in establishing effective backbone connectivity in space,” said Markus Knapek, founder of Mynaric. “SpaceX is probably the highest profile of these and has already trialed lasercom [laser-based communications] between two prototype satellites launched early in 2018. The eventual constellation — planned to consist of just under 12,000 satellites — will employ laser intersatellite links.” For low-Earth-orbit networks, each satellite will host four laser communications terminals: one facing the front, one facing the back, and one on each side. These will connect the satellite to both an intra- and an intersatellite plane. In the upper atmosphere, localized meshed networks of drones or high-altitude platforms (pseudo satellites) form miniconstellations. Maneuvering these miniconstellations above areas affected by natural disasters, for example, can help to reestablish emergency links. An example of this occurred in June 2019 when a magnitude 8 earthquake struck Peru and nearby Ecuador. Within 48 hours, cellphone connection was returned to the stricken area thanks to a network of air balloons powered by solar panels and armed with antennae that could connect to ground stations. “Today’s satellite constellations are good examples of a technology endeavor that will drive growth by bringing broadband connectivity to the forgotten regions of the planet, where people have no access to education or work,” Knapek said. “We see FSOC complementing RF, with a harmonic synergy between a technology that can broadly reach the users (RF) and backbone lasercom technology interconnecting them with a photonic mesh in the sky.” Looking inward toward Earth will undoubtedly benefit Earth observation endeavors, which — together with the increased availability of small portable ground stations — will bring flexibility, especially where natural disasters occur. But experts also see FSOC playing a pivotal role looking outward, to explore deep space, seen as particularly crucial to supporting people or robots that may be sent to remote planets. Current designs of optical communications systems use FSMs and optical closed-loop beacon tracking to provide a tighter optical communications beam in a much smaller package. These compact designs meet the requirements of mobile commercial and military platforms, as well as terrestrial point-to-point and airborne systems. Courtesy of BridgeComm Inc. “We see parallels from times where our forefathers only had a letter every few years to contact their relatives, and today we see our relatives every day by video, all thanks to a global fiber optic network. This makes the world small, so FSOC can bring the stars a little closer,” Knapek said. Blast off for optical communications With demonstrations of the technology heralded a success, market research experts SpaceWorks Enterprises Inc. estimates that about 700 communications nano/microsatellites will be launched over the next five years. These will be used to serve and support the rapidly growing Internet of Things as well as the machine-to-machine market. Practical implementations of current optical communications systems for small satellite applications may reach data rates of about 10 Gb/s, with a terminal weight on the order of 5 kg and a power consumption of about 50 W. The OSIRIS-4 CubeSat, for example, will be offered on the market by the DLR’s commercialization partner Tesat-Spacecom GmbH & Co. K.G. under the market name CubeL. The terminal weighs on the order of 300 g, consumes 8 W of electrical power, and requires only 0.3 U of space within the CubeSat. It reaches a data rate of 100 Mb/s. According to Stefan Seel, a laser expert at Tesat, located in Backnang, Germany, the advantage of FSOC for small satellites and CubeSats is that it is the only way to return the vast amount of data to the ground. The drawback is finding customers for the data from these satellites. “To be honest, [in] 1960, the same ideas floated around that still float around today. In between, [there was] a sobering number of disappointments with failing laser hardware in space and collapsing large constellation projects (such as the civilian Teledesic and the military TSAT [transformational satellite communications system]), both around the year 2000,” Seel said. “The big difference or positive change,” he said, “is the proven demonstrations: SILEX [the Semiconductor laser Inter- satellite Link Experiment], GOLD [the Ground/Orbiter Lasercom Demonstration], and LADEE [NASA’s Lunar Atmo- sphere and Dust Environment Explorer], and the first operational laser communication service [EDRS, the European Data Relay System, dubbed the SpaceDataHighway]. Lasers have become much more reliable, and smaller and more efficient.” This trend looks set to continue, with FSOC initially expected to complement established RF communications. As bandwidth and security demands increase, FSOC will likely eclipse RF for the delivery of services to users and for applications with very high-speed bandwidth requirements where terrestrial fiber infrastructure is either nonexistent or too costly to install.