Free-space optics to connect the world

Free-space optical communication has long been touted as the next big thing in broadband data transmission. Described as ‘fiber without the fiber,’ it can provide point-to-point communication through air, space, and water using infrared, visible, or ultraviolet parts of the electromagnetic spectrum. It can function indoors and out. It has low power requirements and offers high capacity and resistance to electromagnetic interference. And it is fast to install and reasonably cheap.

For these reasons, many see free-space optics (FSO) as a key enabling technology for broadband internet access in developing countries, remote communities, and in disaster response, as well as offering a promising route to the high-speed data rates required of future 6G networks.

FSO communication was first achieved by Alexander Graham Bell in 1880, when the Scottish inventor transmitted sound modulated on a beam of light over a distance of 213 m using his photo-phone. Modern FSO systems consist of a high-power laser source that converts data into laser pulses and sends them through a lens system. The laser travels through air, space, or water before entering a receiver lens system. A high-sensitivity photodetector then converts these laser pulses back into electronic data.

Today, experimental FSO systems on the ground can deliver petabytes per second data transfer rates over several meters and terabytes per second over several kilometers, while their commercial counterparts are delivering gigabytes per second (Gbps) capacity over kilometers. For example, in 2021 Aircision and TNO demonstrated that their FSO systems could reliably transmit 10 Gbps over 2.5 km.

Meanwhile, low-Earth orbiting satellites and ground stations are beginning to be kitted out with space-to-space and direct-to-Earth FSO communication systems, aiming towards global broadband coverage. And space missions such as NASA’s Artemis II crewed mission to the Moon and Psyche orbiter — investigating 16 Psyche, a metal asteroid in the asteroid belt — are launching with onboard laser systems to enable high data rate communication between deep space and Earth. As a result of this growing set of applications, the FSO market is expected to reach $4.1 billion by 2031 at a CAGR of 26.9%.

However, despite decades of research challenges remain. These include beam divergence over long distances, imprecise alignment resulting in pointing errors, strong atmospheric attenuation in inclement weather, and atmospheric turbulence. The Free-Space Laser Communications XXXV conference in January at SPIE Photonics West showcased the latest research and technology advances that address these and other issues associated with realizing the full potential of FSO.

Perfect optical alignment

For instance, Hao Hu of Technical University of Denmark gave a talk on 31 January about his team’s solution to FSO system alignment and tracking. “The narrow optical beam in free-space optical communication means the alignment between the transmitter and receiver needs to be very accurate,” explains Hu. “Traditionally, you would use a mechanical motor to do that alignment, but this method is relatively low speed and requires a very stable fixed platform.”

To address this problem, Hu’s team has developed a new chip-based beam steering technology based on an integrated optical phased array. Used in radio frequency communications for over a century, phased arrays consist of multiple coherent emitters whose interference pattern can be controlled via each emitter’ phase to increase power radiated in desired directions and suppress it elsewhere, essentially steering the resulting beam without having to physically reposition any elements of the system.

Hao Hu holding his experimental integrated optical phased array. Credit: Hao Hu

Until now, optical phased arrays have not found use in FSO systems due to a limitation caused by the tradeoff between field-of-view (FoV) and beam quality. A 180° FoV is possible if waveguide grating array emitters are spaced at a half-wavelength or less, but this causes uncontrollable and strong coupling between adjacent waveguides, increasing background noise. And if emitter spacing is larger than a half-wavelength, strong constructive interference occurs and grating lobes are generated, causing aliasing and limiting the FoV.

To get around this limitation, Hu’s device replaces waveguide array emitters with waveguide superlattices followed by a trapezoidal slab grating as a single emitter. The resulting optical phased array — which the team only recently demonstrated — achieves 2D aliasing-free beam steering with high beam quality across a 180° FoV at high speed (>10 kHz). Moreover, the integrated device is fabricated on a silicon photonics platform and can be produced at high volume and low cost in CMOS foundries.

Hu sees this advance as transformative for FSO communications and a number of other fields: “Free-space optics alignment can be much quicker, but we can also utilize this technology in high-performance light detection and ranging systems and to track fast-moving objects for applications like autonomous vehicles,” he says. “At Photonics West, I’m looking forward to meeting potential collaborators, both academic and industrial, who can help advance this technology.”

Adaptive optics

Elsewhere in the program, a significant proportion of talks were dedicated to a persistent confounding factor in FSO communications: atmospheric turbulence. A result of local weather conditions leading to random variations of the refractive index along the transmission path, turbulence causes beam scintillation, spreading, and wandering, which has a significant impact on beam quality reaching the FSO receiver. Ultimately, turbulence degrades and reduces the reliability of any space-to-ground or ground-to-ground FSO communications link.

Various solutions to the turbulence issue have been put forward. For instance, two talks from French firm CAILabs described the performance of their pilot optical ground station and turbulence mitigation product — a compact, spatial mode demultiplexer based on multi-plane light conversion that has been shown to increase signal collection at the receiver, even in strong atmosphere turbulence conditions. Another successful turbulence solution is borrowed from large ground-based astronomical telescopes: adaptive optics. Adaptive optics measures the signal aberrations from turbulence and utilizes active optical elements (wavefront correctors) incorporated into the receiver or transmitter to correct them.

Schematic of NASA’s LCOT telescope. Credit: NASA

NASA’s Low-Cost Optical Terminal (LCOT) is a prototype ground telescope that will achieve first light in 2023 and contains a novel high-power laser amplifier by OFS and state-of-the-art adaptive optics system from General Atomics. It uses commercial off-the-shelf or slightly modified hardware wherever possible and has a modular design to reduce cost and expand access to NASA’s future optical communications capabilities. “If you invest in a multipurpose design once, then you can build a future to support a wide range of missions,” explains Robert Lafon of NASA’s Goddard Space Flight Center. “And we want this to be something that is universally available to anybody that wants to build an FSO system.”

At Photonics West, Lafon offered an overview and status update on the LCOT project, while in separate talks colleagues Daniel Paulson and Patrick Thompson will provide technical details of LCOT’s adaptive optics and FSO systems, respectively.

What’s more, presentations from three upcoming FSO demonstration missions that may be used to verify the LCOT design are also part of the conference program: the upcoming Laser Communications Relay Demonstration (LCRD), launched on December 7, 2021; the Terabyte Infrared Delivery System (TBIRD), launched on May 25, 2022; and the Orion Artemis II Optical Communications System (O₂O), scheduled for launch in 2024, which will transmit ultra-high-definition video from the Moon during the Artemis II crewed mission. “One of our main goals is to test LCOT against O₂O,” says Lafon. “By that time, I would certainly hope we would have a comprehensive list of changes and improvements that we would want to make toward the final LCOT blueprint.”

Combating atmospheric turbulence

Lafon is a veteran of many Photonics West conferences and says that the event is much more than an opportunity to catch up on the latest missions and research from NASA and other organizations: “Nearly every conference I have attended has led to long-term discussions or formal collaborations with other researchers that have been immensely valuable.”

James Osborn of University of Durham, UK — who attended the event for the first time this year — is hopeful he too can forge partnerships and collaborations. He used to design and build instruments for large astronomical telescopes, including adaptive optics systems. “Now we’re using that technology and knowledge, and moving it to other fields like laser communications,” he says.

Osborn’s team conducts atmospheric turbulence measurements using SHIMM in London’s financial district. Credit: James Osborn

Osborn and his group gave five talks detailing results from various projects to model, monitor, and mitigate atmospheric turbulence for FSO systems. Central to most of these projects is the Shack-Hartmann Image Motion Monitor (SHIMM) — a unique portable instrument developed by the group and capable of continuous vertical turbulence monitoring 24 hours a day. “I really want to showcase SHIMM at Photonics West and find out if other people have some need for this instrument,” Osborn says. “Maybe we can build some collaborations or share data.”

One of Osborn’s PhD students — Lily Westerby-Griffin — presented results from a project where she used SHIMM to model urban turbulence. “A lot of turbulence has been simulated and measured in astronomical contexts, with perfect conditions,” she says. “We took some turbulence measurements in London’s financial district and put them into a simulation to figure out what an optical link would look like if you were to put it in a city center.” The results should help improve the performance of optical uplink and downlink to satellites in urban environments.

Osborn, meanwhile, focused his main presentation on turbulence forecasting. He has developed a tool that marries meteorological data with turbulence models (validated with SHIMM) to predict turbulence strength anywhere in the world. From this, Osborn has created global maps of optical turbulence parameters, temporal sequences, and detailed analyses at specific sites. “The tool gives us site statistics anywhere in the world, which helps us decide where we’re going to place ground stations, and it enables us to optimize the design of the ground station,” he explains. “The other thing we can do is use this forecasting tool to make our networks more resilient to atmospheric turbulence.”

With these and many other advances on show at Photonics West, the future is looking bright for FSO to meet our ever-increasing demand for data.

Ben Skuse is a freelance writer and editor of all things science. A version of this article appeared in the 2023 Photonics West Show Daily.