High speed communications are the backbone of most modern economies. The faster data can be sent to the right place accurately, the better it is not only for the end-user of that data, but also the company that supplies the infrastructure that allows it to happen.
Therefore, lots of effort has been put into optimizing communication networks like Wi-Fi, Ethernet, and Bluetooth. But there’s an alternative wireless tech that blows all of those existing protocols out of the water.
Free-space optical (FSO) communication uses lasers to send data wirelessly at speeds up to 100 times faster than existing radio frequency based wireless systems, and even 40% faster than fiber optic cables. While there are still some technical challenges facing the widespread adoption of the technology, it is poised to make a huge splash on every industry that uses communications soon.
Free-space optical systems
FSO systems are typically made up of a high power optical source, like a laser diode. In some cases, where the information doesn't have to pass as far a standard LED would also work. After the light is generated, it passes through a modulator, which imprints the 0s and 1s of binary language onto the beam, either by rapidly turning it on and off or modulating its phase. Next is a series of mirrors, similar to a telescope, that focuses the beam together to transmit it as a very narrow, tight beam to a receiver on the other end.
That receiver typically consists of an optical filter to block other light sources, like that coming from the Sun. After passing through the filter, the light is picked up by a highly sensitive photodetector, such as an avalanche photodiode, which converts the light signal back into an electrical signal. That electrical signal is then amplified, processed and demodulated to translate it back into the binary 1s and 0s that made up the original digital signal.
Typically, these systems operate in the near-infrared part of the spectrum, especially those that fall into “transmission windows” where the atmosphere the signals travel through is relatively clear. In practice, that means FSO systems operate at a frequency up to 1,000 times faster than the traditional GHz frequencies used in radio frequency communication. That speed difference is the enabling factor for FSO's much higher data bandwidth.
Another feature benefit it has over RF signals is its extremely narrow beam. The series of mirrors and other accessories in the FSO system allows it to concentrate a high amount of power on a very precise target, even over long distances. Compare this to RF protocols, which send signals all over the place, and which also results in a very high amount of energy transmitting signals that will never be used. FSOs also have the advantage of being much harder to hack or block, since any malicious actor would have to directly interdict the line of sight between the transmitter and receiver, which requires knowing where both are. But even if they do manage to interpose themselves, it is still obvious that the communication line has been interrupted, so the transmitter can immediately stop sending onto that channel, ensuring the integrity of the data it's trying to transmit. Unfortunately, this added benefit also ties into one of technology’s biggest engineering challenges.
Wireless challenges
There are two main challenges that face FSO systems today: dealing with weather, and “pointing.” Weather is a common problem for most wireless communication systems. The classic problem is what happens when a satellite signal is lost due to a rainstorm, or signals in a mesh network are degraded due to fog in the morning. With FSO, these problems are exacerbated, as even something as mundane as a shift in the atmosphere can potentially cause a complete disconnect from the communications channel. This issue is caused by a phenomenon known as Mie scattering, which attenuates the laser signal and increases the error rate exponentially as obstacles are placed in the line-of-sight path between the transmitter and receiver. They can also be blocked entirely by a physical object such as a truck or even a bird flying in between them.
An even more difficult challenge is that of “pointing” — for instance, how the transmitter and receiver maintain connection if one of them is moving. The narrow beam application of FSO requires microradians (millionths of a degree) or precision accuracy to maintain the highest bandwidths possible. With moving targets over far distances, such as communications links from satellites or drones to one another, or even to ground stations, this becomes exceptionally difficult. To combat this issue, there is a specialization in FSO system development known as Pointing, Acquisition, and Tracking (PAT) that attempts to maintain a connection even with moving platforms, atmospheric interference or something as simple as someone walking in front of an antenna.
A stack of 60 Starlink test satellites atop a Falcon 9 rocket, close to entering orbit. Starlink is one of numerous companies launching laser communications into space to accelerate wireless data services. Source: CC0 1.0 Universal
Mitigation strategies
The atmospheric challenges facing FSO are shared by another discipline — astronomy. And a potential solution to it appears to come from astronomy as well. In astronomy, the atmospheric scattering the beam undergoes is seen as a rapid fluctuation in the intensity of a received signal — in essence it's what makes stars “twinkle” to a casual observer. Technically known as “scintillation,” this problem has been overcome with a technology known as adaptive optics, which uses deformable mirrors to correct for any atmospheric interference. Ultimately, using this technique in FSO dramatically improves signal quality and stability of communication links.
To solve the PAT challenge, engineers have developed several coping methods. First is using advanced technical hardware, such as fast-steering mirrors and voice-coil motor-based optical beam stabilizers, both of which do their best to track the receiver from the transmitter’s end. On the receiving end itself, the demodulating software can use forward error correcting codes to try to parse out what bits were either received in error or not received at all, as well as fixing them.
One ultimate solution is the development of a hybrid system, which can switch between FSO and traditional RF signal paths depending on the bandwidth and optimization of each at any given time. This technique still reaps the rewards of the faster FSO system, but only if the link is stable. It also mitigates the downsides of what happens if the link isn’t by reverting to more traditional RF systems.
Future space
One of the driving forces behind the development of FSO hardware and systems are space exploration organizations like NASA and SpaceX. SpaceX would love to have an FSO for its Starlink satellite constellation, which involves tens of thousands of individual operating satellites, all of which must talk to one another to be most effective. The U.S.’s Space Development Agency is also looking to leverage satellites in low Earth orbit with operational FSO systems to provide support to deployed warfighters and resilient communications links between satellites.
While it still might be a while before FSO systems become ubiquitous in everyday wireless communication networks, their advantages make them prime targets for further development and derisking. As new algorithms and hardware are developed, and the FSO networks themselves are further tested, we are moving toward an optical communication future with much higher wireless data rates than the early internet pioneers would have ever thought possible — it's likely just a matter of time.
About the author
Andy Tomaswick is an engineer and freelance writer who is passionate about education, space exploration and making the world better through technology. When not engineering or writing something, he spends time with his family or running in circles and throwing plastic discs at people to stay in shape.
