Free Space Optics for Access Networks
Optical signals are moving deeper and deeper into access networks. Achieving the ambitious performance goals of 5G architectures requires more optics than ever between small cell sites. As stated in a recent report by Deloitte, “extending fiber optics deeper into remote communities is a critical economic driver, promoting competition, increasing connectivity for the rural and underserved, and supporting densification for wireless.”
However, there are cases in which fiber is not cost-effective to deploy. For example, a network carrier might need to quickly increase their access network capacity for a big festival, and there is no point in deploying extra fiber. In many remote areas, the customer base is so small that the costly deployment of fiber will not produce a return on investment. These situations must be addressed with some kind of wireless access solution. Carriers have used fixed microwave links for the longest time to handle these situations.
However, radio microwave frequencies might not be enough as the world demands greater internet speeds and simply changing over to higher carrier frequencies will limit the reach of microwave links. On top of that, the radio spectrum is quite crowded, and a carrier might not have the available licensed spectrum to deploy this wireless link. Besides, microwave point-to-point links produce plenty of heat while struggling to deliver capacity beyond a few Gbps. This is where free-space optics (FSO) comes into play.
FSO is a relatively straightforward technology to explain. A high-power laser source converts data into laser pulses and sends them through a lens system and into the atmosphere. The laser travels to the other side of the link and goes through a receiver lens system and a high-sensitivity photodetector converts those laser pulses back into electronic data that can be processed. In other words, instead of using an optical fiber as a medium to transmit the laser pulses, FSO uses air as a medium. The laser typically operates at an infrared wavelength of 1550nm that is safer on the eye.
FSO has often been talked about as some futuristic technology to be used in space applications, but it can be used more than that, including ground-to-ground links in access networks.FSO can deliver a wireless access solution that can be deployed quickly and with more bandwidth capacity, security features, and less power consumption than traditional point-to-point microwave links. Furthermore, since it does not use the RF spectrum, there is no need to secure spectrum licenses.
Overcoming the challenges of alignment, and atmospheric turbulence
FSO struggled to break through into practical applications despite these benefits because of certain technical challenges. Communications infrastructure, therefore, focused on more stable transmission alternatives such as optical fiber and RF signals. However, research and innovation over the last few decades are removing these technical barriers. One obstacle to achieving longer distances with FSO had to do with the quality of the laser signal.
Over time, FSO developers have found a solution to this issue in adaptive optics systems. These systems compensate for distortions in the beam by using an active optical element—such as a deformable mirror or liquid crystal—that dynamically changes its structure depending on the shape of the laser beam. Dutch startup Aircision uses this kind of technology in its FSO systems to increase their tolerance to atmospheric disruptions.
Another drawback of FSO is aligning the transmitter and receiver units. Laser beams are extremely narrow, and if the beam doesn’t hit the receiver lens at just the right angle, the information may be lost. The system requires almost perfect alignment, and it must maintain this alignment even when there are small changes in the beam trajectory due to wind or atmospheric disturbances.
FSO systems can handle these alignment issues with fast steering mirror (FSM) technology. These mirrors are driven with electrical signals and are fast, compact, and accurate enough to compensate the disturbances in the beam trajectory. However, even if the system can maintain the beam trajectory and shape, atmospheric turbulence can still degrade the message and cause interference in the data. Fortunately, FSO developers also use sophisticated digital signal processing techniques (DSP) to compensate for these impairments.
These DSP techniques allow for reliable, high-capacity, quick deployments even through thick clouds and fog. FSO links can now handle Gbps capacity over several kilometers thanks to all these technological advances. For example, a collaboration between Aircision and TNO demonstrated in 2021 that their FSO systems could reliably transmit 10 Gbps over 2.5 km. Aircision’s Scientific Director John Reid explained, “it’s an important milestone to show we can outperform microwave E-band antennas and provide a realistic solution for the upcoming 5G system.”
An alternative for safe, private networks
An understated benefit of FSO is that, from a physics perspective, they are arguably the most secure form of wireless communication available today. Point-to-point microwave links transmit a far more directional beam than mobile antennas or WiFi systems, which reduces the potential for security breaches. However, even these narrower microwave beams are still spread out enough to cover a wide footprint vulnerable to eavesdropping and jamming.
At a 1km distance, the beam can spread out enough to cover roughly the length of a building, and at 5km, it could cover an entire city block. Furthermore, microwave systems have side- and back lobes radiating away from the intended direction of transmission that can be intercepted too. Finally, if an attacker is close enough to the source, even the reflected energy from buildings can be used to intercept the signal.
Laser beams in FSO are so narrow and focused that they do not have to deal with these issues. At 1km, a typical laser beam only spreads out about 2 meters, and at 5km, only about 5 meters. There are no side and back lobes to worry about and no near-zone reflections. The beam is so narrow that intercepting the transmission becomes an enormous challenge. An intruder would have to get within inches of a terminal or the line of sight, making it easier to get discovered. To complicate things further, the intruder’s terminal would also need to be very well aligned to pick up enough of a signal.
Using Highly-Integrated Transceivers in Free Space Optics
Even though fiber optical communications drove the push for smaller and more efficient optical transceivers, this progress also has a beneficial impact on FSO. As we have explained in previous articles, optical transmission systems have been miniaturized from big, expensive line cards to small, affordable pluggables the size of a large USB stick. These compact transceivers with highly integrated optics and electronics have shorter interconnections, fewer losses, and more elements per chip area. These features all led to a reduced power consumption over the last decade. At EFFECT Photonics, we achieve even further efficiency gains by an optical System-On-Chip (SoC) that integrates all photonic functions on a single chip, including lasers and amplifiers.
FSO systems can now take advantage of affordable, low-power transceivers to transmit and receive laser signals in the air. For example, a transceiver based on an optical SoC can output a higher power into the FSO system. By using this higher laser power, the FSO does not need to amplify the signal so much before transmitting it, improving its noise profile. Furthermore, this benefit happens with both direct detect and coherent transceivers. This is a key reason why Aircision has partnered up with EFFECT Photonics to create both direct detect and coherent free-space optical systems, since the startup ultimately aims to reach transmission speeds of 100 Gbps over the air.
FSO has moved from the domain of science fiction to a practical technology that now deserves a place in access networks. FSO can deliver a wireless access solution that can be deployed quickly and with more bandwidth capacity, security features, and less power consumption than traditional point-to-point microwave links. Furthermore, since it does not use the RF spectrum, it is unnecessary to secure spectrum licenses. Affordable direct detect and coherent transceivers based on SoC can further improve the quality and affordability of FSO transmission.