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.
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.
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