Free-Space Optics / Lasercomm

Free-space optical communications (FSO or Lasercomm in short), also called optical wireless communications (OWC) is becoming more and more important in communications, both in the commercial as well as the military sector. In the past, there were primary short-range fixed systems that took advantage of setting up high bandwidth connectivity under fibre-less infrastructure conditions. Now, the focus is shifting to long-range and mobile systems.

Optical wireless can provide an alternative or complement to directional microwave systems with the benefits of optics such as highest data-rates, power efficiency and covert communications with the benefits of no frequency interference or regulation problems. In this way, FSO can overcome today's radio-frequency spectrum and data-rate limitations at lower transmit powers and using smaller communication terminals.

Nevertheless challenges remain, e.g. the requirement for a highly accurate alignment of the transceivers. Further, optical propagation through the atmosphere experiences random variations in phase and amplitude. This is due to turbulence effects caused by fluctuations of the refractive index of the propagation medium as the latter experiences temperature gradients due to solar heating and wind. Models assume that the medium can be described as eddies of different diameter and refractive indices. In the context of geometrical optics, theses eddies may be thought of as lenses randomly refracting the optical beam. This produces a distorted, time and space dependent intensity profile of the beam. The intensity fluctuations (called scintillations) result in a fading of the received power at the receiver which can significantly reduce the transmission quality. The fading time constant is typically in the range of milliseconds while data-rates typically higher than 100 Mb/s are used.

Such fades can produce very long burst errors which can not be rectified effectively by commercial off-the-shelf error protection schemes. Special error correction methods, such as transport layer coding, must be applied in order to correct these fades. For short range links, atmospheric fading can also be overcome by a large link margin. Nevertheless for long range links, where the maximum transmit power is limited by available technology or eye-safety constraints, other methods such as error protection coding or diversity schemes must be applied. Hence, new methods are currently under development in order to bring short range technology into the market which is applicable for long range links.