Optical Communication

Laser beams could soon become a viable alternative to radio waves for the transmission of large quantities of data over long distances through space. High data rates combined with little power consumption and low payload weight make laser communication terminals particularly interesting for application on-board satellites, space telescopes and scientific space probes. The first satellites equipped with laser communication terminals are already orbiting the Earth and more will follow in the coming years.

Laser-based data transmission has several advantages over conventional radio links. Due to the shorter wavelength, lasers can achieve higher data rates than radio signals for the same given aperture. Laser beams are inherently less divergent than radio signals and, therefore, require less power for data transmission. In addition, due to the higher efficiency and the low beam divergence of the laser beam, the laser link is a secure point-to-point connection; a bugging device would have to be in the immediate vicinity of the receiver, or would even have to be introduced into the beam, and this would terminate the connection immediately.

These advantages are particularly useful in space applications. Lasers would be effective when large quantities of data need to be transmitted between satellites. In addition, lasers would be highly functional when data has to be transmitted across vast distances. Among many other applications, laser communication is currently being considered for:

• Data relay services for unmanned aerial vehicles (UAV): UAV inspecting remote areas can send their observation data to a data relay satellite in geostationary (GEO) orbit via an optical link.
• Data relay services for satellites: High-speed laser communication can be used to replace an expensive network of ground stations needed to constantly receive low earth orbiting (LEO) satellites` data. The data gathered by the LEO satellites can by transmitted to a relay satellite in GEO orbit by means of laser communication. The relay satellite then transmits the data to a single ground station thus offering cost savings in operations and infrastructure.
• Inter-satellite links between GEO satellites can be used to share resources and/or route traffic around a satellite network. They are also of interest for intra-continental communications (e.g. between satellites providing services throughout Europe with satellites providing services to Western Europe being linked to satellites providing services to Eastern Europe) and inter-continental links (e.g. between satellites providing services in Europe linked to satellites providing services to the US and / or Asia Pacific rim).
• Deep space data transmissions: The amount of data being collected on exploration missions, such as those to Mars, and are increasing and will soon become limited by RF capacity. This increase may require on-board data processing and coding be introduced, with the resulting loss of access to the raw scientific data. In addition, an increase in the long data transmission times increases operation costs and severely reduces the time available for scientific tasks. By using optical links, the data rate can be dramatically increased, thereby allowing the raw scientific data to be received and resulting in the increased scientific value of future missions.