Optical directional radio

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An 8-beam FSO device that achieves a data rate of 1 Gb / s over approximately 2 km. The large lens in the middle belongs to the receiver, the smaller one on the outside of the transmitter.

Optical radio relay , also known as optical free space (data) transmission , laser link or optical free space communication (FSO for short, from English free-space optical communication ), is data transmission with an unguided light or infrared beam. Distances of a few 100 m to a few kilometers on earth and up to thousands of kilometers in space are bridged. The data are, for example, voice , video signals or digital information.

The term “optical directional radio” was coined because it is very similar to directional radio , which is based on the quasi-optical propagation of radio waves of short wavelengths. The English term Free-Space Optics expresses that, in contrast to fiber optics, these are free-radiating transmitters.

Commercial FSO systems can reach distances of up to a few kilometers with data rates of up to 2.5  Gbit / s .

Optical free space transmission can be used wherever high bit rate connections are required and fiber optic cables are not available or are too expensive.

Applications

Application example for optical directional radio

history

As early as 1880, Alexander Graham Bell applied for a patent for the photophone for transmitting speech using light. However, this development did not catch on because of the boom in electrical telephony.

The German Wehrmacht developed a so-called light communication device , built by Carl Zeiss Jena , and used a light communication device 80/80 primarily in fastening devices, for example for directional transmission on the Atlantic Wall . The units of the Ministry for State Security of the GDR also used their own light telephone in the border area. In both cases, the short range was compensated for by the advantage of security against eavesdropping.

From around 1960 there were handicraft instructions for light telephones with which voice transmission was possible up to about 100 m. Incandescent lamps were used as transmitters. With the development of laser technology in the mid-1960s, the first serious attempts were made with light telephones. These developments were particularly encouraged in the military sector. With the development of more powerful glass fibers for optical data transmission , optical directional radio faded into the background again. However , this development activity was never stopped for the military and space exploration . This is due to a number of advantageous technical properties of FSO, which have also turned out to be interesting for civil use in recent years.

The development of inexpensive laser diodes also advanced the development, as they provide high radiation powers efficiently and easily bundled and can be very easily modulated with very high bandwidths.

Technical characteristics

The free space data transmission with light requires a clear line of sight between the transmitter and receiver. These are point-to-point connections. Both on the transmitter and on the receiver side there are bundling elements (mirrors or lenses) with the transmitter or receiver in their focus. Light-emitting diodes , laser diodes , lasers or semiconductor lasers are used as transmitters . The collimation is done with lenses or mirrors. Photodiodes serve as receivers . In principle, components can be used that are also used for optical data transmission in glass fibers, but transmitters in the visible spectral range are often used to make adjustment easier.

In the case of free space data transmission with light, there are the following interferences on earth:

  • Air shimmer
  • Ambient light
  • Scattering and attenuation by precipitation, fog, smoke

While the ambient light can mostly be compensated, the flickering, but especially the scattering influence from aerosols and precipitation, limits the range or is a source of unreliability. The shorter the wavelength, the more disturbing the haze and fog. These influences affect optical radio relay systems to the effect that the signal is attenuated and / or the error rate in the transmission increases. To avoid these influences out of the way, various technical tricks are used by manufacturers, such as a " Diversity - Architecture " (multiple transmitters and multiple receivers at a distance) and enough "Fade Margin" (power reserve against weather-related signal attenuation ).

The services of the transmitters are limited for security reasons. The lasers should not pose any danger to humans or animals. Commercial systems usually comply with laser classes 1 and 1M, which do not require any safety measures when operating such systems.

With up- and downlinks to / from satellites or between satellites, higher powers are possible and because of the lack of scattering and absorption in space as well as the good focusing of the lasers, very large distances can be bridged. For uplink and downlink, the wavelengths used must lie in an atmospheric window . Compared to radio connections, data rates up to 1000 times higher are possible, whereby the optical systems are more compact.

Advantages and disadvantages

View into the light beam (amateur project RONJA , collimated red LED)

The main advantages over directional radio are:

Directional radio and optical free space transmission have the common advantage over fiber optic cables that the investment is not buried in the ground, so it can be used elsewhere if necessary.

Compared to terrestrial radio relay, the optical transmission path in air near the ground has the following disadvantages:

  • greater dependence on precipitation, haze and fog
  • possible glare effect when using visible wavelengths

The failure of the connections in fog, snow and rain is a major reason why the method is not becoming more widely accepted.

See also

literature

  • Olivier Bouchet, Herve Sizun, Christian Boisrobert: Free-Space Optics. Propagation and Communication . Iste Publishing Company, London a. a. 2006, ISBN 1-905209-02-9 .
  • Hamid Hemmati (Ed.): Deep Space Optical Communications . John Wiley & Sons, Hoboken NJ 2006, ISBN 0-470-04002-5 .
  • Arun K. Majumdar, Jennifer C. Ricklin (Eds.): Free-Space Laser Communications. Principles and Advances (=  Optical and Fiber Communications Reports . Vol. 2). Springer, New York NY a. a. 2008, ISBN 978-0-387-28652-5 .
  • Heinz Willebrand, Baksheesh S. Ghuman: Optical directional radio. Optical transmission of free space in public and private networks. Hüthig, Heidelberg 2003, ISBN 3-7785-3967-1 .
  • S. Heissmeyer; L. Overmeyer; A. Müller: Indoor Positioning of Vehicles using an Active Optical Infrastructure . In: 3rd International Conference on Indoor Positioning and Indoor Navigation (IPIN) . Sydney 2012, ISBN 978-1-4673-1955-3 , pp. 1–8 , doi : 10.1109 / IPIN.2012.6418914 .
  • Regulation D 877/5, instructions for use for light communication device 80/80 mm, 1944

Web links

Individual evidence

  1. see photos on private website: Lichtsprechgerät 1 , Lichtsprechgerät 2 ( Memento of the original from July 24, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. , Light talk device 2 ( Memento of the original from March 2, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.laud.no @1@ 2Template: Webachiv / IABot / www.laud.no
  2. Infrared speakerphone JO-4.03 "Palme". In: German Spy Museum. Accessed June 2, 2020 (German).
  3. https://technologieforum.badw.de/fileadmin/user_upload/Files/Technologie/Praesentationen/2016-Guenther-Satellitenkommunikation-mit-Licht.pdf C. Günther, C. Fuchs, D. Giggenbach, F. Moll, R. Mata Calvo, J. Poliak, R. Barrios, C. Schmidt: Satellite communication with light - at the highest data rates and perfect security , Powerpoint presentation German Research Center for Aerospace