Free-space optical communication
When called optical radio relay, and free-space optical (data) transfer, laser link or free-space optical communication (English free -space optical communication, FSO ) is a technology for transmitting usually digital data by means of light. The data signal may include, but voice or video signals.
Since current systems operate in similar radio relay point-to -point operation, has impressed itself in the German-speaking countries, the term space optics. The English term Free-Space Optics is more appropriate because of the lack of reference to wireless technology within an appropriate context.
Commercial FSO systems achieve distances up to several kilometers at data rates up to 2.5 Gbit / s (STM -16, a network protocol from the SDH world). Free-Space Optics can be used everywhere, where high bit-rate connections are needed and fiber optic cables are not available or too expensive.
In addition, there are systems with lower transmission ranges (in the range of several meters ) based on LED technology that have been developed, for example, for applications in industrial logistics environment.
Applications
- LAN-to- LAN connections on industrial premises (Fast Ethernet, Gigabit Ethernet)
- LAN-to- LAN connections in a city
- Overcoming of roads and obstacles (for example, roads and rivers )
- Quick -deploy broadband access and metro networks of telecom operators ( carriers )
- Temporary network expansion
- Increase the transmission reliability by additional connecting means FSO (redundancy)
- Combined voice and data connections,
- Used to restore disturbed or decomposed compounds ( Disaster Recovery )
- Use for connecting networks and digipeater in amateur radio
- Waiver of renting a dedicated line of a telecommunications provider (see: last mile ).
- Communication between satellites and satellites and ground stations (see: Laser Communication Terminal )
- Industrial logistics environment: Optical data transmission can be used for determining the position of internal transport systems, such as floor-bound discontinuous conveyors such as forklifts. This post local beacons using LED information about their location to a receiving unit on the truck.
Since the beam is relatively small, this type of wireless point-to -point connections is relatively eavesdropping.
History
As early as 1880, Alexander Graham Bell has filed the Photo Phone for transmission of speech by means of light for a patent. This trend, however, due to the boom in electric telephony not.
The German Wehrmacht developed a so-called light -talkie, built by Carl Zeiss Jena, and put a light talkie 80/80, especially in fasteners, so the directional transmission on the Atlantic Wall, a. The units of the Ministry for State Security of the GDR also put a light of its own telephone in the border area a .. In both cases, the short range was balanced by the advantage of security against eavesdropping.
From about 1960 there were DIY instructions for light phones that voice transmission to about 100 m was possible. When the transmitter light bulbs were used. With the development of laser technology, the mid-1960s first serious attempts have been made with light phones. Particularly in the military field, these developments were promoted. With the development of more powerful glass fibers for optical data transmission of the optical microwave fell into the background. For the military and space research, this development activity, however, was never set. This is due in a number of advantageous technical properties of FSO, which turned out in recent years also for civilian use than interesting.
Significant influence was the development of inexpensive diode lasers that can be modulated at very high bandwidths also very simple.
Technological properties
When transmitting data with light through free space there are various influences in the atmosphere, which can be overcome technologically, such as:
- Heat shimmer
- Ambient light
- Shading
- Weathering
- Pollution, for example, Smoke.
These factors have an influence on optical radio relay systems in terms of the signal is attenuated and / or the error rate increases in the transmission. Order to avoid these influences out of the way, various technical tricks are applied by the manufacturer, such as a "diversity " architecture ( multiple transmitters and multiple receivers within a certain distance ) and enough "Fade Margin" ( power reserve against weather-related signal attenuation ). The possible power reserves are restricted for security reasons. Such laser systems should pose no danger to humans and animals. Commercial systems usually keep a laser classes 1 and 1M, which do not require safety measures during the operation of such plants.
Pros and Cons
The main advantages are:
- Very rapid build-up
- Royalty-free operation
- In the narrow light beam reasoned high data security
- High bit rates ( see also: data throughput )
- No relevant influence of the Fresnel zone
- Lesser influence of rain and snow on the transmission
- Full -duplex transmission
- Network protocol transparency
- No interference problems
- Favorable EMC / EMF behavior
In contrast, microwave radio relay also has strengths that often support large distances in professional solutions and have a lower attenuation in fog. Both technologies have the common advantage compared to fiber optic cable, that the investment is not buried in the earth, so it can also be used for other purposes when necessary ( for example, when moving house ).
The main disadvantages are:
- Problems during the approach in aviation
- Problems in the field of stellar cartography.
In certain areas, green lasers are problematic eg in aviation, or Stellar Cartography. Tests in the field of amateur radio have shown that these effects eg can be minimized by the use of green light rather than infrared.