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free space optic (FSO)

In contrast to radio directional radio, optical directional radio works with laser beams instead of radio frequencies. As a result, it has an almost unlimited bandwidth. Optical directional radio or optical free- space transmission, Free Space Optic (FSO) or Free Space Photonics( FSP), is also known as laser link.

The structure of optical systems is the same as that of radio systems. An optical microwave link consists of two stations or radiation heads, the outdoor units, each containing an optical transmitter and optical receiver. On the receiving side, the infrared radiation is bundled in a lens and fed to the O/E converter, which converts it into electrical signals. The photodiode is preceded by a bandpass filter which suppresses the interfering daylight.

Radiation head of an optical point-to-point link, photo: axess-pro networks

Radiation head of an optical point-to-point link, photo: axess-pro networks

The data rates of this optical point-to-point link are several gigabits per second and at distances of up to 4 km. In addition to point-to-point links, FSO systems can also be set up as multipoint links or in other topologies such as star top ology or ring topology.

Structure and transmission interference

FSO systems use power LEDs, but mostly lasers in the wavelength ranges between 780 nm and 900 nm and between 1,500 nm and 1,600 nm. The power is about 1 mW for LEDs and up to 100 mW for lasers. Accordingly, the distances that can be bridged also vary. Thus, simple LED systems reach up to 500 m, laser systems up to 10 km.

Characteristics of FSO systems

Characteristics of FSO systems

Optical directional radio works with line-of-sight and is affected by atmospheric disturbances - rain, fog, snow, thunderstorms - or bird flight; by air flicker, ambient light, weather conditions and environmental pollution. These influences attenuate the laser light, thereby reducing the length of the transmission rate and affecting the error rate. Free space attenuation ranges from 3 dB/km to 20 dB/km, depending on the weather. To avoid interruptions due to weather effects, FSO systems can be designed as multi- beam systems.

FSO technology is suitable for broadband point-to-point connections for LAN-LAN coupling between two buildings or areas. Since it works wirelessly, it can bridge roads and geological obstacles. It can also be used as an interesting alternative in the last mile, providing high- performance connections between the city or wide- area network and the end customer.

Englisch: free space optic - FSO
Updated at: 06.07.2018
#Words: 371
Links: indium (In), contrast, arsenic (As), bandwidth (BW), space (SP)
Translations: DE

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