Free space optics works like the wireless radio connection. The communication is between two points via infrared or invisible (red) light ray on the distance of houndreds of metres up to kilometres. The speed can be up to 1Gbps. Free space optics are always in pair, on the both sides of the connection is one device.
The base of the free space optics is the optical trasnmitter and transciever. In the optical transmitter the laser diode changes the electrical signal on the light signal and sends the data in form of modulated optical ray. Then the optical rays are via optical condensed to the thin light ray with the minimal divergence. The infrared light isn’t visible by the eye, we use the wave-lenght 850 nm, 905 nm, 1200 nm or 1550 nm. In one free sace optics can be used one or more transmitters so that we can increase the communication distance or the reliability of the data transfer. The optical achievement of one laser in the transmitter is in the interval of 5 mW up to 50 mW.
Modulated ray coming from the transmitter comes through the free area and after some hundreds of metres or kilometres strikes upon the transciever. In the transciever is the lens which condensse the ray on the photodiode, which the modulated light ray changes back to the electrical signal.
Most of free space optics are connected in the network with the UTP cable or with the optical cable. Free space optics are against the current data networks transparent and becomes evident only by the increasing the value of the delay of the transferred data. The second way are the free space optics with the integrated ethernet switch and with the possibility of the long-distance monitoring and the administration through the data network.
Advantages of the free space optics against the radio connections:
Limitations of the free space optics against the radio connections:
Atmosferic influence for communication reliability
Free space optics are usually working without the problems in the worsened visibility. Free space optics have enough reserve of the achievement for the function in disfavourable conditions. The greater limitations of optical communication in the free area if the fog. The fog has unusually disfavourable influence on the total rundown of the transfer line, very fast descends maximally possible communication distance. The intense fog is the limitation of the free space optics during the communication on the distances higher than cca 100 m.
The possible time of fails of the optical line during one year can be a few hours and it depends on the local conditions. The fog is more often near the water places or in the valleys. The other condition is the time of the fails. The fog is more often in the early mornings and in the late evenings. In this time, the fails are not so critical and the charge of the network is not so high. These fails are often solved by the back-up radio connection.
In the picture on the right side is possible to see the example of common conditions and pea-souper. Authors of these photos made a measuring of the rundown on the optical line. On the picture is possible to see the optical connection illustrated with the yellow line. The pictures are made from the place where is located the first device. The second device is 2,4 km off. The second optical connection is on the big building – on the right side from the first optical connection – the building is 300 m off.
The first picture was made in the standard conditions which apply most of the time. The total rundown of the optical line is 6,5 dB/km. Both the free space optics works and has enough powerful reserve. On the second picture is the same situation in the forcible fog. The optical line has a rundown 150 dB/km. The optical connection with the communication distance 2,4 km is not working, the optical signal is fully absorbed by the fog. The second connection with the transfer distance 300 m is still working but it’s using maximally powerful reserve. On the third picture is the more powerful fog. The optical line has a rundown 220 dB/km. The forst connection is not working. The second line is not working too, the powerful reserve is empty. The optical signal if fully absorbed by the fog and it’s under the minimal possible detect level. In this unusually forcible fog (rundown 220 dB/km) is maximally optical communication distance cca 50 m up to 100 m.