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FSO Frequently Asked Questions - FAQ Fog and Free Space Optics Free Space Optics uses the infrared portion of the electromagnetic spectrum between visible light and microwaves
With water droplets, the worst attenuation is where droplet size is similar to the wavelength of the infrared radiation - which happens to be that of fog. In very thick fogs, extremely high attenuation is experienced, exceeding 100dB/km. To put that in perspective, a practical FSO system may have a fade margin of 20 - 30 dB when deployed; which would imply deployment at under 500m in these conditions.
Fortunately, extremely thick fog is quite rare, and some FSO vendors have advanced planning tools which use weather databases (over 2,700 measurement points worldwide) to predict the reliable distances for almost any city in the world. For deployment above these distances, microwave backup is used to ensure uninterrupted data transport.
In such regions, microwave systems operating above 18GHz are so badly affected that the bands are virtually unusable. So the relative immunity of FSO to 'rain-fade' makes it an attractive option in high-rainfall regions. To find out more, please visit:
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Heavy
rain, found in the tropics, such as Malaysia, also attenuates the
infrared. However, practical measurements have demonstrated that at
moderate distances (1km) FSO transmission is still uninterrupted.
