FSO and Fog FAQs
Why Fog is critical for Free Space Optics links
Free Space Optics (FSO, Optical Wireless, Laser, Infrared) uses the infrared portion of the electromagnetic spectrum between visible light and microwaves
FSO and Atmospheric Effects
Atmospheric effects such as fog, snow, rain, smog, sand-storms and scintillation (thermal shimmer) all cause attenuation of the infrared signal as it passes through the atmosphere. With the exception of scintillation, the attenuation is caused by particles, or droplets either absorbing the radiation, or reflecting it away from the transmission path.
Effect on Short and Long links
For short links (under 200-500m in most countries), there is no real effect of atmospheric effects on a practical Free Space Optics link. The attenuation is less than the available Fade Margin of the FSO link. Some vendors use Automatic Transmit Power Control (ATPC) to compensate for the increased attenuation by increasing the transmit power during “fade” events.
For longer links (500m up to 4km or more), dense / thick fog may cause serious link attenuation, greater than the available fade margin. These FSO links therefore may suffer brief or longer outages during high fade events.
Diverse Routing & Alternative Media overcomes Fog Attenuation
Resilient Technologies using diverse routes such as Radio, Microwave or Millimeter Wave (MMW) can be used to ensure the total communication link remains uninterrupted during these conditions. CableFree has pioneered “UNITY” platforms to ensure highest uptime, distance and availability by combining FSO with either radio or MMW.
Fog and Rainfall Comparison for Free Space Optics
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.
The following picture is a street in Oxford, UK, on a typical winter morning. Light fog/mist over a country road.
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.
Different types of fog
There are many different types of fog, which are outlined below:
- Radiation fog: Forms when land cools after sunset through thermal radiation under calm, clear skies, causing condensation in nearby air via heat conduction. It is sometimes less than a metre deep but can thicken with turbulence. Common in autumn and early winter, it usually dissipates after sunrise but may persist all day in winter, especially in areas surrounded by high ground.
- Ground fog: Obscures less than 60% of the sky, and does not reach overhead clouds. Often synonymous with shallow radiation fog, it can be as thin as tens of centimetres over specific terrains in windless conditions.
- Advection fog: Forms when moist air moves over a cool surface (e.g. the ocean, areas with snow-pack) via wind, cooling the air to its dew point. Common along the Californian coast due to cold water upwelling, it can be driven onshore by cold fronts, summer low-pressure troughs, or southerly surges. Turbulence may disrupt it, transforming the fog into stratocumulus clouds.
- Evaporation (steam) fog: Occurs over water bodies when cold air overlies warmer water, causing evaporation and condensation. Lake effect fog is this type. it is convective, resulting in denser, deeper fog than stratiform fog. It may produce steam devils.
- Ice fog: Forms in very low temperatures, often from moisture released by animals. It is associated with diamond dust – tiny ice crystals that fall slowly, creating halos under clear skies due to sunlight refraction.
- Freezing fog: Consists of supercooled water droplets that freeze on contact with surfaces, forming rime.
- Precipitation (frontal) fog: Develops when precipitation falls into drier air, evaporation into water vapour that cools and condenses at the dew point.
- Hail fog: Localised, dense fog forming after significant hail accumulation cools the air and increases moisture in a shallow surface layer, often under light winds and humid conditions.
- Upslope fog: Occurs when moist air ascends a mountain or hill, and forms fog due to adiabatic cooling and the drop in pressure with altitude.
Advection fog layer in San Francisco
Rainfall attenuation and Free Space Optics
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.
FSO Units have excellent performance in areas of high rainfall 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 Free Space Optics an attractive option in high-rainfall regions.
For More Information
For more information on Fog, Weather and Free Space Optics link reliability and distance please Contact Us
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