Free-Space Optics (FSO) carries data on a modulated laser or LED beam through the atmosphere instead of a fibre or radio channel. For engineers it offers fibre-class bandwidth (Gbps and beyond), an unlicensed spectrum, strong physical-layer security from the narrow beam, and rapid deployment where trenching fibre is impractical. The trade-off is that the same atmosphere that makes FSO licence-free also makes link availability the central design problem.
The FSO link budget
Every FSO design starts with a power budget. The received power must exceed the receiver sensitivity by a margin that absorbs all losses and fades:
- Geometric (beam-spread) loss — the beam diverges with range; only a fraction of the cone is captured by the receive aperture.
- Atmospheric attenuation — molecular and aerosol absorption/scattering, dominated by fog and haze. Dense fog can exceed 100 dB/km, far worse than rain.
- Pointing and coupling losses — misalignment, building sway, and (for single-mode receivers) fibre-coupling loss.
- System margin — the headroom reserved for turbulence-induced fading and weather, set by the availability target.
Atmospheric turbulence and scintillation
Refractive-index turbulence (characterised by the structure parameter Cn2) causes the received intensity to fluctuate — scintillation — and the beam to wander and break up. The result is bursty fading and a higher bit-error rate, especially over long, near-ground, hot paths. Designing for the worst-case turbulence regime, not the clear-day average, is what separates a link that meets its SLA from one that drops at noon.
Designing for availability
FSO availability is usually quoted against the local fog/visibility statistics. Practical techniques to hit "carrier-grade" targets include:
- Shorter hops and larger receive apertures (aperture averaging reduces scintillation).
- Multi-beam transmitters and spatial diversity receivers.
- Automatic transmit-power control and adaptive coding/modulation.
- An RF backup (hybrid FSO/mmWave) that takes over in dense fog — the single most effective way to reach five-nines.
Where FSO fits
Typical deployments: enterprise/campus building-to-building links, 5G fronthaul/backhaul, disaster-recovery and temporary links, and inter-satellite and ground-to-satellite optical links where there is no atmosphere to fight. If you are evaluating a specific route, the link budget and turbulence assumptions should be modelled before hardware is specified.
Frequently asked questions
Is FSO affected by rain like radio links?
Rain attenuates FSO far less than fog. Because optical wavelengths are much smaller than raindrops, the dominant atmospheric impairment for FSO is fog and low visibility, whereas heavy rain is the bigger problem for high-band RF/mmWave — which is partly why hybrid FSO/RF links are so robust.
How far can an FSO link reach?
Most commercial terrestrial links target a few hundred metres to a few kilometres at high availability; range is traded against availability, aperture size, and transmit power. Free-space optical links between satellites operate over thousands of kilometres because there is no atmospheric loss.
Do I need a spectrum licence?
No. Optical beams fall outside licensed RF spectrum, which removes licensing cost and delay — a key reason FSO is attractive for fast deployment.
SCOPT designs and models FSO links end to end. Explore our analysis & optimization and simulation tools, or talk to our consultants about your route.