Free-Space Optical Communications (FSO)
What is Free-Space Optics (FSO)?
Free-space optics (FSO) is a line-of-sight communication technology that sends data on a beam of light through open space: the air between two rooftops, the vacuum between satellites, or the path from a ground station to orbit. FSO carries data, voice, and video at several gigabits per second without fiber-optic cable and without a spectrum license, which makes it a practical alternative to radio relay links and a fast way to add optical bandwidth where laying fiber is slow or impossible.
780 to 1600 nmTypical operating wavelength band
300 m to 5 kmTypical terrestrial link, with 8 to 11 km possible
Multi-Gb/sUp to 100 Gb/s when paired with adaptive optics
No licenseNo spectrum license and no trenching
Conceptually, FSO is fiber-optic transmission with the glass taken away. Light from a laser or a light emitting diode (LED) is modulated with data through electrical-to-optical (E/O) conversion, sent across the free-space channel, collected by receiver optics, and converted back into an electrical signal (O/E conversion). Because light travels at almost its vacuum speed in air, roughly 1.5 times faster than in silica fiber, a free-space link also has a small latency advantage over an equivalent run of fiber.
The three building blocks of an FSO link
1
Transmitter
A laser or LED source, modulated with the data stream, launches the beam toward the receiver. Lasers dominate longer links because their narrow divergence keeps more power on target.
2
Free-space channel
The atmosphere absorbs and scatters part of the beam, following the Beer-Lambert law, while turbulent eddies driven by temperature variations distort and bend it. Clouds, fog, rain, smoke, gases, and aerosols all play a role.
3
Receiver
A telescope or lens collects the light and focuses it onto a photodetector or into an optical fiber. High-capacity receivers inject the light into single-mode fiber, which is exactly where turbulence does the most damage and where adaptive optics pays off.
FSO compared with fiber and RF links
| Free-space optics | Fiber | RF / microwave | |
|---|---|---|---|
| Medium | Light through air or space | Light in glass | Radio waves through air |
| Licensing | No spectrum license | Rights-of-way and permits | Licensed spectrum in most bands |
| Deployment | Days: mount and align terminals | Weeks to months of civil works | Days to weeks |
| Interception | Narrow beam, hard to intercept | Requires physical access | Broad beam, easier to intercept |
| Main limits | Fog, turbulence, alignment | Cost and time to install | Spectrum congestion |
Link scales: from chip-to-chip to deep space
- Chip-to-chip communication
- Indoor infrared (IR) and visible light communication (VLC)
- Inter-building links
- Free-space laser communications for airborne, spaceborne, and deep-space missions
Where FSO is used
FSO applications by sector
Metro network extensions
Carriers extend existing metropolitan fiber rings and connect new networks without new trenching.
Backhaul
FSO carries cellular traffic from antenna towers back to facilities wired into the public switched telephone network.
Last-mile access
Many city buildings still have no direct fiber connection because fiber arrived long after they were built. FSO connects these end users to service providers or to existing networks at gigabit speeds.
Service acceleration
Customers get service immediately while their permanent fiber infrastructure is being laid.
Fiber backup
A redundant FSO link backs up fiber in place of a second fiber run.
Enterprise connectivity
FSO links install quickly, which makes them a natural way to connect LAN segments in buildings separated by public streets or other right-of-way property.
Point-to-point line-of-sight links
Dedicated optical links between sites for telecommunication and computer networking.
Temporary networks
Fast-to-deploy links for events, temporary installations, and disaster recovery.
Inter-satellite links
Optical links between spacecraft, including the elements of a satellite constellation.
Ground-to-satellite links
Laser links between optical ground stations and satellites, where adaptive optics at the ground telescope corrects the turbulence near the ground.
Airborne and deep-space missions
Free-space laser communications for airborne platforms, spaceborne assets, and deep-space probes.
Military applications
Low electromagnetic emanation makes FSO attractive for transferring sensitive data, including for air forces.
Security applications
A narrow, precisely aimed beam is far harder to intercept than a broadcast RF signal.
Free-Space Laser Communications
The atmosphere is both the advantage and the main challenge of FSO. It needs no cable, but it attenuates the beam, distorts it, and bends it, and when any of these effects grows large enough the result is signal loss or link failure. Two families of effects matter, and they call for very different fixes.
Attenuation: less light arrives
Absorption and scattering by molecules and by particles (aerosols) suspended in the air remove power from the beam. Attenuation lowers the mean received signal. Fog and haze dominate, while rain matters far less at optical wavelengths than it does for radio.
Turbulence: the light arrives scrambled
Fluctuations in the refractive index of the air distort the beam. Turbulence makes the signal vary around its mean through scintillation, beam spreading, and beam wander, and it wrinkles the wavefront the receiver has to focus.
On top of the atmosphere sits geometric loss, set by the beam divergence and the transmitter and receiver aperture diameters. Total link attenuation is the sum of atmospheric attenuation and geometric loss.
Channel impairment explorer
What each effect does, and whether adaptive optics can fix it
Fog and haze
Adaptive optics helps: NoFog and haze droplets are close in size to near-infrared wavelengths, so they scatter the beam strongly (Mie scattering). Most of the scattering on a terrestrial laser link comes from these aerosols, and visibility is the standard way to estimate it.
Effect on the link
A drop in mean received power. Thick fog that cuts visibility below 50 m can push attenuation to hundreds of dB per kilometer.
Typical mitigation
Size the link margin to local visibility statistics, keep hops short, and add an RF or fiber backup path.
Why AO does not help
Light scattered out of the beam is lost power, not a phase error, so no optical correction can bring it back.
Rain
Adaptive optics helps: NoRaindrops are much larger than optical wavelengths, so they scatter light non-selectively, independent of wavelength.
Effect on the link
Modest attenuation on infrared links. Rain mainly affects microwave and radio systems that transmit at longer wavelengths.
Typical mitigation
Standard link margin.
Why AO does not help
Like fog, rain removes power rather than distorting the wavefront.
Wavefront distortion
Adaptive optics helps: YesTurbulent cells with slightly different refractive indices delay parts of the beam by different amounts, so a flat wavefront arrives wrinkled at the receiver.
Effect on the link
At the receiver focus, the tight spot breaks into a spread-out speckle pattern. Coupling into single-mode fiber collapses and the signal fades rapidly.
Typical mitigation
Adaptive optics: measure the distorted wavefront and cancel it with a deformable mirror.
Why AO helps
This is the core job of adaptive optics.
Beam wander
Adaptive optics helps: YesTurbulent eddies larger than the beam deflect it as a whole, so its centroid moves randomly across the receiver aperture.
Effect on the link
The focused spot drifts on and off the fiber core, producing deep, slow fades.
Typical mitigation
Tip/tilt correction and active pointing and tracking.
Why AO helps
Tip/tilt is the lowest-order correction an adaptive optics loop applies.
Beam spreading
Adaptive optics helps: PartlyTurbulence broadens the beam beyond its diffraction-limited divergence.
Effect on the link
Less power reaches the receiver aperture, and the light that does arrive is harder to focus.
Typical mitigation
Larger receive apertures, careful transmitter design, and pre-correction on the transmit side for uplinks.
Why AO helps only partly
Receiver-side AO restores the focal spot, but cannot recover light that missed the aperture.
Scintillation
Adaptive optics helps: PartlyVariations in the refractive index of the air make parts of the beam interfere, so the received intensity flickers rapidly, the same effect that makes stars twinkle.
Effect on the link
Fast intensity fluctuations around the mean, which raise the bit error rate.
Typical mitigation
Aperture averaging with larger receive optics, adaptive optics, and coding and diversity schemes.
Why AO helps only partly
AO corrects the phase across the pupil and steadies the coupled power, while strong amplitude fluctuations remain.
Geometric loss
Adaptive optics helps: NoSet by the link design: beam divergence, link distance, and the transmitter and receiver aperture diameters.
Effect on the link
A fixed loss that grows with distance and adds to the atmospheric attenuation to give the total link loss.
Typical mitigation
Lower beam divergence, larger apertures, and accurate pointing.
Why AO does not help
This is a design budget, not an atmospheric distortion.
Fog and haze loss estimator
How much does visibility cost your link?
Visibility is the standard input for estimating aerosol loss on an FSO link. Pick a visibility and a wavelength to see the specific attenuation predicted by the Kim model, a widely used empirical fit for free-space optical links.
Visibility
Wavelength
340 dB/kmSpecific attenuation at 50 m visibility, 850 nm
170 dBLoss over a 500 m hop
less than 0.01%Power remaining after 500 m
Link likely down: plan an RF or fiber backup
340 dB/kmSpecific attenuation at 50 m visibility, 1550 nm
170 dBLoss over a 500 m hop
less than 0.01%Power remaining after 500 m
Link likely down: plan an RF or fiber backup
84.9 dB/kmSpecific attenuation at 200 m visibility, 850 nm
42.5 dBLoss over a 500 m hop
less than 0.01%Power remaining after 500 m
Link likely down: plan an RF or fiber backup
84.9 dB/kmSpecific attenuation at 200 m visibility, 1550 nm
42.5 dBLoss over a 500 m hop
less than 0.01%Power remaining after 500 m
Link likely down: plan an RF or fiber backup
34.0 dB/kmSpecific attenuation at 500 m visibility, 850 nm
17.0 dBLoss over a 500 m hop
2.0%Power remaining after 500 m
Needs short hops and high margin
34.0 dB/kmSpecific attenuation at 500 m visibility, 1550 nm
17.0 dBLoss over a 500 m hop
2.0%Power remaining after 500 m
Needs short hops and high margin
13.7 dB/kmSpecific attenuation at 1 km visibility, 850 nm
6.8 dBLoss over a 500 m hop
20.8%Power remaining after 500 m
Covered by normal link margin
10.1 dB/kmSpecific attenuation at 1 km visibility, 1550 nm
5.1 dBLoss over a 500 m hop
31.2%Power remaining after 500 m
Covered by normal link margin
6.4 dB/kmSpecific attenuation at 2 km visibility, 850 nm
3.2 dBLoss over a 500 m hop
48.0%Power remaining after 500 m
Covered by normal link margin
4.3 dB/kmSpecific attenuation at 2 km visibility, 1550 nm
2.1 dBLoss over a 500 m hop
61.1%Power remaining after 500 m
Negligible
2.8 dB/kmSpecific attenuation at 4 km visibility, 850 nm
1.4 dBLoss over a 500 m hop
72.7%Power remaining after 500 m
Negligible
1.5 dB/kmSpecific attenuation at 4 km visibility, 1550 nm
0.77 dBLoss over a 500 m hop
83.8%Power remaining after 500 m
Negligible
0.96 dB/kmSpecific attenuation at 10 km visibility, 850 nm
0.48 dBLoss over a 500 m hop
89.5%Power remaining after 500 m
Negligible
0.44 dB/kmSpecific attenuation at 10 km visibility, 1550 nm
0.22 dBLoss over a 500 m hop
95.0%Power remaining after 500 m
Negligible
0.42 dB/kmSpecific attenuation at 23 km visibility, 850 nm
0.21 dBLoss over a 500 m hop
95.3%Power remaining after 500 m
Negligible
0.19 dB/kmSpecific attenuation at 23 km visibility, 1550 nm
0.10 dBLoss over a 500 m hop
97.8%Power remaining after 500 m
Negligible
Indicative values from the Kim visibility model, aerosol scattering only. Geometric loss, turbulence, and molecular absorption are not included. In dense fog the model predicts no benefit from moving to a longer wavelength, which is why fog sets the availability limit for terrestrial FSO.
Fog is a power budget problem that no optics can undo. Turbulence is different: the light still arrives, it is just scrambled, and unscrambling it is exactly what adaptive optics does.
Atmospheric Turbulence Mitigation Using Adaptive Optics
Adaptive optics (AO) removes the effect of air turbulence to improve coupling efficiency into the receiver’s optical fiber. Typically, an AO system delivers an optical gain of at least 10x on the flux injected into the fiber, which can translate into a tenfold increase in link capacity.
Fiber coupling is where turbulence hurts most. High-data-rate receivers inject the incoming light into single-mode fiber, whose guided mode is only about 10 µm across at 1550 nm. A telescope can focus light into a spot that small only when the incoming wavefront is flat. Turbulence wrinkles the wavefront, the focal spot breaks into speckle far larger than the core, and most of the light misses the fiber.
See the correction loop in action
Loop open. The deformable mirror sits flat, so the turbulent wavefront passes through uncorrected. The focal spot breaks into speckle around the fiber core and only a small fraction of the light is coupled.
Loop closed. The wavefront sensor measures the distortion, the controller computes the correction, and the deformable mirror takes the opposite shape. The wavefront leaving the mirror is flat, the spot collapses onto the core, and the coupled flux rises sharply.
Schematic for illustration, not to scale.
How the correction loop works
As shown in the diagram below, adaptive optics systems rely on three components working together to measure and correct air turbulence in real time.
1
Measure
A high-speed wavefront sensor, such as HASO SWIR FAST (1000 to 1700 nm, up to 4.5 kHz), measures the distortion in the incoming beam.
2
Compute
Real-time control electronics convert each measurement into commands for the corrector.
3
Correct
A deformable mirror (DM) reshapes its surface to cancel the distortion, fast enough to keep up with turbulence that changes on millisecond timescales.
Our partner Imagine Optic offers a standard turnkey system called CIAO SWIR that integrates all of these components into a telescope-compatible package. CIAO SWIR was designed with affordability in mind and is based on an efficient, fast combination of deformable mirror, wavefront sensor, and software.
CIAO SWIR for Free-Space Optics (FSO) and SATCOM
With CIAO SWIR, only a few steps are needed to improve your FSO signal:
- Connect the device to your telescope.
- Plug in the fiber.
- Switch on the system.
- Run the automatic calibration.
- Collect the corrected signal at the output of the fiber.
Why optical links for satellite communications
RF frequency bandwidth is saturating, so new methods of communication are needed. RF signals are also broadband, which makes satellite-transmitted information easier to intercept. Laser links can precisely target a station and transmit data in a highly secure way, which makes them ideal for military and security applications. Optical waves can also reach extremely high data rates, up to 100 Gb/s, when paired with adaptive optics. To decode the information, however, the laser light must be injected into a single-mode fiber, and that is the step adaptive optics protects.
First results: fiber injection on a telescope
5xmore photons at the fiber outputOn-sky test, 355 mm telescope, poor seeing (2.2 arcsec)
35 dB to <10 dBsignal loss without vs. with AOLab atmosphere emulator, in partnership with Nokia
10x or moretypical optical gain on injected fluxAdaptive optics for FSO receivers
The signal, acquired with an avalanche photodiode, is proportional to the flux injected into the single-mode fiber. By alternating between opening and closing the AO loop, the gain from the improved point spread function (PSF) can be quantified directly. The first injection test on a 355 mm diameter telescope under poor seeing conditions (2.2 arcsec) showed a gain of 5 in the number of photons at the fiber output.
Another very interesting result is the measurement of signal transmission through the atmosphere, both with and without adaptive optics:
Thanks to a partnership with Nokia, this was simulated in the lab using an atmosphere emulator. Signal losses reached up to 35 dB without adaptive optics. With adaptive optics, losses were reduced to less than 10 dB. That 25 dB improvement means roughly 300 times more optical power reaching the receiver.
CIAO also exists in a visible version, allowing astronomers to do high-resolution imaging: more about CIAO.
Planning an FSO link or an optical ground station? Axiom Optics applications engineers can help you size the telescope, wavefront sensor, and deformable mirror for your link.
Contact Axiom OpticsFrequently asked questions
What is free-space optical communication?
Free-space optical (FSO) communication transmits data on a beam of light through air, space, or vacuum instead of through fiber-optic cable. It provides multi-gigabit line-of-sight links without a spectrum license, typically over 300 m to 5 km on the ground and much longer distances between satellites.
What wavelengths do FSO links use?
Most FSO links operate between 780 and 1600 nm. The 1550 nm band is common for longer links because it shares components with fiber telecom networks and allows higher eye-safe transmit power.
Can adaptive optics compensate for fog?
No. Fog and haze scatter light out of the beam, which is a loss of power rather than a distortion of the wavefront. Adaptive optics corrects turbulence-induced wavefront errors, so fog has to be handled with link margin, shorter hops, or a backup path.
How much does adaptive optics improve an FSO link?
An adaptive optics system typically delivers an optical gain of at least 10x on the flux injected into the receiver fiber. In an on-sky test on a 355 mm telescope in poor seeing (2.2 arcsec), CIAO SWIR increased the photons at the fiber output by a factor of 5, and in a lab atmosphere emulator built with Nokia, signal losses of up to 35 dB without adaptive optics fell below 10 dB with it.
What is CIAO SWIR?
CIAO SWIR is a turnkey adaptive optics system from Imagine Optic, available through Axiom Optics, that combines a deformable mirror, a fast SWIR wavefront sensor, and control software in a telescope-compatible package for FSO and SATCOM ground stations.
How fast does an adaptive optics loop for FSO need to run?
Atmospheric turbulence changes on millisecond timescales, so the correction loop has to run at kilohertz rates to keep up. The HASO SWIR FAST wavefront sensor measures at up to 4.5 kHz across 1000 to 1700 nm.
Sources:
Free Space Optical Communications — Theory and Practices
In: Mutamed Khatib (Eds.), Contemporary Issues in Wireless Communications – November 2014
Adaptive Optics for Free-Space Optics… and for fun! – Imagine Optic