Free Space Optics (FSO) is a telecommunication technology that uses light propagating in free space to transmit data between two points. The technology is useful where the physical connection of the transmit and receive locations is difficult, for example in cities where the laying of fibre optic cables is expensive. Free Space Optics is also used to communicate between space-craft, since outside of the atmosphere there is little to distort the signal. The optical links usually use infrared laser light, although low-data-rate communication over short distances is possible using LEDs. IrDA is a very simple form of free-space optical communications. Distances up to the order of 10 km are possible, but the distance and data rate of connection is highly dependent on atmospheric conditions.

FSO transmitter and receiver.

Applications

Typically scenarios for use are:

• LAN-to-LAN connections on campuses at Fast Ethernet or Gigabit Ethernet speeds.
• LAN-to-LAN connections in a city. example, Metropolitan area network.
• To cross a public road or other barriers which the sender and receiver do not own.
• Speedy service delivery of high bandwidth access to fiber networks.
• Converged Voice-Data-Connection.

Two solar-powered satellites communicating optically in space via lasers.

• Temporary network installation (for events or other purposes).
• Reestablish high-speed connection quickly (disaster recovery).
• As an alternative or upgrade add-on to existing wireless technologies.
• As a safety add-on for important fiber connections (redundancy).
• For communications between spacecraft, including elements of a satellite constellation.
• For interstellar communication.

The lightbeam can be very narrow, which makes FSO hard to intercept, improving security. In any case, it is comparatively easy to encrypt any data travelling across the FSO connection for additional security. FSO provides vastly improved EMI behavior using light instead of microwaves.

History

The reflected sun has been used for communications for thousands of years (heliograph). Alexander Graham Bell developed a light based free space communication system (the photophone).

Beginning with laser developments in the 1960s, the first serious trials started to develop "Lightphones". Military organizations especially were interested and forced some developments. During the boom period of optical fiber installation civil FSO technology lay dormant, but in military and space laboratories the development didn't really stop. Some features of FSO technology were important for the military and again became important for civil use.

FSO can also use red visible light, as in this installation of a RONJA system. The light does not come from a laser, but instead from a high-intensity LED. The intensity vanishes beyond a few steps outside of the narrow beam's path. The range in this case is 1.4 km and speed 10Mbit/s.

Technology disadvantages and behavior

When used in a vacuum, for example for inter-space craft communication, FSO may provide similar performance to that of fibre-optic systems. However, for terrestrial applications, the principal limiting factors are:

• Beam dispersion
• Atmospheric absorption
• Rain (lower attenuation)
• Fog (10..~100 dB/km attenuation)
• Snow (lower attenuation)
• Scintillation (lower attenuation) although to a lesser degree in LED Systems
• Background light
• Shadowing
• Pointing stability in wind
• Pollution / smog
• If the sun goes exactly behind the transmitter, it can swamp the signal.

These factors cause an attenuated receiver signal and lead to higher bit error ratio (BER). To overcome these issues, vendors found some solutions, like multi-beam or multi-path architectures, which use more than one sender and more than one receiver. Some state-of-the-art devices also have larger fade margin (extra power, reserved for rain, smog, fog). To keep an eye-safe environment, good FSO systems have a limited laser power density and support laser classes 1 or 1M. Atmospheric and fog attenuation, which are exponential in nature, limit practical range of FSO devices to several kilometres.

Advantages and Challenges

Main advantages are:

• Quick link setup
• License-free operation
• High transmission security
• High bit rates
• Low bit error rate
• No Fresnel zone necessary
• Low snow and rain impact
• Full duplex transmission
• Protocol transparency
• No interference
• Great EMI behavior
• In some devices, the beam can be visible, facilitating aiming and detection of failures.

Compared to a microwave link, the advantages are that it can support higher bit rates (under good conditions), that its dispersion is lower, and that it is license-free in all jurisdictions.

See also

• Applications of atomic line filters in laser tracking and communication
• Free-space loss
• IrDA
• Laser safety
• Mie scattering
• Extremely high frequency
• Optical telegraph for the early history of optical communication, including semaphore
• Optical window
• Radio window
• Rayleigh scattering
• RONJA Free space optics device with free sources
• Smoke signals
• Visible Light Communications

References

External links

• Harvard Broadband Communications Laboratory: Free-Space Optical Communications

Note: The sites below are vendor sponsored

• http://www.free-space-optics.org (sponsored by fSONA)
• http://www.freespaceoptics.org/ (sponsored by LightPointe)
• http://www.freespaceoptic.com/ (sponsored by System Support Solutions Inc)

Technology Explanation Sites

• Free Space Optics on COST297 for HAPs
• Explanation of Fresnel zones in microwave and optical links