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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.
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
- To cross a road or other barriers
- Speedy service delivery of high bandwidth access to fiber networks
- Converged Voice-Data-Connection
- Temporary network installation (for events or other purposes)
- Re-establish high speed connection quickly (Disaster Recovery)
- As an alternative or upgrade addon to existing wireless technologies.
- As a safety addon 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. 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 organisations 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.
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 principle limiting factors are:
- beam dispersion
- atmospheric absorption
- rain (lower attenuation)
- fog (10..~100dB/km attenuation)
- snow (lower attenuation)
- scintillation (lower attenuation)
- Background light
- Shadowing
- Pointing stability in wind
- Pollution / smog
These factors cause an attenuated receiver signal and lead to higher bit error rates (BER). To overcome these issues, vendors found some solutions, like multibeam or multipath 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.
Advantages and Challenges
Main Advantages are:
- Quick link setup
- license free operation
- High transmission security
- High bit rates
- No Fresnel zone necessary
- Low snow and rain impact
- Full duplex transmission
- Protocol transparency
- No interference
- Great EMI behavior
Also microwave radio has some advantages, which are a longer distance and less attenuation by fog. Both technologies have the economical advantage that a ground cable investment is not required.
See also
- RONJA Free space optics device with free sources
- laser safety
- optical window, radio window, Free-space loss, Rayleigh scattering, Mie scattering
- IrDA, RONJA
- Optical telegraph for the early history of optical communication, including semaphore and
- Applications of atomic line filters in laser tracking and communication
- Smoke signals.
References
- Kontogeorgakis, Christos; Millimeter Through Visible Frequency Waves Through Aerosols-Particle Modeling, Reflectivity and Attenuation
Optischer Richtfunk | Laser langattomassa tiedonsiirrossa (tietoliikenne)
"Free Space Optics"