Free space optics

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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.

FSO transmitter and receiver

Contents

1 Applications
2 History
3 Technology disadvantages and behavior
4 Advantages and Challenges
5 See also
6 References

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.

References

Kontogeorgakis, Christos; Millimeter Through Visible Frequency Waves Through Aerosols-Particle Modeling, Reflectivity and Attenuation