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Free space optical communication

In telecommunications, Free Space Optics (FSO) is an optical communication technology that uses light propagating in free space to transmit data between two points. The technology is useful where the physical connection by the means of fibre optic cables is impractical, due to high costs or other considerations. Free Space Optics are also used for communications between spacecrafts. The optical links are implemented using infrared laser light, although low-data-rate communication over short distances is possible using LEDs. Maximum range for terrestrial links is in the order of 10 km , but the stability and quality of the link is highly dependent on atmospheric factors such as rain, fog, dust and heat. In space range is in the order of several thousand kilometers . IrDA is a very simple form of free-space optical communications.

History

Optical communications, in various forms, have been used for thousands of years. The Ancient Greeks polished their shields to send signals during battle. Later on a wireless solar telegraph called heliograph was developed, that signals using Morse code flashes of sunlight. Alexander Graham Bell developed a light based telephone, the photophone.

The invention of laser in the 1960s, revolutionized free space optics. Military organizations were particularly interested and boosted development. The technology lost market momentum when the installation of optical fiber networks for civilian uses was at its peak.

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.
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.

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

Advantages

Ease of deployment
License-free operation
High bit rates
Low bit error rates
Immunity to electromagnetic interference
Full duplex operation
Protocol transparency
Very secure due to the high directionality and narrowness of the beam(s)
No Fresnel zone necessary

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.