FSO communications systems are wireless point-to-point communications systems that use lasers to transmit and receive communication signals via modulated visible or infrared line-of-sight (LOS) directed beams. Known optical communication systems rely on optical fibers between transmitter and receiver, but free space optical communications systems do not exhibit the limitations associated with the installation and maintenance of such guided wave optical communication systems (a time consuming and expensive process…). Moreover, FSO systems data rates (with very low errors) are comparable to optical fiber transmission systems.
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. By addressing the principal engineering challenges of FSO, the aerospace/defense activity established a strong foundation upon which today’s commercial laser-based FSO systems are based… and some features became important for civil use.
First experimentations on Free Space Optics (FSO) systems were demonstrated by Alexander Graham Bell who developed the photophone. The device converted voice sounds into telephone signals and transmitted them between receivers through free air space along a beam of light for a distance of some 600 feet… Essentially all of the engineering of today’s FSO communications systems was done over the past 50 years with the beginning of laser developments, mostly for defense applications.
- Broadband RF/microwave systems have severe limitations (expensive licensed commodity, low data rates, insecurity, interference)
Among alternatives technologies use today, we can cite:
- Telco/PTT telephone networks still trapped in the old Time division Multiplex (TDM)
- Digital Subscriber Line (DSL) penetration rates throttled by slow deployment and PTTs pricing strategies
- Cable modern access suffering from security and capacity problems
- Wireless Internet still slow
FSO Transmitter/Receiver pairs Global Consumption Market Forecast by Region (Value Basis, $ Million) – ElectroniCast Consultants
According to the 2007 ElectroniCast Consultants study, the global consumption value of Transmitter/Receiver pairs used in fixed-location commercial FSO system equipment was $41.55 million in year 2006. Europe will maintain its market share lead throughout the forecast period (2006-2011), growing from $17.73 million, or a relative market share of 43 percent, in 2006 to $21.35 million in year 2011 (see Figure below). Note that ElectroniCast’s study does not include all of the other parts that compose of the entire FSO system equipment, notably the military applications.
Atmospheric absorption: mainly due to water vapor, rain, snow, pollution.
Fog (10-100 dB/km attenuation): vapor composed of water droplets, which are only a few hundred microns in diameter but can modify light characteristics or completely hinder the passage of light through a combination of absorption, scattering, and reflection. This can lead to a decrease in the power density of the transmitted beam, decreasing the effective distance of a free space optical link
Scintillation: temporal and spatial variation in light intensity caused by atmospheric turbulence. Such turbulence is caused by wind and temperature gradients that create pockets of air with rapidly varying densities and, therefore, fast-changing indices of optical reflection. These air pockets act like lenses with time-varying properties and can lead to sharp increases in the bit-error-rates of free space optical communication systems.
Physical obstructions (birds,…).
Pointing stability: systems must be able to handle the vast majority of movement found in deployments on buildings. Fotr cost raison, note that fixed-pointed FSO systems are generally preferred over actively-tracked FSO systems.
Background light: generally, use of optical filter window to filter all but the wavelength used for FSO communications.
Shadowing: attenuation phenomenon may be caused by the refraction and reflection on barriers.
High bit rates / low bit error rates
Quick link setup
Virtually unlimited bandwidth
Low cost, ease and speed deployment
High transmission security (difficulty to intercept LOS path, possibility to encrypt connection…)
Full duplex transmission
Great ElectroMagnetic Interference (EMI) behaviour
Light beams in FSO systems are transmitted by laser light focused on highly sensitive photon detector receivers. These receivers are telescopic lenses able to collect the photon stream and transmit digital data containing a mix of video images, radio signals, Internet messages, or computer files. Commercially available systems offer capacities in the range of 100 Mbps to 2.5 Gbps, and demonstration systems report data rates as high as 160 Gbps. The optical transmitter can modulate the optical signal to carry data. The optical receiver then collects all of the energy of the optical signal and converts the optical signal into an electrical signal. The optical receiver can operate on this electrical signal recover the modulated data and, in some applications, align the receiver to optimally receive the optical signal.
Typical atmospheric transmission throughout the range of 1 to 3 THz. Note that the major absorption is due to the water vapor.
Most FSO on the market transmit infrared eye-safe (limited laser power density and support laser classes 1 or 1M) light beams from one device, called a "telescope", to another using low power lasers in the TeraHertz spectrum. This region of the Electromagnetic Spectrum (ES) between 300 GHz and 3 THz corresponds to the submillimeter wavelength range between 1 mm (high-frequency edge of the microwave band) and 10 µm (long-wavelength edge of far-infrared light). Terahertz radiations are non-ionizing submillimeter microwave radiation and share with microwaves the capability to penetrate a wide variety of non-conducting materials and can also penetrate fog and clouds…
Optical transmissions provide a wider bandwidth than other wireless communications mediums, such as RF frequency signals. Moreover, optical signals can generally be more focused than RF signals, and are thus more difficult to intercept and less likely to cause interference with other transmissions. In addition, unlike radio and microwave systems, free space optical communications requires no spectrum licensing and interference to and from other systems is not a concern. Moreover, there is no practical limit to the number of separate FSO links that can be installed in a given location, because of the extremely narrow laser beam widths…
FSO communication systems include optical transmitters and receivers that are configured to deliver and receive optical signals propagating in free space, and waveguides are not needed to connect the transmitter and receiver.
FSO use beams of light, such as laser beams, as optical communication signals, and therefore do not require cables or fibers connected between transmitters and receivers. The technology is useful where the physical connection of the transmit and receive locations is difficult, for example in cities where the laying of fiber optic cables is difficult or impossible in some cases…
Communications links enabled by FSO go where fiber cannot cost-effectively venture. (Credit: LightPointe)
The fundamental limitation of free space optical communications arises from the environment through which it propagates:
Low-data-rate communication over short distances is possible using LEDs. As long as there is a clear LOS between the source and the destination, and enough transmitter power, FSO communication is possible. They can function over distances of several kilometers if needed, but the distance and data rate of connection is highly dependent on atmospheric conditions…