What is Free Space Optics?
Free Space Optics is a line-of-sight technology that uses invisible beams of light to provide optical bandwidth connections that can send and receive up to 2.5 Gbps of data, voice, and video communications. It uses the same concept as fiber optics except without the cable. FSO systems run in the infrared (Ir) spectrum, which is at the low end of the light spectrum. Specifically, the optical signal is in the range of 300 GHz to 1 THz (1 TeraHertz = 1 trillion Hz = 1,000,000,000,000 cycles per second) in terms of wavelength.
Free Space Optics was first invented by Alexander Graham Bell in 1880. He called his invention the Photophone. He was able to transmit sound on a beam of light with the help of mirrors and sunlight. It worked by projecting voice through an instrument toward a mirror. Vibrations in the voice caused similar vibrations in the mirror. Bell directed sunlight into the mirror, which captured and projected the mirror's vibrations. The vibrations were transformed back into sound at the receiving end of the projection. Although he couldn't protect the transmission from outside interference, his work was the foundation for fiber optics and now free space optics.
Beginning with laser developments in the 1960s, the first serious trials started to develop “Lightphones”. Military organizations especially were interested and forced some developments. Military and space laboratories continue to develop FSO technologies when the fiber optic boom period came about. Civil uses for FSO have become popular again due to its high data rate throughput. FSO has been used for more than four decades in various forms to provide fast communication links in remote locations.
How it works
Free space optics technology is surprisingly simple. It's based on connectivity between FSO-based optical wireless units, each consisting of an optical transceiver with a transmitter and a receiver to provide full-duplex (bi-directional) capability. Each optical wireless unit uses an optical source, plus a lens or telescope that transmits light through the atmosphere to another lens receiving the information. At this point, the receiving lens or telescope connects to a high-sensitivity receiver via optical fiber. The transmitter and receiver have to have line-of-sight to each other. Trees, buildings, animals, and atmospheric conditions all can hinder the line-of-sight needed for this communication medium. Since line-of-sight is so critical, some systems make use of a beam divergence or diffused beam approach, which involves a large field of view that tolerates substantial line-of-sight interference without significant impact on overall signal quality. Some systems also are equipped with auto-tracking mechanisms that maintain tightly focused connectivity even when the transceivers are mounted on tall buildings that sway.
Characteristics of Free Space Optics
Generally, all of today's commercially available FSO systems operate in the near-IR wavelength range between roughly 750 and 1600 nm, with one or two systems being developed to operate at the IR wavelength of 10,000 nm. The physics and transmission properties of optical energy as it travels through the atmosphere are similar throughout the visible and the near-IR wavelength range, but there are several factors that influence which wavelength is chosen by a given design team.
Atmospheric Transmission Windows
It is important to note that although the atmosphere is considered to be highly transparent in the visible and near-IR wavelengths, certain wavelengths (or wavelength bands) can experience severe absorption. In the near-IR wavelength, absorption occurs primarily in response to water particles (i.e., moisture), which are an inherent part of the atmosphere even under clear weather conditions There are several transmission windows that are nearly transparent (i.e., have an attenuation of <0.2 dB/km) within the 700–10,000-nm wavelength range. These windows are located around specific center wavelengths, with the majority of FSO systems designed to operate in the windows of 780–850 nm and 1520–1600 nm.
These wavelengths are suitable for FSO operation, and several vendors provide higher-power laser sources that operate in this region. At 780nm, inexpensive CD lasers are available, but the average lifespan of these lasers can be an issue and must be addressed during system design (e.g., running the lasers at a fraction of their maximum rated output power, which will greatly increase their life). Around 850nm, reliable, inexpensive, high-performance transmitter and detector components are readily available and commonly used in network and transmission equipment. Highly sensitive silicon (Si) avalanche photodiode (APD) detector technology and advanced vertical-cavity surface-emitting laser.
(VCSEL) technology can be used for operation in this wavelength. Possible disadvantages include beam detection through the use of a night-vision scope, although it is still not possible to demodulate the beam with this technique.
1520–1600 nm . These wavelengths are well suited for free-space transmission, and high quality transmitter and detector components are readily available. The combination of low attenuation and high component availability in this wavelength makes the development of wavelength-division multiplexing (WDM) FSO systems feasible. However, components are generally more expensive, and detectors are typically less sensitive and have a smaller receive surface area when compared with Si APD detectors that operate in the 850nm wavelength. That being said, these wavelengths are also used in long-haul fiber systems, and many companies are working to reduce the cost and increase the performance of 1520–1600nm components. In addition, these wavelengths are compatible with erbium-doped fiber amplifier (EDFA) technology, which is important for high-power (>500 mW) and high-data rate (>2.5 Gbit/s) systems. Finally, 50–65 times as much power can be transmitted at 1520–1600nm than can be transmitted at 780–850nm for the same eye safety classification. Possible disadvantage include the inability to detect the beam with a night vision scope. The night vision scope is one technique that is used to align the beam.
Class 1 lasers are safe under reasonably foreseeable operating conditions, including the use of optical instruments for intra-beam viewing. Class 1 systems can be installed in any location without restriction.
Class 1M laser systems operate in the wavelength range from 302.5 to 4000 nm, which is safe under reasonably foreseeable conditions but may be hazardous if the user employs optical instruments within some portion of the beam path. As a result, Class 1M systems should only be installed in locations where the unsafe use of optical aids can be prevented.
10,000nm (10 mm)
This wavelength is relatively new to the commercial FSO arena and is being developed because of claims of better fog transmission characteristics. At this time, there is considerable debate regarding these characteristics because they are heavily dependent upon fog type and duration. In general, there are few components available at 10,000nm, inasmuch as it is not normally used in telecommunications equipment. In addition, 10,000nm energy does not penetrate glass, so it is ill-suited to behind-window deployments.
The modulated light source, which is typically a laser or light-emitting diode (LED), provides the transmitted optical signal and determines all the transmitter capabilities of the system. Only the detector sensitivity plays an equally important role in total system performance. For telecommunication purposes, only lasers that are capable of being modulated at 20 Mbit/s to 2.5 Gbit/s can meet current marketplace demands. In addition, how the device is modulated and how much modulated power is produced are both important to the selection of a device. Lasers in the 780–850nm and 1520–1600nm spectral bands meet frequency requirements and are available as off-the-shelf products. Within these two wavelength windows, FSO systems should have the following characteristics:
- Ability to operate at higher power levels (important for longer-distance FSO systems).
- High-speed modulation (important for high-speed FSO systems).
- Small footprint and low power consumption (important for overall system design and maintenance).
- Ability to operate over a wide temperature range without major performance degradation (important for outdoor systems).
- Mean time between failure (MTBF) that exceeds 10 yr.
To meet the above requirements, FSO manufacturers generally use VCSELs for operation in the shorter-IR wavelength range and Fabry–Perot (FP) or distributed-feedback (DFB) lasers for operation in the longer-IR wavelength range. Several other laser types are not suitable for high-performance FSO systems.
There are numerous advantages of free space optics.
- Convenience: FSO provides a wireless solution to last mile connection or connection between two buildings. There is no hassle with digging and burying fiber cable. Also free space optics requires no RF license. Therefore you could deploy a FSO system quickly. It is easily upgradeable, and its open interfaces support equipment from a variety of vendors, which helps enterprises and service providers protect their investment in embedded telecommunications infrastructures. It can be deployed behind windows, eliminating the need for costly rooftop rights. It is immune to radio frequency interference or saturation.
- Speed: FSO provides up to 2.5 Gbps of data throughput. This provides ample bandwidth to transfer files between two sites. With the growing size of files, free space optics provides the necessary bandwidth to transfer these files efficiently.
- Security: FSO is a very secure wireless solution. The laser beam cannot be detected with a spectrum analyzer or RF meter. The beam is invisible which makes it hard to find. The laser beam that is used to transmit and receive data is very narrow. This means that it is almost impossible to intercept the data being transmitted. One would have to be in the line of sight between receiver and transmitter to be able to accomplish this feat. Of course if that happens it would cause an alert due to the receiving site has lost connection. There are no security upgrades that are required for FSO.
There are a number of weaknesses with FSO:
- Distance: The distance of FSO is very limited. Operating distance is usually within 2 km. Although this is a powerful system with great throughput, the limitation of its distance is a big deterrent.
- Line of sight: Line of sight must be maintained at all times during transmission. Any obstacle, be it environmental or animals, can hinder the transmission. FSO technology must be designed to combat changes in the atmosphere, which can affect FSO system performance capacity. Among the issues to be considered when deploying FSO-based optical wireless systems:
- Fog: The primary challenge to FSO-based communications is dense fog. Rain and snow have little effect on FSO technology, but fog is different. Fog is 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. The primary answer to counter fog when deploying FSO-based optical wireless products is through a network design that shortens FSO link distances and adds network redundancies.
- Absorption: Absorption occurs when suspended water molecules in the terrestrial atmosphere extinguish photons. This causes a decrease in the power density (attenuation) of the FSO beam and directly affects the availability of a system. Absorption occurs more readily at some wavelengths than others. However, the use of appropriate power, based on atmospheric conditions, and use of spatial diversity (multiple beams within an FSO-based unit) helps maintain the required level of network availability.
- Solar Interference: A FSO system uses a highly sensitive receiver in combination with a large-aperture lens, and, as a result, natural background light can potentially interfere with FSO signal reception. This is especially the case with the high levels of background radiation associated with intense sunlight. In some circumstances, direct sunlight may cause link outages for periods of several minutes when the Sun is within the receiver's Field of Vision. However, the times when the receiver is most susceptible to the effects of direct solar illumination can be easily predicted. When direct exposure (i.e., pointing) of the equipment cannot be avoided, narrowing the receiver field of vision (FOV) and/or using a narrow-bandwidth light filter can improve system performance. It is important to remember that interference by sunlight reflected off a glass surface is possible as well.
- Scattering: Scattering is caused when the wavelength collides with the scatterer. The physical size of the scatterer determines the type of scattering. When the scatterer is smaller than the wavelength, this is known as Rayleigh scattering. When the scatterer is of comparable size to the wavelength, this is known as Mie scattering. When the scatterer is much larger than the wavelength, this is known as non-selective scattering. In scattering — unlike absorption — there is no loss of energy, only a directional redistribution of energy that may have significant reduction in beam intensity for longer distances.
- Physical obstructions: Flying birds or construction cranes can temporarily block a single-beam FSO system, but this tends to cause only short interruptions, and transmissions are easily and automatically resumed. Optical wireless products use multi-beam systems (spatial diversity) to address temporary obstructions, as well as other atmospheric conditions, to provide for greater availability.
- Building sway/seismic activity: The movement of buildings can upset receiver and transmitter alignment. FSO-based optical wireless offerings use a divergent beam to maintain connectivity. When combined with tracking, multiple beam FSO-based systems provide even greater performance and enhanced installation simplicity.
- Scintillation: Heated air rising from the earth or man-made devices such as heating ducts creates temperature variations among different air pockets. This can cause fluctuations in signal amplitude which leads to "image dancing" at the FSO-based receiver end. The effects of this scintillation are called "Refractive turbulence.” This causes two primary effects on optical beams.
- Beam Wander: Beam wander is caused by turbulent eddies that are larger than the beam. Beam Spreading:
- Beam spreading — long-term and short-term — is the spread of an optical beam as it propagates through the atmosphere.
- Safety: To those unfamiliar with FSO technology, safety can be a concern because the technology uses lasers for transmission. The proper use and safety of lasers have been discussed since FSO devices first appeared in laboratories more than three decades ago. The two major concerns involve eye exposure to light beams and high voltages within the light systems and their power supplies. Strict international standards have been set for safety and performance. Laser safety is an important issue. The primary safety concern is the potential exposure of the eye or skin to the beam. High-power laser beams can cause injury to skin, but risks of injury to the eye are more significant because of the eye's ability to focus light and thereby concentrate optical energy. In general, any laser that is considered to be “eye-safe” is also considered to be “skin-safe.” Like sunlight, laser light arrives in parallel rays, which, depending upon wavelength, the eye focuses to a point on the retina, the layer of cells that responds to light. Just as staring at the Sun can damage vision, exposure to a laser beam of sufficient power can cause eye injury. The specific wavelength is important because only certain wavelengths— between approximately 0.4 and 1.4 µm—are focused by the eye onto the retina. Other wavelengths tend to be absorbed by the front part of the eye (the cornea) before the energy is focused and concentrated. The absorption of the eye varies with wavelength. With respect to IR radiation, the absorption coefficient of the cornea is much higher for longer wavelengths (>1.4 µm) than for shorter wavelengths. As such, damage from the UV and visible radiation of sunlight is more likely than from longer wavelengths located in the IR spectrum. Eye response also differs within the range that penetrates the eye (400–1400nm) because the eye has a natural aversion response, which makes it turn away from a bright visible light. Wavelengths longer than 0.7 µm do not trigger an aversion response, because they are invisible. Although IR light can damage the surface of the eye, the damage threshold is higher than that for UV light.
Because of the scalability and flexibility of FSO technology, optical wireless products can be deployed in many enterprise applications including 'last-mile' connectivity, private line replacement, mobile wireless backhaul, metro ring extensions, LAN bridging, high-speed low-interference WiFi/802.11 backbones, remote PBX extensions, remote cellular antenna backhaul, HDTV and other video broadcasting backhaul, building-to-building connectivity, disaster recovery, network redundancy and temporary connectivity for applications such as data, voice and data, video services, medical imaging, CAD and engineering services, and fixed-line carrier bypass.
Hospitals are using leased T1/E1 lines to connect additional buildings. With high resolution MRI and CT images, these leased lines are getting congested. Hospitals and medical groups are turning to high speed optical wireless bridging solutions for buildings that are in line of site to each other. This will allow them to go from 1.54 Mbps to up to 2.5 Gbps. With its inherently secure narrow beam, Optical Wireless is virtually impossible to intercept, assuring HIPAA medical record security.
FSO has the potential to be a highly developed and highly used wireless system. However I don't believe it will happen. The throughput is amazing for a wireless system. It could provide the much needed bandwidth for connecting the last mile or connecting buildings. With its weaknesses I don't think FSO will develop into a mainstream wireless system. There will be those that can take advantage of this system, but with so much dependent on line of sight, I think organizations will stay away from this system. If more development is done such that fog and absorption is not as big of an issue and they can get more distance out of it, then yes FSO will really skyrocket in terms of usage. Until that happens, again I don't see it being a mainstream wireless system.
Posted in: May 2006