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Free Space Optics

 

 

Free Space Optics (FSO)

Technology Basics

Sometimes known as wireless fiber, FSO systems are wireless optical transmission systems. 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 1 THz (1 TeraHertz = 1 trillion Hz = 1,000,000,000,000 cycles per second) in terms of wavelength.

At this frequency, the signal involves a wavelength, or lambda, in the range of (microns), with a micron being 1/1,000,000 meter. Contemporary systems now run signals in the far end of the infrared range, with wavelengths in the 780-850 nm range, and even the 1550 nm range.

FSO is a free space (i.e., wireless) technology, meaning that the signal travels in the free space between transmitter and receiver, rather than through a conductor such as a wire or fiber, or through a waveguide of some sort. Traditionally, such technologies commonly were referred to as airwave systems, since there is air in the free space, at least in the Earth's atmosphere, which is where terrestrial wireless systems are deployed.

The air is a good thing for us humans and other living creatures, since we breathe the stuff. It's not such a good thing for electromagnetic signals, however, as the physical matter in the air attenuates (i.e., weakens) them. The more dense the physical matter between transmitter and receiver, the greater the issue of attenuation, so dust, smog, agricultural haze and even high humidity can be issues.

Precipitation causes problems for airwave systems, in general, through a phenomenon known as rain fade. Fog, in specific, causes problems for optical systems, much as it does for automobile headlights. The fog comprises a great many very tiny droplets of water, each of which reflects the light signal in all directions, attenuating it in the process. The smaller the fog droplets, the greater the impact on the signal. I suppose we could call this phenomenon fog fade, to coin a term.

Needless to say, line-of-sight (LOS) is an absolute requirement, for trees, buildings and other dense physical objects will completely absorb, deflect, reflect and otherwise render the signal useless.

Note: Clear glass windows generally do not affect the signal, depending on the angle of incidence, the physical composition of the glass and other factors. The ability to transmit through glass allows FSO systems to be placed inside offices, rather than being forced to compete with microwave and other RF-based systems for expensive roof rights.

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.

Even under optimum conditions, distances are limited to a range of 2-5 kilometers or so, although most tests show that optimum performance is realized over much shorter distances of a kilometer or less.

Several factors affect the actual link length. Line-of-sight is an absolute requirement, of course, and it certainly helps if the beam is carefully and tightly focused from transmitter to receiver.

Signal strength at the point of origin is a factor, as a stronger light signal will perform better over longer distances. In this dimension, the 1550-nm systems have an advantage, as longer wavelengths are less likely to cause damage to the eye, even at much higher power levels. In high fog areas, distances are severely limited, and some manufacturers offer link redundancy in the form of point-to-point microwave backup systems, which are less susceptible to fog fade.

One of the nice things about FSO is that it does not require licensing, at least not at this point, or any sort of frequency planning. So, the complexities, costs and delays associated with licensed RF systems are not issues. You just point the transmitter at the receiver, fire the system up, and you're in business. (Some systems include an auto-detection feature that lets a single installer deploy the system.)

At some point in time, if and when FSO systems become common, there may be issues of interference between intersecting beams. At that point, licensing or some sort of regulatory mechanism may enter the picture, but that time probably is well into the future.

Also, FSO is unaffected by EMI (ElectroMagnetic Interference). This is a great advantage when compared to both licensed and unlicensed RF systems, which definitely suffer from EMI.

FSO systems also are unaffected by RFI (Radio Frequency Interference), which increasingly plagues RF systems - particularly those running in unlicensed bands. Just so as not to overstate the case, FSO systems can be affected by solar interference, which is a form of EMI, as light is electromagnetic in nature. Various types of optical filters are used to combat this problem, with some blocking all wavelengths other than those used for transmission and others blocking signals that are outside the acceptable range of angle of incidence (i.e., off-axis).

Atmospheric scintillation (i.e., rapid changes in brightness) can be a problem on bright sunny days, but the effects can be overcome through various combinations of filters and clock recovery mechanisms. All of this translates into excellent signal quality and excellent error performance and, therefore, excellent throughput. Reliability is a key selling point of FSO systems, with some manufacturers claiming 99.999% availability.

There is another clear advantage to FSO in comparison to RF systems. Security is much better, as it is much more difficult to intercept the beam. Further, it is much more difficult to jam the signal.

And we can't forget bandwidth. FSO systems routinely run at signaling speeds of 45 Mbps (T3), 155 Mbps (OC-3) and even up to 622 Mbps (OC-12). Systems running at speeds of 160 Gbps have been demonstrated in the labs. At these speeds, RF-based systems simply can't compete.

FSO System Configuration

FSO systems comprise paired transceivers. Some of the transceivers look like small microwave dishes or the VSAT (Very Small Aperture Terminal) dishes used for satellite TV reception. Others resemble telescopes or binoculars, while some are boxy in appearance like those in the figure.

Free Space Optics (FSO)

A focusing lens in the transmitting device serves to tightly focus the light beam on the collecting lens in the receiving device. The transmitted light beam is not perfectly collimated (i.e., parallel). Rather, it naturally spreads out from the transmitting terminal at a divergence angle and may well be several meters wide by the time it reaches the receiving terminal.

Therefore, only a small amount of the transmitted light signal strikes the receive aperture, with the balance of the signal being wasted. By adjusting the focus of the transmitting lens, it is possible to reduce the beam divergence and concentrate more signal on the receive aperture, thereby improving signal quality. The tradeoff is that a narrow beam can easily become misaligned due to external forces such as building sway and strong wind gusts.

Applications

As a transmission technology, FSO systems are agnostic when it comes to the underlying applications supported. Voice, fax, data, video, image and even multimedia all can ride over the system.

At a higher level, FSO systems are effective in short-haul, bandwidth-intensive applications where cabled systems either are not available or are too costly. Traditionally, FSO systems have been used to interconnect high-speed LAN segments. They also are used in disaster recovery applications, and for temporary connectivity while cabled networks are being deployed.

FSO systems increasingly are deployed in local loop applications as an alternative to RF-based systems to extend the reach of optical fiber. Note: Despite all the fiber in the network core and particularly in the long haul, industry studies indicate that only a small percentage (10%-20%) of commercial and industrial buildings are lit (i.e., on-net).

A common configuration involves a central hub, or node, positioned on the roof of a high-rise building with good lookdown for achieving line-of-sight to buildings of lesser stature within the coverage area. The hub building can be connected to the service provider's backbone via FSO or point-to-point microwave.

More commonly and better yet, the hub building is a node of the backbone fiber optic network. The buildings served by the high-speed FSO links may be single tenant or multi-tenant and even mixed use in nature. Within the building, of course, the bandwidth can be subdivided to serve individual tenants or user groups. The network can even assume a mesh or partial mesh configuration, perhaps in consideration of redundancy and media diversity, which yields considerably enhanced network resiliency.

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An application that recently has generated quite a lot of interest is in support of 2.5G and 3G wireless networking, which we discussed in some detail in the last column on Wireless Local Loop (WLL).

As these high-bandwidth networks are deployed, the bandwidth requirements of the links between the base stations and the backbone network and switching centers often exceed the capacity of the existing facilities. As upgrading existing microwave links may not be an option and telco circuits may prove too expensive, FSO is a highly attractive alternative. In fact, it may well be the preferred solution.

Note: FSO systems really aren't a new invention. Alexander Graham Bell invented the photophone in the late 19th century, well before he invented the telephone. Bell demonstrated that the photophone, which he considered to be his greatest invention, could transmit audio signals some 600 feet along a beam of sunlight. As transmission quality was poor, even on sunny days, and the system was delicate, it proved to be impractical. The Nazi military experimented with the concept in tank warfare applications, but that also proved ineffective. More recently, the defense industry refined the engineering specifics to yield contemporary FSO technology.

 

 

 

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