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Optoelectronics & Communications

Into thin air

Under test as an access loop solution, optical wireless technology sends voice and data over free space.

From oemagazine March 2001
28 March 2001, SPIE Newsroom. DOI: 10.1117/2.5200103.0008

Fiber-optic networks transmit enormous amounts of voice/data traffic around the world, but the technology is not the universal telecommunications panacea. Deployment of fiber networks is expensive and time consuming. Particularly in urban areas, laying fiber requires getting permits, tearing up streets, and digging ditches. For enterprise networks and the access networks that connect the edge of the metropolitan area network to the in-building network, the fiber-optic technology can be cost-prohibitive.

Wireless optical networks (WONs) offer an economical alternative to fiber. WONs operate building to building, sending a signal over free space across a clear line of sight. The output beam from a near-infrared (IR) laser source is modulated with a data stream and transmitted. At the other end of the link, a receiver gathers the beam and transfers it to focusing optics prior to detection by a photodiode and conversion to a data stream. A typical system is essentially bidirectional, operating on two separate, closely spaced beamlines. Most frequently, systems operate between rooftops (see figure 1), but they also can be mounted along building façades or behind windows. Depending on local conditions, links can range from 200 to 500 m, with data rates as high as 622 Mb/s (OC-12).

High-bandwidth wireless optical networks deployed building-to-building in an urban area offer fiber-class speeds at about one-fifth the cost of installing fiber. Moreover, network deployment takes days rather than months or years.

nuts and bolts

Most WONs incorporate diode laser sources. The CD-ROM industry and the long-haul telecom industry have both developed diode lasers with broad bandwidths, high powers, and high reliability. Although long-haul dense-wavelength-division- multiplexing (DWDM) systems incorporate components operating at 10 Gb/s, using that technology for WONs is neither straightforward nor necessarily cost effective—the CD-ROM laser costs about $20 (AirFiber uses a 785-nm Class 1 eye-safe laser), and the long-haul DWDM laser (1310 nm or 1550 nm) costs about $1000.

For detectors, systems generally use either p-i-n photodiodes or avalanche photodiode (APD). WON receivers use large-area photodiodes because of beam degradation caused by atmospheric effects. Designs must thus be tolerant of relatively high-capacitance detectors. Generally transimpedance amplifiers are used, but designers must keep feedback resistance low due to high input capacitance. Thus, in many cases, the system becomes limited by detector noise. Designers therefore turn to APDs, which offer internal gain that greatly increases system sensitivity.

Systems suffer geometric loss, defined as the amount of transmitted signal that fails to reach the receiver. The divergence of the transmitter beam essentially sets this fraction, assuming the receiver aperture is smaller than the diameter of the beam at that point. Typical divergence for an AirFiber link is 1 mrad, measured at the 1/e2 point of the Gaussian beam. The fraction of received power is the inverse of the square of the divergence; for example, a system with a 2° beamwidth would suffer about 30 dB more power loss than one with a divergence of 1 mrad.

Most WON products use a combination of spatial and spectral (bandpass) filtering to reject solar interference and increase signal-to-noise ratio. Typical values are rejection of 25 dB or so compared with a nonfiltered system.

In the atmosphere

Figure 1. Mesh architecture provides redundancy for carrier-grade systems.

Atmospheric effects impose loss on a wireless optical link (WOL) through absorption, scattering, and scintillation. A good rule of thumb for determining link range is that if you can see it, you can communicate with it. This rule of thumb also gives potential users an intuitive feel for the relative importance of fog, snow, and rain in preventing link operation. Fog sometimes can obscure the building across the street. Rain hardly ever comes down hard enough to limit visibility severely. And snow lies somewhere in between the two. It turns out that this rule is a bit conservative—a WOL can send data reliably about twice as far one can see.

Many species of gases in the atmosphere can cause absorption, but water is the dominant factor for operating wavelengths of WONs. By using a transmission wavelength outside of the so-called water window and keeping the path lengths short, absorption can largely be ignored.

There are two types of scattering mechanisms: Rayleigh and Mie scattering. Due to its low cross-section, Rayleigh scattering is really only significant for very long path lengths. The effect scales as 1/λ4, making it significant only at shorter wavelengths. Scattering by particles, also known as Mie scattering, is a different story, particularly as the size of the particles approaches the wavelength of the transmitted light. Wavelengths near the particle size are scattered very effectively, hence the deleterious effect of fog, which can cause losses as high as 300 dB/km. To compensate for fog, systems can keep the link length short and increase laser power during episodes of decreased visibility.

Scintillation refers to small spatial variations in the refractive index of the atmosphere. The effect can introduce phase changes in the wavefront of the signal arriving at the receiver causing both null- and high-signal receive levels. Fortunately for short- range systems these effects are relatively small, on the order of several dB for a typical link span of 200 m.

architecture

There are basically three types of network architectures: point-to-point, hub-and-spoke, and mesh. The most common architecture is point-to-point systems, linking buildings of a single enterprise. Network planning is simple and links are independent, but the optical link is a single point of failure without redundancy and is not suitable for carrier-grade systems.

Figure 2. Rooftop-mounted optical wireless nodes communicate between buildings.

The hub-and-spoke architecture allows traffic to be routed easily from a single point to the core network. Each link is a single point of failure, however, and hub location must be chosen to maximize the number of buildings with line of sight. Hub cost is generally high. Given that attenuation effects require short link lengths to ensure carrier-grade service, this is not a very economical architecture.

A mesh architecture (see figure 2) offers a redundant system with near-realtime rerouting. Traffic is collected at specific points for efficient connection to the network. The disadvantage is cost (multiple links per building), and the network requires specialized network planning. Both mesh and the simpler loop architectures are for carrier-grade networks.

WON systems can be engineered to deliver fiber-like bandwidth and carrier-grade reliability at a fraction of the cost and time of laying fiber. The technology can bring OC-12 data rates to the campus and access loop easily and economically. In industrial and urban areas, optical wireless access loop technologies may well be the wave of the future. oe

FOR FURTHER READING

1. L. Andrews and R. Phillips, Laser Beam Propagation through Random Media, SPIE Press, Bellingham, WA U.S. (1998)
2. S. Bloom and V. Chan, Proc. of SPIE 2699, 60-70 (1996)
3. Seth Hartley, internal AirFiber document, August 25, 2000.


Scott Bloom

Scott Bloom is vice president of advanced development, and Janet McVeigh is vice president of marketing at AirFiber, Inc., San Diego, CA.