Selecting the Transmission Wavelength
To select the best wavelength to use for free-space optical communication systems, you must consider several factors. In general, the specific wavelength is not so important as long as the transmission wavelength does not correspond to a wavelength that is strongly absorbed in the atmosphere. As stated previously, Mie scattering is by far the dominant factor as far as attenuation of an IR signal through the atmosphere is concerned. However, applications in dense urban areas with high aerosol contents might slightly benefit from a different wavelength than relatively unpolluted suburban locations.
As has been mentioned, some atmospheric advantages exist for some wavelengths being used for FSO systems, but that is not the whole picture. Another issue has to do with the fact that at approximately 1,550 nm, the regulatory agencies allow approximately 100 times higher power for "eye safe" lasers. This is because at this wavelength, the aqueous fluid of the eye absorbs much more of the energy of the beam, preventing it from traveling to the retina and inflicting damage. The disadvantage of this laser type is mainly cost when compared to shorter wavelength lasers operating around 850 nm. Design engineers must deal with the cost of implementing such a system.
Choosing the correct transmission wavelength involves many factors, such as availability of components, price, required transmission distance, eye-safety considerations, and so on. As noted at the beginning of this chapter, the preferred wavelengths are in the 850 nm and 1550 nm wavelength band. Operation in the longer wavelength transmission windows between 35 μm and 814 μm has also been suggested by the FSO community due to the excellent transmission characteristics of the atmosphere in the mid infrared wavelength range. However, some recent more detailed studies of the Mie scattering coefficients in the mid infrared range suggest that there is no significant advantage in using longer IR-wavelength such as 3.5 μm instead of the 850 or 1,550 nm wavelength ranges to counteract scattering losses. Also, the availability of components such as light sources and detectors in the mid IR wavelength range is very limited. At present, most highly sensitive detectors and light sources in this wavelength range must be cooled to low temperatures. Thermal background noise, which is much higher in the mid infrared when compared to shorter IR wavelengths, impacts the sensitivity and consequently the BER performance.
Current systems rely on mature semiconductor laser technology and devices manufactured to support the fiber-optic cable industry. Can the components be obtained cheaply? Does the technology even exist to use other wavelengths? The engineering challenge is to use the correct combination of existing and novel technologies to achieve innovation at a reasonable price.