Wavelengths of Operation

Glass fiber data links normally operate in one of three wavelength bands (Figure 11.7). These bands are positioned to fit into relatively low-attenuation zones of the glass-fiber attenuation curve. The effects that create most of the attenuation in modern low-loss fibers are Rayleigh scattering , infrared absorption , and absorption by impurities .

Wavelengths much shorter than 700 nm are heavily affected by Rayleigh scattering. This phenomenon is an unavoidable consequence of the amorphous nature of glass, whose random atomic structure causes microscopic variations in the local refractive index. Near the edges of the waveguide , random variations in the refractive index occasionally focus light out of the fiber, never to return. The theoretical loss due to this mechanism (in dB) varies with (1/ l 4 ).

Wavelengths much longer than 1700 nm are most heavily affected by the intrinsic absorption of infrared light by the molecules of glass themselves . These infrared absorption peaks occur between 7000 and 12,000 nm in typical glass fibers [89] . The residual tails from these strong absorption peaks effectively preclude fiber operation above about 1800 nm.

In the central region of Figure 11.7, between 700 and 1700 nm, light is absorbed by various impurities within the glass. Paramount among these is water ( specifically , the OH ion), which in combination with silica produces absorption peaks near 1390, 1240, and 950 nm [90] . Concentrations of OH ions must be reduced below 1 part in 10 8 to achieve the performance illustrated in the Figure 11.7.

Figure 11.7. Attenuation versus wavelength for glass fiber.

What remains are three broad windows commonly used for fiber operation. The first window ranges from 770 nm to 860 nm. The second window ranges from 1270 nm to 1355 nm, and the third from 1500 nm to 1600 nm. Historically, the earliest transceivers used window 1, then window 2, and only recently window 3. This progression of usage corresponds with our ability to produce cheap LED and laser sources operating at progressively longer wavelengths.

Fibers specified for use in both first and second windows are called dual-window fibers. The second and third windows benefit from some peculiar adjustments, called dispersion-shifting, that can be made in the composition of the fiber to improve the bandwidth in one window at the expense of the others.

POINT TO REMEMBER

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