VI. Attenuation in Optical Fibers

Attenuation and pulse dispersion represent the two most important characteristics of an optical fiber that determine the information-carrying capacity of a fiber optic communication system. Obviously, the lower the attenuation (and similarly, the lower the dispersion) the greater can be the required repeater spacing and therefore the lower will be the cost of the communication system. Pulse dispersion will be discussed in the next section, while in this section we will discuss briefly the various attenuation mechanisms in an optical fiber.

The attenuation of an optical beam is usually measured in decibels (dB). If an input power P1 results in an output power P2, the power loss a in decibels is given by

  a (dB) = 10 log10 (P1/P2)

(7-12)

Thus, if the output power is only half the input power, the loss is 10 log 2 » 3 dB. Similarly, if the power reduction is by a factor of 100 or 10, the power loss is 20 dB or 10 dB respectively. If 96% of the light is transmitted through the fiber, the loss is about 0.18 dB. On the other hand, in a typical fiber amplifier, a power amplification of about 1000 represents a power gain of 30 dB.

Figure 7-10 shows the spectral dependence of fiber attenuation (i.e., loss coefficient per unit length) as a function of wavelength of a typical silica optical fiber. The losses are caused by various mechanisms such as Rayleigh scattering, absorption due to metallic impurities and water in the fiber, and intrinsic absorption by the silica molecule itself. The Rayleigh scattering loss varies as 1/l04, i.e., shorter wavelengths scatter more than longer wavelengths. Here l0 represents the free space wavelength. This is why the loss coefficient decreases up to about 1550 nm. The two absorption peaks around 1240 nm and 1380 nm are primarily due to traces of OH– ions and traces of metallic ions. For example, even 1 part per million (ppm) of iron can cause a loss of about 0.68 dB/km at 1100 nm. Similarly, a concentration of 1 ppm of OH– ion can cause a loss of 4 dB/km at 1380 nm. This shows the level of purity that is required to achieve low-loss optical fibers. If these impurities are removed, the two absorption peaks will disappear. For l0 > 1600 nm the increase in the loss coefficient is due to the absorption of infrared light by silica molecules. This is an intrinsic property of silica, and no amount of purification can remove this infrared absorption tail.

Figure 7-10 Typical wavelength dependence of attenuation for a silica fiber. Notice that the lowest attenuation occurs at 1550 nm [adapted from Miya, Hasaka, and Miyashita].

As you see, there are two windows at which loss attains its minimum value. The first window is around 1300 nm (with a typical loss coefficient of less than 1 dB/km) where, fortunately (as we will see later), the material dispersion is negligible. However, the loss attains its absolute minimum value of about 0.2 dB/km around 1550 nm. The latter window has become extremely important in view of the availability of erbium-doped fiber amplifiers.