Basic Principles of Fiber Optics Series: Attenuation

Written by Ben Hamlitsch, trueCABLE Technical and Product Innovation Manager RCDD, FOI

Fiber optic cables have many advantages, but one of the downsides just like with copper cable, is that it can experience what is called attenuation. Attenuation refers to the loss of light as it travels down the fiber. This can be due to a variety of factors: scattering and absorption, intrinsic loss, extrinsic loss, bending losses and more. If you don't know what kind of losses to expect in your system, you won't know how many other components will be needed to compensate for them. In this article we'll discuss attenuation and how it affects fiber optic systems.

Losses In Fiber Optic Cable

• Scattering/Intrinsic Loss
• Absorption/Intrinsic Loss
• Bending Losses/Extrinsic Loss

As we mentioned in the introduction, attenuation is the loss of light as it travels down the fiber. As light travels through the glass core of an optical fiber and is absorbed by the cladding as it passes through, this causes varying amounts of attenuation in the fiber optic cable.

Light can also be scattered by fibers, causing it to be diffused before reaching its destination. This scattering process causes some loss but is not usually considered attenuation because there is no absorption involved in this process.

Losses in fiber optic cables are generally caused by three main problems: scattering, absorption, and bending losses.

Scattering Loss

The scattering of light is a form of intrinsic attenuation. Scattering accounts for the greatest amount of attenuation in a fiber cable, between 95 and 97 percent. Light traveling through the fiber interacts with the densities as shown in the light and is then partially scattered in all directions.

Scattering is caused by atomic structures and particles in the fiber that redirect the light that hits them. This process is called Rayleigh scattering, after the late Lord Rayleigh, a British physicist who first described the phenomenon in the late 19th century.

Rayleigh scattering is also the answer to the age-old question, "Why is the sky blue?" The blue that we see is actually the more prevalent blue wavelengths of light from the sun being scattered by particles in the atmosphere. As the sun moves toward the horizon and the light must pass through more of the atmosphere, the scattering increases to the point where the blue light is almost completely attenuated, leaving the red wavelengths, which are less affected by the scattering for reasons that we'll see shortly.

Rayleigh scattering depends on the relationship between wavelength and the size of the structures in the fiber. Scattering decreases as the wavelength of the light approaches the size of the structures, which means that in fiber-optic communications, shorter wavelengths are more likely to be scattered than the longer wavelengths. This is one of the main reasons that infrared wavelengths are used in fiber optics. The relatively long wavelengths of infrared are less subject to scattering than visible wavelengths. It also explains why the sun turns red on the horizon. The shorter blue wavelengths are more likely to be scattered by the similarly sized particles in the atmosphere than are the red wavelengths.

Absorption Loss

Absorption loss accounts for typically between 3 and 5 percent of the total attenuation in a fiber link. All materials, even the clearest glass, absorb some light. The amount of absorption depends on the type of material and the wavelength of the light passing through it.

You can see absorption easily in sunglasses. Even on the brightest days, only a fraction of the light's energy passes through the tinted lenses. The wavelengths that do not pass through are mostly absorbed by impurities that have been placed in, or coated on, the lens material.

Light travels best in clear substances. Impurities such as metal particles or moisture in the fiber can block some of the light energy, which will then get absorbed. Absorption is the transformation of light into heat as it passes through a dense medium.

Impurities in the glass fiber (such as hydroxyl ions) absorb the light and transform it into heat, causing slight losses in power. These impurities and imperfections absorb light, thus reducing its power as it travels along the fiber. The more light that is absorbed, the more attenuation loss you will have at the other end.

The solution is to use ultra-pure glass and dopant chemicals to minimize the impurities, and to eliminate loss at the peak wavelength during the fiber manufacturing process.

Bending Losses

Because light travels at different speeds through different materials, bending losses are a result of light being scattered and lost when it bends around corners. The more bends in the cable, the greater your losses will be.

Microbending:

Microbending occurs when the fiber optic cable is bent on a small scale, typically at a radius of less than 1 cm. This type of bending can occur when the cable is subjected to small changes in temperature, pressure, or mechanical stress. Microbending can cause the light traveling through the fiber to be scattered, resulting in signal loss and reduced transmission quality.

Microbends are small distortions of the boundary layer between the core and cladding caused by crushing or pressure. Microbends are very small and may not be visible when looking at the fiber optic cable.

Microbends change the angle of incidence within the fiber. Changing the angle of incidence forces high-order light rays to reflect at the angles that prevent further reflection, causing them to be lost in the cladding and absorbed.

Macrobending:

Macrobending occurs when the fiber optic cable is bent on a larger scale, typically at a radius of more than 1 cm. This type of bending can occur when the cable is subjected to more significant changes in temperature, pressure, or mechanical stress, such as those caused by cable bending or tension. Macrobending can also cause the light traveling through the fiber to be scattered, resulting in signal loss and reduced transmission quality.

Both microbending and macrobending can have negative effects on the performance of fiber optic systems, and efforts are often made to minimize both types of bending in fiber optic cables. This may involve using special cable designs or installation techniques that reduce the likelihood of bending, as well as using protective measures such as cable clamps or trays to protect the cables from external stresses.

Compared to a microbend, a macrobend has a much larger radius. Macrobends occur when the fiber is bent around a radius that can be measured in millimeters. These tight radii change the angle of incidence within the fiber, causing some of the light rays to reflect outside of the fiber and, as with microbending, be lost in the cladding and absorbed.

Attenuation is measured in decibels/km, which can be converted to a loss value (in decibels) for a specific length of cable.

The shorter the wavelength, the less light is absorbed. Thus, singlemode has lower losses than multimode at equivalent lengths.

The attenuation of a fiber is affected by the wavelength and length of the cable. The longer its length, the more power it will absorb due to scattering and absorption. Singlemode fibers have lower losses at equivalent lengths than multimode fibers.

Multimode fibers have higher losses at equivalent lengths than singlemode fibers, but less than singlemode fibers do at their respective lengths when used for long distances (i.e., above 100 meters).

• Attenuation increases slightly with increasing temperature but decreases with increasing humidity when dry at the center of the cable.
• When calculating attenuation in a fiber optic cable, it is important to know that attenuation is higher at the center of the cable than at its edges.

Conclusion

The attenuation of a fiber optic cable is an important factor to consider when designing and deploying fiber optic networks, as it affects the maximum distance that a signal can travel without amplification or regeneration. To minimize attenuation, high-quality fiber optic cables and components are used, and the cables are typically installed in a protective sheath to reduce the impact of environmental factors such as temperature and humidity. The more you know about the attenuation in your fiber optic system, the better prepared you will be to plan for it.

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