Intrinsic and Extrinsic Attenuation in Fiber Optic Cables
Attenuation, or the loss of light or signal, is a factor that is almost unavoidable when installing your fiber optic cable network.
Attenuation limits the distance in which the signal can travel through optical fiber and is measured in decibels (dB). It can either be inherent within the glass, known as intrinsic attenuation, or it can be caused by external factors, known as extrinsic attenuation.
There are two different forms of intrinsic attenuation: absorption and scattering. Absorption describes the process of natural impurities, such as hydrogen ions, absorbing the light energy within the glass. The second form is scattering, commonly referred to as “Rayleigh Scattering.” This takes place when the natural minute particles within the fiber scatter the light energy.
Since attenuation is nearly unavoidable when installing a fiber network, there will innately be natural loss values. To optimize on these values, certain fiber types have maximum attenuation rates, or loss values, to prevent a further decrease in optical power. If the signal loss is too high, then it will negatively affect the optical fiber's performance. The typical loss values for intrinsic attenuation for single-mode fibers are approximately 0.40 dB/km at 1310 nm and 0.30 dB/km at 1550 nm. For multimode fibers, the values are a bit higher around 3.50 dB/km at 850 nm and 1.50 dB/km at 1300 nm. However, it’s important to note that these loss values can vary depending on several factors, such as various cable types being used.
Extrinsic attenuation is caused by external factors, such as from people or the environment. This can happen from bending losses, poor splices or poor connectors. Macrobends occur when the fiber is bent so greatly that it changes the critical angle between the core and the cladding, thus causing light to escape. Microbends take place when a lateral stress is applied to the fiber, again causing the critical angle to change and permitting light to escape. Unlike macrobends, which are visible to the human eye, microbends are not.
The typical loss values for extrinsic attenuation are approximately 0.25 dB to 0.75 dB for connector losses, and around 0.05 dB to 0.30 dB for bad splices. Values higher than these recommended ones will result in poorer fiber optic performances due to increased signal losses. Again, these values can vary depending on the reference standard and the type of splicing that is used.
Unless there are installation issues, intrinsic loss will not change. Splice and connector losses, which are allocated in the loss budget, have maximum parameters set by the project engineer, which must be met by the contractor or personnel building the network.
To learn more about fiber optic cable, check out this presentation from Patrick Dobbins, Director of Applications Engineering and Field Engineering for AFL.
For more information on the basics of fiber optic cable, read more about its benefits, basic structure and the differences between single-mode and multimode.