Hello, everyone!
There are three specifications of optical fiber: size (dimension of core and cladding), bandwidth, and attenuation. Today I will explain the attenuation of the optical fiber.
Attenuation
Attenuation occurs during the propagation of the optical signal through the optical fiber. How much the signal will weaken depends on the length of the optical fiber. The attenuation is given in dB/km, it is a longitudinal attenuation of the optical fiber.
This is normal for signal propagation through any medium. If we compare the weakening of cables with copper conductors, the weakening in optical fibers is very small. The amplifier is often required for copper cables, which is not the case with optical fibers. Singlemode fibers require an optical amplifier at about 100km or more. Multimode fibers have a greater attenuation and work up to 2 km.
Fig. 1. Attenuation
The attenuation of the optical signal depends on the wavelength. There are three optical windows: 850 nm, 1310 nm, and 1550 nm, where attenuation and dispersion are optimal. Multimode optical fibers use 850 nm and 1300 nm (sources: LED, VCSEL, and FB laser), and single-mode optical fibers use 1310 nm and 1550 nm (sources: FB and DFB lasers).
The attenuation of multimode optical fibers at 850 nm is 2.5 dB/km, while the attenuation of multimode fibers at 1300 nm is 0.8 dB/km. Singlemode fiber has attenuation on the 1310 nm is about 0.35 dB/km and about 0.25 dB/km at 1550 nm.
There are two main reasons that lead to weakening, these are absorption and scattering.
Scattering
Optical fiber is not perfectly made. There are impurities in it. Therefore, when a light signal propagates through an optical fiber, it can encounter impurities and scatter at that point. It is a phenomenon called scattering. The light is scattered in different directions, which causes attenuation of the optical signal. It is the largest cause of attenuation.
Fig 2. Scattering
Absorption
Absorption most often occurs due to the presence of OH + molecules in the fiber. Absorption depends on the wavelength of light. The strongest absorption occurs at wavelengths of 1000 nm, 1400 nm, and above 1600 nm.
Thank you!
