Fiber optic networks
are networks where the transmission of data is done with the help of optical transceivers and optical cables. The optical transceivers transmit an optical light down an optical cable. As in the case with standard Ethernet copper networks, optical networks are also influenced by exterior stress and interior properties and as a consequence some power is loss. This optical power loss is called Attenuation.
Fiber optic cables consist of fiber optic glass core and cladding, buffer coating, Kevlar strength components and a protective exterior material called a jacket. Depending on the optical cable type. These components can vary in size and strength. Unlike the copper cables which use electricity to transmit data, fiber optic cables use pulses of optical light for the same function. Their core is made of an ultra-pure glass which is surrounded by a mirror like cladding. When the light hits the cable it travels down the core constantly bouncing of the cladding until it reaches the final destination. There are two types of optical cables, Multi-mode and Single-mode. From the outside they look almost the same, however their interior plays a huge role in the optical attenuation. Single-mode fibers are used for a long range, high speed connections because of their tighter core and cladding which improve the light transmission by limiting the light bouncing of the cladding. Multi-mode fibers have larger core thus the light will bounce more and more power will be lost until it reaches the destination.
However, the optical attenuation of optical fibers is not only the lost power due to the core of the cable. High optical attenuation can be caused by absorption, scattering and physical stress on the cable like bending. Signal attenuation is generally defined as the ratio of optical input power to the optical output power. As the names suggest, optical input power is the power injected in the optical cable by the optical transceiver, and optical output power is the power received by the transceiver at the other end of the cable. The unit of attenuation is described as dB/km.
Absorption is one of the biggest causes for optical attenuation. This is defined as the optical power lost due to the conversion of the optical power into another form. Absorption is typically caused by a residual water vapors. Generally absorption is defined by two factors:
Imperfection in the atomic structure of the fiber material
The extrinsic and intrinsic fiber-material properties which represent the presence of impurities in the fiber-material
The extrinsic absorption is caused by impurities like trace metals, iron and chromium, introduced into the fiber during the manufacturing process. These trace metals are causing a power loss during the process of conversion when they are transitioning from one energy level to another.
The intrinsic absorption is caused by the basic properties of the fiber material. If the optical fiber material is pure, with no impurities and imperfections, then all absorption would be intrinsic. For example in fiber optics silica glass is used due to its low intrinsic absorption at certain wavelengths ranging from 700nm to 1600nm.
Scattering losses are caused by the density fluctuations in the fiber itself. These are produced during the manufacturing process. Scattering occurs when the optical light hits various molecules in the cable and bounces around. Scattering is highly dependent on the wavelength of the optical light. There are two types of scattering loss in optical fibers:
Rayleigh scattering- this scattering occurs at commercial fibers that operate at 700-1600nm wavelengths. Rayleigh scattering occurs when the size of the density fluctuation is less than 1/10 of the operating wavelength.
Mie scattering- this scattering occurs when the size of the density fluctuation is bigger than 1/10 of the operating wavelength.
Bending the fiber cable also causes attenuation. The bending loss is classified in micro-bends and macro-bends:
Micro-bends are small microscopic bends in the fiber which most commonly occur when the fiber is cabled
Macro-bends on the other hand are bends that have a large radius of curvature relative to the cable diameter.
Another type of optical power loss is the optical Dispersion. Optical Dispersion represents the spreading of the light signal over time. There are two types of optical dispersion:
Chromatic dispersion which is spreading of the light signal resulting from the different speeds of the light rays
Modal dispersion which is spreading of the light signal resulting from the different propagation modes of the fiber
Modal dispersion is most commonly limiting the maximum bit rate and link length in Multi- mode fibers. The Chromatic dispersion is the main culprit for the attenuation in Single-mode fibers.
Having this in mind we should always consider, test and calculate the possible attenuation of the fibers for deploying a stable network capable for future upgrades.