The extremely low attenuation or transmission loss of optical fibers is one of the most important factors in bringing its wide acceptance as a medium of transmission. Signal transmission within optical fibers, as with metallic conductors, is usually abbreviated as dB. The decibel (dB) is a convenient way of comparing two divergent power levels, say, P1 and P2. This is defined as
Optical fiber attenuation is the measurement of light loss between input and output. Total attenuation is the sum of all losses. So it is the sum of material absorption, Rayleigh scattering in the fiber and waveguide imperfections.
There are other factors which could also cause light loss, such as light leakage when the fiber is under microbending. Attenuation limits how far a signal can travel through a fiber before it becomes too weak to detect.
:: Material Absorption
Material absorption can be divided into two categories. Intrinsic absorption losses correspond to absorption by fused silica (material used to make fibers) whereas extrinsic absorption is related to losses caused by impurities within silica.
A) Intrinsic Absorption
Any material absorbs at certain wavelengths corresponding to the electronic and vibrational resonances associated with specific molecules. For silica molecules, electronic resonances occur in the ultraviolet region ( wavelength < 0.4um), whereas vibrational resonances occur in the infrared region (wavelength > 7um).
Because of the amorphous nature of fused silica, these resonances are in the form of absorption bands whose tails extend into the visible region. The following picture shows that intrinsic material absorption for silica in the wavelength range 0.8~1.6um is below 0.1dB/km. In fact, it is less than 0.03dB/km in the 1.3 to 1.6um wavelength range which are commonly used for lightwave systems.
B) Extrinsic Absorption
Extrinsic absorption results from the presence of impurities. Transition-metal impurities such as Fe, Cu, Co, Ni, Mn, and Cr absorb strongly in the wavelength range 0.6~1.6um. Their amount should be reduced to below 1 part per billion to obtain a loss level below 1dB/km. Such high-purity silica can be obtained by using modern techniques.
The main source of extrinsic absorption in state-of-the-art silica fibers is the presence of water vapors. A vibrational resonance of the OH ion occurs near 2.73um. Its harmonic and combination tones with silica produce absorption at the 1.39um, 1.24um and 0.95um wavelengths. The three spectral peaks seen in above figure occur near these wavelengths and are due to the presence of residual water vapor in silica.
In new kind of glass fiber, known as dry fiber, the OH ion concentration is reduced to such low levels that the 1.39um peak almost disappears. This is show in the below picture. Such fibers are used to transmit WDM (Wavelength Division Multiplexer) signals over the entire 1.30um to 1.65um wavelength range.
:: Rayleigh Scattering
Rayleigh scattering is a loss mechanism arising from local microscopic fluctuation in density. Silica molecules move randomly in the molten state and freeze in place during fiber fabrication. Density fluctuation lead to random fluctuations of the refractive index on a scale smaller than the optical wavelength. Light scattering in such a medium is known as Rayleigh scattering.
Scattering depends not on the specific type of material but on the size of the particles relative to the wavelength of light. The closer the wavelength is to the particle size, the more scattering. In fact, the amount of scattering increases rapidly as the wavelength decreases.
:: Waveguide Imperfections
An ideal single mode fiber with a perfect cylindrical geometry guides the optical mode without energy leakage into the cladding layer. But in reality, imperfections at the core-cladding interface, such as random core-radius variations, can lead to additional losses which contribute to the total fiber loss.
The physical process behind such losses is Mie scattering, occurring because of index inhomogeneities on a scale longer than the optical wavelength.
This has been taken good care of in optical fiber manufacturing and the core radius is made sure not to vary significantly along the fiber length.