ABSORPTION
Absorption is a major cause of signal loss in optical fiber. It is caused by material composition during the fabrication process in fiber optic. As a result, some of the transmitted optical signals have dissipated. It is also known as material absorption. Basically, the optical light was absorb and transferred into another form energy like heat due to molecular and wavelength impurities. Once of the type absorption losses is atom defect. Atom defect is imperfections in the atomic structure of the fiber material such as missing molecules and density of the atom group. Another two types of absorption is intrinsic and extrinsic.
IntrinsicIt is caused by basic atom structure which is there with the material used for the fabrication of optical fiber. If optical fiber a pure glass with no imperfections or impurities then it is an intrinsic absorption. In fiber their use silica. It’s because silica is one of the pure glass and produces low intrinsic material absorption at the wavelength operation. There has three window operation for optical fiber, which is 900, 1310 and 1550 nanometer. The wavelengths of operation range for 700nm to 1800nm.
The measured attenuation in optical fiber communication.There are two regions, which is ultraviolet and infrared. In the ultraviolet region is caused by electronic absorption band. It occurs when a photon interacts with an electron in the valence band and excites it to a higher energy level. At short-wavelength nearer ultraviolet region is more dominant which happens with silica. Another cause is atomic vibrations bands in the infrared region. In the figure, we can observe the absorption peak operating wavelength range from 700nm to 1200nm in the infrared region. It happens that because of the interaction between vibrations of silicon-oxygen (SiO) material band with the electromagnetic field of optical region.
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Extrinsic
Extrinsic has two causes, which is the electronic transitions of the metal impurities ions from one energy level to another and hydroxyl (OH) are introduced into the fiber. Some of the metallic impurities in glasses with the absorption losses caused by one part 109. Table 1.1 Absorption losses by metallic ions in glasses, with the absorption peak wavelengthFrom the table, chromium and copper are inappropriate materials, which can cause attenuation of more than 1 dB / km-1 in the infrared region. The amount of impurities should be below 1 per billion to obtain a loss below 1 dB/km. It is can reduce by refining techniques such as vapor-phase oxidation.Another major extrinsic is caused by hydroxyl ion (OH ion) impurities dissolved in the glass. The silicon-hydroxyl (Si-OH) bond has a fundamental frequency at 2700 nm. However, the harmonics of these fundamental frequencies at 1380 nm, 1250 nm and 950 nm also produces extrinsic absorption.
The three window that use for fundamental frequency. The first window is 950 nm, second window is 1310nm, and third window is 1510nm. Extrinsic absorption can be reduced by reducing the amount of impurities and reduce the level of hydroxyl ions. When the amount impurities and hydroxyl has to be reduced, then the fiber attenuation is reduced.
SCATTERING
Scattering is one of losses signal in fiber optic transmission, it is caused by the interaction of light with density fluctuation in the fiber cable, when it is happen the light energy will scattered into the random direction. In the fiber cable, some of the optical power from one propagating mode gets transferred to another propagating mode, which is the results will be lost signal as leaks or radiation mode. However, the leaks mode does not continue to propagate with the fiber core, but it is radiated out the fiber cable, it is knows as scattering loss. There are two categorize in scattering which is linear scattering and nonlinear scattering.Linear ScatteringIt causes transferred of optical power within one mode another mode without changing the frequency.
There are may be two type in linear scattering which is Rayleigh and Mie scattering.Rayleigh Scattering Rayleigh scattering is caused by interference with the particles smaller than the wavelength of the light, which may be individual atoms or molecules. The lights traveling through the fiber interacts with the particles and the partially scatters in all directions. Figure 1.4, shows Rayleigh scattering in fiber cable. The light has be loss into the classing and backscatter that caused by the particles like bunching of atom the fiber core. Figure 1.4: Rayleigh scattering in fiber cable.
The Rayleigh scattering produces attenuation in the light rays and this attenuation is proportional to 1/λ4, where λ is optical wavelength. Thus, the data transmitted through fiber cable at higher wavelengths, the scattering can be minimized.
There have some other causes which is the result of non-ideal physical properties during the manufacturing process and the inhomogeneities in the core and cladding. The inhomogeneities happen occurs due to fluctuation in refractive index, density, and compositional variations.
However, it is can be reduced by improved fabrication. Mie ScatteringAnother type of scattering is Mie Scattering. It is caused by inhomogeneities which are comparable size with the guided wavelength. This process of optical power always traveling in forward direction. There are some factors in Mie scattering, which is a cylindrical structure of cable is not perfect. Secondly, imperfection of core and cladding interface, core and cladding refractive index is not uniform through out of fiber, and due to bubble or strain fiber. When the size of the defect in optical fiber are larger than one-tenth of the wavelength of light, it is Mie scattering. The result of this scattering is a significant attenuation depending upon fiber material, size, design, and manufacturing process. However, the inhomogeneities can be reduced by removing imperfections during the glass manufacturing process.
Next, controlling the coating of fiber. Lastly, increase the refractive index difference between core and cladding.Non-Linear ScatteringIt causes the optical power from one mode to be transferred in either the forward or backward direction to other modes, at a different frequency. The direction of the scatter can be forward or backward. It produces optical gain with scatter mode but there is a shift in frequency that can be loss of signal and creates attenuation. There are two types of non-linear scattering, which is Stimulated Brillouin Scattering and Stimulated Raman Scattering.
STIMULATED BRILLOUIN SCATTERING
Stimulated brillouin is interaction of the light with the material waves in optical cable. It is happen when a light beam traveling in the optical cable, the variation in the electric field will produce acoustic vibrations. Therefore, the incident photon will produce a phonon of acoustic frequency as well as a scattered photon. Usually, this scattering happen in opposite direction to the incoming light beam. The scattered light will appears as upper and lower sidebands, which are separated from the incident light by modulation frequency. This process a frequency shift is produced, which varies with the scattering angle. This frequency shift is maximum in the backward scattering. When light is a straight line the scattering will appear in the opposite direction to the light at the optical fiber. Scatter in opposite direction at the fiber cable
RAMAN SCATTERING
Raman scattering is the inelastic scattering of photons. When the light has been travel in an optical fiber, the spontaneous scattering occurs. It transfers some of the photons to the near frequencies. The scattered photons can lose energy then it is called a Stokes shift or gain energy then it is called an anti-stokes shift. If a photon of other frequencies already present then the probability of scattering to those frequencies is enhanced. It is known as stimulated Raman Scattering.
In additional, Raman Scattering is a small fraction of Rayleigh scattered light comes off at the different frequency between the applied light and the frequency of a modulation vibration.
