Signal degradation on optical fiber due to dispersion and attenuation
Attenuation
Attenuation is a measure of decay of signal strength or loss of light power that occurs as light pulses propagate through the length of the fiber.In optical fibers the attenuation is mainly caused by two physical factors absorption and scattering losses. Absorption is because of fiber material and scattering due to structural imperfection within the fiber. Nearly 90 % of total attenuation is caused by Rayleigh scattering only. Micro bending of optical fiber also contributes to the attenuation of signal.
The rate at which light is absorbed is dependent on the wavelength of the light and the characteristics of particular glass. Glass is a silicon compound, by adding different additional chemicals to the basic silicon dioxide the optical properties of the glass can be changed.
The Rayleigh scattering is wavelength dependent and reduces rapidly as the wavelength of the incident radiation increases.
The attenuation of fiber is governed by the materials from which it is fabricated, the manufacturing process and the refractive index profile chosen. Attenuation loss is measured in dB/km.
As attenuation leads to a loss of power along the fiber, the output power is significantly less than the couples power. Let the couples optical power is p(0) i.e. at origin (z = 0).Then the power at distance z is given by,
As attenuation leads to a loss of power along the fiber, the output power is significantly less than the couples power. Let the couples optical power is p(0) i.e. at origin (z = 0).Then the power at distance z is given by,
Attenuation is also a function of wavelength. Optical fiber wavelength as a function
of wavelength is shown in Fig
The amount of Rayleigh scattering loss in an optical fiber depends on several factors, including the wavelength of the light, the refractive index of the core and cladding, and the size and concentration of the scattering particles. The scattering loss is proportional to the fourth power of the wavelength, which means that shorter wavelengths, such as those in the blue and green parts of the spectrum, are more strongly affected than longer wavelengths, such as those in the red and infrared parts of the spectrum.
Absorption
Absorption loss is related to the material composition and fabrication process of fiber.
Absorption loss results in dissipation of some optical power as hear in the fiber cable.
Although glass fibers are extremely pure, some impurities still remain as residue after purification. The amount of absorption by these impurities depends on their
concentration and light wavelength.
Absorption is caused by three different mechanisms.
Absorption by atomic defects in glass composition.
Extrinsic absorption by impurity atoms in glass matts.
Intrinsic absorption by basic constituent atom of
fiber.
Rayleigh Scattering Losses
Rayleigh scattering is a phenomenon that occurs when light is scattered by small particles in a medium, such as air, water, or glass. In an optical fiber, Rayleigh scattering causes some of the light to be scattered out of the core and into the cladding, resulting in loss of signal strength.The amount of Rayleigh scattering loss in an optical fiber depends on several factors, including the wavelength of the light, the refractive index of the core and cladding, and the size and concentration of the scattering particles. The scattering loss is proportional to the fourth power of the wavelength, which means that shorter wavelengths, such as those in the blue and green parts of the spectrum, are more strongly affected than longer wavelengths, such as those in the red and infrared parts of the spectrum.
Bending Loss
Losses due to curvature and losses caused by an abrupt change in radius of curvature
are referred to as ‘bending losses.’ The sharp bend of a fiber causes significant radiative losses and there is also a possibility of mechanical failure
Micro bending
Micro bending is a loss due to small bending or distortions. This small micro bending is
not visible. The losses due to this are temperature related, tensile related or crush
related. The effects of micro bending on multimode fiber can result in increasing attenuation
(depending on wavelength) to a series of periodic peaks and troughs on the spectral
attenuation curve. These effects can be minimized during installation and testing
Macro bending
The change in spectral attenuation caused by macro bending is different to
micro bending. Usually there are no peaks and troughs because in a macro bending no
light is coupled back into the core from the cladding as can happen in the case of
micro bends. The macro bending losses are cause by large scale bending of fiber. The losses are
eliminated when the bends are straightened. The losses can be minimized by not
exceeding the long term bend.
Signal Distortion in Optical Waveguide
The pulse get distorted as it travels along the fiber lengths. Pulse spreading in fiber is
referred as dispersion. Dispersion is caused by difference in the propagation times of
light rays that takes different paths during the propagation. The light pulses travelling
down the fiber encounter dispersion effect because of this the pulse spreads out in time
domain. Dispersion limits the information bandwidth. The distortion effects can be
analyzed by studying the group velocities in guided modes.
Group Delay
Consider a fiber cable carrying optical signal equally with various modes and each mode
contains all the spectral components in the wavelength band. All the spectral
components travel independently and they observe different time delay and group
delay in the direction of propagation. The velocity at which the energy in a pulse travels
along the fiber is known as group velocity. Group velocity is given by,
Material Dispersion
Material dispersion is also called as chromatic dispersion. Material dispersion exists due
to change in index of refraction for different wavelengths
Waveguide Dispersion
Waveguide dispersion is caused by the difference in the index of refraction between the
core and cladding, resulting in a ‘drag’ effect between the core and cladding portions of
the power. Waveguide dispersion is significant only in fibers carrying fewer than 5-10 modes. Since
multimode optical fibers carry hundreds of modes, they will not have observable
waveguide dispersion.
Chromatic Dispersion
The combination of material dispersion and waveguide dispersion is called chromatic
dispersion. These losses primarily concern the spectral width of transmitter and choice
of correct wavelength.
Modal Dispersion
As only a certain number of modes can propagate down the fiber, each of these modes
carries the modulation signal and each one is incident on the boundary at a different
angle, they will each have their own individual propagation times. The net effect is
spreading of pulse, this form o dispersion is called modal dispersion. Modal dispersion takes place in multimode fibers. It is moderately present in
graded index fibers and almost eliminated in single mode step index fibers.
Mode Coupling
After certain initial length, the pulse distortion increases less rapidly because of
mode coupling. The energy from one mode is coupled to other mods because of:
Structural imperfections.
Fiber diameter variations.
Refractive index variations.
Micro bends in cable.
Due to the mode coupling, average propagation delay become less and
intermodal distortion reduces.
OTDR (Optical Time Domain Reflectometer)
Optical Time Domain Reflectometer is a common measurement technique used in optical fiber communication systems to measure the optical fiber link characteristics such as attenuation, dispersion, and reflectivity. OTDR works by sending a short, high-intensity pulse of light into the optical fiber and measuring the time and intensity of the light that is reflected back. This reflected light is caused by scattering and reflections from any changes in the fiber's refractive index, such as connectors, splices, or breaks.OTDR measurements provide information on the length of the fiber, the attenuation or loss of the signal as it travels through the fiber, and the location and severity of any discontinuities or reflections along the length of the fiber. By analyzing the reflections and attenuation, OTDR measurements can help identify and locate any problems in the fiber, such as breaks or damage, and provide information on the quality of the fiber link.
In addition to OTDR, other measurement techniques commonly used in optical fiber communication systems include optical power meters, optical spectrum analyzers, and optical wavelength meters. These tools provide additional information on the characteristics of the optical signal, such as its power level, wavelength, and spectral content.
Fabrication of fibers
Fiber optic cables are made of thin strands of glass or plastic, called optical fibers, that transmit light signals over long distances. The fabrication process of fiber optics involves several steps, including preform fabrication, fiber drawing, coating, and testing.Preform Fabrication: The first step in making fiber optic cables is to create a preform, which is a cylindrical rod of glass or plastic that is used to draw the optical fiber. The preform is made by using a high-temperature process called chemical vapor deposition (CVD), which involves depositing layers of glass or plastic onto a cylindrical rod. This process creates a uniform composition and a controlled refractive index profile along the length of the preform.
Fiber Drawing: After the preform is made, it is heated in a furnace to a temperature that softens the glass or plastic material. A fiber drawing tower is used to pull a thin strand of glass or plastic from the softened preform. As the strand is pulled, it is cooled to solidify the material and form a uniform diameter optical fiber. This process is repeated to produce multiple fibers from a single preform.
Coating: Once the fibers are drawn, they are coated with a protective layer of polymer or acrylate. The coating protects the fragile glass or plastic fiber from damage during handling and installation. The coating also helps to reduce signal loss due to microbending or macrobending, which can occur if the fiber is bent too sharply.
Testing: Before the fiber optic cables are packaged and shipped, they undergo rigorous testing to ensure they meet the required performance standards. The fibers are tested for attenuation, bandwidth, and dispersion to ensure they can transmit light signals over long distances with minimal loss or distortion.
The fabrication of fiber optic cables is a precise and complex process that requires specialized equipment and expertise. However, the resulting cables provide high-speed, high-capacity communication links that are critical for modern telecommunications, internet connectivity, and other applications.
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