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Modern Trends in Telecom & Information Superhighway. Institute of Information & Communication Technologies (IICT), Mehran UET, Jamshoro. Signal Loss in Optical Fibers. Attenuation . Attenuation .
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Modern Trends in Telecom & Information Superhighway Institute of Information & Communication Technologies (IICT), Mehran UET, Jamshoro
Attenuation Attenuation is the loss of optical power as light travels along the fiber. Signal attenuation is defined as the ratio of optical input power (Pi) to the optical output power (Po). Signal attenuation (also known as signal loss or fiber loss) determines the maximum repeater-less separation between a transmitter and a receiver. Metallic copper offers attenuation of 5 dB/km while fiber optic cable offers 0.2 dB/km. The total attenuation (A), normally expressed in the logarithmic unit of the decibel, between an arbitrary point X and point Y located on the fiber is: Px is the power output at point X. P y is the power output at point Y.
History of Attenuation End users measure the total attenuation of a fiber at the operating wavelength, λ. At different wavelengths, attenuation offered by fiber optic cable is different
Attenuation Co-efficient In optical fiber communications the attenuation is usually expressed in decibels per unit length (i.e., the attenuation coefficient (α) or attenuation rate), following: L is the distance between points X and Y. α is a positive number because Px is always larger than Py. The attenuation coefficient will also vary with changes in wavelength, λ. Attenuation in an optical fiber is caused by number of mechanisms normally influenced by absorption, scattering, and bending losses (discussed later in this lecture).
Absorption • Absorptionis defined as the portion of attenuation resulting from the conversion of optical power into another energy form, such as heat. • Basically, absorsption occurs when a light particle (photon) interacts with an electron and excites it to a higher energy level. • The main culprits are: the transition metal ions and the OH ions present in the glass. The transition metals are those elements that live between the third and the twelfth rows of the periodic table of the elements. The unwanted elements are mainly Fe, Cu, V, Co, Ni, Mn and Cr. The OH ions come from water. • Absorption in optical fibers is explained by three factors: • Imperfections in the atomic structure of the fiber material • The intrinsic or basic fiber-material properties • The extrinsic (presence of impurities) fiber-material properties
Intrinsic Absorption Intrinsic absorption is caused by basic fiber-material properties when an optical fiber were absolutely pure, with no imperfections or impurities. Intrinsic absorption sets the minimal level of absorption. In fiber optics, silica (pure glass) fibers are used predominately. Silica fibers are used because of their low intrinsic material absorption at the wavelengths of operation. In silica glass, the wavelengths of operation range from 700 nanometers (nm) to 1600 nm. This wavelength of operation is between two intrinsic absorption regions. The first region is the ultraviolet region (below 400-nm wavelength). The second region is the infrared region (above 2000-nm wavelength).
Intrinsic Absorption Intrinsic absorption in the ultraviolet region is caused by electronic absorption bands. The main cause of intrinsic absorption in the infrared region is the characteristic vibration frequency of atomic bonds. The interaction between the vibrating bond and the electromagnetic field of the optical signal causes intrinsic absorption. Light energy is transferred from the electromagnetic field to the bond.
Extrinsic Absorption Extrinsic absorption is caused by impurities (such as iron, nickel, and chromium) introduced into the fiber material during fabrication. Extrinsic absorption is caused by the electronic transition of these metal ions from one energy level to another. Extrinsic absorption also occurs when hydroxyl ions (OH-) are introduced into the fiber. The amount of water (OH-) impurities present in a fiber should be less than a few parts per billion. Fiber attenuation caused by extrinsic absorption is affected by the level of impurities (OH-) present in the fiber. If the amount of impurities in a fiber is reduced, then fiber attenuation is reduced. Water or more precisely (OH-) ion, which is the main fiber impurity, creates an attenuation peak around 1430 nm.
Scattering losses During manufacturing, regions of higher and lower molecular density areas, relative to the average density of the fiber, are created. Light traveling through the fiber interacts with the density areas and as a consequence it is then partially scattered in all directions. Scattering losses in glass arise from microscopic variations in the material density, from compositional fluctuations, and from structural in-homogeneities or defects occurring during fiber manufacture. The molecular density of material used to manufacture fiber cable contains regions in which molecular density is higher or lower than the average density. In addition, since glass is made up of several oxides, such as SiO2, GeO2, and P2O5, compositional fluctuations can occur. These two effects results in the variation of refractive index.
Linear Scattering Losses • Linear scattering mechanism cause the transfer of some or all of the optical power contained within one propagating mode to be transferred linearly (proportionally to the mode power) into a different mode. • This process tends to result in attenuation as the transfer of power is may be to a leaky or radiated mode. • It must be noted that as with all linear processes there is no change of frequency on scattering. • Linear scattering may be categorized into two major types: • Rayleigh (700-nm and 1600-nm wavelength) and • Mie scattering. • Both result from the non-ideal physical properties of the manufactured fiber.
Rayleigh Scattering Rayleigh scattering occurs when the size of the density fluctuation (fiber defect) is less than one-tenth of the operating wavelength of light. Loss caused by Rayleigh scattering is proportional to the fourth power of the wavelength. Rayleigh scattering, which is in almost all directions, is the physical limit to attenuation, increases dramatically when wavelength decreases, following a (1/λ4) variation. This is the main reason why optical telecommunications use infrared light. Rayleigh scattering of sunlight from particles in the atmosphere is the reason why the light from the sky is blue.
Rayleigh Scattering Rayleigh scattering Formula For a single component glass, the Rayleigh scattering coefficient γR formula is: where p = photo-elastic coefficient, λ = operating wavelength n = refractive index of the medium βc = the compressibility coefficient specific to each material. K = Boltzmann’s constant TF = fictive temperature (the temperature at which the material attains isothermal equilibrium). Transmission loss due to Rayleigh Scattering The Rayleigh scattering coefficient is related to the transmission loss factor (transmitivity) of the fiber by:
Mie Scattering • Mie scattering occurs at inhomogeneties in fiber medium which are comparable in size to the guided wavelength. • If the size of the defect is greater than one-tenth of the wavelength of light (λ/10), the scattering mechanism is called Mie scattering. • These result from the non-perfect cylindrical structure of the waveguide and may be caused by fiber imperfections such as irregularities in the core-cladding interface, core-cladding refractive index differences along the fiber length, diameter fluctuations, strains and bubbles. • Mie scattering, caused by these large defects in the fiber core, scatters light out of the fiber core and occurs mainly in the forward direction. • However, in commercial fibers, the effects of Mie scattering are insignificant. Optical fibers are manufactured with very few large defects.
Non-linear Scattering Losses • Optical waveguides do not always behave as completely linear channels whose increase in output optical power is directly proportional to the input optical power. Several non-linear effects occur, which in the case of scattering cause disproportionate attenuation, usually at high optical power levels. • This non-linear scattering causes the optical power from one mode to be transferred in either the forward or backward direction to the same, or other modes, at a different frequency. • The most important types of nonlinear scattering within optical fibers are • Stimulated Brillouin Scattering, and • Raman scattering, • Both of which are usually only observed at high optical power densities in long single-mode fibers.
Stimulated Brillouin Scattering Brillouin scattering occurs when light in a medium interacts with time dependent optical density variations and changes its energy (frequency) and path. Stimulated Brillouin Scattering (SBS) may be regarded as the modulation of light through thermal molecular vibrations within the fiber. The scattered light appears as upper and lower sidebands which are separated from the incident light by the modulation frequency. The incident photon in this scattering process produces acoustic phonon and as well as scattered photon. The frequency shift is a maximum in the backward direction & zero in the forward direction. Brillouin scattering is only significant above a threshold power density. Assuming that the polarization state of the transmitted light is not maintained, it may be shown that the threshold power PB is given by: where d and λ are the fiber core diameter and the operating wavelength, respectively. αdB is the fiber attenuation coefficient and v is the source bandwidth in GHz. Brillouin scattering can also be employed to sense mechanical strain and temperature in optical fibers
Stimulated Raman Scattering Stimulated Raman Scattering (SRS) is similar to Stimulated Brillouin scattering except that a high frequency optical phonon (i.e., quantum of an elastic wave in a crystal lattice) rather than an acoustic phonon is generated in the scattering process. Also, SRS can occur in both the forward and reverse directions in an optical fiber, and may have an optical threshold of up to three orders of magnitude higher than the Brillouin threshold in a particular fiber. Using the same criteria as those specified for the Brillouin scattering threshold, it may be shown that the threshold optical power for SRS PR in a long single-mode fiber is given by: where d and λ are the fiber core diameter and the operating wavelength, respectively. αdB is the fiber attenuation coefficient. SBS and SRS are not usually observed in multimode fibers because their relatively large core diameters make the threshold optical power levels extremely high.
Fiber Bend Loss • Optical fibers suffer radiation losses at bends or curves on their paths. • The loss can generally be represented by a radiation coefficient which has the form: • where R is the radius of curvature of the fiber bend and c1 and c2 are constants which are independent of R. Furthermore, large bending losses tend to occur at a critical radius of curvature Rcwhich may be estimated from: • It may be observed that possible bending losses may be reduced by: • Designing fibers with large relative refractive index differences • Operating at the shortest possible wavelength.
Bending Losses • Bending the fiber also causes attenuation. Bending loss is classified according to the bend radius of curvature: microbend loss or macrobend loss. • Microbends are small microscopic bends of the fiber axis that occur mainly when a fiber is cabled. Microbend losses are caused by small discontinuities or imperfections in the fiber. Uneven coating applications and improper cabling procedures increase microbend loss. External forces are also a source of microbends. An external force deforms the cabled jacket surrounding the fiber but causes only a small bend in the fiber. Microbends change the path that propagating modes take. Microbend loss increases attenuation because low-order modes become coupled with high-order modes that are naturally lossy.
Macrobends Macrobendsare bends having a large radius of curvature relative to the fiber diameter. During installation, if fibers are bent too sharply, macrobend losses will occur. These bends become a great source of loss when the radius of curvature is less than several centimeters. Light propagating at the inner side of the bend travels a shorter distance than that on the outer side. To maintain the phase of the light wave, the mode phase velocity must increase. When the fiber bend is less than some critical radius, the mode phase velocity must increase to a speed greater than the speed of light. However, it is impossible to exceed the speed of light. This condition causes some of the light within the fiber to be converted to high-order modes. These high-order modes are then lost or radiated out of the fiber. Fiber sensitivity to bending losses can be reduced, if the refractive index of the core is increased, then fiber sensitivity decreases. Sensitivity also decreases as the diameter of the overall fiber increases. However, fibers with larger core size propagate more modes. These additional modes tend to be more lossy.
Dispersion • In optics, dispersion is a phenomenon that causes the separation of a wave into components with different frequency. It is most often observed in light waves, though it may happen to any kind of wave that interacts with a medium, such as sound waves. • Rainbows form when light coming from the sun is refracted by the water droplets present in the atmosphere. White light which is a mixture of colors is then separated into its different wavelengths: this phenomenon is called dispersion. • Dispersion-less transmission in general requires a constant group velocity. There are several types of dispersion. • Modal Dispersion • Material Dispersion • Waveguide Dispersion
Dispersion This assumes that the pulse broadening due to dispersion on the channel is τ which dictates the input pulse duration which is also τ. Hence, the maximum bit rate that can be obtained on an optical fiber link is 1/2τ . Dispersion in digital OFC systems cause broadening of transmitted pulses as they travel along the channel. If dispersion is severe, each pulse will overlaps with its neighbor, eventually becoming indistinguishable – Intersymbol Interference (ISI). For no overlapping of light pulses down on an optical fiber link the digital bit rate Bt must be less than the reciprocal of the broadened pulse duration (2τ). Hence:
Bandwidth When a return to zero code is used the binary level is held for only part (usually half) of the bit period. For this signaling scheme the data rate is equal to the bandwidth in hertz (i.e., one bit per second per hertz) and thus: The conversion of bit rate to bandwidth in hertz depends on the digital coding format used. When a non return to zero (NRZ) code is used, the binary one level is held for the whole period τ. In this case there are two bit periods in one wavelength (i.e., two bits per second per hertz). Hence, the maximum bandwidth B is one half the maximum bit rate or
Bandwidth-Length (BL) product The amount of pulse broadening is dependent on the distance the light pulse travels. Thus the measurement of dispersive properties of a particular fiber is usually stated as the pulse broadening in time over a unit length of the fiber (i.e., ns/km) In the absence of mode coupling or filtering, the pulse broadening increases linearly with distance. Thus the actually capacity of fiber is defined as the Bandwidth x Length product. The typical best bandwidth-length products for the three fibers are 20 MHz km, 1 GHz km, and 100 GHz km for multimode step index, multimode graded index, and single mode step index fibers respectively.
Dispersion: The Basics Light propagates at a finite speed fastest ray slowest ray fastest ray: one traveling down middle (“axial mode”) slowest ray: one entering at highest angle (“high order” mode) there will be a difference in time for these two rays
Types of Dispersion in Fibers NOTE: GRIN fibers tend to have less modal dispersion because the ray paths are shorter modal -time delay from path length differences- usually the biggest culprit in step-index material-n() : different times to cross fiber -(note: smallest effect ~ 1.3 m) Waveguide - changes in field distribution -(important for SM) non-linear-n can become intensity-dependent
Maximum Pulse Dispersion B O A θ C v = c/n
Group Delay In optics, the mode contains all of the spectral components in the wavelength over which the source emits. As the signal propagates along the fiber, each spectral component can be assumed to travel independently, and to undergo a time delay or group delay per unit length in the direction of propagation given by here L is the distance traveled by the pulse, β is the propagation constant along the fiber axis, k = 2π/λ , and vg is the group velocity given as:
Group Delay If the spectral width of the optical source is not too wide, the delay difference per unit wavelength along the propagation path is approximately . For spectral components which are apart, the total delay difference over a distance L is: The factor is known as dispersion. It defines pulse spread as a function of wavelength and is measured in picoseconds per kilometer per nanometer.
Effect of Modal Dispersion initial pulse farther down farther still time time time modal example: step index ~ 24 ns km -1 GRIN ~ 122 ps km-1
Material Dispersion • In standard silica fiber, at 1310 nm, waveguide and material dispersions will cancel out each other. This is called the zero dispersion wavelength. • Although material dispersion can not be modified much, waveguide dispersion can be either shifted or optimized to achieve • 1. Dispersion Shifted fiber that has zero dispersion at 1550 nm or • 2. Dispersion flattened fiber that has low dispersion for a wide wavelength range.
