UNIT-II : SIGNAL DEGRADATION IN OPTICAL FIBERS The Signal Transmitting through the fiber is degraded by two mechanisms. i) Attenuation ii) Dispersion Both are important to determine the transmission characteristics of the fiber at operating wavelength 1. Attenuation Signal attenuation also known as fiber loss or signal loss is one of the most important properties of an optical fiber. The basic attenuation mechanisms in a fiber are i) Absorption ii) Scattering iii) Radiation losses or Bending loss. Absorption is related to the fiber material Scattering are due to fiber material and structural imperfections in the optical waveguide Bending loss This losses occur whenever an optical fiber undergoes a bend of finite radius of curvature. Attenuation Units: Total power loss in an optical fiber is A(dB) = 10 Log [P in / P out ] As light travels along a fiber, its power decreases exponentially with distance. P(z) = P(0) e -α z Where, P(0) is the optical power in a fiber at the orgin (z = 0) α p fiber attenuation coefficient. α p = 1/z ln [P(0)/P(z)] The attenuation is denoted by α and unit of db/km α p = 10/z Log [P(0)/P(z)] This parameter is known as fiber loss or fiber attenuation.
1.1 Absorption losses Absorption loss is related to the material composition and fabrication process of fiber. Absorption is caused by three different mechanisms. i) Absorption by atomic defects in the glass composition. ii) Extrinsic absorption by impurity atoms in the glass material iii) Intrinsic absorption by the basic atoms of the fiber material. i) Absorption by Atomic defects Atomic defects are imperfections in the atomic structure of the fiber material such as missing molecules, high density clusters of atom groups in the glass structure. These absorption losses are negligible compared with intrinsic and impurity absorption effects. ii) Extrinsic absorption Absorption is due to impurities in the fiber material. Transition metal impurities(iron, copper, nickel and chromium) OH ions (water) impurities iii) Intrinsic absorption Intrinsic absorption occurs when material is in absolutely pure state with no density variations, impurities and material in homogeneities. 1.2 Scattering Losses Scattering losses in glass arise due to following factors. Microscopic variations in material density. Compositional fluctuations. Structural in homogeneities and Structural defects occurring during fiber fabrication. Types of scattering Losses: 1) Linear Scattering Losses i) Rayleigh Scattering Losses ii) Mie Scattering Losses 2) Non Linear Scattering Losses i) Stimulated Brillouin Scattering Losses ii) Stimulated Raman Scattering Losses
1.2.1 Linear Scattering Losses Linear Scattering transfrs the optical power in one propagation mode to different mode. These losses will occure in the radiation mode or leaky mode. It will not continue to propagate with in the core of fiber and is radiated out from the fiber. Scattering Loss will be more in multimode fibers due to compositional fluctuation Linear processes there is no change of frequency on scattering Rayleigh scattering is an elastic scattering because there is no change in frequency. Rayleigh loss given by α scat = 8π 3 n 8 p 2 k B T f β T / 3λ 4 Where, n Refractive index of silica p Photo elastic coefficient k B Boltzman s constant T f Fictive Temperature β T Isothermal compressibility of the material. Transmission loss due to rayleigh scattering Α = exp (-α scat L) 1.2.2 Non Linear Scattering Losses The non linear scattering transfers the optical power from one mode to the other mode at a different frequency either in forward direction or backward direction. The scattering are observed in single mode fiber at high optical power densities. These scattering are al inelastic due to the shift in the frequency. When the refractive index of the medium depends on the optical intensity of the signal, then these non linear scattering are occurred.
1.3 Bending Loss or Radiative Loss Bending or Radiative losses occur whenever an optical fiber undergoes a bend of finite radius of curvature. Two types of Bending losses i) Macroscopic bending losses ii) Microscopic bending losses i) Macroscopic bending losses These occur when the radius of curvature of band is greater than the fiber diameter. This situation arises when a fiber cable turns a corner. Whenever the radius of curvature is large, the loss is small, when the radius of curvature decreases the loss increases exponentially up to a critical radius of curvature. For multimode fiber the critical radius of curvature R c is given by, The amount of optical radiation from a bent fiber is depending on the field strength at x c and on the radius of curvature R. The effective number of modes (N eff ) that are supported by a curved multimode fiber is given by
Where, α - Graded index profile - Core and Cladding index difference n 2 - Cladding refractive index k - Wave propagation constant N α - Total no.of modes support in a fiber. Macro bending losses may be reduced by, Designing fibers with large relative refractive index difference Operating at the shortest wavelength. ii) Microscopic bending losses Micro bends are due to small-scale fluctuations in the radius of curvature of the fiber axis. This situation arises when the fibers are incorporated into cables. The fluctuations in the radius of curvature are caused either by non-uniform in the manufacturing of the fiber or by non uniform lateral pressures created during thee cable of the fiber. Minimizing of the micro bending losses over the fiber. Micro bending losses can be minimized by introducing compressible buffer or jacket
1.4 Core and Cladding Losses The core and cladding have different refractive indices because they are having different composition. Therefore core and cladding have different attenuation coefficients denotes α 1 and α 2 respectively. Step Index fiber: Where, Total loss of the waveguide can be found by summing over all modes weighted by the fractional power in that mode. Graded Index fiber: Loss at radial distance r from core axis is expressed as, α(r) = α 1 + (α 2 - α 1 ) [n 2 (0) n 2 (r)] / n 2 (0) n 2 2 where, α 1 and α 2 are the core axial and cladding attenuation coefficient.
2. Signal Distortion in Optical Waveguides: An optical signal is distorted as it travels along a fiber. This distortion is due intermodal dispersion and intramodal delay effect. Dispersion The term dispersion refers to spearding if light pulse as it propagates through fiber. It introduces Inter Symbol Interference (ISI). It limits the information carrying capacity of fiber. Dispersion means distortion or degradation of the signal quality at the output end due to the overlapping of the pulses. The dispersion effect can be explained on the basic behaviour of group velocity of the guided modes in the optical fiber. The group velocity is the velocity at which the energy in a particular mode travels along the fiber. Types of Dispersion 1) Intramodal dispersion i) Material or chromatic dispersion ii) Waveguide dispersion 2) Intermodal dispersion 1) Intramodal dispersion Intramodal dispersion is pulse spreading that occure within a signal mode. It arises due to group velocity being a function of wave length. The increasing spectral width of the optical source will increase the intramodal dispersion. i) Material or chromatic dispersion This dispersion arises due to the variation of the refractive index of the core material as a function of wavelength or frequency of light. This wavelength dependence of the group velocity of any given mode (i.e) pulse spreading occurs even when different wavelength follow the same path. Minimize the material dispersion Material dispersion can be reduced either by choosing sources with narrower spectral output widths or by operating at longer wavelength.
ii) Waveguide dispersion Waveguide dispersion occurs when a single mode fiber only confines about 80% on the optical power to the core. Dispersion arises from the 20% of the light propagating in the cladding which travels faster than the core light. The amount of waveguide dispersion depends on the fiber design. Group Delay Dispersion This type pulse spreading occurs when each mode having different value of the group velocity at a single frequency The intermodal dispersion are occurs only in single mode fibers. 2) Intermodal dispersion The intermodal dispersion arises due to the variation in the group delay for each individual mode at a single frequency. Different modes arrive at the exit end of the fiber at different times. This distortion is occurring in multimode fibers. 2.1 Group Delay: In guided optical communication, core is having slow propagating modes and clad is having fast propagating modes. Group delay is caused by the different path lengths with each of the modes of a fiber. Each mode carriers an equal amount of energy through the fiber. The group delay is important parameter in multimode fibers. The transit time or group delay for a light pulse propagating along a unit length of fiber is inverse of the group velocity V g. The group delay is given by
. Group Velocity: Group velocity is the velocity at which the energy in a pulse travels along a fiber. Total delay difference over distance L is ------( 9 )
Pulse
2.2 Material (Or) Chromatic Dispersion This dispersion arises due to the variation of the refractive index of the core material as a function of wavelength or frequency of light. The rms pulse broadening due to material dispersion
Minimize the material dispersion Material dispersion can be reduced either by choosing sources with narrower spectral output widths or by operating at longer wavelength. 2.3 Waveguide Dispersion The waveguide dispersion also intramodal dispersion. This results from the variation in group velocity with wavelength for a particular mode.
2.4 Signal Distortion in Single Mode Fibers The pulse broadening in single mode fibers results from intramodal or chromatic dispersion as only a single mode is allowed to propagate. For single mode fibers, waveguide dispersion is important.
Consider the expression of the factor ua for the lowest order mode
2.5 Intermodal Dispersion (or) Multimode Dispersion The intermodal dispersion arises due to the variation in the group delay for each individual mode at a single frequency. Different modes arrive at the exit end of the fiber at different times. This distortion is occurring in multimode fibers. (i) Multimode Step Index Fiber
(ii) Multimode Graded Index Fiber The refractive index profile of graded index fiber is given by
2.6 Information Capacity Determination A light pulse will broaden as it travels along the fiber. This pulse broadening will cause overlap with neighboring pulses. At certain distance the pulses are not individually distinguished at the receiver and errors will occur. Therefore the information capacity of an optical waveguide is usually specified by the Bandwidth Distance Product (BDP) or Bandwidth Length Product (BLP) As the length an optical cable increases, the bandwidth decrease in proportion. For step index bandwidth distance product is 20MHz.km and graded index it is 2.5GHz.km The information carrying capacity can be determined by short light pulses propagating along the fiber. Dispersion and ISI
2.7 Polarisation Mode Dispersion (PMD) Polarization refers to the Electric Field Orientation of a light signal, which can vary significantly along the length of the fiber. The PMD is similar to wave Dispersion. PMD leads to pulse broadening due to slightly different propagation velocities of the two modes. Signal energy at a given wavelength occupies two orthogonal polarization modes. A varying Birefringence along its length will cause each polarization mode to travel at a slightly different velocity and the polarization orientation will rotate with distance. The difference in propagation time τ between two orthogonal polarization modes will result in pulse spreading. This is the Polarization Mode Dispersion (PMD). If the group velocity f the two orthogonal polarization modes are V gx and V gy then the differential time delay between the two polarization components over a distance L is The PMD varies randomly along a fiber due to the perturbations causing the birefringence effects vary with temperature. The modal birefringence B f of the fiber is given by
3. Pulse Broadening in Graded Index Waveguides The core refractive Index is made to vary as a function of the Radial distance from the center of the fiber. This is called a Graded-Index Fiber. It supports Multimode propagation in a relatively large core together with a low intermodal delay distortion and allows the transmission of high data rates over long distance. If the index profile is carefully controlled, then it is eliminating model dispersion. The rms pulse broading σ in a Graded Index fiber is given by Where, The intermodal delay and pulse broadening are Where, τ g is the group delay for a particular mode having the order (v,m) Where, P vm optical power contained in the mode of order (v,m) M Number of fiber modes
β is the propagation constant and given by, Or Where, M Total number of possible guided modes m The umber of guided modes having propagation constants Where, Let us assume the core cladding index difference << 1 and β 1 = n 1 k.
α opt optimum index profile The Relation between graded index pulse broadening an the step index pulse broadening is given by.
4. Mode Coupling After a certain initial length, the pulse distortion increases rapidly because of mode coupling and differential mode loss The coupling of energy from one mode to another arise because of Refractive index variations Fiber diameter variations Structural imperfections Cabling induced micro bends Irregularities at the core cladding interference The mode coupling tends to average out the propagation delays associated with the modes and there by reduce the intermodal dispersion. The mode coupling and power distribution can occur at connectors, splices and other passive component in an optical link, this can have a significant effect on the overall system bandwidth.
5. Design optimization of single mode fibers The single mode fibers are widely used in the telecommunication application and optical network. The dispersion produced in a single mode fiber is practically very important since the dispersion reduces the maximum bit rate of transmission capacity as well as the bandwidth of the fiber. Zero dispersion in single mode fibers can be achieved at the operating wavelengths by the suitable design of refractive index profile and making the addition of material dispersion and waveguide dispersion as zero. The attributes of single mode fibers - Long life time - Very low attenuation - High quality signal transfer due to absence of modal noise - Large bandwidth distance produce. 5.1 Refractive Index Profiles Varieties of core cladding refractive index configurations are used. 1300nm optimized fibers Dispersion shifted fibers Dispersion flattened fibers and Large effective core area fibers i) 1300nm optimized fibers These are most popularly used single mode fibers in Telecommunication networks. Here two configurations are available. Matched Cladding fibers Depressed Cladding fibers
Matched Cladding fibers: Matched cladding fibers have a uniform refractive index throughout the cladding Typical mode field diameters are 9.5 μm and = 0.35 % Depressed Cladding fibers: In depressed cladding fibers the cladding region surrounding the core has a lower refractive index than the outer cladding region. Mode field diameter are around 9μm, and typical positive and negative index differences are 0.25 and 0.12%. This refractive index profile design is to optimize the waveguide dispersion so as to get zero dispersion at 1.3 μm. ii) Dispersion shifted fibers Material dispersion depends only on the composition of the material, waveguide dispersion is a function of the core radius, the refractive index difference and the shape of the refractive index profile. Total dispersion in a fiber is the sum of material and waveguide dispersion can shift the zero dispersion point to longer wave lengths. The resulting optical fibers are known as dispersion shifted fibers. iii) Dispersion flattened fibers To reduce fiber dispersion by spreading the dispersion minimum out over a wider range. This approach is known as dispersion flattering. These fibers are more complex to design, because dispersion must be consider over a much broader range or wavelength.
iv) Large effective core area fibers Large core area is needed to reduce the effect of fiber nonlinearities, which limit system capacities. The standard single mode fiber have effective core area of about 55μm 2 these profiles yield values greater than 100 μm 2.
Three dimensional refractive index profiles for a) Matched cladding b) Depressed cladding c) Triangular dispersion shifted d) Quadruple clad dispersion flattened 5.2 Cut off Wavelength One of the important transmission parameter for single mode fibers, is cut off wavelength for the first higher order mode. The cut off wavelength of a single mode operation is given by Where, V c is the cut off normalized frequency V c = 2.405 for step index fiber.
TWO MARKS: 1. What are the losses or signal attenuation mechanism in a fiber? The basic attenuation mechanisms in a fiber are i) Absorption ii) Scattering iii) Radiation losses or Bending loss. 2. Define signals attenuation of fiber loss? Signal attenuation also known as fiber loss or signal loss is one of the most important properties of an optical fiber. It is also define as the optical output P out from a fiber of length L to the optical input power P in. 3. Name three different mechanism caused by absorption? i) Absorption by atomic defects in the glass composition. ii) Extrinsic absorption by impurity atoms in the glass material iii) Intrinsic absorption by the basic atoms of the fiber material. 4. What are the Scattering losses? How will Scattering loss occurs? Scattering losses in glass arise due to following factors. Microscopic variations in material density. Compositional fluctuations. Structural in homogeneities and Structural defects occurring during fiber fabrication. 5. Name the types of Scattering losses? 1) Linear Scattering Losses i) Rayleigh Scattering Losses ii) Mie Scattering Losses 2) Non Linear Scattering Losses i) Stimulated Brillouin Scattering Losses ii) Stimulated Raman Scattering Losses 6. Name the types of Bending losses? i) Macroscopic bending losses ii) Microscopic bending losses
7. What is meant my Dispersion? The term dispersion refers to spreading of light pulse as it propagates through fiber. Dispersion means distortion or degradation of the signal quality at the output end due to the overlapping of the pulses. 8. Name the types of Dispersion? 1) Intramodal dispersion i) Material or chromatic dispersion ii) Waveguide dispersion 2) Intermodal dispersion 9. What are the effects of dispersion in optical fiber system? i) Pulse broadening ii) Inter Symbol Interference. 10. What is meant by material Dispersion? This dispersion arises due to the variation of the refractive index of the core material as a function of wavelength or frequency of light. This wavelength dependence of the group velocity of any given mode (i.e) pulse spreading occurs even when different wavelength follow the same path. 11. What is meant by Waveguide Dispersion? Waveguide dispersion occurs when a single mode fiber only confines about 80% on the optical power to the core. Dispersion arises from the 20% of the light propagating in the cladding which travels faster than the core light. 12. What is meant by Group Delay Dispersion? This type pulse spreading occurs when each mode having different value of the group velocity at a single frequency. The intermodal dispersion are occurs only in single mode fibers. 13. What is meant by Group Velocity Dispersion (DVD)? The group velocity is the velocity at which the energy in a particular mode travels along the fiber. 14. What is meant by Intermodal Dispersion? The intermodal dispersion arises due to the variation in the group delay for each individual mode at a single frequency. Different modes arrive at the exit end of the fiber at different times. This distortion is occurring in multimode fibers.
15. What is Bandwidth Distance Product? A measure of the information capacity of an optical waveguide is known as Bandwidth Distance Product (BDP). 16. Define Polarization? Polarization refers to the Electric Field Orientation of a light signal, which can vary significantly along the length of the fiber. 17. Name the configuration of core-cladding refractive index? The core cladding refractive index configurations used in single mode fibers are a. 1300nm optimized fibers b. Dispersion shifted fibers c. Dispersion flattened fibers and d. Large effective core area fibers 18. What are intramodal and intermodal dispersion? Intramodal dispersion are material and waveguide dispersion. These are due to dependence of group velocity on the wavelength. Intermodal dispersion or multimode dispersion is due to variation of group velocity for each mode at a single frequency. 19. Why do we have smaller dispersion in graded index fibers? Due to shaping the refractive index profile in the parabolic manner and by self focusing effect, the dispersion is small. 20. What is meant by Polarization mode Dispersion? Polarization mode dispersion means the dispersion due to the removal of degeneracy between the two orthogonally polarized modes in single mode fibers which causes different phase constants for those two modes.