FIBER PROPERTIES Transmission characteristics of a fiber depends on two important phenomena

1. FIBER PROPERTIES • Transmission characteristics of a fiber depends on two important phenomena • Attenuation • Dispersion Attenuation or transmission loss • Much less than those of metallic conductors • Expressed in logarithmic unit of decibel • Caused by absorption, scattering and bending losses

2. Attenuation - Loss of optical power as light travels along the fiber Defined as ratio of input (transmitted) optical power Pi into fiber to the output (received) power Po Signal attenuation = 10 log 10(Pi / Po) dB Influenced by material composition, preparation and purification technique and the waveguide structure Losses are categorized into 1. Material absorption losses a. Intrinsic b. Extrinsic 2. Linear scattering losses 3. Fiber bend loss

3. Material Absorption Losses Related to material composition and fabrication process Results in dissipation of transmitted power as heat Absorption of light may be Intrinsic – caused by interaction with one or more major components of the glass Extrinsic – caused by impurities within the glass

4. Intrinsic losses • Pure silicate glass has very little intrinsic absorption • In silica glass, the wavelength used is 700 nm to 1600 nm • This is between two intrinsic absorption regions • First in the UV region (< 400nm) • Second in the infrared region (>2000 nm)

5. Intrinsic absorption in the UV band Due to electronic absorption (i.e.,) absorption occurs when a light particle (photon) interacts with an electron and excites it to a higher level. Intrinsic absorption in infrared band Absorption is caused by the vibration of Si-O bonds Interaction between the vibration bond and the EM field of the optical signal causes absorption. Light energy transferred from the EM field to the bond

6. Extrinsic losses • Caused by impurities into the fiber material • Trace metal such as Iron, Nickel, Chromium • Electronic transition of these metal ions from one energy level to another causes absorption • Also occurs where hydroxyl ions (OH-) are introduced into the fiber and peak at 1383 nm, 1250 nm and 950nm • These absorptions define three windows of preferred operation, one centered at 1300 nm, second at 850 nm and third around 1550 nm

8. Scattering Loss • Caused by the interaction of light with density fluctuations within a fiber. • Density changes are produced when optical fibers are manufactured. • 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 • Light is then partially scattered in all directions.

9. LIGHT SCATTERING

10. RAYLEIGH SCATTERING • In commercial fibers operating between 700-nm and 1600-nm wavelength, the main source of loss is called Rayleigh scattering. • Rayleigh scattering is the main loss mechanism between the ultraviolet and infrared regions. • Rayleigh scattering occurs when the size of the density fluctuation (fiber defect) is less than one-tenth of the operating wavelength of light. • As the wavelength increases, the loss caused by Rayleigh scattering decreases.

11. FIBER LOSSES

12. MIE SCATTERING • If the size of the defect is greater than one-tenth of the wavelength of light - the scattering mechanism is called Mie scattering. • Mie scattering, caused by these large defects in the fiber core, scatters light out of the fiber core. • However, in commercial fibers, the effects of Mie scattering are insignificant. • Optical fibers are manufactured with very few large defects.

13. BENDING LOSS • Bending the fiber also causes attenuation. • Bending loss is classified according to the bend radius of curvature: • microbend loss • macrobend loss • Macrobends are bends having a large radius of curvature relative to the fiber diameter • During installation, if fibers are bent too sharply, macrobend losses will occur • Microbend and macrobend losses are very important loss mechanisms.

14. MICROBEND LOSSES • 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 up

15. MICROBEND LOSS

16. MACROBEND LOSS • Macrobend losses are observed when a fiber bend's radius of curvature is large compared to the fiber diameter. • 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 lost or radiated out of the fiber.

17. DISPERSION IN OPTICAL FIBERS Two types of dispersion: • Intramodal dispersion • Intermodal dispersion Dispersion leads to PULSE SPREADING The varying delay in arrival time between different components of a signal "smears out" the signal in time. This causes energy overlapping and limits information capacity of the fiber

19. INTRAMODAL DISPESION • Pulse spreading that occurs within a single mode • Intramodal dispersion occurs because different colors of light travel through different materials and different waveguide structures at different speeds • Also called GROUP VELOCITY DISPERSION (GVD) • Occurs in all types of fibers • Two main causes : Material dispersion Waveguide dispersion

20. Material Dispersion • Arises from variations of the refractive index of the core material as a function of wavelength • Spreading of a light pulse is dependent on the wavelengths interaction with the refractive index of the fiber core • Different wavelengths travel at different speeds in the fiber material and hence exit the fiber at different times • Material dispersion is a function of the source spectral width. • The spectral width specifies the range of wavelengths that can propagate in the fiber. • Material dispersion is less at longer wavelengths.

21. Waveguide Dispersion • Arises because a Single Mode Fiber confines only 80% of the optical power to the core • The other 20% tends to travel through the cladding and hence travels faster • This results in spreading of the light pulses • The amount of dispersion depends on the fiber design and the size of the fiber core relative to the wavelength of operation • In multimode fibers, waveguide dispersion and material dispersion are basically separate properties. • Multimode waveguide dispersion is generally small compared to material dispersion and is usually neglected.

22. INTERMODAL DISPERSION • Intermodal or modal dispersion causes the input light pulse to spread. • The input light pulse is made up of a group of modes (MULTIMODE) • As the modes propagate along the fiber, light energy distributed among the modes is delayed by different amounts. • The pulse spreads because each mode propagates along the fiber at different speeds.

23. Modes travel in different directions, some modes travel longer distances. • Modal dispersion occurs because each mode travels a different distance over the same time span • The modes of a light pulse that enter the fiber at one time exit the fiber a different times. • These conditions causes the light pulse to spread. • As the length of the fiber increases, modal dispersion increases

25. Optical Fiber Connection • Requires both jointing and termination of the transmission medium • Number of connections or joints is dependent upon the link length • Fiber to fiber connection with low loss and minimum distortion is important • Two major categories of fiber joint currently in use: • 1.Fiber splices • 2. Fiber Connectors

26. Splices and Connectors – Ideally couple all light propagating in one fiber into the adjoining fiber Fiber Couplers Branching devices that split all the light from the main fiber into two or more fibers Couple a proportion of the light propagating in the main fiber into a branch fiber Combine light from one or more branch fibers into a main fiber

27. Fiber alignment and Joint losses • Major consideration – Optical losses at the interface • Can be minimized if….. • Jointed fiber ends are smooth • Perpendicular to fiber axis • Two fiber axes are perfectly aligned • Still, a proportion of light – reflected back into the transmitting fiber • This phenomena is called FRESNEL REFLECTION

28. Magnitude of the partial reflection through the interface may be Estimated using the classical Fresnel formula r = [(n1 – n) / (n1 +n)]2