Transmission characteristics of optical fibers

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  • 1. PRESENTATIONTransmission Characteristics Of Optical Fibers PREPARED BY: AIBAD AHMED
  • 2. TRANSMISSION CHARECTERISTICS Transmission characteristics of fiber material interact with the optical signal. Optical signals while transmitting through fiber suffers power losses (Attenuation). Signal attenuation is defined as the ratio of optical input power (Pi) to the optical output power (Po).
  • 3. TRANSMISSION CHARECTERISTICS Attenuation may occur due to following factors:1. Absorption2. Scattering3. Macro bending4. Micro bending Some other factors are: Connectors ,splicing ,imperfect light coupling, radiation and leaky modes.
  • 4. ABSORPTION Some energy of the optical signal is absorbed by the fiber material and converted to heat. It is a cumulative (depends on length) effect. Absorption in fiber material are of two types.1. Intrinsic (pure)2. Extrinsic (impure)
  • 5. ABSORPTIONIntrinsic: This loss is due to interaction of optical signal with electrons & atoms of fiber in it’s pure state. Electrons and atoms of fiber material & optical signal has frequency and wavelength as well. f = frequency of optical signal f1= frequency of electrons f2= frequency of atoms Then process of matching frequencies occur known as “RESONANCE”.
  • 6. CONTINUED If f~f1 then electrons of fiber absorbs optical signal energy called “Electronic Resonance”. If f~f2 then atoms of fiber absorbs optical signal energy called “Atomic Resonance”.Extrinsic: Unintentionally injected impurities in fiber material during fabrication causes absorption.e.g. Metal ions like Fe, Cr, Ni absorbs light as well as OH atoms bonded in fiber absorbs light.
  • 7. PROBLEMQ) When the injected optical power into an 8 km length of fiber is 120 µW, the optical power at the fiber output is 3 µW. Determine the signal attenuation due to absorption in fiber per km.
  • 8. CONTINUEDData:L= 8 kmPi= 120 µWPo=3 µWFormula:Solution: α = 10 log10 (pi / po) = 10 log10 (120 *10^-6/3*10^-6) α = 16 dbnow ,α = (16db)/Lα = 16/8 db/kmα = 2 db/km
  • 9. CONTINUEDnow ,α = (16db)/Lα = 16/8 db/kmα = 2 db/km
  • 10. SCATTERINGIt is the change in the direction of optical signal in thecore of optical fiber.Scattering occurs when an optical signal hits a local RIvariation in the core.Such local RI variations act like small objects in the coreto scatter light.Scattering may change a guided mode into a radiation orleaky mode.
  • 11. TYPES OF SCATTERING RAYLEIGH SCATTERING MIE SCATTERINGRAYLEIGH SCATTERING It occurs when size of local RI variations is less than the wavelength of optical signal (about 1/10 of λ ) .
  • 12. CONTINUED Attenuation due to Rayleigh scattering: Ars = e^(-γL ) Where “γ” is Rayleigh Scattering coefficient α 1/λ^4.and “L” is the length of fiber.
  • 13. PROBLEMQ) Determine the attenuation in db/km due to rayliegh scattering in silica fiber at optical wavelength of 630 nm. Assume the value of constant is 1.895*10^-28.
  • 14. CONTINUEDSolution: γ=constant (1/ λ^4) γ= 1.895*10^-28/(630*10^-9) γ= 1.99*10^-3/mFor 1 km;Ars= e^(-γL )=e^[-1.99*10^-3*10^3]Ars= 0.301/kmIn db attenuation =10 log10 (1/ Ars) =10 log 10 (1/0.301) = 5.12 db/km
  • 15. CONTINUEDIn db attenuation =10 log10 (1/ Ars) =10 log 10 (1/0.301) = 5.12 db/km
  • 16. MIE SCATTERINGWhen diameter of the local variations in RI >wavelengths of guided modesImperfections in core-cladding interface causesMIE SCATTERING .Now a days optical fibers highly purified so MIESCATTERING is impossible
  • 17. MACRO BENDING It is a large scale bending which occurs intentionally by: Wrapping the fiber on spool or pulling around a corner. EFFECTS:1. Under limited conditions, loss of power2. Under extreme conditions , fracture of fiber.
  • 18. MICROBENDINGIt s a microscopic bending and it is unintentional.It may occur during manufacturing, cableinstallation and at service.Factors:During sheathed of fiber in cable, due to stressmicro bends occurs.Due to frequent change of temperature.
  • 19. . MICROBENDINGIrregular external pressure cause micro bendse.g. Heavy vehicle run over it.