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PRESENTATION ON UNIT-IV OPTICAL FIBER

PRESENTATION ON UNIT-IV OPTICAL FIBER. BY DR. S. M. PETHE. What Is Fiber Optics ?. Transmitting communications signals over hair thin strands of glass or plastic Not a "new" technology Concept a century old Used commercially for last 25 years. Fiber Technology. n1 > n 2.

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PRESENTATION ON UNIT-IV OPTICAL FIBER

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  1. PRESENTATION ON UNIT-IVOPTICAL FIBER BY DR. S. M. PETHE

  2. What Is Fiber Optics ? • Transmitting communications signals over hair thin strands of glass or plastic • Not a "new" technology • Concept a century old • Used commercially for last 25 years

  3. Fiber Technology n1 > n 2 n1-refractive index of core n2-refractive index of cladding

  4. Fiber Technology

  5. n1 n2 n1< n2 n1> n2 Rarer to denser denser to rarer

  6. Critical Angle • Ray bends at boundary between materials • Snell’s law • Light confined to core if propagation angle is greater than the critical angle • Total internal reflection (TIR)

  7. Fiber Types

  8. Fiber Types

  9. Standard Single Mode Optical Fibers • Most common single mode optical fiber: SMF28 from Corning • Core diameter dcore=8.2 mm • Outer cladding diameter: dclad=125mm • Step index • Numerical Aperture NA=0.14 • NA=sin(q) • Dq=8° • lcutoff = 1260nm (single mode for l>lcutoff) • Single mode for both l=1300nm and l=1550nm standard telecommunications wavelengths

  10. Standard Multimode Optical Fibers • Most common multimode optical fiber: 62.5/125 from Corning • Core diameter dcore= 62.5 mm • Outer cladding diameter: dclad=125mm • Graded index • Numerical Aperture NA=0.275 • NA=sin(q) • Dq=16° • Many modes

  11. Anatomy of an Optical Fiber • Light confined to core with higher index of refraction • Two analysis approaches • Ray tracing • Field propagation using Maxwell’s equations

  12. Graded Index Multimode Fiber • Higher order modes • Larger propagation length • Travel farther into the cladding • Speed increases with distance away from the core (decreasing index of refraction) • Relative difference in propagation speed is less

  13. Numerical Aperture • The acceptance angle for a fiber defines its numerical aperture (NA) • The NA is related to the critical angle of the waveguide and is defined as: • Telecommunications optical fiber n1~n2,

  14. Fiber Attenuation • Loss or attenuation is a limiting parameter in fiber optic systems • Fiber optic transmission systems became competitive with electrical transmission lines only when losses were reduced to allow signal transmission over distances greater than 10 km • Fiber attenuation can be described by the general relation: where a is the power attenuation coefficient per unit length • If Pin power is launched into the fiber, the power remaining after propagating a length L within the fiber Pout is

  15. Fiber Attenuation • Attenuation is conveniently expressed in terms of dB/km • Power is often expressed in dBm (dBm is dB from 1mW)

  16. Fiber Attenuation • Example: 10mW of power is launched into an optical fiber that has an attenuation of a=0.6 dB/km. What is the received power after traveling a distance of 100 km? • Initial power is: Pin = 10 dBm • Received power is: Pout= Pin– a L=10 dBm – (0.6)(100) = -50 dBm • Example: 8mW of power is launched into an optical fiber that has an attenuation of a=0.6 dB/km. The received power needs to be -22dBm. What is the maximum transmission distance? • Initial power is: Pin = 10log10(8) = 9 dBm • Received power is: Pout = 1mW 10-2.2 = 6.3 mW • Pout - Pin = 9dBm - (-22dBm) = 31dB = 0.6 L • L=51.7 km

  17. Material Absorption • Material absorption • Intrinsic: caused by atomic resonance of the fiber material • Ultra-violet • Infra-red: primary intrinsic absorption for optical communications • Extrinsic: caused by atomic absorptions of external particles in the fiber • Primarily caused by the O-H bond in water that has absorption peaks at l=2.8, 1.4, 0.93, 0.7 mm • Interaction between O-H bond and SiO2 glass at l=1.24 mm • The most important absorption peaks are at l=1.4 mm and 1.24 mm

  18. Scattering Loss • There are four primary kinds of scattering loss • Rayleigh scattering is the most important where cR is the Rayleigh scattering coefficient and is the range from 0.8 to 1.0 (dB/km)·(mm)4 • Mie scattering is caused by inhomogeneity in the surface of the waveguide • Mie scattering is typically very small in optical fibers • Brillouin and Raman scattering depend on the intensity of the power in the optical fiber • Insignificant unless the power is greater than 100mW Geometrical loss- are introduce due to the manufacturing process

  19. Absorption and Scattering Loss

  20. Dispersion • Dispersive medium: velocity of propagation depends on frequency • Dispersion causes temporal pulse spreading • Pulse overlap results in indistinguishable data • Inter symbol interference (ISI) • Dispersion is related to the velocity of the pulse

  21. Fiber Bandwidth

  22. Fiber Bandwidth

  23. THANKS

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