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BTC107 2 Optical Fibres
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BTC107 2 Optical Fibres

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BTC107 2 Optical Fibres BTC107 2 Optical Fibres Presentation Transcript

  • BTC107 TV Systems & Signals Optical Fibre Links Martin Uren
  • Optical Communications  Working in pairs, list 4 examples of optical communications systems  What are the advantages of optical fibre as a communication medium?  What are its disadvantages? 2
  • Optical Fibre Dates  1955: Optical fibre invented in UK by Narinder S. Kapany  1966: First paper to suggest its use in communications  1977: First optical fibre telecomms link, carrying about 2000 telephone calls  1988: BT laid first optical fibre submarine cable between UK & Belgium. Later that year, TAT-8 became first one across Atlantic. It could handle 37,500 telephone calls  1996: 14,000km link between UK, France & USA could handle 120,000 telephone calls 3
  • Optical Fibre Construction 4
  • Propagation in Optical Fibre  Multimode propagation is by ‘Total Internal Reflection’  Limiting angle, θ is where sin (90 - θ) = n2 / n1  Where n2 = refractive index of cladding, n1 = index of core  What is the limiting angle if n1 = 1.45 & n2 = 1.44 Refractive Index = n 0 Refractive Index = n 2 n1 > n 2 > n 0 θ θ c/n 1 θ (c/n 1 )cos θ φ Refractive Index = n 1 θ θ 5
  • Propagation in Optical Fibre  Light waves will only propagate in optical fibre if the path difference after 2 reflections is a whole number of wavelengths  As the Entry Angle θ varies this gives rise to a number of modes, such that sin θ = Nλ / 2dn1  Where, N = mode = 1, 2, 3, etc., n1 = core refractive index  d = core diameter, λ = free-space wavelength 6
  • Propagation in Optical Fibre Source Source Source 7
  • Losses in Optical Fibre  Absorption: Light absorbed by core material  Reflection Loss: Impurities and inhomogeneities at the core/cladding interface  Rayleigh Scattering: Inhomogeneity (variations in density) of core  Impurities: Impurity atoms in the silica cause absorption, particularly water contamination or hydroxyl ion absorption 8
  • Losses in Optical Fibre Fibre Attenuation (dB/km) 100 50 20 10 5 4 2 1 0.5 1 2 0.2 3 0.1 400 6 00 8 00 1000 1200 1400 1600 1800 20 00 1300 nm 1550 nm Wavelength (nm) 9
  • Losses in Optical Fibre  Minimum attenuation is between 1dB/km & 2dB/km at λ of about 1550nm  Next low-loss window gives attenuation of ~0.4dB/km at λ of about 1310nm  What is ‘dispersion’?  Material Dispersion: Caused by variation in refractive index with λ  Intermodal Dispersion: Difference in time for different modes  Minimum Dispersion is at 1310nm 10
  • Losses in Optical Fibre Refractive Index 1.52 1.48 1.44 1.40 0 1000 2 00 0 3 000 4 000 Wavelength (nm) 1300 nm = minimum dispersion 11
  • Optical Fibre Extra Questions  What can be done to increase the data capacity of the fibre?  What is an EDFA (Erbium-Doped Fibre Amplifier)?  What is the maximum distance of an optical fibre communication link?  What is DWDM?  What is a PON (Passive Optical Network)? 12