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           Fiber Optics Fundamentals

                                      Surasak Sanguanpong
                                         nguan@ku.ac.th
                                 http://www.cpe.ku.ac.th/~nguan

                                   Last updated: 25 November 2004




Applied Network Research Group                  Department of Computer Engineering, Kasetsart University




                                                                                                   2/15

Bending of Light Ray

                                                   refracted
                                                                         θi = angle of incidence
 n2 of medium 2                                                          θr = angle of reflection
(less dense medium)                       θt
                                                                         θt = angle of refraction
n1 of medium 1                                      surface
                                                    boundary
(more dense medium)                  θ i θr

                   incident                           reflected


                   refraction index : n = c/v
                            C= speed of light in free space
                            v= speed of light in the medium

Applied Network Research Group                  Department of Computer Engineering, Kasetsart University
                                                                                                  3/15

Snell’s Law
                                                              critical angle


  n1>n2                                                                           θt
                                                1
                                                 2
  n2                                              3                          θi
  n1
                                                                     n1sinθi = n2sinθt
         4                                        4
             3                                                                 θi = sin-1n2/n1
                 2               θc = critical angle                              = θc
                      1



Applied Network Research Group                 Department of Computer Engineering, Kasetsart University




                                                                                                  4/15

How it works
                                                                                       cladding
              Loss                     Total reflection


                                                                          n2
                                                                         n1




                                                                                         core




Applied Network Research Group                 Department of Computer Engineering, Kasetsart University
                                                                                                  5/15


Propagation Modes

      • Finite number of angles at which the rays reflect and propagate
      • Each angle defines a path or a mode




                                                     M = the number of modes
                      πd     (n1)2-(n2)2
                                           2
                                                     d = core diameter (m)
           M=         λ                              λ = wavelength (m)
                                                     n1 = refraction index of core
                                 2
                                                     n2 = refraction index of cladding


Applied Network Research Group                 Department of Computer Engineering, Kasetsart University




                                                                                                  6/15

Fiber classification


                                     Single Mode
                                     Single Mode

 Fiber Optics
 Fiber Optics
                                      Multimode
                                      Multimode


                                               Step-Index
                                               Step-Index

                                           Graded-Index
                                           Graded-Index




Applied Network Research Group                 Department of Computer Engineering, Kasetsart University
                                                                                               7/15


Multimode step-index


input signal                                                         output signal




        n1
                                 Step-change
        n2
                                  • refraction index is uniformly throughout the core




               core/cladding
               characteristics

Applied Network Research Group              Department of Computer Engineering, Kasetsart University




                                                                                               8/15

Multimode graded-index


input signal                                                         output signal




        n1
                                 Gradually change
        n2
                                 • Light begins to bend back toward the center,
                                   eventually reflecting back
                                 • Because the material also less dense, the light
                                   travels faster

               core/cladding
               characteristics

Applied Network Research Group              Department of Computer Engineering, Kasetsart University
                                                                                                   9/15


Single Mode


input signal                                                             output signal




        n1
        n2
                                      • Reducing the core diameter to that of a
                                        single wavelength (3-10 micrometer) will let
                                        the light propagates along a one mode only


               core/cladding
               characteristics

Applied Network Research Group                  Department of Computer Engineering, Kasetsart University




                                                                                                   10/15

Typical Fiber Cable

   Single core

    core       cladding      Jacket
                                                 • Materials for cladding and core:
                                                       glass cladding/glass core
                                                       plastic cladding/glass core
                                                       plastic cladding/plastic core


    Multicore




Applied Network Research Group                  Department of Computer Engineering, Kasetsart University
                                                                                           11/15


Cable Structures Examples


                        Strength                          Outer Jacket
                        Elements
     Color Coded
     Jacket

                                                                          Central
     fiber                                                                Strength
                                                                          Elements

cladding


      core


Applied Network Research Group          Department of Computer Engineering, Kasetsart University




                                                                                           12/15

Optical Fiber Comparison

                         Multimode          Single mode

 Light source           LED/ILD                ILD
 Bandwidth              >1 GHz/km             up to 1000 GHz/km
 Wavelength             850, 1300             1300,1550
 core/cladding          62.5/125*             8/125
 Applications           LAN, backbone         Long distance, Telcom lines

 * options with 50/125, 100/140


         core                            core

                             cladding                         cladding
                62.5/125 micron                    8/125 micron

Applied Network Research Group          Department of Computer Engineering, Kasetsart University
                                                                                                         13/15


      Loss in Glass Fibers
                      2.0
                               0.85μ band         1.30μ band         1.55μ band
                      1.8
                      1.6
Attenuation (dB/km)




                      1.4
                      1.2
                      1.0
                      0.8
                      0.6
                      0.4
                      0.2
                      0
                            0 0.8   0.9 1.0 1.1 1.2     1.3      1.4 1.5 1.6 1.7                 1.8
                                               Wavelength (microns)
         Applied Network Research Group               Department of Computer Engineering, Kasetsart University




                                                                                                         14/15

      Light Source
                                            power
                                                                            ILD
                      LED
                                                                                LED



                      Injection
                      Laser Diode

                                                                                               wavelength



         Applied Network Research Group               Department of Computer Engineering, Kasetsart University
                                                                                          15/15


Fiber in use

  • Plenums or non-plenums
         • plenum cable has a fire-resistant jacket, which will not burn,
           smoke or
         • give off toxic fumes when expose to heat

  • Riser cables
         • cable that runs vertically; e.g. between floors in a building

  • Indoor or Outdoor
         • Indoor cable used in building. Outdoor cable used in
           underground,
         • directed buries, and aerial applications between building.




Applied Network Research Group         Department of Computer Engineering, Kasetsart University

				
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