Transmission Media -Cont

Propagation Modes 

o    Fiber Technology supports two modes for the propagation of light    Multimode    Single Mode Each    of    these    modes    require    fiber    with    different    physical characteristics o    There are two further sub categories of Multimode Fiber:    Multimode Step-Index Fiber    Multimode Graded-Index Fiber    Multimode Fiber o    Multiple beams from a light source move through the core in different paths o    Two types of the Multimode fiber:    Multimode Step Index Fiber    Multimode Graded Index Fiber    Multimode Step-Index Fiber o    Density of the CORE remains constant from the center to the edges o    A  beam  of  light  moves  through  this  constant  density  in  a  straight  line  until  it reaches the interface of the core and the cladding o    At the interface, there is an abrupt change to lower density, that alters the angle of the beam's motion o    Step IndexÆ Suddenness of this change o    Some beams travel straight and reach the destination without reflecting o    Some  strike  the  interface  of  core  and  cladding  at  an  angle  smaller  than  critical angle and penetrate cladding and are lost o    Others  hit  edge  of  the  core  at  angles  greater  than  critical  angle  and bounce  back and forth to the destination •     Disadvantage of Multimode Step-Index Fiber o    Each beams angle is equal to its angle of reflection o    If I is small, R is small and the beam will require more bounces and it will take more time to reach the destination o    If I is large, R is large and beam will reach destination quickly o    In  other  words  there  is  a  difference  is  Path  Lengths  that  results  into  a distortion at the receiver o    This  distortion  limits  the  data  rate  and  make  Multimode  Step  index  fiber inadequate for precise applications ¾   Multimode Graded-Index Fiber o    The solution to the above problem is Multimode Graded Index Fiber o    A grade index fiber is the one with varying densities o    Density  is  highest  at  the  center  of  the  core  and  decreases  gradually  to  its lowest at the edge o    The signal is introduced at the center of the core o    The horiz beams move straight to the receiver o    Beams   at   other   angles   moves   through   the   series   of   constantly   changing densities o    Each density difference causes each beam to refract into a curve o    Signal  can  be  reconstructed  with  far  greater  precision  as  all  the  beams  reach the receiver at almost the same time