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Increase Phase Velocity by Losing Power ??

  1. Jan 10, 2016 #1
    I just read some words from the book Optical Waveguide Theory by Snyder, Allan W., p226. It says that

    "However, although it is possible for the phase velocity of a mode to exceed the maximum speed of light in the cladding (of waveguide), c/n_cl, this cannot occur without losing power to radiation"

    My question is how to understand this? Why losing power can increase the phase velocity of the wave?
     
  2. jcsd
  3. Jan 11, 2016 #2

    Simon Bridge

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    The loss of power is the result of setting up such a high phase velocity wave, it is not the cause of the high velocity.
     
  4. Jan 11, 2016 #3
    Thanks for your reply. I still cannot understand the internal relationship between "losing energy" and "change of phase velocity", could you please explain more about this?
     
  5. Jan 11, 2016 #4

    Simon Bridge

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    If you make the phase velocity too fast, some of the light inevitably escapes.
    It's like if you try to drive too fast around a bend you lose some power to skidding... going fast has more energy but you are also losing energy.
     
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