Need some help regarding phase and group velocity of a wave

  • Thread starter ehj
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ehj
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Main Question or Discussion Point

I'm currently working with lasers and in relation to that, fiber optics. In the book I'm reading there's a section regarding modes and their "group velocity". The text claims that there is the following relation:
[tex] v_g=v_p * sin(A) [/tex]
where [tex] v_g [/tex] is the group velocity and
where [tex] v_p [/tex] is the phase velocity and A is the incident angel as showed on the picture, although denoted as theta there.
From the picture you can see that the relation is reached by using trigonometry, and [tex] v_g [/tex] is in this case simply the velocity in the direction along the fiber. My question is, if this is the general idea of group velocity? I thought group velocity was the velocity of the "amplitude wave" in a wave packet? Am I wrong, or are there just different meanings of group velocity?
 

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  • #2
ehj
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Here is the picture!
 

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  • #3
pam
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v_g found that way would be the velocity of the envelope of a wave packet.
It is the velocity of energy flow along the fiber direction.
 

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