Phase velocity vs Group velocity

Hi,..

What's the difference between Phase velocity(Vp) and Group velocity(Vg).???
Which cause the wave dispersion.???

I searched Pozar but couldn't find useful answers, I don't have Colin.

Thanks verym uch for your help.

Phase velocity:

This term genrally comes into picture when there is a single wave, suppose in optical fiber when there is only one mode (i.e single mode) then your phase velocity is defined and same as group velocity.

Group Velocity:

This term, means suppose when you couple light into a fiber and if the fiber is capable of having different modes, then the input power is distributed over a number of modes, each having a different velocity (modal dispersion). Hence similar to average velocity, a term called group velocity is defined which gives a kind of average velocity of different modes.

So it is clear from above definition that group velocity is the effect of dispersion.

Hope this helps.

Regards,
Prakash.

pthoppay,..

Thanks for your consideration and time.

The group velocity is the velocity of what.???

Do you mean by this that, there's no dispersion for single mode operation.???

Thanks again.

There is no modal dispersion for single mode fiber but there are other dispersion like waveguide dispersion ( the input signal has different frequecies and each frequency sees different refractive index and hence different velocity, there by causing dispersion called as waveguide dispersion).

To conclude group velocity also makes sense for single mode fiber if have different frequecies at the input.

Group velocity is always pictured as a kind of average velocity (not average velocity in iteral sense, assume there 10 students in a class each working on the same project, each student will try to do his part but at different speed, so when you speak as a group you could average the rate of completion of project. Similarly, each mode tries to carry energy from end to other end contributing its part, so to get a picture of how fast energy is propagated you use the term group velocity)

This is the physical meaning of group velocity.

Regards,
Prakash.

this site can give a better understanding
**broken link removed**

Group velocity is applicable when there is information or modulation added to a carrier frequency. As an example, if you send a pulsed carrier thru a network, the group velocity can be derived from the time delay (or difference) between the input rise time to the output rise time and the distance traveled.