# Group velocity dispersion

1. Nov 8, 2013

### eahaidar

I want to ask how does the wave keep the same amplitude if the wave broadens ???

2. Nov 8, 2013

3. Nov 8, 2013

### eahaidar

Thank u I was watching a lecture about self modulation and dispersion I just having a hard time understanding what do they mean when they say pulse shape changes while the spectrum remains the same with distance and vice versa can u elaborate on that ?
Thank you so much

4. Nov 8, 2013

### Simon Bridge

The pulse is made of travelling sine waves.
The particular mix of sine waves that go to make the pulse is it's spectrum.

In a dispersive media, the different sine waves travel at different speeds - making the pulse change shape - but the mixture is still the same because the individual sine waves go through all space.

But I'd have to see the comment in context to figure what they mean.

The vice versa of "the pulse shape changes while the spectrum stays the same" would be

"the spectrum changes while the pulse shape stays the same"

... for the pulse shape to stay the same, in a dispersive media, the spectrum must change.

5. Nov 8, 2013

### eahaidar

You are amazing man I really want to thank you so in SPM the spectrum changes while the time pulse stays the same means. That the even though the summation of the pulses is changing the pulses travelling would stay the same?
Thank you so much I hope I understood u correctly