Measure Phase velocity/group velocity of EM-wave

[spookyfw]

Hey folks,

some weeks ago we had an trial-exam and one of the questions there was: "How do you measure phase and group velocity."

That question really got me. Having a fastly oscillating wave as carrier and then the envelope. Can someone help me? Is it impossible for the phase, as it is usually a harmonic one?

Would be really great if someone would know the answer,
take care,
spookyfw

 

[f95toli]

I am assuming you are talking about EM signals.
How you do it depends on the frequency range. For RF/Microwave you can use a phase detector, these are either dedicated components that you can buy; or you use a saturated mixer.

 

[spookyfw]

Sorry for this very late reply. First of all: thank you very much for your answer. Yes, I was talking about EM-waves. More about a pulse in the visible region though. I wondered how one would then discriminate and measure phase and group velocity. Using a phase detector sounds reasonable, but how does it work?

Would be really grateful if someone would know :).

 

[PhilDSP]

Hi spookyfw,

Except in very special circumstances, measurement of phase velocity is not feasible for a number of reasons:

1. The surface that represents the constant phase of the entire wave packet may not necessarily be a plane.

2. That surface may not necessarily travel in the same direction as the wave.

3. The amplitude of the wave on that surface may be extremely small.

4. That surface may lie outside of the causal influence of the wave.

 

[PJC01]

Hi spookyfw,

Measuring the phase and group velocity of an electromagnetic wave can be done using various methods depending on the specific setup and equipment available. However, one common method is to use a Michelson interferometer.

In this setup, a beam splitter is used to split the incoming wave into two paths. One path travels a longer distance than the other, and then the two paths are recombined. By adjusting the length of one of the paths, the interference pattern can be changed, allowing for the measurement of the phase velocity of the wave.

To measure the group velocity, a similar setup can be used, but with a dispersive medium (such as a prism or diffraction grating) placed in one of the paths. This will cause the different frequencies of the wave to travel at different speeds, allowing for the measurement of the group velocity.

In terms of the wave being harmonic, it is not impossible to measure the phase velocity. The phase velocity is the speed at which a specific phase of the wave propagates, and can be measured by looking at the changes in the interference pattern. However, the group velocity is the speed at which the overall envelope of the wave propagates, and this can be affected by the dispersion of the medium.

I hope this helps answer your question. If you need more specific information, please provide more details about your setup and experiment. Good luck with your studies!

Best,