Do Perfectly Collimated and Monochromatic Beams Show Interference Fringes?

[Gobil]

Hi All,

Just got to thinking about interference, if we have two sources (say a laser beam split in two), and introduce a phase shift between the two by making them travel different path lengths, then recombine them, we see fringes in the wavefront right. (interferometry)

My problem is this, if we have two perfectly monochromatic and perfectly collimated (i.e. no divergence as they propagate out to infinity) beams, the fringes would not be visible right? as the transverse pattern occurs due to the fact that the beams are indeed slightly divergent. You would however see a spot of light whose brightness in proportional to the phase difference between the two beams.

so the only reason we have interferometry is because we can't make perfect beams.
Does this make sense?

Thoughts please.

 

[Andy Resnick]

If I understand you correctly, it is true that in order to use interfereometry as a tool, there needs to be some bias present (some fringes) to increase the accuracy.

But I'm not sure I understood your question.

 

[Dr Lots-o'watts]

I am not saying I agree, and I am not saying I disagree. What I am sure of is that Heisenberg's principle prevents both a perfect monochromatic or a perfectly collimated beam. Both concepts may be useful for our understanding, but neither is physically possible.

In this sense, it's not really a matter of "our ability" to make them, it's just that close analysis reveals that the idea of making such a perfect beam is as absurd as building an infinitely long rod or something.

 

[Gobil]

yes, this is what I was getting at.

The fact that there are transverse modes in any beam means that interfering two identical beams will cause an interference pattern.

If there are no transverse modes, then there will be no spatial variance of interference in the plane of propogation

 

[pmacias]

I would say that your understanding is partially correct. The fringes in an interferometry setup are indeed caused by the interference of two beams that have traveled different paths and have a phase difference. However, even if the beams were perfectly monochromatic and collimated, there would still be a fringe pattern visible. This is because any light beam, no matter how perfect, will have some degree of diffraction. This diffraction creates interference patterns, which can be seen as fringes in the case of interferometry.

In fact, the fringes in an interferometry setup are often used to measure the wavefront of a beam, which can provide information about the beam's quality and divergence. So, while it is true that perfect beams would not produce the same type of fringes as imperfect ones, they would still produce some kind of interference pattern.

Additionally, the concept of a "perfect" beam is somewhat theoretical. In reality, it is impossible to create a beam that is truly monochromatic and collimated. So, the fringes we see in interferometry are a result of the limitations of our technology and the inherent properties of light.

I hope this helps clarify any confusion. Keep questioning and exploring the concepts of interference and diffraction – they are fascinating and important principles in the field of optics.