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Are there special requirements for a laser to create an interference pattern?

janbbeck

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Well, I am trying learn a bit about lasers and optics, so I started out by attempting to make a michelson morely type interferometer - just want to make an interference pattern. So I have lined up the laser, mirrors and beam splitter, but I am not able to get an interference pattern at all. I am suspecting the cheap laser pointer that I am using (cost $2). I have realized that I don't know at all what requirements a laser has to fulfill to be able to make an interference pattern. Is a narrow line width necessary? A certain beam quality?

I have ordered a Helium Neon laser anyway, but even if it works, I wont know why, so:

Please educate me.
 

Cyparagon

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I think coherence length is important, so a HeNe should work much better. You also want to eliminate all sources of vibration. The kitchen table is not good enough. Tiny vibrations from traffic miles away will prevent formation of a stable image. There are elements here similar to holography, so do some research there, too.
 

Blaster

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I think coherence length is important, so a HeNe should work much better. You also want to eliminate all sources of vibration. The kitchen table is not good enough. Tiny vibrations from traffic miles away will prevent formation of a stable image. There are elements here similar to holography, so do some research there, too.
you beat me to my similar reply +1 agree same thoughts
 
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janbbeck

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Ah. Thank you both very much. Some articles about coherence length that cleared up everything:

Coherence length - Wikipedia, the free encyclopedia
(apparently the fringe visibility is a way to measure the coherence length)

Optics basics: Coherence | Skulls in the Stars
(great writeup on the effect of coherence)

thank you guys.
Coherence (physics) - Wikipedia, the free encyclopedia
also explains why putting a pinhole in front of the laser did not help. The coherence length is not affected by a pinhole, only the area.


As a follow up:
Can you give me some direction as to how the quality of the mirrors affect the interference pattern? how is the quality of the mirror specified anyway? rms deviation from desired ideal shape?
 

Cyparagon

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In my several years in the hobby, I've never really read discussions on distinctions of mirror quality... aside from surface position I guess. I've read mirror stats as "scratch/dig" values as a quantitative measurement of smoothness, but It doesn't seem critical for us. As long as you've got a first surface mirror, it should be sufficient.
 
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janbbeck

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In my several years in the hobby, I've never really read discussions on distinctions of mirror quality... aside from surface position I guess. I've read mirror stats as "scratch/dig" values as a quantitative measurement of smoothness, but It doesn't seem critical for us. As long as you've got a first surface mirror, it should be sufficient.
First, I am very thankful to the members that are taking time out of their day to read and answer my questions. This is much appreciated!!

I am curious how the surface finish of a mirror affects the quality of the laser beam on a theoretical level. There was very good theoretical information available on the beam coherence. Isn't there something about how mirrors affect the beam? Is it irrelevant as long as any scratches are smaller than the wavelength?

For example, many highly polished kitchen utensils make mirrors I can easily see my reflection in. Also, many chrome plated (really nickel of course) objects are very good mirrors in term of seeing your reflection. On the scale of a laser beam (less than 1mm accross) they should be very flat too.

so are these cheap first surface mirrors? Why/why not?
 




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