Purchasing Lasers

Holography related topics.
John Howard
Posts: 25
Joined: Fri Jul 06, 2018 11:36 am

Purchasing Lasers

Post by John Howard »

I learned (on this forum) to test a laser reflecting it off of a thin glass slide to see if it would produce interference patterns. Is there a technical term I can use to specify that feature? I find that many people selling lasers have no idea about their suitability for holography. I vaguely recall the term "coherence length", but I don't know if that is related to that "fringe-producing" characteristic. If so, is there a length I should be looking for?

Thanks for any clues
Din
Posts: 402
Joined: Thu Mar 12, 2015 4:47 pm

Re: Purchasing Lasers

Post by Din »

You may be interested in a white paper I wrote for Melles Griot
Melles_WhitePaper.pdf
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John Howard
Posts: 25
Joined: Fri Jul 06, 2018 11:36 am

Re: Purchasing Lasers

Post by John Howard »

Thanks Din. That article tells me a lot about what to look for in lasers for Display Holography. I think coherence length is the term I was looking for.

That leads to another related question: Is there an easy way to measure coherence length of a laser? Thanks again.
Din
Posts: 402
Joined: Thu Mar 12, 2015 4:47 pm

Re: Purchasing Lasers

Post by Din »

John Howard wrote: Fri Jan 06, 2023 9:43 am
That leads to another related question: Is there an easy way to measure coherence length of a laser? Thanks again.
Yes. You set up a Michelson interferometer, and vary the path difference. I've drawn up a diagram. The beams splitter sends the beam along both path a and path b (black lines). On reflection from the mirrors, the beams combine at the output of the splitter (green lines). The combined beams go to a screen. If the paths are equal, ie a = b, you get a strong interference pattern at the screen and a high fringe contrast. As you vary the path difference, a - b, the fringe structure in the interference pattern gets weaker, ie there's less contrast between fringe and not-fringe. So, moving either mirror away from the beam splitter will lower contrast. At some point, the contrast will be too weak to notice. At this point, the coherence length is the path difference between mirrors, a - b (or b - a, depending on which mirror you move). Note, though, you don't want to make a hologram with a path difference between the two beams at the limit of the coherence length. The greater the path length difference approaches the coherence length, the weaker the hologram.

If you want to think of it in this way, a hologram is a photograph of the pattern you see on the screen of a Michelson, albeit on a much smaller scale. The efficiency of a hologram is dependent on the fringe contrast (also known as 'modulation'), so as you get to a path difference of the hologram on the edge of the coherence length, the modulation decreases, and hence the hologram gets weaker.
Michelson20230106_06531341.jpg
Michelson20230106_06531341.jpg (176.97 KiB) Viewed 3284 times
John Howard
Posts: 25
Joined: Fri Jul 06, 2018 11:36 am

Re: Purchasing Lasers

Post by John Howard »

Thanks again, Din. I can (and will) do that. From that explanation, I conclude that - as a general rule - I should aim at having the reference beam and the subject beam be as close to the same length as possible.
Din
Posts: 402
Joined: Thu Mar 12, 2015 4:47 pm

Re: Purchasing Lasers

Post by Din »

Ont thing I forgot to mention is that you need a lens before the screen to expand the combined beam so as to enlarge the interference pattern. The unexpanded beam is too small to see any structure.
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