Tricks for measuring coherence length

Holography related topics.
John Klayer
Posts: 273
Joined: Thu Jan 08, 2015 2:28 am

Tricks for measuring coherence length

Post by John Klayer »

I mean simple and cheap.
pierro787
Posts: 12
Joined: Tue Aug 22, 2017 5:47 am

Re: Tricks for measuring coherence length

Post by pierro787 »

Hello,

Using a Michelson interferometer, getting the frindges with exact same distance beetween beamsplitter and the two mirrors (x1 and x2) and then, move back slowly one of the mirror until the frindges disapear then stop and mesuring one more time x1 and x2 length..

With this simple formula, you get the coherance length of the laser:

∆x = x2 − x1

Pierre
John Klayer
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Joined: Thu Jan 08, 2015 2:28 am

Re: Tricks for measuring coherence length

Post by John Klayer »

If one gets a narrow peak with a Fabry Perot does it necessarily mean that it is spatially coherent? Can't a beam be very monochromatic but still have terrible spatial coherence? Is the Michelson technique the sure fire way?
Din
Posts: 402
Joined: Thu Mar 12, 2015 4:47 pm

Re: Tricks for measuring coherence length

Post by Din »

John Klayer wrote:If one gets a narrow peak with a Fabry Perot does it necessarily mean that it is spatially coherent?
No.
John Klayer wrote:Can't a beam be very monochromatic but still have terrible spatial coherence?
Yes.
John Klayer wrote:Is the Michelson technique the sure fire way?
For holography? Yes, based on the situation. In holography, the fringes, or planes, are formed by two point sources, even though one point source may be at infinity, and a specific frequency. However, if there were more than two point sources, additional fringes, or planes, would be formed. Similarly, if there were more than one frequency, additional fringes, or planes, would be formed. So, in order to get exactly one set of fringes, or planes, you need a source that is highly spatially and temporally coherent. Since no source is infinitely temporally coherent (all real sources have bandwidths), the laser is the best source. But, statements like "You need a laser to make a hologram" are not true. I can make a hologram with a led, albeit very thin hologram!

There are situations where you can relax one or other of these conditions. For example, when you make a shadowgram, the object has a very low spatial coherence deliberately.

If you record one set of fringes (pretty much), then, based on the parameters, every point source of the reconstruction source will "play off" the set of fringes. If the reconstruction source is broad, ie has a low spatial coherence, then multiple images result. It's said that the sun is the "best" source for reconstruction of a hologram. The sun subtends about half a degree at the earth, while some modern leds subtend less.
John Klayer
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Joined: Thu Jan 08, 2015 2:28 am

Re: Tricks for measuring coherence length

Post by John Klayer »

Thanks for the reply. What I'm trying to measure is the residual spatial coherence of a beam after passing thru different lengths of multimode fibers. With a C315M I've made holograms using the up to 50 foot long multimode fibers for both the reference and the object so I know there is some coherence at the end. I just want to be able to measure it. I guess a Michelson with a reasonable length variable arm would do if I use a HeNe that has a short coherence length to begin with. There is nothing about a fiber that would change the bandwidth of the light but it could affect the spatial coherence.
Din
Posts: 402
Joined: Thu Mar 12, 2015 4:47 pm

Re: Tricks for measuring coherence length

Post by Din »

You mean you want to measure the temporal coherence, the range of ω in the beam. Spatial coherence is the Fourier transform of the aperture (the aperture in your case is presumable the face of the fibre), according to the Van Cittert theorem. Can you have one arm of the Michelson as in the input beam, before launching into the fibre, and the other arm as the exit beam, when it leaves the fibre, by splitting the beam,before it enters the fibre?
John Klayer
Posts: 273
Joined: Thu Jan 08, 2015 2:28 am

Re: Tricks for measuring coherence length

Post by John Klayer »

I have a better idea. Come to ISDH 2018 to see.
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