Unwanted fringes

[Peter]

Hello everyone,

I am working on holography of acoustic guitar in our laboratory. I would like to study the vibration modes of the acoustic guitar by real time holographic interferometry. However, I have some problem to observe the hologram with sufficient quality. The problem is that, during reconstruction of the hologram, I observed the unwanted fringes on our objects. The fringes are moving when you moving with your head from side to side during observation of the hologram (see the videos https://www.youtube.com/watch?v=p7dyrlG7UfI , https://www.youtube.com/watch?v=2WRRxxsuDWs).
We used holography plates VRP 63x63 mm from SLAVICH. We made transmission hologram with semiconductor laser with output power 50 mW with wavelength 532 nm. The ratio between reference and object wave was set to 3:1. The polarization of the reference and object wave was set to horizontal. The exposition time was set to 5 min. For developing the plates we used JD-2 kit from Photographers Formulary, Inc. The guitar is placed about 1 meter from the holography plate. The side of the holography plate with emulsion is facing to the guitar. The workplace arrangement is attached. The workplace was set on the Newport optical table.

holography_schema_3.png (104.34 KiB) Viewed 4552 times

I would like to kindly ask you for advice, whether somebody know; why this unwanted fringes are there and how to remove or suppress them. Because of this unwanted fringes it is very hard to see the “wanted” interference fringes on guitar.

Thank you very much for your help.

[Ed Wesly]

Since the fringes are on all the objects in the first video, first explanation is that something is definitely moving, like an optical component. Three minutes is a long exposure time and plenty can go wrong; is your set up enclosed to prevent air currents from blowing across the beam paths?

Make sure everything is bolted to the table well. If you have a big solid piece of metal, try making a hologram with it included in the scene, like: http://edweslystudio.com/Research/Inter ... wn/IS.html And try replacing the reference beam path with beamsplitter to the configuration using only a mirror as a reference in the link above.

But because of the circular nature of the fringes, it seems likely that the laser is to blame. Has it previously made good holograms without fringes? Try a simpler set up, or even place a coin behind and against the holographic plate to make a single beam reflection hologram of it. Relative motion of the object and plate would be cancelled, and if there are still fringes then it must be the laser, as these fringes look like high spatial frequency contouring, indicative of more than one wavelength being emitted by the laser.

If you have access to a scanning Fabry-Perot spectrum analyzer you can find out if your laser is running single frequency. If you do not, try a coherence length hologram: http://edweslystudio.com/Research/Coher ... e2013.html

Good luck on your project, it will be great to see some nice fringe patterns! Are you familiar with the book by Thomas Rossing, The Science of Sound? There are examples of exactly what you are trying to accomplish in it!

[Peter]

Thank you very much for your helpful comments Ed. The setup seems to be solid. We observed exactly the same circular fringes for exposition time 15 s and for 5 minutes. We guess that the stability of the optical components is not our problem.
You are right that the laser could be the weakest point of the system. We have not observed hologram without these fringes, yet. We tried single beam setup without success.
We use laser PHOTOP DPGL - 3050F (http://www.photoptech.com/main/products ... -3050F.pdf).

Could you please recommend us some financially affordable laser for this purpose?

Thanks.

[Din]

What is the difference between the "wanted" fringes and the "unwanted" fringes?

Since you're using an interferometric method, you're going to get fringes, because that's the point of an interferometric system. In this sense, all fringes are "wanted". Any "unwanted" fringes - that is, any structure that is not the result of the interferometric system, but is the result of holographic technique - may be caused by motion or by mode hopping of the laser. If the former (motion), set up a Michelson and check your table/components, if the latter (mode hopping) you may not need a new laser, you may just need to stabilise your present laser. One way to check is to make a hologram of some simple object and note whether you have motion/mode hopping. Mode hopping shows up as a series of straight lines at some angle (usually called "sliced bread" because it looks like sliced bread). If the problem is motion, then usually you get an dark area on the image. Sometimes, motion causes lines on the plate or image, but these lines have a variable spatial frequency. If the spatial frequency is constant, the lines are probably interferometric. Laser142, here on the forum may be able to help with the mode hopping problem, if any (Phil?).

But the fact that the fringes are circular lead me to believe that you're getting interferometric fringes, exactly as you want. The question now is: How do you create the fringes? You say you record a tramsmission hologram of the guitar. Then what do you do to check the vibrational modes? Do you alter the hologram, or the guitar? Also, the fringes are going to move as you swing your head from side to side, because you're changing the angle of vision and so you're changing the phase relationships.

[Ed Wesly]

I looked at the laser’s specifications and the parameter that is most relevant to coherence length, “Longitudinal Mode Spacing”, typically given in Hertz, is missing. The lower the number, the better. And the best figure of merit is “Single Longitudinal Mode”. Check out Sam’s Laser FAQ’s for more info, http://www.repairfaq.org/sam/lasersam.htm.

It looks like your laser may not be suitable for recording holograms, but you can use it for replaying holograms. It would be a good laboratory exercise to set up a Michelson Interferometer with that laser and change the path length in one of the arms to see how the fringes come and go.

There is a member on this forum, Laser142, who sells refurbished lasers. Laser Sam, mentioned above, does the tweaking. I don’t know what your budget is, but a SLM 532 nm laser of 50 to 100 mW might be able to be delivered to you for under $2000 (USD)

Where are you located? I am guessing Eastern Europe from the YouTube title, and there may be other forum members in your area that might be able to help. There is Konrad (konsew) in Poland and Petr Lobaz in the Czech Republic who are very competent holographers, as well as others I might not know. If in Turkey, there is Ahmet Faik (holotarzan).

If you were in Chicago, I could come right over!

[Peter]

Din: Thank you for your reply. The picture and the video which I attached in the first post depicted only reconstructed hologram (without real guitar and objects behind). So the fringes are recorded on the hologram. The guitar was without excitation during exposure which means that the fringes are parasitic. Checking the vibrational modes of the guitar will be the next step. Firstly we want to obtain nice transmission hologram.

We built up Michelson interferometer with the laser and mechanical components seem to be stable. However I noticed blinking of the laser sometimes with changing of the spatial profile of the spot. Maybe this is the problem.

Do you have some picture of the hologram with mode hopping effect, because I have never seen it?

Ed: As I said to Din, we built up the Michelson interferometer and we obtain fringes even the one path was half a meter longer than other one.

What about Lasos Nova 50? Do you have any experiences with this laser? It is suitable for holographic interferometry?

Your guess is perfect, we are located in Czech Republic. Thanks for the contact on Petr Lobaz.

[Ed Wesly]

"However I noticed blinking of the laser sometimes with changing of the spatial profile of the spot. Maybe this is the problem."
How often does this happen? During the length of time of exposure? It is indicative of something not too healthy going on inside of the laser.

"What about Lasos Nova 50? Do you have any experiences with this laser? It is suitable for holographic interferometry?"
I didn't find any Novas listed on their site, but if you are talking about one of these, http://www.lasos.com/products/dpss-laser, then you are on the right track. I have been lusting after one of their 200 mW, 640 nm emitters, but it is $10k USD!

One thing to be aware of with this line of lasers is that the beam is not round! This can sometimes be a nuisance as the ellipticity may not be in the orientation that you would like.

The Coherent Compass 315M does emit a round single frequency beam, but it is only available as surplus. I got mine from Phil on this forum, and I am quite happy with it.

There is a Swedish company, Cobolt, and they sell what you want, SLM with a round beam, but they are a bit expensive. I got to use one of their Flamenco 500 mW units, it was putting out 600, in a nice round single frequency beam. I would like to call them the Rolls-Royce of lasers, but they aren't located in the UK! And I am not so sure if calling them the Volvo of lasers would be all that flattering.

It looks like a new laser is what is needed to get your project back on track! Good luck!

[BobH]

A laser with a very short coherence length will still make fringes in an interferometer with largely unequal path lengths. One needs to move one of the mirrors a very small amount (despite the large difference in the path lengths) to see those fringes change in visibility as the light goes in and out of coherence.

[lobaz]

I have a Lasnova 50 mW from Phil (142laser) and it works fine.
You can contact me at this forum or by e-mail lobaz@kiv.zcu.cz. We can arrange making hologram in your lab with my laser.

By the way, I guess the object moves. After all, a guitar is made to resonate well.

Petr

[Din]

The picture and the video which I attached in the first post depicted only reconstructed hologram (without real guitar and objects behind). So the fringes are recorded on the hologram. The guitar was without excitation during exposure which means that the fringes are parasitic. Checking the vibrational modes of the guitar will be the next step. Firstly we want to obtain nice transmission hologram.

In that case, the fringes are caused by object motion. The front of the guitar was moving during the shot. This is not unexpected, since a guitar is made to have it's surface vibrate. Remember that as you record the guitar, you're also heating it up a little. I don't think it's a laser problem. But, you do need to stiffen the face of the guitar, it's too delicate to shoot for long exposures.

We built up Michelson interferometer with the laser and mechanical components seem to be stable. However I noticed blinking of the laser sometimes with changing of the spatial profile of the spot. Maybe this is the problem.

This does sound like mode hopping. The laser output modes suddenly jump from one mode to another and this is accompanied by a blinking. If this occurs rarely, it may not have happened during your exposure, since the video shows classic object motion. If it mode hops during an exposure, then you see sliced bread. I'm afraid I can't find any pictures of sliced bread, it's a series of parallel lines running diagonally across the plate. You're video did not show sliced bread.

Here is a picture of an interferometer of a guitar (from "Optical Holography" by Collier, Lin and Burckhardt

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