I am having an awful time trying to make a hologram with my 164 SP Argon laser. I ordered the Slavich Ultra fine grain Red and made some nice holograms with my 5 mw HeNe laser. I then ordered some Green Slavich film for my Argon and can not get a hologram. I set up an interferometer to look at the fringes of the Argon even though the holograms I am trying to make are of the Denisyuk type. The fringes are very contrasty but drift. I assumed that this was due to viewing the fringes right after I placed the optics and mounts in the sand (yes I sell have a sand table). I viewed them for only about an hour and they settled a lot but still seemed to drift slightly, first one way then the other but not as far on the return thus a net of drift in one direction. Again I assumed it was the bases in the sand settling. What is tipping me off that it may be the laser is I was viewing the apparently non existent (nothing in the sunlight) reflection holograms I made with the Argon in the spread argon light and I noticed the laser light was sort of flickering. I had the power at only about 2 out of the 10 possible on the power dial on the power supply. This would give me a 1 second exposure on the green film according to the 2 minutes to 2 – 3.5 density developing process.
Also, I can see a hologram of the collimator when viewing the "non-hologram" as a reflection or transmission with the argon light. Obviously the collimator needs to be cleaned. I can see every spec of dust on it in the hologram. So here are my questions if you can help;
1. What does the flickering, although very subtle and relatively infrequent (about every 2 - 3 seconds) mean for the Spectra Physics 164 with an air spaced etalon? Is my laser mode hopping? How can I verify mode hopping? Does it need a couple shots of gas? Could it be that the pressure or flow rate of the cooling water is not enough to keep it stable?
2. Why is it that I can see a hologram of my collimating mirror but nothing of the object? What is being used as the reference beam to get a dim image of my collimator, visible only with laser light? As I view the mirror at all point on the plate I only see one point source of light so I do not think the mirror is so bad that it is making a hologram of itself or there is stray light hitting the plate unless it is the object. The object is an unfired figurine, the kind that you paint then fire to make ceramic. It has a matt, white finish. I have used it before with success.
I guess my next move is to replace the argon with the HeNe and see if I can get a hologram. If not then work out the issue, which would have to be object or plate movement with the HeNe. Then put the Argon back into the picture and see what happens. If I do get a hologram with the HeNe the it must be the Argon laser. Anyway thanks for any info you may provide, specifically on the flickering of the Argon, that is my real question. The rest will be worked out as I back up to the simplest of set-ups. I need to have the confidence in my mind that the Argon laser will work and does not have a problem, the rest will come.
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Well, researching has lead me to beleive that my laser is not mode hopping because the beam position does not seem to jump. BUT....I should now check the voltage across the tube, the voltage of each of the legs going to the power supply. It could be, I beleive, the pressure is too low for the low current I am running the laser at, the current was set too low to be far enough above the threshold or the 3 phase power going to the power supply is very unequal or unstable. Any comment?
What should the tube voltage be for the Spectra Physics 165? I read 232V at 30 amps. Is that correct? At what voltage should I hit the gas fill? I know it takes 10-20 minutes after each fill to check voltage to see if it needs another fill.
Does anyone have the operators manuals for 164 and 165 Spectra Physics Argon Ion lasers? I know there would be a wealth of info in those.
>I then ordered some Green Slavich film
I assume VRP.
>I set up an interferometer to look at the fringes of the Argon even though the holograms I am trying to make are of the Denisyuk type.
Doesn't matter. Just because the fringes are in the body of the plate does not mean the fringes of a Michaelson have no value. In fact, the stability requirements are even more stringent in a reflection. For transmission, the fringes must not move to within about half a fringe in the time of an exposure. For reflection, they must not move to within a quarter of a fringe in the same time. If you're exposure is about a second then watching it for an hour is a bit overkill, you only need to watch it for about 5 seconds.
> still seemed to drift slightly, first one way then the other
If the drift is constant in one direction something is slowly falling. By what you describe, it sounds like something is swaying, maybe because the optic isn't tightly enough attached to it's base. Have you tried wetting the sand? How do you mount the optics?
>I noticed the laser light was sort of flickering.
This could be many things, from power fluctuation to mode hopping. I doubt it's the water supply. The laser would turn off if the water was insufficent. The power supply itself could be damaged. I found that one transistor in a bank of transistors had been removed from the power supply of my laser by the previous owner. It caused some flickering. Another way around this is to detune the laser to a gain line off the center. At the center slight drift will cause the laser to lase at the next gain line but if you run it at a gain line at the edges of the gain slope it can drift a long way without flickering. Of course, the price you pay for this smooth operation is lower power. Mode hopping is seen as a sudden shift in the fringes of a Michaelson, as if the fringes blinked.
>Why is it that I can see a hologram of my collimating mirror but nothing of the object?
Do you mean in transmission or reflection? Do you mean the mirror itself or a point of light that seems to be coming from the mirro? If the latter, you're sure it's the mirror and not a scattered point off a metal base or something? Even so, what may be happening is that you''re not getting an object because the ratio or exposure is not enough. However, it may be enough for a bright point source of light. This may especially be true if you see the point of light in transmission.
I think you should try to make a simple two beam transmission grating at, say 30 degrees with a beam ratio of one-to-one. These are pretty simple and pretty foolproof. The exposure is low and you should see the grationg in the developer. If you can't get a grating, then maybe it's the laser. if you can get a grating, then maybe it's the ratio and/or exposure.
If you want to test for mode hopping, expand the beam and look at the beam profile. "modes" are multiple paths taken by light through the laser tube, ie straight through the center of tyhe tube or one bounce off the sides or several bounces off the sides etc. If more than one mode exists in the tube, there'll be interference between the various modes giving you beam profiles with characteristic dark areas in the main beam. TEM01, for instance, is a line coming straight down the center of the beam so the beam is two ovals on either side of a dark line - "bu*t mode" as a colleague of mine described it. If the beam suddenly looks like this, you're mode hopping.
There is in principle transversal and longitudinal mode hopping.
Transversal hopping is what Dinesh had described - it is associated
with a change of beam profile, and can easily be detected when
expanding the beam to a few inches diameter by a lens. If that
would occur, this would be a disaster for a hologram, as the various
pieces of the beam profile would would not be coherent with respect
to each other. The problem should be more pronounced with higher
laser power. It can be eliminated by a careful adjustement of the
laser (perhaps additionally cleaning if it does not help). I assume
that you have TEM00 mirrors in the laser, which is most likely the
case as you have also an etalon installed, which would not make
sense for a non-TEM00 laser.
Then there is longitudinal mode hopping, and that should be visible
by brief intensity flickerings as well. That always happens and
almost cannot be avoided, but it is not harmful unless you want to
make holograms with great path length differences. Each mode hop
corresponds to a frequency shift corresponding to the resonator
length, so after 10 hops you have an effective coherence
length of roughly of a tenth of your resonator length, which should
still be fine. This can be improved by temperature stabilization
of the etalon.
The best way to identify this kind of problems is to set up a little
interferometer, and watch the fringes over time; also check them
for various different path lengths.
As Dinesh has said, it is very unlikely that the water, or the gas,
would have anything to do with it.
Can you better explain what you mean with "hologram of the collimator"
? Do you see a real 3d holographic image of something, or just
projections of dust spots ? If you see a true 3d hologram, then
the problem seems indeed to be tied to coherence length, ie, mode
stability problems.
I found the problem! I talked Bryan at Cambridge Laser Laboratories (this guy and that company are fantastic) and he exactly told me what the problem was. See my post on Yahoo Holography group titles "Cambridge Laser Laboratories" for another issue resolved by Bryan. http://groups.yahoo.com/group/holography/message/914
We were talking back and forth and he seemed to think the flickering was problem with multiple lines competing with one another. This is after I told him my voltage across the tube (OK), the fact that I could see a hologram of my collimator but not the object, the fact that I made a hologram of the object with a HeNe in the same exact set-up and about the flickering. Come to find out it was my Etalon. It was tuned too close to the flash point. He told me to find the flash point and then adjust the vertical knob on the Etalon to go off the flash, past the next 3 peaks and tune in the fourth peak which I did last night. So Dinesh your solution was correct, “Another way around this is to detune the laser to a gain line off the center." Seems my etalon needed to be "detuned". Also, yes it was the VRP film. And yes WLER your diagnosis was correct, " If you see a true 3d hologram, then the problem seems indeed to be tied to coherence length, ie, mode stability problems."
I then tried an exposure on the Slavich film and I GOT A HOLOGRAM. Funny thing is though the hologram came out very violet and blurry and dim after finishing processing. But when I breathed moist air is turned blue then green then yellowish then orange then red and very bright and crisp. So I knew it was a shrinkage problem. I tried to vary the exposure time to change the color. I didn't have much time so I only increased exposure time. I would have liked to try to decrease exposure time also but the first hologram did not go to the density needed for reflections after 2 - 3 minutes of processing in CW-C2 developer and Potassium Dichromate and Sodium Bicarbonate bleach. So I thought I could not go much further in decreasing exposure. The second exposure came out very violet also with the same slightly blurry and dim characteristics. I called Dr. Jeong as it was his development recipe that I used to develop the green film. He stated I could swell the film with Triethanolamine (TEA) after processing. I told him I did not have that handy at the time. He stated that TEA is an oil that dissolves in water. When you soak the hologram in a concentration of TEA then dry it the TEA remains in the emulsion thus keeping is swelled. So I needed something to dissolve in water and would remain in the emulsion. I tried 1 teaspoon of table sugar in a cup of water. I soaked 1/3 of the hologram in the solution for 2 minutes. Re-dried it and it came out green. I tried to increase the sugar concentration to get it goldish but after 3 teaspoons then 6 total in the 1 cup of water the Holo dried violet again. I am guessing that too much sugar in the emulsion causes a tightening factor when the sugar water dries. It binds to both sides of the fringes and pulls then together. So about 1 to 3 teaspoon of sugar in 1 cup of water for 2 minutes after development raises the replay wavelength about 80 nm.
I am not going to spend much time on refining the green Slavich developing as now I have the laser where it needs to be and I have verified the Denisyuk camera works and is stable. Next phase,,,, Fabrication of DCG plates.
