Hello Holographers,
Any help you could provide would be appreciated.
I bought a Lasos 50 mW 532 nm single longitudnal mode 1 MHz linewidth laser to do DCG holograms because they look awesome and I finally found a great deal on e-bay to get one. I have been excited to make DCG holograms since I learned about them. I am an experienced optical engineer and so I am frustrated I have problems getting this to work. Any information you could provide would be most helpful.
Anyways, I am completely failing to make DCG holograms. When I do the final 100% alcohol bath, basically I do not get hardly any rainbow fringes at all to speak of. This is even after directly taping coins (as a reflection object) to the gelatin emulsion during exposure to try to minimize the distance between the emulsion and the object.
So let me explain the process I am doing. I have read this forum many times and other people's experiences trying to glean what I could be doing wrong. However, after many attempts, I have no clue.
First, I am coating the plates. To clean the plates, I soak for several says in deionized water with 10% sodium hypochlorite bleach added to it. It seems to do a good job of cleaning and etching the plates. I rinse the plates with deionized water and let them dry by placing them into a box where they are placed at about a 30 degree angle to vertical to drip off and dry.
The gelatin emulsion I prepared is 250 grams deionized water, 30 g gelatin, and 5 g potassium dichromate. I have tried two types of gelatin: 250 bloom photographic gelatin and Knox gelatin that is bought for cooking. They both seem to provide the same results. To prepare the gelatin, I have a pot on a hotplate to which I add the water and the gelatin. I heat and keep between about 40 C and 50 C (usually around 45 C to 47 C) and stir using a thermometer to dissolve the gelatin. This takes about a half an hour or so. Once the gelatin is dissolved, I turn the lights off and use a light source I built from red LEDs. I add the potassium dichromate and stir than in until I see the liquid is clear, though with the red light it can be hard to tell if it's completely dissolved, but it usually takes five minutes of stirring or so.
I then pour this into a squeeze bottle using a funnel. I then coat the plates, which are about 10 by 10 cm. I have tried two methods. The first is the veil coating method, where I hold the plate at an angle and use the squeeze bottle and allow the emulsion to run down the plate. The second method is a Meyer bar I made out of a 10 mm diamater stainless steel rod and stainless steel spring of 0.5 mm pitch. I use the squeeze bottle to spread some on the edge of the plate and then use the Meyer bar to draw it over the plate. It is not perfectly even but I think it would work ok.
After coating, I place each of the plates in a plastic light tight box I prepared which has a rail affixed along the bottom so the plate can be tilted to a 30 degree angle to vertical. I let the plate sit for a day or two at room temperature and allow the emulsion to become firm.
To expose the emulsion, I place a 20X microscope objective in front of the laser to diverge the beam. An object is placed on a honeycomb stainless steel optical table with the glass plate leaned up against it. I remove the beamstop in front of the laser. I have read that the exposure level at 532 nm needed is about 200 mJ/cm^2. This corresponds to about a 5 minute exposure time for the plate size I have. I have tried 1, 5, and 20 minutes exposure time. After the exposure is finished, I cover the beam with the beam stop and allow 5 minutes for the dark reaction to occur.
The development I perform immediately. The first solution is 500 mL of deionized water with 5 g of sodium metabisulfite and about 2 g of potassium aluminum sulfate (potassium alum). I keep the plate in this solution for about 2 minutes to be sure all of the remaining dichromate is reduced to Cr(III). I have also tried sodium thiosulfate as a reducing agent because it is used in Kodak Rapid Fixer, but it did not seem to be effective at reducing the Cr(VI). Then I place the plate into a 70% isopropyl/30% deionized water solution for about a minute or so. Finally, I finish with a 100% isopropyl solution for about a minute. After this, I have tried both allowing the isopropyl alcohol to evaporate slowly and naturally, and using a heat gun about about 100 - 150 C to evaporate the isopropyl alcohol. The result rarely if ever shows any fringes at all. I have only one plate that actually worked, and I'm not sure why.
The emulsion on the plates usually looks a little cloudy after drying, and sometimes there is a "dendrite" crackle like pattern, especially if the plate is dried quickly using the heat gun. But any kind of rainbow fringes are absent.
I taped coins to the emulsion so that the distance between the object and the emulsion is minimized in case the coherence properties of the laser are affecting the hologram. The coins themselves produce no hologram, but where the tape was on the gelatin sometimes has a little rainbow interfence to it.
I used a Michelson interferometer to test the laser coherence length, since I bought it off of e-bay and so I could not be sure it was working properly. I had a 25 mm delay in one arm of the interferometer and was able to observe stable interference without any problems. So I would think the coherence length would be well within the distance needed if I was taping coins directly to the emulsion. I don't really have a way to test the full 100 meters or so of coherence length but my guess is that would be overkill for this application. I leave the laser on for about 15 minutes before I take my first hologram to ensure the wavelength has stabilized as well.
If any of the DCG gurus here could help me figure out what mistake I have been making, it would be most appreciated. I have tried most everyhing I can think of and have not been successful yet. Even if my plates were poorly made I think I would observe some kid of interference fringes.
Thank you,
Dan
profdc9@gmail.com