Colin wrote:A hop can not be compensated for because the distance between the fringes changes not the placement. The only reasonable fix for mode hopping is temperature control.
Colin, I agree with this statement and I think I mentioned it previously. It is very plain to see when the laser mode hops. But as I mentioned sometimes the fringes go ever so slightly blurry, then return to nice and contrasty. And sometimes, after the laser has warmed up, it takes minutes for the blurriness to happen. Thus not sure if I have to do multiple exposures or just time the shutter during the stable parts, but it would be a technique that would open some doors.
BobH, I understand now. What causes a repeating coherence like that and what can be done externally to corect it?
I hope to try some exposures this weekend ....on film....
John, are you able to adjust the power of your DPSS laser? I am curious about it's behaviour at very low powers because green DPSS are usually very stable at minum power levels.
Hans, I was thinking of trying the same thing. The laser is supposed to be a 50mW and it is putting out 75mW. There is no external way to decrease power. I wrote the supplier of these lasers and asked for information but they have not gotten back to me yet.
I did manage to get the new "table (if you can call it that)" set back up and I do have a rail inteferometer in which I can view the fringes real time when doing the exposure. So I guess now it is time to make plates and see what I can get.
I mounted the laser to a heat sink and after playing with setting up the new table, an hour or more had passed. At that time, there is virtually no mode hopping at all but I cannot tell of the coherence length/dual line thingy yet.
The stats given above for the laser list it as having CW, Analog or TTL modulation. If this is true get yourself a nice clean 0-5volt adjustable power supply. Hook it up to the BNC connector on the back of your driver and viola you can adjust the beam output from 0 to max (75mw) with an input between 0 to 5 volts. Your power supply needs to be very stable and cannot drift, within reason if you want to maintain an exact output power.
Can I ask what you paid for the DPSS system? I'm asking because the power stability in the specs is +/-2% which doesn't tell me what the baseline stability is. These systems are manufactured in <5% and <10% power stability.
I made 6 holograms last night. All were 2" x 2". 3 were made with Great Lakes and 3 with Knox. The film was sent to me by a fellow hologapher. So I was testing the film, the new laser I am using and a completely 3rd world hologaphy set up.
I basically, set my lab up with minimal components and tried to stay within guide lines that someone with a limited budget and novice, could do quite easily, I will take some photos and post then here. I do not use any vibration isolation table at all.
The film results shocked me. The film was over 24 hours old and the Great Lakes, which is labeled 250 bloom came out milky white not matter what I did. I used 4 alcohol baths starting with 35%, I used all temperatures at 69F. I then did the same for the Knox and the plates were clear. Knox, was told to me to from the manufacturer that it is about 225 bloom. This is very puzzling. There must be something in the way the Great Lakes is made that keeps it softer, or somehow the bloom was mislabeled for that sample. Those were the first two shots. The next two, and then the last two again were first Great Lakes and then Knox but I raised the temperature of the processing baths for each of the pairs unitil I was shooting Fixer - 70F, Water - 78F, 35% - 90F, 70% - 90F, 91% - 95F and 100% - 120F. The Great Lakes continued to get more milky wereas the Knox remained clear, even at the elevated temperatures.
Now for the laser, even after 3 hours I still saw mode hopping and drift in the real time interferometer. In one hologram I saw about a 1/4 wave drift and is evident on the hologram by a single fringe on the object that covers about 1/4 of the object. I also saw some shuttering of the light off to the parallel side of fringes but this would stop. And finally I saw a competition of wavelegths as the fringes would lose contrast for a while and then come back real contrasty. I was able to get a 82 second exposure, just to test a long exposure and did not see any drift, wiggle or dimming of fringes. The hologram came out very bright without any signs of a short coherence length. Out of the 6 holograms there were only two in which I timed the shuttering incorrectly and saw fringe instability in one way or another during the exposure.
These tests are not conclusive yet, but I do believe with a real time interferometer, and a knowledge from studying the fringes and timing the shutter, this laser can be used to make nice holograms.
I will be making some 4x5 or 5x7 plates in which I will be testing the laser futher. I will keep you all posted on my next set of results with this laser.
John
PS: I never saw it as this way before, but in making a hologram, the whole hologram need to be made in a single fringe. While watching the fringes on the interferometer and see it move 1/4 fringe then seeing that exact 1/4 fringe on the hologram, made me see holographic recording in a new way. That is, the entire hologram must be made within a single fringe. Not sure how to explain it, but that is how I see it now....
Boy, what a dumbo I am. It just so happens the object I used was a little building for, what I thought was a non mode hope, non competing line expose but the building, the part that I shot, has contour lines on its wall. I guess to represent some type of siding. Anyway after looking much closer there are lots of sliced bread lines, but just hidden because of the lines already on the building. This also is evident on another hologram I made of part of a porcelain lamp. Only the specular reflection of a Demi Tas coffee cup saucer which is very shallow did not have sliced bread fringes.
This is very depressing. I guess sometimes you want something to work so bad you’re subconsciously subjective instead of being completely objective.
I will still do more testing but unfortunately; it seems the steps I have taken so far do not eliminate the sliced bead phenomena in this laser.
JohnFP wrote:Anyway after looking much closer there are lots of sliced bread lines....I guess sometimes you want something to work so bad you’re subconsciously subjective instead of being completely objective.
I know that feeling well, John. Sometimes those contour lines are not apparent at first, but on closer inspection, they become visible. I went through this routine with diode lasers.
But I think a mode-hopping blue laser can still be very helpful when it comes to DCG. Since DCG requires a considerable amount of experimantation, it can be used as a "temporary" blue laser, if someone doesn't mind seeing the contour bands. The easiest way to make good DCG holograms is with a blue laser, because the film doesn't need any special or unusual preparations. Then it's easier to concentrate on "basic" DCG holography. One can always upgrade to an argon laser or Melles solid-state blue laser at a later date.
On the other hand, we've seen some spectacular MBDCG holograms in photos posted on this forum, which were made with a HeNe.
