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Hi Steven and Dave,

I was using 2mm glass resting on a three prong glass holder

I was able to get to barrow a SFPI some months ago and I did not see any mode hop although it could be a slow drift. I did see mode hops when I first turned on the system for about 30 ceconds or so. So I shot a plate during that period. I watched my scope while exposing the plate and saw there were competing modes for about 10 seconds or so then locked in. I processed the plate and oddly I did not see any ring pattern on the hologram. At first I thought great, perhaps with the longer exposures it was insensitive to a hop or two. But later I thought well maybe I should have tryed it several more times.

My AmDi is pretty low in concentration just below 3% so I don't think there is a lot of stress produced by the film but who knows with the length of my exposure and my bloom strength is at 270.

Yes Dave this was film was a few weeks old since I was going toward narrow band. This could be an issue

I do let my laser warm up for about an hour before using.

It's just an odd thing I get this sort of problem two or three times a year. I think when things go well I might tend to get a bit sloppy regarding settling time and air flow issues.

Anyway thanks all, nice to see some nice activity on the DCG topic

Tony

[quote="Tony DCG"]Thanks Dave
I did clip a thicker piece of glass on the bottom of the plate and the problem went away. I also thought it was glass warp.

I have to admit I was using some thinner glass this last few runs since i was running some narrowband experiments.
I had success before with it but I think it could be one of the sorces of my problems.

Happy Easter all![/quote]
I'm glad you found the problem Tony.

Dave looks to have nailed it. :)
How thick is the glass that you are using?

Most of my glass is 2mm, but I also have some 3mm.
I have shot 6" x 4" using the 2mm glass mounted at three points face down with an exposure of 150mJ without observing any glass bending, but I'm only using a 3% AmDi sensitizing bath. I suspect that if I were to use more AmDi and a longer exposure, I may have problems.

Steven.

[quote="Tony DCG"] I was using some thinner glass this last few runs since i was running some narrowband experiments.
I had success before with it but I think it could be one of the sorces of my problems.[/quote]

In these experiments was the film harder than new films ? I could see harder film as having "extra bending powers" above softer film .....

[quote="Tony DCG"]Here I hope is is a photo of the ring
Sorry if the photo is of poor quality

The rings start in the almost center and are about an half inch apart
The Laser is a Melos Griot 457nm 200mw elipitcal beam[/quote]
Ah, would it be this one?
A very nice laser looking at the specs.

I did a couple of plates some months ago that had similar banding, but my stripes were straight lines.
I don't have a photo to show as they were not capped with a cover plate, so I will have to reprocess.
However, I shot three holograms of the same scene, two came out stripped and one was perfect.
I initially thought that the problem was caused by insufficient settling time to allow the plate to get to ambient temperature,
but all three plates had the same length of settling time.

The fact that the stripes are not equally spaced tells us something important.
It's not the sort of result I would expect to see from a mode hop.
It looks like the sort of result you could get from plate movement, or expansion/contraction of the plate.

Having said that, I did set my 315M-100 on the table with a makeshift interferometer and spent many hours sitting in the dark
watching fringes over a two week period. It turns out that my laser needs quite a bit of time to settle down.
I didn't observe any mode hops, but just a very slow drift in the fringes. It seems to take my laser a couple of hours
to stabilize enough for my exposures, that can be 20 - 40 minutes long.

Could a slow drift in laser wavelength produce the results we are seeing???

I have not had time to shoot any holograms since running tests on my laser,
but I plan to extended my laser warm up time before attempting to make an exposure.

How much time to you leave your laser on before making an exposure?

Steven.

Thanks Dave
I did clip a thicker piece of glass on the bottom of the plate and the problem went away. I also thought it was glass warp.

I have to admit I was using some thinner glass this last few runs since i was running some narrowband experiments.
I had success before with it but I think it could be one of the sorces of my problems.

Happy Easter all!

Thanks for the pix Tony, my vote will go to plate warpage or plate movement? Just for kicks coat a 1/4" thick piece of glass and see what comes, and again try to place a box over it all w the hole cut on to to allow the bean in .....

Thanks for the pix Tony, my vote will go to plate warpage or plate movement? Just for kicks coat a 1/4" thick piece of glass and see what comes, and again try to place a box over it all w the hole in it.....

[attachment=0]move.jpg[/attachment]

Here is a photo that shows my material test. I stuck on various plastic pieces on a metel base and as you can see there are dark areas.
The thinner plastic is problematic. The upper right hand corner is a girls face, note the darkness around her eyes.
I hate photos.....

Today I shot and most of the shots were good except as I tilted the plate there was very very slight dimming.

Thanks all

Here I hope is is a photo of the ring
Sorry if the photo is of poor quality

The rings start in the almost center and are about an half inch apart
The Laser is a Melos Griot 457nm 200mw elipitcal beam

I suspect that the characteristics of the banding would be determined by the size of the mode hop, if the laser was mode hopping.
Smaller change in wavelength = wider banding???

Giving it a bit more thought, banding on the object can only be caused by some sort of double exposure.
Either:
(1)The the object/plate moved, so that it was recorded in at least two different fixed positions. I don't think slow movement would cause this as the banding would be moving during exposure. (2)There was more than one source illuminating the object creating the interference pattern on the object.
(3)A mode hop, effectively giving the object two exposures, but at different wavelengths.

Maybe another possibility if the laser is not actually mode hopping in the usual sense of the term, is a slow frequency drift in the laser between two stable operating areas, not an actual quick jump. Is enough time being given for the laser to warm up/stabilize?

My finger is still tending to point at the laser, but without more info, or a photo, I'm not willing to put money on it.

Steven.