Fringe Lockers and Stability

[Jem]

Hi all

I'm still pondering over my stability issues. I use a small isolation table and have everything well locked down. However, due to the environment i'm working in, a large well insulated wooden shed, I am finding it very difficult to acheive absolute stability.

I was looking for information on fringe lockers as I though perhaps trying one of these may assist in my quest for stability. I came across this webpage: http://www.xmission.com/~ralcon/whylock.html , which has some useful information. However, the page is quite old and i'm not sure if all the information is still relevant.

Does anyone know anything about fringe lockers?, has anyone got experience of using one? How easy are they to set up?, are they very expensive?

Sorry for all the questions, but I want to explore every possibility before I set about the last resort of building a concrete bunker in my garden

Cheers

Jem

[dave battin]

do you have an additonal barrier built arround your entire table ?like a curtain or box? are your interfrometer fringes slowly marching in one direction (delta temps) ?
do a periodic check with a good thermometer and see the fluxation............

can you install rigid foam insulation on the inner walls or do a small heater or ACunit to keep constant? sorry for so many questions, but im in a bit of a hurry and i know you needed this info asap

[Jem]

do you have an additonal barrier built arround your entire table ?like a curtain or box?

I will have by the weekend , i've got a nice custom made curtain/hood with a zip up front for access on order that will effectively seal my holography table from outside light and air currents.

are you interfrometer fringes slowly marching in one direction ?

That would seem to be the main problem that I observed yesterday. I will need to do a few more checks though to nail it down to just that. Strangely, there didn't seem to be too much disturbance of the fringes as I moved about. I did observe a shimmering of the fringes when a noisy wagon drove past on the road which is a good hundred yards or so away.

do a periodic check with a good thermometer and see the fluxation............

Never done that, i'll check it out. Dontcha just love that word... "Fluxation"

can you install rigid foam insulation on the inner walls? sorry for so many questions, but im in a bit of a hurry and i know you needed this info asap

The walls are insulated with 3" of rockwool and the shed is lined with five eighths inch chipboard. So I figure that should be o.k. Normally the temperature is pretty stable, but on very hot days it can creep up considerably.

No worries about the speed of answering, i'm in no hurry. Besides, I really appreciate your help

Cheers

Jem

[BobH]

I use a fringe locker every day at work, but wouldn't recommend one for pictorial work. Unless it's a fixed system, like that for a stereogram. Getting a beam (or pieces of beams) that follow the same path, using all the same optics is difficult and very time consuming. And if you use more than one object beam, the additional beams will not be locked.

If fringes are marching in a unidirectional way, you have a settling or stress related problem. Air currents result in fringes moving in a random way. I'd look for a leaking inner tube under the table, a mirror glued to its mount that's just about to fall off, a burr or particle of metal under a base, or a faulty clamp on the plateholder. Hot glue is terrible to use on a holography system.

[Jem]

Thanks Bob, it sounds like it's a complicated thing to get right. I was just exploring all avenues and didn't want to miss this one before I fully understood it.

As for the moving fringes, i'm going to re-visit this at the weekend when i've a bit more time. I'll report back with hopefully better and more concise information once i've really studied what's happening.

Cheers

Jem

[Ed Wesly]

Air current fringe movement is random, but it's always a good idea to have the table covered, like you said you do. Fringes moving in one direction do indicate something moving in one direction, like stuff ready to fall, or settling. The table I use has a funky epoxy paint on it, and some of the stronger mag bases are squishing the paint over time and moving. Go around the table and tap each optic and see if you hear anything, although if you are as deaf as me, it might be hard to do.

Another thought I have is that you mentioned that you were using a Compass laser, but it was 150 or 200 mW? I have read that some of the higher powered Compasses might not be as stable as their lower-rated brethren, so maybe try lowering the power and see if the fringes stop drifting.

Fringe stabilizers are hard to implement in an object hologram as Bob notes, since you will use some sort of mirror in place of the object in the sort of interferometer set up that generates fringes for the fringe stabilizer to see. So if the object is moving, it's out of the optical path for the FS. But for the transfer set and HOE's it's the bee's knees!

I thought at one time there was a fringe stabilizer DIY on this forum's Wiki? No time for me to check right now. I do have a manual for my MEI fringe stabilizer that includes a circuit diagram and other helpful stuff, so it might be possible to pdf it and send it your way.

Also, Dave, the word is fluctuation, but I like fluxation. I know, I should have been an English teacher, hey, not a bad idea, there are probably more job opportunities out there!

[Joe Farina]

I would suggest trying every approach to stabilize your setup without using a fringe-locker. The rigidity of the table surface is important, for long exposures you don't want it to warp or bend due to fluctuations in temperature or humidity. The mounts for the optics also need to be stable. Vibrations usually aren't much of problem if the table is resting on air, I prefer raquetballs (there is a photo of my table on the forum, posted not too long ago). I use granite for the table-top. Any air currents need to be effectively blocked, I use two-inch styrofoam surrounding the entire table area (the lasers are on the other side of the styrofoam).

If you are handy with electronics, you can build a simple fringe-locker as shown on the wiki, under holography technology, then equipment, then fringe-lockers. It might be interesting to build that simple circuit and lock the fringes of an interferometer, just to see how a fringe locker works. After doing that, it should become clear how complicated it will be to integrate it into a holographic setup. There shouldn't be much of a need for a fringe-locker with a carefully-designed table, but it might be fun to build a simple one to see how it works.

[dave battin]

the word is fluctuation, but I like fluxation. I know, I should have been an English teacher,

Yes sorry about that you guys, it's just my Americanism that continues to butcher up the English language !

Jem when you build an enclosuer/box for your table i will bet you a box of Agfa film there will be a big improvment
in the brightness and sucess of your holograms ................

[Jem]

O.K., so i've carried out more tests tonight. I got my enclosure fitted and that seems to have helped with the fringes 'creeping'. What i'm seeing now is mainly 'shimmering' caused by vibration whenever I hear a noise outside or when I move around. The fringes do stabilise eventually, however i'm a bit concerned about the randomness of the vibrations. I suppose if they occur during an exposure it's a reason for a poor hologram.

I took my table off the special isolation feet that came with it and placed it on to three partially inflated inner tubes. I have to say that I really don't think this has helped much, plus the fact that my laser is 'off' the table so it makes alignment a nightmare as the table tends to wobble each time anything is adjusted. I think i'll go back to the isolation feet. Basically these are purpose made feet for the table with manual levelling adjusters incorporated into them. The feet have a metal base and are filled with hard rubber. Set into the rubber is a threaded bolt(?) that sticks through the table into specially designed holes, thus facilitating levelling adjustment.

I'm now considering drilling (6 x 3" diameter holes) through the wooden floor of my shed... Err... I mean Lab so I can see the concrete slab it's sat on. I then could put some sort of substantial pillars directly onto the concrete to support my table. This would then make it totally independant to the actual shed itself. I figure that this would help the stability no end.

I'm open to all suggestions, so if anyone has any bright ideas please chime in.

Thanks

Jem

[Joe Farina]

I'm now considering drilling (6 x 3" diameter holes) through the wooden floor of my shed... Err... I mean Lab so I can see the concrete slab it's sat on. I then could put some sort of substantial pillars directly onto the concrete to support my table. This would then make it totally independant to the actual shed itself. I figure that this would help the stability no end.

If there's a wood floor between your table and the concrete slab, that's bad. Quite bad. No wonder you're having problems.