Laser Optics

[Dinesh]

Im the new guy at the bottom of the web

Dave, some of the equations on the board behind you are wrong!
E = h-bar*w = h/lambda (should be E = h-bar*w = hc/lambda)
E = 1/2 mc^2 (Jeez! Everyone knows E = mc^2 - whatever that means!)
I = mv (or is it mr). Anyway I = mr^2 or p=mv

Sorry, just joking. Couldn't resist! Bit like a musician who hears a wrong note!

Getting back on point...

Also do I need a special spatial filter since I'm using a nd:YAG DPSS?

The laser doesn't matter insofar as to whether or not you need a spatial filter. A laser beam, in this case the reference beam, picks up the dust in the air and this dust is "carried along" with the beam. This shows up as rings on the plate, so your plate will be covered by a series of rings, like miniature targets. The spatial filter cleans up these rings and gives you a smooth reference beam. So, you either have a fairly dust free room, or you use a spatial filter. Some holographers have used a HEPA filter to clean the room of dust. I personally have no idea how efficient that is, since I've never used one, but some on this forum might be able to comment on the usefulness of a HEPA filter. I would suggest that you start without one, tweak your technique first, then add the necessities for cosmetics. This way, when something goes wrong, you have fewer variables to troubleshoot.

[BobH]

Here's a link to a home holography project I did recently that might serve as an example of what one can do with minimal equipment, listed below:
1 laser
4 mirrors
1 shutter
1 negative lens
1 spatial filter
1 variable attenuator
1 laser power meter

The variable attenuator (a variable beamsplitter disk) was only used to lower the beam intensity so the exposure time was long enough for accuracy with the shutter I used. The shutter could've been a card in the beam. The spatial filter could've been replaced by a simple microscope objective. The mirrors could've been pieces of aluminum front surfaced mirrors. No vibration isolation. You want a power meter to set and adjust exposure times.

https://sites.google.com/site/bobspubli ... ete-canyon

[Jeffrey Weil]

Hello Dinesh,

I think you might have made an error, or course not with the math The dust spatial filters take care of is not in the air and carried along with the beam. If it was the bullseyes we get would be transient. The would move and come and go.

The dust they take care of is on the lenses and mirrors used to deliver and expand the beam. Also the exit window of the laser itself. That's why they don't move. The dust itself is locked in place and so are the bullseyes.

A hepa filter in the room is to keep more dust from settling on the optics after they have been cleaned. Even then you still need a spatial filter as the surface of the mirrors and lenses are not perfect.

I have a whole bunch of spatial filters I've collected over the years but one of these days I'm going to make one myself on my lathe. I made a poor one decades ago when I started but now I'm sure I could make one really pro.

Jeff W

[Ed Wesly]

Here are some ideas for projects using nothing more than a beamspreader and in some cases a flat mirror:

http://nlutie.com/ewesly/PedagogyFrameset.html

Click on Seven Single Beam projects for 3 variations of the same paper.

[holomaker]

Sorry you guys, but for the simplest version i'll stick w/concave mirror. Its the only optic you need for the whole set up!

Instructions;

#1 put sticky clay on rear side of mirror
#2 stick mirror to ceiling
#3 point laser at mirror from 10' or more.
#4 adjust mirror to hit plate nicely
#5 shoot ..........




sorry its such a lousey shot , but you get the idea. with careful area selection a clean and sworl free hologram can be made ....

[Jeffrey Weil]

Nice one Holomaker. Lots of mirror space to find a nice clean spot to get a nice clean expanded beam. Very smart.

There's one setup that's actually more simple. No "real" optics at all! Graham Saxby used to advocate bouncing the raw beam off of an aspirin pill. It's diffuse and convex surface will diffuse and spread out the beam.

I've never tried it, or seen holo's made using it I think, but it has to be the most simple way to make a hologram ever. The pill will de-polarize the beam but what could be easier?

Jeff W

[Ed Wesly]

I had seen at LFC Holography Workshops a student make a SBR hologram by diffusing the laser beam by passing it through a piece of ground glass. The hologram came out OK, but there was a texture to the reference beam, which I think will happen with the pill.

Just checked it out with a laser pointer and the remedy for a hard day’s worth of classes, and it spreads out nicely, with a grainy beam texture.

[Dinesh]

Hello Dinesh,

I think you might have made an error, or course not with the math The dust spatial filters take care of is not in the air and carried along with the beam. If it was the bullseyes we get would be transient. The would move and come and go.

The dust they take care of is on the lenses and mirrors used to deliver and expand the beam. Also the exit window of the laser itself. That's why they don't move. The dust itself is locked in place and so are the bullseyes.

Yes, true. But the dust in the air also has an effect, though the dust on the optics is far more important. I suppose I shouldn't have said "dust in the air" because, as you point out, it's actually mainly the dust on the lenses etc. With beginners I simply use the word "dust". What the spatial filter actually does is act like a low bandpass filter. The bullseyes all have an FT that surrounds the Airy pattern. The hole on the spatial filter blocks out everything but the "zero order". The way I explained it to Joy when I was first teaching her, was to imagine a tree. The various small branches, leaves, twigs etc corresponded to the dust, the irregularities in the optics etc. Basically it corresponds to noise superimposed on the beam. Now imagine you're taking this tree through a circular aperture. The aim is to lose everything but the tree trunk, as far as possible. If the aperture is too large, some of the twigs and smaller branches come through and there is fine structure on the tree trunk; too small and you lose parts of the tree trunk. Just the right size and you get all of the trunk and lose all of the smaller features. She's actually better than me at pinholing now!

Dave, it looks like you have the mirror attached to something above the table. In this case, you can have a slow mirror a long way away. However, nomiss may not be able to do that, he may have to put all the optics on the 36 x 36 table. In this case, you're going to need an awfully fast mirror to cover an 8x10 plate over a distance of 36in. Of course this does bring up a point that none of us has made to nomiss - side ref or overhead ref? If the 36x36 table is all he has and all the optics have to be on it, the train is going to have to be setup sideways or illuminated sideways. One other point, is his laser polarised?

There's one setup that's actually more simple. No "real" optics at all! Graham Saxby used to advocate bouncing the raw beam off of an aspirin pill. It's diffuse and convex surface will diffuse and spread out the beam.

I suspect it could be a real headache trying to mount an aspirin so that it's stable! You may need more than one aspirin!

[Jeffrey Weil]

Hey Dinesh,

I liked the joke about mounting the pill being a headache.

This is how I explain spatial filters to beginners. I tell them the laser produces light as if it were infinitively far away. The dust and other artifacts that mess up the beam are much much closer, just feet away. If you focus the light from far away so it makes it through the pinhole the rest of the stuff is out of focus and will hit the metal disk around where the hole is cut, not making it through.

Since most people understand that when looking at things close up and far away with cameras and binoculars they need to be refocused to see both clear they seem to get it.

Jeff W

[holomaker]

Dave, it looks like you have the mirror attached to something above the table. In this case, you can have a slow mirror a long way away. However, nomiss may not be able to do that, he may have to put all the optics on the 36 x 36 table. In this case, you're going to need an awfully fast mirror to cover an 8x10 plate over a distance of 36in. Of course this does bring up a point that none of us has made to nomiss - side ref or overhead ref? If the 36x36 table is all he has and all the optics have to be on it, the train is going to have to be setup sideways or illuminated sideways. One other point, is his laser polarised?

Dinesh i was thinking on a small table 36x36 (or smaller) the laser and assocated components would be off the table. Then all you need to do is find a place to stick the slower concave mirror ie... ceiling, hanging lamp, plumbing pipe or whatever. I would always be amazed as while shooting test shots (below)the mrs would be directly overhead (in the kitchen) making dinner and you could actually see the beam vibrating from the overhead activities and the test shot still came out ok .....

yes polarisation(time for a 1/2 waveplate),the Coherent 315 has good polarisation too!