Working Hard

Mark Cavin · Post by **Mark Cavin** » Wed May 05, 2004 11:18 am

Rebuilt my table this weekend too; must be spring fever for holographers. Made my first split beam trans and it came out really well. Tried to make a HOE that John helped me with, didn’t come out very well, but I see what it is supposed to do, cool. I was even able to get my lazy arse to check polarity before shooting and discovered that I needed to make adjustments. Hmmm, I see trying overhead lighting for the object beam is going to be a bit of a challenge. Now I know that stability is no issue; if I can get a nice image with the Leith and upatnikes scheme with long beam paths, then I know a simple Denisuk should be very stable setup. Tomorrow I will get my PFG-03C and a copy of Practical Holography and I still have a few plates of VRP-M. Gonna be a bunch of F&F (Fun and Frustration) this weekend.

Regarding HOE:
I have read here that for an object beam, it should be the brighter beam and that the scattered light from the object will turn out less intense than the ref beam. However is this bad for a HOE setup since all the light is reflected by the mirror and will turn out stronger than the ref beam? For this setup is it recommended to use the brighter beam for ref and the weaker for the obj (mirror)?

Flame Away!!

JohnFP · Post by **JohnFP** » Wed May 05, 2004 11:30 am

For and HOE, how do you know which is the object beam and which is the reference beam? It depends on what you want your HOE to do which will determine which is the object and which is the reference. The reference beam, which you are right should be brighter by about 5 - 10 times for a transmission and equal for a reflection, would be the one that will be duplicated in the replay more or less. That is probably the one that is collimated. But not necessarily! It is up to you to decide!

For a reflection HOE make the beams equally intense. For a transmission HOE make one beam 5 to 10 times brighter to start. The brighter will be your reference beam.

Peace!

Dinesh · Post by **Dinesh** » Wed May 05, 2004 2:51 pm

Beam ratios are basically to create the right fringe contrast. If the fringes are too contrasty, you get noise and second orders, if they're not contrasty enough, you get dim images. You want sinusoidal fringes (think of an overdriven amp). The exact ratio depends on the film and the processing, however usually a good rule-of-thumb is 4:1 for laser transmissions, what you call Leith-Upatnieks, and 1:1 for reflections. For HOEs, it depends on a HOE. If you're making a beam splitter, you may need to adjust the ratio for the beam splitting ratio, if you're making mirrors, you need 1:1 ideally. However, if you make the object beam stronger, you'll get "noise". You have to ask yourself what "noise" means on a HOE.

Mark Cavin · Post by **Mark Cavin** » Fri May 07, 2004 10:39 am

Despite the difficulty processing this emulsion, I am getting very good results for reflection holography at 532nm. I get an image that is viewable under dim lighting conditions with very little shrinkage in the emulsion using JD-4 processing, my brightest reflection results to date. If I can get just a little more brightness, just a smidgen of a smudge brighter I will be a smiling happy fool; I will earn two adjectives for my title.

JohnFP · Post by **JohnFP** » Fri May 07, 2004 10:43 am

I will earn two adjectives for my title.

What would they be?

Mark Cavin · Post by **Mark Cavin** » Fri May 07, 2004 10:44 am

Smiling and Happy.

Bob · Post by **Bob** » Fri May 07, 2004 12:13 pm

The only reason to increase the beam ratio for any holographic recording geometry is to reduce intermodulation noise. This is caused by multiple sources of light interfering with each other as well as with the reference beam. We make the reference beam stronger so that the gratings formed with it are of much higher visibility than those formed by object point / object point interference gratings.

All holograms made of point source objects should be shot at beam ratio of one. I can't imagine any reason to purposly reduce fringe contrast! All the stability, material and chemistry hoops we have to jump through are to make the highest contrast fringes possible. It's been my experience that overmodulation of a recording suggests less exposure, not less fringe contrast.

JohnFP · Post by **JohnFP** » Fri May 07, 2004 1:25 pm

Bob this is great stuff. So if I wanted to make a transmission HOE with a collimated beam and a diverging beam I would still want to use 1:1? That makes perfect sense. Boy if only the old timers in holography had this forum can you imagine where holography would be?

Dinesh · Post by **Dinesh** » Fri May 07, 2004 3:36 pm

You want to be careful that the mindset of display holography is different from the mindset of technical (HOE-producing) holography. For one thing, if the fringes, surface modulation, electric field strength or whatever the physical change you're using are not sinusoidal, you'll get higher order Fourier terms coming in. In this case, the exact ratio of the Fourier coefficants determine the relative strength of the various orders. For a pure sinusoidal grating there are only three orders: 0, +1, -1. For any other profile each sinusoidal component will diffract with its own three orders in directions determined by their frequency and with strengths determined by the coefficant of that frequency. In other words, technical holographers go through mathematical hoops calculating the geometry, signal strength, Fourier coefficants, wavefront purity etc etc and think display is "just" a matter of sticking an object in front of the plate - "easy". While display holographers go through hoops getting the lighting, the intermod noise, the background, the angle-of-view etc etc right and think HOEs are just a matter of having two beams both of which may or may not be collimated - "easy". Very, very rarely do you get both talents in one person.

JohnFP · Post by **JohnFP** » Fri May 07, 2004 7:37 pm

But that is why we love you so much. You have the technical and the display with an ability to communicate with both sides. If you quit holography I am going to come to California personally and talk you back into it. My plan is to bring my wife and 3 kids and move into your house until you agree. OK? So be forwarned, don't stop making hologram and don't stop posting on this board.

Ah...hahaha!!!

BTW, not to get off topic but can you ellaborate on the the 3 orders in laymens term. It sound interesting. Or can you provide me with a link or a reading, even if it is elementary physics, that can open my squinty eyes.

Thank you,

John