Speckle noise reduction

Holography related topics.
BobH
Posts: 440
Joined: Tue Jan 06, 2015 10:26 pm
Location: Mesa, AZ

Speckle noise reduction

Post by BobH »

ron olson wrote:I was tempted to try this out and went so far as to offer a swap to Edmund Optics for a large format H1. The product marketing manager declined but I haven't given up on it yet. I saw some of this R&D coming down the pike a couple ears back. I believe it involves ~50 microns of motion at a few hundred Hertz. If anyone gains access to one of these, I'd love hearing of a first-hand trial with a deep-image H1.
http://www.edmundoptics.com/lasers/lase ... ucers/3409

I've used one. They work great.
John Klayer
Posts: 273
Joined: Thu Jan 08, 2015 2:28 am

Speckle noise reduction

Post by John Klayer »

Bob,
Did you use it as the reference beam?

I found a lot more about it on the Optotune web page. They don't mention using it for holography.
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Speckle noise reduction

Post by Joe Farina »

John Klayer wrote:Bob,
Did you use it as the reference beam?
I was wondering about that also.

In reference to split beam holography, I would like to ask another question about "speckle management." If we place a diffuser in the object beam, and if the position of the diffuser is displaced very slightly during the exposure, without introducing any movement to the table (i.e., by carefully suspending the diffuser from the ceiling using nylon threads, etc.), would this work to reduce speckle? Bob had mentioned that the diffuser displacement could be done between a series of exposures, so it seems this would have the same effect.

If an object beam passes through a diffuser, and the diffuser moves during the exposure, will that introduce any kind of path length change? Of course, the diffuser would have to be suspended in air (etc.) as mentioned above. If this does not introduce a path length change, then the diffuser could be displaced many times during the exposure, for example by gently pulling a thread attached to the diffuser (or by using a piece of nitinol wire as part of the diffuser suspension system, and gradually increasing the voltage to the wire so that it moves the diffuser slightly during the exposure).

I have checked the laser speckle distribution as a diffuser is moved, it takes very little movement to change the distribution.

John, on a completely different subject, have you ever made a backlit transmission cave-hologram? In other words, by placing the laser at a distant location (maybe 20 or 30 feet away), and exposing a very deep hologram showing the side(s) of a cave or a series of stalagmites, etc.? The reference beam would hit the plate directly, while the object beam would be the backlit stalagmites, etc. I wonder if a cave is that stable. Maybe you've been doing this all along (or maybe it won't work) but I always wanted to ask.
BobH
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Location: Mesa, AZ

Speckle noise reduction

Post by BobH »

Only in a reconstruction beam to view the hologram. It vibrates, so is no good for recording beams (unless pulsed!).
Ed Wesly
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Joined: Wed Jan 07, 2015 2:16 pm

Speckle noise reduction

Post by Ed Wesly »

Don't forget that the holographic pattern is a form of speckle, or vice versa, or generically, an interference pattern.

Holography "freezes" light waves by recording a stationary interference pattern. If a component moves, the pattern moves, and what was dark might be overwhelmed with bright, and vice versa. No contrast, no hologram.

Moving things during replay will reduce speckle, but not during recording. I have seen beams bounced off of spinning ball bearings, so that the random scratches on the surface of it average out to no speckle.

Multiple recordings of moved diffusers, etc., would reduce speckle by superimposing many patterns, but don't forget, the #1 killer of holograms is movement!
"We're the flowers in the dustbin" Sex Pistols
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Speckle noise reduction

Post by Joe Farina »

Let me clarify what I'm trying to say. I would prefer to do the actual testing, but I'm re-doing my layout at the present time.

Of course, a general rule in holography is that "movement" cannot be tolerated. Path lengths definitely cannot be changed. But I'm talking about a very specific case that does not involve movement in the usual sense. I don't know if this kind of movement will result in a path length difference, that's what I want to test when I'm able.

First of all, think of a single beam reflection hologram. The illuminating laser does not have to be isolated, it can in fact "move" slightly during the exposure.

Now, consider a split-beam reflection hologram. I plan to insert a simple piece of glass (a microscope slide cover with some etching, etc. to make it a diffuser) in the object beam, and this will be strung from the ceiling, with no table contact. Part of the suspension system will include a length of nitinol wire (aka "muscle wire). When a voltage/current is applied to the wire, it will shrink slightly. So when I apply a small voltage/current, the diffuser will move slightly. It takes very little movement to change the speckle distribution, by the way. Probably 0.1 millimeter would change it.

There is no doubt whatsoever that the diffuser movement will cause no disturbance to the table or other components, with careful design. So "movement" in that sense is not the question. The question is: Will the path length of the object beam be increased/decreased by the small vertical displacement of the diffuser? Another way of looking at it might be to forget that the glass has a frosted or etched surface, and just think of it as a plain piece of glass. Will very slight angular displacements (as it moves up or down vertically) cause a path length change for any reason? I really don't know the answer to that question.
John Klayer
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Joined: Thu Jan 08, 2015 2:28 am

Speckle noise reduction

Post by John Klayer »

Now I guess I'll start experimenting with vibrating diffusers for transmission hologram viewing.

As for Joe's off topic question:
John, on a completely different subject, have you ever made a backlit transmission cave-hologram? In other words, by placing the laser at a distant location (maybe 20 or 30 feet away), and exposing a very deep hologram showing the side(s) of a cave or a series of stalagmites, etc.? The reference beam would hit the plate directly, while the object beam would be the backlit stalagmites, etc. I wonder if a cave is that stable. Maybe you've been doing this all along (or maybe it won't work) but I always wanted to ask.

I have thought of doing something like that and maybe calling it Light At The End Of The Tunnel.

John
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Speckle noise reduction

Post by Joe Farina »

John Klayer wrote:I have thought of doing something like that and maybe calling it Light At The End Of The Tunnel.
Very good title! ;)
Dinesh

Speckle noise reduction

Post by Dinesh »

BobH wrote:It vibrates, so is no good for recording beams (unless pulsed!).
Even pulsed, I'm not sure this is true. The vibration frequency would have to be of the same order as the inverse pulse width. In other words, if the pulse width was 20ns and the diffuser moves at a rate of a centimetre a second, it'll move 0.4 microns over the period of the exposure, while the speckle dimension is of the order of a micron or so.
Joe Farina wrote:There is no doubt whatsoever that the diffuser movement will cause no disturbance to the table or other components, with careful design. So "movement" in that sense is not the question.
Not completely true. It'll move the adjacent air. If the beam path is traversing the air surrounding the diffuser, then, as the air moves, there'll be a variation of index. This will cause a phase change and so shift the fringes. In addition there are doppler effects due to the air movement. These effects may be small enough to ignore, however, the effects may be of the same order as the speckle itself.
Joe Farina wrote:Will the path length of the object beam be increased/decreased by the small vertical displacement of the diffuser?
Joe Farina wrote:Another way of looking at it might be to forget that the glass has a frosted or etched surface, and just think of it as a plain piece of glass.
A plain piece of glass is actually quite different from a diffuser. Consider first a plain piece of glass. An object beam for display holography is a divergent field, ie the rays diverge radially outwards. . Consider a typical ray impinging on the glass at some angle, theta. Assuming the glass was absolutely of uniform thickness and both sides were parallel (surface variation of zero rms and zero degrees between the faces of the glass), then the ray will emerge displaced but parallel to the incoming ray, ie theta(in)=theta(out). If there are variations in thickness or non-parallel surfaces, then the ray will emerge at a different angle to the entrance angle. Now sin(theta(out)) = sin(theta(in))/n. Now, as the glass is raised, unless the glass were perfectly flat and perfectly uniform, variations of thickness along the face would cause the ray exiting the system to wobble at a frequency of (and an amplitude given by) the variation in thickness and the disparity from parallelism. You can get a macro model of this if you consider an extreme case of using a prism instead of a plain piece of glass, or two prisms attached by their bases, and move that vertically as you pass the beam through them. You can see that the beam will wobble as a function of the prism angle. In fact, the ray deviation is given by D = (n-1)A, where D is the angle of deviation and A the apex prism angle. Thus, if you modelled the non-uniformity of the glass thickness as a set of prisms with different apex angles, the deviation angle of the ray would vary.

Now a diffuser is a rough surface modelled by amorphous spheroids. Thus, instead of prisms, consider (at a much larger scale) that you glue several glass marbles onto a glass plate and move that vertically. You can see that the beam would wobble with a frequency dependant on the diameter of the spheroids (marbles) and their population density. This effect would be added to the nonuniformity of the glass.
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Speckle noise reduction

Post by Joe Farina »

Thanks Dinesh. Yes, I agree that some air will get displaced when the diffuser moves up and down. But it could be suspended quite firmly in space by using 4 nylon threads, and it would probably only need to move about 0.2 millimeter, so I doubt it would be a practical problem.

That the up/down movement of plain glass will change the direction of an incoming ray due to surface irregularities, and that a diffusing surface would exaggerate these changes in direction, are both very good points, thanks for clarifying them. But I have to wonder how this might be manifest in the final holographic image, and I want to do some testing.

Bob had mentioned earlier that at Simian they moved diffusers between sequential RGB exposures to reduce speckle, so this makes me wonder. They were probably allowing the diffuser (on the table) to have plenty of settling time between exposures. My proposed method with the nylon threads might be too unstable, and it would always be wobbling around somewhat. If that's the case, then I could build a separate (solid) support structure off the table, to support an "arm" extending to the diffuser, and even isolate it from vibrations like the table. Then the diffuser could be moved by the 0.2mm using some electronic means, and when it's not being moved, it won't be wobbling all over the place, like when using the threads. In this case, moving the diffuser during the exposure might be compared to what Simian was doing between exposures?

Too much speculation, I know.
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