Speckle noise reduction

[Joe Farina]

ok, thanks Ed.

[BobH]

At Simian Company, the speckle was decreased by moving a diffuser in the object beam to a random place between each of three exposures (RG&B). Using a diffuser in each object beam will reduce the speckle you record. Larger the diffuser, lower the speckle contrast. More object beams, less speckle. Multiple exposures, less speckle. Look at your holograms in a totally dark room with minimum light to see it, less speckle (pupil dilation).

[Joe Farina]

Thanks a lot, Bob. All of those suggestions make perfect sense, and I have printed a hard copy for my records.

[Joe Farina]

After what Bob said, and after further thought, it seems that measures can be taken to reduce speckle in a split-beam reflection format, but probably little can be done in Denisyuk mode.

Today I was looking at my objects under spatial filter illumination, and all the speckle was clearly visible, just like in the finished hologram. It looks like the hologram simply records what it sees, I didn't quite appreciate this before.

So I assume that in a standard split-beam reflection setup, a shutter could be placed before the laser(s) and the exposure could be broken up into a number of shorter exposures. Then when the shutter is closed, a diffusion screen could be displaced slightly in front of the object beam(s) to change the speckle distribution, then the shutter could be opened again, and the process repeated. Of course, the shutter and diffusion screen movement cannot introduce the slightest disturbance, but I think this can be done with careful design.

Thanks again Bob.

[John Klayer]

I remember back in the paleoholgraphy 1970's some mention of attaching a reference beam mirror to a loudspeaker movement and playing music to cancel speckle. I never tried it.

[Joe Farina]

Thanks, at least there was a mention of it.

[zipsister]

While making Hololujah each metre square hologram frame was exposed using the raw 532nm beam being transmitted through two diffusers (two stationary sheets of frosted glass); two were needed to completely eliminate all speckle. No spatial filter, just the raw beam for less power loss and reduced exposure time.
Dave

[Joe Farina]

Thanks, I will try tests with diffusers. But it sounds like you have more laser power than me

[ron olson]

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

[Joe Farina]

Thanks for posting the link. It's very interesting that there are strategies for dealing with speckle.