Sunday, October 16, 2011

Even More on Despeckling

Here’s another technical note and handy tip from Spencer Luster of LightWorks, this time in relation to some earlier discussion in these pages regarding the issue of laser speckle.

Just when you thought it was safe to turn on the lights, here are even more notes about despeckling laser sources!

Speckle is a function of two kinds of source coherence: 1) Temporal (or spectral) and 2) Spatial. The first refers to the purity of the frequency/wavelength (color) of the light. Spatial coherence refers to how well all the waves of light line up with each other—are they all in phase. The more coherent the light source, the bigger the potential speckle problem.

Temporal (spectral) and Spatial Coherence.
Same frequency/wavelength and all in the same phase. Very Speckly!

If you don't have temporal coherence you can't have spatial coherence, but the opposite isn't true. All the identical frequency light waves could occupy a huge number of different phase positions. You would have perfect temporal coherence, and near-zero spatial coherence. What's more, you'd have little if any speckle problem!

No Temporal Coherence and therefore also no Spatial Coherence. No Speckle.

Temporal Coherence, but no Spatial Coherence. Little or no speckle.

The following techniques can help reduce spatial coherence. These are just a start. I'm sure you'll think of others if you're interested.

Ping pong balls: Drill two 1/8" diameter holes in a ping pong ball, 90 degrees apart. Fire your laser into one hole. After multiple diffuse reflections inside the ball, the light exiting the second hole will have greatly reduced spatial coherence. If you use it as a light source, the image will have very little speckle. (Some will recognize this as a poor man's integrating sphere. It also wastes a lot of light.)

Twisty light pipe: Take a 1/8" to 1/4" diameter acrylic or PETG rod, 2 to 3 ft. long. Polish the ends. Use a very hot water bath, or a heat gun to soften the rod. Carefully bend it into a spiral shape of 2 to 3" radius. Pump laser light in one end. After many reflections inside the spiral, during which many different optical paths are traversed, the output will have reduced spatial coherence. I've built such systems that work well, and are reasonably efficient.

You can pull the same trick with large diameter liquid filled light guides, or even multiple fiber optic strands of different lengths. The point is to significantly alter the path lengths of lots of portions of the light. ("Normal" small core fiber alone usually doesn't let enough path length differences build up.)

The above and similar techniques don't produce collimated beams, but you do get very spectrally pure sources with little or no speckle. Some applications call for this, such as thin film inspection or critical polarization measurement.

Spencer Luster


Vladimír Držík said...

Cool! Thanks for the explanation. We were experimenting with a fibre light guide the other day. Now I know why it didn't work.

Bob der Namenlose said...

Hi, i've been struggeling with the same probles could you upload a picture of the spiral acrylic rod that would help me a loth hanks so much best regards Sebastian

Bob der Namenlose said...
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Bob der Namenlose said...
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