Sciencemadness Discussion Board

Lasers....and dots?

jgourlay - 12-8-2015 at 14:40

When I was but a tadpole, my school had a HeNe laser. The teacher, who really liked me, would let me play with it at will and I did. A lot.

One of things I did all the time was to shine this beast through various lenses and look at the result on the wall. The result, always, was an evenly colored red spot of differing diameters. Let me be very clear: the spot, whether it was a pin point or the size of a volleyball, was ALWAYS a featureless reddish circle.

Come to the future. I know own several cheap chinese laser pointers. The other day I was on the-most-boring-conference-call-in-human-history. I started playing with the lasers and a couple of magnifying glasses and found something I can't explain.

Dots. Dots in the laser. A couple of features of these dots.

1. As the magnification increases, the dot spacing increases. Kind like drawing dots on a balloon, and then putting in more air. The dots themselves, however, are always pin-pricks not matter the magnification or distance to wall.
2. Rubbing, scrubbing, water, and lens cleaner have zero impact on the dots. If they are some kind of lens contamination, they are on the back side of the lens.
3. They are also sharp, pin point, and clear. If I sprinkle dust into the laser, I get more dots. But these dirt dots expand and shrink with distance to the wall. The laser dots THEMSELVES are always a bright pin prick. All magnification does is change the spacing between them.
4. The dot pattern is not on the magnifiers. All magnifiers have the same pattern. Alternately, I can shine the raw laser at the wall, and look at the 'beam spot' with the magnifer, and see the same dot pattern.
5. One of these lasers, when magnified, shows not only dots, but what look just like picture book examples of 'interference patterns'.

What are these dots?

Varmint - 13-8-2015 at 03:04

Dots as in the typical LASER speckle?

I can't understand your description of the original HeNe laser, you say featureless, but speckle gives a grainy appearance, ALWAYS. Basically the bright and dark areas of the speckle are interference between waves, where the coincide in phase, they appear bright, where they are 180* out of phase they are dark, and of course the pattern is filled with all manner of degrees of in and out of phase. To describe it as featureless is puzzling. Side note: speckle can be used as an interesting diagnostic. If you move your head from side to side, if the speckle seems to move faster than and in the same direction as your head motion, you are near-sighted. If the pattern moves opposite your head movement, then you are far-sighted.

Now actual dots (as in beam "fragments"), you can get these from poorly coated optics, poor emitting aperture cleave, and in the odd case, where the collimating lens is lined up so near perfectly that a reflection from the lens surfaces actually makes it back into the cavity and contributes to beam production, much like an external mirror for an experimental (teaching) lab laser.

Sounds to me like you are describing the effects of poor optics, be they the actual die cleaves, poor lens quality/coating, or dirty surfaces of either the laser facet or lens.

[Edited on 13-8-2015 by Varmint]

jgourlay - 13-8-2015 at 04:59

Varmint...extremely helpful, thank you! Maybe I don't remember correctly? 40 years is a long time.

Speckle is exactly what this is.

Side note to the side note: very near sighted and just tried what you suggested. The speckle moves opposite direction when I move side to side...but SAME direction when I move my head up and down!

Poor optics: for $5.99 + free shipping, I'm sure there is a lot of that.

Thank you!

DJF90 - 21-8-2015 at 02:50

Was browsing and found this...

Attachment: A device for laser beam diffusion and homogenisation.pdf (281kB)
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