What Causes the Strange Laser Effects from Interference and Reflection?

[dst]

So I have a couple of cheap <5mw red laser modules. And I also have mirrors. And I'm bored. You can guess what happens next (profit). Some really strange effects that I'd love to know the origin of:

1.) The speckle effect from the laser beams - it doesn't actually matter where your eyes are focused, you will still see it clearly. Which suggests that the interference is created on the retina itself. How the heck?

2.) I have 2 modules, with a hole and a sort of vertical knurling around the top (for grip and to adjust focus). Let's label them X and Y, X is turned on, Y remains powered off. I position X such that half the aperture is blocked by the tip of Y. I get a vertical beam projected instead, not unlike a barcode scanner. There are, I believe, three points of high intensity - one directly from the aperture, another halfway across the "line" and another at the end of the "line". After this third 'point' the beam splits up. http://img527.imageshack.us/img527/1811/kkkkkyv9.jpg"

3.) Shining a laser at itself in the mirror (running it in square beam mode to get the most visible pattern) - http://img301.imageshack.us/img301/1640/untiuz9.jpg . Of course, the interference pattern emerges exactly where the laser light meets with light from the "virtual laser".

This is the strangest one, I heard (on here, most likely) that Paul Dirac held the belief that photons could only interfere with themselves (since annihilation is quite silly). How can you really say that about this case when it seems that there are two classical paths, one taking a slightly longer time than the other (i.e. reflection off the silver coating vs partial reflection off the glass front)?

 

[Andy Resnick]

Briefly:

1) yes, the speckle pattern is on your retina. It's caused by spatial coherence, similar in concept to temporal coherence, but due to the spatial extent of the source. "single mode" (as in single mode fiber, multimode fiber, etc) means that the beam is highly spatially coherent- no speckle. Spatial filtering is done to prepare the light in this fashion.

2) Not clear. What does the undiffracted beam look like? Are you doing grazing incidence on 'module y'? What's the surface topology of module y?

3) A variation of Newton's rings, I'd bet.

 

[dst]

Well, for the second effect, the beam has to interact with a circular hole (http://img518.imageshack.us/img518/5060/72452432ho1.jpg ). I may have got the reflections for the two secondary dots wrong. It also has splines on the outer ring which is where I guess the beam diverges. My guess is that here the distance between the two separate dots on the screen higher up are equal to divergence * distance from wall * diameter of inner circle. I don't see why light should reflect from the inner hole though, which is what I was asking.

Yes, the third effect is Newton's rings (I just looked it up). I don't see why it should happen either. On Wiki and other sources it only lists direct contact between a spherical surface and a flat surface, this only has flat surfaces and no air bubbles are involved.

 

[Claude Bile]

For 3), the curvature of one of the surfaces only has to be slight, especially if you're diagram is accurate and you only see 3 rings. This equates to a path difference of 1.5 wavelengths (which = about 1 micron for a red laser) across the radius of the spot. You therefore only need a change in thickness of a couple of microns from the centre of the beam spot to the outer ring to get this effect.

If I were to venture a guess for 2), the spots are likely due to reflections, whereas the streak is probably caused by the beam grazing a thin horizontal object(s). It's hard to make a defininitive guess without more information.

Claude.

 

[Andy Resnick]

<snip>
Yes, the third effect is Newton's rings (I just looked it up). I don't see why it should happen either. On Wiki and other sources it only lists direct contact between a spherical surface and a flat surface, this only has flat surfaces and no air bubbles are involved.

Your beam is diverging, unless there are some lenses present you didn't mention, and the two surfaces (glass-air, silver-glass) probably aren't perfectly parallel. Even if they were, the beam divergence will still give you varying path differences.

 

[bassplayer142]

This is dirt on the lens. If you clean the lens with a microfiber cloth it mostly goes away.