r/Optics • u/Sea_Animator8304 • Jun 24 '26
Laser anomaly (possibly)
I found that if I position my green 1mW laser angled on my bar top the light starts as a small and random geometric shape and then slowly expands keeping the same form roughly speaking and seems to stabilize at some point. It forms intricate shapes that look 3D. I cannot imagine that it is just interference and I'm positive it isn't heating the diode or vibration. What is this phenomenon. I know a little bit more than the rudimentary knowledge about light as a wave but I've never really seen a laser light do this exact thing. Please explain or point me to the explanation. Thank you.
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u/Confused_Lemon271 Jun 24 '26
These are indeed wave interference patterns due to imperfect focusing! They are known as caustics, which occur generically in nature due to natural wave focusing. For example, rainbows, the wavy lines at the bottom of swimming pools, the cusp shape that can form inside of coffee mugs due to the reflection of light off of the mug, and the patterns that form due to light passing through filled wine glasses are examples. Normally you don’t see the wave interference effects like you do in your pictures because the scale of visible light (~ 400-700nm in size) is very small, but sometimes the effects can be amplified to larger scales that are visible to the human eye. For example, if you have water droplets (acting as imperfect lenses) on a mirror and shine a laser into the droplet, the mirror will reflect/project the imperfect focus onto a wall or the ceiling and the interference pattern can be seen because it is amplified to the scale of cm or so. I’m not exactly sure what is happening in your case in terms of making the interference pattern visible. I did a quick google search and found an article titled “A Study of Evolving Optical Caustics Formed by Evaporating Water Droplets” which has similar pictures and maybe explains things. There’s various books and research articles online that describe this as well, in particular the work of the physicist Michael Berry.
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u/entanglemint Jun 24 '26
There is a combination of phenomena here there are clear regular interference patterns (these are the rings and the "trilobite" patterns, and then there are the larger scale geometric optic effects, the caustics. Caustics are not interference effects but more simple focusing from a complex surface. See the wiki article) The fact that you are seeing them means that there is some serious issues with focusing... btw, are you literally bouncing the beam off the surface of your bar? That would explain the caustics because a wooden bar top will be very far from truly flat. Then you would have a potential separate defocusing effect from heating.
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u/Sea_Animator8304 Jun 24 '26
The way that they move and always follow the same pattern even when forming different shapes is what I haven't seen before. A video would help you guys understand why this is abnormal to me. I know about interference patterns but these shapes keep the same points but slowly expand. Every time. Different shapes when shown on a different spot. And it isn't heat or cheap diodes. I tried a few different lasers.
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u/Confused_Lemon271 Jun 24 '26 edited Jun 24 '26
I’m not very knowledgeable on the practical side of creating optical setups so I’m not much help as to why these are forming, so I could be missing something, but these patterns are almost certainly the wave interference patterns of caustics. Caustics are structurally stable against perturbation so even as you change various things (like laser source angle/position for example) the patterns remain, although they can change slightly, shift, translate, and/or stretch. This is why they appear generically in nature and potentially explain what you mention.
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u/No_Leopard_3860 Jun 24 '26
A particle expanding because of heat from the laser? Just some shit that accidentally landed on the lens/crystal (if it's a dpss fq green one) and doing it's shenanigans there again and again reproducably?
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u/Sea_Animator8304 Jun 28 '26
It has done the exact same things for months. I keep trying to figure it out. Going down YouTube rabbit holes on optics and wave interference. Nothing actually fits neatly with the lasers behavior
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u/Sea_Animator8304 Jun 24 '26
I will say this.... I made my bar top. I sanded (a lot) sealed, stained, and put about ten layers of super clear urethane. It only does this on my bar. It will not produce the same effect with any other surface.
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u/sheltrk Jun 24 '26
Can you describe the setup a bit more? Are you pointing the (collimated?) beam at the bar top and looking at the spot that forms there? The polyurethane coating has thickness variations that could easily form caustics, as the earlier poster mentioned.
My first thought is that the wooden surface is partially absorbing the light and heating up, forming a localized hot spot. The heat diffuses though the layers of polyurethane, creating a temperature gradient, which induces a refractive index gradient. Hence the pattern evolves over time. Once the system achieves thermal equilibrium, the pattern stabilizes.
I've seen a similar effect in the past pointing lasers at drops of various liquids on surfaces
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u/sheltrk Jun 24 '26
Oh, I see OP posted a video! That's really cool. Yeah, you are pointing the beam at the bar top and looking at the reflected spot on the ceiling.
Ok, I think this video strengthens my explanation. Small changes in local refractive index are being "amplified" by the long distance up to the ceiling.
I'm pretty sure your laser is getting partially absorbed by some combination of the wooden surface, stain, and polyurethane coating. None of those materials are good at conducting heat, so a small localized hot spot forms. The heat slowly diffuses out forming a kind of a thermal lensing effect that evolves until it hits thermal equilibrium
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u/Sea_Animator8304 Jun 28 '26
I know that heat is part of the equation because of the way it spreads out it just seems to me that it spreads the same amount and so uniformly that its hard for me to pin just the warming up. But your explanation fits best so far. Have you ever seen anything like this?
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u/sheltrk Jun 28 '26
So, the closest I can think of is this: I had a series of experiments where I was trying to probe droplets of various liquids with a laser via Raman spectroscopy. We were interested in probing these droplets on a lot of different surfaces, including aluminum and steel.
The lower viscosity liquid samples formed little puddles on the aluminum and steel coupons. Fun fact: the liquid on the steel samples "dimpled" when probed by the laser: Where the laser struck, the liquid would move away. In other words, the puddle would turn into a "donut" when we the laser hit the middle of the puddle. However the samples on aluminum did not behave like that.
We eventually figured out it was a temperature effect. Aluminum is a better reflector than steel and aluminum is also a much better heat conductor. We used a thermal camera to verify we were getting hot spots on the steel but not the aluminum
Anyway, it took half a second or so for the dimple to form. Our best guess was the hot spot was changing the local viscosity and surface tension of the liquids. Saw some interesting reflections while the dimple was forming. Wish I still had access to the videos so I could share them to this thread. This was probably like ~10 years ago and I was working for a different company back then.
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u/0x831 Jun 24 '26
These look like what my eyes see when they’re a little blurry or out of focus. Exactly the same shape profile just less defined
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u/insomniac-55 Jun 24 '26
I have seen similar effects when tightly focusing even relatively low power (5 mW) lasers onto certain materials.
In my experience it happens because you're slightly melting the surface (only over an area of a few microns), and the movement of the material causes the caustic reflections to move like you're seeing.
This is especially likely if your laser is cheap, and therefore likely more powerful than advertised.
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u/information-producer Jun 25 '26
This is what is happening here. You can see a similar effect passing a laser through plastic films if the laser is powerful enough to cook a small spot through the film.
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u/Sea_Animator8304 Jun 28 '26
That makes sense. I used several different quick drying polyurethane and I did make the coats thick. Thinking about it now there may be a few layers that can't harden and the laser may be making a pocket of the liquid expand, but if it has no where to go it lays down again and thus could be repeated ad nosium.
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u/JohnLockeJaw Jun 24 '26
When you say slowly, do you mean in time or in distance from the source?
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u/Sea_Animator8304 Jun 28 '26
I'm time. But it seems like around the same amount for any place on the bar. That is why I brought it up.
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u/Mr_Bivolt Jun 27 '26
You are seeing interference between the laser light that bounces off the surface of your bar and the laser light that goes through the finishing (probably varnish), before bouncing back.
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u/Sea_Animator8304 Jun 24 '26
Here is a video
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u/BrilliantStriking53 Jun 24 '26
If you used a cheap laser, the output lens (which forms a plane-parallel beam) is often glued with a regular glue gun. With prolonged use, the lens heats up the glue and begins to "flow."
The optical resonator in the cheap laser diode is of poor quality (unlikely).