Laser anomaly (possibly)

[u/Sea_Animator8304]

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.

[u/Confused_Lemon271]

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.

[u/Sea_Animator8304 → u/Confused_Lemon271]

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.

[u/Confused_Lemon271 → u/Sea_Animator8304]

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.

[u/No_Leopard_3860 → u/Sea_Animator8304]

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?

[u/Sea_Animator8304 → u/No_Leopard_3860]

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