Green laser - red light?

[Lee55555]

Just wondered if anyone can explain this. if I point a 100mW green DPSS laser at a surface and view the reflected spot with green laser safety glasses (i.e. ones that block the 532nm beam) I see a red spot, fairly bright in a darkened room, certainly comparable with a low power presentation style red laser pointer and about the same colour. If I interpose an IR cut off filter (which cuts everything above 700nm) in the beam to eliminate any stray 808 or 1064nm light, the red spot is still seen just as brightly so it is not a case of my eyes seeing further into the i-red than anyone else's!

I thought maybe the intense green beam was exciting a red fluorescence from the target material it struck, but it cannot be this as the colour is independent of the target material (unlike say my violet laser which will give a nice blue fluorescence from some materials, but not others).
So what exactly am I seeing? I thought that lasers were essentially monochromatic, I don't know what the bandwidth of the green liight is but would be surprised to learn it extended all the way from 532 out to somewhere near 700nm! Are there perhaps other transitions going on in the pump diode or the NdYAG giving laser output as other frequencies alongside the main one?

I aee the red spot with a small 5mW greenie too, though in this case it is very weak, so clearly it is only a minor part of the output light, but I am just curious where it comes from.

 

[ped]

The goggles are filtering wavelengths, which is what thier designed to do, i see odd effects with my high power greenie through my expensive wideband goggles.

 

[Lee55555]

Sure but the goggles are only filtering wavelengths out, not changing wavelengths. If they have an O.D. of 2 for the 532nm wavelength, then only 1% of that light will get through, but it will still be green! They won't change the wavelength of that 1% to red. The filter subtracts something from the beamm but (unless it fluoresces itself) it won't put anything new in. The red light must already be in the beam and I wonder where it is coming from.

 

[Toke]

I am not sure where the red light comes from, but it is not part of the original beam.

When I shine a 445nm into a glass of single malt, the beam turn yellow and spread out in a cone with some "thermal blooming" like waving/flutter.

It is likely that whatever is hit re-radiates the light at a different frequency.

 

[dnar]

What you are seeing is normal, there is a small amount of red from the IR led passing through. It is worse with cheaper modules.

If your unlucky enough to damage your cystals then all you will see is this dim red light....

 

[Lee55555]

I just love the idea of the single malt experiment (let's see if it's reproducible, we'd better driink that one and refill it and try again. Well we need to see a result at least three times to believe it is consistent. Better drink that one etc. etc. etc.)!

But sereiously, if whatever the beam hits absorbs it and re-emits it at a longer wavelength as you suggest, that is fluorescence, and whether it happens, and what wavelength it re-emits at is a function of the target material (I'll bet your 445 doesn't give a yellow beam in water, or glass, or perspex).

But my red dot from green laser doesn't appear to depend on target material at all, it can be a white wall, a painted wall, wood, grass, rubber, asphalt, makes no difference.

 

[Toke]

But sereiously, if whatever the beam hits absorbs it and re-emits it at a longer wavelength as you suggest, that is fluorescence, and whether it happens, and what wavelength it re-emits at is a function of the target material (I'll bet your 445 doesn't give a yellow beam in water, or glass, or perspex).

Olive oil gives a green florescence, rapeseed oil just block the light, clear snaps does nothing.
The yellow colour generated by 10 year Laphroaich is apparently not from the alcohol content, but from whatever (im)purities making it whiskey. So yes, it would be interesting to study if different malts give different colours.

If you have eliminated the IR content there is only florescence left, either on the surface you hit, or more likely, in your safety glasses.

 

[Lee55555]

Well thank you all for your opinions. I guess it is either (or both) fluorescence from the glasses themselves (which I must admit I suspect) or the pump diode having a broad bandwidth emission that has some component in the visible even though the peak is in the infrta-red.

Unless someone can think of a simple experiment to distinguish these possibilities I guess the next step would be to trawl through the literature to find details of the fluorescence spectra of the filter materials used in safety glasses and the emission characteristics of pump diodes. Or else to take the laser apart and see if the pump diode plus i-red filter alone gives the red dot (if it does it can't be fluorescence in the glasses as that is always at a longer wavelength than the exciting radiation so the red must already be in the beam. If not then it must be fluoresccence excited by the green).

But I'm afraid I don't have the time to dig out the data (maybe someone has those kinds of facts in their head?) and I don't want to dismantle and probably destroy the laser, so I am content to have pinned it down to two possible causes but not decided between them.

Interesting question though, and I'm just surprised that no-one seems to have asked it before on here, given the number of people who use these high power greenies!

 

[Lee55555]

My son has just suggested an experiment to distinguish the 2 possibilities without dismantling the laser. Since the red goes straight through the glasses but the green is blocked, you put the glasses just in front of the laser and shine it straight through them just in front of a screen. If the red is from the i-r diode and already in the beam it will pass straight through, and the red dot will be visible on the screen just as if you were wearing the glasses looking at a reflected dot. If not, if the red is due to glasses fluorescence, then you will see the red dot on the "outside" face of the glasses (opposite side to the laser) but, being the re-emission of absorbed light, it will be emitted in all directions from the glasses material itself, not in a focussed beam, so you won't see a dot on the screen.

The logic sounds good to me! Tried it out. Clear cut-and-dried answer. Green dot visible on the surface of the glasses facing the laser beam. Red dot visible on the opposite face of the glasses (i.e. behind them). No dot whatsoever on the screen, even though only an inch away.

Conclusion: glasses fluorescence!

 

[Toke]

Good thinking.