Violet light produced from green He-Ne laser?

[elemental09]

I was setting up a 2.0 mW, unpolarized 543 nm (green) He-Ne laser today when I discovered something odd. I was using an iris diaphragm (like these: http://www.thorlabs.com/NewGroupPage9.cfm?ObjectGroup_ID=206) to align the beam parallel to the laser table. I found that if I placed a white cardboard sheet behind the iris, whose aperture was slightly off the beam axis, and closed the iris just enough so that no green light was visible on the white cardboard, a faint purple/violet glow remained. I drew a crude diagram, see:
http://img560.imageshack.us/i/lasery.png/
Note that the shape of the purple/violet glow on the paper was not exactly as pictured. It seemed roughly parabolic, with the glow fading in intensity.
What's happening? I can't explain it.

 

[dacruick]

I can only think of one thing. The laser is emitting a range of wavelengths, highly concentrated around 543nm. If you make an aperture narrow enough, it could block out all of the larger wavelengths and only let through wavelengths around 400nm.

What is the quality of the laser? and what happens if you open the aperture a tiny tiny bit more? I know the intensity will increase, but will the colour change as well?

 

[elemental09]

I'll have to investigate further tomorrow - I've left the lab for the day. Supposedly this is a good quality laser, seeing as it costs nearly 2000 euros. This is the exact model:
http://www.thorlabs.de/thorProduct.cfm?partNumber=HGR020
Then again, as I've come to learn, the pricing and reputability of a manufacturer is a poor indicator of quality when it comes to optics.
I'll also ask my supervisor when I next see him.

EDIT: I don't believe the aperture size could be limiting the wavelengths passed, since the iris remains visibly open (at least 0.5mm) the whole time.

 

[m.e.t.a.]

Could it be an optical illusion? Green and magenta are visually "opposite" colours. If the green light of the laser is somehow scattering/reflecting so as to cast a roughly uniform glow upon the white cardboard, then any areas of shadow would tend to look magenta or purple. http://img46.imageshack.us/img46/8350/lasermagenta.png . If you have a camera with a manual white balance setting, perhaps you could test this?

Incidentally, good question! It's nice when somebody puts the effort into supply as much necessary information as possible.

 

[dacruick]

Yes very good question. I have the same model laser as yours in my lab at 632nm. I know thorlabs to be expensive but good quality.

I am unaware of the effect that m.e.t.a has explained, although if its true, that seems like the most likely solution.

Looking back at your diagram, the purple light seems like it could be created by the shadow of the off axis aperture. Let us know if you find the answer/any new info!

 

[elemental09]

Interesting idea with the optical illusion! Quite remarkable, I opened your image in paint and use the eyedropper, quite hard to believe that the the shadowed segment is grey! Indeed there is a faint, diffuse green background on the cardboard, however I would think the effect here is too strong to be an optical illusion, since the violet glow is quite bright. I took some pictures just now, I'll post them later (don't have my camera cord with me).
I asked my supervisor this morning, and he gave the following explanation: in the He-Ne laser, there are many wavelengths generated from the discharge in the tube. Only one of these is selected by the cavity to qualify as the laser output (coherent, low divergence, etc.). However, in the near field, these extra wavelengths will leak out and appear, in low intensity. Indeed, when today I placed the cardboard backing quite far from the laser (~1.5m), the violet patch was quite large and diffuse. So this seems like the right explanation. Turns out I didn't accidentally discover a new nonlinear optical effect :)

 

[elemental09]

Here are those images, as promised:
http://img28.imageshack.us/i/img0801oo.jpg/
http://img19.imageshack.us/i/img0805tr.jpg/
http://img339.imageshack.us/i/img0806fx.jpg/

 

[Lambduh]

If you have a spectrometer you could see what wavelength it is you're looking at and try to associate an emission line with it. I remember seeing a similar spectra/color when building a He-Ne laser sans enclosure on an optical bench.

Remember that the light coming from the He-Ne is green because the cavity is designed in such a way that the other modes are suppressed in relation to the green line. Spontaneous emission is still going to occur so you'll see various emission lines from both helium and neon. What matters is how you build the cavity to create the highest gain for the wavelength/mode that you do want.

The reason that most He-Ne's are red is that the 632.8nm line is the most likely/efficient and the easiest to build a cavity around.

 

[Claude Bile]

The violet emission is due to the glow of the HeNe mixture being excited via electrostatic discharge. It quite broadband and incoherent, and thus is not an active transition of the laser. It is a remnant of the pumping process.

Claude.

P.S. The red HeNe line is not the most efficient to pump, though it is one of the cheapest ways to achieve visible laser emission, hence their prevalence in teaching labs/alignment setups etc.