What is up with this beam?

I posted some images of this green laser's spot previously to get opinions on it (http://www.laserpointerforums.com/forums/YaBB.pl?num=1221084038/0#0)
but I am still baffled by it.

I have some better images of it now (taken through a red filter to dim the green to a point the camera can capture the spot without the CCD being oversaturated).

Forgive me if I'm wearing this out, but I'm hoping for some opinions based on these new images.

These three images were taken of the spot on flat white paper, after the beam had diverged to ~10mm. The only difference is the exposure settings.







This one is a photo of a piece of thermal (receipt) paper after is was placed directly in front of the aperture for about 20 seconds.




Do these images represent the qualities of a "good" or "bad" beam? What would make the beam shaped like this? How could the last image even be from the same laser as the first 3?

All comments/opinions welcomed. Thanks.

I think the beam is fine.
What it looks like is the IR leakage is creating the oddly shaped profile your camera sees through the goggles.
From the burn mark on the receipt, your laser appears to be running in TEM00.

I think your laser is fine unless the pattern is not IR leakage.

If your laser is bright, has no visible instability (power spikes and drops or flickering) and can burn well, and no noticeable mode shifting occurs, you should be fine.

If you want to determine if you laser is shifting modes, hold a magnifying lens to the laser and project the magnified spot on a blank wall.
If the spot is anything other than a nice round (or slightly elliptical) shape, your laser is likely mode shifting.

I'm 99% sure this is not IR leakage. It appears to have good IR filtration.

It will burn through paper without even being focused (~2mm hole) and is very bright.

What is interesting is that in the thermal paper image, if you leave it in front of the
laser a little longer, it will burn out the white CENTER (or the "bullseye")

Huh... have you tested for mode shifting?

Wikipedia has an article on transverse modes, there examples of a few various modes.
http://en.wikipedia.org/wiki/Transverse_mode

I would check for mode shifts and if your laser is defective, contact the seller and get a refund/replacement.

RA_pierce said:

Huh... have you tested for mode shifting?

Wikipedia has an article on transverse modes, there examples of a few various modes.
http://en.wikipedia.org/wiki/Transverse_mode

I would check for mode shifts and if your laser is defective, contact the seller and get a refund/replacement.

Click to expand...

If I understand the definition of mode shifting, it is not. The mode, whatever it is, stays that way. even if I vary the power the spot still looks the same, etc.

My thermal image looks like the TEM01* from the Laguerre-gaussian chart in the Wiki article (assuming the white part on that chart represents the beam)

For whatever it may matter: this is a 200mW lab style with TEC cooling.

So the white spot in the middle of the burn mark of the receipt is not burned?
The center of the beam should have the highest intensity so I assumed the white spot in the center was the burned paper ash- stuff (for lack of a better term).


If you can get a photo of the beam expanded and projected on a wall, it will be easier to see the characteristics that way.


EDIT: From your description, it doesn't sound like it's running in anything close to TEM00.

Oddly enough:




This is about the best I can do with mag glass. I must be doing something wrong.

That is very confusing.

In the picture with the expanded beam it looks like it is shaped oddly, but it doesn't look like it's mode shifting... a defect in one of the optical elements maybe?
But the pictures with the red filter look like a very weird mode shift...

Is anything obstructing the beam that you can see?
Or scratches on the lens?

Maybe others have more to say... It doesn't look like anything I've seen with any of my lasers.
Even when my laser went through a mode hopping phase (before I fixed it) it never looked like that, even at it's worst. I've gotten TEM00, TEM10, TEM20, and TEM01, but I haven't seen something that looked like that...

So the center was burned by the laser?

Frothychimp or pullbangdead probably have an explanation.

this looks like the classic 'near' tem00 beam. What laser is this? Is it an RPL or similar laser?

I've noticed with the viasho lasers (RPL, prometheus, etc) that if you expand the beam through a lens, the dot is still a circle, appears tem00, but if you look very closely you can see the circle is made of a few different blobs. Maybe this is just my laser though I dont know.... I wouldn't worry about it too much as long as the power is still there and it's not super obvious without a lens.

Hodad said:

For whatever it may matter: [highlight]this is a 200mW lab style[/highlight] with TEC cooling.

Click to expand...

Peace,
dave

I just took this with a different mag glass. It is expanded to about 8 inches in this shot.



The laser is a LAB style with TEC & T03 style pump diode, running stable at 245mW(+/- 1mw).
The beam diameter at the aperture is ~3-4mm and the divergence seems to be about 1.0mRad.

Yes, there is something weird around the edges of the front lens. I have some pics of that I'll try to get up shortly

Here's a photo of the aperture. This is looks to be pretty sloppy work but I thought maybe it was ok since the beam appears to pass through only about the center 3-4 mm of the lens. At least that's about the size of the spot I can see a reflection of on the lens.

There are also several specs of dust behind the lens and what appears to be a tiny "scuff" (for lack of a better way to describe it) about 1.5mm off center of the lens.



Thanks everyone. I really appreciate your input and hope for more.

It's not mode hopping, but I think it's locked in a non-TEM00 mode or some mixture of modes. You can have a laser with multiple transverse modes happening at the same time, and it leads to strange beam profiles like this one. This can be due to poor thermal management or just misalignment of the crystals. I'd say it's a warranty issue, if you have a warranty on this laser.

The lens may be collimating or it may just be a window (or both) but it is designed to keep the innards somewhat sterile. Glue is to most common way of securing such windows and if it is a window it doesn't matter if it is off center. as the lens is parallel across the entire optic. What you are seeing for construction is a basic, general purpose laser.

Do not look at the beam as you will never get a Coherent or Newport or other top-end manufacturer beam out of it but you also did not pay the cost of a Coherent or Newport or other top-end manufacturer. The quality of the optics (material and coatings), the longitudinal modes (spectral width), the shape of the transverse mode, the stability of the power supply, the temperature stability of the crystal set and pump all play a role in the distortion of the beam. What you see with your eye when the beam is highly expanded versus what a beam analyzer sees are two different things.

When visualized at it's normal diameter the beam looks good but if you do see scratches on the inside of the window then that should be repaired. You have the specs of the laser, you could always ask them to give you the results of a beam analyzer and see how it compares or ask an independent party to put it under an analyzer. On your expanded beam photo, if there is no diffraction effects from the aperture (particularly from the glue), the beam quality is probably <2.0 which is somewhat common but the better quality general purpose systems generally have beam qualities <1.2. The analyzer will be able to give you the TEM power distribution and provide a comparison to a Gaussian curve among other things.

Thanks FrothyChimp. That really helps put things in perspective. It isn't a top end laser and the beam quality was supposed to be <2. It has great power and stability. I'm probably looking for perfection where perfection should not be expected (I tend to do that sometimes)

Oh, and after looking closer I think the window or lens I was talking about may just need cleaning. I don't believe it is a scratch or scuff after all.