Lens off center?

[Dkoy]

Hi

I am totally new into this, and I am not even sure this is the right forum.

I bought this cheap laser from AliExpress, but it seems that something went to fast in the assembly? (you get what you pay for I guess).

When I remove the front I can see the beam is reflected inside the tube. Attachment 1 shows this, where you can see the thread from inside the tube reflected on the door.

When I look at the laser diode, it seems that the little lens is off center (attachment 2) Is this as it is supposed to? Should I remove the little lens and glue it back on? Or will that cause me more troubles?

I hope you can help.

BR
Steen

 

[Alaskan]

Is that a photo of the spot taken with the collimation lens off? If so, that appears almost normal to me but not quite right, I have one which looks like that with the collimation lens removed and I think it is a TEM mode issue alone and not related to the position of the expander lens. Looking at your photo, it appears you took the picture with the collimation lens assembly off, otherwise the many concentric rings would not be present in the photo because the lens holder, as a long small diameter tube, normally blocks them from leaving the laser assembly.

If the collimation lens is on and focused to as fine a spot as you can get and is still oblong shaped, then it appears like it isn't operating at the desired TEM00 mode which produces a single spot, perhaps half way between TEM00 and TEM10 making a oblong spot. If not that, possibly TEM10 making two, or even TEM20 instead producing three spots close together which can appear as a oblong spot, but I can't quite see from the photo if there are other spots within it or not.

If you have the collimation lens off and don't see two or more distinct spots, it's probably stuck between modes, neither one or the other but if you start with a cold laser and let it warm up, you ought to see changes to the shape of the spot, over time, this would be a fairly good indication that the centering of the expander lens is not the problem and is a TEM mode problem. My best guess is that laser has the exact same problem one of mine does, is almost a TEM00 mode with one spot, but just a little out from it wanting to divide into two spots but won't quite do it.

If it is a TEM mode problem producing other than a single spot, it could turn into a single spot after it warms up some, others are a single spot to begin with and then turn into a double or triple spot as they become warm. It depends upon the particular crystal for how it acts, I guess. Maybe there are some other factors involved, but none you can do anything about that I am aware of without too many head aches dealing with it. Maybe someone else can suggest something.

If your laser pointer is one of those that sells for about eight dollars on ebay which says it can go 8000 meters, or 5 miles, and has a piece you can screw on to the front to make star patterns, about one out of ten of those cheap lasers act that way.

Here's a link to a photo showing the different TEM modes:

http://en.wikipedia.org/wiki/Transverse_mode#mediaviewer/File:Hermite-gaussian.png

TEM20 Photo:

Here's a video of one of my other lasers which quickly changes from TEM00 to TEM10 mode (mode hopping): http://photos.imageevent.com/qdf_fi...ations/irledandlaserproject/TEM Splitting.mp4

 

[ultimatekaiser]

No. Just leave it alone. It's likely off centered keep the being straight. The reason the beam is kind of ovular, it's probably because The alignment is not quite spot on, or it is being over pumped. Neither are really worth trying to fix in a laser like that.

 

[Dkoy]

Hi

Thank you very much for your answers. Always nice to learn something new.

Yes, the picture is with the collimation lens off.

It is this one - 50W wow (lol) It pop balloons and it will almost ignite a match. I guess maybe 100mw?

http://www.aliexpress.com/item/2015-HOT-50000mw-green-laser-pointer-pen-adjustable-focus-burn-match-5in1-star-converter-free-shipping/2036022073.html

What you describe is exactly what happends. When I turn it on, it looks like TEM01 and after a few seconds it looks almost like TEM00. With the collimation lens it looks much better. Also it takes, maybe 5 seconds, before it is at full power.

BR
Steen

 

[Alaskan]

Yea, it is unethical to lie in advertisements, but many sellers do that with lasers because they know most people don't have laser power meters, and... that it is too expensive to ship them back to China if from there, or just won't bother. OK, it is not the same host as the one I have, here's the module out of mine:

Here's what the unit is on ebay:

http://www.ebay.ca/itm/High-Power-5...-Pointer-Pen-Visible-Beam-Lazer-/141492377648

I suspect many of these inexpensive laser pointers act the same. Your laser might be different and higher power than mine, but I've measured the output of over 50 of these units (as in the photo, above) using this module, to be anywhere from 30 to 80 milliwatts, some even higher but the average output at about 70 mw of which about 25 percent of that power was infrared. No IR filter. On one of these units I removed the expander lens and crystal assembly under it and measured the IR diode output to be about 225mw. If you do that and reassemble the unit without the crystal and expander lens, the collimation lens then won't focus the IR, it is too far away to focus properly but a G2 type of lens holder with a short focal point lens in it can be screwed into the base of the plastic lens holder tube (the inside end of it) with the original lens removed and then it will.

 

[Dkoy]

Very interesting. Thank you. The lens you have attached instead here, which one is that? What should I search for?

BR
Steen

 

[Alaskan]

The above photo in my previous post shows the original lens assembly on the module, the only reason I'd mess with turning the pointer into a IR laser is if the expander lens and crystal assembly were already removed and you don't want to try to put it back together. In that case, here's what I did: I used a S1 glass lens in a G2 lens holder and it worked when close enough to the IR diode to focus the energy into a point, but 225mw of IR just doesn't burn impressively enough for me to think that is a good mod to an otherwise perfectly good 532nm laser which is much harder to make. The S1 lens I used is supposed to pass red well without a lot of loss, but I think the loss was over 25 percent for IR @ 808nm. I don't clearly remember the loss with an S1 lens at IR now but it as probably about that, nothing too far out of the norm but finding a lens made for 808nm would be a better replacement:

Here's one that will fit into a G2 type of lens holder made for IR too: http://www.laser66.com/html/650-G-2.html

If anyone wants to mod one of the laser pointers I showed the link to on ebay, here's what it looks like. It uses the same lens holder tube, but the lens on the small end is removed and replaced with a glass lens on the other end, as in this photo:

 

[Dkoy]

Thanks again. Very informative.

BR
Steen