aligning an external mirror laser

[MichaelH]

Since this may be useful to other people I thought I'd ask here before delving into that most arcane of newsgroups alt.lasers.

I've spent all afternoon trying to align the mirrors on my sp-907 (same as sp-127) to no avail. I just can't get my alignment laser to go straight down the bore no matter how much I fiddle with the height, pitch and yaw.

Does anyone have a design for a rig that can be used to align the mirrors on a (preferably large frame) laser?

[Tony]

Michael-

If the 107 is still lasing, you should be able to tweak the mirrors in without a second laser. This is assuming the bore is pretty straight and aligned. There's lots of info on this one on Sam's page (I'm sure you already knew that). The trick is not to get it so out of alignment it stops lasing.

[MichaelH]

The trick is not to get it so out of alignment it stops lasing.

Too late, but then the "laser down the bore" should have been a dead giveaway.

I've read through Sam's pages and several others but they assume you've already got all the equipment you need to do an alignment.

I don't and my makeshift attempt is about like removing my tonails through my throat.

I'd be up for building a rig unless what I need is reasonably cheap. I imagine this won't be the last time I do this.

[BobH]

Are both mirrors out of the laser? If so, you need to get the alignment beam down the middle of the bore. Try this:
a) split the alignment laser beam so the reflected beam goes off at about 90 degrees,
b) after a suitable distance, fold both beams back toward each other (about 45 degrees) so they are colinear and counter propagating,
c) Mount the head to be aligned so it can be moved vertically ( shims?) and horizontally at both ends,
d) position the laser head in the "bottom" leg of the system above, and fix its position with clamps and/or glue.

Either end of the laser bore can be positioned accurately because a beam at each end points the way. If you place a white card with an aperture cut in it (the size of the beam) into each beam, you'll see concentric rings untill the beams go through the bore. These patterns will guide you as you get close. You'll know when you arrive!

After that, you can put the HR back in, and get it to retro-reflect the beam back through the OC end (again by watching the patterns when you get close). The OC goes in last, and you'll see the reflection off it easily untill you get real close. The OC is by far the easiest. It's all tweaking from there!

[wler]

BTW, it is never needed to precisely position the laser, adjust pitch, yaw etc. All alignments can be done with two (!) extra mirrors between the laser you want to align, and the alignment laser.

The mirror closest to the alignment laser is used to position the beam on the second mirror right "in the middle" -- means, exactly on axis with the laser to be aligned. You can find out this spot by either using the borelight, or some broadband light source behind that laser. Once the alignment beam hits the second mirror correctly, the latter is then used easiest to use the back reflections off the cavity mirrors, and a postcard with a 1mm hole is helpful to align the back reflected beam onto the alignment beam.

[MichaelH]

BTW, it is never needed to precisely position the laser, adjust pitch, yaw etc. All alignments can be done with two (!) extra mirrors between the laser you want to align, and the alignment laser.

I think that's only true if you have mirror mounts with precise x/y controls. Unfortunately I don't.

I'm now four hours into my second alignment session but at least I'm now doing it in a 76deg kitchen instead of a 100deg garage, as I was yesterday.

I'm trying one more time before I give up for the night and either try again in the morning or throw a hammer at the tube.

If I can't get it aligned after a few more tries I'm going to see if I can find someone locally to either do it for me or show me how. Maybe the guy I bought it from hasn't moved away.

Talk about a lesson in why you don't want to fiddle with the coarse alignment controls in one of these puppies.

[Colin Kaminski]

If worse comes to worse you can get an HR mirror that is not frosted and use a alignment laser. A green laser pointer would be great. They are only $70 now. If you go down that path contact Laser Sam. He should have a direct replacement as he takes apart a few HeNes a month. He has had every one I have looked for.

[MichaelH]

Just to make sure I'm doing what people have recommended, here's a picture.



I've set up a hene (smallest divergence I could find) as the alignment laser and using mirrors and a beamsplitter I've aligned the lower leg so the beams are coincident.

I then take the laser to be aligned and place it so those beams are going down the bore. Or as close as I can get. Which is a real bear and I'm not sure I'm doing it right as I can never really get a clean spot.

I then put the HR back in and attempt to align it but again can't be sure because no matter what I do, there's never a clean spot.

My green laser may have a smaller beam and less divergence. I'll test it tonight and if that pans out I'll hook it to wall power and see if I can get the beast aligned using a different laser.

[Ron Michael]

Michael,

I also have aligned with both mirrors in and also remove my HR because it can have a slight wedge. I have been aligning my nd:yag with both mirrors in place but my old technique is to remove the HR and align the OC first with the bore, then place HR in and then align the HR by watching back reflection to the HENE then fire up the laser and adjust HR to be co-axial with the alignment laser.
My old technique for what ever it's worth to you...
http://www.rotorwave.com/resalign.htm
Today I just shoot the alignment laser in the same way from the HR side with all mirrors in and aperture open and watch all my reflections from a black card on the output side until all is colinear, then I adjust the aperture and
the laser will lase every time, I just then tweak to get best beam quality which can be close but not always max output (depends on optics).

To me, the dual beam approach is more time consuming since you have to work harder to align dual beams coaxially when one is enough trouble to geometrically align with your bore. two kinematic mirror mounts to align the alignment laser with your bore is the best approach, but not impossible, just more trouble, to shim both lasers until you get it as long as you keep them from moving afterwards during the procedure.

Before you throw something at your tube,
Hop in your car and come on down, I'll be happy to help you align it for free...

[Martin]

Just to make sure I'm doing what people have recommended, here's a picture.

I've been wondering about the copyright issue you're alluding to in your “Copyright c Michael & Susan Harrison – All Rights Reserved”.

[MichaelH]

I've been wondering about the copyright issue you're alluding to in your “Copyright c Michael & Susan Harrison – All Rights Reserved”.

You can ignore it if you like. It's added by my gallery software where I store my pictures. What's copyrighted is the picture, not the ideas represented. A few of the pictures I have on my web site have been "borrowed" online without attribution and I wanted to put a stop to that.

[MichaelH]

I also have aligned with both mirrors in and also remove my HR because it can have a slight wedge.

Wedge? What do you mean by that?

I have been aligning my nd:yag with both mirrors in place

What power and type of laser are you using for alignment? With both mirrors in place I don't get enough reflection from the HR to tell what needs to be done to it.

I finally got lasing last night and I've adjusted the laser back to 15mW before calling it a night. I'm going to continue adjusting things this morning. I was ultimately successful by using my 5mW green laser pointer, which has a very small beam and little divergence, and using a one-beam method. Shooting down the bore to align the green beam and laser bore, putting HR in and aligning it for best back reflection, putting OC back in and using the reflection from it to get the alignment as close as possible and then twiddling the OC alignment nuts until the beast lased.

At least now I can "read" the reflections just well enough to guess at what's required to align this puppy and the next time I have an entire day free I'm going to build a jig that will make this process easier.

Before you throw something at your tube,
Hop in your car and come on down, I'll be happy to help you align it for free...

Thanks, you're definitely close enough for that and it would give me a great reason to go on a long drive in my Mini.

[Ron Michael]

Michael,

Glad to hear you got it working.
I meant to say if you had a ground HR mirror back you can always come in from the OC side. Thats the typical easy way. Alignment from the HR side is typical for pulse lasers so that the alignment laser can be left it in place.

Most mirrors have at least few arc minutes of wedge and some have much more. This causes the alignment beam to divert from the path slightly when mirrors are out and then in. On the OC side alignment side, you get the HR reflection back so you can compensate for an OC wedge.
On the HR side alignment technique, if you align the OC without the HR then the HR back in is where it's more of a problem, So I outlined a few techniques earlier to compensate for it.

I use a low divergence laser like a HeNe but it depends on the laser medium's absorption, for example ruby would absorb a green laser, it even absorbs it's own wavelength. I was using a 2mw laser but I use a black observation card to reduce the glare and intensity when aligning.

[MichaelH]

Ok, the last 10% is proving to be the hardest.

I've been able (at times) to get the laser back to 20mW but the beam quality is horrendous.

This is a lot like aligning a spatial filter. It gets easier with time but is really hard to describe since so much of it is done by "feel."

Here's a look at the laser quality from a few days ago. (click on the image to get a larger version).



To me, the whorls say that the beam is hitting the inside of the bore which may mean the HR is not at the focal point of the OC. I believe that the HR on this laser is flat with the OC being slightly concave which would imply that the HR should sit at the focal point of the OC for optimum beam quality.

Does that sound right to those that have tinkered with these much longer than I have?

[BobH]

If you're certain the beam is lasing down the middle of the bore, it looks like a dirty surface to me. All that in & out with the mirrors was certain to get particles on the brewster windows. I'd clean both windows (one at a time), the HR, then both sides of the OC. If a surface is dirty, the alignment where best power is achieved might not be down the middle of the bore. After cleaning each surface, realign for best power whee the beam isn't clipping anywhere. Did you try adjusting the bore supports?