aligning an external mirror laser

[MichaelH]

If you're certain the beam is lasing down the middle of the bore...

I'm not. I'm still learning to read the laser and I'm not certain of anything

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?

Yep, I've been all over all the adjustments. Since they all interact, it's hard not to touch them all at some point.

What are the chances that I can remove a mirror and not have to re-align the whole thing? Logically it should be possible... Guess I'm about to find out.

btw, which end of the boot should be pulled back? The end toward the bore, or toward the mirror? I'd figure the mirror but thought it safer to ask.

[BobH]

The boots are not fun. They're probably stuck hard onto the mirror mounts, and harder onto the glass. I might have a set that can be removed off an old dead tube, to replace yours if you need to cut them off. I'll look.

[JohnFP]

I have found that HeNe's of all callibers are very very hard to align when the are out. The main reason is because the bore is so very narrow. What I have done, and am doing now to resurect the second HeNe I disected to try to get one laser modulated by the next with a Piezo on one of the mirrors it to use the smallest of pinholes punched in a card narrow the allignment beam sent down the bore. But is seems you are past that point.

Now what you need to do is walk the laser in. Basically, and this is very time consuming from the way I know how to do it, is to turn one of the axis adjustments so that the laser beam starts to dim. Take it as far as you can but ALWAYS keep the beam lasing. Mark and note what you did with that axis adjustment. Now work with the other mirro and try to tweak in as best you can. If it gets better then you need to go the same way again with the first mirror and repeat. Continue unitil it gets wost. Then go back to the best. Now do the same but adjust the first mirrors other axis. If at any time you cannot get the beam be brightner then try the other way. You should see a definitive and progressive betterness of the beam in both the x adn y directions of the first mirror.

[MichaelH]

The boots are not fun. They're probably stuck hard onto the mirror mounts, and harder onto the glass. I might have a set that can be removed off an old dead tube, to replace yours if you need to cut them off. I'll look.

Thanks. I haven't tried removing mine just yet. I'm going to push things right to the edge before I give that a shot.
At the moment I'm still working through the whole "trying to get the HR at the focus of the OC" theory and at least things aren't getting worse. Not sure they're getting all that much better but I'm getting practice at adjusting the beast.

What are the chances that I can remove a mirror and not have to re-align the whole thing? Logically it should be possible... Guess I'm about to find out.

I've now taken the OC out and replaced it without having to pull any hair out or re-align the whole thing. These seem to have been machined to the tolerances I'd expect. There was a bit of dirt but nothing too horrible. I've re-cleaned it just the same. Now to try the HR.

[BobH]

At the moment I'm still working through the whole "trying to get the HR at the focus of the OC" theory

I wouldn't worry about that! A few millimeters will not make any difference. More important is to be sure all those little springy washers are tight enough to be ultra-stable, but loose enough to be adjustable.

[MichaelH]

At the moment I'm still working through the whole "trying to get the HR at the focus of the OC" theory

I wouldn't worry about that! A few millimeters will not make any difference.

now you tell me. I found that out today. I also found that a straw and my green laser pointer are useful tools in order to learn how to read the reflections off the bore and more easily determine how to move the mirrors. Hasn't made it like a walk in the park but has helped some

More important is to be sure all those little springy washers are tight enough to be ultra-stable, but loose enough to be adjustable.

I see that would be important once it's aligned.

I'm still having trouble getting clean output and the patterns suggest reflections off the inside of the bore. Figuring out if that's due to end mirrors causing the beam to hit the bore or the bore being crooked is a real PITA.

[dcgman]

Just one more thing- once you've cleaned the mirrors/ brewsters windows, give the laser at least half an hour warm-up time before tweaking for max power. The cold laser bore-line may be slighly different from the warm laser bore-line.

[Ron Michael]

Michael,
I saw your request out on Alt.laser. If you don't find someone nearby, here is my suggestion. I suggest starting over. It looks like your right with some light piping and the mirrors are more than likely tilted. LIke BobH said, it's very important to get the beam down the bore correctly and then align the mirrors to the beam. Accuracy here means less beam walking etc. But I will be honest, it's too easy to mess up tweaking. Best to get it right in the first place, then only the very smallest of adjusment is needed for power. Of course if you know your mirrors radius etc it would help in your alignment (thats worth testing each while the mirrors are out). Generally a concave HR steers the laser mode and the OC is adusted for power. You can try the direct beam in method if you have full precision control over the physical mounting of the alignment laser or use a beam steering method.
Get two cheap kinematic mounts $40 each and cheap mirrors for it. Secure your Alignment laser and your main laser in a folded arrangement with two KM each at 45 degree to allow the 180 degree beam steering.
the KM pair allows two main adjustments. First one is for proper height and translation and the second one is for xy tilt. It's important to go in on the OC side if the HR does not have a polished rear surface. The main laser is not to be powered up. It's important to have no resonator mirrors or boots in place. get a clean beam down the bore. It will also allow you to inspect the brewster windows and the output at the other end for dirt diffractions, light piping (light at angles) etc. Once you confirm this place your HR in and observe from the brewster window's side reflection (acts like a beam splitter) that the two beams (primary and HR reflection) are coaxial by HR KM adjustment. then place OC in and again watch for coaxial reflections from the brewster window.
If boots can't be removed, then place alignment alignment laser further away so that you can use a tilted window for a beam sampler. If the HR is polished both sides, then come in on the HR side since you can always just project your beam out the OC side and use a card just past the OC to watch for coaxial adjustments. I always prefer that (looking at light at the end of the bore on a card than using a beam splitter unless you have a wedge type etc so you can distinguish from beam splitter secondaries reflections. (end up sometimes with too many reflections to sort out). Down the bore type off axis reflections are the ones you are trying to eliminate with alignment.
Remember the system uses the bore as it's aperturing (TEM00) so you have to use a smaller (shorter) laser with a 0.5mm tem00 mode size and about 1mrad and try to keep the overall distances short.

If it were me, I would start over. keep the alignment setup in place just incase you feel you need to start over again. Then use accurate alignment watching the reflections and coaxially adjust them. Once you fire up the laser, just be careful about walking/tweaking those mirrors as it will get you back into the condition you are in. Better to use an alignment laser to confirm your geometry than making too much knob adjustment. You can always turn on the laser and let it warm up to see if any bore changes happen while monitoring it with the alignment laser.

[MichaelH]

Michael, I saw your request out on Alt.laser. If you don't find someone nearby, here is my suggestion. I suggest starting over.

I've started over about six times now. The biggest problem is that I'm using ad-hoc tools. I've picked up some relatively inexpensive kinematic mounts from ebay and as soon as those come in I'm going to build a rig for doing this in the future. I'm using the island in the kitchen right now since it's the only surface big and stable enough that's air conditioned (working in 100 degree heat in the garage is much like being in H*LL) but in the future I really need to be able to do this up in my lab area. Wifey would surely agree if she read the forum, of course Andres will probably mention it to her since they work together.

PatB from usenet had a contact 30 minutes from me that I'm going to visit on Thursday. I've had enough joy for now. If I didn't need the laser right now I'd keep playing and learning but I think I'll pay $50 and a hologram or two and get my laser back.

Sometime in the future I'll try it again with the right tools but at a time of my choosing.

How's this for a picture of the alignment you've described? (click for a larger version)



In any case, I've learned a log about aligning this beast, not the least of which is to keep my hands to myself.

Thanks to everyone who have chimed in here and in email. You've all helped to one degree or another. I'm in the process of writing up my trial by fire along with pictures that might be helpful to the next person that comes along, including me the next time I jump into this pool.

[Ron Michael]

Okay you have the beam steering correct. But remember the first KM will adjust both up/down and left/right which translates the beam up/down and left/right of the bore. The second KM is for fine tuning the tilt axis of the beam of either tilt up/down or tilt/left right. Both KM work together.
The card pinhole is not what I would suggest as the pinhole has to be big enough (2x the beam diameter) not to interfere with the alignment beam which negates some accuracy (gross adjustments okay but fine ones are lost in the pinhole).

better like this if coming in from HR side if HR is polished both sides.

* align_laser>----laser_to_be_aligned-------->card


Else if coming on the OC side then use the card/pinhole.

[MichaelH]

The card pinhole is not what I would suggest as the pinhole has to be big enough (2x the beam diameter) not to interfere with the alignment beam which negates some accuracy (gross adjustments okay but fine ones are lost in the pinhole).

I didn't follow this. How does a pinhole before the KMs interfere with fine adjustment?

better like this if coming in from HR side if HR is polished both sides.

* align_laser>----laser_to_be_aligned-------->card

Unfortunately I can't do that as the HR on 907s are ground glass. grrrr.

[Ron Michael]

How does a pinhole before the KMs interfere with fine adjustment?

Michael,

It doesn't interfere with fine adjustment, it's just sets up a diffraction pattern and worse spherical phase shift (diverging wavefront) if the hole is too small which is why 2x the beam size is good. then if you adjust watching the back reflections you get it close until they are lost into the area of the pin hole.

So in your case it would be an acceptable method.

[MichaelH]

Ok, I've had the laser cleaned and tweaked by someone who knows how and while it isn't yet usable, I did learn a lot about cleaning and aligning the monster.

I've written up what I've done over here and intend to put out a formal tutorial once the laser's back to full health.

[MichaelH]

Alright...

First off, thanks to everyone who helped me get the laser back to full health. I couldn't have done it without all of you.

I've cleaned and aligned everything and it's running at 22mW which is better than when I first started messing with it. Yes, I had it going at 25mW just before I killed it but 3mW just isn't enough to fight for right now. The beam quality is back where it should be as well.

I've taken a whole slew of pictures of the process and I'm working on a tutorial which I'll post when it's ready (and mirror on the wiki).

Thanks again!


(click on the image for a full-size view)

[MichaelH]

Version 1 of the tutorial is online at

http://www.dragonseye.com/blog/archives ... laser.html

Any feedback is appreciated.