Aligning an external mirror laser

For several weeks I had a functionally dead Spectra Physics 907 laser. This was especially bad because this laser is the one I use for mastering my holograms which meant I couldn’t make anything new until it was fixed.

During those weeks I picked brains and websites far and wide and managed to get the laser lasing again (several times in fact) but couldn’t get the beam quality back the way it was before I screwed it up. I ended up enlisting the help of a local laser expert and was able to get beam power and quality back where it needed to be.

What follows is a tutorial on how to re-align an external mirror laser. Most specifically the Spectra Physics 907. I also include some tips on cleaning the mirrors and removing stuck boots.

Before I go any further I want to thank the following people:

The guys at the Holography Forum who provided suggestions for getting the beast aligned. Tony, BobH, Wler, Colin, Ron Michael, Martin, JohnFP, dcgman.

Sam Goldwasser for making public such a treasure-trove of information about lasers.

Tom Ehrichs for spending time cleaning and aligning the laser. I gained as much by watching him work as I did by him working.

Tools you need.

  • Clamps.
  • Pliers.
  • 2 Kinematic mirror mount & bases to hold them.
  • 3 mirrors.
  • 1 large flat bladed screwdriver.
  • 1 Laser with a small beam to use for alignment.
  • Lens tissue.
  • Acetone.
  • Wrench for adjusting the coarse alignment nuts.
  • Hex key set for adjusting the fine alignment controls.
  • Can of compressed air.

Step 1 – Build an alignment rig.

An alignment rig should allow you to keep the laser stable, raise the laser off your main work surface so you can get to any alignment controls underneath the laser and be large enough to accommodate alignment mirrors and other tools.

Shown above is a setup that allows me to get the laser high enough that I can get to all eight of the bore alignment screws and allows me to clamp the laser down to a stable surface and hold all but one of the elements needed for me to align the laser.

That one element is a mirror past the HR that can be used to reflect the light from the alignment laser closer to the “operator.” That’s important because when shooting the alignment laser down the bore you really need to be able to get a close-up view of the beam coming out, which is difficult when you’re at the other end of the laser. Fortunately I have an extra tripod to which I hot-glued a small mirror, just for this purpose.

If you have a stable work surface and something equally stable to clamp the laser to, you can certainly use that rather than build a special-purpose rig. I figured I’d do this again some day and decided to build what I needed.

Step 2 – Clamp your laser down.

You want to make sure that you can’t move your laser while you are aligning it. Several steps in this process will apply enough torque to the frame that it is easy to cause the laser to move, undoing all your hard work.

Do not clamp the laser down so hard that you bend the frame either. Just enough that you can’t move the laser easily.

Too much force from the clamps will bend the frame enough that you’ll find your laser goes out of alignment when you release the clamps.

Step 3 – Place your kinematic mirror mounts.

You’ll notice that I’m assuming you have kinematic mounts. If you don’t, take some time to acquire at least two. I’ve tried doing this alignment with ad-hoc mirror mounts that work well for holography but don’t work well for laser alignment.

The mirror mounts I used have three alignment screws for moving the mirror, shown below.

Place the first mount about 6-8″ in front of the laser (this is M1).

Now clamp the mount onto the table so there’s no chance it will move. This is the mirror where you’ll be doing the most tweaking and you want it as stable as possible.

You now have a choice, you can either place the second mirror (M2) off to the side of the laser (see above) or you can place it where my laser pointer is in the previous picture, essentially swapping out the two. It all just depends on what’s more convenient for you. I found that it was easier to tweak the mirrors at the same time if they were a body-width apart from each other and I could reach both comfortably with either hand.

Step 4 – Place your alignment laser.

Shown above is a 5mW green laser pointer. I chose that for two reasons. 1. It’s got the smallest beam diameter of all of my lasers. This is important for getting the alignment beam cleanly down the bore, although you may also be able to reduce the beam diameter by using a card with a pinhole placed after the alignment laser. 2. It’s got a very bright beam which makes it easy to read the various reflections you’ll see when aligning your laser.

So I’ve got my alignment laser aimed at M2, reflected to M1 and then through the bore. I used a small photo tripod for holding the laser which works well but you do need to take care not to move the laser while doing the alignment.

Step 5 – Start your HEPA fan.

If you don’t have one, buy one. You’re about to remove the OC and HR mirrors and clean the Brewster windows. You don’t want any dust getting on them afterwards.

Let the fan run for at least 30 minutes to clean the room you’re in.

Step 6 – Remove the mirror mounts.

On an SP-907 you can remove the mounts by un-screwing the nuts shown above. You may find that the rubber boots have become stuck to either the mounts or the bore or both. Very carefully pull the boots back from the mirror mount until you can slide the mount off the bolts. You’ll probably find that you have to use a tool like a dental pick to get under the boot and gently pry it off the mount. Be very careful not to tear the boot.

Once the first mount is off, repeat the process for the second mirror mount.

At this point you can pull the boots back and get at the Brewster windows or if you do not plan on cleaning the windows, you should touch the boots as little as possible. I recommend taking the boot off and cleaning the windows though.

Step 7 – Remove the boots.

This is even trickier than removing the mirror mount. There is a better than even chance that the rubber boots have become welded to the bore over time as the bore has heated and cooled during use.

One way to un-weld the boots is to take a small tool with a pointed (but not sharp!) tip and gently pry up the boot from the bore, working from the inside of the boot. You should be very careful not to come into contact with the Brewster window. You do not want to scratch the window.

Once the boot is off you may want to wash the boots as they tend to attract a very fine black dust. I washed mine with dish washing liquid and gave them a final rinse in distilled water before letting them sit out (near a HEPA fan) to dry.

Step 8 – Find the light.

Now you need to turn your alignment laser on and bounce its light off M2, to M1 and then at the OC end of the bore. Don’t try to get it down the bore just yet because there’s a better way than doing it by eye.

Now that you’ve got the alignment laser in place, turn your main laser on and using a piece of lens tissue find where the center of the bore light hits the mirror.

You want the alignment laser to hit as near the center as possible.

Now tweak the screws on M2 to put the beam at the center of the bore light.

Step 9 – Shoot the bore.

The next task is to tweak M1 so that the beam goes cleanly down the bore. This process assumes that the laser bore is straight to begin with. If you do not think that’s the case or want to be sure, I’ll have to refer you to Sam’s pages as I haven’t gone through the process of straitening a bore.

As you tweak the screws on M1 you should see patterns that look similar to the following.

Continue tweaking until you get just a spot of light out the other end with no whorls. Depending on your alignment laser you may never reach this latter goal, because of reflections off the inside of the bore due to a beam that’s a bit too large, but you should end up with a fairly clear dot in the center of any swirls.

Go slowly and stay patient. This step takes time unless you’ve done it a hundred times before.

You’re halfway there.

Step 10 – Remove the HR

Now for some real fun. If you examine the HR using your alignment beam and don’t see any dust, skip this and the next step. Otherwise grab a wide-blade screwdriver, some toothpicks and courage.

The pictures show the mount still attached to the laser, which is how I first started out but the technique works as well with the mount laying flat on your table.

Unscrew the retaining ring using a wide-blade screwdriver.

Remove the grommet retaining ring. It should just fall out, or you can pull it out with pliers.

Using two toothpicks, use one to pry up one side of the grommet and the
other to lift it out. You can also use a dental pick if you’re
confident you won’t scratch the OC.

You can now remove the HR using duct-tape on a toothpick or just let it drop out onto some lens tissue.

Step 11 – Clean the HR

Check the HR by putting it in the alignment beam and look for dust and grit on the reflective side. If you see any, try to blow it off with compressed air. If that works, move to the next step. If it doesn’t, get out your acetone and lens tissue.

If you don’t have plastic pliers coat the tips with vinyl tape or plasti-dip so you don’t risk scratching the mirror.

I have my acetone in a dark bottle that has an eye-dropper top. Pick the HR up in your pliers.

Put a drop of acetone onto it.

Place the lens tissue on top of the HR and gently drag it across the mirror to dry it.

Now check the mirror for more tenacious grit and repeat the process until the mirror is clean. I found that for some contaminants it helped to move the tissue back and forth a few times before dragging the mirror dry. Don’t do this on the first try though as you’ll risk scrubbing dust into the mirror.

Put the HR back in the mount, making sure that the reflective side is facing the right way.

Step 12 – Clean the Brewster windows.

Take a careful look at the Brewster windows.

Chances are good that you’ll see signs of dust and dirt on them as the alignment beam shines through the bore.

First try blowing some compressed air at the window and see if the dust is blown off. If it is, you’re done and can move to the next step. If not, you’ll need to use your lens tissue and acetone again.

Take a lens tissue and fold it several times until it’s a flat package about the width of the window while being careful not to touch the central area.

Put a few drops of acetone on the working end of the tissue and gently wipe the tissue across the window just one time.

I sometimes found that there would still be a single piece of lint behind but I could usually remove that by blowing it off with compressed air or with a corner of the lens tissue.

Step 13 – Aligning the HR.

Screw the HR mount back in place (don’t forget to put the boot on first) and tighten the nuts down almost all the way. You want enough room to work but not so much that the laser goes out of alignment any time you move it.

At this point your goal is to tweak the coarse alignment nuts so that the alignment beam is reflected back down the bore at your alignment laser.

Place a white card with a hole in it in the beam path so that you can see the swirls coming back out of the bore. Once you get the HR aligned so the swirls are gone you’ll find that the beam is going back into the alignment laser and you can’t see the dot that represents your target. Take a piece of plain glass or a dichroic mirror and reflect part of the returning beam to a screen where you can see if you’re getting a fairly clean dot of light back out of the bore.

There’s no great magic to this step and really just requires that you experience it in order to gain an understanding of how to tighten/loosen the nuts to move the mirror the way you need.

Step 14 – Aligning the OC

At this point take the time to clean the OC and put it back in its mount. When cleaning the OC be very careful not to scratch the AR coating on the outside.

Tighten down the OC mount almost all the way again, leaving enough room for tweaking.

With the white card from step 13 still in place, tweak the alignment nuts so that the reflected alignment beam coming off the OC shoots right back through the hole in the card. In an ideal world your laser will start to lase again at this point.

Reality will probably be different.

From here, walk the OC mirror so the alignment dot moves back and forth across the hole in the card until you find a point where your laser lases. This is probably going to take some time, just be patient and take it slow. Once you find the point where your laser lases, take a break. You’ve reached an important milestone.

Step 15 – Fine-tuning the alignment

Now for the part that really takes patience.

Your task is to find the point where you get maximum power and best beam quality. To do this, you’re going to be tweaking the alignment nuts at both ends of the frame while watching the output power on your power meter.

Start at one end and carefully tweak each nut until you reach the point where you’re getting maximum power. Now move to the other end and do the same thing.

If at any point in this process you move a nut and lose lasing, move the nut back the other way until you get lasing again. Do not try moving a different nut as you’re just as likely to make the laser go further out of alignment.

Now adjust the fine alignment screws, shown here

to arrive at maximum power. Do the same at the other end.

Now you need to look at the quality of the beam. If you left all the other components alone, the beam should be shining back through the white card. If you turn off the room lights you should get a good idea of beam quality by looking for stray light around the central beam.

If the beam quality is bad you may need to walk the mirrors so that the beam is lasing along a slightly different axis. More information on that process is given here. For example, my laser puts out the strongest and best quality beam when it’s lasing off axis relative to the bore.

At this point you should have a fully tuned laser again.

Added 8/25/2006

Here are some comments from SamG

A green laser pointer may indeed have a lower divergence than other common lasers, though keeping it powered for the time required to do this could be problematic.

On a laser that used to be working well and hasn’t been shipped somewhere or (gasp) dropped!, bore straightening probably isn’t essential. But otherwise, it can result in more improvement than anything else.

For those without HEPA filters and cleanroom-labs, I’d suggest doing the final cleaning of the Brewsters last after everything is back in place and lasing. Just push the boots away from the mirror mounts, then swipe once using a new cotton swab with 1 or 2 drops of pure alcohol on the swab. It will dry almost instantly. When the scatter from the inside of the B-window is equal or greater than from the outside, that’s about as good as it gets. Repeat with another new cotton swab if not adequate. Then, just push the boot back in place.

The mirrors can be easily removed without getting masking tape residue on their outer surface (particularly the OC where it matters) by sucking into a rubber tube and then drawing the mirror out. SP apparently provided this for the SP-125 at some point. Have a tray with lens tissue in it under the mirror mount if it should fall.

Author: Michael Harrison

Husband, Programmer, Irish dancer, tinkerer, astronomer, layabout (as much as possible)

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