Here's the procedure I used to align the laser. You may find a
better one.
- Start with slits in front of the laser wide open. Make sure the
beam is going through the center of the slits and also through the beam
splitter.
- Set the rotating mirror so that the "front" is facing forward
(the front is shinier than the back). Note that there is a lens
in front of the rotating mirror, and it scatters more light than the
mirror itself. The reflected light from the lens is a roundish looking
spot, whereas the light from the rotating mirror is kind of square. You
need to cock the rotating mirror just enough so that the spot from its
lens does not get through the big lens to the M1.
- Slowly rotate the rotating mirror so that you can tell which spot
is the beam and which is a reflection. Note where the beam appears on
M1.
- User the beam adjuster to raise/lower the beam (parallel motion).
Adjust the beam horizontally so that it goes through the center of the
rotating mirror and approximately through the center of M1.
- Look for the beam on M2. Adjust M1 until the beam is centered on
M2.
- Adjust M2 until the return beam is again on the center of the
rotating mirror.
- At this point you may see reflections back near the laser. Forge
ahead!
- Turn on the rotating mirror. Looking through the eyepiece, look
for a bright spot. DO NOT look at the spot UNLESS the mirror is
rotating! You will see a background fields of red light, but
somewhere along the travelling eyepiece you should be able to see a
more intense smudge -- this might be the reflected spot.
- Change the rotation frequency. You should see the spot move.
- Move the eyepiece to get the spot in the center for a given
frequency.