The Mach Zehnder and I got to know each other today. The reason for redoing the alignment was to improve pointing from the PSL table into the MC/IFO in hopes that this would solve the MC unlocking problems that we've been having lately. Since Rana had aligned the IOO QPDs a few weeks ago when all of the alignments and things were good, I used them as a reference for my Mach Zehnder alignment activities.
The order of operations were approximately as follows:
1. Block the secondary (west) arm of the Mach Zehnder using either an aluminum or razor dump.
2. Use SM1 in the MZ to align the beam to the IOO_QPDs (Pos and Ang). I unfortunately also touched BS2 at this juncture, which made the refl path no longer a reference.
3. Make sure that the QPD Sum on both Pos and Ang was sensible. Since there are 2 beamsplitters in a Mach Zehnder, the power on the QPDs should be a quarter when only one beam is on them. Be careful not to allow the beam no clip on anything. The biggest problem was the bottom periscope mirror - if you hit it too high or too low, since it is a very thick optic, you end up coming out its side! This is the frosty part on the edges, totally inappropriate for beams to go through! Since the side of the periscope mirror isn't HR coated, when going through it like this, I was able to saturate the QPDs. Not so good.
4. Also, make sure that this first beam is on the MZ Refl PD. Do this using the steering optics after the beam has left the MZ. Use a viewer to look at the PD, and see the small spot of the beam on the diode. We closed the iris which is present and was standing fully open to remove a spurious beam which was a parallel split-off of the main beam. Since it was very weak, it is fine.
5. Unblock the west arm, and block the east arm of the MZ.
6. Align this arm to both the IOO QPDs and the MZ refl diode using the adjustments on BS1, the PZT mirror and if necessary, BS2. Note that the adjust knobs on the PZT mirror have lock screws. Make sure to unlock them before adjusting, and relock afterward, to avoid slipping while the PZT is moving.
7. Unblock all the beams, and make sure there is only one spot both on the transmission side and the reflection side, i.e. the 2 spots from the 2 arms are completely overlapping. For the Trans side, make sure to look both in the near field and the far field (even after the periscope) to ensure that you really have one spot, instead of just the 2 spots crossing at a single location.
8. Look at the MZ refl DC out and the PD out from the ISS box (which is essentially MZ trans, looking at Morag and Siobhan) on a 'scope.
9. Touch / gently wiggle BS1 or another optic, and watch the 'scope. At the same time, adjust BS1, the PZT mirror and BS2 to maximize the contrast between light and dark fringes. Ideally, the refl PD should go almost to zero at the dark fringes.
10. Check that you still have only one overlapping beam everywhere, and that you're actually hitting the MZ refl PD.
11. Because I was concerned about clipping while still figuring out the status of the lower periscope mirror, I removed the beam pipe holders between the last optic before the periscope, and the lower periscope mirror. The beam pipe had already been removed, this was just the pedestals and the snap-in clamps.
All done for now! Still to be done: Optimize the position of the EOMs. There is a waveplate out front and the EOMs are mounted in such a way that they can be moved in several directions, so that we can optimize the alignment into them. They ideally only should see a single polarization, in order to apply solely a phase modulation on the beam. If the input polarization isn't correct, then we'll get a bit of amplitude modulation as well, which on PDs looks like a cavity length change. Also, the little blue pomona-type box which has the RF signals for the EOMs needs to be clamped to the table with a dog clamp, or better yet needs to be moved underneath the PSL table, with just the cables coming up to the EOMs. The SMA connections and the SMA cable kept interfering with the MZ refl beam...it's a wonder anyone ever made the beam snake around those cables the way they were in the first place. Right now, the box is sitting just off the side of the table, just inside the doors.
Something else that Rana and I did while on the table: We moved the PMC trans optics just a teensy bit toward the PSL door (to the east) to avoid coming so unbelievably close to the MZ refl optics. The PMC trans beam shown in the lowest part of my sketch was very nearly clipping on the MZ refl steering optic just near it. This situation isn't totally ideal, since (as it has been in the past), the first optic which is dedicated to the PMC trans isn't fully sitting on the PSL table. The pedestal needs to hang off the edge of the table a bit to keep this beam from clipping. Unfortunately there really isn't space to make a better beam path. Since we're planning on getting rid of the MZ when the upgrade happens, and this isn't causing us noticeable trouble right now, we're going to let it stay the way it is.
Also, we dumped the reflection from the PMC RFPD onto a razor blade dump. And we noticed that the PZT mirror and BS2 in the MZ are badly vibrationally sensitive. BS2 has a ~400 Hz resonance (which is OK) but a ~150 ms ringdown time!! PZT mirror is similar.
Q = pi * f * tau = 200! Needs some damping.