P. King, R. Savage, J. Oberling, Yuki
This morning we went in and tweaked up the PSL in advance of LHO's entrance into ER10 next Tuesday. The main activity was to improve the mode matching (MM) to the PMC, although the beam alignment into the FSS RefCav was also tweaked. The ISS was turned OFF for the duration of this work.
PMC Mode Matching
Before tweaking the position of the mode matching lenses (L02 and L03), the PMC visibility was measured:
-
Locked: 1.365V
-
Unlocked: 0.425V
-
Visibility: 68.9%
Since L02 was butting up against the mount for M37, M37 had to be moved out of the way; M37 is the final turning mirror in the path that dumps the 1st order diffracted beam from the ISS AOM into the beam dump immediately to the right of the DBB enclosure (see D0902114 for a layout of the PSL table). This beam path was then realigned so that the 1st order diffracted beam was still dumped in the same beam dump.
The power into the PMC was then turned down using the high power attenuator immediatly after the HPO to ~40W incident onto the PMC (in hindsight we should have done the mode matching at the full HPO power, more on this below). Lens L02 was then moved ~1cm closer to the ISS AOM (this was a gross movement along the rail the MM lenses are mounted on, not a fine move with the lens mount's micrometer), the beam was realigned into the PMC and the visibility was measured; visibility came in at 89.5%. At this point we took a micrometer reading from the micrometer attached to the MM lens mounts as a starting point for fine tuning. We moved the lens a few times, realigning the beam into the PMC and measuring the visibility each time. The results are summarized in the below table (the larger the micrometer reading, the closer the lens was to the ISS AOM):
|
Micrometer Positions (mm) |
Visibility (%) |
|
L02 |
L03 |
|
5.0 |
6.32 |
89.5 |
|
8.0 |
6.32 |
90.7 |
|
11.0 |
6.32 |
90.4 |
|
11.0 |
8.0 |
90.5 |
The MM lenses were left in the final position indicated in the table. In summary we moved L02 by 21mm and L03 by 1.68mm, both moved closer to the ISS AOM.
We then increased the power to maximum and measured the visibility of the PMC at 81.5%. This difference was unexpected. The PMC incident power was then lowered to ~50W and the visibility was measured to be 89.6%. What we think is going on is that at higher powers the ISS AOM is heating up and distorting the beam, which changes the MM into the PMC. This is why I said above that in hindsight we should have done the MM at the maximum power incident to the PMC.
Running out of time and wanting to keep as much of the MM improvment we had made as we could, we decided (after consulting with Daniel) to lower the power incident to the PMC to ~80W; this gives ~67W transmitted by the PMC. To make up for the lost power (we were previously transmitting ~100W through the PMC), we asked Travis (who was watching the operator station) to request "Max Power" from the PSL rotation stage; the max power was reading ~40W. We then used IO_MB_HWP1 to return the power available to the IFO to ~63W; with the ISS ON and diffracting ~3.5% this results in a maximum of ~62W available to the IFO, which is close to what we had before lowering the PMC incident power. We then measured the visibility of the PMC with this new incident power (with the ISS OFF):
-
Locked: 0.118V
-
Unlocked: 1.35V
-
Visibility: 86.7%
We left the PMC in this condition and should be sufficient for ER10/O2.
FSS Reference Cavity
The beam alignment into the FSS RefCav was tweaked. In addition, because of the PMC changes mentioned above the half-wave plate WP4 was adjusted for maximum incident power onto the RefCav. This gave us a voltage on the RefCav TPD of 4.2V, which is pretty close to where it's been sitting since we turned the HPO on in April. The FSS was left in this condition and should be sufficient for ER10/O2.
HPO Pump Diode Currents
With time running out for the maintenance period, we decided not to increase the pump diode currents. They should be good where they are for the duration of ER10 and O2a, but will probably need to be tweaked up before the start of O2b.
Other Miscellanea
Peter measured the AC voltage difference between the PSL table top and AC power ground; this was measured at 108 mVAC. He also measured a 19 mVDC voltage difference between the PSL table top and the ISS AOM driver (the driver sits on the Southeast corner of the PSL table).
We left the PSL enclosure with all PSL subsytems locked and operational. We turned off both Mac Minis in the laser room, as well as the Mac in the anteroom, as is required for Engineering and Observing runs. Should anyone do any work in the PSL enclosure that involves turning these computers back on, please be sure to shut them down when you are done. This closes out LHO work permits 6300 and 6301.
