Camilla, Sheila

This morning we aligned the PD for the light rejected by the OFI (B:PD1).  Camilla also aligned the beam dump to catch the reflection.  We measured 13uW on the PD, with the 30dB gain setting we had 0.057V on OFI PD A DC, and about 0.008V of that was offset from the room lights (the cleanroom lights were off so the room lights were dim). With the 2kOhm transimpendance, the responsivity would be 0.06A/W. This seems too low.  Camilla found the ICS link for this pair of diodes S1700519 which indicates that the PD is ffd-200h.

After this we spent some time moving mirrors in the D path for the QPDs, to try to get a longer distance from the VIP to the QPDs.  We are trying to lay out this path without the QPDs, but the space is a little cramped and we want to make sure that we will actually be able to fit the QPDs when they arrive.  60204 describes our first attempt and checks that the beam we are starting with is as expected. Attached are several photos that Camilla took, the second one is annotated.  In these photos ZM5 is not in it's final position, but the cookie cutter shows where it will be). We measured that currently we have 355mm from BS1 to D:M1, 410mm from D:M1 to D:M3, and at most 560mm from D:M2 to D:M3.  This would put the beam reflected off D:M3 4" from the corner of the ZM5 structure, looking at the drawing we think that might be a minimum amount of space to accommodate setting the QPD. That means that the maximum distance from BS1 to QPDB is something like 1.325meters, althoguh shortening this by a few inches would probably make fitting everything easier.

This layout also gives us about 6 inches from the back of the D:M1 pico to the beam around the location where the beam splitter will have to go.  Camilla found that the QPD is approximately 5.5" deep, (D2000246 and D2000373) so this is rather tight. 

Apparently Lee was thinking about this layout at the same time, and emailed us a new proposed lens solution, which I've attached here so that it's easier to find.  The solution in T1900649 figure 19 would require the two QPDs to be closer together than their encolsures will allow, so Lee is proposing that we need a longer lens, ROC 150mm. 

Rich Abbott also let us know in the meantime that the QPDs may be ready to ship in 2 weeks.  As long as the +X door is off, we can work on this path without disturbing ZM5.  The easiest way forward is probably to move on with the alignment of ZM4+5 and come back to this path in ~2weeks once we have the QPDs to place. In the mean time we will look for a 150mm lens to get ready for being installed in chamber.