H:PD1 calibration

This morning we checked the calibration of H:PD1, the filter cavity green beam fiber rejected polarization (H1:SQZ-FC_FIBR_REJECTED_DC_VOLTS).  We measured 640nW on the diode, 0.02492 V with the input beam to the fiber blocked, and 0.0325 V with wave plates rotated to increase power on H:PD1, with 30dB gain setting and 2kOhm transimpedance. This is a responsivity of 0.2 A/W

Beam profile incident on ZM2:

We measured the beam profile of the beam incident on ZM2:

Double check for clipping, finalized alignment of ZM1,2,3

Because we saw some astigmatism, we wanted to check that we were not clipping our beam and were well centered on our clear aperture.  Georgia did another alignment move of ZM2+3 pitch while dithering ZM1 pitch, similar to the way that she and Vicky reduced the clipping on Tuesday afternoon.  We still saw astigmatism after that, and Keita Vicky and I did a double check for clipping by moving ZM2+3 in 100urad steps until the beam is totally clipped. This table shows what each beam clipped on on the SFI, the ones that say aperture stop are the aperture stop at the input to SFI2, several of these Keita said that it could have been either that aperture stop or the polarizer for SFI2.  It seems like a good thing that SFI2 (or it's aperture/polarizer) is most often the limiting aperture, reassuring us that we haven't limited our aperture by mis centering on aperture stops.  The final row of this table shows the center of the two clipped positions, they are close to what Georgia had set the sliders to using the dither technique.  It is reassuring that the two methods agree, so we will keep these slider values for ZM1, 2 and 3.  ZM1 is at 0P,0Y.  These are accepted in SDF, we also looked at the centering of the beam on ZM1,2,3 which seems good.  We will need to move ZM4+ZM5 to center on the irises we set as a reference for our alignment through HAM5.

 Beam size measurements leaving HAM7 towards HAM5

We set up the beam profiler to look at the beam heading towards HAM5, with the idea that we would adjust the voltage on ZM2 to find a nominal value that reproduces the beam that we measured with the flat retro reflector in place of ZM1.  (alog 60665  With 100V on ZM4 and 120V on ZM5, horizontal 13.5% diameter is 2122um, vertical 2148um, with 100V on ZM4 and 60V on ZM5, horizontal 13.5% diameter is 1963um vertical 1988um, this was measured with the nanoscan head 26cm from the edge of the ISI.)  We see that the beam is astigmatic here, more that it was in November and more than the beam incident on ZM2.

Here is a table of measurements where we adjusted the voltage on ZM2, the strain gauge readback is in parenthesis.  We attempted to maintain the alignment by adjusting the ZM2 pitch slider to keep the osem readback the same as we changed the voltage applied to the SAMS.  At the lower voltages on ZM2 we saw poor beam quality, which might be impacting the diameter measurements here.  We expected the lower voltages on ZM2 to be un-usable because we can see that the beam becomes large enough to clip on the VIP, Lee confirmed that this is expected.  Because of the astigmatism we see, we don't have a definitive awnser about where we want to set the ZM2 voltage to be nominal, but we can see that the nominal voltage that we will want will be near the top of the current available voltage (117V is the max we can apply right now).  It seems worthwhile to think about making the changes we need to be able to go to higher voltages on ZM2.

Our plan from this point was to set ZM2 to a nominal voltage, profile a couple of points on the beam reflected off ZM5.  We were then going to use this location (26cm from ISI edge) and do a scan of ZM4 +ZM5 votlages now that the strain gauge readbacks are working (61033) for a calibration.  We ran out of time here, and might not come back to laser hazard in HAM7 until Jan.