Today Rahul and I worked on reducing the last bit of clipping in HAM7. The power budget shows a couple of percent more loss than we've expected previously, but I don't think we can improve it much right now so this is good enough. QPDA segment 3 is railed.
This morning we found that we had some loss on B:L2, see Rahul's power budget alog as of lunchtime: 88971. In hindsight, this was also possible in the power budget that Kar Meng reported before the suspension work this week, 88847. We translated the beam in the -Y direction in the SFI2 aperture, so it would make sense that we might need to translate the lens in the -Y direction. However, there is no space to move it because the mount is already as close as it can be to SFI2 (photo). So, we translated the beam using B:M3, using the power meter to judge when we were improving the alignment. We found that this aperture is very small, there isn't much of a plateau where we aren't clipping.
After doing what we could to reduce the yaw clipping here we did a careful power budget with the thorlabs power meter, we seem to have 8% loss to SQZT7, 11-14% loss from output of the OPO to the homodyne, which is a bit worse than past measurements ( 65066):
- Opo 0.97-1mw
- Input to sfi1 0.97 mW
- Output sfi1 0.95 to 0.97 mW
- Hitting zm1 0.95 to 0.97 mW
- Return from filter cavity after sfi1 0.92 to 0.97 mW
- After BL1 0.93 mW
- Input to sfi2 0.92-0.94 mW
- Output from sfi2 0.91-0.92 mW
- After B:L2 0.86-0.91mW
- After bm4 0.86-0.90 mW
- On the table bottom of the periscope 0.87-0.89 mW
- Before the homodyne 0.86 mW
- Refl power 24mW
We attempted to move B:M3 in pitch to see if we could reduce clipping on B:L2 that way, but that did not improve the transmission. In pitch we also saw that we increased the clipping with small moves in either direction. I will try to look up the beam size and aperture size next week to see if this makes sense.
We adjusted B:B4 and ZM4 a bit to align onto the two irises on SQZT7.
We also walked the two picomirrors used to center the FC QPDs, with the seed beam which saturates the diode. From the control room I reduced the power and was able to mostly center, but it seems that QPDA segment three has been railed at 33.9 since November 29th.
After we finished up, I went back to check the FCGS alignment onto SQZT7, which was not good, so I went back in for a few minutes to get that beam onto the filter cavity reflection diode. I also had a look at the red + green co-alignment, which looks similar to what Kar Meng posted in 88859.
We also checked that the new cable routing doesn't block the beam onto H:PD1, we couldn't see the beam but we think it looks like a clear path. Rahul also took a photo of where the beam exits SFI2.
EPO tagging.
From the alog 59333, the waist (w) is 700um at 54mm away from BL2.
Beam diameter is 1.4mm on the BL2. The diameter of lens aperture is 0.2" (~5mm).
From the Gaussian beam equation, the ratio of of the clipping is given by e^[-2(R2/w2)]. For a beam centered in the iris, the loss is 0%.
Here is a photo of BL2 taken from above. We wondered if the lens mount being tilted could make the aperture smaller explaining losses, but it seems that this tilt is too small to explain a loss ~1%
From the calculation for clipping loss, I plotted the graph for clipping loss for the IR 700um waist versus lens aperture radius. This gives us rough estimate the loss.
