Dave, Alexa, Evan
Summary
We’ve now completed a scan of the equivalent ETMY loss as a function of spot position on the Y optics.
- We locked the Y arm in IR.
- We turned on the REFL WFS loops in which actuate on IM4 and PR2. This keeps the pointing into the Y arm maximized.
- Then, using a spiral pattern, we adjusted the pitch and yaw of ETMY and ITMY in such a way so as to move the spot on ETMY (but not in ITMY). The total amount of excursion was about 4 urad in each direction.
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For each position, we let the configuration settle for 60 s (to let the WFSs catch up), and then recorded the counts on ASAIR using a 5 s
cdsutils.avg. - Afterward, we unlocked the arm and then recorded the power on ASAIR; it equals 1351(15) ct.
- With the above information, we can infer the total loss via the formulas in LHO#15470, again ignoring mode-matching.
Loss vs. alignment is given in the attached plot. The attached zip contains the data and the code used to perform the measurement. The measurement uncertainty is about 45 ppm, and comes from the uncertainty in the number of ASAIR counts with the cavity unlocked. Note that this plot is equivalent ETMY loss; i.e., all the observed loss (including power in the rf sidebands and mode-mismatched light) is assigned to the ETM.
Details
- In order to move the spot on only the ETM, we scaled the angle steps θE and θI so that θE = θI × (4 km − ROCE) / ROCE, where ROCE = 2.24 km.
- For pitch, we stepped the ETM and the ITM with the same sign. For yaw, we stepped them with opposite signs. We think this gives the required soft-mode angular motion in pitch and yaw.
- As for the convention in the plot for the displacement on the face of the ETM: the displacement is found by multiplying the ITM angle by the cavity length. When we push positive in ITM yaw, we simultaneously push negative in ETM yaw. This pushes the cavity mode toward the left when looking at the ETM face-on. When we push positive on the ITM and ETM in pitch, this tilts the optics forward (the tops of each optic approach each other, and the bottoms recede), and thereby pushes the cavity mode downward.
We are repeating the measurement for the ITM. Then the script will try to get a bigger spiral on the ETM.
Please do not unlock the Y arm for the next 200 minutes.
I’ve attached the results of Friday night’s ITM scan data (first attachment). The arm unlocked shortly into the ETM scan, so there is no data there. Note that the axes indicate the ITMY spot position, but the quantity plotted is the equivalent ETMY loss (for the sake of consistency with the previous measurement).
First, the data indicate that the spiral is centered around an especially lossy spot for the arm. Second, it seems that sweeping the spot on the ITM can change the measured arm loss by several hundred ppm. That magnitude seems comparable to the effect of sweeping the spot on the ETM, as we measured on Friday. There are several possible things we might conclude from this:
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The ITMY coating also has anisotropic loss.
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Moving the spot on ITMY is causing clipping somewhere (e.g., the edge of the optic).
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The scan strategy is also moving the spot on ETMY. Since the ROCs of the ETM and ITM are similar (2.2 km and 1.9 km, respectively), we require the alignment sliders to be calibrated to better than 7 % of each other in order for this to work [since (4 km - ROC_I) / ROC_I = 1.07].
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The unlocked value of ASAIR is changing with alignment.
To test the last of these, I locked the Y arm, turned on the WFS loops, and looked at the locked vs. unlocked values of ASAIR for several different arm alignments (separated from each other by 2 urad in pitch/yaw). I first measured ASAIR while locked. Then I held the outputs of the WFS loops, unlocked the arm, and measured the ASAIR value again. I found consistently that the unlocked value was 1370(15) ct. So it seems this is not the issue.
To test the idea that the spot location on ITMY is causing clipping, I adjusted the arm alignment to give lower loss. I ran the ITM sweep again (second attachment). Here the overall loss values are much lower (as low as 550 ppm), but again we find a variation of 100 ppm or so.
The limiting aperture for an ITM scan is probably the BS baffle. Not sure we can (yet) conclude that these losses come from the ITMY.
Here are the expected clipping losses as a function of offset from the mirror center. This assumes a 62mm radius beam on a 326mm radius optic coating. The pdf shows the clipping loss as a function of offset, and the other attachment is an animation showing the intensity spilled over the edge as the offset is increased (since gifs are all the rage these days).
