[Hang, Craig, Gabriele]
Summary
We moved PR3, PRM and the soft degrees of freedom to improve the beam centering on all four test masses and increase the recycling gain.
We started with a recycling gain of about 38.2 and finished with a recycling gain of 47.4 [+24%]
A lock loss terminated our work, so there is still some re-alignment to do to get the test masses centered. And there might be more recycling gain.
We added dithering lines in yaw to all four test masses, and we could improve the centering significantly. At the end all Y2L coefficients are zero, and the dithering signals are quite close to zero. The two ITMs are reasonably centered, as is ETMX. Instead ETMY seems to have a larger mis-centering.
If we need to put back the Y2L coefficients, we probably only need to set ETMY to about 4.
Details
- We moved PR3 as we did in the past days.
- We had to compensate the PR2 alignment offset since the offload was clipped at the limiter
- From time to time we also adjusted the soft degrees of freedom position, by opening the loops, optimizing the power, and adjusting the QPD offsets
- We did the same with the POP QPD. Finally we were hitting a large offset, so we pico'ed the QPD
- We also moved the ALS POP QPD to keep the green beams in ggo shape on the cameras
- We also updated the green references, by moving the green QPD offsets and adjusting the camera set points [not quite to the last position, since we lost lock, but it should be quite close to good...]
We finally lost lock after more than 6h continuous lock.
Wow, very impressive. Hiro's simulations (G1700140) gave a PRG of only 39, though that was including an ad-hoc loss per arm of 50 ppm.
This is awesome!
Our PR gain is POP_A_LF / IM4_TRANS_SUM with calibrations to take into account reflectivity branching ratios, but we're quite close to the edge of IM4 trans. I think our normalized yaw number is more than 0.8. (We haven't pico'ed yet, to keep an alignment reference, but I think it might be about time to pico and say that this is better than O2.) I'm suspicious that our PR numbers that we've been reporting might be systematically high, if the number we're dividing by is smaller than the total actual amount of power in transmission of IM4.
That said, this has been true since our vent recovery (and also to a lesser extent during O2), so the +24% is certainly real, but the absolute values we've been reporting for PR gain might be high. We should pico and check the calibrations to be sure.
Looking at TR_X_NORM and TR_Y_NORM (which are normalized so that the single arm build ups are 1, at the time of the best recycling gain the build ups were 1646 and 1732 relative to the single arm, which would indicate a recycling gain of 45. Gabriele pointed out that the reflectivity of PRM is 3.1% according to galaxy, which means that this gives a recycling gain of 53.0
Another option for estimating the circulating power would be to try this method which has been done at LLO: 36745
WOW! Amazing!! Congratulations!!
Here's a quick computation of the power recycling gain as a function of the round trip losses in the arm (in ppm, sorry I forgot the units in the plot), assuming Tprm = 3.1%.
The two marked points correspond to the values of recycling gain estimated with POP_LF and with the arm transmission.
Dark offsets for TR_X and TR_Y are not very well set. The table below shows the TR values in different configurations, and the corresponding estimated recycling gain. Taking into account the dark offsets now the X and Y arm transmission give consistent recycling gains.
| Dark | Single arm | Full IFO | Recycling gain | |
|---|---|---|---|---|
| X | 0.045 | 1.00 | 1648 | 53.5 |
| Y | -0.007 | 0.98 | 1736 | 54.5 |
The REFL_A_LF also goes up by about 5% (relative to unlocked so it measures the ifo reflectivity) as expected. Great job!
From the level of reflected power (w.r.t. the off resonance state) we can estimate again the round trip losses.
The plot below shows that the REFL level we measured after re-aligning (8.3%) corresponds to about 61 ppm round trip losses, and a recycling gain of 53.4, which is very close to the recycling gain we can estimate from the arm power buildup.
The recycling gain improvement is seen in the CARM offset reduction sequence too. I ran the script that Sheila and Gabriele posted in alog-43344 on data from yesterday (starting at t = 1220674303).
This plots the reflected power (normalised) vs the arm build up (transmission normalised to a single arm). As we step through the CARM offset reduction, the TR_X increases and the REFL_DC decreases. The slope is an indication of the power recycling gain.
As of yesterday we're going better than the previous measurements, including compared to O1.
In addition to Georgia's analysis, here is a similar plot, with also some fit lines. The new curve is again consistent with no losses in the PRC and arm round-trip losses of about 66 ppm.
