craig.cahillane@LIGO.ORG - posted 23:06, Friday 04 June 2021 - last comment - 16:51, Thursday 02 December 2021(59142)
Power in the interferometer - Pre-O4
Recently, we reported the results of the arm power measurement during a time with high-noise DARM to be around 330 kW (alog 58804). However, later measurements did not confirm these results. After some thought, 330 kW in the arms was impossible (with 40 W input). We agreed with them too quickly when they agreed roughly with our "back of the envelope" calculations, which ran as follows:Back of the envelope -------------------- Input power on PRM: 39.2 W Power recycling: 57.2 Arm gain: 265 Estimated arm power = 300 kWThe latest arm power measurements suggest an arm power closer to 261.0 +- 1.3 kW. Full results are reported at the bottom of this alog. Varun has upgraded the arm power measurement to use every segment of the two QPDs at the end of each arm (16 total TFs). I've processed these measurements, removed TF bias by estimating the TF via the excitation, and found the weighted average of their results. The fourth attachment shows the arm power estimate TF for just the Y-arm A QPD, to give the idea behind the estimate. All results are the on LHO CDS machines at/ligo/gitcommon/labutils/arm_power_measurement/plots. The code is committed at https://git.ligo.org/aligo_commissioning/labutils/-/tree/master/arm_power_measurement.Coupled cavity --------------The issue with the back of the envelope above is theArm gainnumber, which assumes perfect coupling of power into the IFO. We have to consider the coupled cavity, and the new lower arm losses. Our current PRM transmission is 3.1%, and the ITM transmission 1.5%. Given these parameters, and our measured PRG, our round-trip arm loss is 34 ppm. Using the arm power, our round-trip arm loss is 46 ppm. Using the estimated reflected power of 9% from alog 59058, the round-trip arm loss is 40 ppm. These arm losses are remarkably low: Advanced LIGO design documents anticipated 75 ppm round-trip losses in e.g. DCC T1000298, Section 2.2.PRM Transmission ----------------Finally, for any of these given arm losses, it's clear that our PRM transmission is too high. The PNG attached below indicates that if we assume 20 ppm single-testmass losses (yellow line), our reflected carrier is minimized for Tprm ~ 1.7%. As it stands, ~9%, or 3.5 W, of our incident carrier light is promptly reflected. If we do transition to a PRM with transmission 1.7%, it's possible that Parm = 305 kW could be achieved (with the same input power of 40 W). The mathematica notebook is committed at https://git.ligo.org/aligo_commissioning/power_budget, and also attached below.New arm power results --------------------- DARM/H1:ASC-X_TR_A_SEG1_OUT Arm Power Estimate = 258.649 +- 3.567 [kW] DARM/H1:ASC-X_TR_A_SEG2_OUT Arm Power Estimate = 272.490 +- 7.995 [kW] DARM/H1:ASC-X_TR_A_SEG3_OUT Arm Power Estimate = 265.565 +- 6.931 [kW] DARM/H1:ASC-X_TR_A_SEG4_OUT Arm Power Estimate = 257.086 +- 5.114 [kW] DARM/H1:ASC-X_TR_B_SEG1_OUT Arm Power Estimate = 262.114 +- 3.735 [kW] DARM/H1:ASC-X_TR_B_SEG2_OUT Arm Power Estimate = 276.812 +- 5.655 [kW] DARM/H1:ASC-X_TR_B_SEG3_OUT Arm Power Estimate = 274.728 +- 5.965 [kW] DARM/H1:ASC-X_TR_B_SEG4_OUT Arm Power Estimate = 254.638 +- 3.776 [kW] DARM/H1:ASC-Y_TR_A_SEG1_OUT Arm Power Estimate = 255.051 +- 4.339 [kW] DARM/H1:ASC-Y_TR_A_SEG2_OUT Arm Power Estimate = 261.821 +- 4.145 [kW] DARM/H1:ASC-Y_TR_A_SEG3_OUT Arm Power Estimate = 270.828 +- 4.383 [kW] DARM/H1:ASC-Y_TR_A_SEG4_OUT Arm Power Estimate = 248.732 +- 5.074 [kW] DARM/H1:ASC-Y_TR_B_SEG1_OUT Arm Power Estimate = 254.859 +- 4.259 [kW] DARM/H1:ASC-Y_TR_B_SEG2_OUT Arm Power Estimate = 268.231 +- 5.189 [kW] DARM/H1:ASC-Y_TR_B_SEG3_OUT Arm Power Estimate = 269.522 +- 3.745 [kW] DARM/H1:ASC-Y_TR_B_SEG4_OUT Arm Power Estimate = 251.185 +- 3.503 [kW] X Arm Power Estimate = 262.3 +- 1.7 [kW] Y Arm Power Estimate = 259.9 +- 1.5 [kW]References ---------- ## DCC design documents - Advanced LIGO Length Sensing and Control Final Design - Arm Cavity Finesse for Advanced LIGO - Advanced LIGO Systems Design ## papers - Demonstration of light recycling in a Michelson interferometer with Fabry-Perot cavities ## alogs Pre-O4 RF powers (alog 59058)
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Comments related to this report
These loss numbers do not include the transmission through the ETM, which I have assumed is T_ETM = 5 ppm. Trans through the ETM is usually grouped in with arm losses, so add 5 ppm to all the results above.
Sheila, Craig
In my above post, I assumed carrier PRC losses = 0.5%. This assumption is founded on very little, and matters for what the PRM transmission should be for critically coupling the IFO (not that we want to critically couple the IFO).
Sheila made some contour plots of the power in the interferometer (power recycling gain, arm power, and reflected power), vs arm losses on the x-axis and BS losses on the y-axis.
Then we compared each of them with the actual measurements made for each of them above.
Measurements
============
Input power = 39.2 W
PRG = 57.2
Arm power = 261 kW
Reflected power ratio = 9% of input power
Model
=====
Again, the model we used was just a simple coupled cavity (three mirror cavity). We also assumed plane waves, i.e. no mode mismatch between input->prc->arms.
In this case, the reflected power, PRG, and arm power give us degenerate information. Unfortunately, all disagree, with the PRG and arm powers giving a 12% difference.
Measurement methods
===================
Daniel estimated the reflected carrier power: https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=59058
I verified the PRG: https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=58327
twice: https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=52267,
and above I reported Varun's arm power measurements.
The arm power measurement may be the most accurate here, because it is based on differential power fluctuations in the arms causing the mirrors to move via radiation pressure.
Therefore, only TEM00 light in the SRC due to the DARM offset can return to the arms with sufficient efficiency (and in the right quadrature) to actually create motion.
PRG is a power ratio measurement between single arm and full ifo, but could be adversely affected by HOMs due to thermalization, i.e. junk light causes us to overestimate the true TEM00 light in reflection or the PRC.
Reflected power is also a power ratio estimate, done by changing the RF modulation depths, and should be pretty accurate except for HOMs.
Finally, for our arm power estimate models we need in the input power very accurately to infer losses.
We don't know our input power very well, because of uncertainty in how much light makes it through the IMC and input optics.
For instance, according to our model, if the input power is 12% lower than what we think (34.5 W instead of 39.2 W) then the arm power and PRG measurements agree.
The last time input power was calibrated was here: https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=46577.
Future work
===========
1) Verify the input power calibration while the HAMs are open
2) Use 9 MHz to estimate PRC losses (for 9 MHz at any rate, which may not be the same as PRC losses for carrier)
3) Analytic mode matching model between input -> PRC -> arms (could we use some of this SRC -> arms MM stuff going around?)
4) Finesse model of a three mirror cavity
Git repo
========
https://git.ligo.org/aligo_commissioning/power_budget
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