Sheila, Thomas, Elli, Evan
We locked the Y arm in IR, and then turned on WFS loops which feed back to IM4 and PR2 in order to keep the buildup in the arm maximized. We measured the dc counts on ASAIR_A_LF. Then we unlocked the arm and measured ASAIR_A_LF again. The results are as follows:
- ASAIR_A_LF_OUT, locked: Pon = 1205(5) ct
- ASAIR_A_LF_OUT, unlocked: Poff = 1300(10) ct
- LSC-Y_TR_A_LF_OUT, locked: 11.4(1) ct
Using the formula in LHO#15470, the locked and unlocked values of ASAIR give an equivalent loss of 267(31) ppm on ETMY.
To account for the power in the sidebands, we use the modulation depths given in LHO#15674: Γ9 = 0.219(12) and Γ45 = 0.277(16). Then the power in the sidebands is PSB = Poff × (Γ92+Γ452)/2 = 81(7) ct. Then using our new value for the power fraction, A2 = (Pon − PSB)/(Poff − PSB), we get an equivalent loss of 286(33) ppm on ETMY, not accounting for mode mismatch.
We also took loss scans by moving spot on ETMY in a spiral pattern, as in LHO#15476. The sideband power is subtracted here as well. It appears that judicious alignment of the arm may give us lower loss (something like 140 ppm), compared to the number reported above.
In the attached plot, I've masked out data points for which the transmitted power was below 11 ct.
As before, the zero point of the displacement is somewhat arbitrary; we performed the usual initial alignment sequence for the arm (baffle PDs for TMS and the ITM, then maximize the buildup of the green power), but didn't attempt to determine the location of the spots on the test masses.
Also note that for the formula in LHO#15470, the physically meaningful solution requires us to take the negative branch of the square root when computing A (so substitute A → −A in this formula).