It turned out the 'ringing' was caused by the respective other ETM still being aligned. For these reflection measurements both test masses of the other arm need to be misaligned. For the ETM it's sufficient to use the Misalign button in the medm screens, while the ITM has to be manually misaligned to move the reflected beam off the PD.
I did another round of armloss measurements today. I encountered some problems along the way
- Some time today (around 6pm) most of the front end models had crashed and needed to be restarted GV: actually it was only the models on c1lsc that had crashed. I noticed this on Friday too.
- ETMX keeps getting kicked up seemingly randomly. However, it settles fast into it's original position.
General Stuff:
- Oscilloscope should sample both MC power (from MC2 transmitted beam) and AS signal
- Channel data can only be loaded from the scope one channel at a time, so 'stop' scope acquisition and then grab the relevant channels individually
- Averaging needs to be restarted everytime the mirrors are moved triggering stop and run remotely via the http interface scripts does this.
Procedure:
- Run LSC Offsets
- With the PSL shutter closed measure scope channel dark offsets, then open shutter
- Align all four test masses with dithering to make sure the IFO alignment is in a known state
- Pick an arm to measure
- Turn the other arm's dither alignment off
- 'Misalign' that arm's ETM using medm screen button
- Misalign that arm's ITM manually after disabling its OpLev servos looking at the AS port camera and make sure it doesn't hit the PD anymore.
- Disable dithering for primary arm
- Record MC and AS time series from (paused) scope
- Misalign primary ETM
- Repeat scope data recording
Each pair of readings gives the reflected power at the AS port normalized to the IMC stored power:
which is then averaged. The loss is calculated from the ratio of reflected power in the locked (L) vs misaligned (M) state from
![\mathcal{L}=\frac{T_1}{4\gamma}\left[1-\frac{\overline{\widehat{P}_L}}{\overline{\widehat{P}_M}} +T_1\right ]-T_2](https://latex.codecogs.com/gif.latex?%5Cmathcal%7BL%7D%3D%5Cfrac%7BT_1%7D%7B4%5Cgamma%7D%5Cleft%5B1-%5Cfrac%7B%5Coverline%7B%5Cwidehat%7BP%7D_L%7D%7D%7B%5Coverline%7B%5Cwidehat%7BP%7D_M%7D%7D%20+T_1%5Cright%20%5D-T_2)
Acquiring data this way yielded P_L/P_M=1.00507 +/- 0.00087 for the X arm and P_L/P_M=1.00753 +/- 0.00095 for the Y arm. With  and  (from m1=0.179, m2=0.226 and 91.2% and 86.7% mode matching in X and Y arm, respectively) this yields round trip losses of:
 and  , which is assuming a generalized 1% error in test mass transmissivities and modulation indices. As we discussed, this seems a little too good to be true, but at least the numbers are not negative.
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