[Aso, Nakano, Michimura]

We measured the optical throughput from BS output to the DARM photodiode (AS_PDA1).
Using independently measured PRM, ITMX and BS reflectivity/transmission, the measured value was about 4.6 %.
We have unknown 60% optical loss.

Details:
- Measured the power at AS PDA1 and PDA2 with ITMX single bounce (see attached).

- From K1:LAS-POW_IMC_DC_OUTPUT, the IMC output power during the measurement was 3.7 W, which gives 0.38 W at BS, assuming PRM transmission is 10.35% (JGW-L1605744) and there's no other large loss. This also gives 95 mW at BS output in ITMX single bounce.

- The measured power at AS PDA1 and PDA2 was 4.4 mW and 2.0 mW, respectively, as reported in klog #11756.

- This means that the optical throughput from BS output to AS PDA1 is 4.4 mW/95 mW = 4.6%.

- Since we have T=30% SRM, we don't currently have OSTM installed, and we have two beamsplitters in the AS table before PDA1, the expected optical throughput is 0.3*0.5*0.5 = 7.5%.

- This means that we have unknown 4.6/7.5 = 60% loss, which sounds kind of OK since there's OFI, viewports etc.

Next:
- Update the shot noise of the noise budget