[Betsy TJ Koji]

We measured the optical powers in HAM6 before OM1-3 and OMC as well as at the transmission of OM1 and OM3.
Taking the ratio of these powers and AS_C QPD SUM, we get the calibration of the optical powers in HAM6.
This will be used for throughput calculation of the entire IFO output optics.

The ratio is

(Power into HAM6)/(AS_C QPD SUM) = 0.0162 +/- 0.0005 [mW/cnt]

Caveat: Our measurements were dominated by the systematic error of the power meter (as usual). Estimated size of the error is +/-3%.
Without having a better power meter, I don't expect I can improve this number.

As you can see in the spread sheet, each measurement has small statistical error like 0.1~0.5%. The numbers for AS_C QPD SUM was taken by the data downloaded from CDS.

However, two measurements of the optical power before OM1 (blue) showed 5% discrepancy. Also tracing the power along the main HAM6 path, the fluctuation of the power measurement (blue plot) exceeds the expected reduction of the power (orange plot) by measured transmissions. (Note that OM1 and OM3 has 800ppm and 1.6% transmission, that is consistent with the measurement in 2015.)

Therefore, this big fluctuation was concluded to be a systematic error of the measurement, particulary of the power meter (Ophir diode type sensor head), considering the stability of the AS_C QPD SUM.
From the two measurement of the optical power before OM1, and the plot, the systematic error was estimated to be +/-3%.