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 Thu Sep 29 20:21:29 2016, Johannes, Update, General, YARM loss measurement Mon Oct 3 21:24:02 2016, Johannes, Update, General, XARM loss measurement Tue Oct 4 22:18:24 2016, Johannes, Update, General, X/YARM loss measurement Wed Oct 5 19:10:04 2016, gautam, Update, General, Arm loss measurement review Mon Nov 14 19:15:57 2016, Johannes, Update, General, Achievable armloss measurement accuracy Tue Nov 15 20:35:19 2016, Johannes, Update, General, Achievable armloss measurement accuracy Thu Nov 17 21:54:11 2016, Johannes, Update, General, Achievable armloss measurement accuracy Thu Jan 12 02:45:53 2017, Johannes, Update, General, Next armloss steps Fri Jan 13 08:54:32 2017, Johannes, Update, General, DC PD installed
Message ID: 12614     Entry time: Mon Nov 14 19:15:57 2016     In reply to: 12533     Reply to this: 12618
 Author: Johannes Type: Update Category: General Subject: Achievable armloss measurement accuracy

Looking back at elog 12528, the uncertainty in the armloss number from the individual quantities in the equation for $\mathcal{L}$ can be written as:

$\delta\mathcal{L}^2=\left(\frac{T_1(1-\frac{P_L}{P_M}-2T_1)}{4\gamma}\right)^2\left(\frac{\delta T_1}{T_1}\right)^2+T_2^2\left(\frac{\delta T_2}{T_2}\right)^2+\left(\frac{T_1(1-\frac{P_L}{P_M}-T_1)}{4\gamma}\right)^2\left(\frac{\delta\gamma}{\gamma}\right )^2+\left(\frac{T_1}{4\gamma}\right )^2\left[\left(\frac{\delta P_L}{P_L}\right )^2+\left(\frac{P_L}{P_M} \right )^2\left(\frac{\delta P_M}{P_M}\right )^2\right ]$

Making some generous assumption about the individual uncertainties and filling in typical values we get in our measurements, results in the following uncertainty budget:

$\delta\mathcal{L}^2\approx\left(12\,\mathrm{ppm}\right)^2\left(\frac{\delta T_1/T_1}{5\%}\right)^2+(0.7\,\mathrm{ppm})^2\left(\frac{\delta T_2/T_2}{5\%}\right)^2+\left(2\,\mathrm{ppm}\right)^2\left(\frac{\delta\gamma/\gamma}{1\%}\right )^2+\left(140\,\mathrm{ppm}\right )^2\left(\frac{\delta P/P}{2.5\%}\right )^2$

In my recent round of measurements I had a 2.5% uncertainty in the ASDC reading, which completely dominates the armloss assessment.

The most recent numbers are 57 ppm for the YARM and 21 ppm for the XARM, but both with an uncertainty of near 150 ppm, so while these numbers fit well with Gautam's estimate of the average armloss via PRG, it's not really a confirmation.

I set the whitening gain in ASDC to 24 dB and ran LSC offsets, and now I'm getting a relative uncertainty in measured reflected power of .22%, which would be sufficient for ~25ppm accuracy according to the above formula. I'm going to start a series of measurements tonight when I leave, should be done in ~2 hours (10 pm) the latest.

If anybody wants to do some night work: I misaligned ITMY by a lot to get its reflection off ASDC. Approximate values are saved as a restore point. Also the whitening gain on ASDC will have to be rolled back (was at 0dB) and LSC offsets adjusted.

ELOG V3.1.3-