Kiwamu, Jeff, Evan

The calibration group has known since O1 that Hanford (but not Livingston) has an anomalous loss of gain in the DARM optical plant at 10 Hz and below. This can be explained by 0.5° of positive (antispring) SRC detuning away from pure RSE.

The first attachment shows a loop-corrected pcal sweep from O1, calibrated into mW/pm. On top of this I have plotted my guess at what theory curve this corresponds to (from Rob Ward's thesis), using 700 W of beamsplitter power, an arm pole of 42 Hz, an SRM transmissivity of 37 %, a homodyne angle of 90°, and a one-way SRC carrier phase of 90.5°. (The theory curve is pretty much insensitive to variations in the homodyne angle at the few-degree level, and we know that the homodyne angle deviates from 90° by less than 3°, since we ran with 20 mA of dc offset light and there is less than 1 mA of contrast defect light.)

To test this, I took a new set of pcal sweeps at 10 W of input power, with several different SRC detunings. The result is shown in the second attachment, again with guesses about the theory curves. All are with 350 W of beamsplitter power, 42 Hz arm pole, 37 % SRM transmission, and 90° homodyne angle. 0 ct of SRCL offset corresponds to the green (90.8°) curve, –200 ct corresponds to the blue (90.1°) curve, and +200 ct corresponds to the red (91.5°) curve. The implied calibration (0.1 ct / pm for the SRCL error point) is consistent with SRCL OLTF budgeting. The fact that 0 ct of SRCL offset produces a nonzero RSE detuning is perhaps not surprising, since we have never had good angular control of the SRM.