For the TCS+ISC team
With the success of increasing the power last night to about 17 Watts input, the chance to make an absorption measurement for the ITMs became a reality and we were able to use both Hartmann sensors to extract a spherical power which we fit to a Comsol model, this technique is the same as Aidan's and Georgia's method.
ITMX absoprtion: 206 +/- 1.8 ppb
ITMX_RH tuning for 50 Watts: 0.44 Watts Total
ITMX_CO2 pre-loading: 1.03 Watts Central
ITMY absoption: 454 +/- 1.5 ppb
ITMY_RH tuning for 50 Watts: 4.19 Watts Total
ITMY_CO2 pre-loading: 2.27 Watts Central
Plan for tomorrow:
- We'll plug in these values for the ring heater tonight, by the morning they should be relatively close to thermal equillibrium.
- Turn up the power of the CO2 lasers, and measure the simple michelson contrast defect. If it works, we should be able to lock at 2 Watts.
Details
I was hoping to figure out a way to estimate the uncertainty in this measurement but it's a little bit more involved than I thought, the method is essentially using a least squares fit to get the coefficients for the exponential decay of the thermal lens after losing lock, but the residuals seem to be pretty good which leads to about a 1% accuracy which should be taken with a large grain of salt because the absorption estimate is made by interpolating the scale of the thermal lens' exponential decay and then dividing the coefficient by the arm power in Watts. However, the arm power is an estimate as well with its own error bars which I'm still trying to figure out.
P_arm = 17 Watts * 45(PRC_Gain) * 0.5(BS) * 282(Arm_gain)
Once we got estimates for the absorption (1st and 2nd attachment), we re-ran the calculations which gave ring heater settings to compensate for the 50 Watt nominal power based off the absorption. ITMY ring heater requirement is particularly high because of excess absorption. Also to pre-load the system, the CO2 laser settings are also calculated (3rd and 4th attachment).
Some extra Hartmann plots from last night's power up:
1. Spherical power for ITMX and ITMY as we power up and cool down. ITMX has glitches when the laser power is changed.
2. PRISM values for ITMX and ITMY as we power up and cool down (second and third row of plots) with spherical power (top row) for reference, and power and POP 18 sidebands for reference.
3. Contour plots of the ITMX cool down. Top plot is using "current" and reference times both in the cool down, bottom plot using a "current" time just before we lost lock, when the spherical power was ~5e-6, not sure if this is legit.
4. Contour plots of ITMX cool down.
Last night before we lost lock we increased the power to 19 W and decreased it to 17 W shortly after. This made the absorbed power in the optic a little confusing. This evening we powered up to ~15 W and stayed there for an hour before losing lock, to let the thermal lens settle before powering down.
The ITMY spherical power for this cool down was consistent with previous measurements however ITMX is even more confusing, with the exponential decay not obvious in the spherical power, see attachment which can be compared to the second attachment of my last comment which talks about last night's data.
The ITMX Hartmann has an iris directly before the camera which is blocking a ghost beam, and the beam also reflects off the beamsplitter, which underwent some alignment changes recently. Perhaps we should check the iris centering on table again. A quick stream of the Hartmann image (with the plate still on, so it only shows an array of dots) shows nothing amiss though.
For now TVo has used last night's 17 W cool down data to measure the ITM absorption.
