Here are the results from EndX ETMX BSC9.  The HEPI Tilt Decoupling improvement approaches x10.  Looks like the crossline still couples at 0.1 hz though.  Maybe further tuning could be of benefit.

Stefan, Evan

Since we turned off the TCS last night and left PRMI locked on carrier, we have roughly 8 hours of good data that tells us the (1) the contrast defect and (2) the behavior of the ITM thermal lens as a function of time. To extract these quantities, we did the following:

• We extrated minute trends of LSC-POP_A_LF_OUT, LSC-REFL_A_LF_OUT, and LSC-ASAIR_A_LF_OUT from last night.
• We calibrated POP_A power to PRC power as follows:
  • FM10 already calibrates the POP_A counts into microwatts of power incident on the PD.
  • In Kiwamu's recycling gain measurement (LHO#15793), he misaligned PRM and locked MICH on a dark fringe. There was 7.2 μW on POP_A. Assuming a PSL power of 10.7 W (measured today), an IMC transmission of 83 % (LHO#9954), and a PRM transmission of 3.0 % (galaxy), this means there was 260 mW of power in the PRC.
  • The calibration PRC / POP_A is therefore 3.7×10⁴ W/W.
• We calibrated ASAIR_A power to SRC power as follows. Note that what I call "SRC power" is power leaving the BS going toward SRM (this is to distinguish it from "AS power" seen after the SRM).
  • ASAIR_A is already calibrated into microwatts of power incident on the PD.
  • In Stefan's contrast defect measurement (LHO#15332), he found the maximum SRC power from the free-swinging michelson was 1890 ct on ASAIR_B (not A). Today I measured the calibration to A is ASAIR_A / ASAIR_B = 3.52 ct/ct. The maximum SRC power from the free-swinging Michelson I take to be 10.7 W × 0.83 × 0.03 × 0.986 (this last number is the ITM reflectivity).
  • The calibration SRC / ASAIR_A is therefore 39 W/W.
• We calibrated REFL_A to REFL (i.e., the power heading from the PRM to IM4) as follows:
  • FM10 already calibrates the REFL_A counts into milliwatts of power incident on the PD.
  • In yesterday's RFAM measurement (LHO#15781), we measured 81 mW on REFL_A with the PRM aligned and the ITMs misaligned. The REFL power I take to be 10.7 W × 0.83 × 0.97.
  • The calibration REFL / REFL_A is therefore 106 W/W.
• It can be shown that the ratio P_SRC / P_PRC = T_M R_I, where TM is the carrier power transmissivity of the Michelson, and R_I is the power reflectivity of each ITM. I assume R_I = 0.986. Therefore, we can solve for T_M as a function of time given the calibrated power measurements. This is given in the upper plot of the attachment. From last night's data alone, the starting value of T_M is 1.115×10⁻³. However, Daniel has previously corrected the contrast defect for the presence of the 45 MHz sidebands (LHO#15432), finding a defect of 0.99×10⁻³. In the plot, I have rescaled T_M by 0.99 / 1.115 to reflect this.
• To convert ths into a change into diopters according to Stefan's formula, we need to know the "best" contrast defect achievable (i.e., the residual defect assuming no RoC mismatch). Stefan's measurement (LHO#15332) showed a minimum in ASAIR_B of (−0.2 + 1.78) ct with the Michelson locked on a dark fringe. According to Daniel's analysis, 0.17 ct of this is due to the 45 MHz sideband. So the net minimum is 1.41 ct. With a bright fringe power of 1890 ct, I take the "best" contrast defect to be 8×10⁻⁴.
• By using this "best" defect and Stefan's formula above, we compute change in the ITM thermal lens (in diopters) as a function of time. This is given in the lower plot of the attachment.

In principle, we can also use the above data to extract the mode-matching into the PRMI as a function of time. Perhaps we will pursue this later.

The carrier recycling gain was measured to be 45 with the three ASC loops closed in PRMI.

Please forget the previous measured recycling gain (alog 15527).

(Some numbers)

• POP_A increased from 7.2 uW (which I subtracted a dark offset of -6.3 uW) to 10350 uW
• According to the galaxy web page, I assumed Tp to be 3.1 %.
• Therefore Gp = 10350 uW / 7.2 uW x Tp =  45

• Also, for cross-check, I looked at POPAIR_A as well.
• POPAIR_A increased from 29.6 uW (with an offset of -0.8 uW being subtracted) to 43200 uW.
• This gives a recycling gain of 43500 uW / 29.6 uW x Tp = 45

Note that the IMC incident power was at 10 W during the measurement. REFL_LF dropped from a nominal of 83 mW to 6.3 mW when the PRMI was locked. The dark port ASAIR_A_LF stayed at 12000 counts during the lock. We could see a donuts mode at the dark port digital camera.

We shut off the ITMX CO2 laser at 20:00:40 in local time (4:00:40 UTC) for tomorrow's HWS project. We are leaving the PRMI locked on the carrier to see what happenes.

The lock held for about 8 hours, from about 8 PM to 4 AM local time.

We locked the PRMI on carrier. The carrier recyclying gain was measured to be 35 at highest. However, since the alignment was not perfect, it probably would go up. To be continued.

After playing with the gain settings, we eventually became able to lock the PRMI on carrier. However the alignment was not stable to keep it locked with high build-up. I think this needs more study to understand what is going on. Anyway, so far, the highest buidl-up in POPAIR_A_LF we had tonight was about 3.5x10⁴ uW. When the simple MICH without power-recycling was locked, POPAIR_A_LF was about 30 uW. Assuming that there is no mode-mismatch and Tp=0.03, we get a recycling gain of 3.5x10⁴ / 30 * Tp = 35. 

LSC settings:

• set LSC_CONFIGS to PRMI_SB_OFFLOADED
• flip the PRCL control sign so that the PRCL_GAIN is -11
• switch the sensor for MICH from REFL45 to AS45Q with an input matrix element of 1.
• set LSC-MICH_GAIN to -100.
• flip the sign of POPAIR_B_RF18_I in the trigger matrix so that it locks when RF18 is negative.

Attached are the OMC scan results for a PRMI sideband lock, compared to a scan from a single-bounce beam.  The first plot shows the results of three single-bounce scans and three PRMi scans (100 second ramps of PZT2); the second plot has averaged the traces.  The PRMI data appears to be shifted upwards compared to the single-bounce data, by about 1V in the PZT2 output.  We expect some drift and hysteresis in the PZT, but the single-bounce data was taken immediately after the PRMI lock, and a shift of this size is...surprising.

Using Kiwamu's expression from alog:14532, I calculate the PRC gain of the 45MHz sideband to be about 11.8 or 14, depending on which sideband peak you use.  I think this is lower than we expect.  The PRMI sideband lock was quite wobbly with a lot of angular motion, we might get a more robust measurement by locking the OMC to a particular mode and maximizing the transmission.

Here is a table of peak heights:

With a Schnupp asymmetry of 9.5cm the gain, for example for the upper 45MHz sideband, is (4.7/0.31) * (0.03*0.5*0.5) * (1/sin(2*pi*0.095*5*9100230/c)**2) = 13.9.

Just for a book-keeping purpose:

Two weeks later from this entry, we have measured the recycling of the carrier with the ASC loops fully engaged. We measured it to be 45 (see alog 15793).