This is a brief and preliminary update of the calibration activity from today. I calibrated the ITMX and ETMX reponses using the usual free-swing Michelson fringe.

If I believe the measurement, I must have underestimated the ESD response by a factor of 5.3 (!?) in the previous calibration which is hard to believe for me. I would like to repeat the measuerment perhaps with different conditions (e.g. opelv on/off, L2P off, linearization off/on, different bias, different frequencies and etc) and on ETMY as well.

(MICH free swing)

The method is the same as what Joe described in LLO alog 14135. To obtain the ASQ_pkpk value, I did not run the fancy matlab code or anything, but I just picked up a highest peak value and lowest one in H1:LSC-MICH_IN1_DQ. The alignment was adjusted beforehand by locking MICH. The pk-pk value was measured to be 27.0 counts. Using the relation, d (ASQ)/d(ITMX) = 2 * pi * ASQ_pkpk / lambda, I get

ASQ optical gain = 1.59 x 10⁸ cnts/m

The input power to IMC was at 9.6 W, measured at the periscope bottom PD. ASAIR_ALF could get to 4550 counts at maximum and ASAIR_B_LF 1500 counts when MICH was freely swinging. The below are some dtails:

• ASAIR_A_RF45 whitening gain was set to 30 dB
• Whitening stage 1 and 2 were engaged (probably this was too much as it seemed already limited by some elecronics noise).
• I attach the time series.

(ITMX L2 stage calibration using MICH)

After locking MICH, I shook ITMX L2 stage at H1:LSC-SUS_ITMX_L2_LOCK_L_EXC with a drive level as high as possible without DAC saturation. I did a swept sine measurement to check how high frequency I would be able to get without loosing good signal-to-noise ratio. It seems that exctiation above 20 Hz is hopeless -- the drive signal dives into sensor noise. From this measurement I picked up one frequency point, 13.05 Hz where the ITM response was measured to be

ITMX L2 response = 8.41 x 10^-18 m/cnts @ 13.05 Hz

• L2 coil driver state was set to 2 to get the maximum range.
• No oplev damping loops were engaged.
• MICH loop correction was applied in the estimation of the L2 stage response as was done in Joe's alog 14135. Abs(1/(1+G)) = 0.817 @ 13.05, measured.
• I did not oberve a DAC saturation during the measurement.
• I attach a dtt file of the open loop measurement and ITMX response measurement.

(ETMX calibration using X arm)

Keeping the 9.6 W incident power, I locked the IR laser to the X arm with a UGF of 100-ish Hz. I did a swept sine measurement on ITMX and ETMX at different times but in the same lock strech. On ITMX, the L2 stage was driven again with the same setting as that of the MICH locking. On ETMX, I had set up the suspension filters such that they are the same as the full locking condition (e.g. drive signal goes not only ESD but also L1 stage and so on). Neverthelss, since my swept sine measurement does not go below 10 Hz, the ETMX response essentially represents the ESD response (with a small effect from the L2 stage which is almost two orders of magnitude smaller than the ESD in terms of displacement).

Taking the ratio between the two actuators, I confirmed that the ratio goes as f^2 as expected in a frequency range from 10 to 60 Hz. The ETMX/ITMX ratio was measured to be

ETMX_L3 / ITMX_L2 = 1.70 x 10² @ 13.05 Hz

ETMX_L3 response = 1.43 x 10^-15 m/cnts @ 13.05 Hz. This is almost 5.3 times stronger than what we have in the CAL-CS calibration.

• IM4 and PR2 were controlled by the REFL WFSs in order to maitain the high build up in the X arm all the time during the measurement.
• ETMX oplev pitch damping loops was on at the L2 stage with a gain of -9500.
• The swept sine excitation was injected at SUS_ETMX_L3_LOCK_L.
• The L2P and L2Y were kept engaged on L3 stage on ETMX.
• The L1 coil driver state was set to 2.
• The bias voltage was set to 9.5 V.
• The ESD liearization was on with a force coefficient of -1.8e5.
• I did not see a DAC saturation on ETMX.
• I attach the dtt files.