Sheila, Evan, Rana, Eli, Kiwamu, Alexa

Today we were able to reduce the CARM offset from 800pm to 100pm. We made three major changes to acheive such an offset.

1. We noticed that the sqrt(TRX+TRY) counts into the CM board was only about 100 pk-pk. We increased this to about 800pk-pk by reallocating some gain. We decreased the LSC-REFL_SERVO_IN1_GAIN from 0dB to -16dB, and increased the LSC-REFLBIAS_GAIN from 8 to 50.4.
2. We added a z20:p40 to REFLBIAS in order to get more phase and push the UGF of TR CARM from 200 Hz to 150 Hz. This filter is only engaged after we have already turned off ALS COMM, at which point we do not need to compensate for the COMM PLL VCO.
3.  We noticed that ASAIR_RF45_Q was rather noisy as we were reducing the CARM offset. We added an integrator FM2 (z2:p0) to ALS DIFF to reduce low frequency noise. This helped make TR CARM more stable.

Transition Process from 800 pm to 400pm

To start,

• LSC-TR_CARM FM1(2pm), FM9(z35:p1k^2) ON
• LSC-TR_CARM_OFFSET = -0.5
• LSC-REFLBIAS FM9 (z40:p1.6), FM3 (z40:p1) ON, GAIN 50.4
• LSC-REFL_SERVO_IN1 GAIN -16dB
• ALS-C_REFL_DC_BIAS GAIN 28
• TRAMPs at ~13 s

With some TR buildup (approximately 1ct on LSC-X/Y_TR_A_LF_OUT),

• LSC-REFLBIAS FM6 (z1:p0), FM4(z1:p40) ON, GAIN 28.0
• Ramp down LSC-REFL_SERVO_IN2 Gain from 16dB to -32dB then turn off ALS-COMM

With CARM on sqrt(TRX+TRY),

• LSC-REFLBIAS FM10(z20:p40) ON, GAIN 28 at -0.5 cts offset (Note: the gaurdian does up to this point)
• LSC-REFLBIAS GAIN 39 at -1.0 cts offset

Transition Process from 400pm to 100pm

With all the settings as above, we bring the CARM offset to -3.2 (or ~150pm). At this point we transitioned the in-air TR PDs to the IR TR QPD_Bs.  For reference, ASC-X/Y_TR_B_POW_NORM =1,1  and LSC-TR_X/Y_QPD_B_SUM_GAIN = 0.222, 0.175 respectively. Also, the QPD NSUM was about 400cts here. We continued to reduce the CARM offset to -4.5 cts (or ~80pm). At first,t the ASAIR_RF45_Q looked like a decent signal and responded well to a DIFF offset change. We turned on a servo for about 1.5min to ensure ASAIR was around zero (z servo -r LSC-ASAIR_A_RF45_Q_NORM_MON -g -11111 ALS-C_DIFF_PLL_CTRL_OFFSET).  We measured ASAIR_RF45_Q/DARM_IN1 = 3e7 (at this point DARM_IN1 is just ALS DIFF signal). Note: we added a -160dB FM10 filter in LSC-ASAIR_A_RF45. Then the PD_DOF_MATRIX for ASAIR_RF45_Q is 3.3. So far, we were able to transition from ALS DIFF to RF DARM 50% of the way, and then we broke the lock. We started to reconsider whether we were actually in the linear range for RF DARM. Maybe we should be using AS45Q/SQRT(TRX) at this point? This requires a model change; however, this is already impleneted in the l1lsc.mdl that is in the SVN, so we could just copy that.

A little more ...

The farthest CARM offset we reached was -7 cts or ~60pm on sqrt(TRX+TRY) with the QPDs controlling CARM and ALS DIFF for DARM. Here we had 26 x single arm power, and REFL DC dropped from 80mW to ~75mW.