Sheila, Kiwamu, Evan, Nergis, Alexa,

Since we got ALS diff working again, we moved onto the full lock attempt. So far we got back to a point where DRMI was locked with the arm cavities held by ALS at a off resonance. We locked DRMI twice this evening and each time DRMI dropped the lock right after the ASC loops came in. We need to look into this issue.

Even though we had aligned the DRMI before attempting the full lock, the DRMI looked misaligned a lot when we restored PRM and SRM. It seems that we ran a wrong ASC offloading script by accident in the initial alignment process which left a unneccessary offset in the M2 stage of PRM. We realigned PRM by misaligning SRM and locking PRMI while keeping the arm cavities locked. This gave us back good alignment in DRMI. However it took more than 15 minutes to get DRMI locked for some reason in both times. The DRMI unllocked right after the ASC loops came in both times. According to a brief look at the first lock loss, it seems that all three DRMI length error signals showed monotonically-increasing-oscillation for 0.5 sec before it unlocked (see the attached 0.5 -ish sec time series). Interestingly, IMC transmitted power increased at the same time by 10% or so. It is unclear what was going on at this point. Another thing is that it seems that the DRMI pulled ALS diff down when it unlocked and therefore this resulted in unlock of IMC.

I set all channels to be sdf monitored on h1psliss,fss,pmc,dbb using the "sdf_set_monitor 1 " command. I then unmonitored the 4 channels which are changing daily on PSL ISS

H1:PSL-ISS_REFSIGNAL, H1:PSL-ISS_SECONDLOOP_REF_SIGNAL_ANA, H1:PSL-ISS_SECONDLOOP_SERVO_ON, H1:PSL-ISS_SECONDLOOP_PD_SW

I loaded all four tables into the four FE models.

By switching the RY blend from 01_28 to 250mHz, we make the 0.6Hz peak disappear (see this log).

Until we find from where this peak is coming from, we'll stay in this new configuration on HAM3. The performance seems pretty similar, except in X where we reinject more noise at low frequency.

Expect the usual day shift and swing shift 1-3 Hz noise on Friday.  No weekend Hanford transportation activity is scheduled.  It's possible that Hanford hauling through the Wye barricade could decrease by 30-40% after March and make another significant decrease in July as excavation operations in the 300 area wind down.  Hauling from the 618-10 remediation area on Stevens will continue.  The uncertainty associated with these forecasts is large, however.

Sheila, Kiwamu, Evan, Nergis, Alexa

We have been able to lock ALS DIFF for +20 min.

• Daniel's tidal feedback is engaged for both the X and Y-arm. This is successful in bringing the DIFF PLL into range. We need to update the SUS screens to show that the tidal feedback is engaged; right now it just shows a large L drive align input even if the L1_LOCK_OUTPUT is zero.
• We increased the LSC-DARM gain from 400 to 600. The OLTF has a UGF of 8 Hz with a phase of 75 deg; consistent with what we had previously.
• We decreased the L1_LOCK_L_GAIN from 0.5 to 0.28 (reverting an increase we had done previously), to bring the UIM/ESD crossover to 1Hz.
• The L2P and L2Y filters are just a flat gain of 5.2, and -0.7 respectively, with FM10 (-60dB) enagaged. The L2P is not that great. Some times we have more success with FM1(L2P), FM2(L2P2) ON, FM10 OFF and a gain of -1. However, this has a high Q, and we tend to kick ETMX more when we lose lock. 

I have attached screen shots of the ALS DIFF filter configuration. In addition, I have attached the ALS_DIFF Spectrum, UIM/ESD crossover, and OLTF. The ALS DIFF spectra shows that the noise is suffeciently low to move onto locking.

[Koji, Dan]

This is a followup entry for LHO ALOG 16034.

Summary

- The OMC cavity was locked with PDH locking by implementing a bypass optical path from at the OMC REFL to the AS resonant RF PD.
- The OMC cavity length displacement was measured. It is found in the 4th attachment.
- It is mostly consistent with Zach’s measurement LLO ALOG 8674 and has x3 better floor level at some frequencies.
- There is a forest of peaks above 400Hz to 1.3kHz. They were very easily excited by light tapping on the HEPI crossbars

Motivation

- The length noise of the Output Mode Cleaners at LHO and LLO were so far locked with the transmission DCPDs with length dither or mid fringe with CDS.
- The measurement bandwidth with these techniques was limited by the CDS bandwidth (8kHz) or the dither frequency (2~3kHz). The cavity length noise above these frequencies wer e unknown.
- The measurements were also prone to the intensity noise on the beam. As the base band is at audio frequency in either cases, it is hard to be shot noise limited without proper setting of the intensity stabilization. Some features in the spectrum were not distinguishable from the intensity noise.

- PDH locking of the OMC was expected to provide an independent measurement of the OMC length noise with possibly better sensitivity, as the PDH locking is in principle insensitive to the intensity noise.
- In fact, the most of the conmponents for the PDH locking were already there. If we use the single bounce beam from one of the ITMs, the beam is already phase modulated. An RF PD is at the same table with the OMC REFL beam. The detection system and actuator are on the field racks next to HAM6. Therefore the effort of the PDH locking was minimal.

Configuration

- The ITMY single bounce beam was guided to the OMC. i.e PRM/SRM/ITMX/ETMX/ETMY were misaligned.
- The beam alignment to the OMC was controlled using OMC QPDs. The dither alignment servo has not been configured and was not functional at the time.
- The OMC REFL beam was aligned to the OMCR path on ISCT6 by moving an in-vacuum picomotor as Dan described as Dan described.

- The OMCR beam was introduced to ASAIR_A PD without moving existing optics on the table. As found in the figure (attachment 1), an additional optical path was added to the OMCR path. The OMCR beam was deflected between two lenses and brought to AS45 PD going through the space between the mirrors in the AS path. The beam on the PD was focused by a lens with the focal length of 150mm. This made the spot sufficiently small for the 2mm aperture of the PD.

- With the single bounce configuration, the optical power from the chamber was ~10mW.

- The servo configuration is found in the figure (attachment 2). The AS 45MHz demodulator was used for the PDH sensing (i.e. no rewiring was necessary). We found our bad luck that the proper demodulation phase was about 45 deg off and the signal size in the I and Q phases were almost the same with opposite sign. This meant that we could combine these two with another SR560. But we decided to use the I signal for the error signal.

- Since there is no digital signal path from LSC outputs to the OMC PZT, we implemented an analog servo. The error signal from the demodulator I-phase monitor channel was fed to an SR560 with gain of -2 and LPF (-16dB/Oct, fc=300Hz). The 50 Ohm output of this SR560 was fed to another SR560 with the gain of the unity. The second SR560 was used as a summing point for an openloop TF measurement. The 50Ohm output of the second SR560 was connected to an aux drive port of the HV driver.

Servo modeling

- You may wonder how just a 300Hz 2nd order LPF could make the servo stable!? In fact, we could lock the cavity even with gain of -1 with flat response. This is a subtle combination of the dewhitening and the poles and zeros formed by the PZT capacitance and the output RC network of the driver.

- The open loop transfer function of the servo was measured (attachment 3) by injecting the excitation at the second SR560 while the "after sum" (denominator) and "before sum" (numerator) signals were observed with SR785.

- Driver/actuator response: The HV Piezo driver (D060283) has two dewhitening stages and an output RC network. The dewhitening stage, which are common for the digital and external analog inputs, have two poles at 0.923Hz and two zeros at 10.15Hz with the DC gain of the unity. Note that the signal is reduced by a factor of 0.9989, as the input impedance of the driver (47.5kOhm) and the 50Ohm output impedance of the SR560 form a voltage divider. The main HV stage has the gain of 10. The output stage has the output series resister of 50k (R51) and then the parallel capacitors including the PZT capacitance of 0.51uF (Noliac NAC2124). (C11 - 0.47uF // C26+R55 - 0.47uF // Cpzt - 0.51uF). This imposes two poles at 2.19Hz and 502.1Hz, and one zero at 338.6Hz. Finally the OMC PZT2 has the calibration of 12.9nm/V (measured at Caltech), and the beam incident angle of theta = 4.04deg, and parasitic mechanical resonance of the PZT tombstone (pole at 9.5kHz Q=100 and zero at 11kHz Q=100). Don't forget that the factor of 2 i.e. cavity length change = 2/cos(theta) * PZT displacement

- The model of the openloop transfer function agrees exteremely well with the measurement. From this model, we determined the slope of the PDH signal to be 4.0e9 V/m.

Cavity displacemen noise

- Calibrated cavity displacement noise is found in attachment 4.
- The red curve is the error signal calibrated in the unit of displacement. The compensation of the loop supression was applied to this red curve in order to obatin the "estimated free running motion" of the cavity (Blue curve).
- The estimated cavity displacement seems to have better floor level by a factor ~3 compared to the half-fringe measurement at LLO. Also the spectrum below 300Hz looks cleaner and smoother. We wonder what is the cause of this noise.
- Similar to the LLO measurement, the spectrum has forest of peaks from 400Hz to 1.3kHz. There is very eminent peak at 9.5kHz which is associated with the prism resonances of the cavity.

- The dark noise was estimated to be 3.3x10^-17m/rtHz. I made the simplest estimation of the shot noise level. The dark noise was assumed to be limited by the PD noise. The shot noise intercept current is 2mA and the photocurrent was ~8mA. Therefore the shot noise level was estimated to be 3.3x10^-17x Sqrt(8/2) = 6.6x10^-17 m/rtHz.

Peaks between 400Hz and 1.3kHz

- It is unlikely that the OMC cavity itself has such many mechanical resonances from 400Hz to 1.3kHz. It is known that the OMC cavity has one high Q resonance at 1kHz (body bending mode). But any other resonances are above 3kHz.

- We tapped the ISCT6 tables, theHEPI crossbars, and chambers in order to see if we can excite these forest somehow.
- Basically everything is accoustically coupled. But we dare to say that the table does not excite the noise much. The most sensitive one was the HEPI crossbars. Just light touch of a HEPI cross bar excited the modes nearly x100 (attachment 5). This excitation was more eminent at the HEPI crossbars than at the chamber or the flange for the windows.

Still to do

- The displacement data is to be compared with the measurements with the other techniques.
- The displacement with PDH while the cavity is locked with the dither locking.
- Noise coupling from the OMC ASC.
- Evaluate frequency noise coupling.
- Actuator noise from the PZT driver.

It appears that we don't have a lot of coherence between RZ and YAW. Thus, even if the in-loop sensors (T240s) show good improvement with the substraction ON, it won't impact the overall chamber motion.

Also, knowing that fact, it doesn't seem relevant to let the RZ loop OFF anymore... I would turn back ON the RZ loop on ST1.

As we paid some attention to the ETMX baffle diode (as we wanted to obtain some data for the IR scattering in X arm) we have found that the output of these go negative (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=16083), unlike all other PDs.

In the attached trend for the past 300 days, the last time a reasonable positive current was observed for EX baffle PDs (ch6 and 7 in the plot) was June 21 2014 during one of the BSC9 vents. When we peeked down the arm after that vent (pink vertical line), the sign was already wrong.

Daniel remembers that an amplifier box was replaced around that time, so we'll swap the box with a spare.

The clean room over BSC 9 was turned off at 1048 hrs PST.

I have turned off the Mid station chillers.

ISS AOM Diffracted power was up ~11.3%. REFSIGNAL was also set to ~2.13 from the ~2.23 that It was set to yesterday to maintain roughly 7.5% diff. This morning I adjusted it back down to ~7.1% with a REFSIGNAL of -2.25. Is the Diff Power purposely being raised during commissioning at night?

We have had at least three incidents today where MC1 and MC3 both tripped, we haven't had time to investigate why but this is a new phenomenon.  

Sheila, Kiwamu, Alexa, Evan, Nergis

After locking the x-arm in green (alog) and testing for the x-arm losses (alog), we repeated our usual initial alignment procedure. Some comments:

• With the x-arm locked on green, we adjusted PR3 to increase the COMM beatnote from -1dBm to 4.5dBm.
• We had to move SR2 by a lot to get the AS_C QPDs centered, as well as the ASAIR camera.
• We restored HEPI RX, RY offsets for HAM4, HAM5
• The RM's master switches, dampimg filters, and M1_LOCK filters were off, which are all needed for the DC centering servos.
• Kiwamu turned off the POPB offsets that he and Stefan had added about a month ago when they found the position that gave the best DRMI buld up.  Currently the beam is well centered on POP_B, but without the offsets on.

We were able to lock ALS COMM very easily; however, we had difficulties locking DIFF. At first we thought we had found our culprit: LSC-DARM had FM10 engaged, which should not have been; the LSC-DARM filters should be FM5, FM7, FM8 with FM3 turned on later. Even after we noticed this, we still could not lock DIFF. As soon as the DIFF engages (even with gain's of 1, relative to the nominal 400), we would immediatly lose lock. We ensured that the ETMX ESD was working by sending in an excitation to the bottom stage in both pitch and yaw; this was successful and we did not see any large Y2P or P2Y coupling. We did not take L1, L3 crossovers, which we should do tomorrow. We also had trouble getting the DIFF PLL in range -- the old guardian tidal scripts the Nick wrote have been decommissioned, and we were not yet ready to implent Daniel's new tidal feedback, so we were left to using an ezcaservo in the test offset. This did not work very well, and limited us from figuring out what was wrong with DIFF. It's possible that there are still some more inccorect filter/gains setting.

Elli, Nergis, Daniel, Evan

Today we did a preliminary measurement of the Schnupp asymmetry to be 9.0cm +/- 1cm.  We injescted a beam from the auxiliary laser on IOT2R which can be offset in frequency from the main laser.  We measured the power of this beam at the AS port using a 1611 photodiode on ISCT6.  With the michelson locked to a bright fringe, we measured the variation of auxiliary laser power with frequency at the AS port.  Due to the Schnupp asymmetry, the power should change as the auxiliary laser offset from the carrier increases as cos(scnupp_assymetry*2*pi*f_offset/c).

By fitting a curve to our measured data, we can get an estimate of the Schnupp asymmetry.  I did a fit using least squares fitting to a quadratic to get 9.0cm +/-1cm Schnupp asymmetry.  The measured data and curve fitting codes are attached.  Evan hopes that we can reduce our error estimet by doing the curve fitting more elegantly.

Sheila, Keita, Nergis, Evan, Alexa

With our beloved x-arm back, we began the alignment process:

1. Aligned TMSX with ITMX baffled PDs (and ETMX misaligned). For reference:

2. Aligned ITMX with the ETMX baffle PDs (and ETMX misaligned). Again, for reference:

Yes, the ETMX baffle PDs have a negative voltage!!

3. We realigned ETMX by hand. We had to increase the max pitch PV limit from 440 to 500. We hope those were set arbitrarily; the DAC was not saturating with these higher slider values. We were able to lock the x-arm on green to a buildup of 1.4-sih, which is good.

Unfortunately the green wfs servo did not wok and would misalign the arm. We did have to turn on the ITMX ISC_INF_LOCK and MO_LOCK, and TMSX M1_LOCK filters. Unfortunately this was not the culprit. It appeared that the wfs error signal had an offset, but we did li. We want to continue with locking. 

Removed one of the PEM AA chassis to troubleshoot some bad channels. Troubleshooting/repairs is taking longer than expected but should have the unit back in service sometime tomorrow morning.

D1001421 Serial Number S1001053.