[Yuta, Anchal, Radhika]

Yesterday we attempted to lock MICH and BHD using the BH55_Q_ERR signal. We adjusted the demodulation phase to send the bulk of the error signal to the Q quadrature. With the LO beam misaligned, we first locked MICH with AS55_Q_ERR. We tried handing over the feedback signal to BH55_Q_ERR, which in theory should have been equivalent to AS55_Q_ERR. But this would not reduce the error and would instead break the MICH lock. Qualitatively the BH55_Q signal looked noisier than AS55_Q.

We used the Moku:Lab to send a 55 MHz signal into the demod board, replacing the BH55 RF input [Attachment 1]. The frequency was chosen to be 10 Hz  away from the demodulation frequency (5x Marconi source frequency). However, a 10Hz peak was not visible from the spectra - instead, we observed a 60 Hz peak. Tweaking the frequency offset a few times, we realized that there must be a ~50Hz offset between the Moku:Lab and the Marconi.

We generated an X-Y plot of BH55_Q vs. AS55_DC with the MICH fringe: this did not follow a circle or ellipse, but seemed to incoherently jump around. Meanwhile the X-Y plot BH55_I vs. AS55_DC looked like a coherent ellipse. This indicated that something might have been wrong with the demod board producing the I and Q quadrature signals.

We fed the BH55 RF signal into an unused demod board (previously AS165) [Attachment 2] and updated the channel routing accordingly. This step recovered elliptical I and Q signals with Moku input signal, and their relative gain was adjusted to produce a circle X-Y plot [Attachment 3]. C1:LSC-BH55_Q_GAIN was adjusted to 155.05/102.90=1.5068, and measured diff C1:LSC-BH55_PHASE_D was adjusted to 94.42 deg.

Now BH55_Q_ERR was able to be used to lock the MICH DOF. However, BH55 still appears to be noisy in both I and Q quadratures, causing the loop to feedback a lot of noise.

Next steps:

- Amplify the BH55 RF signal before demodulation to increase the SNR. In order to power an RF amplifier, we need to use a breakout board to divert some power from the DB15 cable currently powering BH55.

[Radhika, Anchal]

We have added an RF amplifier to the output of BH55. See the MICH signal on BH55 outputs as compared to AS55 output on the attached screenshot.

Details:

• Radhika first tried to use ZFL-500-HLN+ amplifier taken out from the amplifier storage along X-arm.
• She used a DB15 breakout board to source the amplifier power from PD interface cable.
• However, she reported no signal at the output.
• We found that BH55 RFPD was not properly fixed tot eh optical table. We bolted it down properly and aligned the beam to the photodiode.
• We still did not see any RF output.
• I took over from Radhika on this issue. I tested the transfer function of the amplifier using moku:lab. I found that it was not amplifying at all.
• I brought in a beanchtop PS and tested the amplifier by powering it directly. It drew 100 mA of current but showed no amplififcation in transfer function. The transfer function was constant at -40 dB with or without the amplifier powered.
• I took out another RF amplifier from the same storage. This time a ZFL-1000-LN. I tested it with both benchtop PS and PD interface power source, it was wokring with 20 dB amplification.
• I completed the installation and cable management. See photos attached.
• I also took the opportunity to center the ITMY oplev.

Please throw away malfunctioning parts or label them malfunctioning before storing them with other parts. If we have to test each and every part before installation, it will waste too much of our time.

1) please remember to follow the loading and power up instructions to avoid destroying our low noise RF amplifiers. Its not as easy as powering up any usual device.

2) also, please use the correct decoupling capacitors at the RF amp power pins. Its going to have problems if its powered from a distant supply over a long cable.

[Anchal, Radhika]

We selected a 102K (1 nF) ceramic capacitor and a 100 uF electrolytic capacitor for the RF amplifier power pins. I soldered the connections and reinstalled the amplifier [Attachments 1, 2].

[Yuta, Paco, Anchal]

BH55 RFPD installation was still not complete until yesterday because of a peculiar issue. As soon as we would increase the whitening gain on this photodiode, we saw spikes coming in at around 10 Hz. Following events took place while debugging this issue:

• We first thought that RFPD might be bad as we had just picked it up from what we call the graveyard table.
• Paco fixed the bad connection issue at RF out and we confired RFPD transimpedance by testing it. See 40m/17159.
• We tried changing the whitening filter board but that did not help.
• We used BH55 RFPD to lock MICH by routing the demodulation board outputs to AS55 channels on WF2 board. We were able to lock MICH and increase whitening gain without the presence of any spikes. This ruled out any issue with RFPD.
• Yuta and I tried swapping the whitening filter board but the problem persisted, which made us realize that the issue could be in the acromag that is writing the whitening gain for BH55 RFPD.
• We combed through the /cvs/cds/caltech/target/c1iscaux/C1_ISC-AUX_LSCPDs.db file to check if the whitening gain DAC channels are written twice but that was not the case. But changing the scan rate of the whitening gain output channel did change the rate at which teh spikes were coming.
• This proved that some other process is constantly writing zero on these outputs.
• It tuned out that all unused channels of acromags for c1iscaux are still defined and made to write 0 through /cvs/cds/caltech/target/c1iscaux/C1_ISC-AUX_SPARE.db file. I don't think we need this spare file. If someone wants to use spare channels, they can quickly add it to dB file and restart the modbusIOC service on c1iscaux, it takes less than 2 minutes to do it. I vote to completely get rid of this file or atleast not use it in the cmd file.
• After removing the violating channels, the problem with BH55 RFPD is resolved.

The installation of BH55 RFPD is complete now.

[Radhika, Paco]

Optical path setup

We realized the DCPD - B beam path was already using a 95:5 beamsplitter to steer the beam, so we are repurposing the 5% pickoff for a 55 MHz RFPD. For the RFPD we are using a gold RFPD labeled "POP55 (POY55)" which was on the large optical table near the vertex. We have decided to test this in-situ because the PD test setup is currently offline.

Radhika used a Y1-1025-45S mirror to steer the B-beam path into the RFPD, but a lens should be added next in the path to focus the beam spot into the PD sensitive area. The current path is illustrated by Attachment #1.

We removed some unused OPLEV optics to make room for the RFPD box, and these were moved to the optics cabinet along Y-arm [Attachment #2].

[Anchal, Yehonathan]

PD interfacing and connections

In parallel to setting up the optical path configuration in the ITMY table, we repurposed a DB15 cable from a PD interface board in the LSC rack to the RFPD in question. Then, an SMA cable was routed from the RFPD RF output to an "UNUSED" I&Q demod board on the LSC rack. Lucky us, we also found a terminated REFL55 LO port, so we can draw our demod LO from there. There are a couple (14,15,20,21) ADC free inputs after the WF2 and WF3 whitening filter interfaces.

Next steps

• Finish alignment of BH55 beam to RFPD
• Test RF output of RFPD once powered
• Modify LSC model, rebuild and restart

[Radhika, Paco, Anchal]

I placed a lens in the B-beam path to focus the beam spot onto the RFPD [Attachment 1]. To align the beam spot onto the RFPD, Anchal misaligned both ETMs and ITMY so that the AS and LO beams would not interfere, and the PD output would remain at some DC level (not fringing). The RFPD response was then maximized by scanning over pitch and yaw of the final mirror in the beam path (attached to the RFPD).

Later Anchal noticed that there was no RFPD output (C1:LSC-BH55_I_ERR, C1:LSC-BH55_Q_ERR). I took out the RFPD and opened it up, and the RF OUT SMA to PCB connection wire was broken [Attachment 2]. I re-soldered the wire and closed up the box [Attachment 3]. After placing the RFPD back, we noticed spikes in C1:LSC-BH55_I_ERR and C1:LSC-BH55_Q_ERR channels on ndscope. We suspect there is still a loose connection, so I will revisit the RFPD circuit on Monday. 

A design flaw in these initial LIGO RFPDs is that the SMA connector is not strain releieved by mounting to the case. Since it is only mounted to the tin can, when we attach/remove cables, it bends the connector, causing stress on the joint.

To get around this, for this gold box RFPD, connect the SMA connector to the PCB using a S shaped squiggly wire. Don't use multi-strand: this is usually good, since its more flexible, but in this case it affects the TF too much. Really, it would be best to use a coax cable, but a few-turns cork-screw, or pig-tail of single-core wire should be fine to reduce the stress on the solder joint.

[Paco, Anchal]

We followed rana's suggestion for stress relief on the SMA joint in the BH55 RFPD that Radhika resoldered. We used a single core, pigtailed wire segment after cleaning up the solder joint on J7 (RF Out) and also soldered the SMA shield to the RF cage (see Attachment #1). This had a really good effect on the rigidity of the connection, so we moved back to the ITMY table.

We measured the TEST in to RF Out transfer function using the Agilent network analyzer, just to see the qualitative features (resonant gain at around 55 MHz and second harmonic suppression at around 110 MHz) shown in Attachment #2. We used 10kOhm series resistance in test input path to calibrate the measured transimpedance in V/A. The RFPD has been installed in the ITMY table and connected to the PD interface box and IQ demod boards in the LSC rack as before.

Measurement files

Since we need more signal for both BH55 and BH44 to compare LO phase locking scheme, BH55 and BH44 RF outputs are now amplified with ZFL-1000LN+ and ZFL-500HLN+ respectively (see Attachment #1).
The amplifiers each draw ~0.1 A current of 15V DC power supply, and Sorensen power supply now reads 6.9 A (see Attachment #2).
With ITMX single bounce and LO beam fringing, BH55_Q (45 dB whitening gain, C1:LSC-BH55_PHASE_R=-110 deg) gives ~500 counts in amplitude, and BH44_Q (24 dB whitening gain, C1:LSC-BH44_PHASE_R=4.387 deg) gives ~100 counts in amplitude (and they are orthogonal) (see Attachment #3).

Ideally, BH55 and BH44 should give orthogonal signals to lock LO phase (40m/17302).
This was checked with various interferometer configurations.
BH55 and BH44 are indeed orthogonal in ITM single bounce and MICH, but was not measurable in FPMI.
Maybe we should investigate BH44 in MICH BHD configuration first to see why BH44 is very noisy in FPMI.

Method:
 - X-Y plotted BH55_Q and BH44_Q and fitted with an ellipse to derive amplitude of each signal and phase difference between them.
 - Amplitude and phase differences are calculated using the following equations, where (ap, bp) are the semi-major and semi-minor axes, respectively, and phi is the rotation of the semi-major axis from the x-axis. (Thanks to Tomohiro for checking the calculations!)

 xAmp = np.sqrt((ap * np.cos(phi))**2 + (bp * np.sin(phi))**2)
 yAmp = np.sqrt((ap * np.sin(phi))**2 + (bp * np.cos(phi))**2)
 phaseDiff = np.arctan(bp/ap*np.tan(phi)) + np.arctan(bp/ap/np.tan(phi))

Jupyter notebook: /opt/rtcds/caltech/c1/Git/40m/scripts/CAL/BHD/MeasurePhaseDiff.ipynb

 - This was done un following 4 configurations.
  - ITMX single bounce vs LO
  - ITMY single bounce vs LO
  - AS beam in MICH locked with AS55_Q vs LO
  - AS beam in FPMI locked with REFL55/AS455 vs LO
 - For each configuration, RF demodulation phases were tuned to minimize I.
 - Statistical error was estimated by calculating the standard deviation of 3 measurements.

 - Also, FPMI BHD sensing matrix was measured when FPMI is locked with REFL55/AS55, and LO_PHASE is locked with BH55_Q or BH44_Q.

Jupyter notebook: /opt/rtcds/caltech/c1/Git/40m/scripts/CAL/SensingMatrix/ReadSensMat.ipynb

Result of BH55/BH44 orthogonality check:
 - ITMX single bounce vs LO
            Demod phase         Amplitude       Phase Diff
    BH55_Q  -94.4 +/- 0.2 deg   600.4 +/- 0.6   
    BH44_Q  -9.0 +/- 0.2 deg    124.3 +/- 0.2   -86.7 +/- 0.1 deg
 - ITMY single bounce vs LO
            Demod phase         Amplitude       Phase Diff
    BH55_Q  -92.9 +/- 0.3 deg   588.0 +/- 0.3   
    BH44_Q  -8.9 +/- 0.3 deg    123.0 +/- 0.1   -87.2 +/- 0.1 deg
 - AS beam in MICH locked with AS55_Q vs LO
            Demod phase         Amplitude       Phase Diff
    BH55_Q  -68.7 +/- 0.8 deg   44 +/- 1   
    BH44_Q  -28.5 +/- 1.7 deg   10.3 +/- 0.1   -84 +/- 2 deg
 - AS beam in FPMI locked with REFL55/AS455 vs LO
            Demod phase         Amplitude       Phase Diff
    BH55_Q  35 +/- 3 deg        257 +/- 4   
    BH44_Q  -16 +/- 3 deg       44 +/- 1        -77 +/- 3 deg
 - Attachmented pdf contain example X-Y plots from each configuration. For ITM single bounce and MICH, BH55 and BH44 seems to be orthogonal, but for FPMI, ellipse fit does not go well.
 - Difference in the BH55 demodulation phase for ITMX single bounce and ITMY single bounce (1.5 +/- 0.4 deg) agrees with past measurement and agree marginally with Schnupp asymmetry (40m/17274).
 - Maybe we can derive some length differences using these demodulation phases.

Result of FPMI sensing matrix measurements:
 - Below is the sensing matrix when FPMI is locked with REFL55/AS55, and LO_PHASE is locked with BH55_Q. BH44 is noisier than BH55, and the response to LO1 is consistent with zero. This is also consistent with BH44 being orthogonal to BH55, but the error bar is too large to say.

Sensing matrix with the following demodulation phases (counts/m)
{'AS55': -168.5, 'REFL55': 92.32, 'BH55': -110.0, 'BH44': -8.93097234187195}
Sensors       DARM @307.88 Hz           CARM @309.21 Hz           MICH @311.1 Hz           LO1 @315.17 Hz           
AS55_I       (-2.49+/-8.35)e+10 [90]    (+2.36+/-0.85)e+11 [0]    (-0.64+/-3.99)e+10 [0]    (+0.57+/-4.07)e+09 [0]    
AS55_Q       (-3.50+/-0.08)e+11 [90]    (+0.09+/-1.20)e+11 [0]    (-0.79+/-8.66)e+09 [0]    (-0.70+/-5.96)e+08 [0]    
REFL55_I       (+0.72+/-8.09)e+11 [90]    (-1.42+/-2.75)e+12 [0]    (+0.00+/-1.37)e+11 [0]    (-0.38+/-2.78)e+09 [0]    
REFL55_Q       (+0.19+/-1.93)e+11 [90]    (-2.14+/-6.92)e+11 [0]    (+0.00+/-3.16)e+10 [0]    (+0.17+/-1.19)e+09 [0]    
BH55_I       (-1.41+/-0.55)e+11 [90]    (+1.46+/-2.28)e+11 [0]    (-1.60+/-3.72)e+10 [0]    (-0.07+/-3.05)e+09 [0]    
BH55_Q       (+2.05+/-3.10)e+10 [90]    (-1.72+/-4.86)e+10 [0]    (-0.31+/-2.19)e+10 [0]    (-3.06+/-0.87)e+09 [0]    
BH44_I       (-0.41+/-2.03)e+11 [90]    (+0.10+/-2.39)e+11 [0]    (+0.06+/-1.31)e+11 [0]    (-0.01+/-2.71)e+10 [0]    
BH44_Q       (+0.14+/-3.23)e+12 [90]    (+0.02+/-3.67)e+12 [0]    (+0.07+/-2.03)e+12 [0]    (-0.02+/-4.22)e+11 [0]    
BHDC_DIFF       (+8.49+/-0.47)e+11 [90]    (-0.06+/-2.93)e+11 [0]    (-0.16+/-1.01)e+10 [0]    (-0.27+/-2.04)e+09 [0]    
BHDC_SUM       (-2.30+/-0.11)e+11 [90]    (+0.68+/-7.92)e+10 [0]    (-0.44+/-3.33)e+09 [0]    (-0.63+/-5.63)e+08 [0]    

 - Below is the sensing matrix when FPMI is locked with REFL55/AS55, and LO_PHASE is locked with BH44_Q. BH44 response to LO1 is again consistent with zero. Locking LO_PHASE with BH44 is not robust. Also, BHDC_DIFF response to DARM is less, compared with LO_PHASE locked with BH55_Q. This means that BH55 is somehow better than BH44 in our FPMI BHD, which contradicts with simulations (with no contrast defect and DARM offset).

Sensing matrix with the following demodulation phases (counts/m)
{'AS55': -168.5, 'REFL55': 92.32, 'BH55': -110.0, 'BH44': -8.93097234187195}
Sensors       DARM @307.88 Hz           CARM @309.21 Hz           MICH @311.1 Hz           LO1 @315.17 Hz           
AS55_I       (-7.56+/-4.89)e+10 [90]    (+1.61+/-1.05)e+11 [0]    (+0.51+/-2.48)e+10 [0]    (+0.88+/-8.02)e+08 [0]    
AS55_Q       (-3.62+/-0.05)e+11 [90]    (+0.02+/-1.23)e+11 [0]    (+0.67+/-3.73)e+09 [0]    (+0.02+/-1.28)e+08 [0]    
REFL55_I       (+1.09+/-8.12)e+11 [90]    (-1.47+/-2.82)e+12 [0]    (+0.01+/-1.34)e+11 [0]    (+2.20+/-5.29)e+08 [0]    
REFL55_Q       (+0.22+/-1.93)e+11 [90]    (-1.83+/-7.23)e+11 [0]    (+0.02+/-3.18)e+10 [0]    (+0.56+/-1.17)e+08 [0]    
BH55_I       (-1.21+/-0.08)e+12 [90]    (+0.17+/-4.31)e+11 [0]    (-1.24+/-3.02)e+10 [0]    (-0.30+/-3.55)e+09 [0]    
BH55_Q       (-3.83+/-0.30)e+11 [90]    (-0.12+/-1.42)e+11 [0]    (-0.61+/-1.96)e+10 [0]    (-0.21+/-1.49)e+09 [0]    
BH44_I       (-0.22+/-2.01)e+11 [90]    (-0.07+/-2.30)e+11 [0]    (-0.02+/-1.27)e+11 [0]    (+0.08+/-2.62)e+10 [0]    
BH44_Q       (-0.77+/-8.27)e+11 [90]    (-0.13+/-9.51)e+11 [0]    (-0.04+/-5.23)e+11 [0]    (+0.02+/-1.08)e+11 [0]    
BHDC_DIFF       (+1.94+/-0.81)e+11 [90]    (-0.58+/-1.84)e+11 [0]    (+0.18+/-3.53)e+10 [0]    (-0.49+/-3.84)e+09 [0]    
BHDC_SUM       (-2.22+/-0.12)e+11 [90]    (+0.66+/-7.70)e+10 [0]    (-1.04+/-3.97)e+09 [0]    (+0.31+/-6.10)e+08 [0]

Other notes:
 - TRX and TRY are noisier at ~28 Hz when locked with REFL55/AS55 than when locked with POX/POY. DARM signal seems to be contaminated with broad 28 Hz noise. Needs investigation of the cause.
 - BS oplev loops seem to be close to unstable. When FPMI is unlocked, BS is kicked significantly.

Next:
 - Repeat measurement in 40m/17351 with BH44.
 - Compare LO phase noise in MICH configuration when LO_PHASE is locked with BH44 and BH55.
 - Investigate 28 Hz noise in FPMI
 - Tune BS local damping loops

[Yuta, Paco, Anchal]

BH55 meas diff

We estimated meas diff angle for BH55 today by following this elog post. We used moku:lab Moku01 to send a 55 MHz tone to PD input port of BH55 demodulation board. Then we looked at I_ERR and Q_ERR signals. We balanced the gain on I channel to 1.16 to get the two signals to same peak to peak heights. Then we changed the mead diff angle to 91.97 to make the "bounding box" zero. Our understanding is that we just want the ellipse to be along x-axis.

We also aligned beam input to BH55 bit better. We used the single bounce beam from aligned ITMY as the reference.

LO phase lock with single RF demodulation

We attempted to lock LO phase with just using BH55 demodulated output.

Configuration:

• ITMX, ETMs were significantly misaligned.
• At BH port, overlapping beams are single bounce back from ITMY and LO beam.

We expected that we would be able to lock to 90 degree LO phase just like DC locking. But now we understand that we can't beat the light with it's own phase modulated sidebands.

The confusion happened because it would work with Michelson at the dark port output of michelson, amplitude modulation is generated at 55 MHz. We tried to do the same thing as was done for DC locking with single bounce  and then michelson, but we should have seen this beforehand. Lesson: Always write down expectation before attempting the lock.

After the amplifier was modified with a capacitor, we continued trying to approach locking LO phase to in quadrature with AS beam. Following is a short summary of the efforts:

• To establish some ground, we tested locking MICH using BH55_Q instead of AS55_Q. After amplification, BH55_Q is almost the same level in signal as AS55_Q and a robust lock was possible.
• Then we locked the LO phase using BH55_Q (single RF sideband locking), which locks the homodyne phase angle to 90 degrees. We were able to successfully do this by turning on extra boost at FM2 and FM3 along with FM4 and FM5 that were used to catch lock.
• We also tried locking in a single ITMY bounce configuration. This is a Mach-Zehnder interferometer with PR2 acting as the first beam splitter and BHDBS as the recombination beamsplitter. Note that we failed earlier at this attempt due to the busted demodulation board. This lock worked as well with single RF demodulation using BH55_Q.
• The UGF achieved in the above configurations was ~15 Hz.
• In between and after the above steps, we tried using audio dither + RF sideband, and double demodulation to lock the LO phase but it did not work:
  • We could see a good Audio dither signal at 142.7 Hz on the BH55_Q signal. SNR above 20 was seen.
  • However, on demodulating this signal and transferring all signal to C1:HPC-BH55_Q_DEMOD_I_OUT, we were unable to lock the LO phase.
  • Using xyplot tool, we tried to see the relationship between C1:HPC-BHDC_DIFF_OUT and C1:HPC-BH55_Q_DEMOD_I_OUT. The two signals, according to our theory, should be 90 degrees out of phase and should form an ellipse on XY plot. But what we saw was basically no correlation between the two.
  • Later, I tried one more thing. The comb60 filter on BH55 is not required when using audio dither with it, so I switched it off.
    • I turned off comb60 filters on both BH55_I and BH55_Q filter modules.
    • I set the audio dither to 120 Hz this time to utilize the entire 120 Hz region between 60 Hz and 180 Hz power line peaks.
    • I changed the demodulation low pass filter to 60 Hz Butterworth filter. I tried using 2nd order to lose less phase due to this filter.
    • These steps did not fetch me any different results than before, but I did not get a good time to investigate this further as we moved into CDS upgrade activities.

give us an animated GIF of this cool new tool! - I'm curious what happens if you look at 2 DoF of the same suspension. Also would be cool to apply a bandpass filter before plotting XY, so that you could look for correlations at higher frequencies, not just seismic noise

I added the EPCIS channels for the c1omc model (gains, matrix elements etc) to the autoburt such that we have a record of these, since we expect these models to be running somewhat regularly now, and I also expect many CDS crashes.

[JC, Yuta]

To circumvent IPC error sending BHD DC PD signals from c1sus2 to c1lsc, DB9 cable from BHD DC PD box sent to c1sus2 is now split and sent also to c1lsc.
They are now available in both

c1sus2 ADC1
C1:X07-MADC1_EPICS_CH16 (DC PD A) and CH17 (DC PD B)

c1lsc ADC1
C1:X04-MADC1_EPICS_CH4 (DC PD A) and CH5 (DC PD B)

Next:
 - Add battery powered SR560 to decouple c1sus2 and c1lsc to avoid the ground loop

[Paco, Yuta]

We removed splitter to route BHD DC PD signals to c1hpc and c1lsc. This was necessary to circumvent IPC error, but this is no longer necessary. Now BHD DC PD signals are ADC-ed with c1hpc, and sent to c1lsc via IPC.
We also found that BHD DC PD signals have whitening filters as described in LIGO-T2000500 (Readout board is LIGO-D1400384).
We added unwhitening filter zpk([151.9;3388],[13.81],1,"n") to C1:HPC_BHDC_A and B, based on measured whitening stage gain (see Sec 3.1 of characterization reoprt in LIGO-T2000500).
This solved the signals leaking to minus (40m/17068).

Next:
 - Modify c1hpc model to send BHD DCPD signals to c1lsc after unwhitening. (Note added on Nov 15: The same unwhitening filter is also added to C1:LSC-DCPD_A and B for now. See attached.)
 - Redo visibility measurements,

After discussing with Anchal, we decided to route BHD related PD signals directly to ADC of c1sus2, which handles our new suspensions including LO1, LO2, AS1, AS4, so that we can control them directly.
BHD related PD signals will be sent to c1lsc for DARM control.

Re-cabling was done, and now they are online at C1:X07-MADC1_EPICS_CH16 (DC PD A) and CH17 (DC PD B) with 15ft DB9 cable.
Here, DC PD A is the transmission of BHD BS for AS beam, and DC PD B is the reflection of BHD BS for AS beam (see attached photo).

[Paco, Tega, Yuta]

Today, we made a custom MEDM screen for the BHD Homodyne Phase Control, which is basically an overview of the c1hpc model. See Attachments 1 & 2 for details.

[Paco, Yuta, JC]

We did some easy tests on the BHD readout in preparation for BHD MICH. With the arm cavities and LO beam misaligned, but the MICH aligned, we measured the transfer function from C1:LSC-DCPD_A_OUT to C1:LSC-DCPD_B_OUT to get a rough estimate of the gain balance: 1.8 * DCPD_A = DCPD_B. We then locked MICH using REFL55_Q and looked at

• A=C1:LSC-DCPD_A_OUT
• B=C1:LSC-DCPD_B_OUT
• 1.8 * A - B (which we encoded using C1:LSC-PRCL_A_IN1)
• 1.8 * A + B (which we encoded using C1:LSC-PRCL_B_IN1)

namely the DCPD BHD signals. After turning the MICH_OSC on (2000 gain @ 311.1 Hz), we took some power spectra under the following three configurations:

1. LO misaligned, no MICH offset.
2. LO overlap, no MICH offset.
3. LO overlap and MICH offset.

For 1. the expectation was that since LO is misaligned and the AS port is dark, we would get no signal. In 2., however both A and B would might see some incoherent signal, but still no MICH. Finally in 3. all signals should be able to see MICH, including A-B. Attachment #1 shows the measurements 1, 2, and 3 (offset = -5.0). Then, with increasing offset values, the BHD MICH signals increased as well; discussion to follow.

[Yuta, Paco]

TL;DR Successfully controlled LO phase, and did BHD-MICH readout with various MICH offsets and LO phases.

Today we implemented a DCPD based LO phase control. First, we remeasured the balancing gain at 311.1 Hz (the MICH oscillator freq) and combined C1:HPC-DCPD_A_OUT with C1:HPC-DCPD_B_OUT to produce the balanced homodyne error signal (A-B). We feed this error signal to C1:HPC-LO_PHASE_IN1 and for the main loop filters we simply recycled the LSC-MICH loop filters FM2 through FM5 (we also copied FM8, but didn't end up using it much). Then, we verified the LO phase can be controlled by actuating either on LO1 or LO2. For LO2, we added an oscillator in the HPC LOCKINS at 318.75 Hz (we kept this on at 1000 counts for the measurements below).

The LO phase control was achieved with a loop gain in the range of 10-30 (we used 20), no offset, and FM4, and FM5 engaged. FM2 can be added to boost, but we usually skipped FM3. Then, we went through a set of measurements similar to the ones described in a previous elog. A key difference with respect to the measurements from before is that we locked MICH using AS55Q (as opposed to REFL55Q). This allowed us to reach higher MICH offsets without losing lock. After turning on the MICH oscillator at 3000 counts, we looked at:

1. LO misaligned + MICH at dark fringe (offset = -21).
   • Here, we don't expect to see any MICH signal and indeed we don't, except for a small residual peak from perhaps a MICH offset or slightly imbalanced PDs.
2. LO aligned, but uncontrolled + MICH at dark fringe (offset = -21).
   • Here we would naively expect MICH to show up in A-B, but because of the uncontrolled LO phase, we mostly see the noise baseline (mostly from LO RIN? ...see measurement 3) under which this signal is probably buried. Indeed, the LO fringe increased noise in A, B, and A-B but not in A+B. This is nice.
3. LO aligned, but uncontrolled + MICH with dc readout (offset = +50).
   • Here we expected the MICH signal to show up due to the large offset, and we can indeed see it in A, B, and A+B, but not in A-B. Nevertheless we see almost exactly the same noise level even though we allow some AS light into the BHD readout, so maybe the noise observed in the A-B channel from measurements 2 and 3 is mostly from LO RIN. This needs further investigation...
4. LO aligned, controlled at no offset + MICH with dc readout (offset = +50).
   • In general here we expected to see a noise reduction in the A-B channel since the LO fringe is stable, and a MICH signal should appear. Furthermore, since LO phase is under control, we expect the LO2 Oscillator to appear which it does for this and the following measurements. Because of the relative freedom, we tried this measurement in two cases:
     1. When feeding back to LO1
        • We actually see MICH in the A-B channel, as expected, after the noise level dropped by ~ 5. We also observed small sidebands +- 1 Hz away from the MICH peak, probably due to local damping in either LO or AS paths.
     2. When feeding back to LO2
        • We also see MICH here, with a slightly better drop in noise (relative to feeding back to LO1). Sidebands persisted here, but around at +- 2 Hz.
5. LO aligned, controlled (offset = 10) + MICH with dc readout (offset = +50). *
   • Here, we expected the A-B MICH content to increase dramatically, and indeed it does after a little tuning of the LO phase . The noise level decreased slightly because LO phase noise is decreased around the optimal point.
6. LO aligned, controlled (offset = 20) + MICH with dc readout (offset = +30). *
   • Here, we naively expected A+B MICH content to decrease, but A-B remain constant. In order to see this we tried to keep the balance between the offsets, but this was hard. We don't really see much of this effect, so this also needs further investigation. As long as we keep controlling the LO phase using the DCPDs because the offsets tend to reduce the error signal we will have a harder time.

* For these measurements we actuated on LO2 to keep the LO phase under control.

Note that the color code above corresponds to the traces shown in Attachment #1.

What's next?

• Alignment of LO and AS might be far from optimized, so it should be tried more seriously.
• What's the actual LO power? How does it compare with AS power at whatever MICH offsets?
• Try audio dither LO phase control.
  • With MICH offset.
  • Without MICH offset, double demod (after dolphin fix )

Please see the attached doc. 

I think the conclusion is that if the AS1 RoC error is not significantly more than 1%, then with some adjustment of the AS1-AS3 distance (~ 1 cm), we could find a solution that simultaneously makes the AS path mode-matching better than 99% for the t- and s-planes. 

The requirement of the LO path is less strict and the current plan using LO1-LO2 actuation should work. 

Optics --> Cabinet at south end (Attachment #1)

Scanned datasheets--> wiki. It would be good if someone can check the specs against what was ordered.

Basically, they repeated our specs and showed the coating performances for HR/AR for 10deg P and PR/AR for 45deg P. There is no RoC measurement by the vendor.
Nevertheless, their RoC (paper) specs should be compared with our request.

Stephen and I discussed the in-vacuum OMC wiring.

- One of the OMCs has already been completed. (Blue)
- The other OMC is still being built. It means that these cables need to be built. (Pink)
- However, the cables for the former OMC should also be replaced because the cable harness needs to be replaced from the metal one to the PEEK one.
- The replacement of the harness can be done by releasing the Glenair Mighty Mouse connectors from the harness. (This probably requires a special tool)
- The link to the harness photo is here: https://photos.app.goo.gl/3XsUKaDePbxbmWdY7

- We want to combine the signals for the two OMCs into three DB25s. (Green)
- These cables are custom and need to be designed.

- The three standard aLIGO-style cables are going to be used. (Yellow)

- The cable stand here should be the aLIGO style.

[Koji, Stephen - updated 30 September]

Cable lengths task - in vacuum cabling for the green section (new, custom for 40m) and yellow section (per aLIGO, except likely with cheaper FEP ribbon cable material) from 40m/16198. These arethe myriad of cables extending from the in vacuum flange to the aLIGO-style on-table Cable Stand (think, for example, D1001347), then from the cable stand to the OMCs.

a) select a position for the cable stand.

 - Koji and I discussed and elected to place in the (-X, -Y) corner of the table (Northwest in the typical diagram) and near the table edge. This is adjacent to the intended exit flange for the last cable.

b) measure distances and cable routing approximations for cable bracket junctions

- Near OMC bracket to the cable stand, point to point = 17.2, routing estimate = 24.4.
- Far OMC bracket to the cable stand, point to point = 20.5, routing estimate = 32.2.

  - Recommendation = 48" for all green section cables (using one length for each OMC, with extra slack to account for routing variation).

c) (outdated - see item (b) and attachment 3) measure distances (point to point) and cable routing approximations for all items.

 +X OMC (long edge aligned with +Y beam axis) (overview image in Attachment 1)

- QPDs to the cable stand, point to point = 12, routing estimate = 20.
- DCPDs to the cable stand, point to point = 25, routing estimate = 32.
- PZTs to the cable stand, point to point = 21, routing estimate = 32.

+Y OMC (long edge aligned with +Y beam axis) (overview image in Attachment 1)

- QPDs to the cable stand, point to point = 16, routing estimate = 23.
- DCPDs to the cable stand, point to point = 26, routing estimate = 38.
- PZTs to the cable stand, point to point = 24, routing estimate = 33.

Cable stand to flange (Attachment 2) (specific image in Attachment 2)

- point to point = 35, routing estimate = 42

  - Recommendation = 120" for all yellow section cables, per Koji's preferences for zigzag cable routing on stack and coiling of slack.

[Don, Koji]

Don is working on finalizing the BHD Platform design. All the components on the BHD platform are almost populated and aligned.

Don is still working on the table legs so that we can detach the legs when we need to float the table in the future.
The BHD BS mount will have a third picomotor so that we can steer 3 dof with the mount while the remaining dof needs to be provided by the OMC.
The BHD BS position is going to be adjusted so that the incident and trans beams have sufficient clearance.
The OMC legs (kinematic mounts) need more work so that we can adjust their positions for initial setup while they can be the reference for the reproducible placement of the OMCs.
The OMCs are rigidly held with the legs. For the damping of the 1-kHz body bode, which has a relatively high Q, there will be a dissipative element touching the glass breadboard.

I quickly ran the FEA model to check the resonant freqs of the BHD platform.
The boundary conditions were:

• The platform was not loaded
• FIxed constraints were given to the five legs

Don has optimized the cut-out size for the OMCs to increase the rigidity of the plate. Also, the ribbed grid is made at the bottom side.

The lowest mode is at 168Hz. Because there is no leg around, it seems reasonable to have this kind of mode as the fundamental mode.
The other mode lined up at 291Hz, 394Hz, 402Hz, ...
The mode freqs will be lower once the platform is loaded. But as the unloaded platform mode, these mode freqs sound pretty good numbers.

Stephen and I discussed the nominal heights of the BHD platform components.

• The beam height from the stack is 5.5"
• The platform height is 1.5" and the thickness of 0.4", according to the VOPO suspension, which we want to be compatible with.
• Thus the beam height on the BHD platform is 4".
• The VOPO platform has a minimum 0.1" gap from the installation surface when it is suspended.
• When the BHD platform is fixed on the table, we'll use positioners that are fixed on the stack table. Then the BHD platform is fixed on the positioner rather than fixing the entire platform on the stack. This leaves us the option to suspend the platform in the future. The number of the positioners is TBD.
• Looking at the head size for 1/4-20 socket head screws, It'd be nice to have the thickness of 0.5" for the positioners. This makes the thin part of the stiffener to be 0.6" in thickness.
   
• The numbers are nominal for the initial design and subject to the change along with FEA simulations to determine the resonant frequency of the body modes.

I'm planning on simulating the BHD readout noise in a manner very similar to the ALS noise model using Simulink. I've made a sketch of the model for the longitudinal DOFs (attached). A model for ASC will be similar but with more measurement devices (OpLevs, QPDs, WFSs).

I'm not pretending to simulate everything in this diagram on the first go, it is just a sketch of the big picture.

I feel like there is an instability in my thought process on this. Before my tendency to try to scale and generalize this problem brings me to a full stop I will make small but quick progress.

First thing is to calculate the noise budget for a simple Michelson. The involved optics are:

• ITMX
• ITMY
• BS
• LO1
• LO2
• AS1
• AS4

all sensed with OSEMs and OpLevs only.

Things to fetch:

1. OSEM sensing noise. Where do I get the null stream (AKA butterfly mode)?

2. Oplev noise, look at the SUM channel (or this elog)

3. Actuation TF. Latest elog.

4. Coil driver noise. Going to take the HP supply curve from this elog.

5. Seismic noise + Seismic stack TF. Or maybe just take displacement noise from gwinc.

6. Laser noise. Still need to search.

7. DAC noise. Still need to search.

I have made a Simulink diagram to use in the MICH modeling (attachment) for the homodyne angle detection scheme. The model will be used for each optic separately and the noises will be combined in quadrature.

I gathered some more bits of info to fill the Simulink boxes. This is what I have so far:

[JC, Paco]

We installed a steering mirror intersecting the BHD beam path and put the AS beam on the ITMY Oplev QPD (see Attachment #1 for a photo of this temporary hack) . This is done to do coil balancing of AS1/AS4, LO1/LO2. QPD sees ~ 10000 counts when the beam is centered.

[Paco, Yuta]

We follow this procedure -- but with different sensors for all BHD suspension coil output balancing.

AS1/AS4

We dither BUTT first, lock the LO-AS fringe (DC lock), and look at the residual LO_PHASE spectrum to minimize POS coupling. We then unlock, misalign LO beam and look at the hijacked Oplev (ITMY) while dithering POS to minimize PIT and YAW couplings.

LO1/LO2

We dither BUTT first, lock thChangeset summarye LO-AS fringe (DC lock), and look at the residual LO_PHASE spectrum to minimize POS coupling. We then unlock, misalign AS beam and look at the hijacked Oplev (ITMY) PIT/YAW residual noise while dithering POS to minimize PIT/YAW coupling.

Changeset summary

The new coil output gains are summarized in the table below:

Finally, I reverted the hacked QPD setup to restore the ITMY OPLEV.

[Anchal, Paco, JC]

We had to open ITMY, ETMY chamber doors to get the cavity aligned again. Once we did that, we regained cavity flashing and were able to align the input injection and cavity alignment to get transmission flashing to 1.0 (C1:LSC-TRY_OUT_DQ). JC later centered both ITMY and ETMY oplevs. The ITMY oplev had become completely out of range.

We also opened ITMX, ETMX chamber doors to get Xarm alignment. Again, it seems that ITMX had moved a lot due to cable post installation.

To be continued

Listing some talking points from the last week of activity here.

1. LO delivery fiber cable may be damaged.
   • The throughput itself doesn't suggest any problems, I get almost all the light I put in out the other end.
   • However, even when I slightly move the fiber, I see huge amplitude fluctuations in the DCPD readouts. This shouldn't be the case, particularly if the light is well matched to one of the special axes of the PM fiber. I checked with a PBS at the output that this is indeed the case, so something else must be funky?
   • In any case, I don't think it's a great idea to use this 70m long fiber for bringing the light from the PSL table to the adjacent AP table. Chub has ordered a 10m patch cable.
   • I was a bit too hasty this morning, thinking we had a patch cable in hand, and so I removed the fiber from the AP table. So right now, the LO beam doesn't make it to the BHD setup. Depending on the lead time for the new patch cable, I may or may not resurrect this old setup.
   • I have also located some foam and rigid plastic tubing which I think will help in isolating the fiber from environmental length(phase) modulation due to acoustic pickup.
2. BHD commissioning activities
   • Basically, I've been trying to use the Single Bounce ITM reflection/ Michelson / PRMI with carrier locked to get some intuition about the BHD setup. These states are easily prepared, and much easier to understand than the full IFO for these first attempts.
   • One concern I have is the angular stability (or lack thereof). When the PRMI is locked, the DC light level on each DCPD fluctuates between ~0 (which is what it should be), up to ~30 cts (~85uW).
   • Using the empirically determined attenuation factor between the DCPDs and the dark port of the beamsplitter, I estimate the power can be as high as 20mW. This is a huge number, considering the input to the interferometer is ~800mW. I assume that all the light is at the carrier frequency, since the PRC should reject all the sideband light in this configuration. In any case, the total amount of sideband light is ~20mW, and the carrier stays resonant in the PRC even when there are these large ASDC excursions, so I think it's a reasonable assumption that the light is at the carrier frequency. Moreover, looking at the camera, one can see a clear TEM10/01 profile, indicative of imperfect destructive interference at the beamsplitter due to beam axis misalignment.
   • The effect of such excursions on the BHD readout hasn't yet been quantified (by me at least), but I think it may be hampering my attempts to dither the homodyne phase to estimate the LO phase noise.
3. High voltage coil driver project - see thread for updates.
4. Trek HV driver has arrived.
   • I haven't opened the box yet, but basically, what this means is that I can dither the mirror intended for homodyne phase control in a reasonable way.
   • Previously, I was using the OMC HV driver to drive the PZTs - but this dither signal path has a 2kHz high pass filter (since the OMC length dither is a kHz dither). I didn't want to futz around with the electronics, particularly since the unit was verified to be working.
   • So the plan now would be to drive the input of the Trek with a DAC output (an appropriate AI chassis has been prepared to interface with the CDS system).
   • Hopefully, there's enough DAC dynamic range to dither the PZT and also do the homodyne phase locking using a single channel. Else, we'd need to use two channels and install a summing amplifier.
   • We definitely need more high-voltage amplifiers/supplies in the lab:
     • Any Thorlabs HV drivers we can recover? 
     • Eventually, we will need HV for coil drivers, OMC PZTs, steering PZTs, homodyne phase control PZT. 
5. PMC bases have arrived.
   • Joe Benson from the machine shop informed me today afternoon that the bases were ready for pickup.
   • We have 3 bases in hand now. The finish isn't the greatest in the world, but I think it'll work. You can see some photos here.
   • I will hold off on putting this together while I work on the basic airBHD commissioning tests. We can install the PMCs later.
6. AS port WFS project
   • We now have in hand almost all the components for stuffing the ISC whitening and LSC demod boards.
   • Rich, Chub, Luis and I had a call on Monday. The advise from Rich/Luis was:
     • Choose an inductance that has Z~100 ohms at the frequency of interest, for the resonant transimpedance part.
     • Choose a capacitance that gives the appropriate resonant frequency.
     • Don't stuff more notches than you need - start with just a 2f notch (so 110 MHz for us), and make sure to place the highest frequency notch closest to the photodiode.
     • Rich also suggested looking at the optical signal with a non-optimized head, get an idea of what the field content is, and then tune the circuit as necessary. There are obviously going to be many issues that only become apparent once we do such a test.
   • The aLIGO modulation frequencies are only 20% different from the 40m modulation frequencies. So I thought it is best if for our first pass, we stick to the inductance values used in the aLIGO circuits (same footprint, known part etc etc). Then, we will change the capacitance so that we have a tuning range that is centered our modulation frequencies.
   • The parts have been ordered.
7. ISS project
   • Half of the LO light on the BHD breadboard is diverted for the purpose of sensing the LO intensity noise, for eventual stabilization. Right now, it is just getting dumped.
   • A PD head has been located. It has a minimalist 1kohm transimpedance amplifier circuit integrated into the head.
   • Our AOM driver has an input range of 0-1V DC. We want to map the servo output of +/-10V DC (or +/-4V DC if we use an SR560 based servo for a first pass) to this range.
   • I wanted to do this for once in a non-hacky way so I drew up a circuit that I think will serve the purpose. It has been fabricated and will be tested on the bench in a couple of days.
   • Once I get a feel for what the signal content is, I will also draw up a interface board to the PD head that (i) supplies the reverse bias voltage and +/-15 V DC to the PD head and (ii) applies some appropriate HPF action and provides a DC monitor as well.
8. Summary pages are dead.
   • See issue tracker.
9. General lab cleanup
   • I moved all the PPE from the foyer area into the designated cabinets along the east arm.
   • Did some basic cleanup of the lab in preparation for crane inspection. Walkways are clear.
   • I de-cluttered the office area a bit, but today I received ~10 packages from Digikey/FrontPanelExpress etc. So, in fact, it got even more cluttered. Entropy will go down once we ship these off to screaming circuits for stuffing the PCBs.

[Yuta, Paco]

Today we calibrated the actuation on BHD suspended optics: LO1, LO2, AS1, AS4.
Actuation transfer functions for these optics look good.

ITMY actuation

For a reference we locked LO-ITMY single bounce using the LSC MICH loop. The error point was BH55_Q, the whitening filter gain was 45 dB, IQ demod rotation angle = 151.061 deg, the servo gain was -10, and the actuation point was ITMY. The measured UGF for this loop was ~ 150 Hz when FM2, 3, 4, 5 and 8 were all enabled. Note FM8 is an elliptic low pass (600 Hz cutoff).

LO1, LO2, AS1, AS4 actuation

We then lock the LO phase by feeding back BH55_Q_ERR to the actuation points under test with exactly the same filters but a servo gain of 0.6 but otherwise we are using the same servo filters FM2, 3, 4, 5 and 8 for this controls. The measured UGFs were all near ~ 70 Hz.

Here we had to be careful not to excite mechanical (?) resonances similar to the previously observed "violin" modes in LO1. In particular, we first noticed unsupressed 816 Hz noise in AS1 was being reinjected by the loop sometimes tripping the local damping loops, so we added bandstop filters at the AS1_LSC output filter bank. The resulting loop was then allowed to increase the gain and turn on FM2 and FM3 (boosts). This was also the case in AS4, where 268 Hz and second + third harmonics appeared to be excited by our feedback control. Finally, AS4 also displayed some mechanical excitation at 96.7 Hz, which seemed too low to be a "violin" mode, and its "Q" factor was not as high. We added a bandstop for this as well.

Attachment #1 shows LO_PHASE OLTFs when actuating in the different optics. By taking the actuation ratios (Attachment #2) with respect to our ITMY actuation reference and which had previously been calibrated to be 4.74e-9 / f^2 m / cts, we now have estimated our BHD suspension actuation calibrations to be:

• LO1 = 3.14e-8 / f^2 m / cts
• LO2 = 2.52e-8 / f^2 m / cts
• AS1 = 3.14e-8 / f^2 m / cts
• AS4 = 2.38e-8 / f^2 m / cts

This magnitudes are consistent with the expected coil driver ranges (about a factor of 10 difference).

[JC, Paco, Yuta]

After the IFO recovery (elog 40m/16881), we installed an analog camera for BHD fringe using a BNC cable for old SRMF camera so that we can see it from the control room.
We also aligned AS-LO using LO1,LO2 and AS4.
We then aligned GRY injection to get maximum GTRY.

Maximum TEM00s right now are
 C1:SUS-ETMX_TRX_OUT_DQ ~0.1
 C1:SUS-ETMY_TRY_OUT_DQ ~0.05
 C1:ALS-TRX_OUT_DQ ~0.20
 C1:ALS-TRY_OUT_DQ ~0.18

Since there has been a proliferation of BHD Google docs recently, I've linked them all from the BHD wiki page. Let's continue adding any new docs to this central list.

Last week, we could find an alignment which realizes LO beam and AS beam both unclipped, but it was not consistent with an alignment which realize BHD fringe (40m/17046).
Today, we tweaked the alignment of SR2, AS1, AS4 to have BHD fringe with reduced LO and AS beam clipping.
AS beams on AP table and BHD both still look clipped, but much better now.
Ideally, SR2 and AS1 will unclip AS beam, and LO1, LO2, AS4 would make BHD fringe, but it is hard right now since LO beam seem to have little room and LO2 have little actuation range.
BHD optics on ITMY table, including camera, and AS55/ASDC were realigned after the aglinment work (Note that DCPD_A path have a pick-off for camera path, and this pick-off mirror have quite significant incident angle dependence of R/T ratio).

Current alignment scheme:
Current alignment scheme I figured out is the following.
 - Check Y green. If it is transmitted at good spot on GTRY camera, Yarm is OK. If not, tweak ITMY/ETMY. alignment.
 - Mis-align AS4, align TT1, TT2, LO1 to have DCPD_A_OUT of ~130 and DCPD_B_OUT of ~125.
 - Align PR3, PR2 to maximize TRY_OUT to ~1.05.
 - Tweak ITMY/ETMY if the beam spot on them are not good.
 - Align BS, ITMX to have good MICH fringe and TRX_OUT to ~1.1.
 - Tweak ITMX/ETMX if the beam spot on them are not good.
 - Misalign ETMY, ETMX, ITMY to have LO-ITMX fringe in BHD DCPDs, and align AS beam with SR2 and AS4 differentially, with ratio of AS4/SR2=3.6.

DC PD values in various configurations:
Both arms locked with POX/POY, MICH free, PRM/SRM misaligned
                          Mean     Max      Min
C1:IOO-MC_TRANS_SUM :     14088.57 13947.52 14167.04
C1:LSC-ASDC_OUT16 :           0.16    -0.02     0.34
C1:LSC-POPDC_OUT16 :        369.34   -74.88   854.34
C1:LSC-REFLDC_OUT16 :         0.03    -0.00     0.06
C1:LSC-TRY_OUT16 :            1.00     0.95     1.04
C1:LSC-TRX_OUT16 :            1.07     1.04     1.08

Only LO beam to BHD DCPDs
                          Mean     Max      Min
C1:IOO-MC_TRANS_SUM :     14121.32 14057.71 14159.38
C1:HPC-DCPD_A_OUT16 :       129.80   128.37   130.68 (Consistent with, 40m/17046. Power as expected within 20%. Squashed shape)
C1:HPC-DCPD_B_OUT16 :       123.42   121.92   124.48

ITMX single bounce (ITMY, ETMX, ETMY, PRM, SRM, LO misalgined)
                          Mean     Max      Min
C1:IOO-MC_TRANS_SUM :     14105.13 14000.89 14171.91
C1:HPC-DCPD_A_OUT16 :        92.54    91.45    93.30 (Consistent with 40m/17040, Power as expected within 40%. Clipped to the left in camera)
C1:HPC-DCPD_B_OUT16 :       137.70   136.55   138.53 (Note that DCPD_A/B ratio is different from LO, due to BHD BS R/T unbalance; 40m/17044)
C1:LSC-ASDC_OUT16 :           0.10     0.09     0.10 (Power as expected 40m/16952. Clipped to the right in camera)
C1:LSC-POPDC_OUT16 :        309.19   288.93   327.10 (Power as expected within 30% 40m/17042.)
C1:LSC-REFLDC_OUT16 :         0.02     0.01     0.02

ITMY single bounce (ITMX, ETMX, ETMY, PRM, SRM, LO misalgined)
                          Mean     Max      Min
C1:IOO-MC_TRANS_SUM :     14112.09 14025.37 14154.51
C1:HPC-DCPD_A_OUT16 :        92.58    92.01    93.26
C1:HPC-DCPD_B_OUT16 :       137.68   136.81   138.27
C1:LSC-ASDC_OUT16 :           0.10     0.09     0.10
C1:LSC-POPDC_OUT16 :        308.48   290.49   319.73
C1:LSC-REFLDC_OUT16 :         0.02     0.01     0.02

MICH fringe only (ETMX, ETMY, PRM, SRM, LO misalgined)
                          Mean     Max      Min
C1:IOO-MC_TRANS_SUM :     14090.34 13979.15 14143.86
C1:HPC-DCPD_A_OUT16 :       325.60    91.92   714.57
C1:HPC-DCPD_B_OUT16 :       400.27    18.37   762.57
C1:LSC-ASDC_OUT16 :           0.19    -0.05     0.41
C1:LSC-POPDC_OUT16 :        595.66  -119.21  1334.11
C1:LSC-REFLDC_OUT16 :         0.03    -0.01     0.07

LO-ITMX fringe only (ITMY, ETMX, ETMY, PRM, SRM misalgined)
                          Mean     Max      Min
C1:IOO-MC_TRANS_SUM :     14062.58 13968.05 14113.67
C1:HPC-DCPD_A_OUT16 :       224.31    89.57   371.66
C1:HPC-DCPD_B_OUT16 :       259.74    85.37   421.86

Next:
 - Measure contrast (40m/17020) and estimate mode-matching of LO-AS again (40m/17041)
 - Now that we have better LO-AS fringe, lock LO phase in MICH (40m/17037)
 - Now that Dolphin issue was fixed, try double-demodulation to lock LO phase

I aligned today using this scheme. I couldn't seem to get C1:IOO-MC_TRANS_SUM above 13400 by using WFS or manually aligning. The original state before was the following:
                  Pitch       Yaw     
C1:SUS-MC1:     -0.4672     -0.7714
C1:SUS-MC2:      4.0446     -1.3558
C1:SUS-MC3:     -2.0006      1.6001

[Anchal, Yehonathan, Yuta]

We did the constrast measurement with the method same as 40m/17020.
Contrast between ITM single bounce and LO beam increased to 74% (we had 43% before unclipping LO beam in 40m/17056).
From equations in 40m/17041 and measured ITM sigle bounce power (93 or 138 counts @ BHD DCPD) and LO power (130 or 124 counts @ BHD DCPD) from 40m/17056,  expected visibility for perfectly mode-matched case is 99%.
Measured constrast of 74% indicate mode-matching of 56%.

Both arms locked, MICH fringe (20% percentile)
Contrast measured by C1:LSC-ASDC_OUT is 80.66 +/- 0.20 %
Contrast measured by C1:LSC-POPDC_OUT is 92.27 +/- 0.66 %
Contrast measured by C1:LSC-REFLDC_OUT is 89.59 +/- 0.84 %
Contrast measured by all is 87.51 +/- 1.69 %

Both arms misaligned, MICH fringe (20% percentile)
Contrast measured by C1:LSC-ASDC_OUT is 82.50 +/- 0.61 %
Contrast measured by C1:LSC-POPDC_OUT is 94.18 +/- 0.26 %
Contrast measured by C1:LSC-REFLDC_OUT is 92.78 +/- 0.19 %
Contrast measured by all is 89.82 +/- 1.75 %

ITMX-LO fringe (40% percentile)
Contrast measured by C1:HPC-DCPD_A_OUT is 73.93 +/- 1.52 %
Contrast measured by C1:HPC-DCPD_B_OUT is 73.56 +/- 1.22 %
Contrast measured by all is 73.74 +/- 0.98 %

ITMY-LO fringe (40% percentile)
Contrast measured by C1:HPC-DCPD_A_OUT is 73.45 +/- 0.61 %
Contrast measured by C1:HPC-DCPD_B_OUT is 75.27 +/- 0.50 %
Contrast measured by all is 74.36 +/- 0.54 %

For both 40m/17020 and 40m/17024, what does the contrast mean if the numbers are leaking out to ~-100cnt?
Also how much is it if you convert this contrast into the mode matching?

BHD fringe visibility was measured again with unwhitening filters on on BHDC_A and B, which removed signal leakage to zero (40m/17265).
The result didn't change much from previous measurement (40m/17067) thanks to using the 'mode' of signal to calculate visibility.
Measured constrast of 74% indicate mode-matching AS beam to LO beam of 56%.

ITMX-LO fringe (10% percentile)
Contrast measured by C1:HPC-BHDC_A_OUT is 74.46 +/- 0.07 %
Contrast measured by C1:HPC-BHDC_B_OUT is 74.25 +/- 0.07 %
Contrast measured by all is 74.35 +/- 0.07 %

ITMY-LO fringe (10% percentile)
Contrast measured by C1:HPC-BHDC_A_OUT is 74.01 +/- 0.10 %
Contrast measured by C1:HPC-BHDC_B_OUT is 73.85 +/- 0.09 %
Contrast measured by all is 73.93 +/- 0.08 %

Errors are from standard deviation of 3 measurements.
The notebook lives in /opt/rtcds/caltech/c1/Git/40m/scripts/CAL/BHD/measureContrast.ipynb

I wanted to pass along a complication pointed out by K. Thorne re: our plan to use Gen1 (old) Dolphin IPC cards in the new real-time machines: c1bhd, c1sus2. The implication is that we may be forced to install a very old OS (e.g., Debian 8) for compatibility with the IPC card driver, which could lead to other complications like an incompatibility with the modern network interface.

I have a query out to Dolphin asking:

1. Have they done any testing of these old drivers on Linux kernel 4.x (e.g., Debian 9/10)?
2. Is there any way to buy modern IPC cards for the two new machines and interface them with our existing Gen1 network?

I'll add more info if I hear back from them.

Answers from Dolphin:

1. No, and kernel 4.x (modern Linux) definitely will not work with the Gen1 cards.
2. No, cards using different PCIe chipsets cannot be mixed.

Since upgrading every front end is out of the question, our only option is to install an old OS (Linux kernel < 3.x) on the two new machines. Based on Keith's advice, I think we should go with Debian 8. (Link to Keith's Debian 8 instructions.)

I went through the optics list (in the BHD procurement google spreadsheet) and summarized how to build them.

The red ones are what we need to purchase. Because of the strange height of the LMR mounts, the post needs to have none half-integer inch heights.

They need to be designed as the usual SS posts are not designed to be vac compatible (not because of the material but the design like screw hole venting).

We also need to check how many clean forks we have.
-> The components were ordered except for the custom posts.
 

[Jon, Tega, Hang]

We proposed a few BHD mode-matching telescope designs and then preformed a few monte-carlo experiments to see how the imperfections would change the story. We assumed a 2 mm (1-sigma) error on the location of the components and 1% (1-sigma) fractional error on the RoC of the curved mirrors. The angle of incidence has not yet been taken into account (no astigmatism at the moment but will be included in the follow-up study.)

For the LO path things are mostly fine. We can use LO1 and LO2 as the actuators (Sec. 2.2 of the note), and when errors are taken into account more than 90% of times we can still achieve 98% mode matching. The gouy phase separation between LO1 and LO2 > 34 deg for 90% of the time, which corresponds to a condition number of the sensing matrix of ~ 3. 

The situation is more tricky for the AS path. While the telescopes are usually robust against 2 or 3 mm of positional error, the 1% RoC does affect the performance quite significantly. In the note we choose two best-performing ones but still only 50% of the time they can maintain a power-overlap of > 99%. In fact, the 1% RoC error assumed should be quite optimistic... Not sure if we could achieve this in reality. 

One potential way out is to ignore the MM for the first round of BHD. Here anyway we only need to test the ISC schemes. Then in the second round when we have the whole BHD board suspended, we can then use AS1 and the BHD board as the actuators. This might be able to make things more forgiving if we don't need to shrink the AS beam very fast so that it could be separated from AS4 in gouy phase.

It would be good to have a corner plot with all the distances/ RoCs. Also perhaps a Jacobian like done in this breathtaking and seminal work.

As Rana suggested, we present the scattering plot of the AS path mode matching for various variables. The plot is for the AS path, Plan 2 (whose params we summarize at the end of this entry).

In the corner plot, we color-coded each realization according to the mode matching. We use (purple, olive, grey) for (MM>0.99, 0.98<MM<=0.99, MM<=0.98), respectively. From the plot, we can see that it is most sensitive to the RoC of AS1. The plot also shows that we can compensate for some of the MM errors if we adjust the distance between AS1-AS3 (note that AS2 is a flat mirror). The telescope is quite robust to other errors.

The compensation requirement is further shown in the second plot. To correct for the 1% RoC error of AS1, we typically need to adjust AS1-AS3 distance by ~ 1 cm (if we want to go back to MM=1; the window for >0.99 MM spans also about 1 cm). This should be doable because the nominal distance between AS1-AS3 is 115 cm. 

The story for plan1 is similar and thus not shown here. 

==============================================================

AS path plan2 nominal params:

label     z (m)     type             parameters  
-----     -----     ----             ----------  
SRMAR          0    flat mirror      none:     
AS1       0.7192    curved mirror    ROC: 2.5000 
AS2       1.2597    flat mirror      none:     
AS3       1.8658    curved mirror    ROC: -0.5000
AS4       2.5822    curved mirror    ROC: 0.6000  
OMCBS1    3.3271    flat mirror      none:   

I've started a spreadsheet for the BHD optics specifications and populated it with my best initial guesses. There are a few open questions we still need to resolve, mostly related to mode-matching:

• PR2 replacement: What transmission do we need for a ~100 mW pickoff? Also, do we want to keep the current curvature of -700 m?
• LO mode-matching telescope: What are the curvatures of the two mirrors?
• Lenses: We have six of them in the current layout. What FLs do we need?

The spreadsheet is editable by anyone. If you can contribute any information, please do!