Repeatedly in the past we had found the ISS PD's DC level jump to a state past 10Volts to somewhere arount 12V. The last time this occured last night (alog 9637). Staring at the PSL ODC screen, it told us that two things were wrong: BIT7 "ISS actuation saturation" and BIT11 "NPRO Relaxation Oscillation" were red.

Last night we fixed the ISS actuation saturation by readjusting the set point, but the PDs remaind in the odd 12V state.

Staring at the ODC screen this morning, BIT11 "NPRO Relaxation Oscillation" remained red. Taking that suggestion by ODC seriously, I went out to toggle the NPRO noise eater. Sure enough - the the ISS PD's switched back to the normal "<10V" state, and BIT11 went green.

Conclusions:

- Our long-standing ISS PD mystery is caused by noise eater oscillations.

- The PSL ODC system correctly announced this problem.

FYI, The ODC overview screen is available from the SYS tab on the site overview, and has links to all subsystem screens. The subsystem screens have a description string for each bit. Currently fully operational -including meaningful thresholds -are PSL, IMC, ISI and HPI for all currently commissioned systems, as well as SUS for MC1, MC2 and MC3. (All SUS's have a working ODC system installed, but the good settings are still a moving target due to commissioning.

I put the ODC master screen on video5. Due to the many not yet commissioned systems, there is still a lot of red there. In the attached plot, there is even more red due to the ongoing IMC length measurement. (stay tuned)

Related previous alogs:

alog 9637

alog 7077

Richard & company are moving this Dust Monitor to HAM4 (since it's an open chamber).  Still working on bringing it online (it's been INVALID).

This was Dave & Jim doing work with the hardware watchdogs and they said it shouldn't have affected anybody.

I have been silently checking the signal chain of the REFLAIR and POPAIR RFPDs using the AM laser (a.k.a. PD calibrator) to make sure that they are functional expectedly.

Summary

• REFLAIR_A and POPAIR_A looked good as they showed reasonable signal gains
• On the other hand, REFLAIR_B and POPAIR_B RFPDs showed relatively large discrepancy from the expected values. I might have missed some gain factors somewhere. I will revisit these RFPDs later when I get a chance.

The RF frequency of the AM modulation was adjusted in each measurement such that the demodulated IF signal was below 50 Hz.

Calibration of the amplitude modulation depth

We recalibrated the AM laser.

The current setting of the laser was changed recently because we opened up the current driver when we thought the laser diode had been dead in the early December. Then the laser head and its current driver were sent to Rich at Caltech for his extensive testing although the laser magically fixed itself and he didn't find anything wrong. So this was the first time for us to use the AM laser which had been fixed. Because of that mysterious event, I wanted to recalibrate the laser. First of all, Yuta and I measured the power to be 2 mW with an Ophir Vega without the attenuation filter. Then we measured the modulation depth for the amplitude modulation by using a Newfocus 1611 as a reference.

The new calibration for the amplitude modulation is:

P_am =  5.13 mW x (P_dc / 1 mW) * (1 V / V_drive)

where P_dc is the laser power at DC and V_drive is the drive voltage when it is driven by a 50 Ohm source. For example, if one puts this laser to a PD which then shows a DC laser power of say 2 mW, the AM coefficient is now 5.13 mW x ( 2 mW / 1 mW) /V_drive = 10.26 mW/V_drive.

• Some details for derivation:
  • DC transimpedance of NF1611= 10k Ohm
  • AC transimpedance of NF1611 = 700 Ohm
  • Responsivity of NF1611 at 980 nm = 0.5 A/W
  • Measured DC voltage = 7.36 mV
    • => DC laser power of 7.36 mV / 10kOhm / 0.5 A/W = 1.47 uW
  • Measured AC voltage when driven by 0.5 Vpp at 10 MHz = 1.32 mVpp (measured by a 50 Ohm oscilloscope)
    • => P_am = 1.32 mV / 700 Ohm / 0.5 A/W = 3.771 uW
    • => AM coefficient is 7.54 uW / V_drive
  • Therefore the AM calibration is:
    • 7.54 uW / 1.47 uW x 1 mW = 5.13 mW x (P_dc / 1 mW) * (1 V / V_drive)

REFLAIR_A_RF9 (S1203919)

Remarks:

The signal chain is OK. The PD response is smaller by 15% for some reason.

It seems as if the transimpedance is smaller by 15% than what had been measured at Caltech (LIGO-S1203919). The cable loss from the RFPD to the rack was measured to be 0.47 dB. Be aware that the demod gain is half of the quad I/Q demodulator because this is a dual channel demod (see E1100044). The demod conversion gain is assumed to be 10.9 according to LIGO-F1100004-v4.

• Measured DC at BNC jack = 108 mV
  • DC laser power = 108 mV / 100 Ohm / 0.67 A/W = 1.61 mW
• Expected AM signal when driven by 0.1 Vpp = 5.13 mW x (1.61 mW / 1 mW) x 0.1 Vpp x 0.67 A/W x 245 Ohm = 135.6 mVpp
• Measured AM signal at the SMA jack of the RFPD enclosure = 115 mVpp
  • This is already smaller by 15%.
  • If we blame the transimpedance, it must be 208 Ohm rather than 245 Ohm.
• Measured AM signal at the rack = 109 mVpp
  • cable loss = 0.47 dB
• Expected IF signal = 135.6 mVpp x 10.9 / 2. = 622 mVpp
• Measured IF signal = 570 mVpp
  • If we take the 15% and 0.47 dB losses into account, we would expect this to be 527 mVpp.
• Expected ADC counts = 135.6 mVpp x 10.9 x 2^16 counts /40 V x 1/2 = 1209 counts pp
• Measured ADC counts = 900 counts pp
  • If we take the 15% and 0.47 dB losses into account, we would expect this to be 973 counts pp.
  • This is within 10% divation. Good enough.

REFLAIR_A_RF45 (S1203919)

Remarks:

The signal chain is healthy.

Found cable loss of about 1.5 dB. The measurements excellently agree with the loss-included expectation.

• DC laser power = 1.61 mW (the same as REFLAIR_A_RF9)
• Expected AM signal when driven by 0.1 Vpp = 5.13 mW x (1.61 mW / 1 mW) x 0.1 Vpp x 0.67 A/W x 341 Ohm = 188.7 mVpp
• Measured AM signal at the SMA jack of the RFPD enclosure = 190 mVpp
  • Very good agreement !
• Measured AM signal at the rack = 160 mVpp
  • Corresponds to loss of 1.5 dB
• Expected IF signal = 188.7 mVpp x 10.9 = 1028 mVpp
  • If the loss is taken into account, it is going to be 1028 mVpp x -1.5 dB = 865 mVpp
• Measured IF signal = 880 mVpp
  • Consistent with the loss-included expectation
• Expected ADC counts =1028 mVpp x 2^16/40 x 2 = 3369 counts pp
  • If the loss is taken into account, it should be 3368. x -1.5 dB = 2837 counts pp
• Measured ADC counts = 2784 counts pp
  • Good agreement with the loss-included model

POPAIR_A_RF9 (S1300521)

Remarks:

The signal chain is healthy.

The measurement suggests that there is loss of 1 dB somewhere. I didn't measure the cable loss this time.

• Measured DC at the BNC jack = 95.6 mV
• Estimated DC laser power = 95.6 mV / 100 Ohm / 0.67 A/W = 1.42 mW
• Expected AM signal = 5.13 mW x (1.42 mW / 1 mW) x  0.1 Vpp x 0.67 A/W x 741 Ohm = 361.7 mVpp
• Expected ADC counts = 361.7 mVpp x 10.9 x 2^16/40 counts/V = 6454 counts pp
• Measured ADC counts = 5705 counts pp
  • If we assume this discrepancy is from a loss, it corresponds to loss of 1.07 dB.

POPAIR_A_RF45 (S1300521)

Remarks:

The signal chain is OK. Though loss sounds a bit too high.

The measurement suggests a possible loss of 2.6 dB somewhere. I didn't measure the cable loss.

• Estimated DC laser power = 1.42 mW
• Expected AM signal = 5.13 mW x (1.42 mW / 1 mW) x 0.1 Vpp x 0.67 A/W x 837 Ohm = 408.5 mVpp
• Expected ADC counts = 408.5 mW x 10.9 x 2^16/40 counts/V = 7295 counts pp
• Measured ADC counts = 5391 counts pp
  • If the discrepancy is assumed to be from loss somewhere, the loss should be 2.6 dB.

REFLAIR_B_RF27 (S1200234)

Remarks:

The signal gain is bigger than the expectation by a factor of 2.3.

• Measured DC at the BNC jack = 836 mV
  • This corresponds to a laser power of 836 mV / 2000 Ohm / 0.4 A/W = 1.045 mW
• Expected AM signal = 5.13 mW x (1.045 mW / 1 mW) x 0.05 V_drivepp x 0.4 x 1000 Ohm = 107.2 mVpp
• Expected ADC counts = 18 dB x 107.2 mVpp x 10.9 x 2^16/40 counts/V = 15206 counts pp
  • I assumed that the gain factor of the diplexer (LIGO-D1300989) to be 18 dB at 27 MHz.
• Measured ADC counts = 35431 counts pp
  • This is bigger than the expected by a factor of 2.3

REFLAIR_B_RF135 (S1200234)

Remarks:

The signal gain is bigger than the expectation by a factor of 1.5

• Measured DC laser power = 1.045 mW
• Expected AM signal = 5.13 mW x (1.045 mW / 1 mW) x 0.0014 Vdrivepp x 0.4 A/W x 1000 Ohm = 3 mVpp
• Expected ADC counts = 43 dB x 3 mVpp x 10.9 x 2^16/40 counts/V = 7573 counts pp
• Measured ADC counts = 11689 counts pp
  • This is bigger than the expected by a factor of 1.54

POPAIR_B_RF18 (S1200236)

Remarks:

The signal gain is bigger than the expectation by a factor of 2.3

• Measured DC at the BNC jack = 744 mV
• DC laser power = 744 mW / 2000 Ohm / 0.4 A/W = 0.93 mW
• Estimated AM signal = 5.13 mW x (0.93 mW / 1 mW) x 0.1 Vdrivepp x 0.4 A/W x 1000 Ohm = 191 mVpp
• Estimated ADC counts = 191 mVpp x 10.9 x 2^16/40 counts/V = 3404 counts pp
• Measured ADC counts = 7803 counts pp
  • This is bigger than the expected by a factor of 2.3

POPAIR_B_RF90 (S1200236)

Remarks:

The signal gain matches with the expected value, but I don't believe this.

• DC laser power = 0.93 mW
• Estimated AM signal = 5.13 mW x (0.93 mW / 1 mW) x 0.1 Vppdrive x 0.4 A/W x 1000 Ohm = 191 mVpp
• Estimated ADC counts = 191 mVpp x 10.9 x 2^16/40 counts/V = 3408 counts pp
• Measured ADC counts = 3549 counts pp
  • This suspiciousely shows an excellent agreement with the expected one.

I've attached a plot showing that the IMC has 'not been in a good state' (i.e. red) for the last 21 hours due to the ASC control switch not being on (summary bit 1). I can't see anything obvious in the alogs to determine why this is, anyone know? Thanks

(Correction Jan/31: Sheila's made a similar plot for a period (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=9691) where the cavity was locked all the time, and it seems like the plot presented here was sampling only a small edge of the 00 lock range.)

As of now, it seems like both ETM and ITM should be within 0.3 urad pk-pk from the optimal alignment to stay locked on 00 mode.

Plot 1: I took OL and green transmission data for 5 minutes when the cavity was going back and forth between 00 and 01 mostly without losing lock.

In the top panel, the transmission time series is actually the sum of the SHG and the transmission, but it's on 00 mode when it was 1400 counts, 01 when it was between 1000 and 1200 counts.

Plot 2: 2D color map of the above data.

X axis is ITM OL PIT, Y is ETM. I arbitrary set the threshold of 1300 counts for green transmission and subtracted that from the transmission data, and set the color map such that orange-red color is above 0 (i.e. locked to 00 mode), yellow-green is negative  (01 mode).

Each cell is normalized by the number of data points in that cell. Blue means there's no data point in that cell.

Sorry that X and Y axis scaling is different. To aid your eyes, I put white and pink diagonal lines showing the direction of common (soft) and differential (hard) misalignment.

Plot 3: 2D plot of where the OLs were staying.

You cannot tell from plot 2 where the OL was staying the most, so here is the number of data points in each cell divided by the total number of data points. Adding everything on this map together you'll get 1.

On average EX=-28.12, IX=-6.28.

Conclusion:

1. Thin band on 00 mode area, elongated along the white (soft or common) line,  shows that the problem is along the pink line (differential or hard) due to cavity geometry discussed before by people.
2. In order to stay locked on 00, both ITM and ETM should be within 0.3 urad pk-pk or somethibng, otherwise you'll drop out of the locking range along the pink line.
3. The DC alignment was not optimal despite dither alignment. [ITMX, ETMX] could have been centered around [-6.4, -27.2 urad] (instead of [-6.28, -28.12]). Not sure what to make out of this, as the dither servo was going on and off each one second.
4. A factor of 2 or 3 improvement will make ITM great, but ETM needs more (but note that the data was when it was bad).

Note that this is dependent on RF sideband frequency.

This morning Filiberto, Travis and I made grounding checks of the SUS cables on the ISI.  We also check a few of the TMS cables that were easy to access near our bundle.  Coery checked some prior as well.  Long story short, we only found 1 cable route that has a ground issue and it is on a TMS segment that runs to BOSEMs.  It is the connection with the D100225-S1104782 cable in it.  This cable has been through testing, so likely we can find and remedy this ground.  Coery said he'll add it to the TMS hit-list.

We've observed some burn marks on the shielding of the ETMy ring heater cables which occured sometime during the 3 prior weld sessions (May 2012, Dec 2013, Jan 2014).  The burns are on the lowest ring heater cable that laces around the test mass and makes a connection between the test mass and the PUM.  There is a burn on the right segment and the left segment.  I dug up a picture that shows that the burn on the "right" segment (as viewed from the back of the suspension) was there just after the May 2012 weld session, so that one is not new.  However the "left" cable burn is new from the Dec or Jan welding.  We did not see when this actually occured.

Filiberto tested the ring heater cable and found that all pins are operating as per spec, although he thinks pin 1 is shorting.  We are tracking down what this means (did we test it correctly, when was it last tested, is it possible that the burn is contributing to the short, even though it does not look like it is, etc.).

I'm making a measurement of the ITMX direct reflected beam size on ISCT1. PRM is misaligned, sending the beam to the parking dump for this mesurement. Please do not align PRM as this will make the REFL beam too powerful for the beam analyzer and may fry it.