This entry summarized the status of REFLAIR_B signals. REFLAIR_B is the broadband photodetector, D1002969, mounted in reflection of the interferometer which is responsible for measuring the 3f signals at 27 MHz and 136 MHz in DRMI. The 3f signals are relatively weak, especially at low modulation depth. This has lead to a low signal-to-noise ratio at sub milliampere photocurrent and distortion problems above a milliampere. The problem of saturation by out-of-band intermodulation products was recognized early on a LLO and fixed with a diplexer amplifier, D1300989.

The situation at LHO and LLO are essentially the same.

• LLO operates with 0.2 mA of photocurrent and can lock both the DRMI and the full interferometer. The signal-to-noise is not great and sensor noise dominates the 3f locking signals above 20 Hz. The modulation depth is three times the design value and will generate some excess light at the antisymmetric port.
• LHO operates around 4 mA of photocurrent and can lock DRMI with a better signal-to-noise ratio in the 3f locking signals. However, the signals have a strong dependency on the incident power, see here.

Koji has recently measured the second order intermodulation products of the RF chain in the broadband photodetector and found the second order intercept point to be rather low, around 30 dBm. This is maybe not too surprising considering these RF amplifiers are single transistors or Darlington configurations operated around a fixed DC working point. No specification of the IP2 is given in the datasheet.

This leads to the conclusion that 3f signals at LHO are predominantly due to intermodulation distortion in the RF amplifier chain and not due to the optical signals. This obviously works just fine for the DRMI, but probably doesn't give the required immunity to the carrier mode for full interferometer locking.

Possible solutions:

• Try to operate LHO at low photocurrent and re-optimize the DRMI locking signals.
• Removed the first amplifier stage from the broadband photodetector. This will remove 22 dB of gain and hopefully greatly reduce the RF distortion. The dark noise of the photodetector will increase by about 3 dB. At 27 MHz we should be shot noise limited with 0.2 mA of photocurrent. So, this should have little effect on the sensor noise. However, at 136 MHz the shot noise limited sensitivity is around 2 mA, so we should see a ~1 dB increase in the noise level.
• Get the 3f photodetector which is based on a tuned design into fabrication.

Increasing the modulation depth and reducing the photocurrent at the same time will not improve the situation, since the distortion depends on the absolute signal strengths of the individual RF lines. However, removing the first amplifier stage should give us enough room to further increase the modulation depth and improve the signal-to-noise ratio. Using a high modulation depth during locking may require an adjustable EOM driver, such as the, D0900760.

Betsy, Travis

This alog represents the last ~month of intermittent assembly work on the 3IFO QUAD unit "Q7".

After switching out 3 problematic wire segments and the entire set of top stage blade cartridges, we have finally launched a first round of testing on Q7 this morning.

Details:

After assembling and fully suspending Q7, we had a difficult time bringing the 6 DOFs of each chain into coarse alignment.  We discovered that the top most blades used in the QUAD were the softest of any QUAD unit thus far, as per the stiffness catalog T1000068.  In fact, this set had a much wider variation in stiffness than any other unit as well which was adding to our challenge.  We decided that we should delegate this set of blades to the spare bin and swap them with another set.  After swapping in a set from the middle of the stiffness range (Tops Set 7), we saw the QUAD chain alignment was much improved in side shift.  We then continued to struggle with differential height.  We decided that we might have had a wrong wire segment length lower in the chains so we first switched in new main chain PUM loop which had no effect, and then switched in a new set of reaction bottom and final wires which did help our differential height issue.  (We confirmed the removed sets were ~1-2mm longer than the new set although we really don't know which is "correct".)   With all of this new hardware, we were able to align the chains to within the coarse specs.  We laced up the reaction chain lower stages and readjusted alignments and suspension mechanical groundings.  Yesterday we assembled the top mass tablecloth and cabled/attached/aligned top 12 BOSEM sensors.  Today we are running TFs.

Over the past 2 days, there has been a series of earthquakes (26 so far, ranging from 2.5 to 4.7 magnitude) near Lakeview, OR, approximately 400 miles south of LHO.  Something for commissioners to keep in mind as they struggle to lock the IFO.  See attachment.

re WP 4913

I switched the HEPI pump Servo to run on the true differential pressure as LLO has been doing forever.  I did glitch the system through my sillyness.  I was all ready to do it without any serious glitch and next time it will be better.  Still, the HEPI position loops on the platform did not trip--pretty good support for the hydraulic system.  The differential pressure now does look slightly noisier based on the time series.  But of course this is now a sum of two noisy signals, maybe time for some smoothing and serious grounding study.

Attached is 45 minutes of second trends for the Pump station channels and some cartesian basis HEPI L4C signals.  I don't see any ill affects of this transition so far.

So the alarm manager is okay as I changed the database alarms.  The medm though is also common so if you open the EndY HEPI Pump Servo medm, it will show pressure not okay.  When it is deemed okay to continue down this path, I'll update the medm when I install the same database changes at EndX.

no restarts reported

Alexa, Nic, Evan, Dan, Sheila

Tonight we went back to trying to get ALS DIFF working well.  We aren't sure what the problem is. 

First Nic checked that the gain of the 2 ESDs were the same.  The old X gain in L3 LOCK L was 0.2, the new one is 1.12 The old Y gain was 0.7 the new one is 0.384.  We also checked the crossover by using the green control signal. 

Attached are some plots, based on the suspension model that Jeff used to design the DIFF loop, and filters downloaded from foton. This is using none of the boosts for DIFF, which is how we have been running the last few nights without being able to use the ESD.  The two plots on the left are the cross overs for our loop, and the open loop.  We can use these plots to compare what we have now to what Jeff originally designed, (G1400146-v2)  We also copied the livingston filters, see alog (12590), their crossover is the third plot while their open loop (we added a scaling factor to get the right ugf) is the last plot on the right. 

Btoh ESDs are driving, but we seem unable to use them in the loop. 

Nic, Alexa, Sheila, Evan, Dan

We wanted to measure the crossover between L1 and L3 of ETMY. We took a transfer function from LOCK_L1/L3 to ALS-Y_REFL_SERVO_CTRL_OUT with green locked to the yarm. We found an old template made by Sheila and Stefan for the L1 stage. We repeated the measurement for a few data points (green trace), and it seemed the same as the old measurement (blue) so we aborted the measurement and went with the assumption that it was still a valid measurement. We then measured for the L3 stage (red trace). Clearly the crossover between L1 and L3 is about 2 Hz as expected.  Nic plans on dividing these two transfer functions tomorrow morning... 

Fo reference the xml file is located here: /ligo/home/sheila.dwyer/ALS/HIFOXY/Y_UIM_2.xml

After spending far too long scratching my head about calibrating the OMC DCPD signals, I've measured the RIN of the OMC transmitted beam, and it's not pretty.

To make the measurement I locked the OMC on the side of a carrier TEM00 fringe, as described here.  I measured the open-loop transfer function; this is the first plot attached (UGF is 100Hz, which is about the same as when we're dither-locked on the peak of the fringe with much higher gain in the LSC servo).  To calibrate OMC-DCPD_NORM into true RIN I divided by 1/(1+G) in DTT so that the loop suppression is accounted for.  To calculate the RIN seen by other PDs, I divide by the PD sum at DC.

The results, in the second plot, show that the beam incident on HAM6 has some issues.  At high-ish frequencies, 100Hz and up, the OMC Trans intensity noise is due to noise out of the mode cleaner.  It's coherent with IM4 Trans and the ISS second loop PDs.  Probably this noise can be mitigated using the ISS second loop and also reducing the IMC angular fluctuations that were described by Gabriele.

At low frequencies, between about 0.2 and 3Hz, there is huge intensity noise on the input beam to HAM6 beam that is not seen just after the IMC (neither IMC_TRANS or the ISS second loop PDs).  The noise is seen in HAM6 by the OMC QPDs (in green - limited by electronics noise above 50Hz?) and by ASC-AS_C (dashed black).  At high frequencies the intensity noise in HAM6 is coherent with the noise out of the IMC, but the loud stuff around 1Hz is likely due to the clipping that Keita measured yesterday.

The third plot shows an attempt at characterizing the length noise of the OMC; the loop-corrected RIN was converted into meters using the derivative of the usual Fabry-Perot transmission formula at half-resonance (the conversion factor is 3.7e8 RIN/meter).  This is a first step towards building an OMC noise budget, following the work by Zach at LLO.  At high frequency (around a kHz), the noise begins to drop below the limit of 3x10^-16 meters/rt[Hz] prescribed by Valera's estimate in G1100903, but there's a forest of lines around 1kHz that's not unlike what was observed at L1.  At lower frequencies, the noise seen at the DCPDs is dominated by the input intensity fluctuations, and it's not possible to measure the length noise that's intrinsic to the cavity.  The photocurrent during these measurements was about 3mA for each DCPD (shot noise RIN of 10^-8/rt[Hz]).

Evan, Alexa

Even after the ETMY L2P work performed yesterday (alog 14832), last night we found the L 2 angle for ETMY was still bad. So today we did some more investigation, again. I don't think we made any big head-way, but I will post what we did and the results anyways...

Setup: We excited ETMY_L1_LOCK_L at various frequencies with an amplitude of 5e5 cts and 0 deg phase. The LOCK filters that are nominally installed were left on. Using the pitch lock-in, we demdoluated the pitch oplev signal at the respective frequencies and noted the I and Q signals. We then turned off this excitation and excited ETMY_L1_DRIVEALIGN_Y2P_EXC. This Y2P filter bank was empty, and we changed the gain from 0 to 1. The phase of this excitation was also set to 0 deg. We adjusted the gain such that the magnitude of the I,Q signals was close to that induced by the LOCK_L excitation. Then, we adjusted the phase of the Y2P excitation until we reduced both I, Q signals to zero. In other words, we drove in length with the LOCK_L, and examined the motion in the pitch oplev. We reduced this motion with an excitation in the Y2P drive align matrix. We had hoped this information would help us create a patch and improve our L2P in L1. 

Results:

0.8 Hz

With both excitations, and Y2P phase set to 55 deg, then I = -0.026, Q = -0.006

0.6 Hz

With both excitations, and Y2P phase set to 1 deg, then I = -0.01, Q = -0.01

Alexa, Evan

The ETMY oplev damping was off. We have turned both picth and yaw damping back on.

Additionally, I have tweaked the filters and the gain slightly in order to reduce gain peaking:

• Pitch now has an elliptic low-pass at 15 Hz rather than a Chebyshev at 10 Hz. The gain is now -0.3 ct/ct.
• For yaw, the filters are a 0:0.1 z:p pair, a broad notch at 0.6 Hz, a 15 Hz elliptic low-pass, and a notch at 1.33 Hz (this peak shows up in the yaw spectrum and I assume it is the PUM pitch mode of the suspension).

According to conlog, the loop gains appear to have been randomly adjusted several times over the last week.

We hope that this might improve the robustness of the ALS locking. If not, we might try a similar recommissioning of the ETMX oplev damping (since this are also off).

I and Daniel were talking about the reason why there are bounce and roll coupling, and the bounce should be the local gravity axis versus the arm angle.

I looked at old DCC document (T980044) and found that Y arm is almost parallel to the vertex local Y axis while X arm is almost parallel to the X end local X axis.

Below is a table of angle deviation from pi/2 between the relevant arm and the local vertical defined by the gravity.  Deviation=0 means that the arm is orthogonal to the local gravity. FYI, the difference in the local gravity angle over 4km is about 2*pi*4km/40000km = 628 urad.

The bounce coupling is directl proportional to these numbers.

LHO EY is 80 times worse than LHO EX. At LLO EX is worse than EY, but LLO EX should be a factor of 2 better than LHO EY.

By Keita.

Jeff pointed out that some incoherent sensor noise (I think that's the phrase he used) in Krishna's post from earlier today. He suggested this may be due to RY coupling to X. We might be able to fix this by pushing up the blend on RY or use a blend the rolls off the seismometers at low frequency. RX and RY already use Ryan's 250 mhz blends, so at first glance we only have 1 filter with a higher blend. I've attached 4 plots, the first three of which show CPS, L4C and T240 components respectively of our available blends. The fourth plot compares the T01_28 (dashed) components and the current 250mhz blend (solid). It looks like the T01_28 maybe (?) fits the bill for more low frequency roll off of the seismometers.

9:15 Fil Aaron to EX working on cabling
10:00 Alastair to LVEA TCS-Y table
10:15 Hugh, Gerardo to LVEA
10:30 Alexa transitioning EY
11:30 JeffB to LVEA, moving SUS pallets for 3IFO
11:00 Kyle to EY cleaning up tools, done 12:00
13:00 JeffB, Bubba to LVEA looking at craning operations around High Bay, out 13:15
14:00 Gerardo to LVEA
14:30 Hugh, Suresh to HAM2 HAM3 for oplev work

K. Venkateswara

I had installed temperature sensors on BRS and GND_T240 yesterday as described in 14825. The first plot shows the trend over a day along with the PEM_VEA temperature sensor. The count to Kelvin conversion was expected to be 1.56e-3 K/count. This seems roughly consistent with the temperature of the T240. The BRS temperature sensor shows a lower magintude and a phase offset due to it's extra thermal shielding and larger mass.

The attached pdf shows the ASD of the temperature sensors and the coherence between them and their respective instruments. The temperature sensors are mostly limited by ADC noise. An op-amp based pre-amp of 50-100 gain would be useful. BRS_RY_Out shows a little bit of coherence near few mHz while T240 X, Y and Z show no coherence with the temperature sensor.

J. Warner, K. Venkateswara

We have turned on sensor correction on Z and X at ETMX since noon today.

Z on ETMX had been switched to TBetter blend by an unknown person during the weekend. Jim switched it back to the 90 mHz blend(Ryan's LLO blend). We then also turned on the X sensor correction which is currently using the tilt-subtracted super sensor.

The first two pdfs show the before and after plots for the two configurations. Before was taken ~3 am this morning and after was at ~12 noon. Note that the improvements in Yaw/Pitch at low frequencies were largely due to going to the 90 mHz blend and then using Z sensor correction.

The third plot shows the improvement in X and Z transfer functions from the ground to Stage 1 before and after the configuration change.

It seems to be helping over a large frequency range above 10 mHz and not hurting anywhere, so we will leave it on for overnight monitoring. If it affects commissioning in any way, it is easy to turn off through the ISI screens.

edit: Z sensor correction is to HEPI_IPS and X is to Stage1_CPS

As the wandering laser intensity continues I intend to learn more about these systems. I can only report the findings and make the adjustments. ISS diffracted power is down to ~5% this morning. REFSIGNAL was at -2.04V as it was set by me yesterday. Today I have adjusted REFSIGNAL to -2.02V to bring the diff power to ~8.6%. Attached is a past 5 day trend. the spikes, i am told,  are likely caused by activations on MC2? (for example)