Estimated Length Motion of the JAC

The calibrated error signal and feedback signal spectra were measured to estimate the free-running length motion of the JAC. Below the unity gain frequency (UGF = 400 Hz), the feedback signal represents the cavity length motion, while above the UGF the error signal represents the motion. The estimated length noise is well below the design assumption and does not appear to limit the JAC performance.

Method

The spectra of the calibrated error signal and feedback signal were measured to estimate the free-running length motion of the JAC.
- Below the UGF (400 Hz), the feedback signal represents the cavity length motion.
- Above the UGF, the error signal represents the cavity length motion.
Around 400 Hz, the spectrum appears slightly inflated because of the phase bubble, but the actual noise level is expected to be approximately flat in that region.

Observed Features in the Spectrum

- Above 30 Hz
Above 30 Hz, the spectrum becomes flat. This is likely not real cavity motion, but instead electronics noise from the readout chain. A more careful calculation is needed to identify the exact source, but it is most likely the photodiode or the ADC.
Since the incident power is currently 1 W, this noise floor is expected to decrease if the input power is increased.

- Below 10 Hz
Toward DC, the spectrum rises approximately with an f^-3 slope. This is interpreted as drift in the PZT control signal caused by temperature drift.
Therefore, the actual cavity length variation at low frequencies is expected to be smaller than what appears directly in the measured spectrum.

- Estimated Length Noise
Looking at the spectrum around 10 Hz, where the above effects are not expected to dominate, the cavity length noise is estimated to be approximately 2 × 10^-14 m/rtHz.
In the design, the cavity length motion was conservatively assumed to be 1 × 10^-12 m/rtHz at 10 Hz.
Therefore, the measured result is well below the design assumption, indicating that the loop design and the JAC do not introduce problematic intensity noise or phase noise.

Sanity Check with the FSS Unlocked

As a sanity check, the same measurement was repeated with the FSS unlocked. In this measurement, the vertical axis was converted into laser frequency noise by multiplying the calibrated length signal by
FSR / (lambda / 2)
where the free spectral range is given by
FSR = c / L
with c the speed of light and L the cavity round-trip length. For the JAC, L = 2.02 m.

The resulting spectrum, shown in the second plot, is approximately 100 Hz/rtHz at 100 Hz. This is consistent with the typical frequency noise of the NPRO laser.
This also confirms that the JAC is sufficiently quiet compared with the NPRO noise level.

Today we have been having some difficulty in locking the IMC, which might be related to MC1 an MC1 suspension problem.  

With the mode cleaner aligned and flashing, we checked the IMC REFL RF24 I and Q signals to check that the PDH phase looks OK, it does.  This is a similar amplitude to what we saw in trends from a time when the IMC was locking, here.  

WP13043 h1sush6 front end install

Daniel, Erik, Fil, Jonathan, Dave:

The new h1sush6 front end system is running with its full card complement and a basic IOP model.

On Wednesday afternoon we got the computer booting and seeing most of its chassis, detailed in alog 89376

The outstanding issues were: timing card was not receiving a timing signal, 4th Adnaco BP was not seen.

On Thursday we tracked the fibre issues to a not-quite-seated MTP on the MSR's MER patch (port 3). Once this fibre was reseated correctly the timing card received its signal and the 4th Adnaco BP was seen.

At this point I built up the IO Chassis with the correct card layout, using the ADC and 16bit-DACs provided by the BHD group. We supplied the Interface cards and ribbon cables from stock.

As of end-of-business Thursday the IO Chassis was almost complete, I had miscounted the 16bit-DACs and we were one card short. I built h1iopsush6 with this partial layout and we got the model running.

Friday lunch time I installed the 5th 16bit-DAC and added it to h1iopsush6. The system is now complete as-per drawings G2301306

H1SUSH6 IO Chassis Layout

This morning I went to the bifurcated laser hazard and adjusted alignment of the ALS SHG path on ISCT1, picking up where Jenne left off yesterday.  Drawing here

The IR beam was reasonably centered on the bottom periscope mirror and the 1" steering mirror right after it, it was clipping on the 1" BS used for the monitor PD.  I yawed the 1" mirror right after the periscope to reduce the clipping, seen on a card right after the BS.  Then I continued to yaw that first steering mirror to get a beam transmitted through the SHG, as you translate the beam you can see lots of glints transmitted which aren't the actual beam.  As soon as there was full beam going through the SHG, the green beam was alinged onto the COMM A broadband PD, so I didn't move any steering optics other than the first one.  

The monitor diodes do not agree with the power levels that they showed before the table move.  The IR one shows 6 times more power than before the table move, which makes me think the diode may not have been well aligned at the end of O4.  The beam in reflection off that first beamsplitter has two lobes visible on a card, possibly the front and back surfaces of the BS.  The beam was clearly missing the green power monitor PD, (before photo), I moved the diode to center, but the beam is still too low.  Nearly a year ago TJ and I wrote that this diode wasn't working, 84558, it does but seems to have been working since June 9th 2025, a day when the table was realigned as described in 84900. Since this alignment puts the beam onto the COMM diode, which is quite far from the SHG, I think it must be similar to previous alignments, and we probably need to adjust the height of this diode.

The power on the COMM A PD is about 300 counts, similar to what it was in O4.  So, this is probably a good enough alignment for us to proceed with locking with.  

Summary

The JAC length servo was designed to set the unity gain frequency (UGF) at 400 Hz. Additional low-frequency boost was implemented to improve suppression below 50 Hz. The open loop gain (OLG) was measured and compared with the servo model.

Details

• PZT actuator compensation

  The PZT driver has poles at 1 Hz and 400 Hz, and a zero at 10 Hz.
  To compensate for this response, a zpk(400, 10, 40) filter was implemented in the servo.
  With this compensation, the actuator response becomes approximately a first-order low-pass with a cutoff frequency of 1 Hz.

• Unity gain frequency

  The servo gain was then set so that the unity gain frequency (UGF) is 400 Hz.
  The phase margin at 400 Hz is 37 degrees, which includes the phase contribution from the first-order low-pass (90 degrees) and the additional phase delay in the system.

• Low-frequency boost

  A 1–50 Hz boost filter (first-order pole-zero boost filter) was implemented to improve low-frequency suppression.
  The phase contribution of this filter is –7 degrees at 400 Hz, resulting in a total phase margin of about 30 degrees at the UGF.
  Also, the integrater was implemented below 1Hz. Combination between boost and integrator gives f^{-2} slope below 50Hz.

• Open Loop Gain measurement

  The open loop gain (OLG) has been measured. A comparison between the measured OLG and the model is shown in the attached plot in the left panel. The right panel is the TF of the PZT.

• MEDM interface

  The OLG measurement template can be opened from the spectrum/OLG button in the bottom-right corner of the MEDM screen.

Summary

The calibration of the PDH error signal and the feedback path was derived using the Guardian-based signal normalization. The normalized PDH error signal allows the optical gain to be calculated analytically. The transfer function from L_SERVO_OUT to the cavity length actuation was measured and modeled, separating the optical gain and the PZT actuator response.

Details

Normalized PDH error signal

With the Guardian normalization, the PDH error signal at L_SERVO_IN1 can be written as
V = x / (1 + x^2)
where V is the signal at L_SERVO_IN1, and
x = l / HWHM
where l is the cavity length fluctuations and HWHM is the cavity half-width at half-maximum.

Using the finesse F, the cavity HWHM is
HWHM = lambda / (4F)

At the lock point (x = 0), the slope of the error signal is
dV/dx = 1

Optical gain

Therefore, the optical gain is
dV/dl = dV / d(x * HWHM) = 4F / lambda

Using F = 125, lambda = 1064e-9 m, the optical gain becomes
dV/dl = 4.70e8 cnts/m

Error signal calibration

To convert the signal at L_SERVO_IN1 to cavity length, we apply the inverse of the optical gain.
Calibration factor = 2.128e-9 m/cnts

Plant measurement

After locking the cavity with a provisional filter, the transfer function from L_SERVO_OUT to L_SERVO_IN1was measured and treated as the plant (see attached plot).
Since this plant includes both the optical gain dV/dl and the PZT actuator response, L_SERVO_IN1 was converted into meters using the calibration factor above before the measurement.
Also, the servo output is calibrated in V (and converted into cnts at the PZT_DRV filter). That means, the measured plant represents the transfer function from the PZT driver input to the actual cavity length actuation with the Unit of m/V

Plant model and actuator calibration

The optical gain and PZT actuator response are implemented in the servo model as FM9 and FM10 of L_SERVO.
In addition, a zpk(-800, 800) filter is included to emulate the phase delay.
The comparison between the model (FM9*FM10) and the measured plant (uncalibrated) is shown in the second plot. This response includes the PZT driver transfer function. That has two poles at 1 Hz and 400Hz, and one zero at 10 Hz. The DC gain estimated from the measured TF is 2.57 nm/V. This is comparable to the value measured with the scan of the JAC (2.97 nm/V).

Error signal normalization consistency

The error signal at JAC-L_SERVO_IN is normalized by the power at output of JAC_REFL_A_RF43. Therefore, once the guardian normalization procedure has been executed, the same calibration factor should remain valid.

I was adjusting the alignment of MC1 pitch earlier, when the suspension had a large alignment shift that wasn't caused by a change in the requested drive to the DACs.  The attached screenshot shows my requested alignment shifts in opticalign, and the resulting change in the requested DAC counts on T2 + T3.  There is a sudden 420 urad jump in the osem readbacks from pitch, and a 133 urad jump in roll at the first time cursor.  The master outs only see a few cycles on oscillations, which probably is from the damping loops responding to the jump, but they settle to the same requested drive as before the jump.  The jump seems to be real as the IMC flashes disappeared at this time.  

Fri Mar 06 10:17:49 2026 INFO: Fill completed in 17min 45secs

Gerardo says "long fill due to change of configuration, GVs are open"

Summary

The JAC cavity was locked in vacuum for the first time. Alignment was performed using the JM1 suspension and PSL PZT scan. After lock acquisition, the demodulation phase was optimized and a new Guardian state (NORMALIZE_SIGNALS) was implemented to automatically normalize several signals based on a slow PZT sweep.

Details

First in-vacuum lock

• The JAC cavity was successfully locked in vacuum.
• Prior to lock acquisition, the cavity alignment was adjusted using JM1 suspension and PSL PZT scanning.
• The PSL PZT was scanned with a 10 s period, and the alignment was tuned so that the TEM00 peak became dominant.
• After locking, the alignment was fine-tuned by maximizing the transmitted power.

Demodulation phase optimization

• A 300 Hz excitation was injected into L_SERVO.
• The demodulation phase was adjusted to minimize the signal appearing in the Q phase.
• The optimized demodulation phase is 63.37° (MEDM value)

Guardian update: NORMALIZE_SIGNALS state

• A new Guardian state NORMALIZE_SIGNALS was added.
• In this state, a 100 s PZT sweep is performed and used to normalize several signals automatically.

Normalization steps

1. JAC_REFL_A_RF43 input offset
The median value of JAC_REFL_A_RF43_I/Q_IN1 during the scan is taken as the offset
The negative value is applied as the input offset.

2. PDH error signal normalization
The peak-to-peak value of JAC-L_SERVO_IN1 during the scan is measured.
The signal is normalized so that the peak-to-peak becomes 1, by setting
REFL_A_RF43 filter gain = 1 / (max − min). The normalized signal is shown in the attached plot.

3. PZT scan trigger normalization
The peak photocurrent during the scan is measured.
This value is written to the Beckhoff channel JAC-TRANS_A_DC_NOMINAL to normalize the current signal.

4. Normalization report
A summary plot is automatically generated and saved as
/opt/rtcds/userapps/release/ioo/h1/medm/plots/normalize_report.png
You can open the plot from the button in the bottom-right of the MEDM screen

TITLE: 03/06 Day Shift: 1530-0030 UTC (0730-1630 PST), all times posted in UTC
STATE of H1: Planned Engineering
OUTGOING OPERATOR: None
CURRENT ENVIRONMENT:
    SEI_ENV state: EARTHQUAKE
    Wind: 20mph Gusts, 16mph 3min avg
    Primary useism: 0.34 μm/s
    Secondary useism: 0.18 μm/s 
QUICK SUMMARY:

• More alignment and commissioning work planned today
• We are still ringing down from a 6.3 from the Solomon islands

Jason and Jennie found that there's no digital signal coming out of JAC WFS DC no matter what.

I and Jason went to the floor, I flipped the gain switch of the WFS interface from low to high, but there was absolutely no response from any of H1:JAC-WFS_[AB]_SEG[1234]_INMON.

We confirmed that interface cables are connected from the IOT1 to the WFS interface (ISC-R1 slot U10), the interface is connected to the AA (ISC-C1 slot U30), and the AA chassis is connected to ADC5. These all agree with D1900511 floor wiring diagram. I even disconnected the cable coming to AA (ISC_373) and WFS DC signals didn't respond.

It seems that there's discrepancy between the floor wiring diagram and the model which was made according to T1100472 where JAC WFS DC signals are routed to ADC3 (see attached).

Good news is that RF channels are working.

As of now, the light is hitting WFSA and I was able to phase RF using an excitation injected into JAC PZT while JAC itself was locked. WFSB is dark now. 

S Muusse, C Compton

Alignment of the visible laser was completed on the X table. This means there is very little optical setup that is left to do before moving the tables into the LVEA and aligning the QPD pickoff when the viewports are ready! Still to do is to check beam pointing and size with the movement of L2 on the translation stage.

Feedthrough panels for both tables were constructed as per D2500305 and have been installed onto one of the tables. Currently, we are determining how the laser cables will be installed in the panel and the power supplied for the fan, visible laser and the flipper which will use some combination of the top 3 currently empty ports. The chassis are also being mounted on the racks that live under the tables. We are steadily progressing from testing into getting the electronics and cables in there forever homes before the tables are moved to the LVEA. 

[Jenne, Corey, RyanS, Jane Glanzer]

After EE fininished cabling ISCT1 and IOT1, Keita and I put up laser barriers to make the South Bay of the LVEA local laser hazard ("bifurcated"), so that both tables can be opened at will.  

Please pay attention to the signs on the laser barriers, and if the barriers are closed and / or the signs show Hazard, then wear laser safety goggles beyond the barriers.

The Vac team opened the gate valves, and we ran the baffle align scripts.  After the baffle align scripts completed, I did not touch ITMX again.  

Corey and I moved PR3 so that we could see beams on the cameras (we later reverted PR3's sliders), and moved ETMX and TMSX until we had nice TEM00 flashes.  We were then able to lock it and run then offload the ETM_TMS_WFS.  

With Xarm nicely aligned and PR3's sliders reverted, we had great transmission on ALSX and a familiar looking spot on the camera.  However we didn't see anything on the Yarm PD or camera.  So, I moved the BS until I could see something (also having increased the exposure of the camera so we could see the straight leakage beam), and then moved ETMY until we started to get flashes.  I aligned ETMY and TMSY until we could hold lock on TEM00, and moved the BS until we had ~max transmission on ALSY's transmission PD.  We then ran then offload the yarm's ETM_TMS_WFS. 

I then (since HAM1 is at much lower pressure than 10 mTorr) opened up the ALS light pipe, and went to check that the alignment on the table looked "reasonable".  Indeed, it looks reasonable, in that the PSL beam seems to go nicely through the SHG and is roughly centered on downstream optics.  However, we don't have good beatnotes.  I had a quick look at the beams headed toward the beatnote PDs, and at least one is easy to see that there are 2 distinct beams headed toward the PD.  I don't recall on the top of my head if this is the Diff beatnote though; if it is, then it depends on the BS alignment, so we likely want to hold off on doing any on-table alignment until we have a better idea of the BS alignment relative to the ITMs. 

At Sheila's suggestion, I also reset the green initial alignment references (SDF table attached just before accepting them), so that we have an easier time coming back to where we are now.  As usual, we'll have to re-reset them after we achieve full 2W lock.

In summary: ITMs were set with the BafflePD script.  Green ETM_TMS WFS were run and offloaded.  Nothing on IST1 was touched (yet), but it's clear that we'll likely need to do some on-table alignment.

Proposed next steps: Check BS alignment by looking at MICH (so, after JAC is locked, then IMC locked, then XarmIR aligned).  Then, consider aligning the ISTC1 beatnotes.  Then, lock the whole IFO!

TITLE: 03/05 Day Shift: 1530-0030 UTC (0730-1630 PST), all times posted in UTC
STATE of H1: Planned Engineering
INCOMING OPERATOR: None
SHIFT SUMMARY:

The arms were opened up today! 

• And this afternoon, we started work on the green arm alignment and after about 2-3hrs Jenne got both arms aligned with WFS (not with spool cameras).  Woo Woo!  
• Alignment work on JAC in-air table also started in the afternoon.

I got pulled from my shift for a good chunk of the 2nd half of my shift, and so Ryan Short helped me out and will post about afternoon LOG of activities. 

LOG:

• 1551-1651 LVEA technical cleaning (nellie.kim)
  • 1654-1741 MY/FCES cleaning (nellie)
  • 1700-1807 MX cleaning (kim)
  • 1808 H2 room (kim)
• 1551-1603 Hauling parts into East Bay (randy)
• 1605-1632 EY cleanroom curtains (randy)
• 1644 First Contacting BHD optics (camilla)
• 1710-1735 Unlocking HEPI of BSC2 (jim)
• 1723-1729 Checking HAM1+7 (travis)
• 1720-ish Cabling up ISCT1 & IOT1 (marc.fil)
• 1739 tour (kissel)
• 1751-1807 IOT1 work (daniel.jennie)
• 1753 CHETA table alignment (sophie)
• 1806-1809 grabbing a dust monitor in cer (ryanC)
• 1806-1816 checking elevator arm in east bay (randy)
• 1813-1822 turn off turbo H7 isolate aux cart H2 (jordan)
• 1815 wipe off optics table for BHSS in optics lab(oli)
• 1820 chatting w/ fil at isct1 (richard)
• 1830-2000 Moving BHSS from Staging to Optics Lab (elenna,mitch,betsy,corey)
  • ...RyanS covered Ops during this time & for a good chunk of the rest of the afternoon  (THANKS, dude!)
• 2046-2059 continue elevator arm work in east bay (randy)
• 2119 Checking electronics for JAC wfs at the rack (daniel)
• 2122 Working on IOT1 (new little JAC table) to look for beam (jennie.jason)
• 2140 HAM1 relay work & climbing around HAM1 (gerardo,fil)
• 2115-0020 Green arm alignment (jenne, I wrote down the steps she took & will post later...probably in the wiki)