The current driver now in the EE shop for troubleshooting (chassis S1500267/PCB S1500270) looks to be a D1200719-v3 PCB that has been modded to v4. The mod to v4 is also documented in the DCC traveler.

I repeated Zach’s 2017 coarse current adjust knob response test with this unit driving a 25 Ω dummy load, and got slightly different results. There’s a bigger offset at the zero knob setting, and saturation sets in above a knob setting of ~7.

It may be that Zach followed through on his stated plan to mod the circuit with a voltage limiter, like the one added in v7 of the schematic.

Next steps are to check the board for signs of this possible mod, and see if the observed misbehavior can be reproduced.

Other mods in v6/v7 of the circuit address high frequency oscillations and crossover issues, and we may want to implement those as well.
 

In the loop model I had posted earlier a part of the discrepancy in the calibration of the delay line can be explained because I had not used a buffer at its output. After adding an SR 560 buffer while the calibration is closer than expected, there is still some discrepancy even in the DC MHz/V conversion which I am still investigating.

Nonetheless, I have attached the latest laser TF in Attachment 1 and using the same calibration, a measured laser frequency noise spectrum in Attachment 2.

[aaron, shruti]

Yesterday (Thursday Feb 17), I installed the rails for the PSOMA enclosure's acrylic panels. Meanwhile, Shruti used the delay line box to measure the frequency noise of the NxS beat note. Also measured the open loop transfer function from current modulation voltage to beat note frequency. Adding an SR560 as a unity gain buffer between the delay line box and the moku brought the measured curve closer to the expectation, at least at low frequency. I think the next step is to repeat the direct measurement of current noise from the ITC502 and ITC510 so we can put it on the same plot as the laser frequency noise.

(Probably Shruti is working on an update about this, but I wanted to document it before I forget)

Instructions from liquid instruments (via Rana) on controlling Moku from the ipad while it is connected to the internet for data uploads:

1. hook up Moku to ethernet via the wire. Make sure the Ethernet lights are blinking. Moku can be DHCP, or whatever.
2. Connect the ipad to the cryo-fi wifi network so that iPad and Moku are on the same cryo network. i..e do not connect ipad to the 'Moku-XXX' access point.
3. Should be able to coontrol Moku from iPad and also connect to internet

- this is looking great. I think all of the important terms are there.

It looks like something may be iffy with the placement of the poles/zeros. Usually when the magnitude slopes down, the phase also goes down. i.e. 1/f means the phase should be -90 deg, not +90 deg. Sometimes this is a real physical situation like in the RIO TF, but for most electronics, cavities, etc. its just a syntax error and you have to make sure the poles and zeros are in the left-half of the complex plane. Might be syntax weirdness in some python packages.

Loop Model

• The script used to generate the plot in Attachment 1 can be found here and the data is in the data repository.
• The different elements of the loop  I have considered are:
  • Laser driver [mA/V]
  • Laser [Hz/V]
  • Cavity [V/Hz]
  • RFPD, mixer and low pass filter [V/V]
  • Attenuator: 10 dB [V/V]
  • Servo LB1005 [V/V]
  • Pomona filter [V/V]
• I made separate measurements of the following:
  • Laser and driver combined: I used this script and Aaron's delay line setup. The script shows both the driver and laser separately, but I have shown them together here in the pink trace. The measurement of the driver separately was a gross estimate since I used the 'ANALOG OUT' port of the driver instead of its actual output which needed me to turn off the laser (which I can do at a later time if needed).
  • LB1005: I measured its transfer function by the method outlined in *Attachment 2* to obtain the green solid line in the graph. But in the estimates of the open loop TF I have used the analytic function (green dashed line) of 1/f with 21 dB gain at 3 kHz which corresponds to the knob setting, since they agree roughly in magnitude and phase to upto almost 1 MHz.

OLTF direct measurement (solid grey)

• This is the open loop transfer function that I directly measured earlier on 7th Feb

OLTF estimated 1 (dashed black)

• Here I considered only a constant magnitude for the laser and driver
  • Laser driver 20 mA/V
  • Laser 150 MHz/mA
•  For the cavity, I used information from the measured PDH signal, and considered it to be a single pole low pass with gain 132 mV/MHz and pole at 2.3 MHz
• I only considered the low pass filter (ignoring mixer and RFPD) and approximated it as where = 5 MHz
• The attenuator was a constant 10 dB attenuation with no phase change (With DC coupling, and 50 Ohms input and output I measured this to be true enough)
• For the LB1005, I used the analytic form shown by the green dashed line
• I have also used the analytic form of the designed filter shown by the purple dashed line

OLTF estimated 2 (solid black)

• The only change in this from 'OLTF estimated 1' is that I have added the phase information of the laser and driver transfer function I had measured. I kept the magnitude of this transfer function to be the same constant calibration factor used in 'OLTF estimate 1'.
• Using this, the phase of the model matches the measured one quite well.

OLTF estimated 3 (dotted black)

• Everything the same as in 'OLTF estimated 1' except for the laser and driver transfer function
• I used both the magnitude and phase of this measurement (shown in pink on the plot).
• I have to re-check the delay line calibration because there is a constant factor discrepency between this and the measured OLTF for the most part

[shruti, aaron]

Shruti set up a transfer function measurement using Anchal's 'phasemeter TF' script: while the moku phasemeter monitors the beat note frequency, the moku output drives one of the laser currents with a sine wave. By stepping the driving frequency, one can measure the transfer function from voltage at the laser driver 'mod in' port to laser frequency.

The script worked fine, but the measurement did not. By manually setting up the same measurement at a fixed frequency through the ipad interface, we found that the beat note frequency is drifting too much for the phasemeter to keep up. With even small excitations to the current drive (>5 mV at mod in), the frequency moves too much for the phasemeter's loop to keep up, and the phasemeter loses lock.

Instead, we are setting up the delay line frequency discriminator box. We will directly measure the transfer function from laser current drive mod in to the output of the discriminator box. To get the transfer function of just the current driver and laser, we will need to independently measure the transfer function of the frequency discriminator (including RF amplifier and delay line), but that's no problem.

After setting up the DLFD, we tuned the beat frequency to observe the max, min, and null response of the delay line. The peak/trough of the discriminator signal are at 150 MHz and 77 MHz. The delay line output is nulled at 118 MHz. The noise on the beat note (with no additional modulation applied) causes 56 mV fluctuations pk-pk on the delay line output. We used the moku output to drive the laser current mod in, and found that a 5 mV excitation at 1 kHz was visible above the noise but still not using much of the discriminator's full range (not very quantitative on whether we're completely in the linear regime, since the noise on the beat note is already quite large).

For the loop mode, I suggest plotting all of the components as separate traces in the Bode plot first. e.g. cavity, driver, mixer LP, LB box, etc. Then we can see which of the components make sense. For the magnitude, each one will have different units, but you can indicate those in the legend (i.e. W/m, Amps/V, Hz/Amp, etc).

When doing the swep sine TF, I suggest going from 10 Hz - 10 MHz, and setting the Moku to use more cycles, so that the low frequency coherence is increased. You can also stitch together a few different measurements: this allows to use different magnitude at different frequencies. For 10-1000 Hz, it may be easier to use the CDS system since that makes it easy to use varying amplitude v frequency.

Attachments 1 and 2: Pomona Filter

• I was able to lock the cavity at the same gain with the updated pomona filter circuit (shown in Attachment 1) and no 10 dB attenuator before the current driver.
• This new filter has a pole a little above the cantilever resonance (~50 Hz) and zero at 1.5 kHz in order to remove the occurrence of the many peaks seen earlier between 1-10 kHz separated by roughly this cantilever resonance frequency.
• The resistors were increased in order to incorporate a ~10dB attenuation and remove the Mini-Circuits attenuator which was at its output. The Mini-Circuits attenuator was not properly impedance matched.

Attachment 3: Loop Model first attempt

• I tried measuring loop transfer functions and noise spectra once again but my choice of injection of the small signal (LB servo's -B port) resulted in the lack of coherence in the measurements at lower frequencies, hence I tried to roughly estimate it. Code found here
• I have yet to include the phase information of the laser and current driver, which may seem to matter significantly above 1 kHz
• The magnitude at lower frequencies don't seem to match. Could this be because the cantilever's optomechanical response is quite high and therefore modifies the error signal?
• I also forgot to include the low pass filter after the PDH demodulation mixer which is at 5 MHz

Attachment 1: New open loop transfer function

• After adding the pomona box filter to the loop, I tried locking again. The max gain I was able to set corresponded to a UGF of 13 kHz -- I was unable to recover the previous UGF of 22 kHz.
• I think the phase plot makes sense with the addition of another zero and pole, but I'm not sure if the phase margin is to be measured from +180 degrees in this case?
• Also, I had to do this with the 10 dB attenuator before the modulation input of the current driver. Even with adjusting the servo offsets the modulation without the attenuator was high enough to reach the current limit for the diode nearly every time.
• Attachment 4 contains estimates of the resistors inside the Mini-Circuits 10dB attenuator using measurements provided by Rana. Attachment 5 contains the filter TF measured with and without the 10dB attenuator at its output - the 45 Hz pole is now at 250 Hz which is not what we want
• The solution is probably to modify the pomona box with higher series resistance, and lower capacitance, and use the box without an attenuator at its output.

Attachment 2:

This contains the calibrated noise spectra without accounting for loop suppression for before (blue) and after (red) the pomona box filter was added. Since the servo gain is not as high as required and filter pole is much higher than we want this may not be a valid comparison.

Attachment 3: Corrected calibration for noise spectra

Chris pointed out that I was not inverting for the loop suppression correctly in the previous calibration.

After now incorporating the phase of the open loop transfer function and using the right magnitude, the closed and open loop spectra match at frequencies above the UGF (which was 22 kHz in this case - before adding the pomona box filter)

The script I used for generating the plot can be found here and also here with results.

[rana, shruti]

Today we measured the transfer function of the pomona box I had made earlier using a tee at the output port and splitting it between a 10kOhm resistor [to simulate the modulation input of the ITC 502 current driver] and the input of the Moku kept at 1 MOhm impedance and DC coupled. The output amplitude was 700 mV with 50 Ohm impedance. for the measurement

I used the script found in labutils to quickly fit the zero and pole to the measured TF. The pole is 45 Hz, and zero at 1.5 kHz, which is more or less what was expected.

The heat capacity of aluminum is ~1 J / (g K) or 1000 J/(kg * K) in SI units. We might have about 10 kg of stuff in the chamber, so thats 10000 J * 200 K = 2 MJ.

That would correspond to a boiling off of ~10-15 L of LN2 if we go all the way to 100 K, but only half of that if we just use it to get from 300 to 200 K.

Since the Debye temperature of most common metals is ~400 K, we know the heat capacity of Aluminium will drop by a factor of ~2-3 by the time it gets to ~150K (due to the freeze out of some of the high frequency phonon modes), so 2 MJ would be an overestimate.

This assumes that the radiative heat load is negligible. If our shielding has an emissivity of 0.03, the radiative heat load from the vac chamber onto the cold stuff will be ~10 W, or ~1 MJ in 24 hours.

Timing signals have been restored and the models on the cymac are running again.

Timing for the cryo lab CyMAC is supplied by a Silicon Labs 5340 eval board. It needs to be initialized by a Raspberry Pi. As far as I can tell, the root cause of this problem is that the Pi has not been configured to run the necessary initialization script automatically on startup. The other issue is that we have two Pis for timing in the lab (the second one controls the GPS receiver), and at first I had them mixed up.

The timing board connects to the Pi over the I2C bus. The command i2cdetect -y 1, on the correct Pi, confirmed a device with address 77 was present. This is an uninitialized timing board. After running the initialization script (./si5340init.py Si5340-RevD-RTSCLOCK-Registers.txt, found in /opt/rtcds/tst/x1/target/timing), the I2C address updated to 75, representing an initialized timing board. Then models on cymac1 could start and run normally.

To be fixed next time: run the init script automatically on startup, give the two Pis descriptive hostnames to make it clear which one controls what, and put the timing board and its Pi into a proper enclosure.

Calibrating the noise budget

1. I measured the noise spectra at the control point (after the servo) and the error point (before the servo and input attenuator) using the Moku. See these (loop, setup) diagrams for a better description. For the previous measurement (elog 2775 and elog 2776) we had chosen the ambiguous units of dBm/Hz, but this time we used the noise in dBV/rt Hz which does not require the impedance. I took separate measurements for the different frequency regions and stitched the data together. The raw noise spectra at both these points are shown in Attachment 1.
2. Using the PDH signal on the oscilloscope I estimated the cavity pole frequency using the same process outlined in the table here. I found the cavity pole frequency to be 2.3 MHz and the cavity response to be 132 mV/Hz -- the two quantities required to estimate the cavity transfer function (modeled as a single pole low pass filter). [update 27-Jan-22: Added Attachment 3 with the crude cavity pole and response measurements]
3. I also used the open loop transfer function measured earlier to correct for the loop suppression
4. I used the updated noise calibration notebook that I uploaded here and the data (uploaded here).
5. Attachment 2 shows a calibrated noise spectra using only the noise measured at the PDH error point. The red curves is the measured noise in-loop calibrated to the frequency noise at cavity input, the blue curves incorporate the inverted loop suppression factor. The lighter curves show the previously measured noise spectra without the cantilever.

All this was measured without adding the pomona box filter which would be my next step.

[Aaron, Stephen]

Aaron preliminarily estimated the load for the PSOMA experiment to be 40 MJ. The first nitrogen boil off calculation for 40 MJ of cooling energy is a LN2 volume of ~250 L, as follows:

• The heat of vaporization of liquid nitrogen is 199 kJ/kg = .199 MJ/kg
• The density of liquid nitrogen at atmospheric pressure is .806 kg/L
• Volume_LN2 = Cooling_Energy / ( Heat_of_Vaporization / Density_LN2 )

Thoughts on making a better estimate of current QIL Cryostat thermal load, to inform the PSOMA thermal load estimate.

• I think Radhika and Aaron are doing some estimates based on QIL Cryostat cooldown curves, but I'll do some eyeballing below in the meanjtime.
• Collecting cooldown parameters
  • Eyeballing a representative cooldown curve the QIL Cryostat coldhead temp seems to pass from room temp to 60K in about 2 hours.
  • Eyeballing the capacity map from Sumitomo, there seems to be an average power of 70 W for those first 2 hours, then an average power of 35 W (crossover at ~50K) for the remaining duration.
    • We have about a 36 hour conductive cooldown period to reach steadyish state at below 80K.
    • We have about a 24 hour conductive cooldown period to reach 100K (looking at Inner Shield temperature, dominated by coldplate thermal mass)
• 36 hour cooldown to 80K steady state:
  • 2 hours at 70 W and 34 hours at 35 W gives a total cooldown load of 4.8 MJ. We'll make this the current estimate for the QIL Cryostat cooldown, with ongoing load of 0.12 MJ every hour.
  • This would require an LN2 volume of 19.4 L, with ongoing consumption of .48 L/hr.
• 24 hour cooldown to 100K:
  • 2 hours at 70 W and 22 hours at ~40 W (higher average power than above case) gives a total cooldown load of 3.7 MJ. In the case of PSOMA measurements at 123 K, this is roughly the point at which the LN2 would cease to be used and the cryocooler would take over.
  • This would require an LN2 volume of 14.9 L.
• 64 hour period such as a weekend:
  • 2 hours at 70W and 62 hours at 35 W would make a total cooldown load of 8.3 MJ, with those first 2 hours costing about 0.5 MJ (allowing a refill to make a dent in the required capacity.
  • The required LN2 volume to sustain through 64 hours would be 50L, and for reference the largest LN2 volume offered by IRLabs standard is 8 L.

Some observations while measuring the noise spectra [Rana, Shruti]:

• The cantilever mode at 45 Hz is the dominant source of noise in the measured noise spectra. Attachment 1 shows the noise spectrum at lower frequencies and Attachment 3 shows this signal in the PDH signal, and cavity transmission and reflection.
• The contribution of the noise of the second order (90 Hz) is roughly 35 dBV smaller than the fundamental, and the third harmonic at 135 Hz seems virtually non-existent visually.
• There are present many peaks from 1 kHz  to tens of kHz. The smaller spacing between them seems to be around the cantilever resonance. (Attachment 2). Although very similar looking peaks do seem to be present in noise spectra measured before we added the cantilever (such as in elog 2776).

In order to suppress these noisy peaks, following the suggestion in elog 2838, I made the pomona box filter shown in attachment 4. The is the output impedance of the LB1005 servo and is the input impedance of the analog modulation port of the ITC502 current driver being used.

[Update 31st Jan 22] is shown incorrectly in Attachment 4 - it is supposed to be in series, not in parallel to the remaining circuit.

The cymac crashed. I restarted it so backups could resume. No models are running yet—there may be an issue with the timing signals.

While I played around with locking the cavity I was not able to see the cantilever mode dominate the PDH signal at most settings. The only times I did see it was sometimes when the cavity was losing lock after being locked for a while.

There were some current and temperature settings at which I was not able to lock the cavity with the same overall gain (4.40 on the knob = -6 dB which I did not change) but at the following two settings I was able to lock the cavity with the residual noise being ~30% the PDH pk-pk:

• I ~ 114 mA, T=8.193 kOhms. PDH pk-pk was 370 mV and the locked residual pk-pk was 130 mV
• I ~ 130 mA, T=8.356 kOhms. PDH pk-pk was 400 mV and the locked residual was 154 mV

I measured the current using the 'ANALOG OUT' port of the ITC 502 on an oscilloscope using the conversion factor of 20 mA/V.

At the second setting mentioned above I also quickly measured the open loop transfer function similar to elog 2761  and elog 2759 (shown in Attachment 1 and data saved here). Since the gain was lower this time, the UGF was a smaller value of 22 kHz with a larger phase margin.

Message from dan Kozak:

The cryo lab rsyncs are failing:

        ldas-cit# rsync rsync://cymac1.caltech.edu:2045/ldasaccess/trend/minute/
        @ERROR: max connections (4) reached -- try again later
        rsync error: error starting client-server protocol (code 5) at main.c(1658) [Receiver=3.1.3]

You might check to see if there are hung rsync processes using up the
connections.  Also, 4 is a _very_ small maximum, so you might want to
edit /etc/rsyncd.conf to have the line

        max connections = 64

Let me know when you've had a chance to sort this and I'll try again.

I've alerted ChrisW, so he can coordinate w Dan

The scripts for the PSOMA cryostat model can be found in mariner40/CryoEngineering/ (can be moved to somewhere more fitting). There are 2 scripts: PSOMAstatParams.ipynb (for specifiying physical system parameters) and PSOMAstatCooldownEstimation.ipynb (for simulating the model). I used the tentative parameter values and geometry found here: https://dcc.ligo.org/LIGO-D2100770.

Attachment 1 shows the resulting cooldown curves. The system is being cooled by a LN2 resevoir fixed at 77K. The model has been handed off to Aaron for testing and tweaking of parameters.

Aaron and I previously saw that the 40 Hz cantilever mode was using up most of the PDH signal's linear range while in lock.

This means that the loop has just enough gain to stay locked. Increasing the gain seems to make it unstable, so we want to lower the UGF.

So perhaps a 3x reduction in the UGF and a 10x increase in the gain at 40 Hz? To do this we want to shape the loop with a pole:zero boost having a 30x gain at DC relative to the high frequency part.

We can do this by making a pole zero section in a Pomona box with a pole at 40 Hz and a zero at 1200 Hz. This should be stable, since I expect our UGF is > 3 kHz.

To figure out the component values, we need to now the output impedance of the LB box and the input impedance of the current driver. Any ideas?

[rana, aaron]

We investigated further the 144 kHz oscillation. The line is not visible in the beat note power spectrum (between N and S lasers). However, when we block the beam at the REFL PD, there is a -120 dBV/rtHz peak at 144 kHz. Because the full range of the PDH signal is about 0.5 V, but the line is about 1000x lower (~40 uV), we're not too worried about it for now.

Next, we checked out the cavity lock. The gain was set too high, such that the servo frequently locked onto a weakly resonant mode, while in the primary mode the laser maintained lock but the PDH signal saturated its range. When we lowered the gain, the 41 Hz cantilever resonance appeared not only in the control signal but in the PDH error signal, indicating that the loop gain is insufficient at low frequency for the laser to track the cavity. We need to change the loop shape to give us enough phase margin to increase the gain at low frequency.

With the gain knob set to 5.08, the cavity maintained a stable lock despite insufficient low frequency gain (at least near 40 Hz). While locked, both the PDH signal and the cavity transmission had a 41 Hz oscillation. This indicates that there is a DC offset in our lock setpoint. A fluctuation at frequency f_0 in the error signal should cause an f_0^2 fluctuation in the transmission signal on resonance; off resonance, the leading order term is instead f_0.

I removed the 10 dB attenuator between the LB box output and the laser driver's current modulation input. The ranges of the LB box output and ITC 502 input are matched, so there should be no need for an attenuator. I also slightly reduced the servo gain to 5.3. There is still a 10dB attenuator between the PDH mixer/lowpass and the LB box input, to avoid saturating the servo's input stage (+- 330 mV).

After moving most electronics to the same power strip as in the above elog and tying the table to ground, I no longer see a 144 kHz oscillation in the PDH error signal (oscilloscope). I do see a ~40 Hz oscillation in the PDH error signal... could that be the fundamental mode of the cantilever? Zach's thesis quotes a fundamental resonance at 45 Hz. The 40 Hz oscillation is present even while unlocked (but with the servo engaged).

Though the error signal looked clean, the lock was not steady for more than 10s of seconds. I suspected that the current control loop was far from the center of its range. I am monitoring the current control signal from the LB box Aux output, which picks off the signal before the final summing amplifier stage. Therefore, in order to adjust the temperature setpoint to 'zero' the control signal, I needed to first disconnect the LB box inputs then adjust the output offset until the voltage at aux output matched that at the final output. Then, with the laser locked, I adjusted the temperature setpoint until the control signal as measured at aux output was zero (plus some offset that was present at both the main output and aux output even with the inputs disconnected).

After adjusting the temperature setpoint manually, the beamspot is noticeably brighter on the trans mon camera, and the lock is stable for minutes.

I tried setting up the digital control to handle the temperature loop, but the realtime system is in a bad state and I didn't attempt to diagnose it.

confirming fiber path

I traced the fiber beam path and confirmed that the system is set up as in elog 2833. I add a FC/PC-to-FC/APC patch cable between the 50-50 beamsplitter and the 1611FC-AC photodiode. We've been meaning to add that patch for a while to avoid damaging the connection.

I'd like to also see a beat note so we can hunt for this 140 kHz oscillation tomorrow.

I opened the connector housings on the cable for the N laser TEC and readjusted them such that the internal clamp bites down on the jacket of the cable, rather than closing over the conductor wires themselves (see attachments 1 and 2). Wiggling the cable no longer causes an alarm on the TEC.

I noticed that the FC 1611 photodiode was being powered from a GlobTek +-12 V DC supply with 0.25A current limit; however, those photodiodes require +-15 V DC with 0.3A current limit. I located the proprietary cable and am now powering that photodiode from the New Focus 0901 power supply located on the PSOMA rack.

I also noticed that the SMA cable from the fiber box panel to the spectrum analyzer that should have been carrying the beat note was connected to the wrong port! That's probably why it wasn't visible before.

With those changes, I could see the beat note on the spectrum analyzer .

I wanted to add a cable from the beat note PD DC output to the fiber box panel, so I could check that the DC power on that photodiode makes sense. But, I couldn't locate one in cryo or EE. Maybe the 40m or another WB lab has a spare?

labwork to do

• posting table of important parameters (like those in elog 2830) in wiki, to be printed and posted in the lab
• psoma servo drawing

done

• AC power for (most) electronics involved in locking share a single power strip. The laser drivers, LB box, function generator (connected to LB box), photodiodes supply, oscilloscope, spectrum analyzer (HP 8560E), and moku all are now powered from the long power strip ('A') located under the optics table. The OCXO preamplifier box supplying ~30 MHz modulations for PDH is powered by the Sorensen DC supplies... which are not powered from the same strip. The Sorensens are on the Aux rack, while the other electronics are on the PSOMA rack. It would be somewhat involved to power the Sorensens from the same supply (need a longer power cable sent across the walkway), but can be done.
• I tied the optical table to ground through a long (springy) wire with 1 MOhm resistor soldered inline near the halfway point. One end of the wire is bolted to the optical table, and the other is bolted to the enclosure frame. There are two power strips bolted to the enclosure... but the 'enclosure' power strips are plugged in to the wall on the opposite side of the room from the 'under the table' power strip powering the PDH electronics. That's not great. I could move the PDH electronics over to the 'enclosure' power strips, but it would require some cable rerouting.

notes

• The DB9 cable connecting the North TEC driver to the laser is flaky (moving the cable causes the TEC alarm to trip on the driver). The rubber of the cable is visibly separated from the casing, and the ground shielding is exposed . I have powered down the North laser, and suggest we don't run it until replacing or repairing the cable

I powered on the 'North laser' using an ITC 510, and make set up the fiber optics for the beat note as depicted in Attachment 1:

• Powered the Newfocus 1611 FC (fiber-coupled). The rack mount box (Thor Labs RBX32) does not yet have an adapter for the PD power supply so I wired it through an open slot.
• There are 90-10 beam splitters after the faraday isolators of both the lasers with the 10% coupled output going to a 50-50 beam splitter to form the beat note.
• The transmission of the 50-50 beam splitter is sent to the 1611-FC
• I'm not sure how to dump these beams appropriately within the fiber box: the 90% output in the path of the north laser and the anti-symmetric port of the 50-50 BS. Currently, they are just being dumped into metal/ rubber covers.
• Still cannot find the beat on the spectrum analyzer (either the connections on the 50-50 BS are weird, or I have not properly explored the relative setting space to get a beat within 1 GHz]

Locking updates:

• The LB box seems to also output some modulating current when the lock is turned off ~ 3mA pk-pk change which is weird
• I've added a 10dB attenuator at the input of the LB1005 servo box to make sure the PDH signal is within 100 mA (the input limit of the LB1005)
• I experimented with changing the attenuation at the output at input but realized that without a 10-20 dB attenuation the current ends up being too high in the loop and rails. Currently there is a 10dB attenuator at the output.
• The output (after attenuation) is now also split with a T to be sent to the Moku for troubleshooting and offset correction
• I've been trying unsuccessfully to get data from the scope through a LAN connection

I'm collecting useful calibration/gain parameters in Elog 2830 for quick reference.

1. check to see if the 140 kHz noise is on the laser already. Post spectrum of beatnote in ELOG.
2. Check error signal of cavity with a beam block placed inside the cavity. This allows the reflected beam to still hit the PDH PD to check for electronics pickup, but there is no cavity signal.
3. Check error signal with beam to REFL PD blocked. also post to ELOG.
4. Let's discuss about the 50 Ohm termination stuff. Generally, the 50 Ohm or not issue is not critical in these applications where the signal is mostly less than 2 MHz (after demodulation); the wavelength for 2 MHz in cable is ~(c*2/3)/(2 MHz) ~ 100 m. The main purpose of checking impedances is to make sure that the sources can drive the load, whether its 1 Mohm, 50 ohm, or 25 ohm. So it would be fine to have a T going to a 1 Mohm scope input.

[aaron, shruti]

Shruti has observed a 144 kHz oscillation on the PDH error signal from our cavity-with-cantilever. The oscillation was railing the PDH signal, making it impossible to maintain lock for more than a few seconds. I came in to troubleshoot.

1. Observed a flaky lock with PDH signal railing
   • initial settings on the LB box were LFGL = 90 dB, G = 6.76, PI corner = Int
2. Adjusted gain on the LB box to G=5, then back to 6.76 after observing no change
3. The PDH signal was being split with a BNC T junction after the lowpass, sending one end to the LB box channel A input (50 Ohm) and the other to the oscilloscope (1 MOhm). I removed the T junction and sent the PDH signal directly to the LB box, and instead sent the LB box's 'Error Monitor' output to the oscilloscope (still 1 MOhm). I observed no significant change
4. I noticed that the current drive output of the LB box was being split with two T-junctions, each terminated with 50 Ohm, resulting in the LB box's output driving 25 Ohm. I removed the unnecessary T junction, so the output is now sent to a 20 dB attenuator, followed by a T-junction with one end terminated in 50 Ohm and the other sent to the current drive mod in.
   • The cavity can now remain locked for about 10s
   • After reducing the gain to G=5.70 and tweaking the laser temperature, the PDH error signal is not quite railing (oscillating at 142 kHz with 500 mV pkpk)
5. I realized I should have changed the input impedance for the oscilloscope channel monitoring the PDH error signal in step 3. 'Error Monitor' output wants to drive 50 Ohm.
   • After adding adding a T-junction to 50 Ohms between the error mon output and the 1 MOhm scope, I observed 300 mV pkpk oscillations at 142 kHz and stable locks for up to a minute.
6. I removed some extraneous BNC T-junctions (were just open ended), and disconnected aux out from the back of the LB box. No change in the 142 kHz oscillation.
7. I next checked the LB box offsets by
   • turning off the servo and terminating both inputs with 50 Ohm.
   • Tuned the input offset to null the LB box error monitor point (viewed on Moku scope) with the servo off.
   • Also tuned the sweep offset to null the LB box output with the servo off, though this should not matter for the in-loop behavior    
   • No change for the 142 kHz oscillation

That's all for me this morning. I think the oscillation is sufficiently low we could try using the DAQ to feed back to temperature as we were before. It would be useful for diagnostic purposes to maintain a more extended lock, and I'm finding I need to tune temperature anytime I reacquire lock. Maybe we're just always sitting close to the edge of the current control loop.

WIP

Videos of flashes and a very noisy lock.

In Attachment 1 and the video showing a lock, the PDH signal essentially goes to its rails. Most of the noise is in oscillations that are roughly at 143 kHz.

Cavity and Lock Parameters

This elog (2815) shows the cantilever with its mount and a top view of the cavity.

Optics

• FSR: ~ 600 MHz
• Finesse: [Input coupler transmittance: <4% , Cantilever transmittance:]. Check polarization - measure Finesse for both.
• cavity pole: also depends on polarization
• DC optical gain (measured as the PDH slope): W/m, W/Hz

Electronics

• Laser resistance->temperature conversion chart
• Laser temp->frequency to be measured using beat (has not been done consistently in the past since it varies with current)
• Actuation Coefficient
  • Modulation coefficient for ITC 502: 20 mA/V
  • ITC + RIO coeff = (2 GHz laser freq shift) / (Volt of input to ITC)
• Signal path calibration (first should optimize demod phase)
  • New Focus 1811 40,000 V/A, ~0.85 A/W ?
  • Mixer conv loss: 5 dB
  • 50 Ohm term before LP (6 dB attenuation)
  • compare the W/m calibration to expectation based on:
    • input laser power
    • RF modulation depth
    • cavity Finesse
• LB lock box transfer function with & w/o Boost engaged.
• Useful dynamic ranges
  • LB box input stage: +- 10 V. The input stage is a difference amplifier buffer, allowing +- 10 V common mode signals to be subtracted before the filter stage
  • LB box filter stage: +- 330 mV
  • LB box summing stage: +- 10 V
  • LB box output drive: +- 20 mA or +- 10 V
  • ITC 502 current modulation input: +- 10 V
• Measure transfer functions of laser
  • Measure ITC current driver separately, then ITC+laser
  • Measure phase response using beat and phase lock

In order to improve the acoustic isolation of the box I used rubber tubing cut as depicted in Attachment 1 and attached to the edge of the plastic box (as seen in Attachment 2).

Attachment 2 also shows one of the four dog clamps I used to secure the box. Finally I placed an oscilloscope on top of the box to provide some weight.

The box has 4 holes for the input, reflected, transmitted-to-PD, and transmitted-to-camera beams.

PDH signal

In elog 2731, the PDH signal is almost 1 V pk-pk, but recently I had only been seeing a very noisy signal ~10 mV pk-pk (purple trace in Attachment 4. Transmitted light is yellow, reflected light is blue). I adjusted the lengths and removed the attenuators in the path of the EOM but it did not seem to change.

While going through the path of the cables once more, I realized that signal from the OCXO was being sent to the wrong port on the rack mount (Attachment 5). It was earlier on D3, and I later changed it to A4 which was labelled 'S EOM', but the actual location was D4 which I identified from this diagram. Now I see a reasonably large PDH signal as expected with ~800mV pk-pk (video here).

Since clamping the cantilever, I began to align the triangular cavity with the cantilever.

Attachment 1 shows the setup of the cavity with the input coupler at M1, curved mirror at M2, and a cantilever with a flat optic at M3. The cantilever does not possess any knobs for fine adjustment and its initial mounting is fixed. M1 and M2 can be adjusted, but to get to TEM00 resonance, and any fine alignment, the two steering mirrors before the cavity were used.

Weird things happen while aligning:

1. While aligning, I began to see resonant peaks corresponding to the cantilever's mechanical resonance showing that the cantilever has a very high Q. I mistakenly thought the cavity was close to being aligned to resonance in this configuration while also being very stable, but later understood that the strong oscillations actually seemed more like accidental misalignments creating an optical lever. The green traces in the videos are the measured transmitted light through the cantilever. When the cavity is really aligned close to the TEM00 mode the transmission looks like the green trace here, and the reflection (from M1) is the blue trace.

2. Another time I thought it was aligned I was looking at the camera at the transmission of the cantilever [M3 in Attachment 1] and could see a single flashing spot but when I moved the camera around I actually ended up seeing two beams instead, simultaneously flashing. Minor changes in the alignment resulted in the beams individually and simultaneously turning into higher order modes. Initially I thought that the situation was as depicted in Attachment 2 and found that it was a consistent geometric solution, but later Koji pointed out that was the geometry that creates the TEM01 mode. In Attachment 2, the initial beam (red, M1->M2) overlaps not with the beam in that path from the second round trip (yellow, M1->M2) but with the one from the third round trip. Was it possible that this misalignment was too severe that instead of being TEM01 two separate, but identical, resonant cavities were created? A measurement of the FSR, which I did not do, would have proved that that was the case.

Since the transfer of fiber components into the box, I have re-aligned the cavity to the TEM00 mode. Note: replacing the fiber at the fiber launch and also changing the polarization at the fiber launch moves the beam around.

PDH locking stuff:

[more information and data coming soon]

1. The PDH signal seems too low, possibly because the LO and RF relative phase must be re-adjusted.

We've been learning the various processes required to fabricate silicon cantilevers following Zach's recipe.

The last step involves etching cantilevers out of the silicon wafer using KOH. Specifically, Zach's etch recipe for a 500 um wafer coated with 400 nm SiNx on both sides is:

1. Submerge wafer in 30% KOH solution at 80 C for 6 hours
2. Remove and rinse the wafer, then scribe and break along the lines between the cantilevers
3. Return the separated pieces to the KOH bath for an additional 2 hours
4. Rinse, dry, and briefly HF etch the resulting cantilevers

something wrong with the KOH etch

I tried to follow this recipe last Wednesday, with the following modifications:

• Our wafer is only 300 um thick
• KNI now stocks 50% KOH instead of 30% KOH. This is probably good for us, since higher molarity KOH results in more specular etched surfaces. But it does change the etch rate
  • According to Fig 2.28 in Silicon Micromachining (fig from Seidel et al), The etch rate for Si in KOH should decrease from 50 um/hour in 30% KOH to about 30 um/hour in 50% KOH

Because the wafer is 3/5 the thickness but the etch rate is also 3/5 as fast, I anticipated that a 6 hour etch would be appropriate to produce something for scribing and breaking... however, after 4.5 hours, the entire wafer was almost entirely dissolved. All that remained were thin, fragile sheets of Si or SiNx. What's going on? Some possibilities

possible explanations

Expected rate of etching was off

I have since perused the KNI wiki for more resources on etch rates through Si. The most extensive data seem to be from the BYU page on KOH etching. It suggests that 30% KOH at 80 C etches through 100 Si at 80 um/hour (compared to 60 um/hour that Zach was using), while 50% KOH etches through 100 Si at 45 um/hour. This doesn't really explain the results. Even at 45 um/hour, we should have been left with 100 um of Si after 4.5 hours, or 1/3 of the initial material. If we take Zach's 60 um/hour at face value and apply the data from BYU as a 'relative rate of etching,' we would be scaling by a factor pretty close to that suggested in Silicon Micromachining. 

I wasn't able to find good data on the rate of KOH etching through SiNx depending on temperature or concentration.

We are using doped silicon, but I found reference online to boron doped silicon etching more slowly.

The bath temperature could also have been systematically higher than 80 C, or just not well controlled around 80 C.

To test this, I'm planning to do a shorter etch of a sacrificial piece of Si under the same conditions as before (30% KOH, 80 C). I'll remove any oxide layer with HF beforehand, and check the bath temperature with a thermometer. 

We didn't really have 400 nm of hard SiNx on both sides of the wafer

I used the standard SiNx deposition recipe on both sides of the wafer, but did not check the resulting mask with an ellipsometer. We should in the future do this after most previous steps: PECVD, optical lithography, DRIE etching, etc.

I'm being trained on the ellipsometer this afternoon, and plan to measure the thickness of SiNx on some 4" wafers Zach had left over. 

Wafer was thinner than 300um

Our cleaning process involves a few minutes in an HF bath, and we use DRIE etching during the optical lithography step. Either of these processes could thin the wafer. In particular, during lithography I noticed that my photoresist was a bit thinner than I'd intended. Perhaps the I etched through the exposed photoresist more quickly than anticiapted, allowing the DRIE etch to reach the underlying silicon for longer. 

In the future I'll measure the thickness of the edges of the wafer (where there is no cantilever) with a micrometer before etching.

Update

This afternoon, I tried to measure the etch rate of the KOH bath. I did the following:

• Auto-tuned the PID parameters of the KOH bath's temperature controller. The settings ended up being unchanged from before
• Heated the bath to a nominal temperature of 80 C. I put a thermometer in the bath and found the true temperature to be 85 C. After finishing for the day, I set the calibration offset of the temperature controller such that the thermometer reads the true temperature. 
• Tested the rate of etching with a wafer I'd recently RCA cleaned
  • The wafer is from the same batch as the previous wafer: 3" diameter, boron doped with resistivity 1-10 Ohm-cm, and double side polished
  • Before starting, I used a micrometer to measure the thickness of the wafer at 0.30 mm. 
  • Prepared a solution of 2.5% HF, and etched the wafer for 2 minutes then rinsed with DI water
    • After the HF etch, I measured the thickness of the wafer at 0.29 mm. The purpose of the HF etch was to remove any SiO2 from the surface of the wafer before bathing in KOH.
  • Place the wafer in KOH bath (85 C true temperature) for 35 minutes. The wafer was completely dissolved in this time.

I had intended to remove the wafer and measure its thickness again, so unfortunately can only place a lower bound on the etch rate. Nonetheless, the implied rate of etching is >16.5 um/minute (etched through at least 290 um in 35 minutes, from both sides of the wafer). This is more than an order of magnitude faster than expected, even allowing for the increased bath temperature. Clearly I am missing something -- is the KOH actually being diluted due to some additional DI water in the pumping system? Is the boron doping really increasing the etch rate by that much? Did the wafer just fall off of its holder and get lost in the murky KOH (I did fish around for several minutes, no sign of a wafer in the bath)?

I mounted the fiber components (north and south Rio lasers, Faraday isolators, 90-10 beamsplitters, 50-50 beamsplitter, fiber EOM, 1611FC-AC) inside our breadboard-in-a-box from Thorlabs (attachment 1). 

Along the way

• Inspected and cleaned all fiber tips that I connected. Fibers that are not connected have not necessarily been cleaned. I took some photos using my phone and a 15x macro lens (attachment 2 is of a clean fiber), but the camera kept focusing on some schmutz on the microscope's eyepiece, so they're not very illuminating. 
• Turned off the S laser that Shruti was using, disconnected its cables, and reconnected the cables through the DB9 feedthrough on the box
• Removed from cryo cavs table the beamsplitter, photodiode, and fiber connectors that we were borrowing for the three corner hat measurement. These are now inside the box.
• labelled all fiber ends
• Used the 10m patch cable that was running from cryo cavs table to PSOMA table to carry the beam from the fiber box to the beam launch on the PSOMA table. Note that one end of the patch cable had dust or damage on its face over the fiber core that I couldn't manage to clean off. Could try again, or get another (shorter) patch cable). 

Though I set up the beamsplitters in the box approximately where we will eventually want them, the beam currently does not pass through any beamsplitters. I left the optical layout for the south laser identical to how it was on the table, with the addition of a patch cable between the EOM and the launch: laser -> faraday -> EOM -> patch cable -> launch. The north laser is not connected to its drivers. 

26 Oct 21, Tue

Yesterday while the power was out I turned off laser drivers and other powered electronics that I could think of. When the power was back I rebooted the computers (checked that I was able to ssh into cymac) and the electronics that I had turned off. I noted that the cavity was back on resonance as indicated by the forest of peaks that we were seeing.

Today when I returned to the lab the forest was missing and Aaron noticed that the cantilever seemed to be tilted in the clamp. Attachment 1 shows the exact angle at which it was found. A small part at the end is chipped off but it otherwise appears usable to me.

Attachment 1: The cantilever at the angle at which it had tilted to with the chipped bits, after just removing the top piece of the clamp.

Before re-clamping, we (Aaron and I) decided to use a variable torque socket wrench to test for optimal clamping torques on another sacrificial cantilever. I decided to abandon this experiment after Rana pointed out at the group meeting that this would not give any really reliable number for the required clamping torque since breakage torques may have a wide range of random values for different cantilevers.

28 Oct 21, Thu

Attachment 2: Mobile phone image (without external lens) of the cantilever's clamping surface. It shows some pits and scratches, one larger than 100 microns.

For reference, the dimensions of the clamping area are 1cm x 1cm.

Attachment 3: The clamping surface of the smaller (top, while clamping) piece of the clamp, that contains the indentation where the cantilever sits.

[aaron, shruti]

Aaron removed the plastic wrap and began powering and reconnecting all electronics.

I drilled three holes into the plastic box that covers the cavity, reconnected the south laser diode and TEC to the ITC 502 combi controller and set the laser temperature to 9.888 kOhms and current to 126 mA (until the beam was visible).

21-Oct-21

Continued with alignment of the cavity until I observed a forest of peaks again while sinuoisoidally oscillating the temperature, although they can be observed even without temperature cycling now that we ahve a cantilever.

Next steps:

• Mode-match to TEM00 better to be able to observe something on the IR camera
• Drill holes for both the transmission PD (behind cantilever) and transmission monitor (behind curved mirror)
• Set up locking electronics once again
• Weigh and clamp down the plastic box
• Procure new mount for the cube PBS to mount it in path so s-polarization transmits
• Add PBS and HWP to the input path
• Set up beat with another free-running laser to get a sense of the laser noise when measuring the noise spectra

[aaron, shruti, raj]

We added the acrylic framing that supports the HEPA FFU, as well as two of the 3 panels covering the top of the enclosure. Raj and Shruti also unwrapped and added handles to the orange acrylic doors.

However, the last acrylic panel for the roof of the enclosure is too small. Though the part is cut as specified at 23" on the short side, the counterbore holes on the long side of the panel do not span the distance between the two rails where the panel mounts. The counterbore holes are 0.75" from the edge of the panel, which means the two lines of holes are 21.5" apart. However, the distance between the center of the rails is specified at 22.5". The distance between the rails is appropriate for the size of the HEPA FFU, and the 0.75" gap along the edge of the panel is standard across the other roof panels. Therefore, I suspect the overal width of the panel was just specified 1" too short.

I'll alert F&L and have them send another panel ASAP

[aaron, shruti, chris, raj, radhika]

On Tuesday, Shruti and I did fit checks of all connectors for the enclosure. We received the remaining parts according to the parts list from F&L, so should have everything we need. We requested clarification on where to use a few connectors, though haven't yet received a reply (our contact at F&L was unavailable).

1. Add the feet to the vertical bars (E)
2. Connect cross struts to the horizontal bars (U and V to G and T)
3. Slide horizontal bars (G and T) onto the vertical bars (E), starting with the short side (G)
4. Add the plate brackets to the framing bars
5. Connect the framing on the top of the enclosure (A) to the horizontal bars along the top of the enclosure (B)
6. Connect the remaining framing (F and A) along the top of the enclosure, first with the end connectors then with brackets
7. Add the plastic panels to the top of the enclosure
8. Add acrylic doors

With some adjustments, we completed steps 1-5. The frame of the enclosure is around the table. Tomorrow we'll try to complete the rest of the build, which includes adding the roof and panel doors to the enclosure.

Backups were restarted for the cryo lab computers gaston, spirou, and cominaux. A 4TB USB drive was connected to cominaux, mounted under /backup, and rsnapshot was configured to run on a nightly basis. It does not back up the full disk, but only those directories where user-generated files are kept (/home, /etc, /usr/local, /opt, /ligo). rsnapshot's configuration files are: /etc/rsnapshot.conf and /etc/cron.d/rsnapshot.

For the cymac, configuration and minute trend files are backed up by rsnapshot to a 1TB disk, mounted as /backup on cymac1.

[shruti, aaron, raj, chris, ian]

1. Prepping
   1. Powered off lasers and other electronics around the table, disconnected power cords, and separated the PSOMA rack several inches from the table
   2. Wrapped the table in plastic in preparation for assembly
2. Assembling the frame
   1. Brought in the 80-20 bars, and set the long 2x1 bars along the sides of the table ('x' is 95" long, 'y' is 50", 'z' is 90.25")
   2. Identified the enclosure's feet, which are PN #65-2191 (bolt with a flat hex head). We're pretty sure from the enclosure diagram that these feet mate with the rectangular block PN #2130. Unfortunately, the bolt is M10 while the threaded block is 3/8-16... happily, we found some other rubberized feet with a 3/8-16 bolt in the EE shop, and were able to mount those on the vertical bars.
   3. Next up, we noticed that none of the 45 degree cut cross bars have arrived. As far as I can tell, we have all other parts, but I have

I've asked F&L for a parts list so we can do a proper inventory before starting assembly. Also requested a quote on purple panels, update on shipment for the cross bars, and imperial threaded feet.

Shruti and I transferred the materials for our new enclosure from receiving to the subbasement hallway. I noticed that the acrylic panels we received are amber, not purple as we specified... unfortunately it looks like the error was already present in the quote, which listed acrylic #2422 (in my emails with their rep, we'd both confirmed acrylic #2424). Because it was in the quote, I'm not sure there's anything we can do, but I'll ask. 

Ian, Raj, Shruti, Chris, Aaron, and possibly other grads will meet at 10 tomorrow for assembly.

We returned CFC-2X-C into the same cabinet it was borrowed from. Thanks CRYO!

Today I began to see a forest of peaks in the transmission (transmission through the cantilever optic) while aligning meaning that the cavity is around resonance. I then adjusted the Watec camera to the transmission of the curved mirror and began to see some flashes. I took a video showing this which can be found here.

I tried activating the feedback loop with the settings we had on earlier but while it did seem to increase the peak powers of the peaks it did not seem to lock. The steering mirror to the refl photodiode (Newfocus 1811) and the corresponding lens needs to be re-adjusted.

Similar to elog 2767, before changing out the mirror from a rigid mounted one to the cantilever, I measured the loop transfer functions and noise spectra since we had not done so since we moved the table.

The only change between the current and previous version was that I tried to make the low frequency phase offset zero.

Attachment 1 shows different estimated open loop transfer functions corresponding to ratios of specified closed loop transfer functions (as mentioned in the labels). The UGF was lower than what was previously acheived.

Attachment 2 shows the fit of the slope of the open loop transfer function as done previously in elog 2776.

Unfortunately, I did not notice the issue with binning when I measured the noise spectra which resulted in a discontinuous spectra corresponding to the different regions I measured separately on the Moku. The data for this and everything else mentioned in this post is in Attachment 3.

In short: Between Friday and today, I de-bonded the mirror from the broken cantilever, picked two cantilevers and bonded both the mirrors to them with the AR surface being the one on the bond side, clamped and mounted on a post one of these cantilevers, and placed it in the cavity replacing one of the flat mirrors (M3).

1. De-bonding from broken cantilever

For this, I used methanol and soaked it for around 30 min similar to the procedure here. Since this bond was weaker, it took lesser time.

2. Bonding both mirrors to cantilevers

I picked one cantilever from the previous collection and another from the dish shown in Attachment 1. This second cantilever did not seem to have the back surface passivated with SiNx and looked like it was just oxidized silicon.

I used the GE cryo-varnish at four points around the mirror placed on the etched edge of the cantilever for both and let it dry for a few hours (Attachment 2). The bonding was performed such that the AR surface was on the side of the cantilever.

3. Clamping and aligning

Using the alignment jig on the table, I picked the cantilever without the SiNx passivation on both sides (identifiable as with one non-glossy side) to mount to the clamp. I tightened it not too strongly and then used a 3/4" post of suitable height to get the center of the mirror to a height 4" from the table.

I placed it roughly where I thought it should be on the table replacing the flat mirror M3 from the previous setup. I got the beams from two round-trips to overlap visually and added a PDA10CS to look at the transmission from the cantilever.

The aim now is to do as we did earlier - sinusoidally change the temperature by driving the TEMP TUNE input with a function generator and slowly tweak the alignment of the steering mirrors to find the cavity resonance.

I also measured the transmission of the cantilever as roughly 0.1% using the power meter.

I broke the cantilever while fastening the fork clamp. Afterwards, I used methanol to remove the varnish and separate the mirror from the cantilever shard. I then used a cotton-tipped swab soaked in methanol to clean the varnish from the sides of the mirror. I drag wiped the HR and AR surfaces of the mirror with methanol followed by isopropyl alcohol. Finally, I bonding the mirror to a different cantilever (this one with somewhat more pitting than the previous) -- again bonding with cryo varnish at four points on the sides of the mirror, but on recommendation from Chris this time with the mirror AR surface touching the cantilever.

This morning, I clamped the cantilever-with-mirror that we bonded yesterday.

I first used a broken cantilever to practice the clamping. I clamped the test piece first in the nominal position, then with slight alignment errors in the available degrees of freedom to see what those errors would look like. If the cantilever is misaligned such that it does not rest in the clamping groove, the clamping block will show a larger than usual gap on at least one side (attachments 1, 2). If the cantilever is not entirely 'in' the clamp, the thicker part of the cantilever will be visible above the clamp (attachment 3). Attachment 4 and 5 show the test cantilever in good alignment.

Next, I clamped the cantilever-with-mirror by 

1. Insert the alignment pins into the clamping block
2. Place the bottom clamping block flush with the aligning piece
3. Place the cantilever on top of the clamping block and aligning piece, such that its long edge is flush with the aligning edge and the thick part of the clamped Si is just at the end of the clamping block
4. Set the top clamping block into place, holding the bottom block in place and lining up with the alignment pins. Make sure the gap between the two clamping blocks is even, and that the thinned part of the cantilever ends just at the edge of the clamping blocks.  
5. Add the clamping screws and tighten enough to secure the cantilever. Attachment 6 is the final result. 

The Q may improve with more clamping force, but I'd like to do a more controlled test of how much torque can safely be applied to the clamping screws. We don't want to break any usable cantilevers. 

I've borrowed 2 collimating lenses (for fiber input) for use with the free-space AOM in the DOPO lab.

[Shruti, Koji, Aaron]

Today Koji guided us to (1) remove the mirror I had contacted to the cantilever that later broke, and (2) clean and contact the second flat (1550 nm wavelength coated) mirror to another cantilever using cryo varnish.

(1) De-bonding the mirror

After my failed attempts at using methanol and acetone along the edges followed by soaking the contacted-to-silicon optic in isopropanol, today we were finally able to de-bond the mirror by soaking in methanol. We used two washers to raise the optic from the aluminum surface and folded foil pieces to keep everything in place. The setup shown in Attachment 1 was covered with another aluminum foil dish and let sit. It took at least an hour of soaking to completely de-bond.

Koji then drag wiped the optic with pure isopropanol to clean any remaining residue and cryo-varnish hairs.

(2) Bonding a different mirror to a cantilever

First we tested drag wiping the broken silicon cantilever with methanol and isopropanol. Then we selected a reasonably looking cantilever that was not chipped at the edges, though it had an uneven surface, and drag-wiped both surfaces with isopropanol.

We drag-wiped the HR surface of the optic a few times and then placed it on the portion of the cantilever that had the square etched-out region. While Koji held the mirror in place I applied cryo-varnish to four points around the mirror. This is now set to dry.

Update: Photos are available on the ligo.wbridge google drive.  I uploaded everything, but could pare down to save drive space. 

I borrowed this free-space AOM (1550 nm) for use in the DOPO lab.

Attachments 1 and 2:

The cantilever clamp with a cantilever on it to be used on an optical post. This will replace the non-transmitting flat mirror in the PSOMA cavity. There are two screw holes to mount the optical post, but it would only be possible to use one, therefore this will be initially mounted asymmetrically.

Attachment 3:

Cantilever with the two flat mirrors from Zach's setup. Without having it properly mounted, I could only very crudely measure its reflectivity as R>81% using a power meter.

The one on the left is shown with (what I believe, by comparing to the curved optic in our present setup, is) its HR surface facing up and the one on the right with its AR coated surface shown.

Attachment 4:

I attempted to bond the mirror to the cantilever with cryo-varnish after testing that it bonded two pieces of silicon.  The HR surface which would be facing the interior of the cavity was the side I decided to put the varnish on because the pictures from Zach's thesis depicted the cantilevers that way. I coated the edge of the square-shaped hole on the cantilever head with the varnish and placed the mirror on it. I placed a lens wipe on top of the mirror and then another flat optic over it. While applying pressure on it with a tweezer the cantilever broke (possibly because the aluminium foil below it was very crinkly which I didn't think too much about beforehand).

While the mirror seems to have bonded after a couple of minutes of holding it down, I probably have to remove the varnish (with IPA/acetone I think ?) and re-bond to a different cantilever.