For reference I attach to this entry the retardation fit with and without taking into account the temperature of the LC.

Note that here and in previous entry I used the mean of the 3 data taken at every voltage step.

Taking into account the temperature in the fit reduce the residual rms by a factor 2 and the peak to peak by 50%.

I fit the unwrapped retardation of the LC (expressed in nm) as a function of temperature and voltage.

The peak to peak uncertainty is surely about 20 nm due to our 2.5deg azimuth angle we measured with 0V applied to the LC.

Indeed, the largest peak in the residuals appear at the folding voltages.

The rms of the residual is 2.6nm.

[Marc, Rishabh, Shalika]

After Takahashi-san opened the END gate valve, we recovered the GR beam on 2nd target.

We corrected the H3 sign issue and could realign the END reflection on 2nd target. We did some offload in pitch and yaw without issue.

We also realigned the END oplev.

Finally, we realigned INPUT by overlapping the input and reflected beam on the GR Faraday Isolator.

After some alignment tweaks, we were able to see GR fundamental mode in transmission of the FC but were not able to engage the lock.

We will investigate this issue.

[Marc, Rishabh, Shalika]

We first tried to rotate the LC while monitoring both azimuth and ellipticity.

It seems that we have a minimum azimuth angle of 2deg..

We changed the mounts of QWP and HWP for motorized ones.

By rotating the HWP we can now bring back the azimuth angle after the LC to 0deg...

It is not clear to me how this can happen..

We acquired some sweep in this condition but found similar levels of retardation.

Then, we installed and aligned a PBS before our camera to acquire data in the crossed polarizers technique.

FInally we heated the LC to about 40deg and acquire data with 0Vrms applied while letting cool down to about 25deg as shown in figure 1.

We have to add input power monitor to get more precise estimation.

I kept looking into the IRMC locking issue. I decided to start by splitting the measurement of the control loop transfer function

Figure 1 shows a model of the control loop for the electronics boards that we use in TAMA (Eleonora Capocasa thesis appendix D). We can extract the following transfer functions using noise injection and frequency response measurement:

• Servo transfer function: Noise to PERTURB IN, Frequency Response PZTmon/EPS2 - This is the transfer function of the electronics components inside the circuit board. Generically, it has an integrator response at low frequency (can set to single integrator 1/F or triple integrator 1/F^3), is flat or low slope across the UGF of the cavity in question, then rolls off at high frequency.
• "Optomechanical" transfer function: Noise to RAMP, Frequency Response EPS2/PZTmon - This is the transfer function of the light going in and out of the cavity when the piezo is excited. It should be generally flat until the cutoff frequency, i.e. low pass filter. The noise injection to the piezo will excite the mechanical resonances of the piezo and invar spacer, so it becomes a low pass filter with mechanical peaks. Actually, I don't really like this "optomechanical" terminology, to me it means 3 different things: 1) Thorlabs' terminology for moving mounts of optics such as translation stages, 2) The effect of spurious mechanical vibration of tables and housings on the amount of power going in and out of a cavity, 3) The direct interaction of quantum radiation pressure with moving mirrors (Braginsky and Khalili yellow book, etc).
• "Open loop" transfer function: Noise to PERTURB IN, Frequency Response EPS1/EPS2 - This is the open loop transfer function of the controlled cavity on lock. But again, I don't like how it is often generically called "open loop transfer function", because the above two are also open loop transfer functions

I measured the open loop transfer function for the optical cavity, as well as the electronics. The cavity transfer function looks fine (figure 2 - note that the appropriate noise excitation level is 100x lower when injecting to RAMP vs PERTURB IN). Basically the same as the reference version in the wiki. The electronics transfer function (figure 3, 4) is very low though. It has a -20dB/decade slope across the supposed UGF. I adjusted the gain of the servo and it didn't make much difference in the shape. Looking at the shape a bit more, I figured that it might be an issue with the integrator, since it is missing a lot of low frequency gain. So I tried to do the measurement again switching to 1/F^3 integrator, but the IRMC mostly refused to lock even when I tried different gain settings. I did see a bit of a flash of transmission (servo gain = 4) so I don't think it is just the 1/F^3 switch. Maybe it is something else along the chain that is badly behaved when in the presence of a triple integrator.

Some reference curves for the GRMC are shown in figure 5, from Yuhang's thesis. These are for the GRMC, but it has the exact same geometry as the IRMC. The overall unity gain frequency of the loop is 2 kHz. The servofilter has a -10 dB/dec slope across this frequency. The servofilter itself has a UGF of 200 Hz and a stronger slope at lower frequency. by comparison, the IRMC servofilter pretty much just strongly suppresses the signal even at 10 Hz.

Next, I tried looking at the RF sideband level. For the EOM used in the IRMC locking (QUBIG PM8-NIR_88), we have the following parameters:

• Resonant frequency: 88.3 MHz
• Applied signal (DDS1 channel 0): Output -8.5 dBm -> + 14.1 dB (RF amplifier board at bottom of FC cleanroom electronics) -> 5.6 dBm 
• Modulation depth at 1064 nm, 5.6 dBm: 0.2 rad (guess based one some generic datasheet, I don't have the actual one)

Using these, I estimated the amplitude of 88 MHz sidebands applied to the beam, with the following relevant parameters:

• 1064 nm photoelectric response: 0.6 A/W
• Transimpedance gain (50 Ohm load): 5.0 x 10^3 V/A
• -3 dB bandwidth: 150 MHz

If the bandwidth specification actually means a linewidth of 75 MHz, then having the 88 MHz sidebands out of the linewidth would result in an extra -2.7 dB attenuation past the 3 dB point (assuming first order rolloff for the photodiode), i.e. -5.7 dB power. Using the transimpedance gain there is 3.0 V/mW from optical power to PD signal. 

I measured the RF sidebands directly from IRMC REF RF (i.e. the cable from the PD that is filtered with a DC block) with the IRMC in scan mode, applying a T before the electronic signal goes to the mixer with the demodulation signal from DDS1. The sidebands have a level of approximately 60 mV. This implies a power level of 0.02 mW. The sideband relative intensity at 0.2 rad modulation depth is J_1(0.2) = 0.1. We start with 1.68 mW x J_1(0.2), then there is an extra -5.7 dB attenuation = x 0.27, and then divide again by 2 for 2 sidebands. This gives 0.023 mW converted by the PD into voltage for one first order sideband. So it seems consistent with us having -5.7 dB attenuation of sidebands at the PD due to being out of band.

[Marc, Shalika]

We used our new VI to characterize our LC (previously described in elog 3157 or 3155)

First, we aligned HWP and QWP to read 0+/-0.1 deg ellipticity and 90+/-0.1 deg azimuth angle with our camera.

We installed the LC and rotated it to maximize the ellipticity (-12deg). Note that we expect the LC to not affect the azimuth angle in this configuration but we measured 87.5 deg.

This could mean that our LC axis is not perfectly aligned with our input polarization/camera. We will try to further check this behavior.

In any case we then saved several sets of sweep as in figure 1.

we performed the ellitpticity unwrapping as in figure 2 by flipping twice the ellipticity compared to its maximum value (as expected as we have 2 wrapping points of the ellipticity).

Also note that we had to remove +/-5 points around the end of a sweep which exhibit strange behavior (spikes in ellipticity, azimuth and power).

This could be because the Vrms applied to the LC is quickly change from its maximum to minimum values as we do not see this feature with slow sweep by hand (if I remember correctly).

We measured retardation between 949.9 to 15.7 nm.

The descrepancy with our previous measurements and Thorlabs measurement could be due to this 2.5deg offset in azimuth that might indicate an improper alignment of the LC axis or due to long term fluctuations of the LC response.

Finally, figure 3 reports our 10 sweeps unwrapped.

All these steps are done in Python codes saved in LC-experiment folder.

As the temperature was changing during the measurement, we can see that the main effect is at really low Vrms applied to the LC only.

We plan to further characterize the temperature effect on maximum LC retardation with 0Vrms applied.

Note that one sweep (ie in future corresponding to 250 polarization states) took 25s but this is not limited by VI execution time nor LC.

[Marc, Shalika]

We merged our 3 VIs into a global one to perform LC calibration.

We followed a similar structure as for the PCI VI : a time structure where we first initialize parameters of VI, a while loop to do measurements and change LC voltage or temperature, and finally exiting all VI.

We plot and save data in a parallel while loop to the one where we set up the LC parameters.

Both these loops have different execution time. This is also because we need to be able to sweep the LC voltage at a different time interval than data acquisition or save.

Now we have one issue with LC temperature control where it stops if the target temperature is too far from the actual temperature. We have to enable/disable the temperature control few times to reach the wanted temperature.

While this can be done with one button of our front panel, we want to find a way to do it automatically.

Next steps :

- synchronize data saving and LC voltage sweep

- improve temperature control

- implement filtering

- add rotator control

We found that the issue of not being able to move END yaw is similar as in elog 2995, namely the sign of H3 is flipped...

Also, we now have about 2mm range for the pitch picomotor in one direction but it should be enough for our offload.

[Takahashi, Marc, Rishabh]

We opened the chamber to investigate the suspension in the west end.

• All actuator magnets are still on the mirror.
• There is no rubbing point.
• The pitch-yaw motion of the mirror with the picomotors looks fine.
• We found that the OpLev beam hits the cables for the actuator coils. We treated the cables so as to avoid the beam.

Using the power meter in reflection I saw that the mode matching of IRMC was > 95%.

Flipping the INV/NON INV switch makes the IRMC lock to the mode with output power up to 1.01 mW (/1.68 mW input = 60%). So that works now. Actually, I did the same thing for SHG previously when it was showing similar behaviour...

Adjusting the gain on the IRMC servo shows very little change in the transfer function. For gain increase from 1.1 to 7, the unity gain frequency increases to a rather underwhelming 15 Hz. Now that I think about it, the IRMC PD (Thorlabs PDA05CF2) is specified for a -3dB bandwidth of 150 MHz (75 MHz linewidth). 

IRMC error signal has a DC offset of 150 mV for some (probably not good) reason (figure 1). Green mode cleaner error signal suddenly went to about 60% of what it was yesterday (figure 2).

[Marc, Rishab, Shalika]

TLDR :

PR and BS are offloaded and fine ; INPUT need offload ; END can not move in yaw

cameras are fine ; can not connect to pico server or 2nd target

First we wanted to do suspension healthcheck but had again time-out issue.

It was solved by restarting the standalone pc. We found out that the BS response was really strange (eg fig 1) and that a END had a too low magnitude (fig2).

We went to tried to realign BS (even though was already good enough for the previous measurement) and went to END room where we found that the oplev gain was 10 instead of 100.

We put the proper gain and also realigned slightly the END oplev.

Before, we were not able to see camera image from 2nd target or end room in central building so we confirmed that END room cameras are working properly.

In central building, we restarted the quad camera board that solved this issue.

We could see green flash on GR transmission.

We then offloaded PR by keeping the GR beam on second target and acting on picomotors to make the coils offset go to 0.

We did the same for BS. However, at first both pitch and yaw of coils moved in diagonals in the camera. We found out that once the coils offset absolute value is below 200 this coupling is removed.

Now both PR and BS are working fine and offloaded.

We have to do the same for END (but now yaw coil offset is 0 and it seems far off good alignment and coils do not work) and INPUT.

We found that the ethernet cable that connect the picomotors pc to the switch was squash below a monitor. We replaced the cable but still are unable to connect to picomotors nor 2nd target...

Rishabh, Michael

We intended to go and tweak the IRMC control loop gain for better control bandwidth and stability, as well as measuring the GRMC/MZ transfer functions. But there were a few problems.

The IRMC seems a bit misaligned. We only have about 0.33 mW/1.66 mW = 19.9% transmission. We tried unlocking and relocking several times. Trying to view the free spectral range on the IRMC reflection PD showed some weird jagged spectrum with not very high amplitude. Power meter showed nothing, although Marc later said this is because the bandwidth of the power meter needs to be set to High. Perhaps the PLL alignment process affected the IRM alignment again. The injection and reflection power are basically the same as when it was working properly though.

On the green path after the SHG, the 90/10 green beamsplitter sends reflection to the filter cavity green AOM, which was optimised as per recent elog entries. Accordingly, due to optimisation of the 90/10 BS reflection alignment, there will be a small amount of change in the transmitted path length to the GRMC and MZ. Indeed, some change in the mode spectrum could be seen in the GRMC transmission (figure 1). We used the steering mirror after MZ and now the mode matching is (1.25/(1.25+0.40)) = 96.9% (figure 2, 3). We also tweaked a bit the PDH error signal - the phase was adjusted from 164.993 to 185 degrees (DDS2 Channel 2 GRMC Demod), so the error signal looks a bit more symmetrical (figure 4).

We couldn't lock the GRMC though. According to a previous entry from Yuhang regarding this issue, we looked at several troubleshooting points:

0. grmc has a good alignment.
97% mode matching (figure 2, 3)

1. PDH signal has 316mV pk-pk checked from EPS1.
120 mV pk-pk (figure 4)

2. grmc has loop sign of INV, which is as design.
Yes

3. The RF source phase is reloaded. The phase of RF source is 125deg. When it is changed to 35deg, the signal around resonance becomes flat. This indicates the RF signal phase is still a good one.
Due to the change of EOM the DDS phase is different, but we flipped by 90 degrees and saw that the signal around resonance became zero so indeed the error signal and DDS configuration are fine.

4. There is a switch which has +/- sign. This doesn't decide the sign of control loop. But when we use this type of servo for CC1/2 controls, we need to flip this switch. I tried to flip this switch, but it doesn't help to close loop.
I tried this but no change

5. grmc transmission is checked to have 1.13V peak. This is two times smaller than the value written by Pierre.
The main peak reads 1.25V from the PD signal (figure 3)

6. Loop gain is 3 as usually used.
Yes. Likely this will need to be changed later though due to the EOM change.

7. Threshold for peak identification is -0.55V. This is as required.
The threshold knob was set to 3.3 but I tried changing it to a few values and didn't get any lock.

8. The GR power reaching AOM is measured to be 44mW, whose nominal value is 50mW.
We have 47 mW reaching the AOM

We have good mode matching and a good error signal shape. So the alignment, transmission PD, DDS configuration and PZT are all fine. It seems like the issue is coming from the servo module, or perhaps the amplitude of the error signal. Hopefully it's just to do with the settings and not something for which we have to take the board outside of the cleanroom.

[Marc, Michael, Rishabh, Shalika]

We tweaked the last mirror on green injection into PR and recovered targets on PR tank.

We moved PR coils and recovered BS target. Actually, the gate valve between BS and INPUT is open so we could not use it to fine tune this alignment.

However, we found out that centering the beam on the 2inch mirror inside BS chamber was also a good way to have the beam centered on the first target.

We tried to control the 2nd target and look at its camera from remote but we could not connect to them. Maybe related to the on-going fiber installation in the arm?

We went to 2nd target and could recover alignment by moving BS pitch and PR yaw. It now seems that at least one of the yaw magnet of BS fall down as the beam is not moving properly.

Then we tuned END to recover the back-reflection on the 2nd target. However, it also seems that END yaw is not working properly..

We tweaked a bit INPUT and the green beam is now back to the injection FI.

We will have to offload several suspension using pico-motors as current coil offset is quite large for several dofs.

I checked the alignment to the green AOM (FC first stage length control).

Initially the following signal was applied to the AOM: 109.036 035 615 MHz, 5 dBm signal generator amplitude + 18 dB RF amp, 24V 0.5A power supply to RF amplifier

I used the " BSN10 GR 90/10" (as it is labelled) to move the beam into the AOM, then kept going to align the first order diffracted beam into the iris (figure 1). The following power levels were measured:
Power from SHG: 270 mW
Power into AOM: 47.0 mW
Power after AOM (no signal): 46.0 mW
Power after iris (23 dBm signal): 22.0 mW

(c.f. reference levels 2764: 48.6 mW in, 24.3 mW out)

I took a few different measurements of the first order power for different RF signal amplitude - not really a comprehensive characterisation, just a quick check. Seems roughly consistent with old measurements (531 1679), but I wasn't trying to be too precise for now. I didn't rotate the AOM at all though.
22 dBm -> 18.6 mW (40%)
23 dBm -> 22.0 mW (47%)
24 dBm -> 26.0 mW (55%)
25 dBm -> 30.0 mW (67%)
26 dBm -> 33.2 mW (70%)

The green beam is roughly aligned close to the PR targets (figure 2). There shouldn't be too much adjustment needed, though it is a 2 person job.

Through the time I was in the cleanroom the SHG unlocked a few times, maybe once every 45 minutes or so.

I measured the OLTF of the SHG and IRMC using the old spectrum analyser. I used two methods:

1) In FFT instrument mode, apply white noise to Perturb IN, measure Ch2/Ch1 (EPS1/EPS2) frequency response and coherence using a range of noise injection levels. The noise -> FFT method is fast but less precise when it comes to measuring characteristics about peaks.
2) In swept sine instrument mode, apply swept sine to Perturb IN, measure Ch2/Ch1 frequency response and cross spectrum (swept sine doesn't have coherence option)

In the case of white noise injection, lock at 500 mVpk was a bit temperamental but the spectrum could still be measured. Swept sine worked for an initial injection level of 20 mVpk. For values higher than that, it would unlock when the measurement reached ~ 100 Hz (it starts at the max frequency and goes downward).

Magnitude, phase and coherence for the two cavities are plotted for various excitation levels (fig 1-6), and the best result is collected in a 3x1 image (fig 7, 8)

SHG has a somewhat low unity gain of 600 Hz, nearly 100x lower than previous (50 kHz). Phase margin is about 50 degrees. Maximum coherence is obtained for about 200 mVpk noise injection.

IRMC is very low. It seems a bit strange actually. UGF is around 10 Hz and coherence is quite bad for excitation < 500 mVpk, but it also unlocks really easily when excited at this level.

[Marc, Shalika]

Yesterday we checked again DDS3 outputs and got similar results as last time (3178).

We opened the box to check the +12V/GND provided to each amplifiers. We found that DAC0 and DAC3 were not receiving the expected voltage.

DAC3 +12V cable got easily disconnected and we found that DAC0 +12V pin was extremely tilted..

We removed DAC0 amplifier and soldered a new one. We also resoldered DAC3.

Our guess is that all the +12V/GND cables have a strong strain and twist due to the limited space in the board. So we replace the connection to the board output from a SMA female to female connector to a about 5cm length SMA cable.

All the ouputs of the board are now as expected :

DAC0 8.8dBm

DAC1 9 dBm

DAC2 8.6 dBm

DAC3 8.6 dBm

Maybe the strong twist of DAC0 amplifier +12V pin was the reason for all our issue?

Also the board inside is now quite messy. If we want to tidy it we should prepare some custom length SMA cables.

Marc, Shalika, Michael

We looked a bit at SHG transfer function using the new spectrum analyser (Source -> Perturb IN, Ch1 -> EPS2 Out, Ch2 -> EPS1 Out). Just a quick check using FFT with white noise injection showed that the gain of the loop is somewhat lower than before. The frequency response Ch2/Ch1 in the reference level is flat at 20 dB until 10 kHz, but now we drop off fairly fast at about 100 Hz. Switches are set to NON INV, 1/F3, DIF OFF, SIGN -

I found the SHG wouldn't lock properly even though the servo light was green. First I checked the power compared to reference levels. Input IR was fine (~700 mW) but reflected green was very low, < 1 mW. Next I tried checking the mode spectrum but it didn't show anything at all. After flicking the lock switch a few more times I noticed the green power actually dropped when it locked, so based on that I turned the INV switch to NON INV and now it locks as it should (270 mW green). Basically, we have to do some tweaking of the servo because of changing the EOM

We recently purchased a new Stanford Research SR785 Dual Channel Signal Analyser to be used in the FC cleanroom.

It is a bit complicated. One of the main features is that the measurement is independent of the display. This allows us, for example, to have a really precise measurement but with a really rough display, among other things. It can also save a lot more data into its internal buffer in one go, so we can then transfer to disk without having to redo a measurement.

By default it had a really irritating alert message that sounds like a phone ringing, so I turned it off in "Preferences -> Alarm noise -> Quiet". There is another menu option which says "Alarms -> off" but that controls the display of error messages. Very strange UI in my opinion.

It can write data to USB, however, due to weird stuff, it won't recognise storage devices over 8GB. This seems to be a common issue with making old tech forward compatible with FAT32 USBs. The grey USB drive in the FC cleanroom works fine and I already formatted it.

The spectrum analyser formats the USB to pretend that it is several hundred 1.44MB floppy disks. But unfortunately Windows only recognises the first "disk" in the sequence (labelled 000 in red digits on the front panel of the device next to the USB port), so it still runs out of space. Also at one point the spectrum analyser would refuse to re-format the USB, so I had to Full Format on Windows (slow...) and then re-format on the analyser. Shalika says that apparently you can program it to send data over wifi to the computer, which would be better. The manual talks about cable connections only (RS232 and GPIB) but maybe we can find a wifi attachment.

To do: Figure out a good way to remotely import data from the analyser using programmable commands, and then batch convert into some convenient data format. Personally I want to separate headers and data, and the delimiter doesn't matter to me. MATLAB also seems to not mind this operation. But maybe others also have preferences for dealing with data.

Objective: [In continuation to elog 3177]

1. Measuring ringdown decay time

Details:

1. The new RF switch's output was attached to AOM. The diffraction efficiency remained the same as noted previously, i.e. 57% (as mentioned in 3131)

2. The PD used at the transmitted path is  PDA20CS-EC (see elog 3139 for properties). The PD at the transmitted beam is set with a 10dB attenuator.

3.  The amplitude of TEM00 mode in the transmitted signal was 4V. The demodulation phase was set to 185º. The mode matching was observed to be 4.88/(4.88+1.82)=72%

4. The filter cut-off frequency was 3 Hz and the gain was 100. 

5. The laser temperature was 8400±20. The offset voltage varied from -1 to -0.8. 

6. The cavity was able to lock the TEM00 mode (see Fig 1). The voltage of the RF switch control was set to low to cut off the input to AOM. The possible fitting obtained gives a decay time of 1.45 µs (see Fig 2)