We found the IRMC transmission was 1.28mW while it was about 2mW in the past.

We remove a quite transparent square ND filter and could recover about 1.8 mW transmission.

We tried to tweak a bit alignment (both PBS and lens before the IRMC) but it did not change the transmission.

[Marc, Michael, Yuhang]

As reported in previous elog (eg 3189), the error signal of GRMC is about 10 times smaller than the nominal value.

We checked the RF signal used to lock about found that it is about -36dBm.

So we decided to use an unused amplifer from the amplifier rack (previously used for EOM FC/GRMC and providing 20.8 dB amplification).

After that, our RF signal is about -9dBm.

With this higher RF power, the error signal is now 800Vpk and we can lock GRMC and MZ with no particular issue.

The GRMC transmission with MZ offset of 4.2V is now about 26mW (25mW nominal).

I found that the dry-pump (ACP15) in the south end has failed. I replaced it with the spare pump (DSP500).

Yuhang, Marc, Michael

The lock of both the p-pol and CC phase locked loops was recovered. 

Details

Yuhang adjusted a bit the fiber coupling and noticed a bit of an issue with touching something somewhere and the fiber signal immediately disappearing.

Yuhang adjusted the ML-ppol fiber coupler position while Marc and I looked a bit at the green mode cleaner. The mode matching is 95.4% but the error signal is also a bit low, about 120 mVpk (in the past it was 10x larger). This situation is similar to the IRMC and SHG. We seem to be losing a bit of power from the transmission of the BS90/10 to the input of the MZ. The beam seems quite large and passes close to a mirror post on the way to the MZ input.

Eventually we certified a satisfactory level of fiber coupling for all 4 fibers. Taking the signal from the fiber PD shows a 45 MHz peak at -20 dBm and we could see some optimisation on the spectrum analyzer using the half wave plate.

We were a bit confused about which DDS RF signals required an attenuator before injection to the PLL servo, and by chance found that the DDS3 DAC3 is controlling the p-pol local oscillator - it should be controlling homodyne LO, with p-pol LO coming from DAC0 - with our recent PLL issues we didn't have the correct PLL LO frequency (was 7 MHz, should be ~ 20 MHz) and didn't notice this cause until now. But we decided to leave it for now and just try locking the PLL with the DAC3 port.

We found that some ad-hoc DC blocks (grey boxes on the bottom of the rack labelled #4, #5, #6 etc) used for monitoring the PLL signals were improerly connected. So we double checked all the connections coming from the fiber PDs and confirmed the proper placement of signals going to the "BEAT" port on the servo, and the "MON" wires near the spectrum analyzer. Then we confirmed the sign of the signal on the PLL software and eventually could lock p-pol PLL with fast and slow loops.

For the CC p-pol, we discoverd some silly mistakes like mixing up the CC/p-pol lasers and a disconnected signal wire. Then we saw that the CC LO PLL signal is very large (14 dBm) so we put the -12 dB attenuator back in. There was some issue again with the PLL software but it is now fixed and CC p-pol lock was recovered.

Yuhang, Marc, Michael

We tweaked a lot of the infrared mode cleaner control loop, from optics to electronics.

Details

First of all, the power to the IRMC_REFL PD was decreased using an ND1 filter, since the voltage was near saturation. A lens was also placed to reduce the beam size on the PD.

We investigated a lot of connections coming from the photodetector and going to the servo board, cables, servo settings, DC blocks. The reference level for the IRMC error signal was about 1.2 V before the replacement of the main laser EOM, and right now we have 120 mV. By comparison, the SHG error signal only went down by about a factor of 4 after the EOM change. They each use the same PD (Thorlabs InGaAs RFPD PDA05CF2) so it seems that there was some other source of reduction. We scanned and fixed all of the settings of the IRMC servo board, particularly the servo gain, detection threshold and PZT scanning speed. Halfway through we had some trouble locking, so decided to check with SR560 instead of the servo, and indeed it could lock with quite low gain. However, for the servo the gain is not sufficient to provide a good signal. By slowing the scan speed on the servo and moving the scan window around using the PZT offset knob on the high voltage driver, it could be seen that the servo is attempting to lock with a flashing red light, so it is detecting the PDH signal and the reflected peak crossing the locking threshold.

Eventually we decided to consult Pierre Prat regarding the electronics board. We propose to replace resistor R33 of figure 1 which determines the amplification of the gain potentiometer.

Marc, Michael

We attempted to fix the CC PLL locking issue but no luck. It seems that the issue is on the electronics side as the optical level coming out of the fibers is quite good.

Details

I aligned a bit the ML->AUX2 fiber. I made about a 20% increase in the voltage on the oscilloscope coming from the fiber PD monitor. Then, we checked the couplings. The power levels are as follows:

ML in 4.6 mW
ML out 0.75 mW
ML coupling = 0.75*2/4.6 = 33%
ML scope 7.5 mV
CC in 1.0 mW
CC out 0.3 mW
CC coupling 60%
CC scope 2.0 mV

Seems like way too good CC fiber coupling and yet not much signal on the oscilloscope. I tried doing a full rotation of the waveplate in from the the CC fiber but it didn't change noticeably. I also tried switching PDs but the result was the same. Likewise, both PDs showed no appreciable signal on the spectrum analyser.

I tried calculating the expected voltage from the PD. The sensor is not transimpedance (Thorlabs DET01CFC/M) and the output voltage depends on the load resistance V_out = I_out * R_load. The current conversion is ~ 0.7 A/W at 1064 nm. We use 50 Ohm terminators, so the expected voltage would be about 26 mV for ML and 10.5 mV for CC. But the measured value is about a factor 4-5 below this. For comparison, the p-pol PLL has a total voltage of about 45 mV (~ ML 20 mV + ppol 25 mV). 

[Marc, Shalika]

We modified the temperature controller VI to be able to change quickly the LC temperature.

Indeed, up to now the controller would disable if the requested temperature was too far from the actual one.

Now, we pause the controller for 2s to prevent the controller to disable. It is now far quicker to reach the requested temperature but there could be some issue with data saving during the pause.

Then, we measured the LC retardance as a function of its temperature with 0V applied.

As shown in attached figure, we varied the temperature between about 25degC to 45degC.

To fit over a broad temperature range, we used a*10 + (T-T0)*b + c*np.exp((T/T0)*d) (in blue).

As we mainly care about low temperature where we have the largest retardance, we also tried a 1st order polynom up to 30degC (in red).

We now have -16.2 nm/degC (the minus sign coming from the unwrapping).

We tried to fit with various function the LC retardation as a function of applied voltage.

Up to now we were using R = a + b/(1+(V/c)**d)**e (1pole in attached figure) but it seemed not optimal as seen from the residual shape.

We now tried the 2 poles function : R = a + b/(1+(V/c)**d)**e + b/(1+(V/f)**g)**h and 2 different poles :  R = a + b/(1+(V/c)**d)**e + i/(1+(V/f)**g)**h.

These 2 functions improved a lot both residual rms and peak-to-peak values. We are now able to reconstruct the LC retardance better than for polarimetry.

New step is to add the temperature dependence.

I remeasured the transfer function whose resonant frequency is set around 0.5Hz.
I measured it from 2Hz to 0.1Hz.
The peak of resonant looks like 0.4Hz.

This time, peak of resonant is crushed by range of photo sensor.
When I measured it around resonant frequency, displacement is bigger than linear range of photo sensor.

What I did: measure transfer function of Roberts linkages.
 

I measured transfer function of Roberts linkages.
I attacehd two transfer function.
Resonant frequency are set at 1.5Hz(Fig 1, 2) and around 0.5Hz(Fig 3, 4) respectively.
The set up of these experiments are also attached.

The experiment whose resonant frequency is set at 1.5Hz probably have peak at 1.5Hz(Fig 1).

The peak on experiment whose resonant frequency is set around 0.5Hz is 3Hz(Fig 3). This peak is yaw motion.
The reason that I didn't measure transfer function surrounding resonant frequency is the range of photo sensor.
I realized that when I measure the transfer function, I need to measure mutiple time.
 

What I will do: I will remeasure the transfer function whose resonat frequency is set around 0.5Hz.

I tryed to check how much the shaker works.

I attached the pictures of the fransfer function from input of the shaker's power amplifiler to read out from the accelerometers' gain amplifiler.
The accelerometers are on the shaker's floor.

I didn't tune read out voltage from these accelerometers, so I only be able to refer the shapes of transfer function.

The shaker is MEE-035 made by Akashi Corporation (Fig 3).
The power amplifier for shaker is AME-100made by Akashi (Fig 4).
The accelerometer on shaker's floor are 707LFZ and 710B190 made by TEAC (Fig 5, 6). The manual says that these accelerometers have frequency response from 0.2Hz to 8000Hz and from 0.02Hz to 200Hz respectively.
The gain amplifier for accelerometer is SA-16P and SA-16U made by TEAC (Fig 7).

The input for power amplifiler is 1Vpp. The gain for accelerometer is 200 for 707LFZ and 10 for 710B190.
I set lowpass filter at 100Hz on the gain amplifiler for accelerometers.

I tuned accelerometer's resonat frequency to measure seismic noise and know the mechanics of accelerometer.
In this time, I tried to make resonant frequency be close to around 0.2Hz.

The pictured and transfer function was attached.
The weight attached accelerometer is 341.2g.

The transfer function is from actuator voltage to accelerometer's LDVT read out voltage. 

[Marc, Shalika]

We were guessing that part of the peak-to-peak uncertainty is due to the unwrapping of the ellipticity.

We decided to switch to cross-polarizer characterization as there is only 1 unwrapping compared to 2 in the direct characterization.

We tuned HWP and QWP to generate an input polarization state with ellipticity = 0.1deg and azimth = 0.05deg.

We installed a power meter to monitor a pick-off of the input beam. We measured input power of 3.6 mW and after HWP/QWP 0.8 mW.

This powermeter readout is also implemented inside our VI.

We installed a TAMA polarizer before the camera (NEWPORT 10GL08AR.33) before the camera and rotated it to minimize the transmitted power (36 nW)

We installed our LC and rotated it to maximize the transmitted power (23 uW).

We took some measurements in this configuration but it does not seem to improve our fitting uncertainty.

Maybe we should try to tweak our fitting function as it seems that most of the fitting error arises around our inflection point.

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