Matteo and Yuhang

Based on Aritomi-san's code, I add the degradation from mode-matching to the CCFC error signal. The simulation result is in attached figure 1. From this simulation, worse mode-matching makes CCFC error signal degrade around resonance. But mode-matching doesn't affect the CCFC error signal's offset.

Based on this simulation, we sent BAB to the filter cavity and checked the mode-matching was about 0.75. We found the IR drift happened only in the yaw direction. After optimizing yaw, mode matching increased to about 0.9. When we checked the CCFC error signal's pk-pk value, we found some issues with this signal's demodulation. After optimizing the mixer, we saw an even better CCFC error signal. The comparison of CCFC error signals before and after optimization is in attached figure 2.

I compared the mm-optimized/mm-original CCFC error signal's minimum. In the simulation, the ratio is 0.64. While in measurement, it is 0.58.

We took a spare PSD and replaced the old one for PR Oplev. The spectrum of PSD was measured and shown in figure 1. We can see that the new PSD has higher noise than the reference. Apart from that, the new PSD also shows different peaks, which needs to be further examined.

TAMA PSD for PR pitch show excess noise again, the situation is shown in the figure 1.

I attached OLTF of CCFC and green lock. Note that I flipped the sign of measured data to match the measurement and theory. The measured phase is not consistent with theory.

In elog1727, I tuned CC PLL frequency from the fitting of CC separation frequency and CC PLL frequency, but the error of the fitting parameters is quite large with respect to optimal CC separation frequency 108 Hz. So this method is not precise to decide the correct detuning.

As written in elog2294, current CCFC error signal is not consistent with theoretical plot with optimal detuning, but instead it is similar to the theoretical plot with 25 Hz detuning.

If the current detuning is 25 Hz, we have to change the detuning by 29 Hz to obtain optimal detuning 54 Hz. Using the formula in elog1727, the CC PLL frequency has to be changed by 2*29 Hz/1.907605 =  30.41 Hz. Since the current CC PLL frequency is 6.99704303 MHz, optimal CC PLL frequency should be either 6.99707344 MHz or 6.99701262 MHz. I checked both cases by looking at CCFC error signal and found that 6.99701262 MHz is correct one (In DDS, 6.99701253 MHz was set). 

Here is the new CC PLL setting. I saved this setting as 20201126_dds3_CCFC.

Attached plot shows CCFC error signal with different CCFC demodulation phase. Amplitude of the CCFC error signal is normalized with 83mV which is the amplitude of CCFC error signal when CCSB are off resonance of FC and CC1 is scanned.

Now the shape of CCFC error signal is similar to theoretical plot. In addtion to that, zero crossing point of blue curve in second plot is around 58Hz which is almost optimal detuning.

CCFC error signal with 25 Hz detuning is very similar to the measurement.

Actually, the errors of the fitting parameters are -1907605 +/- 36859 and 13347486 +/- 257882. This error is quite large with respect to 108 Hz. We need to fine tune CC PLL frequency by looking at CCFC error signal.

By the way, optimal detuning should be 54 Hz. I attached CCFC error signal with optimal detuning. Normalized offset for I phase is 0.81 in the plot.

I checked that for optimal detuning (70Hz), the expected in-phase demodulation CCFC error signal should have normalized offset 0.91.

In this measurement, we got the in-phase demodulation CCFC error signal normalized offset to be about 0.69. I checked that for this normalized offset, the detuning set by PLL will be 43deg.

The calculation is in the attached figure.

By looking at the normalized offset-removed plots, it seems for different demodulation phase, they have the 'same' peak but just somtimes folded.

If we compare it with calculation, it seems the measured peak is a factor of 2 smaller than calculation.

It has been a long time that we found CCFC error signal measurement doesn't match well with calculation. Recently, we found that this might be due to an offset, as reported in elog2286.

The CCFC error signal is equation 14/15 in Aritomi paper, which can be written as proportational to sin(a_p-a_m+a0_m-2d_p). Please check details from arxiv:2004.01400.

Here a_p and a_m are the upper and lower sidebands phase change caused by the filter cavity. When the filter cavity has detuning much larger than linewidth (70Hz), a_p-a_m will be close to zero. a0_m is the lower sideband phase change caused by the filter cavity when carrier detuning is optimal. When we change the demodulation phase of CCFC, we add another term phi_d to the sine function.

Therefore, in the case that FC is locked and detuned to 1kHz (CC1 locked), the CCFC error signal will be sin(a0_m+phi_d). Since a0_m is a fixed number, we expect to measure a shifted sine wave when scanning phi_d (from 0 to 2pi). For each phi_d, we expect an 'offset' from zero. We got a 'shifted' sine wave from the experiment. The result of this measurement is in the first attached figure. There is also a sinusoidal plotted in this figure. We could see that the measurement matches with sinusoidal. The difference still needs to be investigated (one further check could be a measurement of error bar for each point, the other could be to plot a histogram of measured data)

To double-check this expected offset, we performed the filter cavity scan around resonance (CC1 locked, FC locked while AOM scanned). The CCFC error signal for different phi_d is in the attached figure2. If we subtract the measured offset from figure2, we got figure 3. Not surprisingly, all offsets of CCFC error signals were removed. Thus we know this offset agrees with the calculation.

Then the question comes: why we saw a different CCFC error signal in an experiment different from a calculation that is not caused by an offset problem?

If we compare figure 2 with the calculation, we could see that every time the CCSBs cross resonance of the filter cavity, the appeared peak seems to be not deep enough. So we guess the problem is related to how the filter cavity changes the CCSBs phase when they cross resonance.

This not ideal CCSBs phase change could be caused by mode mismatch/misalignment or PLL setting. More investigation is required.

Recently we are investigating HAMAMATSU PSD to be used for Oplev. This is due to the noise increase observed for BS Oplev spectrum, which was pointed out to be caused by PSD.

Last Friday, a similar problem was found also in PR Oplev. As shown in the attached figure 1 and 2, the PR Oplev noise has different noise increase at different time.

I also checked PR Oplev spectrum this Monday, the spectrum became normal as attached figure 3.

Oil under the rotary pump was there from the old days. I replaced the rotary pump to new dry pump (ACP15). The TMP with the dry pump is working now at the mid point.

DDS3 CH2 (14MHz) was split to used for both CC1 and CCFC demodulation so far. To change these demodulation phase independently, we used DDS3 CH3, which is usually used for CC2 demodulation, for CCFC demodulation. We changed CC1 and CCFC demodulation phase independently and checked CCFC error signal. We confirmed that changing CC1 demodulation phase is identical to changing CCFC demodulation phase for CCFC error signal.

To see the possibility of using HAMAMATSU PSD for INPUT mirror oplev, I did the comparison between Thorlabs PSD (with amplification) and HAMAMATSU PSD (without amplification) for INPUT mirror oplev.

The comparison result is shown in the attached figure.

Upper figure: (for INPUT pitch) REF0 is INPUT oplev spectrum with Thorlabs PSD. REF 2 Thorlabs PSD electronic noise after amplification. REF 4 INPUT oplev spectrum with HAMAMATSU 04 PSD. Red line: HAMAMATSU 04PSD electronic noise.

Lower figure: (for INPUT yaw)REF0 is INPUT oplev spectrum with Thorlabs PSD. REF 2 Thorlabs PSD electronic noise after amplification. REF 4 INPUT oplev spectrum with HAMAMATSU 04 PSD. Red line: HAMAMATSU 04PSD electronic noise.

It can be seen that Thorlabs PSD has a bit better SNR. This better SNR is proven to come from the amplification (proved in elog2114).

As reported in elog2281, the HAMAMATSU PSD electronic noise level is similar with DGS ADC noise. In this entry, I report a futher investigation of them.

1. I had a closer look into these two noise spectrum. As shown in the attached figure 1 (RED is elec noise, BLUE is ADC noise, GREEN/BROWN are integrated noise), elec noise is a bit higher than ADC noise but not a factor of  2. Therefore, to investigate further the electronic noise, we should amplify signal from PSD before it goes inside DGS system.

2. Before amplification of PSD individual signal, I checked the time series of individual signal from PSD. This is shown in the attached figures 2. We could see that it has very large noise. Therefore, it is very diffcult to amplify such large noisy signal. Indead, I found SR560 could only give a factor of 2 amplification for it before saturation. But the good thing is that, after the combination inside DGS, this noise is cancelled and resulted in a clean pitch/yaw electronic noise.

So if we really want to check better the electronic noise, we should combine PSD individual signal before they go inside DGS system.

By replacing 03PSD with 04PSD for BS oplev, I compared their performance.

The measured oplev spectrums are shown in the attached figure 1 (RED 04PSD, BROWN 03PSD), while the electronic noise comparison with ADC noise is shown in the attached figure 2 (RED is elec noise, BLUE is ADC noise, GREEN/BROWN are integrated noise).

We could conclude that both 03PSD and 04PSD have almost the same performance. The difference in position resolution is not affecting their performance now. So I guess this position resolution maybe related with beam size.

I tried to measure the residual gas molecules by a mass spectrometer.
The Q-mass could be operated by a front panel, and the pressure was 3.0*10^-5 Pa, 7.8*10^-7 Pa for H2o and N2 respectively.

Hoever, I could not connect to PC, and could not do degas.
This Q-mass needs degas but in order to do that, the connection to PC is needed...

I will ask the company.

Since we have problem of TAMA PSD, we considered to buy new PSD. Matteo asked two PSD from HAMAMATSU company for test. They are C10443-03 (we call it 03 later) and C10443-04 (we call it 04 later). The information of them can be found from this link.

From the datasheet, 03 and 04 PSD only have difference in position. I think the integrated noise spectrum can represent a position resolution. If so, the one with better position resolution should have a lower noise spectrum. Therefore, I firstly tested 03 which has a better position resolution. If it is better than the old PSD, we should use it in the future.

Together with Matteo, we made a customized circuit based on Mammoth connectors, bananna connectors and lemo connectors. In this way, the eight channels from HAMAMATSU PSD can be connected to power supply and ADC of DGS system. Four channels from PSD are used for power supply, which contain one plus(12V), one minus(-12V) ,and two grounds. Another four channels are called x1, x2, y1, and y2 separatly. The experimental set-up is shown in attached figure 1.

To convert four signal channels into pitch and yaw, I realized a matrix inside simulink file. This matrix is enclosed in a block called 'BS_test', as shown in the attached figure2.

Since old measurements have been saved in DGS system, I just plot new PSD data with old ones. As shown in the attached figure 3, there is comparison between old low gain TAMA PSD and 03PSD. We can see

1. REF0/1 (BS angular motion sensed by low gain TAMA PSD) is comparable with REF16/17 (BS angular motion sensed by 03PSD). This means that two PSD have the same gain.

2. REF8/9 (electronic noise of low gain TAMA PSD) is lower than REF24/25 (electronic noise of 03PSD).

3. Red lines in this figure shows the ADC noise. Actually this measurement is a bit strange. It shows that ADC noise is even higher than the electronic noise of low gain TAMA PSD. This can be correct if the ADC noise really becomes worse by itself.

4. Since the Red lines overlap with REF 24/25, it means the measured electronic noise maybe just ADC noise. So if we want to check better mirror angular motion, we need to amplify the signals coming from PSD.

I also compared the electronic noise between high gain TAMA PSD with 03PSD. The result is shown in the attached figure 4. Red lines are 03PSD electronic noise, which is lower than REF8/9 (high gain TAMA PSD electronic noise). 

We will also test 04 PSD to justify the relationship between PSD position resolution and noise spectrum.

I pumped down the cryostat and turned on the Q-mass.
The LED lamp next to "POWER" turned on, though that of "Pa" did not.
It seems that this Q-mass may not be used anymore.

Furthermore, in order to degas the Q-mass, we need to pump down below 10^-4 Pa, though it cannot reach below 6*10^-4 Pa...
I gave up the measurement.