Yuhang and Michael

This measurement is more for the purpose of the KAGRA filter cavity project. We would like to optimise the design of the length control for the filter cavity. This involves setting the parameters of the control loop (bandwidth, filters, RMS noise etc) as well as choosing a control scheme of one of the three available for frequency dependent squeezing (A+ Resonant Locking Field, AdV+ Subcarrier, TAMA CCFC).

The performance of the filter cavity length control is in principle limited by two noise types:

• backscattering - the length noise of the filter cavity causes fluctutations of incident coherent fields, most notably the residual interferometer dark port power that leaks through the squeezing path Faraday isolators. This light has length noise imprinted on it by the filter cavity which degrades squeezing at frequencies where mirror motion is dominant (~ < 10 Hz). Backscatter can be decreased by increasing the gain or bandwidth of the control loop. However, increasing the gain at low frequency causes issues with stability, and increasing the bandwidth causes issues with...
• sensing noise - primarily caused by phase (frequency) noise of the voltage controlled oscillator that generates the sensing field for filter cavity locking. The phase noise of the sensing light causes locking fluctuations of the filter cavity length, which in the case of sensing noise can't be suppressed by increasing the gain of the control loop. In the A+ case, the phase noise of the AOMs that generate the resonant locking field is quite severe and the reinjection of sensing noise becomes prominent at and above about 10 Hz.

Changing the control bandwidth will raise one of these noises and lower the other, and the "bucket" is of the order 10-30 Hz for ~ 100m filter cavities with km scale GW detectors. Thus, we would like to measure the frequency or phase noise of the TAMA PLL at low frequency, as a placeholder for the KAGRA filter cavity sensing control noise. The PLL generates a sine wave that is the frequency difference of the main laser and CC/p-pol. Obviously we would like to keep this difference as stable as possible. We can beat this sine wave with a different one in order to check the stability. When the frequency of the signal and LO is the same, provided that the LO is stable enough, the voltage coming out of the mixing between the local oscillator and PLL is proportional to the phase noise. The relevant equations for this principle of measurement are given in Yuhang's thesis, 4.25 to 4.31. By default, we are using 190 MHz for the CC PLL and 250 MHz for p-pol, but the digital system in TAMA can only generate sufficiently clean signals up to 100 MHz. So we lowered them to 50 MHz. 

The spectrum of the phase noise is related to the spectrum of the mixed voltage by:

PSD (rad/rtHz) = PSD (V/rtHz) / Apk^2

Where Apk^2 is the amplitude of the oscillation that occurs when we change the LO by a small amount.

For the actual measurement, I used the P-POL PLL MON channel as the signal and the CC PLL LO (DDS3 board, Channel 0) as the local oscillator. The level of the DDS3 Ch0 output at 50 MHz was found to be 7.61 dBm after removing the attenuator that is normally attached to it. The signal and LO were send to a small mixer. By setting DDS3 Ch0 to 50.0001 MHz, the Apk is found to be 0.083 V. Then, resetting it back to 50 MHz, the signal and LO are combined at the mixer to give the phase noise spectrum. Of course, you should make sure that the PLL has remained locked throughout the measurement :).

Figure 1 shows the spectrum of PLL phase noise. Figure 2 shows a previous measurement from Yuhang's thesis. In general seems mostly consistent but I think we have more noise in the tens of Hz band right now.

Marc, Yuhang

As described in McKenzie PhD, a possible coupling mechanism of pump RIN coupling to squeezing measurement is the presence of a spurious IR seed.

In our case, this could come from spurious LO back-reflected from Homodyne to the OPO or from IR coming from the SHG.

We started investigation on the first case by checking if the FI after the OPO has good attenuation factor.

We removed the Homodyne BS and installed a reflective mirror before the AMC.

We injected BAB and overlapped these 2 beams.

Then, we removed BAB and check the LO power around the FI.

We checked the OPO Trans PD and found really low power. We tuned the steering mirror just before and got TEM00 peak height at about 2V.

We got about 420 uW incident, 400 uW reflected (to the top) but only 10 nW transmitted... Despite alignment tuning we could not find where the missing 20uW are..

We removed the OD filters on the LO path but we had similar power ratio.

We tried to tune the polarizer of the FI (furthest from OPO) by about 20 deg but the transmitted power did not change at all...

In the past, we always demodulate the reflection of GRMC at 78MHz to get PDH and lock GRMC. However, the 15.2MHz modulation on the main laser which is well within the linewidth of SHG, which makes this 15.2MHz modulation present also on green. To check what is the quality of this modulation, we did a demodulation of GRMC reflection at 15.2 MHz today.

The PDH demodulated at 15.2MHz and 78MHz are shown in attached figures. We can see that the PDH at 15.2MHz is even larger than the signal at 78MHz. This indicates that the SHG process seems not to degrade the sideband of green. This could make the lock of GRMC easier in the future.

Marc, Yuhang

We investigated the possible RIN and phase noise coupling from the LO to the squeezing measurement.

Pump RIN :

To make this measurement, we injected 900mVPk of white noise to IRMC.

Results are attached in the first 5 figures. We can see a huge degradation of the squeezing level below 8kHz. However, the coherence is quite low (about 0.4).

In the 5th figure we also showed the contribution of this noise to the squeezing level but the shape is too different to precisely estimate the 'real squeezing' we are generating.

Pump phase noise :

To make this measurement, we injected 20mVPk of white noise to CC2.

Results are attached in figure 6 to 11.

This time, there seems to be no contribution to the squeezing measurement.

Marc, Yuhang

We investigated the possible RIN and phase noise coupling from the green pump to the squeezing measurement.

Pump RIN :

We repeated the measurement of elog 3065 but injected this time larger noise (400mVPk instead of 100mVPk).

Results are attached in the first 5 figures. The results are compatible with the previous measurement : to match the projection of pump RIN to our estimation of classical noises, we should have about 9.4dB of squeezing instead of the 6dB that we are seeing at high-frequencies.

Pump phase noise :

To make this measurement, we injected 20mVPk of white noise to CC1.

Results are attached in figure 6 to 11.

This time, we can see a broadband degradation of the squeezing level (by at least 2dB). However, the coherence is really low between our excitation and the squeezing measurement..

Also, it seems that the contribution of pump phase noise create some similar pattern to the contribution of pump RIN to the squeezing measurement..

Marc, Yuhang

These past days it was not possible to access the clean room pc from remote.

We found out that the was turned off, the DDS was emitting a really loud noise and lasers were all off.

It might have been caused by the typhoon this past week-end which might have created some electricity shut-down.

We restarted the pc and DDS rack before reloading the appropriate configuration files.

Everything seems fine for now.

I checked the RF driver(1080AF-AEN0-2.5) working in atc.

Tuning Voltage was fixed 10V, and Analog input was set from 0.1V to 1.0V.
I recorded voltage and frequency of AO Modulator, and also recorded voltage and ampere of Power Supply.
Before recording it , I ran electricity only from Power Supply for 5 minute to warm up the RF driver.
And I used a 20dB/50OHM attenuator(Mini-Circuits model CAT-20) to prevent the oscilloscope from saturating.

The result about Analog input and AO Modulator was as follows.

I graphed and fitted linearly with respect to voltage of the AO Modulator and voltage of the Analog input.
The figure was attached. The width of error bars is 1σ.

The result about Analog input and Power Supply was as follows.

Marc, Yuhang

First, we checked BAB and LO alignment inside the AMC.

LO mode-matching is about 99.9% and BAB is 99.12 +/- 0.3 %.

We measured visibility by installing a PD before the AMC and got 95 +/-1.7 %, but we expect to find same value as mode-mismatch into the AMC..

We then measured the visibility directly from the homodyne and got 98.1 +/- 0.4%.

While this value is closer, there is still some discrepancy and we are not so sure about the reason for that.

We measured squeezing and anti-squeezing with higher pump power (45mW, 50 mW, 55 mW).

The result is attached in figure 1.

From the fit, we have optical losses = 20.74 +/- 1.16 % and phase noise = 34.64 +/-3.15 mrad.

We tuned PLL frequency and OPO temperature before each measurement but forgot to record their values..

DDS RF channels are summarized in elog1488.

This information is summarized in my Ph.D. thesis Appendix B. I take them and put them here in a table.

Yuhang and Marc

As reported in logbook3064, the noise coupling from 532nm pump relative intensity noise (RIN) to anti-squeezing was characterized. In this logbook, we report the same noise coupling way, but it's from RIN to squeezing.

For two situations of no noise sent to MZ and 100mV sent to MZ, we measured four spectra, including RIN spectrum, squeezing spectrum, transfer function spectrum (from RIN to squeezing), coherence spectrum. The comparison of each spectrum in the two situations is shown in attached figure 1 to 4.

We can see that the noise injection is coherent below about 10kHz. So the transfer function measurement should be reliable below this frequency. Therefore, we take the measured transfer function when 100mV is sent to MZ for calculation in the next step.

The next step calculation is firstly a subtraction of measured squeezing spectrum (orange in Fig. 5) and expected squeezing level (black in Fig. 5). This expected squeezing level is an estimation of a constant value of 6.4 dB since squeezing is frequency independent, considering the flat region of measured squeezing spectrum above 20kHz. The result of the subtraction is the green curve in Fig. 5, which should represent all the calssical noise, such as homodyne electronic noise, pump amplitude/phase noise, and LO amplitude/phase noise. On the other hand, we use the transfer function to transfer the RIN, measured when no noise is sent to MZ, to squeezing measurement. Thus we get the blue curve in Fig. 5, which should represent the classical noise coming from the pump amplitude noise (RIN). However, we see in Fig. 5 that the green curve is even lower than the blue curve, which seems not very reasonable for me since total noise (green) should not be smaller than a signle noise component (blue).

To solve the mismatch of green and blue curves issue in Fig. 5, we adjusted the expected squeezing level by hand to 9.4 dB. Then we get Fig. 6 which makes the derived total noise (green curve) overlap with a single RIN noise (blue curve) at peaks around 10kHz. We should note that this is just a guess of squeezing level of 9.4dB, but it could explain better the classical noise coupling to squeezing. The green curve has some frequency regions higher than the blue curve, which could be also reasonable if the pump phase noise, LO amplitude/phase noise can contribute to that. However, the noise contribution from these other noise sources needs to be verified.

Yuhang and Marc

It was found that the relative intensity noise (RIN) contribute substantially to the squeezing noise spectrum, such as the peaks around 10kHz in the squeezing noise spectrum reported in logbook 3062. Thus we suspect that the amount of RIN could limit the squeezing measurement at TAMA.

In my understanding, the RIN could introduce noise through the noise amplification of the bright field of CC sideband. On the other hand, the squeezing field is generated from the DC power of the pump. Therefore, the noise from CC field and RIN should be uncorrelated with the squeezing field.

To investigate the effect of RIN, we took anti-squeezing noise spectrum measurement in two conditions. One is the measurement as usual. In the other condition, we inject 100mV white noise (from ~50 to 51200 Hz) to the 'pertubation in' point of the MZ locking servo. In both condition, we took four measurement. They are the spectrum of anti-squeezing, the spectrum of RIN (without normalization), the transfer function from RIN to anti-squeezing, the coherence between RIN and anti-squeezing.

As a fast analysis, we attach in this logbook the comparison of those spectra. A detailed analysis will follow after considering more carefully the noise coupling mechanisms.

Sorry for my late reply.

It's better to leave the actual value of beam radius [mm] with an error.

Marc, Yuhang

First we checked the LO alignment into the AMC and found misalignment in both pitch and yaw.

We tuned the Homodyne balance and checked the visibility.

After some tweaking we measured visibility about 0.92 % which is lower than expected (99%).

We found that the BAB was heavily misaligned in the AMC and after tweaking we could reach about 95.7 % visibility.

However, because our goal is to make sure that the system is in an understood state we decided to go towards FIS measurement.

(Actually we also found out that there were large scattering noise below 100 Hz, maybe due to grass cutting.. Even if the situation improved with the better homodyne visibility, we should also try to check this measurement at lower frequency)

We measured FIS with following conditions :

Results are computed in figure 1 where we could reach 5.9 dB squeezing with 40 mW of green power.

With the fit of ASQZ versus SQZ in figure 2, we have optical losses = ( 20.9+/-0.93 ) % and phase noise = (25.46 +/-3.14 ) mrad.

From our wiki, we expect 19.1% optical losses but taking into account the lower visibility, the expected losses become 23.6 %.

For the phase noise, we have quite large uncertainty because from 45 mW of green power the CC1 became really noisy. We will investigate this issue tomorrow.

During the alignment tuning, I made a mistake and moved the pitch of the steering mirror close to the mirror labelled 'FIS'. Before measuring FDS we should make sure to recover its alignment using the irises on the table.

Marc, Yuhang

We found that one SR560 that was used for the input oplev yaw was broken (increasing a lot the noise above 10 Hz) so we replaced by the one used for the length sensing oplev.

We want to find the good PSD position and first tried to measure the natural length mechanical resonance in the tilt PSD without excitation but we could not see any signal at the expected frequency (0.94 Hz).

We decided to inject white noise in the length of INPUT at this frequency.

First we used 5000 counts amplitude but we could see harmonics so we decreased the amplitude to 1000 counts.

The attached figure report the 0.94 Hz peak amplitude in the pitch and yaw spectrum while moving the tilt PSD (larger position correspond to moving the PSD closer to the INPUT mirror).

For the yaw, we can see the expected 'V shape' in addition to some constant noise which might be due to non perfectly decoupled driving.

The minimum of this 'V shape' is at about 7.3 cm which is really close to the waist position.

For the pitch, we can clearly see the 'V shape' but the minimum position is about 3 cm which seems to be really off.

We have to investigate the reason for this discrepancy.

Marc, Yuhang

In order to prepare for OPO measurement, we measured some preliminary FIS spectrum.

New measurements with better tuning are planned.

We checked the OPO non-linear gain after realigning the BAB and GR beam into it.

we measured without green 550 mV in transmission and with 25.6 mW of green 2540 mV which corresponds to non-linear gain of 4.47.

This is a bit lower than expected (should be about 5).

We tried to tune the OPO temperature (starting at 7.130 kOhm with 10 Ohm increment) but it seems we were already around the optimal temperature. The p pol PLL frequency in this condition was 225 MHz.

We also had to realign the GRMC and had some issue with the error signal sign before locking it.

We tuned the Homodyne balance by injecting noise into the IRMC (sine with 1.2 V Pk2Pk at 800 Hz) and trying to minimize it from the homodyne sub DC channel.

We checked the LO and BAB matching into the AMC and after some tuning could reach about 99.9 % mode-matching for each field into the AMC.

Finally we measured shot-noise and FIS.

We tried to change the phase of the FIS but could not reach more than 4 dB.

Possible reasons are that we forgot to optimize the OPO non-linear gain or the Homodyne visibility.

Furthermore, we had to tweak alignment at several places so we might have to retune it today to have a more stable alignment.

Despite the fact that they have a power knob, the laser model that we use for oplev don't allow to tune the output power. 

Aritomi, Marc, Michael, Yuhang

We reset the DDS rack, restarted the control PC and reloaded all configurations.

We checked input oplev and found that the laser output was really low (sum read by PSD was about 150 counts while usual value is about 5000).

We replaced the laser head by a new one and could reach up to 7000 counts.

To avoid the saturation, we tried to reduce the laser power, but we couldn't tune the laser power at all...

In any case, we installed 2 OD1 and could reach about 5000 counts on the PSD sum.

What I did : I measured gaussian beam radius and fited beam propagation using beam profiler and Python.

Why I did: trying for Pound-Drever-Hall method.

Experiments: I set up the following in a attached picture.

Result:The result of fitting is also in a attached picture.

Conclusion: I could have measured the beam radius.

Problems:There are some possibility that I mixed optical elements in different wavelength bands (1550nm and 1064nm) in ATC.

What I did: I checked the suspected  optical elements  used for 1550nm or not.

Subjects: Two HWP and one QWP that were suspected in 1064nm,but one HWP(named "1") is cleaned by using first contact. So I checked one HWP(named "2") and one QWP(named "3").

Experiments: I set up the following in attached pictures. In experiment for the suspected HWP(named "2"),maximum mW and mimimum mW showed in Power Meter was as follows.

In experiment for the suspected QWP(named "3"), maximum mW and mimimum mW  showed in Power Meter was as follows.

Conclusion: Based on the above results, it can be concluded that HWP(named "2") and QWP(named "3") are for 1550nm wavelength.

Next: I will check the suspected HWP(named "1").




Addition: I checked WHP(named "1"),WHP(named "2"),and QWP(named "3")again.I set up the following in additional attached pictures.Additional experiments were set up the same as before.

 In experiment for the suspected HWP(named "1"),maximum mW and mimimum mW showed in Power Meter was as follows.

 In additional experiment for the suspected HWP(named "2"),maximum mW and mimimum mW showed in Power Meter was as follows.

In additional experiment for the suspected HWP(named "3"),mW showed in Power Meter at point A,B,and C was as follows.

Based on the above results, it was concluded that HWP(named "1"),HWP(named "2") and QWP(named "3") are for 1550nm wavelength.There was no mixing.