Aritomi, Eleonora, Yaochin, and Yuhang

For the adjustment of OPO/FC matching and FC/LO matching, we always do the following.

1. Send BAB into the filter cavity. Align IR into the filter cavity while filter cavity alignment is kept in the best condition. Check IR 0/1/2 order modes by changing AOM frequency.

2. Align BAB reflection into AMC while the filter cavity is locked.

However, we found that the matching from BAB_ref into AMC is quite different when the filter cavity is locked and detuned. And we know that we measure FDS while FC is detuned, so we decide to align BAB_ref into AMC while FC is detuned. So now the procedure is as follows.

1. Send BAB into the filter cavity. Align IR into the filter cavity while filter cavity alignment is kept in the best condition. Check IR 0/1/2 order modes by changing AOM frequency.

2. Align BAB reflection into AMC while the filter cavity is detuned.

However, we are not very sure why the matching can be so different when FC is locked and detuned. Because I think the BAB_ref should be quite similar in both cases of lock and detuned since we had 94% of matching from OPO/FC. I put the reason for my thought as following. Please correct me if there is something wrong.

Yao-Chin and Yuhang

When IRMC locking, we monitored yaw & pitch of LO light jittering by position sensitive detector(PSD) as shown in fig 1. PSD was put before alignment mode cavity(AMC). We also measured yaw & pitch of PSD dark noise.

In addition, we sent 200mV Vpp of random noise to IR phase shifter(IRPS) and measured again yaw & pitch of LO light jittering as shown in fig 2. Magnitude value increased more obviously above 60Hz than without random noise situation.

When BAB light sent to filter cavity, we monitored yaw & pitch of reflect light by PSD as shown in fig 3. Because suspension mirror jittering, The jittering magnitude of low frequency range (<200Hz for yaw, <500Hz for pitch) is huge.

Simon, Pengbo

We finished the measurement of the beam profile using the beam profiler.

The beam profile could not be measured at the waist due to the space limits, and the results may be not that accurate.

But as can be seen from the attachment, the diameter of the waist is 81 mum roughly, which didn't offset very much compared with the result before. 

I designed a glueing jig for fused silica mirrors.
There is a through hole for putting a mirror and its holder.
A space aroung the center is in order to prevent sticking the jig to the mirror due to the leaking glue.

I will ask Sato-san to check the design.

sqz_dB = 12.5;                    % produced SQZ

L_rt = 100e-6;                    % FC losses

L_inj = 0.20;                     % Injection losses

L_ro = 0.11;                      % Readout losses

A0 = 0.1;                         % Squeezed field/filter cavity mode mismatch losses

C0 = 0.1;                         % Squeezed field/local oscillator mode mismatch losses

ERR_L =   5e-12;                  % Lock accuracy [m]

ERR_csi = 80e-3;                  % Phase noise[rad]

phi_Hom = [0, pi/2*105/90];       % Homodyne angle [rad] (you can input a vector of values)

det =  360;                       % detuning frequency 

Yao-Chin and Yuhang

When IRMC locking and only LO light hit into homodyne detector, we aligned LO light to arrive dc balance signal of homodyne shown in fig 1 and measured shot noise using analyzer (Aglient 35670A). Using low frequency, which range from 125mHz to 200Hz, random noise from analyzer source sent to high voltage amplifier (Gain: x15) of IR phase shifter(CC2). With different Vpp values of random noise (its spectrum shown in fig 2), we measured the RF spectrums of homodyne detector. Below 100mV Vpp, the spectrums are similar from 20Hz to 200Hz range shown in fig 3. The magnitude value of shot noise increased when above 500mV Vpp of random noise. 

Note: We average spectrum magnitude value from 20Hz to 200Hz.

Pengbo, Simon

We finished the measurements of the beam-profile after the telescope.
The horizontal profile could not be measured directly at the waist (as we put the blade too much on the sensor-side). However, the fit with a Gaussian diffraction development gives a good result in terms of the expected waist diameter of 70 mum (~ 72 mum from the fit, see attached picture).

For the vertical measurements, we put the blade ~2cm closer to the pump-beam side and started another run. This time (see the second figure attached) we could cross the beam's waist in the expected position which is in correlation with the horizontal measurements. Also here, the distribution has been fitted and the diameter estimated to be ~74 mum at the waist, also in agreement with the horizontal measurements.

Eleonora, Raffaele, Yaochin and Yuhang

Yesterday night we came to TAMA and improved especially the matching and alignment from OPOtra to filter cavity. We moved the lens position and also tried to align a bit the steering mirror. After this work, the matching level is estimated as following

TEM00: 460

TEM10: 108

LG10: 108

offset: 100

So the mis-matching level is about 5%

This matching level is better than the measurement of locking accuracy of last time. And we think the matching level will also influence the locking accuracy. So we characterize the locking accuracy again. This time we used the same setting as last time. The AOM was scanned with a modulated sine wave. The modulation is 2000Hz/s. We measured the PDH signal and use it for calibration. The measured PDH signal pk-pk is 234mV with a seperation of 118ms.

So the calibration is 2000/(2*234/118) Hz/V

We measured the demodulated BAB reflection spectrum, the locking accuracy is integrated as 5Hz. But now we have some new peaks.

Today, the spacer for cryogenic cavity was delivered and I did brief inspection of it with half inch flat mirror.
I just checked whether the reflected beam can pass through the output hole of the spacer or not.
The reflected beam could pass through, but there is a undesirable feature due to my mistake of design.
Fortunately it can be easily removed by an additional machining.
I will ask the company to do it.

The measurement of CC2 loop opto-mechanical transfer function, CC2 open loop tranfer function and CC2 error signal spectrum is attached as attachement.

The good news is that the OMTF is very flat.

The first obvious oscillation we could find is ~23kHz(the last attachement). While we could see from the error singal spectrum, we have two narrow peaks at ~15.5kHz and ~22kHz.

Aritomi, Eleonora, Yaochin and Yuhang

We found that fc tra is more than 3000 counts. And this is more than we had, so we did the following check. And it shows that the more green power seems to be from a higher conversion efficiency of SHG.

green before EOM 268mW

before AOM 48.4mW

before MZ 198mW

AOM frequency 109.03575MHz

AOM amplitude 2.5dbm (then go through zhl-2 amplifier to AOM)

[Aritomi, Yuhang, Yaochin, Eleonora, Matteo, Raffaele]

First we aligned IR into filter cavity. Current mode matching is around 95.8%.

Then we succeeded in locking CC2 with filter cavity with new phase shifter. This time CC2 testmass feedback worked well and CC2 correction signal and IRMC reflection became more stable. Gain of testmass feedback is -2.

We measured FDS at 600Hz and CC2 phase noise (Pic.1,2). CC2 demodulation phase is as follows. CC2 error signal is 76mVpp. At high frequency, we had 3dB squeezing, but the spectrum was not clean. Below 100Hz, there was large bump.

After this measurement, we found that DC balance of LO was bad. We measured frequency independent squeezing before/after DC balance (Pic.3). After DC balance, squeezing spectrum and squeezing level got better, but still not clean as before.

Then we measured FDS again with/without testmass feedback (Pic.4). Low frequency bump became lower down to 60Hz and it doesn't change with/without testmass feedback. It seems that DC balance improved low frequency bump.

I roughly investigated the transmittance of fused silica mirrors which will be used for input and output couplers.
I measured transmitted beam power for 2 of 4 mirrors.
The trasmitted power was 1.8uW for both of them with respect to 10mW incoming power.
This value corresponds to T=0.018% and R=99.982% assuming no loss in mirrors.
Then the finesse can be estimated as 1.7*10⁴.
This value is reasonable since the designed finesse was 1.5*10⁴.

I gonna construct FP cavity with these mirrors and try to lock TEM00.

I removed the gauge from the chamber.

Baking time today (11/26): 9:30-10:30 (1 hour)

After the temparature settled, I opened the gate valves for the TMP. The pressure was decreased from 1x10^-6 mbar to 2x10^-7 mbar.

[Eleonora, Federico, Matteo]

The zero-detect board of the rampeatuo (Pic1) reads a signal (usually the cavity transmission) and compares it with a treshold signal in order to engage the lock. The threshold signal can be manually set with a potentiomenter and goes from -15 V to 15 V. Pierre installed a probe to monitor this threshold some times ago.

In the current configuration the rampeauto sums the transmission signal and the threshold and engages the lock if this sum is > 0. This means that if we don't connect the transmission signal, the cavity gets locked when we set the threshold above 0.  

By connecting the transmission and keeping the threshold below zero we can assure the the lock is engaged only when the transmission is higher than the absolute value of the treshold. This means that we can prevent the servo from locking on HOMs. In the current configuration, the cavity transmission (when it is locked and well aligned) is ~1.5 V. The threshold is set at -0.5 V so that the lock is engaged only when the transmission is higher than 0.5. 

In order to remotely control the lock, we used a stanford to subtract an offset (generated by the recenty installed DAC) to the transmission signal, before the rampeauto. If such offset is 0 the cavity stays locked, if it is larger than 1 V the trasmission signal sent to the rampeauto is lower that 0.5 V and the servo stops the lock. We set the offset at 1.5 V and we verified that the cavity lock can be controlled by adding and removing it.

I added a button on the main MEDM screen to control such offset (pic 2). From now on please try to use remote lock of the filter cavity as much as possible and keep the threshold knob where it is.

The attached figure shows the remained peaks in the squeezing/anti-squeezing spectrum.

In the anti-squeezing spectrum, we could see that all the peaks appear at a harmonics of 50Hz.

In the squeezing spectrum, the peaks appear mainly at harmonics of 50Hz. Only 3 out of 14 peaks are not harmonics of 50Hz but they are close.

Eleonora and Yuhang

After Takahashi-san glued the PZT on the new mount. We soldered the PZT to a BNC connector and replaced it with the old IR phase shifter. (attached figure 1)

The alignment to IRMC was recovered. Then we locked it and tried to see the effect of scanning IR phase shifter. We sent a sine wave to IR phase shifter from 25-125V, which is almost the same output range of our servo. Then we found IRMC transmission is modulated by almost 12% in the pitch direction. (attached figure 2)

The reason why pitched is mainly modulated is guessed as that the mirror on top of PZT is a bit heavy so it has some pitch tilt, the PZT motion creates mainly pitch misalignment. Also, the beam is tilted hitting on IR phase shifter is tilted in the pitch direction.

FC IR TRA channel (ADC CH1) is not working well. We used ADC CH16 (FDS-SEISM_Z_IN1) for FC IR TRA instead.

Pengbo, Simon

Today, with the very much appreciated help from Manuel, we could start the translation-scans of the razor blade in horizontal direction at different Z-values in order to analyze the beam-profile automatically.
For the measurements itself, the powermeter is connected via a BNC cable to the DC-port of the LabView program.

Pengbo has written a Python-script to fit several of those scans at once so that we can easily get the results.

The laser-power of the pump beam is set to be ~110 mW for the entire measurements.

We started with our measurements actually in the area where the waist of the beam is supposed to be (Z-distance = 2mm, dZ = 0.1 mm, Zcenter = 74.1).
The next set of measurements will be over a wider range in Z further away from the waist.