Michael and Yuhang

Today we found a new issue about new OPO component No.28. The issue is described in the attached PDF file. The issue is that a counter bore hole is made from a wrong side of no.28 component.

We also took photos about no.28 component with wrong and correct holes, as shown in the attached Fig. 1 and 2. (The one with correct hole actually has wrong thread as reported in elog2724)

For solution, we found that a longer screw seems to be able to solve the problem. We will look for some longer screws and clean them. Then try to use them for the new OPO.

Again issue with the translation stage so we have to remove the mirror to reset the motors reading from Zaber..

I deactivated the windows update for 1 month

I checked data especially of the measurement with 45deg input polarization angle.

It seems there was some issue with the calibration of the DC signal (the DC with approximately 45 deg input polarization is higher by a factor 2 than the expected maximum value when we inject pure p polarization).

We will redo this measurement again today.

To realize the CCFC+green OLTF with green only, we need the following green filter.

1 + G_CCFC/G_green = 1 + P_CCFC/P_green*G_f*P_f 

where G_CCFC, G_green are the OLTF of CCFC and green, P_CCFC, P_green are the cavity pole of CCFC and green, and G_f = 1000, P_f are gain and pole of the CCFC filter. The pole P can be written as

P = 1/(1+i*f/f_p)

where f_p is the pole frequency. f_p for P_CCFC, P_green, P_f are 57 Hz, 1450 Hz, 30 Hz, respectively.

This ideal filter is complicated to realize with combination of zeros and poles. Instead, we can use a simple filter which is the combination of 2 poles and 2 zeros. The pole and zero frequency are 30 Hz and 1000 Hz, respectively, and gain is 1000. As you can see in the attached figure, the simple filter is similar to the ideal filter. 

Abe, Katsuki, Marc

This entry reports the birefringence measurements of spare ETMY without roll rotation (ie with the 2 ears flats).

delta_n and theta seems quite consistent over all measurements.

Michael and Yuhang

We have measured residual amplitude modulation (RAM) for green beam using the reflection from GRMC.

We use a DC PD to monitor the GRMC reflection power. An RF PD in reflection of GRMC is used for measuring RAM.

We performed PD spectrum measurement several times for different green power going into PD. The result is shown in Fig.1.

The equation used to find relation between frequency and phase should be restricted inside cavity because it comes from the term phi = 2*pi*(f*L)/c. Since we assume the cavity is kept on resonance, we have relation between f and L. So we don't compare the phase of laser inside and outside cavity.

We measured the phase noise introduced by AOM. According to Fourier transform, the frequency noise is phi/f.

Taking phi/f and cavity pole, we get the frequency noise introduced by AOM as Fig.1. We can see AOM introduce negligible frequency noise of only 25 uHz.

Abe-san, Marc, Matteo

Yesterday we installed the spare ETMY on the translation stage.

With eye inspection, we could see dusts on both surfaces that should be cleaned before the absorption measurement.

Using the probe laser we found X_center = 397.5 and Y_center = 111.635.

The Z position is 80 mm which corresponds to beam waists about 1 cm in front of the mirror center.

After alignment and calibration, we took a first measurement with 70 mm radius with s polarization.

A next measurement with polarization angle about 30 deg is on-going

Michael and Yuhang

We measured AFG3251 phase noise in elog2745, which shows that it has higher phase noise than DDS (about a factor of 3). Especially, taking the phase noise into account using f = phi*(fsr/2*pi)/2 (wrong) and infrared cavity pole p (transfer function = sqrt(p^2/(p^2+f^2))), we find it gives locking error even larger than what we observe (Fig. 1). The green cavity pole is neglected since it acts at relatively high frequency which almost doesn't contribute to locking error. For the moment, we don't know why it happens.

If the phase noise of AOM driving signal gives any limitation, a less noisy driving signal would provide less locking error. As we know from M. Vardaro thesis, DDS provides signal with less phase noise. Therefore, we measured filter cavity locking error (IR) using DDS and AFG3251 to driving AOM separately. We forgot to calibrate this signal (will be done later), but the comparison of the locking error in these two conditions are as shown in Fig. 2. This indicates AOM driving signal phase noise maybe not a limiting noise source.

Abe, Marc

In order to get the correct limits of the translation stage we had to home every motors.

Thanks to the help of Michael and Yuhang we removed the shinkosha 7 and placed it back after this operation.

Then we set correct Z limit (25 mm to 100 mm).

We checked the AC (measuring s pol) and DC (measuring p pol) alignment, maximal and minimal values without mirror.

We installed the mirror and realigned the 2 PSDs.

We started a polarization measurement with s polarization at the input and from X = 398 to 470 mm and Y = 20 to 235 mm that should allow us to see border effects.

(the mirror center is X = 398 mm and Y = 122 mm).

Michael and Yuhang

We have optimized in-vacuum Faraday rotator and achieved a reduction of in-vacuum propagation losses from ~15% to ~11% as reported in elog 2727 and 2729.

To confirm this lower losses, we locked filter cavity and used BAB to measure it again. Especially, the measurement when chamber opened was done with a HR mirror just after dichroic. We made BAB reaching input mirror this time.

After earthquake and alignment of GR beam, we can still find IR resonance for filter cavity. The achieved filter cavity IR transmission was ~300. This indicates a mode matching level of (300-100)/(550-100) = 44%. Since we don't need IR to be resonant inside filter cavity to measure in-vacuum propagation losses, we didn't optimized mode matching yesterday. The AOM frequency was found to be 110.037371 MHz to have IR resonant for filter cavity.

We made mistake of not removing the CC pick-off mirror at the beginning. After removing the pick-off mirror, we measured filter cavity injection and reflection BAB power as Fig. 1 and Fig. 2. During the measurement, we took care to try to center beam on the sensor of power meter. This implies that the in-vacuum propagation losses are 11.6%. This is in agreement with elog 2729. 

Michael and Yuhang

Following the method provided in Marco Thesis, we made a phase noise measurement for AFG3251. We take signal from DDS as a reference signal, since it was reported from Marco thesis that this is a stable signal source. We will check the phase noise of DDS later.

The measurement was to use DDS and AFG3251 to generate 20MHz signal and acquire their beatnote after low pass. We tried to synchronize them with another DDS channel to provide a 10MHz signal to the 'external reference input' channel of AFG3251 (Fig. 1 and 2). However, the 20MHz between them cannot be synchronized. In the end, we tune DDS to be 20.00073506 MHz to have beatnote signal as flat as possible. In the end, there was still some very low frequency component was not removed (Fig. 4). Then we took measurement of beatnote with different frequency band using network analyzer.

To calibrate the measured beatnote signal, we increased DDS signal frequency by 1kHz. Note that we used DDS channel with amplitude reduced by a factor of 2 to avoid saturation. And we found pk-pk beatnote is 0.316V. Therefore, the measured beatnote spectrum need to be divided by (0.316/2).

The phase noise is shown in Fig. 3. The integrated phase noise from high frequency to 10Hz is 399urad, which is slightly higher than the measured DDS phase noise (reported to be 117urad from Marco thesis).

Abe, Marc, Matteo

We restarted the setup after the electrical blackout.

We installed shinkosha 7 on the translation stage and started to check the input polarization.

However, it seems that there are some issues with the Zaber encoder so we need to home every motors of the translation stage.

The GRMC/MZ servo was suspected to have issue, but it was found that GRMC works well in elog2741. This indicates the problem comes from MZ servo.

I asked Pierre for some suggestions. Especially, I found the enable signal from MZ servo is always around zero. Pierre suggested me to check the resistance of MZ servo while MZ servo is not powered. I did such check and found there is a resistance of 67kOhm. At least there is no short cut.

I did similar check as elog2713, I found that the signal coming from GRMC ref PD has no signal. Then I checked the PD and found it is not powered! This is very strange because the switch of this PD is very hard to reach. But anyway, by switching on GRMC ref PD, I could lock GRMC and MZ again. Now there is no problem at all for GRMC and MZ.

Although this is just a stupid issue, but during the check of GRMC, I found out it is very necessary to record the level of signals for each control servo. These values will be very essential for trouble shooting in the future. I plan to make a list of all useful channels of these servos soon. On the other hand, elog2741 is a standard way to check servo and can be useful for future check as well.

Michael and Yuhang

Yesterday, we finally aligned back filter cavity. During the recovery phase, we found the camera driver used for intra-arm second target is broken. We replaced it with a driver labelled with 'PRM surface'.(fig1,2,3)

We also found the motor C of END mirror picomotor is broken. This motor was used for adjusting END mirror pitch (fig.4). But we can use the motor B for pitch temporarily since these motors are just used for sending signals with particular IP address (fig.5). The signal to be sent to picomotor is decided by ourselves from LABVIEW.

Michael and Yuhang

We reported an issue about not being able to lock GRMC one month ago in elog2713. We asked Pierre for some suggestions of testing the servo. He recommended to take GRMC servo out and use a SR560 instead of the plant in the opto-mechanical control loop. The setup of this test is sketched as the attached figure 1.

The results are briefly summarized in Fig.1 as well. We found that if we don't send signal to TRANSMIS IN and use MAN mode, we can successfully lock control loop. However, using  a positive 0-2V sinusoidal signal with period of 1s can cause the lock/unlock of the loop in AUTO mode. Note that a threshold is chosen at -0.4V. We also found that if we use a positive 0.9-1.1 V sinusoidal signal with period of 1s, we can keep the loop locked in AUTO mode. This test verified that GRMC control servo should work well.

In fact, we succeeded in locking GRMC afterwards without the signal from MZ servo going into GRMC servo's ENABLE IN. Once the ENABLE IN channel is connected to MZ servo, GRMC loop cannot be locked. We checked also that this signal is always zero. Therefore, we suspect the issue comes from MZ servo now. More test will be done to test MZ servo.

Yuhang and Michael

During the recover of filter cavity facilities, we found a router is broken. This router is used for intra-arm second target remote control and pico-motor control. It is essential to repair it.

Accidently, we found a new router around. After just replacing the old router with a relatively new one, the router works again. (new and old routers are shown in the attached figure 1, we can see the old one shows very yellow)

The old router is labelled with a piece of pink tape with broken written on top.

Same as previous entry but the sample was rotated by 180 deg (estimated by eye)