IR alignment inside FC recovered : ir locked again

Marc, Yuhang

Today we check the centering of the ir beam from M0 to M3 (see fig 1 of entry 2452) and added an iris just after M0.

It means that we now have several references to check the ir alignment injection inside the FC :

1) FI aperture after the OPO

2) new iris after M0

3) targets on PR window

We are still not sure about the reason of this misalignment (even though the ir injection inside the FC has not been checked during several weeks where 2 earthquakes & opening of the PR & BS chambers happened...).

But if it happens again, we can use these 3 references to try to find the reason.

We locked the p-pol PLL to 160 MHz to have ~7mW of ir power incident on PR chamber.

Then, we moved M2 and M3 to recover the ir beam on the PR window target. As explained in entry 2453, we used this time the weaker ir beam.

Then, we decided to move the dichroic mirror (SM2) picomotor.

We tried to use several references :

1)  camera looking at an ir sensor card on the ir reflection of the FC on the squeezing bench

2) camera looking at the 2 " mirror inside BS chamber.

3) 2 cameras looking at the FC targets

 

1) was useful to get back the beam on PR targets (even though we found out later on that the beam was not the reflection of FC)

2) was useful when moving SM2 picomotors.

After several motions using the Labview vi where we couldn't see changes on 2), we used the joystick.

We could see the beam hitting the edge of the 2" mirror.

After a rough centering (we also had to move slightly in yaw) we could get the beam on the FC 1st target.

We locked FC with green to insure a proper position of BS and unlocked it. We superposed green and ir on this target and then on the 2nd one by cutting alternatively green or ir.

We could finally locked back both green and ir (see fig 1).

As a reference, fig 2 and 3 shows the camera looking at the 2" mirror with green respectively on and off.

ir re-alignment inside the FC & reference points on PR chamber

Marc and Yuhang

Before recover BAB to filter cavity, we checked again M0 to M3, we tried to make BAB roughly centered on mirrors M0 to M3.

Then we tried to recover reference points on PR chamber. As attached photo 1, we noticed the reference points IR1 can be either weak or bright depending on the tilt of incident beam. Especially, when a weak IR light hits on IR1, the light on IR2 can be closer to the reference point.

Therefore, we think we should make a weak light hit on IR1 while a brighter light hit on IR2, which should be the good reference.

ir re-alignment inside the FC & loose screw on MM1000s

Marc, Yuhang

Today we continued the ir alignment started yesterday.

Based on the labeled presented in figure 1, we could see that the beam was misaligned horizontally on M2, the 2 following lenses and M3.

We acted on M1 to recenter the beam there.

Then, we wanted to recover the beam on the target outside PR chamber by acting on M3.

Even tough I only moved slightly the horizontal screw, it might have been too much as it became easily loose.

As a reminder M3 is mounted using a holder MM1000s from First Mechanical Design compagny (http://www.1md.co.jp/mm1000_E.php)

To recover this screw, it was needed to tighten another small screw pointed by the red arrow in figure 2.

Realignment is on-going (one reference back on PR chamber)

Modification of TAMA PD (2)

As reported in elog2437, I modified TAMA PD based on Pierre's design. But since we don't have an exact capacitor of design value (the capacitor in parallel with the photodiode, we call it Cpp), we don't have the resonant peak at the design frequency.

As mentioned in elog2437, Cpp's design value is 160pF while the one in place is 150pF. So I tried to put a capacitor with 7pF and 10pF in parallel with the 150pF Cpp. However, after putting them, I found the noise spectrum has changed from the measurement on 20210405. The noise spectrum measurement is shown in the attached figure.

I will investigate more why this happens. In the worst case, I will solder this circuit again with a new op-amp. Then check if the noise spectrum becomes reasonable.

Knife edge measurement in Y direction

[Aritomi, Abe, Marc]

We did knife edge measurement of IR and red beams with a knife at the edge of the 2'' holder towards the imaging unit. 

Fitting result is as follows:

IR : waist size = 40.8 um at z = 30.6 mm

red : waist size = 52.8 um at z = 39.8 mm

Note that Z in this plot is raw data and thickness of 2'' inch holder is not considered. The fitting for red doesn't match the data very well. 

Second figure is an example of knife edge measurement of red in Y direction at Z = 44 mm. There is a peak when the knife starts to cut the beam and this may underestimate the red beam radius. 

IR filter cavity alignment check

Michael and Yuhang

We have reported that BAB was misaligned for filter cavity.

To increase BAB power, we are amplifying BAB with OPA. The pump phase is scanned with frequency around 100Hz and amplitude of 1V signal sent to high voltage driver.

At the beginning, we were trying to find where the IR beam was located by using camera. Then we found we can see from camera but the position where IR beam hits is not clear and depends on where we put camera. Also we tried to use IR viewer to find IR inside PR chamber, it didn't work either. So we can use camera to check, but this is not really helpful.

On the other hand, we should rely on the reference points outside the PR chamber window. We found we could easily recover the IR beam for these reference points. However, we found reflection was extremly weak (less than 1%).

To maximize IR reflection, we followed the method we used three years ago. We put a sensor card in reflection and set up a camera to monitor the beam on sensor card (attached photo 1 is setup, attached photo 2 is image from camera). By doing alignment with two mirrors, we found reflection power cannot be increased a lot. In addition, after this alignment, we made IR go away from reference points. We also made IR hits almost the edge of last bench mirror. This means we are not going to the good direction.

So the starting point, where IR beams hit on both reference points, should be quite close to good injection. If we can rely on this, the only problem we are having should be inside vacuum chamber. The IR mirror equipped with picomotor before GR/IR overlap is the dichroic mirror. Actually this is the place where IR and GR overlaps, but the tilting of this mirror doesn't affect GR. So we think we should do like this.

1. Align the last two steering mirrors on bench to hit on PR chamber IR references as good as possible.

2. Move yaw of dichroic mirror while checking the first target inside arm and reflection together.

3. In the end, we should see: (1)IR/GR overlap on the arm first target (2)reflection power increase while reflection reaching nominal position

Many misalignments are found recently

Before measuring FIS in elog, we found OPO is misaligned for p-pol, CC and BAB beams. To check FDS, we injected BAB into vacuum chamber. However, we could find this BAB in FC reflection or in the first target on PR chamber.

We will check the first target in arm, also check inside PR chamber with camera soon.

AA new driving matrix and loop closed

Marc, Yuhang

The past trials to measure the AA sensing matrix taking into account possible coupling between pitch and yaw of both input and end mirrors were all based on injecting 2 Hz lines on each degree of freedom (dof).

This gave us strange results, especially visible in the TF phase between each excitation and sensing QPDs (ie far from +/-180 or 0 degrees).

As suggested by Raffaele in last FC meeting, this could arise because 2Hz is too close to the mechanical resonance of the mirror.

Therefore, we decided to use 15 Hz lines to measure the TFs.

We also tried to measure the demodulation phase between I and Q of each QPD segment using a 15 Hz line but this was not so much conclusive ( the line amplitude might have been too low and also hard to distinguish in the time series from the natural 11 Hz pitch resonance). So we used 2 Hz line to tune this demodulation phases.

They are now :

segment	QPD1 1	QPD1 2	QPD1 3	QPD1 4	QPD2 1	QPD2 2	QPD2 3	QPD2 4
phase [deg]	130	120	120	120	-10	0	0	0

Then we injected a line at 15 Hz on each mirror dof with amplitude 1000 except for end pitch which was 1500 (without particular reason...)

Figure 1 to 4 present the TF magnitude (top left) phase (bottom left), coherence (top right) and time serie (bottom right) for respectively input pitch excitation, input yaw, end pitch and end yaw.

The TFs magnitudes give the sensing matrix absolute value while the phase the sign of each element. We used only TF when the coherence was above ~0.4. From this all phases was +/-180 or 0 deg within less than 10 degrees.

This gave the following driving matrix :

QPD1 pitch	QPD2 pitch	QPD1 yaw	QPD2 yaw
8.6	6.5	0.3	2.6	In pitch
7.1	-15.2	1.2	-0.6	End pitch
1.6	0	-10.8	-7.7	In yaw
-1.4	1.4	-5.5	16.6	End yaw

Note that the actual sign of the computed matrix is the opposite but we used a negative gain.

Using gain of -0.05 for in and end pitch and -0.05 for in and end yaw we could close the AA loop.

The comparison between the QPD and OpLev signals are presented in figure 5 (pink is live QPD, green is live OpLev).

We can see that there is almost no coupling between the various dof !

However, the AA loop gain needs to be reduced a bit more as we can see that it introduces noise (eg comparing the green with the brown := OpLev reference)

ir realignment

Abe, Aritomi, Marc

Previously we were using the pinhole to characterize the beams.

We found out that this technique is heavily dependent on the alignment on the pinhole and that the power fluctuations affect strongly the quality of the measurement.

Also, as the translationStage.v3.vi that allows to automatically perform many scans along the Z direction is now available, we switched to using the razorblade with this vi.

The 2 lenses have been moved so that L1 (closest to the periscope) and L2 (closest to the imaging unit) are now respectively :

L1->L2 = 21.2 cm

L2-> z=0 of translation stage = 23.4 cm

With this configuration, we characterized the red and ir beams when cutting the beam vertically. For this, we placed the razor blade as close as possible to the 2" holder at the edge towards the lenses.

The measurement and fit are presented figure 1 (red := red beam, black := ir beam) where the z position corresponds to the real one (ie the half width of the 2" holder (9 mm) has been substracted)

It gives :

ir : waist size = 37.5 um at z=38mm

red : waist size = 76.6 um at z=42.8 mm

During the week-end, we started several measurement of the red and ir beams with the blade at the edge of the 2" holder towards the imaging unit (ie 1.8cm closer to the imaging unit than the measurement presented in this entry). This will allow to check the crossing point of these 2 beams.

We should get all the required measurements on Monday.

Note that the height of the ir beam has been corrected following this measurement as shown in elog2444.

FIS measurement after IRPS improvement and homodyne lens flipping

Marc and Yuhang

I measured frequency independent squeezing on Thursday (20210408), the difference between this measurement and last measurement is the improvement of IRPS. The result is shown in the attached figure 1.

After this, I also flipped lens inside homodyne. The minimum ROC of gaussian beam with w=390mm is (x_R+x_R^2) = 650mm. This fits better the flat surface of lens, which makes it reasonable to make light hits on lens flat surface. After this work and tilting lens a bit (few degrees), I got squeezing spectrum as the attached figure 2.

Figure 2 represents the cleanest squeezing spectrum we have ever got.

IR beam height adjustment

[Aritomi, Abe, Marc]

Since IR beam height was higher than red beam by ~3 mm as shown in elog2446, we adjusted the height of two lenses for IR beam. We made the first lens lower by 3 turn of screw to make the beam lower after the first lens. By doing this, the IR beam height just after the second lens changed from 55 mm to 52 mm. Then we made the second lens lower by 6 turn of screw to make the beam flat.

We did knife edge measurement in X direction at Z = 25 mm and 70 mm to check if the IR beam is flat. Attached figures show knife edge measurement at Z = 25 mm and 70 mm. The beam height are X = 316.96 mm and 316.85 mm, respectively.

Now IR beam is flat and the IR beam height is almost the same as the red beam height.

Comment to OPO replacement - characterisation of beam placed in ATC cleanroom (Click here to view original report: 2421)

Axis of figure 2 should be "beam radius"

OPO replacement - ATC cleanroom setup

I checked some optics in the ATC cleanroom to prepare for the construction of the new OPO.

We can use the Faraday isolator Thorlabs IO-5-1064-VLP - aperture 5mm diameter, 9cm length, maximum power 1.7 W, 25 W/cm^2 (blocking), 100 W/cm^2 (transmission).

Using an f = 100 mm lens placed as shown in figure 1, we can maintain the beam waist below 1mm diameter over a distance of [distance] from the 100mm lens, so it is sufficient to use for the FI.

I did a rough estimate (figure 2) and measurement (figure 3) of the beam size to make sure. We can maintain < 1mm beam diameter to about 35cm from where I placed the lens.

Setting for knife edge measurement for IR

First picture: X axis

Second picture: Y axis

DDS board 1 channel 2 or 3 has phase change

Today, I tried to lock OPO but not successful. While checking OPO error signal, I found the error signal didn't seem to be reasonable. By adjusting only the phase of channel 3 (LO for OPO PDH demodulation), I found the optimal phase changed from 135deg to 205deg. This change of 70deg probably comes from the 90 phase change of DDS, combined with the original phase was not optimized.

Attached figure shows the PDH signal when new optimal phase is set.

p-pol PLL fibers rearranged

Marc and Yuhang

Yesterday, we found p-pol PLL couldn't be locked. While checking it, we realized that it will be better to put p-pol PLL on the second layer breadboard.

The new location of p-pol PLL fibers are shown in the attached figure.

IRMC Phase shifter optimisation

Yuhang and Michael

The IR phase shifter position was shifted to the beam waist as per 2435. The arrangement is shown in figure 1, 2. The half wave plate is contained inside the lens mount. We then measured again the noise spectra using the PSD after transmission through the IRMC.

The results show that there is now considerably less change in the noise spectrum when excitation is applied to the phase shifter, versus when there is no excitation. In figures 3 and 4 we show the normalised spectra for pitch and yaw. The X/T and Y/T noise look to have a noise floor of about -30 dB, versus -20 to -25 dB as shown in 2422. The broad peak at ~2.3 kHz also looks like it is suppressed. Figures 5 and 6 show the absolute measurements from the PSD compared to the dark noise. The result is improved versus that in 2407, we don't seem to have mysteriously high pitch noise anymore.

Modification of TAMA PD

Based on the design from Pierre and me, I modified the TAMA PD today. The modification is as following:

1. Change op-amp to LMH6624

2. Set R1 to be 33Ohm

3. Set R2 to be 330Ohm

4. Put 3pF capacitor in parallel with R2

5. Change photodiode parallel capacitor to be 150pF

Figure 1 is after I remove op-amp. Figure 2 is after I put the new op-amp. Figure 3 is after I put R1, R2, 3pF and 150pF. Figure 4 shows the label I put on this PD.

I also measured the TAMA PD noise level with bandwith of 2GHz and 10MHz. The 10MHz measurement is around 14MHz. I found:

1. The offset is about 0.1V

2. Figure 4 shows the main oscillation is around 268MHz with amplitude of 5uV_rms/sqrt(Hz). 

3. Figure 5 shows the noise peak is a bit higher than the designed value. This is due to the photodiode parallel capacitor was chosen to be 150pF, but the designed value is 160pF. We can choose to put a 10pF in parallel with the 150pF one.

Improvement of IRPS driving to reduce LO amplitude noise

Michael and Yuhang

We moved IRPS to be very close to the waist position. According to a simulation I have done, this will reduce quite a lot the amplitude noise.

To test if this really solves problem. Firstly, after realigning everything, we got the IRMC spectrum as shown in attached figure 1. Then we gave a high voltage difference of 70V, which is almost the maximum voltage we give for IRPS while CC2 loop is locked. In this case, we got IRMC scanning spectrum as attached figure 2. We computed misalignment to be 4.5%.

Compared with the result we had from elog2424, the LO amplitude noise should be reduced by a factor of ~5. From this result, combined with input mirror feedback, we can completely avoid amplitude noise coupling at low frequency when CC2 loop is closed.

Filter cavity alignment recovered

Michael and Yuhang

We recovered filter cavity alignment. While doing the recoverment, we found some issues of picomotors. Especially, the BS yaw had problem. The problem is when we move yaw, the actual motion was pitch. The solution is to disconnect the pitch control wire. Then when we move yaw, we can really move it.

After recovering mirrors positions, we got cavity flash as attached figure 1. Then we centered all oplev signals on each PSD. After that, the medm interface is shown in the attached figure 2. In the end, we also measured the spectrum of each oplev signal. FIgure 3 shows the comparison with old measurement. No issues were found.