HS mirrors are installed and pre-aligned, Ilaria will cross check under vacuum.
Everything was smooth but at the evening I was not satisfied by some details concerning the relative preset of the suspended elements (actuation cage/marionette/mirror). And we want to further cross-check tomorrow morning after breathing some fresh air.
The vacuum can start after lunch.
I am very thankful to all the people involved.
notice that now
TX_MAR-TX_MIR_ITF = + 3 mrad
TY_MAR-TY_MIR_ITF= + 2.2 mrad
meaning that marionette sensor has to be reset.
Last Wednesday, we worked on the improvement of the error signal coming from the IMC end mirror quadrant photodiode which looked spoiled by scattered light noise. Indeed, we already identified at the installation some extra light coming from MC tower which was focused in a point not far away from the main beam focus and it was somewhat difficult to get rid of it. We have made several checks (using a video camera) without understanding well the origin of this light. In order to get rid of part of this spurious light, we have installed a diaphragm of 50mm clear aperture right at the output of the tower window. We accurately positioned it by looking at the beam on the west wall of MC building. In the end, we installed a new quadrant photodiode on the bench the replace the old one which was a temporary one. The improvement on the error signals is significant even if it looks that there is still some residual scattered light noise. We are planning some noise injection to try to understand the origin of this noise and maybe on a longer term to install some baffles inside MC tower to kill the scattered light coming from the tower.
The IMC AA loops have been left open for the night since we couldn't complete the activity planned around IMC end mirror quadrant
which consisted of studying the spurious light reaching the quadrant and replace the quadrant by a new one. The work will be completed tomorrow.
IB and MC are under local control and BPC loop is closed.
Now that the new dihedron has been installed we have repeated the absorption measurements that were carried out in logbook entry 31732 with the "Virgo " dihedron. As before, the meausrement consisted of changing the laser power at the IMC input and we measured the frequency shift of the HG10 and HG01 mode. The results are reported in the following tables and the frequency shift versus input power is plotted in the figure.
In the logbook entry 31732 we saw that for the HG10 mode the frequency shift induced by absorbed power is almost the same for both the end mirror (~32.5Hz/mW)Â and the dihedron mirrors (~35Hz/mW for each dihedron mirror). As we cannot distinguish effects caused by individual mirrors we will consider that a shift of 34Hz corresponds to a total absorption of 1mW in the three mirrors. The measured frequency shift change from 3.43W to 7.38W input was 177.5Hz. To account for this 177.5Hz, total absorption needed is 177.5/34 = 5.22mW.
The cavity power change is (7.38-3.43)*(1160/pi)=1458 W, so the total absorption is 5.22e-3 / 1458 = 3.6ppm. This compares to 3.1*3=9.3ppm that was found using the "Virgo+" dihedron. We may therefore assume that the absorption of each dihedron mirror has reduced by (9.3-3.6)/2=2.9ppm.
The slopes of HG10 and HG01 in the figure are almost the same except for the first point of HG01. This indicates that the absorption by the end mirror is the dominant thermal effect. If they were of the same size, the slopes would be different. This is because we saw in logbook entry 31781 that the slope for HG01 with absorption in the dihedron is different. The absorption of the aLIGO test mass coating shows that the LMA coating absorption is ~0.3ppm. Therefore if we assume the absorption of the DIHEDRON is 0.3ppm, the absorption of the end mass is 3ppm. If we were to assume the DIHEDRON absorption is 1ppm, the end mass absorption would 1.6ppm but then we would expect to see a difference in the slopes of the frequency shifts of HG10 and HG01. It is difficult to quantify, but we are able to say that the absorption by the DIHEDRON mirror is much smaller than that of the end mirror.
A FFT simulation has been used to calculate the shift in frequency of the HG10 and HG01 modes due to the deformation of the IMC mirror surfaces caused by coating absorption. The simulation includes the DIHEDRON geometry, the baffle with two 40mm holes in front of the DIHEDRON and phase maps (provided by LMA) of all three mirrors. The thermal distortion of the mirror surfaces was calculated using the Hello-Vinet formula, which was confirmed to be accurate enough for all three mirrors by comparing with COMSOL calculations.
The results of this simulation are shown in the figure. We see that the frequency shift for the end mirror heating is the same for both HG10 and HG01. The frequency shift for the DIHEDRON is almost the same as that of the end mirror for the HG10 mode (horizontal plane). However, the two frequency shifts are different for the HG01 mode (vertical plane). It is not easy to give a simple explanation. One key issue is that the thermal bump of the DIHEDRON is almost axi-symmetric, and the size of this axi-symmetric bump looks like a horizontally wider oval seen by the beam.
It took quite some time to find the right way to measure the position of the HG01 and HG10 modes. The setup used for the measurement is depicted in the first picture. The IMC cavity is locked as usual phase modulating the laser beam at 22.304MHz. The new feature used consisted in using the second EOM to scan the cavity while being locked using a second pair of sidebands with much smaller modulation depth (a sweep is generated using a HP network analyzer). Looking at the RF IMC error signal (in reflection) looking in the Free spectral range around the modulation frequency (above 21 FSR) you can see that when you tilt the input beam horizontally by adding an offset on the Beam Pointing control system (opening the AA loop of the IMC cavity), the 10 mode amplitude increases. An example of the HG10 amplitude displayed on the spectrum analyzer is shown in the second picture. The experiment we did consisted in changing the laser power at the IMC input (from 4.9 to 7.2W) and we measured the frequency shift of the HG10 mode. The results are reported in the following table.
NB: the FSR has been measured (see entry #31678 (https://logbook.virgo-gw.eu/virgo/?r=31678)) to be 1045125.3Hz.
A FFT-based simulation was used to estimate the absorption rate of mirrors based on this frequency shift. The simulation includes the DIHEDRON geometry, the baffle with two 40mm holes in front of the DIHEDRON and phase maps (provided by LMA) of all three mirrors. The thermal distortion of the optic surface was calculated using the Hello-Vinet formula, which was confirmed to be accurate enough for all three mirrors by comparing with COMSOL calculations.
The HG10 frequency shift was calculated for different cases:
1) By heating the end mirror by 1mW absorption, the frequency shift is -32.5Hz.
2) By heating two DIHEDRON mirror by 1mW each, frequency shift is -70Hz (~2x of the end mirror effect)
3) By heating all 3 mirrors by 1mW each, frequency shift is -102Hz (essentially the sum of case 2) and case 3))
These results show that the thermal distortion changes the frequency by ~34Hz/1mW per mirror. The current experiment cannot distinguish effects caused by individual mirrors and 102Hz / 1mW-per-mirror is used assuming the absorption rates are the same for all mirrors. The measured frequency shift change from 4.87W to 7.22W input was 270Hz. To account for this 270Hz, total absorption needed is 270/102 = 2.65mW-per-mirror.
In order to estimate the power change in the cavity, we have used the last numbers which were measured extracted form the cavity pole and the transmission of Input and output dihedron mirrors.
T=81.3%+/-2.3% (Throughput)
Matching=0.945 (IMC matching)
PoutIMC=7.6W*Matching*T=5.84W+/-0.17W -> this corresponds to about 7.44V on IMC_TRA photodiode.
This means that the conversion factor for the photodiode is about 0.785W/V. Thus, during the experiment the power was varied between 4.87 and 7.22W-> DP=2.35W.
The cavity power change is 2.35W x (1160/pi) =867 W, so the absorption is 2.65e-3 / 867 = 3.1ppm / mirror. The throughput and the mode contents are calculated for the case that each mirror absorbs 100mW, for 90W input power, and the thermal effects were found to be negligible. With 150mW absorption, for 135W input, the throughput of the TEM00 mode drops by 3%. With the simulation setup described above, the round trip loss is calculated to be 80ppm and the observed 400-600ppm loss cannot not be explained. A small misplacement (several mm) of the baffle in front of the dihedron increases the round trip loss only by several 10ppm and doesn’t seem the be to cause of the extra losses since we measured similar round trip losses without this baffle (see logentry 31636 # https://logbook.virgo-gw.eu/virgo/?r=31636).
The simulated value of the HG10 frequency does not match with the measurement. The RoC of the end mirror based on the frequency measurement is 183.7m with 4.9W input power, but the RoC using the frequency shift by the simulation is 185.4m at the cold state, with 185.1m RoC being used in the simulation. This discrepancy has to be further understood. Similar measurements will be carried out on the IMC cavity with the new dihedron in the next days.
The simulation was made for a varying radius of curvature of the end mirror. This is a degree of freedom that would be possible using a CHRoCC if necessary. Due to a slight confusion with end mirror orientation a simulation has been made with the end mirror rotation about the optical axis of both 0 degrees and 180 degrees.
The two attached plots show the two main criteria used for the IMC requirement: Throughput loss on the TEM00 mode and fraction of power that is retroreflected by the IMC due to end mirror backscattering.
We see from these two plots that the IMC is well within specification for both possible end mirror orientations. This is mainly due to the fact that, by chance, there is very little light which is backscattered by the end mirror and coupled into the reverse cavity.
We have measured the angle of the dihedron to be 89 deg 59' 15'' which is close to what was expected (89 deg 58 58 2).
NB: the precision in the measurement of our auto-collimator for this measurement is 15'' (1 unit in the reticle) but the accuracy is much better (probably a few seconds).
Then we have measured the tilt of each mirror respect to the horizontal direction.
To do this measurement, we have considered the plane of the bench as a flat reference.
The measurement has been performed following the steps described in the first attached drawing.
The results are the following:
S1 : -30''
S1': 0''
S2 : -13'30''
S2': +13'
Consider that we measure on the autocollimator 2 times the angle. This means that the tilt of the shorter plate respect to the other is of 6'15'' 15'' (< 1.9 mrad) (see second drawing).
On the pedestal, with PEEK springs screwed, the tilt of the dihedron is fortunately slightly smaller.
we measured a residual tilt of the dihedron respect to the SIB1 surface < 1 mrad.
This tilt seems not a problem for the first phase of INJ installation and commissioning (IMC locking, IMC commissioning) but is not acceptable for INJ commissioning with the ITF and SIB2.
A new dihedron should be completed as soon as possible in order to install it after IMC commissioning activity and before completing the installation of INJ parts.
We visualized the beam on a camera in order to extract the parameters and have the size of the beam at this position (figure 3).
The Zemax simulation gives us (at the position of the screen) a theoretical size of 22.8mm if the incident beam waist is 2.65mm. If the incident beam waist is 2.7mm, the output beam (at the position of the screen) has to be 23.3mm.
The analyze of the beam analysis gives a size of 23.6mm on the y direction and 25mm on the x direction. The error is 1.5% along the y direction and 7% along the x direction.
These errors seem reasonable, knowing that the error on the parameters of the incident beam is of the order of 5% (between 2.6 and 2.7mm).
The parameters of the incident beam will be re-measured and the very fine tuning of the telescope could be done once the output beam will be sent to the interferometer (figure 4 - figure 5 - figure 6).
we have cleaned several mounts that will be used on RFC reflection optical path and we have assembled all the parts (except the optics that we will install just before mounting the parts on the bench). A view of those parts can be seen on the first two pictures.
SIB1_M14 mirror has been been installed on the bench and well aligned (see third picture).
We have also checked that all the parts we have installed so far were well screwed (in particular we have tightened better all the picomotors).
In the end we had to realign a bit the beam on the bench.
In the optics lab, we have measured the polarization quality of the beam transmitted by the polarizers.
We characterized polarizer #8 at different incidence angles (between 54 to 62 degrees).
The polarization quality of the beam transmitted is very good (up to 47 dB).
We also check that the polarization quality is not depending on the power (there is only a slight change of the polarization state by about 1dB
between 10W and 150W).
Next week will be dedicated to the reference cavity alignment.
A scan has been launched for the night. Bas has made a loop that compensates input beam drift by using the beam splitter to keep the beam in the center of B1p camera.
The activity will be completed tomorrow. This will help us for the future tests on the IMC Chrocc.
Note that the mirror just in front B5 has been reinstalled but is not bolted on the bench.
We reduced the input power from 14 Watts to 7 Watts.
We have had difficulty in finding a signal to characterize the end mirror RoC change.
Today we looked at the beam size on cam B5.
This seems to give information on how the RoC is changing. See figure.
The time constants are very long so this weekend we will scan the temperature of the CHRoCC to have data for Monday.
The test has started at 6:30pm LT and should last about 14 to 15 hours.
The installation is now completed.
The results of the CHRoCC characterization will be soon presented in a dedicated logbook entry.
To center the heat pattern, the CHRoCC has to be tilted of several millimeters by raising its front side with some spacers.
The adjustments will be made tomorrow morning as well as the final tests.
All power cut.
Internal shutters of CO2 lasers closed.
All water cooling valves closed
The CHRoCC was tested under atmospheric pressure and the heater was set to 440 C (2.5 V, 5A): the surface of the window was heated until the heating pattern given by the IR was stable (figure 2). The amplitude of the temperature of the heating pattern is about 2 C.
Then a circle of the dimensions of the IMC mirror was printed thanks to the heat of the fingers. It makes the reference for the data analysis (figure 3).
Profile of the experimental heat pattern is compared to the expected results from Zemax simulations (figure 4). The shapes of the temperature profile and the intensity profile are matching, that is a first confirmation of the good behavior of the system. The system is now ready for an installation in IMC tower.
We are ready for an installation in IMC tower from next Tuesday on.
To perform the tests we modified the NI optical banch switching off the power stabilization loop and superimposing a spot beam to the ring using the same path as the cross hair. To focalize the beam we used a lens to make the dimension of the spot of about 1cm on the test mass, as verified with the thermal camera.
In the following the list of the GPS times corresponding to the different activities performed on the system:
ON GPS=1003674027 central spot, power beam 23.6 mW
OFF GPS=1003674670
ON GPS=1003675396 central spot, power beam 10.2 mW
OFF GPS=1003676470
ON GPS=10036777462 central spot, power beam 16.4 mW
OFF GPS=1003678352
we moved the spot on the TM surface (inside the phase camera image)
ON GPS=1003681242 central spot, power beam 23.6 mW
GPS=1003681648 up
GPS=1003681769 up
GPS=1003682303 up
GPS=1003682719 down
GPS=1003683290 down
GPS=1003683598 down
OFF GPS=1003683956
ON GPS=1003752189 central spot, power beam 23.6 mW
OFF GPS=1003752394
ON GPS=1003753475 central spot, power beam 23.6 mW
GPS=1003754027 right
GPS=1003754604 right
GPS=1003755242 left
GPS=1003755801 diagonal
GPS=1003756467 diagonal
GPS=1003757053 diagonal
GPS=1003757604 central spot but the ITF unlocked
OFF
ON GPS=1003759248 central spot, power beam 23.6 mW
OFF GPS=1003759630
Measure with different power beam with central spot:
ON GPS= 1003762863 1.6 mW
GPS= 1003763459 3.17 mW
GPS= 1003764072 5 mW
GPS= 1003764617 7.4 mW
GPS= 1003765138 10.2 mW
GPS= 1003765590 13.2 mW
OFF GPS=1003766132
At this point we removed the lens to focalize the beam and put a mask with a central cross before the BS in the OBJ plane of L5. On the TM the cross dimensions are comparable with the YAG.
ON GPS=1003771585 power beam of about 25 mW
OFF GPS=1003779536
In this case most of the beam power was stopped by the mask itself.
A detailed analysis of the data will follow.
We changed the position of the two setups on the detection bench with respect to the past and we had some troubles in obtaining a good reference beam for both the cameras. Therefore, we decided to remove the collimator at the output of the optical fiber and to put two lenses (converging lens f = 30 mm and diverging lens f = - 250 mm) and we obtained a good reference beam on both B1p and B5 PC photodiodes.
In order to verify both the sidebands during the alignment we connected PC B5 photodiode to B1p electronics.
Figure 1 shows the images (amplitude and phase) of B5 carrier and sidebands.
Figure 2 shows the powers of B5 carries and sidebands.
Work in progress in B1p PC alignment.
Figure 1 shows the ring before the intervention.
Figure 2 shows the ring after the intervention (the image is comparable to the reference).
Figure 1 shows images taken today.
Figure 2 shows reference image.
WI appears to be unchanged.
NI appears to be misaligned with respect to reference image.
Figure 1 shows the dark fringe spectrum before and after the switch off confirming that this was the source of the bump.
