I took a picture of the setup in PSL lab, and drew a line for laser path. I omit the mode cleaning part since it's not in use now.
I'm working on estimating aLIGO sensitivity when material uncertainties are taken into account. I have a result for a reference cavity, uncertainty due to Ta2O5's Young's modulus might have smaller effect than we previously expected. All plots and code are attached below.
GWINC does not take any uncertainties in material parameters into account, so its noise budget does not have any error bar. We want to know how the noise budget might change due to imprecise knowledge of the material parameters. One particular issue is coating thermal noise that is dominating around 30 - 200 Hz, so we want to know how its level will change with material parameters. Some import ant parameters are loss angles and Young's moduli of each material.
In Hong et al 2013 paper, there is a plot of the calculated coating Brownian noise vs Ta2O5's Young's modulus (YH). The calculated coating BR noise is calculated with the corresponding YH while other parameters are fixed. This would be ok if each parameters were independently measured. In reality, loss angles are measured from ring down measurements, and YH and YL are used to calculated the material loss angles (phiH/phiL), see Penn et al. 2003. So to make the calculation reflects the real situation, we should take the correlation between phiH/phiL and YH/YL into account when we calculate coating BR noise. So the goal is to estimate coating BR noise for aLIGO with some uncertainties from loss angles and Young's moduli of the coatings.
calculate BR noise vs YL and YH (see PSL:1408 for the original code) using numbers from our setup (these can be changed later when we want to apply for aLIGO calculation). The code calculates BR noise with phiL = 1e-4, phiH = 8e-4. the numbers are from our measurement and another ring down measurement. This does not take the correlation between loss any YL/YH into account. I do this to compare my code to Hong's result, and they agree. YL and YH are varied between 80% and 120% of their nominal values (YL = 72 GPa, YH = 140 GPa).
above: Fig1:Thermal noise level of 28 Layer QWL structure, spot size = 180 um as a function of YH and YL
above:Fig2: three slices from the 3-d plot for different values of YL, Y_L min and Y_L max are 80% and 120% of the nominal value.
above:Fig3: three slices from the 3-d plot for different values of YH, Y_H min and Y_H max are 80% and 120% of the nominal value.
Next, let's assume that the values of SiO2 are well measured and the error is much smaller than those of Ta2O5, so we can fix phiL and YL. Then recalculate BR noise when phiH and YH are correlated. I use a calculation from ring down measurement (see PSL:1412 or Harry 2002 or Penn 2003). The equation is
constant = phi_parallel = (YL*dL*phiL + YH*dH*phiH) / (YL*dL + YH*dH)
from this equation, we can write phiH as a function of YH assuming that other parameters are constant. Currently, I'm using numbers from CTN setup.
above: Coating BR noise as phi_H is varied along Y_H (green) compared with the previous calculation (Blue) from fig 2. The two traces cross at YH = 140 GPa. Note that in this plot, YH is varied between 50% and 200% of the nominal value. We see that the uncertainty of coating noise due to YH becomes smaller compared to the previous calculation done in Hong paper.
==what to do next==
From fig3, uncertainty in YL does not change the BR noise level that much, but this calculation assumes no correlation between YL and phiL. I have not been able to include uncertainty in YL and see the effect on phiL yet, because that will need more constraint equation. But I should check if it will greatly change phiL and affect the total BR noise calculation or not.
I'm estimating the BNS range of aLIGO. Here is a quick note about the calculation.
For example, the normal configuration for aLIGO will have BNS in spiral range equal to 178.29 Mpc (based on the current code available on gwinc.
That GWINC link is more than a year old. You're best off just updating your CVS checkout of the code, or getting a new zip file from someone else if your CVS is broken. When I run gwinc with nomm.m, I get R_BNS = 189.5 Mpc.
I just saw you comment. I'll find an update version for GWINC.
Anyway, I have a code to plot the result. I will use it on an updated code.
Some material parameters in the calculation are:
In the IFOModel_rnd.m file which is a copy of IFOModel.m for material params, I use normrnd(mean,sigma) to generate the random value of the material parameters.
Note, for loss angle of SiO2, I have to use abs command to make sure that all the generated values are greater than zero.
This is because the mean is comparable to the standard deviation, and sometime it gives negative values.
Note: I have not taken the coherent between the loss and Young's modulus of Ta2O5 into account yet. I have to read how they measure this more carefully.
Here are prelim results from the above numbers.
above: a histogram of BNS range, due to uncertainties in loss angles/young's moduli of the coatings.
The mean of the histogram is slightly less than the nominal value from GWINC because the mean values of loss angles for fused silica (6e-5) is slightly higher than the original value (4e-5) used in the code.
above: histograms and Gaussian fits for BR noise (blue/green) and total noise and its Gaussian fit (red/cyan) at 100 Hz in the strain unit.
The calculation for LIGO astronomical reach with uncertainties are updated. See the details below.
I got the updated GWINC from Nic. When I run the nomm file, the BNS range is 189.5 Mpc. The nominal value of the refractive index for nH in the code is 2.06 which is the value for the pure Ta2O5. The refractive index for Ta2O5 doped with TiO2 25% (Ta2O5:TiO2) is 2.119 (see Harry et al 2007 paper). So when I changed nH to 2.119, the BNS range became 192 Mpc due to the thinner coating.
The ring down measurements from Harry 2007 paper measure parallel loss of the coating from multilayer coating, then extract PhiH from the measurement. Again, this is done by assuming the knowledge of phiL, YL and YH. The loss angle of silica (phiL) used in the paper is 1e-4 (from Crooks 2006 paper) while the value in GWINC is 0.4e-4. In this calculation, I use phiL = 1e-4 because of a couple reasons:
So the value phiL = 1e-4 is only used for extracting phiH as a function of YH, while phiL = 0.4e-4 is used as a nominal value in noise budget calculation.
Running the code
The calculation is done in the code name BNS_score.m (see the attached zipped file). The file calls on IFOModel_rnd2.m that generates random material parameters by normrnd command. The plot can be made by running plot_hist.m file.
fig1: histograms and gaussian fits for coating BR noise and total noise at 100Hz.
fig2: The astronomical range for BNS, in MPC unit.The histogram is from 20k samples. The average is at 189.2 Mpc, while the mode is around 188 MPc.
last night one of the DAQ cards failed and the acav crate stopped working, so also the temp stabilization of the analyzer cavity stopped woking. I restarted everything this morning and the setpoint should be reached again by lunch time or so
I'm working on characterizing ACAV loop to check if the loop is good enough for the coating noise measurement. The results show that there we need better improvement on this part.
I realigned the cavity, and the visibility is only 85%. I have not figured out yet why the visibility decreases from 90%. Then I measured dark noise @ error point, inloop noise with different gain setting, and error signal's slope ( all with 50 ohm system).
Error slope (with 2V DC on RFPD) =122.7 kHz. The visibility is @ 85%
1) Dark noise and inloop noise: The in loop noise changes with gain setup on UPDH box. I changed the gain from 3 to 7 and made sure that there was no oscillation (oscillating @ gain7.5).
With the error signal slope, I can calibrate in loop noise to absolute frequency noise from ACAV loop and plot them on noise budget. On the figure below, we can see that at high frequency noise from ACAV loop is dominating.
Notice the bump around 3kHz. It comes from the phase noise of the marconi. If I change the tuning range from 10khz to 1kHz the in loop noise also change (blue to green in the below figure). It is a trade off between lower frequency noise, less gain. The peaks around 80 Hz go up because we have less gain to suppress the noise.
2) OLG TF at different gain setup
Back to the 10kHz input range. Since the in loop noise is getting worse with more gain, I look into the OLG TF of the loop. The UGF is around 3-10kHz, depending on the gain. TF looks ok at gain 3-5, the magnitude increases along with the gain. At gain 7, the TF starts to deviate at high frequency (it might be oscillating at high frequency).
The in loop noise should be suppressed with 1/(1+G) factor, but the results above do not tell the same story. The gain increases but the in loop noise gets worse. So the next step is to look deeper into this problem. I'll use SR650 instead of UPDH and check if the same problem occur or not. This should verify if the UPDH box is bad or not.
Note: I'll calculate what we need for ACAV loop, and check if LIGO homemade VCO will be good enough for ACAV loop or not.
We measured the TF of marconi using PLL loop. Marconi has flat response up to around 200kHz. This is quite good and we can certainly use it in ACAV loop.
==block diagram of ACAV loop==
The whole OLG TF was measured in PSL:... This time we looked into the marconi to see if its TF has bandwidth high enough for ACAV loop or not. We know that Marconi has lower phase noise than LIGO homemade VCO (Megan's elog), but we have not learned about its bandwidth yet.
==PLL setup for Marconi TF==
The actual magnitude TF at DC can be determined by using a voltage calibrator to inject DC signal and measure how much the frequency of the output changes. This depends on the tuning range setup on the Marconi. However, we don't know the bandwidth of the TF, so we use PLL to find out. The setup is shown below. The gain from SR560 was set to be low, so that the signal at high frequency will be the TF of the Marconi + frequency discriminator.
Since the mixer output gives 1/f response (flat in [rad/rtHz] unit), we corrected the TF by multiplying back with f to get the TF of the marconi. The magnitude on the plot below has arbitrary unit. We are only interested in its shape. We tried 100Hz, 1kHz(not shown), 10kHz tuning range. The magnitude varies with the tuning range as expected. The phase does not change that much.
If we want to have phase margin of 45 degree, assuming other components in the loop have no phase lag. The best UGF we can do is upto 200kHz, according to the phase response of the Marconi (the phase drops by 135 degree around 200 kHz). Therefore, using a Marconi as an oscillator for driving the AOM is also possible because its bandwidth is high enough for measurement up to 1kHz.
Note: we will check what is the TF of the amplifier (H) used in our setup to make sure that it is not the limiting component.
We can definitely use Marconi as a VCO in our ACAV loop.
NExt: The next step is checking the UPDH box. At a glance, we found that the TF shape of the current UPDH is not suitable for our requirement.
I checked the TF of the amplifier used in the ACAV loop because I did not measure that yesterday. The amplifier's TF is flat at least up to 200 kHz. The Bode plots between the loop with and without the amplifier are pretty much the same. Thus, it is ok and won't cause a problem in the loop.
The setup is similar to what I did in the previous entry, except the output of the marconi is connected with the amplifier. See blue arrow.
The TFs between the two cases have similar shape. So the amplifier will not be the limiting component. The magnitudes (in arbitrary unit) are slightly different because I did not attenuate the power by the same factor it was amplified.
The UPDH box will be modified next to see if we can increase the loop bandwidth.
TF from all components except the sevo is measured (collectively). This will help us to determine what kind of servo(UPDH) we want for ACAV.
TF from other components in ACAV loop except the servo (collectively) (frequency discriminator, marconi, amplifier) and fit]
TF from ACAV loop without servo. The red curve is fitted with 1pole at 50kHz with 70kHz delay (exp(-i*f/70kHz)) . Since our bandwidth of interest extends upto only a few hundred kHz, this fit is good enough for a model. It starts to diverge from the data at 300kHz.
Note: The TF looks ok, it is flat as expected from most of the parts (frequency disc/Marconi/amplifier). The pole and time delay is from the AOM. We can see the phase changes as we change the AOM position so that the beam is closer to the PZT side. We gain ~5 degrees from adjusting the position.
[current updh schematic]
The TF has a pole around 50kHz. C18 with 3300pF gives a zero at that frequency and cancels the pole. We are designing the TF of the servo that is suitable for our need (UGF ~100kHz, with 1/f roll off at UGF, and ~45 degree phase margin, and, 1/f^2 at low frequency)
Problem with UPDH, on stage 2, the resistors' values might be wrong. We will check and fix it tomorrow.
acav is now re-aligned. As Tara stopped aligning the AOM i will use the NPRO pzt to lock it (instead of the VCO). Then i will also see where the shit of the other tf comes from (hopefully).
previous measurement is invalid - due to changing in the alignment from floated table to unfloated table over night we don't know what we really measured.
What we can say is that the beat frequency changed from 181.265MHz to 185.9MHz, so by 4.66MHz for a 0.2K increase of the shield using 1.53V heater voltage.
aligned everything properly, locked both cavities, changed tuning input range to 100kHz and turned heater off. We might stop the new measurement at any time as we got the basic information we need
Today we installed two accelerometers on the table, for vertical and horizontal(beamline) positions.
The TF between the PZT's driving V and the signal from the accelerometer are plotted below.
I did some calculation on frequency noise due to beam/cavity mismatching in translational and pitch which causes the natural axis to change from the designed value. For example, if the beam translate away from the center by a little, same angle, the cavity length as seen by the beam will be shorter because of the curve mirrors.
It looks comparable to what we see on the beat signal.
Frank found me 2 of Breul & kjaer 8318 accelerometer. I installed them on the table for vertical and beamline horizontal directions.
I chose vertical direction because the spring was removed from the cavity suspension and become susceptible to vertical seismic.
For beamline horizontal direction, I chose it because it is the same direction the PZT pushes the table, so I expected to see some strong signal.
The signal from 8318 is sent to SR560 preamp, ac couple(doesn't change between dc/ac), roll off at 1kHz, gain200.
The signal from SR560 is sent to response (B channel on SR785)
The source is sent to HV amplifier that drives the PZT and reference (A on SR785).
The integration cycle is 50, with 10 settle cycles. I tried 100 integration cycles (quickly cheking between 100Hz and 1kHz again), but there was no significant change.
No strong signal on the horizontal direction. The magnitude is even smaller than that of vertical one.
I'm not sure how valid the TFs are. I tried changing gain on SR560 and looked at high frequency, 100 - 1kHz,TF the magnitude changes correspodingly
with the gain, the shape looks the same.
The next plan will be measuring the TF between cavity motion (shadow sensing technique)and acceleration on the table. I'll also compare with the seismometer on the table.
sandwich structures using plexiglas : "Damped Windows for Aircraft Interior Noise Control" http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.79.9176
bare plexiglas: http://www.eplastics.com/Plastic/Plastics_Library/Plexiglass-Noise-Reduction
We started making the acoustic enclosure around the input beam area (second half of the table, before the chamber). The frame is done. We haven't received all items for the panels yet, so we just tried to use the aluminum bubble wrap as test panels. And we just used a piece of plastic planes to cover the top. There is no improvement in acoustic coupling yet.
We added the lid on top of the enclosure. More work is needed to complete the box.
We made the closing lids by cutting a 1/8" acrylic panel. A strip of soft foam was added between the frame and the lids to form a seal.
We did a qualitative test by placing a white noise source inside the box and listening. The aluminum bubble wrap we used did not provide good noise reduction. So we replaced one side by a plastic piece (~1/8" thick) with a damping pad on. It could damp the noise pretty well. I'll borrow a blue bird microphone from Den tomorrow, so we can measure the TF or just the relative noise signal to check how much attenuation we get from our structure.
We measured the frequency response(microphone out/signal to speaker) to see how well we can shield the outside acoustic. The test panel did help reducing the acoustic coupling, but there is still room for improvement.
Den lend me a blue bird microphone from 40m. So we setup a measurement to compare two panels we have. The first one was what we made yesterday (plastic with damping pad), the second one was the aluminum panel(1/8" thick) with soft foam on the inside and foam strip on the edge where the panel met the frame.
We measured the frequency response between the microphone signal and the speaker driving signal. The source was white noise (band limit) 100Hz - 6.5kHz, 1.4V. The output has a T so that one was sent to the speaker, another one was for chA. The SR785 chB input for microphone signal was floated since the mic gave differential output. This should prevent the pre-amp output to see "ground" at the output and break the opamp.The measurement was average over 5000 samples.
We measured with the speaker on and off (but the white noise ref to chA was still connected) to check we have a good SNR for every setup. Three setups were:
fig1: setup, with the panel remove.
fig2: two panels for testing. Left, a plastic piece with damping pad attached on (from yesterday). Right, an aluminum panel with soft foam
fig3: panel under test.
==conclusion + plan==
From the plot, it is not very clear if the aluminum panel (panel2) is better than the plastic one (panel1). It might be that noise coming from other panels(which we have not changed) is the dominating signal. We will put the mic in a smaller container surrounded by acoustic damping with an opening for the material/structure to be tested. Then we can test a sample easily without removing/installing the panel all the time.
For now, we are planing to use another kind of foam to put inside the box. We check by ears and found that it is better than the current foam we use with the aluminum panel.
We are still working on the acoustic shielding panels. The work should be done by tomorrow.
Fig1: Left, one panel with damping foam + black pad on top( to prevent scattered light). Right,a panel with two layers of good damping material with a damping pad under it. This type can damp acoustic noise pretty well.
We prepared all sides of the acoustic box. However, we don't have enough damping materials, so all the panels are not similar, but they all have some soft foam to provide acoustic damping. All the holes for cables/ beam are marked and will be drilled tomorrow.
The enclosure box for input optics are done. We still need to order more of the nuts for one panel, but the box should provide certain acoustic shielding for now.
We will measure the beat signal once the temperature stable.
The box has four 1-inch diameter holes, 2 for periscope, one for input beam, another one is for the beam to RCAV curve mirror which we cannot fit in the box.
We had to rearrange the cable for ACAV AOM to have fewer cables going in and out the box. The cable for driving the AOM was remade so that it did not block the panel.
I check the performance of the enclosure box for input optics. It neither improves the beat signal that much.
==Is the acoustic box good? No==
To check how good the acoustic shield can be, I measured the beat signal and feedback signal to ACAV AOM when the lid were on and off. There were no much improvement in both signals, see fig1 below.
fig1, beat signal and ACAV feedback, converted to frequency noise. Beat signals between the lid close and open (red, purple) are very similar. Feedback signal to AOM are also the same (blue, cyan). I plot the 4 traces together to see if there are any coincided peak, so I can know where it happens (beat path or input optics). Note the peak at 280 Hz in Cyan trace is not real, it pops up after ~50Avg. I could not find its origin yet.
==Is it really acoustic coupling? Yeah, kind of==
The results were so similar between the lid open and close, so I wondered if those were really acoustic. To test this, I turned off the two computer (PC and fb2) and remeasured the beat. Those computers' fans are quite loud when they are on. For fb2, the fans still work even when it is shut down, but definitely much quieter. The beat signal was improved a bit, see figure 2. The results were real, I repeated them twice. Note that the room are still not totally quiet with the two computers off, sounds from sun machine and electronic rack are still there, and they are as loud as the two computer and closer to the beat setup as well.
fig2: beat signals when the computer are on (blue) and off (red), several peaks are obviously reduced when the computers are off.
==discussion and plan==
Since the computer are sitting on the floor, it is not certain if the peaks due to the computers are from acoustic transferred through air or vibration transferred through the ground. But the peaks in question are at high frequency (almost 1kHz), and we have 3 stage seismic isolation on (except floating table). It is very likely that these peaks are caused by acoustic. To make sure that they are really acoustic, I'll float the table and repeat the measurement again.
We started installing the new acoustic enclosure box for the beat path. It covers the whole beat setup and the part for power detection. The walls are installed. The lid will be made later.
The mirror mount in front of new focus 1811 PD has long knobs which extend into the wall, so I replace it with another mount that has no knobs, and the panel can fit without obstruction.
The current box has only three holes, 2 for input beams, one for output beam at the beam splitter. Windows might be installed in these holes for better acoustic enclosure.
The beat signal is measured with an adhoc lid covering the enclosure on the beat part. There are no significant improvement in the acoustic region of the beat signal yet.
The closing lid is the only missing piece for our acoustic box, so I just use smaller pieces of acrylic planes I can find to cover the top, and measure the beat signal. There is not much improvement in the acoustic region, see fig2 below for a closer look.
There might be dust on the optics somewhere, because the signal at low frequency is not as stable as it used to be. I could see scattered light bump coming up when I averaged for too long (this did not happen last time). I'll check all optics and make sure that they are clean.
NOTE: The temperature servo is back on. We had to disconnect it in order to install the acoustic box. I set C3:PSL-RCAV_TEMPAVG to 32.5 ( the previous value was 35.03). I changed it because I noticed that as the temperature reached up to the set point, the beat frequency went up as well. I'll check if this set point will reduce beat frequency down to ~ 160 MHz or not.
ordered thin (1in) t-channel aluminum frame parts from mcmaster (similar to bosch framing, but cheaper, link) and other stuff to build a nice acoustic enclosure with access from all sides. Bought the 1/4" plexiglas parts for the lid from drillspot (much cheaper and free shipping). For the side panels i'm thinking about using thin aluminum panels mounted to the outside of the frame (not in the slot) and then using the full depth of 1in of the frame structure for acoustic damping using a combination of sound absorber-barriers, rubber foam, lead, mass loaded vinyl/Nitrile closed cell foam or anything else. However, putting it in the slot would be easier. Didn't buy foam and stuff. Will wait for the frame being on the table to see what to do.
Acromag1 and ws3 have both been reconnected to the network and booted.
New ModbusApp session is started in tmux and all channels look accessable.
No access to outside world, external IP address seems to be down ATF:2178.
Access to the network is now restored ATF:2179.
A photodiode for measuring the laser power reflecting from the Faraday isolator's in port is added.
The cable is prepared and connected from the PD to DAQ.
The channel name will be C3:PSL-NPRO_PWRMON.
Last week I accidentally changed the polarization of the beam to the 35.5 MHz EOM. So I optimize it again to minimize an RFAM effect.
I used the signal of the transmitted beam behind RCAV on the PD for beat signal. Since the cavity pole is around 37 MHz, I should be able to see the
signal at 35.5 MHz easily. I connected the RF signal from the PD to a spectrum analyzer and adjusted the 1/2 wave plate to minimize the peak at 35.5 MHz.
However I also notice two peaks at 35.29 and 35.71 MHz (35.5 +/- 0.21 MHz) which are approximately the same size as the 35.5 MHz peak.
I'm not sure where they come from.
machined all steel parts myself this morning and submitted them for welding to Mike. He said we can pick it up at the end of the week, so if we are lucky we can install the stuff Friday afternoon already.
New beam height will be ~6.5in -> 5-7/8 + 3/8 for the beam + 1/4 clearance
after talking several times to the machine shop guys and checking stock at central we came up with the following design for the two beams supporting the vacuum chamber including thermal insulation. The main requirement was to not raise the beam height too much to not make everything to unstable. We also can only raise it about 1in before reaching the upper limit of the existing periscope. The current design adds 3/8" of height from the beam plus additional space below (~1/4). The width is designed to accommodate a second thermal insulating box around the existing setup if required for more stability.
got some T-connectors and connected all four springs to the lab air supply. The pressure of 32PSI is barely enough to float the chamber. The side with the horizontal extra flange is almost too heavy to float, but it's OK. The other side is much better. But overall the chamber is pretty good leveled. Tara re-aligned the laser to both cavities and the beam height is ~6.5in as designed.
we measured the TF from the table to the vacuum chamber. Sensor is attached to the top of the ion pump in the center (left picture), the other one on the aluminum block next to the chamber (see picture). Resonance frequency is at 5.19Hz and has a transmissibility of ~10. Both numbers are slightly higher than expected from the datasheet.
[Tara, Frank, Zach, Jenne]
i've picked up all parts, tapped a few more holes and assembled the two beams. Zach helped us lifting the (really heavy) vacuum chamber to install the two beams. I had to cut one side of the center insulation (the one with the flange on the side) as it was impossible to install with the chamber resting on the two beams. Even after cutting it it was really difficult to install. There is barely enough space for the other side. The other pieces are easy to install.
Had a hard time to find an adapter for the schrader valve. Have been to 3 bicycle stores, three car supply stores, home depot, osh etc. - the simply don't have an adapter which one can screw on the valve and connect a hose or anything like that. Ended up making one out of some valve extensions. Jenne helped me installing the air supply for one side to check how much pressure we need to lift it. It turns out that the air supply in the lab is sufficient. Will get all missing parts tomorrow from the hardware store to finish installation and measure the TF of the new isolation.
I finished aligning the beams from Ref cavity and Acavity, broke a connector on the PD we were going to used to measure the beat .
Frank borrowed another PD to use for now and ordered the replacement for the broken connector which should arrive next week.
Now I'm waiting for the temperature to settle, so both cavities can be locked and see the beat.
by now all parts are baked and ready for assembly. The only part we couldn't bake is the chamber itself. But Bob said that putting everything together and using the heaters on the chamber to "bake" the whole thing at lower temperature would be ok. So the plan is to assemble everything including an old 8l ion pump and bake the whole thing. As soon as all the insulation, temp sensors and stuff is ready we move the cavity into the (hopefully) clean chamber and replace the 8l pump by the 20l pump currently used on the other chamber.
I think ants are now building their colony in the lab, even though there is no garbage, they can find some food around here.
That seems like a cricket's leg (see attached pic). A few of them walk around the optic table/ optics too.
I think we should let an aardvark roam around the lab for a day.
any results from the poison from 40m they tried in the TCS lab? Did it work?If yes we should get some for the other labs too....
we cleaned and assembled the new vaccum chamber. we installed an electical feedtrough (for sensors we might wanna have inside, e.g on the thermal shields or the stack), an electrical isolator part for the ion pump( to avoid ground loops with the high voltage source for the pump) and the first AR-coated window. We will replace the second one after we moved the cavity. Right now we only have to mount the ion pump and then we can bake the whole thing before we finaly move the cavity to its new home.
The mechanical workshop has to rework the new insulated feet we got today. The upper part where the cavity sits on was machined really badly. It didn't fit to the chamber at all...
We are still waiting for the additional heaters. They claim that they already shipped them last week so we don't know what happened to them. A week from San Diego to Pasadena? Who knows where they shipped it to (Antarctica?)
We've cut the last two parts of the insulating foam and glued everything together. Next we have to cut the holes for the flanges and feet of the chamber. The hole for the beam will be cut after we aligned the beam into the cavity
[Peter, Tara], we assembled 2 short reference cavities today. The bonding between the spacers and mirrors are strong and holding the mirrors nicely.
I got the cavity fixtures (made from delrin) from the machine shop today, so I asked Peter to help me assembling the cavities. All picture can be found here
I tested the bond by lifting the whole cavity by handling at the mirror on top only, and wiggling it a little bit. The bonds weren't broken. The hardest part was cleaning all surfaces to make sure that there was no dust.
From hindsight, I don't really need to see the fringes to do the bond. If the surface is clean, the pieces will be bonded instantly after a light pressure. If there are particles on the surface that cause fringes, the bond will not form anyway. So for Si cavity, Dmass can try to do optical contact without a setup to see the fringes.
fig1: the mirror is placed in position by the fixture. The mirror is not pressed on the spacer yet. Fringes can be seen on the polished ring on the mirror. See the video to see how the fringes vanish after applied pressure.
We noticed wide angle scattered light behind the PBS in front of RCAV. The scattering source is probably the curved mirror behind RCAV AOM. We borrowed the similar mirror from 40m and will try to compare them.
The wide angle scattered light behind the PBS in front of RCAV might contribute to the noise in beat signal. The picture shows the scattered light with area larger than the half inch PBS cube. This picture was taken when the beam's polarization was changed to P-polarization so that most of the light was reflected from the PBS. With small transmitted light through the PBS, the scattered light can be seen clearly behind the PBS, see here.
After the inspection, it is very likely that the curve mirror behind RCAV AOM is the source. So we borrowed another R=0.3 mirror from 40m to see if it will be better or not, this will be done soon.
Note: during the inpsection, we also identified another bad PBS,pic. This is the one in front of RCAV AOM. Its center surface looks dirty, so we replaced it with a better one.
Update, beat measurement after several optics replacement. Peaks around 10 Hz, 35 Hz show up this time.
Optics that we replaced are:
The problem with the curve mirror from last entry has not been fixed yet. It turns out that the mirror we borrow from 40m is worse than the one we have (surface is more milky), so we leave the original mirror as it is.
Note: The beat measurement was done when the air springs were inactive. Noise at high frequency goes down a bit.
The power input to each cavity is 1mW, setup on PLL is 1kHz input range, with gain = 200.
please be careful near the HEPA bench in 058E (PSL lab). I started the baking of the plate. The plate is sitting on the bench and is hot. It's powered by a HP power supply with 70V sitting on the floor. Warning signs are posted. The heater is insulated but anyways, be careful when near.that area. Setup will be removed Friday around noon. Same for the baking of the viton in Vladimir's lab. Heater and vacuum parts are hot, also the pump itself! Everything is hot, not just warm! Don't touch it.
The beam is sent to 2nd cavity (RCAV). The beam is mode match roughly, since there is no PMC, the exact beam size is hard to measure. The laser resonance when RCAV_SLOWOUT @ 0.2383V. There is enough transmitted beam for alignment the beat setup behind the cavity.
measured the beam pointing caused by driving the AOM frequency modulation input. data is uncalibrated so far, just a screenshot of the dataviewer. PDHOUT is the VCO input signal...
I'm trying to re-align the beams to the cavities. Due to the new RTV springs for the seismic stack, the cavities' natural axes shift by ~1/4 " with respect to the previous position.
I had to adjusted the height of the top mirror of the periscope before I could align and lock RCAV (visibility ~ 95%) again. The pictures below show the position of the current beam. With the previous setup, the beam position was almost at the center of the holes. Now, for RCAV, the axis shifts closer to the edge. RCAV might yaw with respect to the previous position. Left picture shows the incoming beam position, Right picture shows the outgoing beam position.
For ACAV, however, it seems that the position changes a lot and the beam clips on the outer edge of the top mirror before I can even find TEM00. I think I'll have to add a spacer between the mirror mount and the vertical plate in order to re align the beam.
I think we can keep the stack position as it is for now, if I can lock both cavities and the transmitted beams can be adjusted on the breadboard for beat path. We might also have to increase the hole size on the insulation cap as well depending on where the beam position of ACAV will be.
I realigned ACAV and found TEM00, but now the transmitted beam is completely missed the opening on the insulation, it is off from the center by ~ 1 cm.
we replaced the mount for the combining beam splitter in the beat setup as it caused a large, broadband peak in the spectrum around 1.4kHz. The new mount is one of the old, fixed turning mirror blocks they used in initial LIGO at LLO as far as i know. After replacing the mount the peak is entirely gone. I've used two springs instead of one to increase the pressure. We could not determine the resonance frequency of the new mount. Tapping the mount excites only known mechanical resonances from the surrounding mirror mounts. Tara posted a plot for comparison before and after replacing that mount (see here). He also has prepared a nice plot combined with a drawing which mount corresponds to which resonance we see in the spectrum. We will use this to start reducing (or even eliminating) those resonances starting with the most dominant ones close to 1kHz
Attached a copy of the drawing.
It's a quiet night, so I went down the lab to measure the beat signal. We are getting close. I think I have to review my noise budget calculation and estimate the error in the measurement carefully.
So after a few things Evan and I did a few days ago:
Then I measured the beat signal.
We reduce some noise from scattered light at frequency below 100 Hz, we are limited by some white noise at high frequency ~ above 1 kHz.
fig1: measurement vs noise budget
fig2: zoom in. The slope of the measured signal agrees well with the slope of thermal noise.
Short note from tonight measurement:
1) scattered bump from dc to 100Hz is mostly from seismic. It is worse during the day. It gets smaller at around 3-4 am. Unless we have a better seismic isolation, we might not be able to see anything below 100Hz.
2) RIN shape from RCAV changes, reasons still unknown. (DC level 0.7 V)
3) I might see the effect from RIN induced TO noise at frequency ~ 1-3 kHz. (compare RIN and beat).
I'll get into details tomorrow.