We've set up the PEM monitor computer to broadcast the temperature channels to EPICS.
We'll find a permanent home for the machine tomorrow.
The (Windows 7) computer that runs the OPC server is in the TCS Lab. The EPICS server on this machine needs rebooting at the moment.
More critically, we need a framebuilder running on the network in order to save these channels to file for future trending. A slow EPICS framebuilder is all that is necessary.
We finally got the temperature sensors broadcasting to EPICS channels again - well, in part anyway. There are a lot of configuration issues to work out (refresh rate, saving to frames, license for OPC server, battery monitors, data precision). But at least we can now see a temperature sensor channel in EPICS that corresponds to a live measurement. The configuration to get the data from the remote unit to EPICS is shown in the attached block diagram.
More details can be found here:
For one, we removed the QWPs which were the first optics in the transmission paths. These had been necessary for the prior cavities where the Silica Tantala mirror coatings were not birefringent. The circular polarization which was transmitted needed to be turned into linear polarization to get the beat note on the PD. Now, because the cavities with AlGaAs coatings are birefringent, the resonant and transmitted light is already linearly polarized and the QWPs unnecessary. Before removing them, the power on the main readout PD, a PD1811, was 208 mV. Afterwards, it was 194 mV.
On the south path, we have placed a HWP so that the transmitted beams can have their polarizations matched. It is on a 1" post and held down with a fork.
In the longer term, this should probably be replaced with the solid metal blocks that were used to hold the QWPs. If these blocks are reinstalled, the waveplate mount should be twisted slightly in yaw in order to reduce the amount of backscatter into the cavities.
The laser beam entering the first Faraday isolator appears to be 1–2 mm too low. It is clipping on the input aperture, and the transmitted beam looks like crap.
When Aidan and I turned on the south laser today, we found that the transmitted beam out of this Faraday was entirely crap. It was blindingly obvious on an IR card, and only 50 uW was making it to the input of the PMC. The rest was scattering at wide angles at the Faraday output port.
It is not clear to me how the pointing through the Faraday could have deteriorated, since it is on a solid metal mount and is only 10 cm from the output of the laser.
At any rate, I was able to "recover" the previous performance (i.e., crappy but workable) by placing the Faraday isolator slightly further down in the optical path. Before, the layout was:
Laser -> QWP -> HWP -> Faraday -> lens -> HWP -> steering mirror -> PMC EOM,
and the HWP angles were -1 deg and 167 deg, respectively. Now the layout is
Laser -> QWP -> HWP -> lens -> steering mirror -> Faraday -> HWP -> PMC EOM,
and the HWP angles are 341 deg and 167 deg. The first HWP angle is chosen so that 20 mW is transmitted through the Faraday (the rest is dumped at the Faraday's various output ports). The second HWP angle is chosen to send s polarization through the PMC EOM. I then had to resteer through the PMC EOM and through the PMC. With 20 mW incident on the PMC, the transmission is 11 mW. Not great, but about the same as the previous situation.
I remark that the south optical path between the laser and the PMC should be reworked as soon as is feasible, because what I've done is a hack job to keep things moving. Either the Faraday mount needs to be remachined, or the optical path needs to be redesigned to allow for proper steering through the Faraday. Additionally, the table surface next to the laser mounts is noticeably warm to the touch, so I do not recommend trying to shim up the laser (as it may negatively impact the heatsinking).
Some tasks not included on the list:
The beat signal looks awful. It has some amplitude modulation at 6.75MHz and looks like it has some strange saturation effects going on. This is too much noise for the PLL to lock to.
We thought, for a minute, that the reason for this may be related to one of the HV supplies for the RCAV locking. The needles on the front of the positive supply unit +150V and 0mA current drawn. The other 3 HV supplies in use all show around 20-25mA current draw when used with the TTFSS boards.
We popped the top of the RCAV TTFSS box on the table and looked at the TP4 output on the HV/Interface board (this looks at the signal coming out of the high voltage amplifier that feeds the EOM, but reduced a voltage divider to 1/10th the value). It was freely swinging between +/-4V, so the HV amplifier seems to be happily getting both +ve and -ve voltages. There might be a problem with the needle on the HV supply.
We investigated the way we were locking the PDH loops using the TTFSS boxes. Here's what we previously did:
We realized, yesterday, that this wasn't the correct way to lock the loop as box now expected the gain settings for the loop to be set remotely (and we were providing none of that to the unit). Still, the default gain in REMOTE was enough to provide a stable lock and we didn't understand exactly how that box worked (which is obvious in retrospect). So, yesterday, we pored over the schematics for the TTFSS boxes (Rich is drawing a very nice block diagram to show the loop structure), and realized our error. The correct way to lock is the following:
From here, we were able to play with the common mode gain settings reduce the noise of the beat note between the lasers. And we were able to lock the PLL. The main evidence for the latter is the fact that we can change the DC value of the control signal in the PLL by varying the carrier frequency of the Marconi.
We want the elog process to run in verbose mode so that we can see what's going. The idea is to track the events that trigger the elog crashes.
Following an entry on the Elog Help Forum, I added this line to the elog starting script start-elog-nodus:
./elogd -p 8080 -c /cvs/cds/caltech/elog/elog-2.7.5/elogd.cfg -D -v > elogd.log 2>&1
which replaces the old one without the part with the -v argument.
The -v argument should make the verbose output to be written into a file called elogd.log in the same directory as the elog's on Nodus.
I haven't restarted the elog yet because someone might be using it. I'm planning to do it later on today.
So be aware that:
We'll be restarting the elog today at 6.00pm PT. During this time the elog might not be accessible for a few minutes.
I tried applying the changes but they didn't work. It seems that nodus doesn't like the command syntax.
I have to go through the problem...
The elog is up again.
Today I took more measurements after reflecting off the beam by 90 degrees to another direction and using the Beam Profiler Dataray Beamr2-DD. I used the InGaAs detector with motor spee dof 11 rps and averaging over 100 values.
Following is the fit with and without the new data taken. Data1 in the graph is the earlier data taken using razor blade and Data2 is the data taken today using beam profiler.
The two fits estimate same waist positions and waist sizes within error bars of each other. However, the reduced chi-square is still pretty high.
I've also added the data file and code in the zip.
I've implemented all the proper analysis norms that Jon suggested and are mentioned in the previous post. Following is the gist of the analysis:
with shear loss angle taken from Penn et al. which is 5.2 x 10-7. The limits are 90% confidence interval.
The analysis is attached. This result will be displayed in upcoming DAMOP conference and would be updated in paper if any lower measurement is made.
Thu Jun 4 09:17:12 2020 Result updated. Check CTN:2580.
If all layers have an effective coating loss angle, then using gwinc's calculation (Yam et al. Eq.1), we would have effective coating loss angle of:
This is worse than both Tantala (3.6e-4) and Silica (0.4e-4) currently in use at AdvLIGO.
Also, I'm unsure now if our definition of Bulk and Shear loss angle is truly same as the definitions of Penn et al. because they seem to get an order of magnitude lower coating loss angle from their bulk loss angle.
I realized that in my noise budget I was using higher incident power on the cavities which was the case earlier. I have made the code such that now it will update photothermal noise and pdhShot noise according to DC power measured during the experiment. The updated result for the best measurement yet brings down our estimate of the bulk loss angle a little bit.
The analysis is attached.
If all layers have an effective coating loss angle, then using gwinc's calculation (Yam et al. Eq.1), we would have an effective coating loss angle of:
Also, I'm unsure now if our definition of Bulk and Shear loss angle is truly the same as the definitions of Penn et al. because they seem to get an order of magnitude lower coating loss angle from their bulk loss angle.
Automatically updating results from now on:
I added the possibility of having a power-law dependence of bulk loss angle on frequency. This model of course matches better with our experimental results but I am honestly not sure if this much slope makes any sense.
Auto-updating Best Measurement analyzed with allowing a power-law slope on Bulk Loss Angle:
RXA: I deleted this inline image since it seemed to be slowing down ELOG (2020-July-02)
I have switched on HEPA filters to high, both on top of the main table and on top of the flow bench.
Continuous measurement is stopped hereby. This experiment is finished.
I shorted the interlock terminals on the North laser power supply and still as soon as I turn the key to 'ON' position, the south laser drops to standby mode and the north laser power supply display does not switch on with the status yellow led blinking asynchronously. I still do not understand why the two laser operations are coupled. South laser power supply does not share anything other than the power distribution board with the North power supply. Could it be that something in the north power supply has created a short circuit in the power drawing portion?
As Koji suggested, I can use a spare LWE NPRO controller but do we want to put more resources and time into this experiment? We have acquired loads of measurements over 4 months in the quietest environment already. So I'm not sure if it is worth it.
I noticed small amount of water on the floor (Attachment 1) on the west end of the lab. Immediately above it is a pipe which I don't know what it does. One can see another drop forming at the edge of this pipe (Attachment 2). This water is slowly dripping on the side of the pipe (Attachment 3). I could trace it out to coming from somewhere on the top (Attachment 4 and 5).
Maybe this is just some condensation because of increased humidity in the air. But maybe this is some troubling sign. What should I do?
I added script SRIMD.py in 40m/labutils/netgpibdata which allows one to measure second order intermodulation product while sweeping modulation strength, modulation frequency or the intermodulation frequency. I used this to measure the non-linearity of SR560 in DC coupling mode with gain of 1 (so just a buffer).
Edit Wed Feb 17 15:34:40 2021:
Adding self-measurement of SR785 for self-induced intermodulation in Attachment 3 and Attachment 4. From these measurements at least, it doesn't seem like SR785 overloaded the intermodulation presented by SR560 anywhere.
I followed the analysis of this recently published paper Jan Meyer et al 2022 Class. Quantum Grav. 39 135001 to calculate the birefringence noise in the CTN experiment. Interestingly, the contribution from birefringence noise after my first attempt at this calculation looks very close to what we were calculating as coating thermo-refractive noise before. If this were true, our experiment would have seen it much before. In fact, we wouldn't have seen thermo-optic cancellation as Tara experimentally verified here. So something is missing
After going through some literature and reading properly Meyer et al, I have the following understanding of the birefringence noise (and why it is called so).
This is a question I am still not sure how to answer. My understanding is that the common mode change in refractive indices of both axes drives the thermo-refractive noise. This means I should be able to derive the coefficient of thermo-refraction using the same formalism.
Both thermo-refractive noise and thermo-photoelastic noise show up as dn/dT terms in the thermo-optic noise summation, just through different physical processes. This could mean that experimentally measured coefficients of thermo-refraction already include birefringent contribution if any. In my calculations for the plots presented here, I got the following values of the two coefficients:
Coefficient of thermo-refraction (Effective for coating): 8.289e-05
Coefficient of thermo-photoelastic effect (Effective for coating, using Eq.11 of Meyer et al.): 8.290e-05
It was very surprising to me to see that both these coefficients came out to be within 1% of each other.
Because of this, when we add the noise sources coherently (since they are all driven by the same thermal fluctuations), the thermo-optic cancellation that we have experimentally proved does not work anymore. So something must be wrong with my calculation.
I made a few changes in my calculations today, which changed the noise contribution of this photoelastic noise (coatTPE) to roughly half of the individual contribution from coating thermo-refractive (coatTR). If this was true, it would significantly affect thermo-optic optimization, although not totally destroying it. I admit there is an outcome bias in this statement, but this noise estimate fits very well with the noise floor measured by CTN lab.
I made two changes in total:
So now, the noise calculation is as follows:
I think we need to regroup and discuss this further.
The photothermal transfer function measurement made back in 2014 showed some cancellation of thermo-optic noise, but there were some irregularities with the modelled transfer function even back then. Here in attachment 1, I have plotted the measured photothermal transfer function, along with the estimated transfer function with and without adding a term for thermal photoelastic (TPE) channel.
I was wondering if photothermal noise would get amplified due to the TPE effect. We were not using a measured photothermal transfer function in our noise budget for this noise contribution and relied on a theoretical model instead. For comparison, I added noise traces for three cases, Estimated photothermal noise with and without PTE, and photothermal noise using measured TF. In all these cases though, the ISS in the experiment suppressed RIN enough that photothermal noise did not matter to beatnote frequency noise.
The obvious go to measurment here would be two-lasers-one-cavity to measure the residual between the two polarisaiton modes of one of the cavities. Is the experiment in a state where this could be done easily?
Not easily, but it is doable if we resurrect the south path only. I estimate ~1 month of work for that if things go fine.
If I recall correctly Tara had this set up with an optical circulator on the input side which Antonio and I switched to linear polarisasion with Faraday isolator. The mode splitting of the AlGaAs coatings would take care of only selecting one polarisation mode, but is it posisble that the latter measurments sampled a different polarisation to the original thermo-optic measurment? Just a thought.
With circularly polarized light, Tara could be addressing any of the two possible resonances, with only effect of suffering in modematching with the cavity. So it should be a 50/50 chance that they measured it in a different polarization. However, the nature of thermal photoelastic measurement is same in both polarizations. The photoelastic tensor for GaAs (cubic symmetry), in theroy, does not create birefringence, or afect different polarizations differently. The source of birefringence in these coatings is not known.
Martin Fejer recently gave two talks in a coatings workshop where he showed calculations regarding the thermal photoelastic channel. I have not been able to under the logic behind some of the calculations yet, nevertheless, I used his formulas for our coatings to get an alternative idea of this noise coupling.
I just received more calculation notes of Fejer (through Yuta) which I'll study and try to make more sense of this calculation. It also contains the calculations of sough-after birefringence noise.. But in his presentation as well, he stated that birefringence noise is not sourced through termperature fluctuations and is not part of thermo-optic noise (something I didn't understand again).
Today, we did the beam profiling for the beatnote detector just before the photodiode. I have attached the data taken. The z values mentioned are from a point which is 2.1 inch away from a marked line on the stage.
However, the analysis concludes that either the beam radius changes too slowly to be profiled properly with given method of measurement or something else is wrong. Attaching the the z vs w(z) plot from this data and few fit plots.
I borrowed the following components from PSL lab to QIL lab
1. Mixer (Minicircuit, ZFM-3-S+)
2. RF amplifier (Minicircuit, ZFL-500LN)
3. IFR/Marconi 2023 A (# BD9020)
The light transmitted from both the cavities has been monitored while the cavities where locked (Vacav = 1.369 V, Vrcav = 5.7909 V) and beats on RF photodiodes where visible. Power on the three photodiodes PD-Rcav (North cavity), PD-Acav (South cavity) and PD-RF decreases of about 10% from its maximum value on PD-RF and about 5% on the others photodiodes periodically every ~7 minutes.
I also notice that:
PD-Rcav = ~60mV (+- 2/3%);
PD-Acav = ~170mV (+-2/3%);
3. Enabling/disabling the boost switch on the FSS box does not give any improvement;
4. Pressing the red botton (gain) on the FSS box neither;
In the same condition of locking the control signals of both PDH loop have been monitored too.
Here we can see that:
It is worth also monitoring the error signal while the cavities are locked and when they are not (with a triangular wave applied at the laser PZT).
On saturday a qualitative effect of the modulation produced by the EOM located in the PDH-north loop has been checked.
The goal was to have a look at the error signal of the PDH-north while the laser PZT was scanning frequecies around the two s-p TEM00 resonances. Because a that time I did not find the right error-signal connections on the FSS board (next elog will clarify where it is) I have demodulated the signal with an external mixer (and with a low pass filter) and monitored it. The picture shows the error-signal that we have with this setup:
In order to have a better understanding of the FSS electronic boards we decided to take
open loop transfer functions (OLTF) of the FSS North cavity loop. In the same time we took
the opportunity to verify that the broadband EOM is working. However this measurement will
be done again for a better quantitative analysis of the loop performances.
OLTF measured in the common path shows the shape due to the PZT and due to the EOM
paths as expected.The UGF has been pushed up to 170kHz. In the picture below we see the
traces corrisponding to the two different gain settings.
Network Analyzer FSS
RF -----------> EXC (Common)
R -----------> OUT2 (Common)
A -----------> OUT1 (Common)
These measurements have been done with the Switch Exc on located on FSS interface board
and with the following gain settings:
OLTF measured in the FAST path have the connections applied at the Fast and the switch EXC on
the front panel turned on. This measurement has been taken with "high gain settings" of the previous
measurement.In the following pictures can be seen the previous OLTF too (green/red).
RF -----------> EXC (Fast) (front panel switch Exc ON)
R -----------> OUT2 (Fast)
A -----------> OUT1 (Fast)
PLEASE note these are not the total OLTF (relative to their path) because is still missing the transfer function
between OUT1 and OUT2 in both the paths Common and Fast.
I would like to model this loop, I need to figure out the best way to do it.
As described in the elog entry n. 1577 we were not able to lock the PLL as has been described in elog n. 1570. I have started by playing with the two PDH gains of both loops (North and South) as this could have been causes of non tolerable noise in the PLL loop. I have also monitored the peaks described in entry n.1577 as we were suspicious for their preventing the PLL locking.
The PLL loop has been locked repeatedly after locking the cavities multiple times. This result has been achieved by setting the PLL gain on the SR570 at 20 (PLEASE NOTE I was not able to lock the PLL with any other gains settings).
The peakes of entry n.1577 are not preventing us to lock the PLL.
North FSS interface: Common gain =700; Fast gain = 450; PID = 4.451V;
South FSS interface: Common gain = 850; Fast gain = 250; PID = 0.5527V;
Beat frequency = 69.6 MHz;
However these are not the only allowable settings for the gain, but the PDH loop gains are crucial for the PLL locking. Later I am going to give a quantitative analysis for our PDH loops in order to have them in a more stable and/or less noisy locking point.
The goal of the TCN experiment is to measure the TC noise. This requires to lower down the noise level that we have at
the PLL output. I have decided to take noise measurement today in order to have a reference from the level we are starting with.
We should start to implement changes in order to lower the noise at the output and keep monitoring it.
I report a plot of the PLL noise in Vrms/sqrt(Hz) as it is not clear yet how to convert the units in Hz/sqrt(Hz). From older Elogs ID 889,
I see that there is available a calibration for the IFR 2023a, and this depends on the input range. I am not sure on what the input range is.
The measurement has been taken in dirrent frequency range in order to have a better resolution. For now, when we are at this level
of noise it is not worth it.
I have tried to lock the PLL with a FM deviation which is less than 300kHz but it was not possible. There is too much noise.
Target noise and noise taken sometimes in the past by Evan’s (in Hz/sqrt(Hz)).
Current measurements in Vrms/sqrt(Hz);
Beat freq = ~ 65 MHz;
FM deviation = 400kHz
gain = 20;
note: something happened after the first measurement, but at this point is not so important.
Data are on the lab control/home/data/20150912_PLL_noise
Measurement of the PLL noise have been taken at different PDH gains settings. Noise start to increase when the gain on the oh North loop
are below North: Common/Fast=500/300 and South: Common/Fast=420/420. Stays fairly the same with higher values.
The measurement are still in V/sqrt(Hz), but for now I use them only for comparison purpose.
Noise comparison at different gains: In the legend are the "values" of the gains (the numbers on the knobs)
Current measurements in Vrms/sqrt(Hz)
Data are on the lab control/home/data/20150912_PLL_noise/
I needed to have some data of the current OLG of one of the two PDH loop in order to fully model the electronic (+ optic setup).
The measured transfer function of the north loop has a unity gain around 150kHz at maximum gain settings.
The description of the measurement is in ID 1575. However now I know that the TF from Comm out1 to Comm out2 has gain of 1, this means
that we do not need to add this part of the electronic path when we measure the OLG.
---> At moment I did not succed yet with modeling the OLG.
it shows the TF of the north loop PDH at different value of the gains.
Data are on the lab control/home/data/20150910_NorthPDH_transfer_functions/
In order to have a better understanding of the gain associated to the Knobs on the interface PDH box, I took some measurements from TestInput to Out1Fast on the FSS field box
at different gain values for the common and fast knobs and measured the gain of the transfer function at DC. From fitting dB vs Knob counts
Common Knobs dB/100Counts = 2.35dB
Fast Knob: dB/100Counts = 2.49dB
I took more measurements than I needed of course, I did it to check while I was taking measurement that some unwanted electronic effect was happening.
Data are on the lab control/home/data/20150831_Knobs_calibration/
Before starting to work on reducing the noise, we decided to revise all subsystems in order to make sure that we know and understand the current state of each of them.
I made some noise measurements of the Marconi with two different carrier and different frequency range, obtaining the same results of I.D. PSL 816, 828, 874. Nothing
new at moment, so I will be very short and allocate the proceedure adopted in dokuwiki soon (https://nodus.ligo.caltech.edu:30889/ATFWiki/doku.php?id=main:experiments:psl:menu).
I resume the following results:
The way I have measured is the same as described in the above mentione ELOG I.Ds.
I have tried to measure the electronic noise from the Marconi but I am around 6 dB off. I would like to understand why...
I have used the same procedure that will be used to measure the photodiode electronic noise by disconnecting the feedback
in the PLL loop, measuring the noise of the control signal (Vfb) when no carrier is activated and by using the open loop TF = DGA (D Discriminator G gain A actuator):
Vfb = 20e-6;
%Vfb = sqrt(Vfb^2 + Vfb^2)
sr = 10^(6/20);
A = 71.1; % Hz/V
G = 2e3; %
D = DGA/A/G;
noise = Vfb/D/G
All data are in the lab computer: controls/home/data/20151010_PLL_TF and 20151011_Marconi noise
I add a PLL transfer function taken with the SR785: It shows a unity gain frequency of ~54kHz instead of ~27kHz.
This is due to the AG4395 50 Ohm impedence (+ the 50 Ohm impedence at the output of the SR560). A factor of 2 is missing.
New PLL noise measurements have been made with Fdev=1KHz and Fdev=10kHz at the input range of the Marconi.
There is an improvement of a factor ~10 for frequencies above 200Hz. However the two set of measurement show
the same noise in Hz/sqrt(Hz) between them. The lock of the PLL has been done keeping the same gain; when Fdev = 10KHz
the sr560 gain is 50, and when Fdev = 1kHz the SR560 gain is 500. The latter setting provokes an increase of the noise in V/sqrt(Hz)
which compensate for the reduction in the input range when Fdev goes from 10kHz to 1kHz. It is not clear to me what is happening.
Additionally it is not clear what is the “extra” noise that we see in the following noise budget as the sum does not match with the measurement.
1. The first plot shows the noise budget with the measurement taken in September 12 (2015). We see that the PLL noise is dominated by the
Marconi and photo thermal noise;
2. The second plot shows the noise budget with the PLL noise measured today with Fdev=1kHz; Here I do not understand
what is happening above ~200Hz; the total noise is off from what has been measured. I also should check the power on the photodiode and
see if it matches the level (2dB) I have used for the Marconi noise measurement.
3. The same as in point 2. but here we have Fdev=10kHz; here the total noise is closer to match the measurement,
4. The last plot is just the comparison between the PLL noise taken in the past September and the one measured today.
Please note: ISS is off
Data are stored in TCN lab computer: controlfb2/data/20151014_PLL_noise
Today the beat frequency was very difficult to find. Something changed, and I am not sure yet how to drive
the frequency of the beat. However the beat frequency is at 50MHz while so far it was at 64MHz. The PLL lock
more difficult and noisier.
Sometimes today or Yesterday we have increased the temperature of the lab (~ 2 degrees F).
lab computer/control fb2/home/data/20151016_PLL_noise_50MHz
The work done yesterday and today gave us a working ISS in the North path with a different optical setup.
Yesterday I have made several attempts to make the ISS working again in the North path. I have failed until I have noticed
the following setup for the AEOM:
lambda/2 ———p————> AEOM —-—> lambda/4 ——> PBS —> lambda/4 ———s—>EOM…..
Because I was not in agreement and I have asked Evan and I have figured out that this setup was not what it was meant to be.
Today I have replaced the second lambda/4 with a lambda/2 which made the ISS North working. Now the setup is:
lambda/2 ———p————> AEOM —-—> lambda/4 ——> PBS —>lambda/2———s—>EOM
The spectrum shows the intensity noise measured at the photodiode in transmission of the North cavity. Not clear what is happening
with the dark noise.
Loop setup: PD North ---> SR560 --->AEOM
SR560 setup: Gain at 5e4, HP=30Hz; LP=1kHz;
From Yesterday I have also measured:
1. TF from AEOM to PD North;
2. TF from PD North to PLL control signal (injecting noise on AEOM);
SOME of the THINGS that I think need TO BE DONE in a short term:
1. We should implement the second AEOM in the South path. My plan is to replace the EOM that currently is in the path with the
AEOM because the beam shape requirement are the same, so it will be faster, given that we will not use the PMC.
2. We need to check the shape of the beams at the modulators (ALL) in order to figure out if the (A)EOM requirements are respected.
This could be important for beam deformations which affect the mode matching at the cavities.
3. Need of a working dataviewer/IRcamera and a beam profiler;
4. It is very important that we spend some time in organizing the lab. The amount of time spent for looking for things is becoming an
obstacle for a proper lab work.
5. We also need to consider about the height of the table. Aside note: After two days in the lab my back is very painful.
6. Implement two Faraday Isolators in order to use only one polarization; for the moment I am even tempted to use a 50/50 BS, j
ust for the moment.
So far we have been locking the cavities on the resonance given by the S-polarized light.
Before the installation of the AEOM in the South cavity I wanted to have look to the beam profile along the paths. EOMs provokes distortion of the beam shape which may affect our mode-matching. It is important to keep the beam very small (200-500um diameter).
I think they are ok in the North path, a bit less good for the south path. Anyway I am going to use the beam as it is for the AEOM in the South path, replacing the EOM 21MHz used for the PMC with the AEOM that will be used for the ISS.
The pictures show the beam profile with the measurement done and with some ABCD matrix simulation for North and South path. They should come with an optical layout which I will make as soon as I will get OMNIGRAFFLE. I use inkscape but I will avoid that in order to be compatible with Rana and Aidan.
The AEOM has been installed in the South path replacing the EOM 21MHz used for the PMC. There is a high noise that I clearly see at the photodiode in transmission.
When I have placed the AEOM in the path I have decided to take the alignment of the previous EOM as reference. Not ideal because the reference should be the incoming beam. The beam is not parallel to the table and it was decided to be as less as possible invasive. The mode matching and the alignment gave at that time 20% of visibility (at each polarization). After the installation parameters where unchanged. Later I have improved the alignment bringing the visibility at 30% for both the polarizations. After that, when everything was in place I have easily locked the cavity but the power in transmition was showing a very high noise. I have spent all the day trying to twick the alignment because and servo loop gain, but we need to solve this before going further. My back does not allow me to proceed for today.
I have also noted that the South Laser which is labeed 2W laser has the lambda/4 and the lambda/2 rotated in a way that at the output of FI we had few mm. I am not sure if damping the power at the FI is a good thing.
In order to debug the intensity noise that I found after the installation of the EAOM in the South path I have removed it from the path. The ASD measured at the ISS photodiode
located in transmition of the South cavity is anyway higher than what we have in transmition at the north cavity. Tomorrow I will try to optimize the other two EOMs alignment located
in the South path and then implement the EAOM again. However I see a very high drifting in the beate note.
While I was debugging the "high" intensity noise at the ISSPD north i have noticed some scattering from the FI (north laser). It seems relatively well aligned but I did not want to touch it for now.
However I have measured the power emitted by the north laser and it is 306mW. The current setup provides ~99.3% dumping of the light into it. It means that only 2mW is at the output of the FI
while all the rest is dumped in the way out ---> I want to change it as soon as possible.
TO BE DONE
The laser setup will be changed in a way that the lambda/4 will maximize the linearization of the light (whatever angle is) and lambda/2 will maximize the power in transmition at the FI. A lambda/2 and a PBS
will be placed either before or after the FI in order to send 2mW to the rest of the setup. Now the question is to take care of the type of PBS because the damage threshold can be too low:
I consider the following two options:
1. PBS with damage threshold of 100W/cm^2 @(532nm) --> The minimum radius of the beam at 306mW is r ~ 628um (taking care of a factor 2 of safe margin and a factor 2 for 1064nm)
2. PBSO with damage threshold of 1MW/cm^2 @(1064nm) --> The minimum radius at the same 306mW is r ~ 4um;
I do not know the size of the beam. I do not have the optics to measure it and at moment I am not sure about previous measurements.
I have measured the power of the North laser vs the power on the display.
Power Display = 479mW; Measured P = 306mW
DC = 2.12A
T + 34.4965
Power Display = 68mW; Measured P=230mW
DC = 2.08A
T + 26.465
The EAOM is again in place but not the lambda/4 after it yet.
I found out that the intensity noise issue that I found after the implementation of the EAOM
is due to the lambda/4 implemented after it. I did not figured out what it is happening there.
At moment with the only EAOM there is no intensity noise issue.
(As reminder the lambda/4 is supposed to be rotated to produce circular light, splitting the light
in s and p at the output of a PBS located after it)
However I have optimized the setup of the lambda/4 soon after the laser. The light coming
out from the laser is optimized to be linear and the lambda/2 after it is rotated to produce 1mW
after the Faraday Isolator (for the moment). This setup will be changed with the use of PBSO to
dump the light.
Scusami :-(. D'ora in poi usero' solo PDF :-)!!!
Per favore, utilizzare solo PDF.
The EAOM has been implemented again in the South path. I still see an intensity noise effect in transmission, but it is much less than what I have seen from the previous days.
However the loop is suppressing most of it (but I am not happy about this). The figure shows the intensity noise and its suppression (Vdc = ~80mV).
The setup is the one that it is currently in use in the North path:
I have also tried a different setup with light entering in the EAOM at 45 degrees, but the loop does not show suppression of intensity noise. I do not explain why at moment.
Note: When the light entering the EAOM is p (the current setup) the light coming out of the modulator
is circular polarized. 15% of ‘p’ goes in ’s’. This is not happening in the north path where the light
remains mostly linear. I am not convinced that this EAOM is properly functioning.
I have measured the noise at the beat note with both the ISS servo activated —>
NO improvement compared to the past. It is actually a bit worse. However with the EAOM
in the South path I had to change sluggishly the PDH gain settings.
- I took two measurements with different Fdev at the Marconi (1kHz and 10kHz) and the noise
is the same mostly. It seems that the limitation at moment relies NOT in the Marconi noise.