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.

Quote:

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.

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:

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).

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:

With the loop open and the TTFSS interface set to LOCAL & TEST, we adjusted the temperature of the laser until we were close to TEM00 resonance.

Then we set the first switch from LOCAL to REMOTE.

This locked the loop.

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:

Set to LOCAL & TEST

Adjust the temperature so we were close to TEM00 resonance.

Scan through the two polarization TEM00 eigenmodes (separated by ~1MHz)

We can sit (a) outside the mode with big transmission (big mode), (b) in between the two modes or (c) outside the small mode.

We sit outside the big mode and then switch TEST to OFF to turn the loop on (I have been cursing about the naming conventions on these boxes for the last two days).

This locks the loop.

With the gains set locally.

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.

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:

All measurements taken to date are sifted through and the sum of PSD bins between 70 Hz to 600 Hz (excluding 60 Hz harmonics and region between 260 Hz to 290 Hz (Known bad region)) is summed. The least noise measurement is chosen then.

If time-series data is available (which at the moment is available for lowest noise measurement of May 29^{th} taken at 1 am), following is done:

Following steps are repeated for the frequency range 70 Hz to 100 Hz and 100 Hz to 600 Hz with timeSegement values 5s and 0.5s respectively.

The time series data is divided into pieces of length timeSegment with half overlap.

For each timeSegment welch function is run with npersegment equal to length of time series data. So each welch function returns PSD for corresponding timeSegement.

In each array of such PSD, rebining is done by taking median of 5 consecutive frequency bins. This makes the PSD data with bin widths of 1 Hz and 10 Hz respectively.

The PSD data for each segement is then reduced by using only the bins in the frequency range and removing 60 Hz harmonics and the above mentioned bad region.

Logarithm of this welch data is taken.

It was found that this logarithm of PSD data is close to Gaussian distributed with a skewness towards lower values. Since this is logarithm of PSD, it can take both positive and negative values and is a known practice to do to reach to normally distributed data.

A skew-normal distribution is fitted to each frequency bin across different timeSegments.

The fitted parameters of the skew-normal distribution are stored for each frequency bin in a list and passed for further analysis.

Prior distribution of Bulk Loss Angle is taken to be uniform. Shear loss angle is fixed to 5.2 x 10^{-7} from Penn et al..

The Log Likelihood function is calculated in the following manner:

For each frequency bin in the PSD distribution list, the estimated total noise is calculated for the given value of bulk loss angle.

Probability of this total estimated noise is calculated with the skew-normal function fitted for each frequency bin and logarithm is taken.

Each frequency bin is supposed to be independent now since we have rebinned, so the log-likelihood of each frequency bin is added to get total log-likelihood value for that bulk loss angle.

Bayesian probability distribution is calculated from sum of log-likelihood and log-prior distribution.

Maximum of the Bayesian probability distribution is taken as the most likely estimate.

The upper and lower limits are calculated by going away from most likely estimate in equal amounts on both sides until 90% of the Bayesian probability is covered.

Final result of CTN experiment as of now:

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.

Final result of CTN experiment as of June 4th 9 am:

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.

Adding Effecting Coating Loss Angle (Edit Fri Jun 5 18:23:32 2020 ):

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.

Final result of CTN experiment as of June 11th 2 pm:

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.

Adding Effecting Coating Loss Angle (Edit Fri Jun 5 18:23:32 2020 ):

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:

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 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.

Final result of CTN experiment as of June 15th 5 pm:

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.

Adding Effecting Coating Loss Angle (Edit Fri Jun 5 18:23:32 2020 ):

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:

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 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.

Final result of CTN experiment as of June 23 5 pm:

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.

Adding Effecting Coating Loss Angle (Edit Fri Jun 5 18:23:32 2020 ):

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:

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 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.

Final result of CTN experiment as of June 24 9 pm:

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.

Adding Effecting Coating Loss Angle (Edit Fri Jun 5 18:23:32 2020 ):

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:

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 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.

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)

Major Questions:

What are the known reasons for the frequency dependence of the loss angle?

Do we have any prior knowledge about such frequency dependence which we can put in the analysis as prior distribution?

Is this method just overfitting our measurement data?

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?

Is it worth it to fix this path?

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).

IP2 Characterization

Generally the second order intercept product increases in strength proportional to the strength of modulation frequency with some power between 1 and 2.

The modulation frequency strength where the intermodulation product is as strong as the original modulation frequency signal is known as intercept point 2 or IP2.

For SR560 characterization, I sent modulation signal at 50 kHz and set intermodulation frequency to 96 Hz.

The script sends two tones at 50 kHz and 50khz -96 Hz at increasing amplitudes and measured the FFT bin around 96 Hz with dinwidth set by user. I used 32 Hz bin width.

In attachment 1, you could see that beyond 0.1 V amplitude of modulation signal, the intermodulation product rises above the instrument noise floor.

But it weirdly dips near 0.8 V value, which I'm not sure why?

Maybe the modulation signal itself is too fast at this amplitude and causes some slew rate limitation at the input stage of SR560, reducing the non-linear effect downstream.

Usually one sees a straight curve otherwise and use that to calculate the IP2 which I have not done here.

IMD2TF Characterization

First of all, this is a made up name as I couldn't think of what else to call it.

Here, we keep the amplitude constant to some known value for which intermodulation signal is observable above the noise floor.

Then we sweep the modulation frequency and intermodulation frequency both, to get a 2-dimensional "transfer function" of signal/noise from higher frequencies to lower frequencies.

Here I kept the source amplitude to 0.4V and swept the modulation frequency from 10kHz to 100kHz and swept the intermodulation frequency from 96 Hz to 1408 Hz, with integration bandwidth set to 32 Hz.

I'm not completely sure how to utilize this information right now, but it gives us an idea of how much noise from a higher frequency band can jump to a lower frequency band due to the 2nd order intermodulation effect.

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

What is birefringence noise?

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).

The temperature fluctuations cause length fluctuations in the coating layers (through the coefficient of thermal expansion)

The length fluctuations cause stress fluctuations in the coating layers (through Young's modulus Y).

The stress fluctuations get converted into refractive index fluctuations through the photoelastic effect (through photoelastic tensor)

This induces refractive index fluctuations that are different for the fast and slow axes. The difference between the two fluctuations causes a phase shift in reflected light from each layer. That's why this can be birefringence noise. In an unstressed isotropic material, this pathway should not exist.

Is this different from thermo-refractive noise?

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.

Calculation:

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.

Possible explanations:

Calculaiton error in my code. I'll double check tomorrow.

Somehow the thermorefractive noise already takes into account the birefringent noise, through the coefficient of thermo-refraciton that we use as seed in our thermo-refractive noise calculation. This would explain how the witnessed themo-optic cancellation was achieved.

Meyer et al. is calculating birefringent noise for the substrate. Maybe the tensorial calculations are different for coatings.

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.

Changes in the calculation:

I made two changes in total:

I'm using original coefficients of thermal expansion for each layer instead of the "effective" coefficients used in calculations of thermo-optic noise as per Evans et al. PRD 78, 102003 (2008)

I removed the use of young's modulus and the crystal's elasticity tensor.

So now, the noise calculation is as follows:

The temperature fluctuations cause isotropic strain fluctuations in the coating layers related through coefficient of thermal expansion

The strain fluctuations cause changes in the refractive index of the layers through photoelastic tensor

In the last step above, I assumed isotropic bulk strain in the layers (which is expected for this cubic lattice), thus

The product of the above two numbers give the coefficient of thermo-photoelastic effect as:

I averaged this coefficient over all coating layers weighted by their thicknesses.

The noise contribution comes same as coatTR term as they both are channels causing dn/dT.

Notes:

The above calculation does not take into account any birefringence in the layers that could be caused by this effect. In fact, the cubic crystal symmetry of GaAs does not allow for birefringence to occur in usual formalism and the only way it could happen is due to a large strain in one direction breaking the symmetries. Thus, I would not call this noise "birefringence noise", but it is a credible noise source in it's own right.

Note that the themo-optic cancellation is only partially happening now, but the thermo-optic noise is still much less than the simple quadrature sum of the noises. We can maybe check back our measurements in our previous paper if the measured photothermal transfer function allows this.

Maybe this noise source is not perfectly coherent with coatTE and coatTR and needs to be added a bit differently.

About the plot:

The trace marked "Coating Thermo-Optic" is a coherently summed noise of coatTR, coatTE, and coatTPE.

The trace marked "Coating Thermo-elastic + Thermo-refractive" is what we previously used to calculate as thermo-optic noise.

"Measured Beat" is the best measurement we made and is a median over 50 lowest noise measurements made in June of 2020.

"Coating Brownian" trace is calculated using bulk loss angle value of 4.878e-5 which was measured by Penn et al. in indirect measurement.

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.

Notes:

The estimated transfer function without TPE (as was estimated back then) does match well with the measured transfer function on the south cavity below 200 Hz.

However, the north cavity measurement did not match well.

The estimated transfer function with TPE (green) is in between south and north measurements at least in magnitude above 200 Hz.

However, the phase of estimated transfer functions (with or without TPE) do not match well with any of the measurements. This phase discrepancy is worrisome.

Looking at these estimated transfer functions and measured transfer functions, which model do you think explains the measured data better?

Updated noise budget:

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.

Quote:

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.

Major difference

Fejer argues that free body thermal expansion does not generate any strain, and it is only when the substrate is present to counteract with it, that such strain is generated.

Hence, the calculation goes as: thermal expansion -> stress in presence of substrate -> strain -> photoelastic effect.

So instead of the simple contribution for photoelastic tensor and thermal expansion that I take, the term is:

This gives an effective (averaged with layer thickness weighting) coefficient of thermal photoelasticity of 1.45e-5 K^{-1} instead of 4.30e-5 K^{-1} from my calculations. That's a reduction by a factor of roughly 3.

Updates

Attached is the photothermal transfer function calculated with TPE contribution as calculated by Fejer. This makes the situation bit more messy on what to trust.

I updated the noise budget with two new noise traces, the thermo-photoelastic contribution as calculated by Fejer and the total thermo-optic noise as calculated by Fejer.

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.

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:

The PD-Rcav trace is noisier than PD-Acav trace;

The mean voltage values of the two photodiodes are way different:

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:

the PDH-North loop is noisier;

A step occurred in the cian trace (at the second division) without causing variation in the transmitted light;

Enabling/disabling the boost switch on the FSS box does not give any improvement;

Pressing the red botton (gain) on the FSS box neither aside from the fact that when the botton is kept pushed it slightly "cleans" the trace (not quantified);

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).

Control signal of PDH-North (yellow); Control signal of PDH-South (cian); PD-RF power (pink)PD-RFPD-Rcav (lower) , PD-Acav (upper)

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:

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.

Conclusion

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.

Some settings:

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.

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

we get:

North:

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

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 calibration of the Marconi (South) for convertion in Hz units is : 711Hz/V;

Noise coming from Marconi can be reduced by lowering the input range independently from the carrier;

PLL open loop TF measured;

Electronic noise from Marconi measured.

--------------------->

The way I have measured is the same as described in the above mentione ELOG I.Ds.

Electronic noise:

=======

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

Data:

=====

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.

Quote:

Summary

=========

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 calibration of the Marconi (South) for convertion in Hz units is : 711Hz/V;

Noise coming from Marconi can be reduced by lowering the input range independently from the carrier;

PLL open loop TF measured;

Electronic noise from Marconi measured.

--------------------->

The way I have measured is the same as described in the above mentione ELOG I.Ds.

Electronic noise:

=======

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

Data:

=====

All data are in the lab computer: controls/home/data/20151010_PLL_TF and 20151011_Marconi noise

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.

NOTE:

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.

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).

Setup

=====

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.