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R&D (FilterCavity)
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YuhangZhao - 22:50, Wednesday 16 May 2018 (781)Get code to link to this report
Measurement of contrast of MZ after installation of mode cleaner

After the installation of mode cleaner, we measure the contrast again. This method is like this

We use the high voltage driver of MZ as an offset adder. By giving different offset to it, we can have almost all light transmit or almost no light transmit.

Then we record the highest peak hight while almost all light transmit. Record the highest peak hight while almost no light transmit(actually, this is the time when TEM00 almost vanishes).

We got result like attached picture 1.

However, I don't know if this is the right calculation method. (Maybe I should do like Marc did for FC scan, intergrate all the peaks in-between a FSR?)

Images attached to this report
781_20180516155005_contrast.png
R&D (FilterCavity)
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YuhangZhao - 21:45, Wednesday 16 May 2018 (780)Get code to link to this report
Comment to Comparison of old and new servo (Click here to view original report: 760)

Corresponding to the comment of Eleonora, the bandwidth of filter cavity for infrared is 114Hz but not 55Hz. Then I think we can explain the result (almost).

bandwidth=FSR/Finesse=500000/4355=114

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YuhangZhao - 21:40, Wednesday 16 May 2018 (779)Get code to link to this report
Comment to FC scan (Click here to view original report: 776)

According to the signal we send to DDS board, the modulation frequency of we give to EOM is 15.2MHz. The FSR is 0.5MHz.

15.2/0.5=30.4

So we will have additional 0.4 of FSR. This exactly explains the result we have on the oscilloscope.

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YuhangZhao - 21:29, Wednesday 16 May 2018 (778)Get code to link to this report
The misalignment effect brought by AOM modulation

Participaint: Marc, Matteo

We used PSD(position sensitivite detector) to detect the beam dittering incured by AOM modulation.

The modulation information of AOM: 2MHz(pk-pk), 10Hz(50ms for a half period). This gives us AOM scanning velocity of 40MHz/s. (this is really a fast scan!)

We put PSD inbetween the telescope after AOM. Although this gives us diffcuilty, but this is the only proper place to put PSD. See attached picture 1 to know how can we propagate the dittering back through the lens.

The calibration we use the result of entry 276. It is 0.0071[m/V]. We use it to transfer the voltage change of PSD to the beam dittering in meter.(Then we got "x" in attached picture 1)

 

Conclusion: See attached picture 2. We have 10.7e-6[rad] angle change corresponding to frequency change of 2MHz.

Images attached to this report
778_20180516142813_474292500.jpg 778_20180516142827_aom.png
Comments related to this report
YuhangZhao - 22:54, Thursday 24 May 2018 (786)

The serial number of AOM is MT110-A1.5-VIS. I checked the manual today. The best input modulation voltage is 1V. There is also one information about the seperation angle's relationship with wavelength. However, the manual is so sketchy that I cannot understand clearly. 

Bad thing: The operating manual is only availabe for someone has account of that company!

YuhangZhao - 14:34, Monday 28 May 2018 (790)

I find there is a mistake in the calculation, see attached picture 1. I used a wrong unit for a number. After the correction, I plot the result again(see attached picture 2)

Now the error on AOM is 1.07mrad.

R&D (FilterCavity)
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YuefanGuo - 18:50, Wednesday 16 May 2018 (777)Get code to link to this report
Couple beam into the fiber progress

Participants: Yuhang, Yuefan, Matteo

Today we tried again to couple the laser beam into the fiber.

But first, we moved the two steering mirrors and collimator closer to each other. Since the further the distance, every time the movement on the collimator input is larger.

Then by adjusting the two steering mirrors and the x, y screws of the collimator, we are able to reach 10V(maximum) on the photodiode which corresponds to 5.5mW which is even larger than the input laser power.

We found out this is because the oscilloscope has a high impedance which will amplify the power. So we add a 50ohm resistor and we could get 10% beam power coupled into the fiber and after that, it became difficult to improve more.

According to Thorlab website, this collimator should have 2.09mm at the output, but now with a 200mm focal length just after the laser output, we have a beam at the collimator around 400um in radius.

So we did a bit simulation, in order to get the right size of the beam we should combine one -100mm and one 200mm lens. We will try to change the setup tomorrow.

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MarcEisenmann - 17:52, Wednesday 16 May 2018 (776)Get code to link to this report
FC scan

Participants : Marc, Yuefan, Yuhang

 

Here is presented the FC scan.

Compared to entries 771 and 775 few corrections have been implemented.

There was a mistake in the first estimation of the mode-mismatch (2 order 0 have been taken into account as well as the 2 sidebands).

There was another mistake done while doing the fitting : Now we used 1 Airy function defined by the 2 order 0 and then only changed the normalization value (T0).

We obtain the following results :

r : 0.9992

FSR : 125 s (the AOM frequency modulation have been changed but we still have FSR=500 kHz)

T0 (normalization factor) : 3.6374    0.0164    0.0604    0.0274    0.0119    0.0262    3.3774

x0 : 65.62 89.52 115.155 115.208 164.78 164.88 190.62

The mode-mismatch was evaluated using the ratio sum of higher order modes T0/ fundamental mode T0.

This leads to a mode-mismatch of 3.9%.

 

Few things to notice :

The position of the sidebands needs to be confirmed

While modulating the AOM frequency to realize the scan, the beam exiting the AOM is tilted which leads to the different heights of the fundamental order peak (?)

Images attached to this report
776_20180516105215_fcscan.png
Comments related to this report
YuhangZhao - 21:40, Wednesday 16 May 2018 (779)

According to the signal we send to DDS board, the modulation frequency of we give to EOM is 15.2MHz. The FSR is 0.5MHz.

15.2/0.5=30.4

So we will have additional 0.4 of FSR. This exactly explains the result we have on the oscilloscope.

MarcEisenmann - 17:38, Monday 21 May 2018 (783)

The total amount of power not coupled inside the FC is composed of the mode-mismatch and the sidebands.

The sidebands maximum are respectively : 0.1462 and 0.1422 mV (this takes into account the background value).

This means that 11.84% of the light is not coupled inside the FC.

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MarcEisenmann - 23:10, Tuesday 15 May 2018 (775)Get code to link to this report
FC scan

Participants : Marc, Yuefan, Yuhang

 

We did another FC scan.

Using 2 differents scales to see the fundamental mode and the other higher order modes we could have a quite good precision on every order peak.

 

By fitting every resolvable peak by an Airy function and using the ratio Area of fundamental mode / Area of fundamental mode + higher order mode,

We found that 95.24% of the light was coupled into the fundamental mode of the FC.

Images attached to this report
775_20180515160708_fcscanbigscale.png 775_20180515160713_fcscansmallscale.png
R&D (FilterCavity)
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MarcEisenmann - 16:38, Tuesday 15 May 2018 (774)Get code to link to this report
Fiber alignment

Participants : Marc, Yuefan, Yuhang

Today we continue the alignment of the collimator and the fiber.

Because last time wasn't successful, we decided to use a 1550 nm fiber laser and try to align it at the opposite of its use (inject the laser with the fiber and try to align the output of the collimator).

If we could align it using an IR card, it was not possible to use a power meter nor the beam profiler due to this wavelength.

 

We decided to go back the first way of aligning this collimator (laser at the input of the collimator and check the power at the output of the fiber).

We decided to use 2 sterring mirrors (actually one mirror and one 98/2 BS).

We got the following power

output of the Auxiliairy laser : 0.873A -> 29.7mW

After 1 mirror + BS (98/2) : 28.7mW

After collimator : 28.7mW

After the fiber 4.5 uW

We can see some light at the output of the fiber (on IR card, power meter and fiber photodiode).

One problem is that as soon as we touch the fiber or the collimator, we see power fluctuations.

This means that between each time we screw we need to wait for few seconds...

It should be useful to find a way to fix the fiber.

Images attached to this report
774_20180515093753_powerfluctuationswithtouch.png
R&D (FilterCavity)
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MarcEisenmann - 11:57, Monday 14 May 2018 (772)Get code to link to this report
Coherence between input laser power and reflection from the FC

Participants : Marc, Yuefan, Yuhang

 

In order to study the coherence between the input laser IR power and the reflection from the filter cavity, we installed a PBS on the IR injection path and a photodiode at its reflection.

 

We studied 3 cases :

1) Both IR and Green resonants inside the FC (greenir.png)

2) No beam resonant inside the FC by misaligning the EM and making sure that there was no attempt to lock on the servo (no.png)

3) Only Green resonant inside the FC by detuning the EOM frequency (green.png)

 

We were expecting to find similar results between the cases 2 and 3 which is not the case.

We were expecting to find coherence at high frequencies due to power fluctuations only but it only appears in the only green resonant case (maybe the coherent length of IR is too short?)

The no beam case shows clearly a 200Hz peak (ventilation of the clean booth) and a 600Hz peak (turbo pump of BS).

Images attached to this report
772_20180514044918_green.png 772_20180514044931_greenir.png 772_20180514044955_no.png
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R&D (FilterCavity)
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EleonoraCapocasa - 08:04, Monday 14 May 2018 (773)Get code to link to this report
Comment to Comparison of old and new servo (Click here to view original report: 760)

The error signals for green and infrared account for the closed loop laser frequency noise filtered by the pole of the cavity  (which is different for green and IR).

In the fist attached plot, the error spectra has been divided for the corrispondig pole in order to go back to the close loop laser frequency noise.

freq nois =  err sig * ( sqrt( 1+(f/f0)^2)     with f0 = 55 Hz for IR and 1.45 kHz for green

The two curves obtained shoud be coincindents. The discrepancy (about a factor 2.5) suggests that there is maybe an issue with the calibration.

Images attached to this comment
773_20180514005813_irvsgreefn.png
R&D (FilterCavity)
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MarcEisenmann - 21:25, Saturday 12 May 2018 (771)Get code to link to this report
FC scan

Participants : Marc, Yuefan, Yuhang

 

Last Friday we did a scan of the FC for IR.

 

[ In order to obtain a good beam position, the BS pitch correction is close to saturation (9V). It might be useful to move some picomotors in order to have a better beam position and a lower correction]

The scan was performed by modulating the frequency sent to the AOM.

We used the following parameters : 1 MHz for the half peak-to-peak value over a half-period of 2 mHz (the lowest value permitted).

[The "half" values are used as they are what we provided to the AOM frequency generator]

We obtained the result presented in the first picture (FC_scan.png).

Few points to notice :

there is a symmetry at the second peak (if we zoom in we can distinguish 2 peaks)

The vertical scale was not good enough to have a good resolution of the small peaks -> on Monday we will do another scan with 2 scales : 1 to resolve the fundamental mode and another one for the higher order modes.

 

Anyway, as a preliminary analysis of this FC scan, we fitted the first 2 peaks with an Airy function (fig. FCscan_fit) which gives us a FSR of 62.57 s.

By doing :  1MHz / 2mHz * FSR[s] = FSR[Hz] we find a FSR = 500.560 kHz which is in good agreement with the logbook entry #668.

Another preliminary analysis was trying to estimate the losses due to coupling with higher order modes.

The fundamental peaks were fitted with a gaussian function while the (poor resolved) small peaks were fitted by triangles.

By comparing the ratio between the areas of fundamental mode / fundamental mode + higher order modes, we found a mode-mismatch of roughly 10%.

This analysis will be more precisely performed on Monday.

Images attached to this report
771_20180512142108_fcscan.png 771_20180512142123_fcscanfit.png
R&D (FilterCavity)
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EmilSchreiber - 12:11, Saturday 12 May 2018 (769)Get code to link to this report
Mode matching telescope for green mode cleaner finished
Participants: Yuhang, Matteo, Yuefan, Marc, Emil
 
We designed and set up the mode-matching telescope for the green mode cleaner. First measurements indicate a very good mode matching and alignment to the mode cleaner.
 
To get good starting parameters we did a new beam profiling measurement of the green beam after the first 50:50 beamsplitter. An old measurement from last year (entry 613) could not be used, because the beam was changed in the meantime. The measured parameters varied quite significantly between minor and major axis (probably a measurement artefact, not strong indication of real astigmatism), but we could confirm that this does not significantly effect the mode matching solution. As a result we get a waist of 26µm roughly 10cm before the 50:50 beamsplitter (7cm after the EOM).
 
We did the beam fitting and and telescope optimization with the MATLAB toolbox 'a la mode'. There are several possible solutions with the available lens set and we chose one based on overall beam size (not too large) and by minimizing the sensitivity of the mode overlap to the lens position.
 
While setting up the beam path we reconfirmed the beam parameters with another profiling measurement while one of the two lenses was installed.
 
With the mode cleaner in place we performed a mode scan to do the final alignment. Both paths of the Mach-Zehnder interferometer were aligned individually to achieve very good contrast (first measurement indicates >95%).
 
Based on the mode scan, the final misalignment is <1% and the 2nd-order mode mismatch is about 1%.
 
During our work we had noticed that the beam shape was not quite Gaussian (some interference fringes and indication of clipping). This was traced back to a point before the first green Faraday, so it probably originates inside the SHG. We still need to see wether this significantly affects the mode cleaner transmission. This could be done by measuring the reflected power on and off resonance.
During our work we had noticed that the beam shape was not quite Gaussian (some interference fringes and indication of clipping). This was traced back to a point before the first green Faraday, so it probably originates inside the SHG. We still need to see wether this significantly affects the mode cleaner transmission. This could be done by measuring the reflected power on and off resonance.Participants: Yuhang, Matteo, Yuefan, Emil
 
 
We designed and set up the mode-matching telescope for the green mode cleaner. First measurements indicate a very good mode matching and alignment to the mode cleaner.
 
To get good starting parameters we did a new beam profiling measurement of the green beam after the first 50:50 beamsplitter. An old measurement from last year (entry 613) could not be used, because the beam was changed in the meantime. The measured parameters varied quite significantly between minor and major axis (probably a measurement artefact, not strong indication of real astigmatism), but we could confirm that this does not significantly effect the mode matching solution. As a result we get a waist of 26µm roughly 10cm before the 50:50 beamsplitter (7cm after the EOM).
 
We did the beam fitting and and telescope optimization with the MATLAB toolbox 'a la mode'. There are several possible solutions with the available lens set and we chose one based on overall beam size (not too large) and by minimizing the sensitivity of the mode overlap to the lens position.
 
While setting up the beam path we reconfirmed the beam parameters with another profiling measurement while one of the two lenses was installed.
 
With the mode cleaner in place we performed a mode scan to do the final alignment. Both paths of the Mach-Zehnder interferometer were aligned individually to achieve very good contrast (first measurement indicates >95%).
 
Based on the mode scan, the final misalignment is <1% and the 2nd-order mode mismatch is about 1%.
 
During our work we had noticed that the beam shape was not quite Gaussian (some interference fringes and indication of clipping). This was traced back to a point before the first green Faraday, so it probably originates inside the SHG. We still need to see wether this significantly affects the mode cleaner transmission. This could be done by measuring the reflected power on and off resonance.
Images attached to this report
769_20180512050823_beamprofiling.png 769_20180512050843_telescopeplot.png 769_20180512050955_modescan.gif
Non-image files attached to this report
KAGRA MIR (Absorption)
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ManuelMarchio - 00:06, Thursday 10 May 2018 (767)Get code to link to this report
Simulations of the bulk reference sample and the sapphire for different thicknesses

We are still investigating the reason for the calibration problem in the bulk absorption that gives a factor of 3 between the measurements done with our experiment and at LMA or Caltech on the same samples.

In order to understand how the interaction length (the crossing area of probe and pump) affects the measurement, I made some simulations of the scan changing the thickness of the samples and keeping constant the absorption/cm rate.

Images attached to this report
767_20180509170500_bulksample6338mm.png 767_20180509170508_sapphiresample6338mm3mm616mm.png
KAGRA MIR (Absorption)
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ManuelMarchio - 23:49, Wednesday 09 May 2018 (764)Get code to link to this report
Problem with the SURFACE reference sample

Before measuring the coating absorption of LMA samples and crystalline coating I wanted to test the surface reference sample in order to check if it had any damage. So I made a map of it.
The map shows a regular pattern of absorption that oscillates by a factor of 2.

I made some checks to understand the nature of this strange behavior. I report the steps in the attached pdf slides.

I'm going to look for a better alignment

Images attached to this report
764_20180509132306_surfrefproblem.png 764_20180509164639_43.png 764_20180509164644_30.png 764_20180509164648_44.png 764_20180509164655_26.png 764_20180509164701_35.png 764_20180509164706_45.png 764_20180509164714_00.png 764_20180509164720_57.png 764_20180509164725_53.png 764_20180509164730_18.png 764_20180509164738_36.png 764_20180509164809_41.png 764_20180509164813_43.png
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R&D (FilterCavity)
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MarcEisenmann - 17:55, Wednesday 09 May 2018 (765)Get code to link to this report
Alignment of the collimator

Participants : Marc, Yuefan

 

To have a lower beam divergency (so easier working conditions) we installed a lens f=200mm 6 cm after the output of the auxiliary laser 1.

We also increased the laser power to 44.3 mW (0.9 A for the pump).

By carefully aligning the lens and the collimator vertical and transversal position (X and Y) we obtain a good aligment on this 2 directions.

As the beam is quite diverging after the collimator, this was done checking the beam positions around 20 cm after the collimator.

 

By unscrewing the 3 screws moving the Z position of the collimator (its distance to the laser),

we reach a power of 44.1 mW at the output of the collimator meaning 99.55% transmission of the collimator.

This seems good enough to go on the others steps (installation of the fiber, add another collimator at the output of this fiber and check the beam size at its output).

We also found another holder for the second collimator meaning we have all the needed components for the next steps.

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YuhangZhao - 11:17, Wednesday 09 May 2018 (763)Get code to link to this report
Comment to Comparison of old and new servo (Click here to view original report: 760)

Here I attach the rms integration of the four error signal curves.

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763_20180509041738_rms.png
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MarcEisenmann - 23:33, Tuesday 08 May 2018 (762)Get code to link to this report
Auxiliary Laser 1 installation and pre-alignment of the collimator

Participants : Marc, Yuefan

 

Auxiliary Laser 1

Today we installed the auxiliary laser 1 on the squeezer bench.

Compared to the lens on the IR path far from the FC, its output port is at 14 holes (vertically toward FC) and 4 holes to the left (toward the main) laser.

This position seems convenient to avoid to twist its power supply while letting some space for the future optics to be installed.

Using 2 mm spacers, the beam height at the output of this laser is 7.5 mm.

 

Pre alignment of the collimator

Because of the divergency of the laser and the divergency of the not-aligned collimator, the beam at the output of the collimator is quite diverging.

In order to follow the alignment proposed by Thorlabs, we need to measure the output power of the collimator and incremantally find its maximum.

The powermeter had to be placed few cm after the collimator making this task quite painfull...

We could reach an output power of 10.35 mW (compared to the 11.3mW of the laser). However, a sad mis fixation of the collimator post made it moved quite a bit.

Tomorrow we will add a converging lens between the laser and the collimator in order to have a lower divergency of the beam and better work condition.

Images attached to this report
762_20180508163247_20180508172616.jpg
R&D (FilterCavity)
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YuhangZhao - 23:03, Tuesday 08 May 2018 (761)Get code to link to this report
Mach-Zehnder installation simulation

After the test of Mach-Zehnder, we need to install it in reality. But we change the original design. Thanks to the optocad code of tomuta-san, I revise it and do the simulation.

 

We have considerations:

1. The stable lock of filter cavity. We decided to use MZ only for the Mode cleaner and OPO. 

2. The focal length of lense is limited. So I asked yuefan to give me the list of lense we have. I select focal length of 50mm and 75mm

3. We need to adjust the position of these two lenses to have a good mode matching.

 

The change can be seen in the attached picture 1. We will start to install it tomorrow.

Images attached to this report
761_20180508160323_benchchange.png
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YuhangZhao - 19:19, Tuesday 08 May 2018 (760)Get code to link to this report
Comparison of old and new servo

The result of previous comparison is wired. So we decide to change back the old servo and measured error signal again. At the same time, we also did calibration again.

Note here, the calibration method for green likes entry 750. The only difference is that we consider frequency much higher than unity gain frequency. Then we got result like attached picture 1 and 2.

The calibration for infrared got from attached picture 3 and 4. For the problem of saturation, we changed the demodulation phase.

Finally we got result as attached picture 5. We can make them overlap by multiplying a factor of 11 like picture 6.

As a result, we find the new servo make noise level lower than before. So now I put back the new servo, we can get the green transmission as 1.5V and very stable like picture 6.

Images attached to this report
760_20180508121118_greenca.png 760_20180508121133_greencafinal.png 760_20180508121341_calibrationinfrared.png 760_20180508121357_infraredca.png 760_20180508121525_comparison.png 760_20180508121538_overlap.png 760_20180508121949_721319689.jpg
Comments related to this report
YuhangZhao - 11:17, Wednesday 09 May 2018 (763)

Here I attach the rms integration of the four error signal curves.

EleonoraCapocasa - 08:04, Monday 14 May 2018 (773)

The error signals for green and infrared account for the closed loop laser frequency noise filtered by the pole of the cavity  (which is different for green and IR).

In the fist attached plot, the error spectra has been divided for the corrispondig pole in order to go back to the close loop laser frequency noise.

freq nois =  err sig * ( sqrt( 1+(f/f0)^2)     with f0 = 55 Hz for IR and 1.45 kHz for green

The two curves obtained shoud be coincindents. The discrepancy (about a factor 2.5) suggests that there is maybe an issue with the calibration.

YuhangZhao - 21:45, Wednesday 16 May 2018 (780)

Corresponding to the comment of Eleonora, the bandwidth of filter cavity for infrared is 114Hz but not 55Hz. Then I think we can explain the result (almost).

bandwidth=FSR/Finesse=500000/4355=114

Matteo Barsuglia - 05:58, Friday 18 May 2018 (782)

I think there is a factor 2 missing in the formula: the pole of the cavity is FSR/(2*F) = 500000/(2*4355) = 57 Hz

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EmilSchreiber, YuhangZhao - 10:06, Tuesday 08 May 2018 (759)Get code to link to this report
Mach-Zehnder control loop design and testing
We finished and tested the electronics for the Mach-Zehnder controller. The loop performs well with a control bandwidth of 3.2kHz. We will need to further investigate whether the resulting amplitude stability of the transmitted green beam is sufficient.
 
The servo electronics are designed to give a simple 1/f open-loop transfer function. We use the fact that the PZT driver already includes a low-pass filter with a corner frequency of 77Hz (see entry 585). Additionally, the servo includes a low-frequency integrator to give infinite gain at DC, which can be switched off for lock acquisition.
 
We found that a mechanical resonance at 12.4kHz was limiting the achievable unity-gain frequency. Including a low-Q notch filter for this frequency allows for a slightly higher UGF, but it is then limited by another mechanical feature at 4.9kHz. There is yet another instability at around 600Hz, but this only occurs if the gain is set too low and can thus be completely avoided.
 
I measured noise spectra of the error signal for the in-loop case, free running noise (set to the same operating point by hand) and dark noise (green beam path blocked). The residual in-loop noise is dominated by features at 600-1000Hz. At these frequencies there is currently not much loop gain to suppress the noise. The dark noise is surprisingly high, in particular at low frequencies. We will have to look into this further.
 
Things to check:
  • Can we identify the 4.9kHz resonance and maybe reduce it mechanically (e.g. by tightening screws)?
  • Are the 600Hz noise features actual amplitude fluctuations coming from the SHG or are they added by the Mach-Zehnder? (This can be tested by putting the PD before Mach-Zehnder.)
  • Is the low-frequency dark noise caused by ambient light or electronics? Can it be reduced?
- Is the low-frequency dark noise caused by ambient light or electronics? Can it be reduced?We finished and tested the electronics for the Mach-Zehnder controller. The loop performs well with a control bandwidth of 3.2kHz. We will need to further investigate whether the resulting amplitude stability of the transmitted green beam is sufficient.
 
The servo electronics are designed to give a simple 1/f open-loop transfer function. We use the fact that the PZT driver already includes a low-pass filter with a corner frequency of 77Hz. Additionally, the servo includes a low-frequency integrator to give infinite gain at DC, which can be switched off for lock acquisition.
 
We found that a mechanical resonance at 12.4kHz was limiting the achievable unity-gain frequency. Including a low-Q notch filter for this frequency allows for a slightly higher UGF, but it is then limited by another mechanical feature at 4.9kHz. There is yet another instability at around 600Hz, but this only occurs if the gain is set too low and can thus be completely avoided.
 
I measured noise spectra of the error signal for the in-loop case, free running noise (set to the same operating point by hand) and dark noise (green beam path blocked). The residual in-loop noise is dominated by features at 600-1000Hz. At these frequencies there is currently not much loop gain to suppress the noise. The dark noise is surprisingly high, in particular at low frequencies. We will have to look into this further.
 
 
Things to check:
- Can we identify the 4.9kHz resonance and maybe reduce it mechanically (e.g. by tightening screws)?
- Are the 600Hz noise features actual amplitude fluctuations coming from the SHG or are they added by the Mach-Zehnder? (This can be tested by putting the PD before Mach-Zehnder.)
- Is the low-frequency dark noise caused by ambient light or electronics? Can it be reduced?
Images attached to this report
759_20180508030432_transferfunction.png 759_20180508030444_noisespectrum.png 759_20180508030451_machzehnderservocircuit.png
Comments related to this report
EleonoraCapocasa - 10:46, Friday 20 July 2018 (897)

We verified that the noise feature at 600 Hz which appears in many error signal spectrum: SHG (entry #620), MZ (entry #759), IR FC error signal (entry #750) and some power spectrum (entry #772) is coming from the turbo pump of the BS in the central bulding.