NAOJ GW Elog Logbook 3.2
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.
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
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.
Here I attach the rms integration of the four error signal curves.
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.
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.
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.
Here I attach the rms integration of the four error signal curves.
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.
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
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
- 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?
It seems like the peak of infrared error saturates on oscilloscope from picture 3. Maybe we can put an attenuator for it?
Participaint: Emil and Yuhang
After change the new servo and new infrared demodulation board, we did a rough phase adjustment. We decided to make it as best as possible, so we changed the green and infrared demodulation phase.
For infrared phase, we change the demodulation phase and find a really small error signal. Then we add 90 degree to get a good phase. However, we cannot get a clear green error signal and we cannot see the difference when we change the phase. For the green locking, we did like this:
1. We lock green.
2. Change the phase until we can see the oscillation of error signal.
3. Decrease the gain till oscillation disappears
Because the gain of SR560 is 1 now, so we connect the demodulation signal directly to Rampotu servo. Now the gain is 4.5 on the Rampeauto board.
After change the demodulation phase, we measured open loop transfer function and error signal again. I put the result here.
(Note: this time I also attach the error signal before calibration)
It seems like the peak of infrared error saturates on oscilloscope from picture 3. Maybe we can put an attenuator for it?
Here is the 2nd version :
Changes :
OPO, homdyne and squeezed vacuum beam path have been added
For the article preparation as well as for future meetings, it could be useful to have a simplified optical scheme of the experiment.
Here we tried to do one following Oelker example ( Audio-Band Frequency-Dependent Squeezing for Gravitational-Wave Detectors ).
In the scheme attached to this entry is a preliminary scheme.
The "Not Yet installed" part should contain OPO PLL (?) and homodyne readout.
We haven't yet put the OPO, the PLL and the homodyne readout. The question being how to add them without making the scheme to difficult to read.
All the other main components are indicated.
Maybe it could be also useful to add the control loop?
Here is the 2nd version :
Changes :
OPO, homdyne and squeezed vacuum beam path have been added
Participants : Yuefan, Yuhang
When we tried to recover the FC lock, we had to act quite a lot on the BS control.
The pitch was saturating below -0.7 so we had to play with BS and IM yaw in order to reduce the saturation on the BS control while keeping a good beam position.
[ When the IM is misaligned it is really difficult to see the green transmitted beam because of another beam splitter has been installed on the green path in the squeezed bench]
We could finally performed a losses measurement still using the lock/unlock technique which gives us : 60.4 ppm +/- 7.3
With 2.54% misalignment and 0.25% mode-mismatching.
We also plotted the SHG stability over 1 000 s ("shgstability.png")
It seems to be quite stable around 1.5V even though some low frequency variations can be seen.
The last part from around 750s corresponds to the time we started to try to lock the FC.
It seems that some of the light came back towards the SHG.
When the "noisy measurement" lock was performed I forgot to check if the error signal was at 0 ...
Participants : Eleonora, Emil, Yuhang
We performed 3 new losses measurements using the "lock-unlock" techique.
The IR reflected power can be seen in fig "23to26.pdf".
To extract the losses value for the measurement made on April 23d 2018 I only extracted the firsts values to avoid the noisy part of the signal.
The April 26th measurement was really noisy. However, 3.69% of the power were coupled to the FC 1st order mode and 2.49% with the 2nd order.
In order to better investigate the source of this noise, we will add a beam splitter and a photodiode on the IR injection path in order to see if there is any coupling between the laser power fluctuations and the IR reflected power or if we should investigate other sources.
It also seems that the IR reflected power fluctations are higher when the frequency of the AOM is such that the IR 0 order mode is resonant in the FC.
However, if we change slightly this frequency, the fluctuations seems to disappear.
We will install a camera on the IM viewport where a 2" mirror was installed to see if we can extract some informations about the scattered light.
The computation of the 3 measurements can be seen on "23to26meas.png".
The noisy measurement leads to the huge error bar.
When the "noisy measurement" lock was performed I forgot to check if the error signal was at 0 ...
After change the new servo, we tried a lot to make it work well. Now we can get a stable operation by using this new loop. So it's time to characterize our new locking.
The green calibration factor was got in this way:
1.Measure the ratio between the point before servo(Y) and the point after servo(X). Actually the second point is PZT monitor, so we need to multiply PZT monitor by 100 to get X.
2.According the loop flow chart, we can present Y/X by using transfer function of plant and filter. And we know the laser will actuate with 10^6 V/Hz. The SHG gives a factor of 2. We also measured the open-loop transfer function and we call it G. You can refer to attached picture 1, the blue line is Y/PZTmonitor. We can see from the phase, only high frequency has good shape. So we trust only high frequency and we use it for the calculation afterwards.
3.Now we can get the the correction of PZT(in other word, the slope of error signal). The unit of it is V/Hz. The equation to get it is K=(Y/(X*100))*(1+abs(rho(G)*e^(-iphi(G))))*sqrt(1+f^2/f_0^2)/10^6/2
The result is calibration factor(green)=1/K=552 Hz/V.(Here f is frequency, f_0 is the pole of filter cavity)
The infrared calibration factor was got in this way:
1.We give a tri-angular modulation for AOM, it is pk-pk 4000Hz, T 5s. This means 1600Hz/s.
2.We save the error signal of infrared, we calculate the pk-pk of this error signal. Then we divide it by their corresponding time. Let's say the result is slope(V/s). The calibration factor is 2*slope(V/s)/1600(Hz/s)
The result is calibration factor(infrared)=13.33 Hz/V
Finally, we get the error signal.
Participant: Emil, Eleonora, Marc, Yuefan,Matteo
Yesterday, We used the green light to test the Mach-Zehnder. After giving a ramp to its PZT, we can get the picture shown as below. After adjusting, we can maximize it to this case shown in the picture. The contrast is 4.04/4.72=85.6%
We discussed about the possible interaction between the SHG control loop and the Filter Cavity control loop to explain the 6.56 kHz oscillation at the error signal of the FC control loop.
If noise appears on the SHG control loop, the power level of the green beam will vary.
The gain of the FC control loop is power beam dependent and can vary.
As the FC control loop is controlled to 0 crossing of the error signal, a variation of gain has a 2nd order influence.
But if the mixer produces an offset, this influence increases with the level of this offset.
Things to do:
0) Measure the FC loop offset (at EPS1 divided by 26.5)
1) Inject a DC into PERTURB to see the influence on the oscillation
2) Measure noise in SHG without and with FC control loop
3) Match the impedance (50 Ohm) at output of the LP filter (set at the output of the FC demodulation miser)
4) Measure LO and RF levels on FC demodulation mixer
5) Remove cables between LP filter and the input of the Stanford Research SR560 amplifier (connect directly Mixer, filter and SR560 without cable)
The gain of the input gain G1 has changed and is set to G1 = 5.1
So the output EPS1 (AC2) shows the input signal with a gain G4 = G1 * G2 = 5.1 * 5.2 = 26.5
If you measure the offset at EPS1 output, it should be divided by 26.5 to obtain it at the input (Detect Sig).
Remark:
The attenuator "Gain Piezo" has an effect on the dynamic of "Piezo Sig" output.
For instance, if "Gain Piezo" is set to position 0.7 as currently (on 10 max), the dynamic will be:
D = +/- 0.7/10 * 14 * 10 = +/- 10V
(Gain Ampli HT = 10)
The sample we are going to measure is 0.5mm thick and 2inches diameter large. Our mount has a grub screw that is not able to secure such a thin sample. So I had to add a thick ring behind the sample. I did a mounting test with a "not for use" (not polished) sapphire piece with the same dimensions as the sample. I add a ring (which is the 1.5-to-2 inches adaptor), leaned the ring on the sample, and fixed the ring with the grub screw. The ring doesn't push the sample, it only touches it. This reduces the risk of breaking the sample.
In order to test the IR probe on the LMA samples I previously cleaned them and applied the first contact.
I checked the crystalline coating sample (the one on silica substrate) under a powerful green light. I carefully blew some spray air on it to remove the dust and the best I could do is showed in the picture.
To check which side is the coated one, I put the sample on a clean tissue on the table, and I looked at the shadow below the border of the coating. We the shadow is larger, the coating faces up.
