NAOJ GW Elog Logbook 3.2
Participaint: Aritomi, Eleonora, Matteo and Yuhang
First we checked the homodyne dark noise. The purpose is to compare this result with Henning's result. We are using network analyzer, but itself's noise level is not low enough to see the dark noise of homo-dyne. So we decided to use Stanford research SR560 to amplify the homodyne noise. We gave it a factor of 100(40dB). Then we got the result as shown in attached figure 1 and 2. The difference is they have a marker located at 1kHz and 10kHz, others are the same. We can see below 3kHz, the noise should be dominated by electronic noise. And above 3kHz, the noise level should be the noise level of homodyne. In this case, the real noise level of homodyne should be around -122-40 = -162dBV/sqrt(Hz). However, the result from Henning is
-156dBV/sqrt(Hz). This is not a neligible difference.
We are arriving to the point to operate homo-dyne. We power up homo-dyne with a DC voltage supply at +/- 19V. For checking both the alignment of the beam inside homodyne and also if the homodyne work well, we send an amplitude modulation to the main laser beam. At the beginning, we send a 100mV pk-pk and 1kHz signal into the current control of main laser. However, this modulation seems not stable. Then we decide to modulate the PZT of IRMC, in this case, we moved the locking point of IRMC around the TEM00 peak. Then the modulation became very clear. Here the clear or stable means if the signal we see on the oscilloscope shows clear line or can be triggered.
We also find out that the the even we send the voltage +/- 19V to homodyne. The current going inside seems like zero.
And when we see the 1kHz amplitude modulation signal on the monitor PD with amplitude of several volts. We can only see the same signal on homodyne with amplitude of several millivolts. Seems like we didn't switch on homodyne.
The last figure shows the homodyne on our bench.
Participaint: Aritomi, Eleonora and Yuhang
According to the optical layout, the lens should be 250mm before beam waist. And the beam waist is 390um. By putting a 30mm lens, we found the beam waist after lens will be 22um. The distance of the waist from the lens is 29mm. The corresponding rayleigh range is 1.4mm. The aperature of homodyne detector is 0.5mm, so we should make the going inside beam smaller than 100um. The range of beam smaller than 100um is 12mm.
The measurement result agrees with the simulation. The result is attached in the figure2.
The attached figure and PDF is the same. They show the space we may have the leg for breadboard. The space in this case is roughly 90cm*75cm. It covers all the space for PLL and two auxiliary laser heads.
1. The p-pol beam is very sensitive to the mirror mount. Even you touch the mirror mount, the alignmet condition will be changed.
2. The OPO's BAB transmission is also very sensitive to mirror mount.
3. The PLL locking is not robust enough. It can only last for several tens of minutes. And very diffcult to acquire, we can close and open software again to make this better.
4. The second Faraday's mount keeps moving. And its transmission has a lot of scattering.
Participaint: Eleonora and Yuhang
We convert BAB into p-pol and then measured the beat between it and the LO at the homodybne BS.
The result is shown in the attached picture. The visibility is (Vmax-Vmin)/(Vmax+Vmin) = (4.26-1.26)/(4.26+1.26) = 0.5435
The BAB power is 0.125mW and LO power is 1.24mW. The expected visibility is 2*sqrt(P1*P2)/(P1+P2) = 0.5768
So from these values, we should have 0.5435/0.5768 = 94.22% of matching of two beams. This is complaint with the matching we measured before.
There is one thing block us from measuring visibility, it is the use of lenses. Before we used lense, we can see the beam is flashing but cannot see this signal on oscilloscope.
By considering the level of visibility, I did the simulation of degradation. As shown in the attached figure two. Now because of only the reason of visibility, we degrade squeezing by 2.74dB.(I assume here the initial squeezing level is -10dB, the degradation is different for different initial squeezing level)
Participaint: Aritomi and Yuhang
We decide to change the position of OPO transmission PBS because of the space limitation. We also put the ''flip'' mirror on the translation stage. By moving this mirror, we will switch its direction to homodyne or to filter cavity. The scheme now is to use the two steering mirror before this ''filp'' mirror to do alignment for filter cavity. And adjust this ''flip'' mirror to align the beam into homodyne. Because this ''filp'' mirror has three adjustable knobs.
According to the design shown in the first attached figure, we aligned the BAB into AMC. However, the matching is not as good as LO.
From all the peak values, we can compute the matching is (0.51-0.0065)/((0.0115+0.0153+0.0173-0.0065*3)+0.51-0.0065) = 95.34%
We didn't see the visibility and found out the reason is we forgot to convert BAB into P-pol.
Participaint: Aritomi and Yuhang
Since we found PDA05CF2 (thorlabs InGAS PD) is more suitable for infrared signal. We decided to use it for OPO locking. Before we took the filter cavity locking PD for OPO locking because it has a DC channel. Since we have a better one, we decide to put this qubigPD (with DC) back for filter cavity locking.
Besides, according to simulation, the OPO reflection error signal is 5 times smaller than transmission. And we improve the error signal by a factor of 10. So in principle, now the error signal from OPO reflection should be also enough to lock OPO.
This PD has only one output channel. RF and DC components are separated by using a minicircuit Bias-Tee ZFBT-4R2G+ (datasheet uploaded on the wiki).
The RF part is sent to the mixer for PDH demodulation, the DC is used to monitor the lock status.
Actually we did this replacement long time ago. But I would like to put here some reference value of this PD. We should have error signal pk-pk value more than 120mV. And DC value more than 3.3V. Note here the impedence is .
Besides, we can also see that this PD has a better performace than the Qubig we had. There is a result of previous measurement of Qubig PD. Compared with that, we improve the error signal by a factor of 10.
Actually, there is only a factor of 3 or 4 difference between Qubig PD and thorlabs PD's resoponsivity.
This PD has only one output channel. RF and DC components are separated by using a minicircuit Bias-Tee ZFBT-4R2G+ (datasheet uploaded on the wiki).
The RF part is sent to the mixer for PDH demodulation, the DC is used to monitor the lock status.
Participaint: Aritomi and Yuhang
This beam splitter will be used for homodyne. And the balance of these two power value is very important. So we decide to measure this value.
We bought two BSW41-1064, according to specfication. The overall performance is T=50+/-5% while R=50+/-5%. So R/T should be between 0.905 and 1.105.
This power ratio depends also on polarization. According to thorlab website, p-pol should be closer to 50:50. We measured the power ratio relationship with polarization for one mirror. The result is shown in the attached figure. It seems there is relationship between polarization and power ratio. But it is small dependence. We can also see this mirror is still within the error range of power ratio provided by thorlab. But it is not as Matteo suggested.
Then we changed it to the second mirror. Then the power ratio becomes R:T=607.5/615.4=0.987. This is much better.
Matteo and Aritomi recovered the vacuum pump for filter cavity. The procedure is to first bring back rotative pump and after it reachs a certain level. Turn on the molecular pump. There are some valves to seperate pump. According to the sequence of the pump on, open them.
Manuel, Victor
1310nm probe
we aligned the pump beam to maximize the AC signal of the surface reference sample, and we made a calibration scan with pump power 35mW.
Then we increased the power at the maximum and measured the crystalline coating with three different probe power: DC=1.01, DC=1.62, DC=2.54.
Then we plot the three scans together and the result is that the AC/DC overlaps.
We also repeated a scan with higher resolution in z.
Participaint: Aritomi and Yuhang
We aligned the local oscillator into the alignent mode cleaner(AMC). The method is to remove input and output mirror and then make sure the beam going through MC through the center. And the beam should be flat horizentally and vertically. The reason why we aligned LO first is it is easier because it is much brighter than the OPO transmission. Then we can use this as a reference for the alignment of the second beam.
The resule of this match is shown in the first and second attached figure. The matching is (1.75-0.012-(0.02-0.012))/(1.75-0.012)=99.5%
Participaint: Aritomi and Yuhang
According to the design, we implement the telescope which is very close to OPO. However, I found I can improve the mode matching by moving the first lens close to OPO. However, after movement of 1cm closer, I cannot go on because of the limitation of space. Then I did the simulation by Jammt, it proves that the waist position should be farther away the first lens.
So I did simulation again(with waist position 1.5cm back) and also leave more space for the lens to be moved. We will implement this design tomorrow.
[Aritomi, Yuhang]
Here is information of lock of IRMC.
Input power: 2 mW, output power: 1.56mW (transmissivity: 78%)
Filter for lock of IRMC:
SR560 with low-pass 30 Hz, gain 10
40dB attenuation
We put an half-wave plate on the path before homodyne BS to get p-pol.
Then we measured reflection and transmission from BS for homodyne detector.
Reflection: 0.74mW, Transmission: 0.82mW
BS (BSW41-1064) somehow does not split the beam half and half now.
[Yuhang, Eleonora]
We have tried to recover the alignment and the lock of the filter cavity.
The alignment was easily recovered even if we could not use the local control for the telecope. See entry #1139. (We just tweeked a bit the BS position wiith picomotors)
We optimized the aligment with the input and output mirrors local control and it seems it is quite fine (flashes movie here:https://drive.google.com/open?id=1045raQj_n84CjnnUuYfYBT6Kqngv-id8) nevertheless we could not lock the cavity. We didn't have much time to investigate. Some possibilties:
1) The power is too large (a factor 3 higher than before), so the gain of the loop could be too high (we tried to change the gain value and also put some attenuation but it didn't work)
2) The beam fluctuation it too large since the BS and PR are not controlled (but this should be visible in the flashes).
3) Since the power is higher the camera in transmission could saturate more than before and mask the presence of HOM flashes. (We suspect there coud be a higher mismatching due to change of the beam from SHG)
The initial value of the gain were: input attenuaion = 5 and PZT gain = 4. As mentioned before we tried to change them but still we coudn't achieve the lock.
We will try to go back to the previouse power value and setting configuration to see if we can lock in this condition.
[Eleonora Yuhang]
While trying to recover the filter cavity alignement we found that the target PC used for the telescope local control had died.
We already had this issue which was solved by changing the power box. See entry #1035. We tried it again with another power box. It seemed to work fine and the supervisor PC could connect to it but we were not able to read and write any signal. We suspect a problem with the ADC/DAC unit connection. We will try another power box.
[Takahashi-san, Yuhang, Eleonora]
With the help of Takahashi-san, we have switched off all the instrumentation in TAMA in view of the power shutdown planned for tomorrow.
In particular we switched off the vacuum system in the central area and south arm and the compressor in the elecshop. We also closed the gate valves between the TM towers of the south arm and the pipe.
Some details of the swiching off precedure are reported in the attached file.