IR locking accuracy

[Aritomi, Yuhang, Yaochin, Eleonora]

We measured IR error signal of filter cavity using BAB with TAMA PD which is same as CC1 PD in the reflection path from filter cavity. Pic. 1 shows IR error signal and IR transmission when filter cavity is scanned with AOM (setting is Pic. 2). Calculation method of calibration factor is same as entry 750.

Note that factor of 2 in calibration factor is because slope of error signal on resonance is as twice as slope of peak-peak of error signal.

Then we measured spectrum of IR error signal when IR is locked on resonance and calculated rms (Pic.3). IR locking accuracy is 4.2 Hz which corresponds to 4.5 pm of locking accuracy. Pic. 4 shows squeezing degradation with this locking accuracy. In addition to mode mismatch, locking accuracy is also dominating. We need IR locking.

Work on Cryostat

[Namai-san, Ueda-san from KEK, Sato-san, and Tomaru-san]

We worked on the cryostat in ATC to install adapters for windows on 80K shield.
What we did is as follows:

1. Attached the adapters on 80K shield
2. Adjusted the insulator on 80K shield not to touch the 300K shield
3. Removed 300K shield for temporary and install 80K shield
4. Re-installed 300K shield
5. Re-located the chamber to install the cold head and bellows
6. Installed an optical breadboard and connected to cold head with grease

Since we don't have indium sheet, we used grease to contact with cold head.
Though this grease can be used in vacuum (~10^-7 Pa), if the contamination on mirrors become a problem, we will remove the grease and use indium sheet.

The remaining tasks are:

• Install modified 4K shield, and windows on 80K shield
• Cabling
• Vacuum check

14MHz signal at homodyne

I checked homodyne RF signal if I can get 14MHz CCSB beat note, but I couldn't find 14MHz signal in homodyne RF. 14MHz signal may be filtered out by low pass filter after demodulation inside homodyne. We have to modify homodyne if we want to detect 14MHz at homodyne...

Squeezing degradation budget

[Aritomi,Yuhang,Yao-chin, Eleonora]

The attached plot shows a very tentative degradation budget for the squeezing injection into FC detuned at 50 kHz (entry #1751)

PRODUCED SQUEEZING: 16 dB  (from anti squeezing measurement) (entry #1587)

TOTAL INJECTION LOSS = 33 %

- from inside OPO to 1st PBS = 18% (loss of frequency independent squeezing: 21% - visibility: 2% - quantum efficiency: 1%)

- after 1st PBS before PR chamber = 10%

- from PR viewport up to FC  = 5 %

TOTAL READOUT LOSS = 12 %

- from FC to PR viewport: 10%

- after viewport to Homodyne: 1%

- Homodyne Q. E. = 1%

MISMATCH from SQZ to FC = 10 %

MISMATCH from SQZ to HOM = 5 %

FILTER CAVITY LOSSES : 100 ppm

PHASE NOISE = 100 mrad (very tentative, when CC2 doesn't unlock)

LOCK ACCURACY = 5 pm (very tentative, to be measured)

Conclusion: it seems that the degradation is dominated by mismatching 

Comment to BS TMP trouble (Click here to view original report: 1761)

CCSB error signal for filter cavity locking (CCFC)

[Aritomi, Yaochin]

We put TAMA PD in the reflection path from filter cavity and detected CCSB 14MHz beat note and demodulated it with TAMA demodulator. Filter cavity is locked with green and CCSB are off resonance. For demodulation LO, we used CC1 demodulation LO. Then we scanned green phase going to OPO which scans relative phase of CCSB. This relative phase scan of CCSB emulates CCFC error signal.

Note that we only have 90:10 RF BS and when we used this RF BS to divide CC1 LO and used 10% of CC1 LO for this filter cavity error signal, there was no error signal. We'll buy 50:50 RF BS.

Pic.1: CCFC 14MHz signal 

Pic. 2: CCFC 14MHz signal after RF amplification (34dB)

Pic. 3: CCFC error signal for filter cavity when CCSB are off resonant and green phase is scanned

Nikhef quadrant characterization

[Yuhang, Yaochin, Aritomi, Eleonora]

We wanted to check the gain of the quadrant PD by shining some light on it and look at the response of the RF channel with a spectrum analyzer. We assumed that amplitude fluctuations at that high frequencies are shot noise dominated and thus have a flat spectrum. It this way it would be possible to measure the gain TF of the PD and indentify its resonances.

We sent to one sector of the quadrant about 1.5 mW of green light in reflection from FC and we amplified the RF signal of 32dB. We couldnt' see any difference in the spectrum with respect to the case when we don't send any light, except for the light at the modulation frequency that we could not switch off (15 MHz, needed for SHG).

We decided to take the same measuremnt with a red laser diode, in the same configuration for differen power value. We found that the RF spectrum level increases with the power showing a peculiar shape.

We observed that the RF signal stop increasing already at about ~250uW, while DC seems linear (see first plot) 

Not that the responsivity of the sensor is similar for green and red (0.2 [A/W] wrt 0.25 [A/W])

In order to confirm that the laser diode has not any strange amplitude feature at high frequency, we tried it on a QUBIG PD and found a reasonable RF spectrum with a cut off at 100 MHz ( as reported by the data sheet)

Conclusion: something is wrong. The quadrant seems to repond too differently to green and red light. But the gain doesn't show any resonace. We will investigate more.

BS TMP trouble

(Log on 21st)

As reported in the Elog, the BS TMP was failed on 18th. The pump was restarted, but it has problems.

1. Started the RP in the back side of TMP.
2. Opened the angle valve of RP.
3. Reset the TMP and started it.
4. The pressure in BS chamber was 1.8x10^-6 Torr.
5. After the rotation speed of TMP reached 600 Hz, opened the gate valve for BS chamber.
6. The pressure was increased to 2.0x10^-6 Torr. This means that the outgassing rate of TMP itself is too large.
7. Closed the gate valve and the angle valve.
8. Stoped the TMP and the RP.
9. Checked the RP. The inline filter between the TMP and the RP was filled by oile. The vapor of oile might contaminate the TMP.
10. Replaced the RP system (RP itself, inline filter, bellows, angle valve) to the other from the EW arm.
11. To declease the outgassing rate of TMP, a baking is necessary. Binded the TMP with the ribbon heater and attached the thermometer.
12. Restarted the TMP and the RP closing the gate valve.
13. The temparature must be lower than 120 ^oC. The heater was turned on/off manually around 110^oC for 30 min.
14. According to the manual of TMP, 4 hours are necessary for the baking. Will continue the baking on 23rd.

IR reflection measured on 20191015 after closing chamber

Injection: 234 uW

Reflection: 200 uW (off resonance)

Reflection: 135-139 uW (on resonance)

Off resonance reflectivity is 85.5%. 

SHG, OPO parameters after typhoon

SHG temperature: 3.115 kOhm

green power (mW)	OPO temperature (kOhm)	p pol PLL (MHz)	BAB maximum (V)
0		305	0.088
20	7.17		0.416
25	7.18		0.576
30	7.18		0.792
35	7.18		1.06
40	7.19	160	1.5
45	7.19		2.06
50	7.2		2.82
55	7.209		3.52
60	7.209	160	4.36

Correction of PD's position for FC_tra IR / IR transmission issue

Yao-Chin and Yuhang

We found filter cavity transmission IR PD is placed not on the waist of the detection beam. To avoid the clipping noise and also increase the detection ability, we corrected it. Look at the attached picture 1 and 2. Now the PD is closer to BS and the beam is well within PD.

Also, we measured the filter cavity transmitted power.  Compare this transmitted 0.5uW with incident 400uW. The measured transmission is about 0.6%. While the calculated value from the LMA specification is 1%. Since the measured IR after Dichroic and laser line filter goes thorough a lot of optics, this should be consistent.

Sudden failure of BS turbo pump

P-polarization Maps of OSTM (SK)

[Simon, Pengbo]

We have started to analyze the coated OSTM.

First, we took out the OSTM and inspected the mirror visually. We found pencil marks on the barrel and among them an arrow that indicates the thicker side of the wedged substrate and shows toward the HR side of the mirror.

The orientation which we choose for the measurement is to put the thicker side upside first and then rotate it by 90 degrees(with the thinner side facing the PC). Thus we only need to consider the horizontal deflection of the beam. After the adjustment of the sample, we found no output beam from the other side of the sample which mainly due to the coating absorption. So we removed one of the two ND filters to increase the laser power.

Second, we started the adjustment of the IU.

Due to the deflection effect, we first changed the lens position to make sure the beam goes through its center. Secondly, we rotated the second PBS slightly to have a perpendicular incident of the beam. Then, we adjusted the position of the two photodiodes so that the s-polarized beam and the p-polarized beam can hit the center area, respectively. Because we were doing the P-polarization measurement, the s polarized beam is nearly invisible, so we used the oscilloscope to finish the adjustment.

Then we ran the polarization map scan in the center with a 15mm radius. Attached to this report is the P-polarization map.

Not responding coil circuit seems open

[Yuhang, Yaochin, Eleonora]

We continued investiagation about not responding coil of the input mirror. (See entry #1742)

We measured the resistance across each coil loop at the flange. For all the working coils it was about 2.4 Ohm for the not working one we found the loop is open.

This suggests the magnet coud be fine and the problem could come from the coil circuit. 

Measurement of current loss budget

Aritomi, Eleonora, Yao-Chin, and Yuhang

We summarized the loss budget in the first attached figure.

From the power measurement as following, we could know this part contributes about 10% of loss

measured after first PBS after OPO	265uW
measured after last mirror before PR chamber	198uW

From the power measurement as following, we could know this part contributes about 1% of loss

measurement of IR reflection after PR chamber	221uW
measured before homodyne	218uW

We also measured the IR reflection(IR_ref) beam shape as in the attached figure 2. It has a bit ellipse in the horizontal direction. Because of this elongation, we had a second order peak when we match this IR_ref to alignment mode cleaner(as shown in the attached figure 3,4). By comparing this higher-order peak(12mV) and TEM00 peak(276mV), we could deduce the homodyne readout efficiency should be (1-12/276)^2 = 91.5%. But this IR_ref has beam jittering, so the real readout efficiency should be less than 91.5%.

We also measured the filter cavity round trip loss again. By using the same method we used last year, we found now this value is around 100ppm. We will do more investigation to make sure of this. The measurement is attached in the figure 5. 

In the last figure, the achievable squeezing without considering phase noise is attached.

Saturation of CC2 correction signal

[Aritomi, Yuhang, Yaochin, Eleonora]

We have saturation of CC2 correction signal (8Vpp) and it seems coming from servo since we have this saturation without connecting piezo. HV amplifier we are using can take up to 10V and the gain is 30 and piezo for phase shifter can take up to 1000V.  So we are using 240V out of 1000V piezo range.

Frequency dependent squeezing at 50kHz

[Aritomi, Yuhang, Yaochin, Eleonora]

Today we measured frequency dependent squeezing at 50kHz.

Setting is same as yesterday (AOM frequency is 109.13698MHz). Since CC2 lock is unstable, we made average number 5 while it was 50 yesterday.

Frequency dependent anti squeezing at 50kHz

[Aritomi, Yuhang, Yaochin, Eleonora]

Today we measured frequency dependent anti squeezing at 50kHz.

We set AOM frequency 109.13684 MHz which is larger than carrier frequency by 100kHz and corresponds to 50kHz detuning. CC2 demodulation phase is 105deg for squeezing, 155deg for anti squeezing and 135deg for intermediate.

Power measurement when PR chamber is open(measured on 20191011)

Aritomi, Matteo, Yao-Chin, and Yuhang

All the measurement is inside PR chamber. We sent BAB to do this measurement.

injection power	394uW
before dichroic	379uW
after dichroic	375uW
transmission from the first PBS	3.4uW
transmission from the second PBS	3uW
before going back to bench	334uW

Injection loss: 4.8%

Reflection loss: 10.9%

IR reflected from filter cavity cut issue solved

Aritomi, Matteo, Yao-Chin, and Yuhang

We opened the PR chamber on last Friday. But moving the last IR back reflection steering mirror, we solved the cut issue of this beam.

Attached figure 1: We just opened PR chamber, we checked with IR viewer. We could see a beam on the right side of chamber window.

Attached figure 2: After moving the last IR back reflection steering mirror, we made IR back reflection hitting on the window. And this IR back reflection is between IR(injection) and GR.