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ID Date Author Type Category Subject
2483   Wed Nov 27 15:17:51 2019 anchalDailyProgressFSSFSS Diagnostics - Quick HF OLTF of NFSS

I quickly took a high-frequency Open Loop Gain measurement of NFSS loop at 10 dB COM Gain and 10 dB FAST gain, using the same measurement method as in CTN:2443. The UGF has not changed much but there is a dip at 435 kHz. This was there before too, I was just not paying enough attention to this part of OLTF before. So, we can say with some confidence that the 435 kHz signal seen in the oscilloscope in CTN:2482 at TP1 is actually due to some non-linear effect most probably and does not get suppressed at all. The phase margin near UGF looks about 135 degrees so there is no solid reason to believe this could be due to loop oscillation.

So I got to think of what combination of RF frequencies might be mixing down to create this oscillation and where. This oscillation is also visible in Plot 6 and 7 of NFSS_RFPD_Output_Oscilloscope.pdf of the measurements done in CTN:2470.

Data

Attachment 1: NFSS_OLTF_HF.pdf
2486   Wed Dec 4 17:09:44 2019 anchalDailyProgressTempCtrlIncreased range of out-of-loop temperature sensor

This has happened few times now that acromag channel for the can heater driver stopped updating according to the PID script and the can gets heated to a very high temperature. This pushes the temperature out of the ranges of the current AD590 temperature sensor board. I have changed the range of channel 2 (this was being used for out-of-loop) to ensure we can still see some meaningful temperature value when such incidents happen. I have replaced R18 from 100k to 27k. The updated table is:

CH No on Board EPICS Channel Name Temperature Conversion Function (ºC) Range (ºC)
1 C3:PSL-TEMP_TABLE = V/0.810875 + 27.30248869 13.860-40.745
2 C3:PSL-TEMP_VACCAN_OOL = V/0.43875 + 33.3115385 8.450-58.155
3 C3:PSL-TEMP_VACCAN_INLOOP = V/1.625 + 33.3115385 26.604-40.020

### Weird phenomenon?

• I'm not sure this problem occurs though. Right now the out-of-loop temperature sensor shows that the temperature of the can is 55 Degree Celsius.
• This is also not cooling down fast. Last time it took days for it to cool down to set value.
• The alignment of the cavities change vertically when the can temperature is significantly different from the set value of 34.38 Degree Celcius.
• This hinders them from locking properly unless I tune the alignment back. But when the can will cool down finally, they will be misaligned again.
• Also, I just refuse to believe it is actually that hot, but I check the voltage after the transimpedance amplifier in temperature sensor boards and two independent AD590s are reporting this.
• And anyways, why is the acromag output channel getting frozen anyways.
• That being said, this is an irreproducible but non-harmful problem yet, so lower priority than the FSS saga going on.
2487   Wed Dec 4 18:11:09 2019 anchalDailyProgressFSSFree running laser frequency noise spectrum

I took a spectrum of PMC error signal when the FSS loop is not closed. This should provide a rough estimate of the free running laser noise. We had earlier seen a peak at 435 kHz in the Northside, hence I wanted to take this data with some references. First of all, this peak is very similar in the description of relaxation-oscillation peaks of these NPRO lasers mentioned on page 52 of this manual. The "Noise Eater (NE)" is supposed to suppress this peak significantly. However, in the spectrum of the PMC error signal, there is no difference when noise eater was ON or OFF.

I took a spectrum of Southside as well, just to see if I could see action of Noise eater there. For south laser, the noise eater suppressed noise only till 100 kHz or so and probably this side also has a similar relaxation-oscillation peak problem but is shadowed by a large feature at 30 kHz. Not, the absolute value of the spectrum between North and south are vastly different due to different amount of light, different transimpedances od the PDs and different gain values in the feedback circuit.

However, the noise eater is supposed to reduce relative intensity noise only. And the error signals of PMC should really be telling us noise in the frequency of the laser. So maybe I'm connecting two dots in different Hilbert spaces. But Rana suggested that a busted Noise Eater could be the reason for the 435 kHz peak, I just do not understand how RIN would cause frequency noise so badly. I thought photothermal transfer functions from RIN to frequency noise were very small.

Attachment 1: PMCErrorSignals.pdf
2490   Mon Dec 9 19:54:44 2019 anchalDailyProgressFSSFSS Diagnostics - How much distortion affects the functioning of PDH?

I'm trying to think hard with my small brain how the distortion would affect the PDH functioning and inject noise in the frequency of the laser. I have a line of reasoning which starts with a question.:

• The two RF sidebands of laser that fall on the cavity, upon reflection, do they destructively interfere? Because the calculation I did in CTN:2481 suggests that the power at second harmonic (2-Omega frequency) is so high because of the sidebands beating with each other.
• In case they do not destructively interfere, the generated second-order harmonic (2-Omega) will have laser amplitude noise on it. This when it mixes with the (1-Omega) signal which actually carries the cavity length noise at the inputs of MAX4107, the distorted 1-Omega created would have laser amplitude noise on it.
• This might be the reason why FSS loops get overwhelmed with seeing a lot of laser amplitude noise which ideally it is not supposed to see. And it tries to correct this noise in phase quadrature making the situation worse.
• Since Andrew left, I increased the laser power reaching the cavities to go above the shot-noise limit of the photodiodes. Maybe, this increased light level increased this effect to a point where we are witnessing the problem.

Of course, all this depends on the RF sidebands interfering constructively upon reflection. I remember (I don't know from where) that it is the opposite. Either there is a fault in my calculations or this is indeed what is happening. I need to understand this properly to go further. Need help.

2492   Tue Dec 10 17:03:41 2019 anchalDailyProgressLaserLaser Settings back to defaults

I put laser settings on both North and South Cavities back to default. From this point onwards, all settings about the lasers would be known and kept track of. The red values are the settings that were changed.

### NPRO Laser Settings

Property Display Symbol North South Units Notes
Laser Model - M126N-1064-700, SN 5519, Dec 2006 126N-1064-500, SN 280, Nov 1997 -
Diode Temperature DT 22.3 28.7 $^\circ C$ Informational only.
Diode TEC Voltage DTEC 0.8 0.7 V Informational only. +ve -> cooling, -ve -> heating.
Measured Laser Crystal Temperature LT 40.8 55.2 $^\circ C$ Informational only. Calibration dependent.
Laser TEC Voltage LTEC 0.0 -0.5 V Informational only. +ve -> cooling, -ve -> heating. Manual says typically should be 0.0V.
Target Laser Crystal Temperature T 40.087 -> 40.0010 48.0010 $^\circ C$ Changed back to factory set value on North Side.
Laser Head Power Level PWR 66  ->  624 92  ->  101 mW Calibration dependent. CHanged the calibration to meet the power meter but even then, power meter says a maximum 500 mW, so North Side is not entirely correct. On the south side, it was difficult to mount power meter perpendicular to the beam, so there might be some clipping loss in calibration.
Power Adjustment ADJ 0 -2  ->  0 - From -50 (off) to +10. Changed the diode current around set value.
Diode Current DC 2.06 2.04 A It can be changed to change power level. Reflects measured value.
Diode Power Monitor DPM 0.00 0.00 V

Calibration Dependent.

Noise Easter NE ON ON - -
Laser Diode Status LD ON ON -
Nominal Diode Current - All the way clockwise All the way clockwise - It can be changed by turning the left potentiometer from the back of the laser head. Factory default is all the way clockwise. I have set both North and South Lasers to this point.

Note:

While turning the nominal diode current of south laser all the way clockwise, I found that the laser power peaks before the maximum diode current is reached. This diode current is about 1.9 A. This is unexpected. Any explanations on this would be helpful.

2495   Thu Dec 12 19:15:55 2019 anchalDailyProgressFSSFSS Diagnostics - Beatnote Spectrum wrt Laser Power and Modulation Index

I took beatnote spectrums in the current modulation index (was set to around 0.3 earlier). Then I took spectrum after attenuating the modulation signal power to both EOMs on North and South Path by 3 dB and 6 dB. This should reduce the modulation depth by $1/\sqrt{2}}$ and 1/2. After every change, a small displacement happens in the thermal control of the cavities, so I had to wait for some time to let it settle. The gains of the FSS loops were kept constant to make sure only the modulation depth is the parameter that is changing. Gain values were 24 dB and 16 dB for South COM and FAST gains, and 11 dB and 10 dB for North COM and FAST gain.

Inference:

• There is almost no difference in noise upto1 kHz.
• There is a new bump I'm seeing between 100 and 200 Hz though which was there on Dec 12th but not on Dec 13th measurements.
• This bump might have been reduced due to a decrease in laser power or something might have changed between the two days. I'll check with increasing power what happens.
• Above 1 kHz, we see that as modulation depth is deceased, we see a reduction in noise. The same goes for power.
• Overall, this does show that the effect of these changes is more or less as expected and the effect of non-linearity, if any is not much.
• It is still valuable to quantify properly how much the non-linearity might be affecting the final beatnote spectrum though. Working on it.

Data

Attachment 1: BeatnoteSpectrumwrtPowerandModindex.pdf
2496   Fri Dec 13 16:24:02 2019 anchalDailyProgressFSSFSS Diagnostics - RFPD Circuit Latest Available Photos

I found the latest available photos for the Cavity Reflection RFPD circuits.

There were minor changes made after taking these photos which are logged at: https://nodus.ligo.caltech.edu:30889/ATFWiki/doku.php?id=main:experiments:psl:rfpd

I'll update these above photos whenever the next time I get a chance. They would be present at:

Attachment 3: SN009.jpg
Attachment 4: SN010.jpg
2497   Fri Dec 13 17:44:05 2019 anchalDailyProgressFSSFSS Diagnostics - RFPD RF Coupled Output Offset

I rechecked the 20dB coupled RF output of SN009 (RFPD on north Cavity reflection) and SN010 (RFPD on south Cavity refleciton). The following are the mean values over 10s taken at DC coupling with 50 Ohm input impedance, +/- 40 mV input range with two different oscilloscopes.

SN009 RF Out 20dB Coupled
Shorted SN009 RF Out 20dB Coupled SN010 RF Out 20dB Coupled
TDS 3052B -0.2 +/- 0.01 mV 7.0 +/- 0.1 mV 7.3 +/- 0.1 mV
TDS 3034C 0.5 +/- 0.01 mV 7.4 +/- 0.1 mV 7.6 +/- 0.1 mV

Note that these are 20dB coupled values, so the actual offset reaching the FSS board is about 70 mV. The PD input is AC coupled through a transformer so it shouldn't be reaching further than there but ideally MAX4107 at the RFPD is supposed to have a maximum input offset voltage of 3 mV which at gain 10  should look like 30 mV. So what we are seeing (7.0 mV) is more than twice the rated maximum input offset. I'm not sure if this means our MAX4017 is busted or something is wrong in the loop. Help needed in understanding this result.

2499   Tue Dec 17 16:55:07 2019 anchalDailyProgressFSSFSS Diagnostics - TTFSS North U7 Boost Stage Input-Output TimeSeries

As suggested few group meetings ago, I took time series data at the input and output of boost stage opamp U7 (at TP15 and TP16) using TDS 3034C. I know trusting oscilloscope for synchronous measurement of two channels is not a great idea, but this is a good zeroth order approximation for this approach. 500 MHz 10MOhm impedance probes were used for the measurement of the signals. The data is taken at two different acquisition rates and I used basic math to calculate the expected ideal output. Following formula was used:

$\inline \dpi{150} V_{Ideal}(t) = V_{TP16}(0) \ + \ V_{TP15}(0)\frac{R_{29}}{R_{37}+R_{39}} \ - \ V_{TP15}(t)\frac{R_{29}}{R_{37}+R_{39}} \ -\ \frac{1}{C_{35}(R_{37}+R_{39})}\int_0^tV_{TP15}(t)dt$

Ofcourse the offset point above wouldn't work. Also, I have not included the notch filter that is also present at this stage.

The results of this analysis are attached. I also, calculated the required slew rate of the AD847 at this position. At the 100 MSa/s sampling rate, we saw that maximum required slew rate for ideal signal was around 110 V/us, much less than rated 300 V/us. I don't see a bad change in shape of the waveform. At 1 GSa/s sampling rate, we see that  required slew rate reached about 220 V/us at indeed oscillations at high frequencies are limited even though this is below the rated value.

Looking from data taken at CTN:2474 present in this directory, these fast oscillations that we are seeing on the oscilloscope are around the modulation frequency 36 MHz.  These leftover downconverted 2-Omega from teh RFPD might be saturating slew rate limtis of many opamps in the TTFSS box. I think I should look back into Andrew's implimentation of the elliptical filter right after the mixer in the RF board. He did say that we need to make the filter lossy to ensure the reflected signal at 36 MHz gets absorbed at a 50 Ohm resistor. For this we need to add a resistor to gnd at node rfbn2. Seeking permission to make modifications to board.

Data

Attachment 1: NFSS_UT_TS_Analysis.pdf
2500   Tue Dec 17 18:29:58 2019 anchalDailyProgressFSSNeed for ISS?

If we are going to keep the same laser with busted Noise Eater, should be go in the direction of implementing the ISS?

In Oct, 2018, I along with Johannes developed ISS boards and photodiode transimpedance amplifier boards. These were characterized by Johannes at Cryo_Lab:2180 and Cryo_Lab:2181. However, Johannes was using AOM while we have EOAM here. There are slight differences which should be addressable given we have an offset port also in the ISS board. But the expected open loop gain needs to be worked out along with right choice of transimpedance and cavity pole neutralizer stage in ISS.

Johannes gave me his M2ISS photodiode mounts before leaving, so we have them too. This looks like a 1-week project from start to finish. So with the thumb rule of "nothing goes as expected" and me being a grad student, this should take 2 weeks. Is it worth it? Should I start working on this? Or can I just get a laser with working NE?

Also, there is no real documentation of why we stopped using the existing ISS in CTN? Last mention of ISS is at CTN:2132 and after that I came and was working on ISS boards which we never installed in CTN since or FSS, RFPDs and thermal controls were priority then.

2501   Wed Dec 18 14:04:43 2019 anchalDailyProgressLaserSouth Laser Power Dropped.

After the last calibration, South Laser power at Laser head determined by its internal photodiode and show as setting PWR today (noticed today) dropped to 80 mW with none of the other settings changed. The diode current is still the same, so this could be one of the following two things:
1) The internal circuit to measure laser power at head went wrong.

2) Something is wrong with the laser crystal.

The laser power as seen by the reflection photodiode at South PMC captured a glitch on Dec 17th, 2019 8:30 am (might be 7:30 am in PST). The glitch shows that PMC went out of lock due to it but then returned to a laser power lower than before. The voltage level decreased from 2.72 V to 2.3 V after lock, which is equivalent to a drop of ~15.4%. The laser head power level according to PWR monitor dropped from 101 mW to 80 mW, which is equivalent to ~20.8%. While these don't match, PMC reflection is also not a true measure of power level. But I am sure this power has actually reduced and even if the laser head power meter is off a little bit, we have witnessed a big drop in power.

Restarting the laser (Put it on Standby and switch it back on) didn't change the power level.

Restarting the laser with turning the key off and on also didn't change the power level.

Even though 80 mW is enough for our experiment, this sudden decline in power shows there is something happening with the laser that we do not understand.

Data

Attachment 1: SouthLaserPowerDropped.pdf
2502   Wed Dec 18 18:40:32 2019 anchalDailyProgressFSSMaybe the NPRO Laser isn't bad after all

Today I inserted two photodiodes, Thorlabs PDA10CS at (110,44) at dumped end of a PBS before PMC and at (64,28) at dumbed end of INput PBS of Faraday Isolator after PMC. These photodiodes were set at 20dB gain which according to the manufacturer gives us a bandwidth of 1.9 MHz.

Then, I just took Noise Spectrums using AG4395A with the parameter file attached in the data directory, from 0 to 1 MHz at 1 kHz ResBW and 50 averages. The output was divided by the DC level of the photodiode which was measured using TDS 3052B averaged over 10s. This measurement was done with FSS ON or OFF.

### Inference

• Whenever FSS is ON, the loop is completed, so noise due to PMC and FSS is present in the measured RIN.
• But when FSS is OFF, before PMC, the noise should be just of free running laser even though PMC is locked downstream.
• The most important observation is that when FSS is OFF, before PMC, the RIN is pretty small with no peaks in 100s kHz, particularly the notorious 433 kHz peak.
• The dome from 1kHz to almost 100kHz peaks at 10kHz is most probably due to FSS as it goes away whenever FSS is OFF.
• The peak at 433 kHz persists even after switching off FSS and is present only after PMC. So this peak is clearly due to PMC and not due to laser as was thought before.
• Conclusion is, that laser is free of any large RIN and most of the RIN introduces is due to FSS or PMC.
• I've left the PDs in there with termination. I'll change some PMC loop and FSS loop parameters tomorrow to see nature of their contribution to RIN.

North Laser is hereby acquitted.

Data

Attachment 1: NorthLaserRINAnalysis.pdf
2504   Fri Dec 20 15:00:30 2019 anchalDailyProgressFSSPZT, EOM to RIN TF Measurement

I took some TF measurements, but I'm not sure if I used the right method to do this. All the resutls look essentially the same, so maybe I'm just measuring instrument noise and nothing else. Regardless, I' posting the results.

Measurement details:

For both measurements, I sent the source signal, first directly to PZT or EOM and second with a 50 Ohm termination in parallel. In case of EOM, I also did another measurement with low power to see if I can uncover any saturation effects my high powered source might be causing.

The output of two photodiodes, Thorlabs PDA10CS at (110,44) at dumped end of a PBS before PMC and at (64,28) at dumbed end of INput PBS of Faraday Isolator after PMC was measured and I took DC level of the outputs averaged over 10 s right after taking TF measurement. This was divided by the measured TF to get units of 1/V i.e. from PZT/EOM to RIN.

The parameter configuration files for the measurements are in the data directory.

What might have gone wrong:

I suspect that maybe AG4395A is unable to drive the capacitive load of PZT and EOM after a certain frequency. I need to find a better way to actuate the PZT and EOM in a known fashion, possibly through the FSS box or some buffer driver.

In the case of EOM, I'm also uncertain if the amplitude of actuation would be enough to do anything whatsover. Maybe the transfer function needs to be taken with high voltage driver.

Any comments on my measurement techniques are welcome as I'm surely not doing this right.

I concluded that PMC is causing the 433 kHz peak because the only time I do not see it in CTN/:2502 is behind PMC when FSS is OFF. I couldn't think of a way to check if PMC Servo is causing it on its own. Can I do that without closing the loop somehow?

Data

Attachment 1: NorthLaserPZTEOMtoRINTF.pdf
2505   Fri Dec 20 18:08:44 2019 anchalDailyProgressFSSPZT, EOM to RIN TF Measurement Through FSS

### TF Measurement Method

• I switched on Test1 switch on RF Board to cut off RFPD input to FSS.
• I switched on Test2 switch to connect the excitation port at the summing point.
• I switched on Ramp Engage for FSS to ensure paths to PZT and EOM are connected.
• Then, I disconnected and shorted EOM input when taking the measurement for PZT and vice versa.
• COM Gain was 6dB and FAST gain was 10 dB.
• TF was measured from Test2 EXC Port to the output of PDs. This was divided by 10s averaged DC value of PD output to get TF to RIN.
• Measurements were taken in turns for low and high frequencies with IFBW 10Hz and 1 kHz.
• Again, PDs are  Thorlabs PDA10CS placed at (110,44) at dumped end of a PBS before PMC and at (64,28) at dumped end of Input PBS of Faraday Isolator after PMC.

### Calculations:

• The noise spectrum in FSS loop was measured at OUT2 port on the common board.
• This was divided by 50/(453+50) voltage division caused by 50 Ohm input impedance of AG4395A. This gives spectrum at the output of the U3 adder stage in the common board.
• The TF measured above are divided by 392/1200 which is the gain from Test2 EXC to output of U3. So the TFs are no referenced from the output of the U3.
• The measured noise spectrum is multiplied by the TF from that point to RIN at After and Before PMC due to PZT or EOM.
• These are summed quadratically to get total estimated RIN ASD (before and after PMC) due to the noise from FSS.
• Finally, I also plotted the RIN ASD measured in CTN:2502 before and after PMC when FSS was ON.

### Inferences:

• The measured RIN, both after and before PMC are much higher than the estimated noise due to FSS.
• In particular, the bump around 10kHz in measured RIN before and after PMC is not present in transfer functions from FSS to RIN or the noise spectrum itself in FSS.
• The notorious 433 kHz peak is present everywhere, but there's order's of the magnitude of difference between FSS contribution and seen RIN. So it is mostly the other way round, the RIN causing these peaks in FSS.
• Either that, or there is something wrong with the way I took measurements or did the above calculations.
• Also, I'm aware that the RIN is actually fed back into the FSS through PD and it is not an open-loop. But I did this simpler analysis first. Maybe that's what is wrong.

Data

Edited on Mon Dec 23 15:21:50 2019 .

Attachment 1: NorthLaserPZTEOMtoRINTFThruFSS.pdf
2506   Mon Dec 30 10:33:33 2019 anchalSummaryOtherSummary of questions asked in December

• CTN:2492 Why are output powers of these lasers much lower than rated power when diode current is maximum? And why do I see a maxima in South Laser Power when diode current is increased, shouldn't it just increase with diode current monotonically?
• CTN:2495 Comments on beatnote noise differences with different laser power and modulation depth. Can I infer some good choice of laser power and modulation index from this? Suggestions for more tests on this line?
• CTN:2496 Rana asked for the latest available photos of FSS RFPDS. We want to add an active notch in the final RF amplifier stage.
• CTN:2497 There is indeed actual DC offsets at the output of FSS RFPDs RF out. Please cross-verify my measurement method and does this mean the MAX4107s are busted. Should I replace them?
• CTN:2499 Time series measurement of the signal before and after the summing stage in FSS on Northside. There is significant leftover 1-Omega frequency even after the elliptical filter. I think I need to make the input end lossy to absorb back reflections from the elliptical filter. Need permission to modify the circuit and test if this helps.
• CTN:2501 There is a logged event of South Laser losing about 20% of its output power with the same diode current. Is this laser dying? What is happening?
• CTN:2505 As instructed, I have taken transfer functions through FSS from PZT path and EOm path to RIN before and after the PMC. I need validation on method used. My conclusion is the noise in FSS is due to RIN and not the other way round (FSS causing RIN).  I tried taking an OLTF of the PMC loop but the NPMC Loop unlocks as soon as I connect the source port of AG4395A to an excitation port in a summing stage. So I've been unable to verify the source of this RIN yet, but previous measurement (CTN:2502) suggests it is PMC.

HAPPY HOLIDAYS

2507   Tue Jan 7 17:59:20 2020 anchalDailyProgressFSSNPMC TF

### Method:

• I injected excitation at FP1Test and readout at Mixer Out (FP3Test).
• All TF were taken in three stages encompassing the whole frequency range. The same configuration files were used.
• First, I took TF measurement when the loop was closed and the variable gain is 6 dB.
• Then I opened the loop by removing mixed down PD signal from FP2Test and shorting it. This gave direct transfer function from FP1Test to Mixer Out as in second plot.
• Using the two measurements, I calculated the open-loop transfer function for the NPMC Servo Loop.

### Inference:

• My hope was to see some activity or unity gain around 433 kHz where we see the spike of noise.
• But the unity gain is much before around 6 kHz with phase margin of about 90 degrees.
• There is a small dip around 433 kHz but again it seems like a reaction to the problem rather than the source of the problem.
• The measurement, however, is pretty trashy. I could have used SR785 for the lower frequency region to make it better.
• I think we are back to square one. Any comments to infer more or suggest further tests are welcome.

Data and Analysis

Attachment 1: NPMC_TF.pdf
2508   Thu Jan 9 19:00:19 2020 anchalDailyProgressFSSTTFSS OLTFs

I repeated this measurement to compare later after changes to RFPDs.

Data

Attachment 1: FSS_OLTF.pdf
2509   Mon Jan 13 19:04:38 2020 anchalDailyProgressFSSFSS Diagnostics - SN009 (North RFPD) Notch Improvement Attempt

### Circuit changes (Updated Schematic)

I made the following changes to SN009:

• R15 -> 10k Ohms for increasing Test Input Power.
• R1   ->  226 Ohms
• R2   ->  25 Ohms (Two 50 Ohm resistors stacked in parallel)
• Added Ln, 2.2 uH fixed inductor.
• Added Cn 0.5-1.5 pF Trim Tunable Capactior.
• Ln and Cn are in series such that Ln + Cn is parallel to R1.
• Rewired the 5V line from one end of C3 to one end of C14 (U8-14) with yellow 20 AWG single core wire.

### Measurements

• I used the setup in 40m to take light transfer function of the modified SN009.
• Method described in CTN:2247 was used.

### Inference

• It turns out, however, that the placement of "active" notch didn't help at all. Infact, the ratio between peak and notch went from 30 to 20 :(
• I needed to reduce the values of R1 and R2 to ensure the notch doesn't effect the peak transimpedance much.
• During the tuning, I found that as I turned the screw on the capacitor, the transimpedance will just cycle through phases of low and high impedance. I dont' think I ever saw a very clear notch because of the new changes.
• I also tweaked the passive notch slightly to bring it closer to 72 MHz.
• I'll think more about what has happened and will try another pair of inductor and capacitor.

Data

Attachment 2: SN009_D980454-00-C.pdf
Attachment 3: SN009_Notch_Improvement_Attempt.pdf
2512   Tue Jan 14 18:19:33 2020 anchalDailyProgressFSSFSS Diagnostics - SN009 (North RFPD) Notch Improved!

### Circuit changes (Updated Schematic)

I made the following changes to SN009:

• R1   ->  100 Ohms
• R2   ->  10 Ohms
• Added Ln, 220 nH fixed inductor. (Coilcraft 1206CS-221X_L_)
• Added Cn 4.5-50 pF Trim Tunable Capactior (GKY50086).
• Ln and Cn are in series such that Ln + Cn is parallel to R1.

### Measurements

• I used the setup in 40m to take light transfer function of the modified SN009.
• Method described in CTN:2247 was used.

### Inference

• The ratio of resonance peak at 1-Omega (36 MHz) to 2-Omega (72 MHz) has been imporved from about 30 to about 200!
• Overall gain has been reduced but we can just increased the laser power to compensate for that.
• The problem from last time got resolved by chosing a combination of L and C such that C is significantly more than any stray capacitances.

Data

Attachment 2: SN009_D980454-00-C.pdf
Attachment 3: SN009_Notch_Improvement_Attempt.pdf
2513   Thu Jan 16 11:17:15 2020 anchalDailyProgressFSSFSS Diagnostics - SN010 (South RFPD) Notch Improvement Attempt

### Circuit changes (Updated Schematic)

I made the following changes to SN009:

• R15 -> 10k Ohms.
• R1   ->  100 Ohms
• R2   ->  10 Ohms
• Added Ln, 220 nH fixed inductor. (Coilcraft 1206CS-221X_L_)
• Added Cn 4.5-50 pF Trim Tunable Capactior (GKY50086).
• Ln and Cn are in series such that Ln + Cn is parallel to R1.

### Measurements

• I used the setup in 40m to take light transfer function of the modified SN010.
• Method described in CTN:2247 was used.

### Inference

• The ratio of resonance peak at 1-Omega (37 MHz) to 2-Omega (74 MHz) has been improved from about 20 to about 120.
• This is less improvement than what we saw on the SN009 case.
• Also, the measurement through test port and using lasers is not matching up. This could mean that my estimation of DC transimpedance is wrong.
• In the last plot, I tried multiplying the measurements of the result by a factor of 3.56 and it makes the transimpedance measurements match with test port measurements.
• This shows there is a factoring error in my estimation of dc transimpedance or maybe the whole thing is damped down when light falls on the photodiode instead of test signal.
• Also, there is a significant oscillation peak at around 200 MHz. Maybe that is the reason for the damped overall transimpedance. Working on this next.

Data

Attachment 2: SN010_D980454-00-C.pdf
Attachment 3: SN010_Notch_Improvement_Attempt.pdf
2514   Thu Jan 16 20:18:44 2020 anchalDailyProgressFSSFSS Diagnostics - SN010 (South RFPD) Notch Improved

### Circuit changes (Updated Schematic)

I made the following changes to SN009:

• R1   ->  200 Ohms
• R2   ->  10 Ohms
• Ln   -> 470 nH r. (Coilcraft 1206CS-471X_L_)
• Replaced U1, MAX4107.

### Measurements

• I used the setup in 40m to take light transfer function of the modified SN010.
• Method described in CTN:2247 was used.

### Inference

• The ratio of resonance peak at 1-Omega (37 MHz) to 2-Omega (74 MHz) has been improved from about 20 to about 230.
• Also, the measurement through test port and using lasers is still not matching up. This could mean that my estimation of DC transimpedance is wrong.
• In the last plot, I tried multiplying the measurements of the result by a factor of 3.56 and it makes the transimpedance measurements match with test port measurements.
• This shows there is a factoring error in my estimation of dc transimpedance or maybe the whole thing is damped down when light falls on the photodiode instead of test signal.
• But the oscillation peak problem has been resolved. So the photodiode is good to be used. Minor tweaks might be required to see why test port estimation is different from laser measurement.

Data

CTN:2516 : Notch improved on FSS RFPDs. Gain values increased as well.

CTN:2517: TTFSS OLTFs with Maximum Gain

Attachment 2: SN010_D980454-00-C.pdf
Attachment 3: SN010_Notch_Improved.pdf
2515   Wed Jan 22 14:16:28 2020 anchalDailyProgressFSSTTFSS OLTFs

Repeated these measurements after changes to the RFPDs (Notch improved).

### Inferences:

• No significant shape change is observed in the Open loop Transfer Functions (OLTFs).
• Only minor absolute value changes have happened which just indicate a change in overall transimpedance at omega frequencies of the RFPDs.
• This indicates that if non-linearity was affecting anything, it at least wasn't the shape of the OLTFs or the suppression of laser noise.
• Also, these measurements endorse that no major harm has been done to the circuits in the process of modification (as should be the case).

Data

Attachment 1: FSS_OLTF.pdf
2516   Wed Jan 22 15:20:00 2020 anchalNotesBEATNotch improved on FSS RFPDs. Gain values increased as well.

### New Gain Values:

South Common Gain: 24 dB , Fast Gain: 18 dB

North Common Gain: 14 dB, Fast Gain: 14 dB

I incrased the above values as much as I could without getting oscillations in the loop.

### RFPD Changes:

• The North FSS RFPD SN009 (CTN:2512) and South FSS RFPD SN010 (CTN:2514) have been improved in terms of ratio of their peak transimpedance to their notch transimpedance by implementing an active notch in the output RF amplifier.
• This was supposed to reduce non-linear effects (if any) because less of the 2-Omega frequency would go through the amplifier.
• It seems like it didn't help at all as the beatnote noise spectrum is at the same place as before.
• While the gain values of FSS have increased, the transimpedances of the RPFDs decreased after the changes,

Latest BN Spectrum: CTN_Latest_BN_Spec.pdf

Daily BN Spectrum: CTN_Daily_BN_Spec.pdf

CTN:2514 : FSS Diagnostics - SN010 (South RFPD) Notch Improved

CTN:2517: TTFSS OLTFs with Maximum Gain

2517   Wed Jan 22 16:00:26 2020 anchalDailyProgressFSSTTFSS OLTFs with Maximum Gain

Repeated these measurements with maximum possible gain values in the FSS loops.

Data

CTN:2514 : FSS Diagnostics - SN010 (South RFPD) Notch Improved

CTN:2516 : Notch improved on FSS RFPDs. Gain values increased as well.

Attachment 1: FSS_OLTF.pdf
2518   Thu Jan 23 19:23:03 2020 anchalDailyProgressFSSNFSS Boost Stage

Today Koji came to the lab to help me out with the FSS and give me few tips.

We made some changes to the boost stage at U7 in North FSS Box board D040105-C. All the changes were made by Koji personally. He replaced Koji showed me how to solder wires and SMD components so that I can do it better next time. We soldered the wires for the boost switch, replaced the resistor R29 with a thin film 5.6 kOhm resistor and replaced the capacitor (however the older capacitor was fine too). Overall, we made the arrangement of the components neater in the stage.

We figured that the only way to measure the transfer function of this stage when boost is on is to provide some reasonable offset as well so that the opamp doesn't saturate at DC. Hence, I do not have a good measurement of the stage before the changes were made. But after the touch-up, the stage is very close to the expectation as shown in the plot.

Data

Relevant elog posts:

CTN:2384 : Mentioned that something is wrong with the boost stage. But It was just the issue of measuring it wrong.

Attachment 1: NorthFSSBosstStageTF.pdf
2519   Mon Jan 27 18:29:19 2020 anchalDailyProgressFSSFSS Diagnostics - RFPD RF Out Spectrum

After the Notch improvement, I took 20 dB coupled outputs of the RF out ports of the FSS RFPDs, SN009 (North) and SN010 (South), when cavities were locked or unlocked.

### Measurement:

• Configuration files for all measurements are present in the Data directory.
•  ZFDC-20-5-S+ 19.5 dB directional coupler was used.
• 19.5 dB was added to all measurements to correct back from coupled value.
• Note that in case of measurement with unlocked cavities, attenuation of 10 dB was used for North side as it was overloading Ag4395A. hence different noise floors.

### Inference:

• No significant oscillations above 100 MHz, in particular around 300 MHz or 350 MHz.
• When unlocked, the second harmonic is 50.7 dB lower than the first harmonic for North side and 53.7 dB lower for South side.
• However, when locked, the second harmonic is only 18.1 dB lower than the first harmonic for North side and 6.9 dB lower for South side.
• Last measurement was taken in CTN:2470 where the difference between first and second harmonics during lock was ~12 dB for North and ~17 dB for South.
• But I don't have very good confidence with the last measurement as it was foolishly taken in Noise mode and had different IF Bandwidth than the present measurement.
• With -50 dBc SFDR (Spurious Free Dynamic Range) of MAX4107, I expect that the distortion signal generated at the 1-Omega frequency would be -90.95 dBm for North and -96.5 dBm for South.
• This suggests that the non-linear distortion component in the error signal would be about 59 dB lower than the actual signal for North side and 53 dB lower than the actual signal for South side.

Data

Relevant elog post:

CTN:2470 FSS Diagnostics - RFPD RF Ouput under inspection

Attachment 1: FSS_RFPD_RFOut_Spectrum.pdf
2521   Wed Jan 29 14:49:48 2020 anchalDailyProgressBEATBeanote Spectrum vs FSS Gain Values

In a discussion with Craig sometime back, it was brought up what happens when I lower the gains of the FSS loops. So today I did a test which lowers the Common and Fast Gain values on the FSS boxes by 3 dB in each step and sees what happens to the beatnote.

### Measurement

• The measurement was done solely by bnvsFSSgains.py script present in the Data folder.
• Before each measurement, the gain values are stepped and the script waits for 5 s before measurement is attempted.
• Measurement is done through the usual mokuPhaseMeterTimeSeries.py script which is used for all beatnote measurements of the lab. Recently I was able to make it more robust with some discussion with liquid instruments application engineer.
• All default settings were used except for duration which was kept to 10s to keep reasonable time series data file size.
• SN101 detector's RF out coupled through a 10 dB directional coupler was used to make the measurement (as usual).
• I ensured by watching the error signal on an oscilloscope that none of the FSS loops went into oscillation during the measurement. Its is hard to say for lower gain settings though if that happened or not.

### Inference

• The beatnote spectrum noise does not start going up until after the gains have been reduced significantly by around 12 dB at each stage.
• Also, weirdly the beatnote spectrum was actually lower for 2 particular settings of gain which were midway. There the beatnote spectrum is only 2 times as much as estimated!
• I'll repeat this measurement again to make sure this optimal gain setting region was not a coincidence due to some other parameter out of my control.
• But since we do not see a clear scaling of beatnote noise with lowering of FSS gains, I think it is safe to infer that we are actually not limited by residual NPRO noise as has been the thesis for quite some time.

Data

Attachment 1: Beatnote_vs_FSSGain.pdf
2522   Wed Jan 29 15:58:26 2020 anchalDailyProgressBEATBeanote Spectrum vs FSS Gain Values
 Quote: I'll repeat this measurement again to make sure this optimal gain setting region was not a coincidence due to some other parameter out of my control.

### Measurement

• Everything is almost the same as the last measurement.
• This time, if you notice from orange to yellow curve, I had to manually lower down the gain on the south common path by 12 dB instead of 3 dB to ensure the south path doesn't go into oscillations.
• From there onwards, the gains were reduced normally by 3 dB in each step/

### Inference

• The beatnote spectrum indeed drops down to lowest when the gains are 3 dB below the maximum gains where I keep the loops.
• Maybe keeping a large gain margin is important. But I need help to understand this result.
• Also, it is kind of clear that the laser noise starts showing up from the yellow curve onwards only. So in current gain settings, the noise floor is probably due to something else.
• Another weird fact I saw was that the RMS of error signal taken at "Mixer" port which is the TP5 point in LIGO-D040105-C_CTN_North increases significantly only after the gain values of the loop is decreased by more than 15 dB (to -1 dB each on FAST and COM).
• The same thing can't be said for the south path where I see a gradual increase in this error signal RMS value as the gains are reduced.
• If someone has a better idea of understanding these results or has some suggestions on further tests or a combination of parameter changes I should do, please let me know.

Data

Attachment 1: Beatnote_vs_FSSGain.pdf
2523   Thu Jan 30 10:55:22 2020 anchalNotesBEATLowest ever beatnote spectrum today.

Today we have measured the lowest beatnote spectrum till now. This happened because I set the FSS gain values to lower than the maximum I could reach.

FSS North South
Common Gain (dB) 11 21
Fast Gain (dB) 11 15

Latest BN Spectrum: CTN_Latest_BN_Spec.pdf

Daily BN Spectrum: CTN_Daily_BN_Spec.pdf

Relevant Elog Post:

CTN:2518: NFSS Boost Stage

CTN:2514: SN010 (South RFPD) Notch Improved

CTN:2512: SN009 (North RFPD) Notch Improved!

Attachment 1: CTN_Daily_BN_Spec.pdf
2524   Fri Feb 7 09:57:32 2020 anchalDailyProgressBEATSpanned gain parameter space in NFSS and checked beatnote

Yesterday I took beatnote measurements and spanned gain values (COM and FAST) to see the variation in beatnote with them.

### Method

• I used BNspec.py with default settings (10 kHz bandwidth at 15.625 kSa/s for 60s) while spanning the parameter space of NFSS gains.
• Every time the gain values are changed, a gainCycle.py is done and script waited for 10s before attempting beatnote measurement to let loops settle down.
• The beatnote frequency was robustly within 2 kHz of 27.34 MHz during this whole measurement.
• Overall experiment was run by spanParamSpace.py script.

### Inference

• I calculated the integrated total beatnote frequency noise in the frequency range 200 Hz to 1 kHz where the total noise is expected to be dominated by coating Brownian noise.
• Interestingly, the beatnote noise in this range is not so much dependent on the NFSS loop gain values.
• I notice a largely flat heatmap with no significant local minima.
• This indicates that beatnote is indeed not limited by North NPRO residual frequency noise in this region at least.
• I'm repeating this measurement today while scanning SFSS loop gains.

Data

Attachment 1: NFSS_Spanned_Beatnote_Variation.pdf
2525   Fri Feb 7 18:12:03 2020 anchalDailyProgressBEATSpanned gain parameter space in SFSS and checked beatnote

I took beatnote measurements and spanned gain values (COM and FAST) on South side to see the variation in beatnote with them.

### Method

• I used BNspec.py with default settings (10 kHz bandwidth at 15.625 kSa/s for 60s) while spanning the parameter space of NFSS gains.
• Every time the gain values are changed, a gainCycle.py is done and script waited for 10s before attempting beatnote measurement to let loops settle down.
• The beatnote frequency was robustly within 2 kHz of 27.34 MHz during this whole measurement.
• Overall experiment was run by spanParamSpace.py script.

### Inference

• I calculated the integrated total beatnote frequency noise in the frequency range 200 Hz to 1 kHz where the total noise is expected to be dominated by coating Brownian noise.
• Unstable regions in the parameter space are clearly visible.
• But as to the stable region, once the loop gets onto that, the noise does not change much, similar to North side.
• So here as well, it seems like the beatnote noise is largely independent of the gain settings.
• I'm unsure if I should look at this and the last measured data sets differently. Maybe I'm missing something.

Data

Attachment 1: SFSS_Spanned_Beatnote_Variation.pdf
2526   Mon Feb 10 11:07:33 2020 anchalDailyProgressBEATSpanned gain parameter space in PMCs and checked beatnote

I took beatnote measurements and spanned gain values in the PMC to see if stability issues in PMC loops can be affecting the FSS downstream.

### Method

• I used BNspec.py with default settings (10 kHz bandwidth at 15.625 kSa/s for 60s) while spanning the parameter space of PMC gains.
• The beatnote frequency was robustly within 2 kHz of 27.34 MHz during this whole measurement.
• Overall experiment was run by spanParamSpace.py script.

### Inference

• I calculated the integrated total beatnote frequency noise in the frequency range 200 Hz to 1 kHz where the total noise is expected to be dominated by coating Brownian noise.
• Unstable regions in the parameter space are clearly visible.
• Ideally, the noise in beatnote frequency should have very minimal coupling with the gain values in PMC loops.
• This has been found true indeed for the south side but not so much on the North side.
• Very clearly there is a corridor from -3 dB to 8 dB in NPMC gain where noise remains low. We generally use 7 dB value.
• But this might just be too bad PMC loop causing light amplitude noise in the output.
• There is a bulge at 400 Hz in the beatnote freqeuncy noise. I can also see a broadened oscillation in PMC error signals around that frequency on both sides.
• I'm afraid that maybe the North PMC is causing big mode mismatch at 400 Hz causing the noise in the beatnote frequency. Or is that not possible?

Data

Attachment 1: PMC_Spanned_Beatnote_Variation.pdf
2528   Mon Feb 10 17:41:25 2020 anchalDailyProgressscatterCleaned up table; Installed hex beam dumps

Today I cleaned up the table, removed Scott's RFAM measurement setup and installed hex beam dumps on the input rejection of faraday isolators.

### Recalibration

• I measured the laser power at (50, 26) and (50, 29) when the lasers were detuned from the cavities using a power meter. Then I updated the ratio of DC readout of SN010 and SN009 with these power levels to infer reflected power in mW correctly.
• Similarly, I measured the laser power at (9, 26) and (9, 29) right after the cavities and updated ratio with dc readout of cavity transmission photodiodes at (4, 37) and (9, 34) to infer transmitted power correctly.
• I also adjusted horizontal tilt of periscope top mirrors at (46, 26) and (46, 29) to improve modematching to about 68% on North side and 75% on South side (numbers calculated by ratio of transmission to reflected power).

### Table cleanup and new beam dumps

• Not complete, but I removed a lot of unused optics, beam dumps, clamps etc from the table.
• I removed the photodiodes I put in before and after PMC to measure the intensity noise in the north path and its origin (CTN:2502).
• I dismantled Scott's RFAM measurement set up (CTN:2377) on the South path which used pick off from the input rejection of the faraday isolator at (58, 19).
• I have now installed new hex beam dumps at the input rejection of the faraday isolators on path path at (62, 16) and (65, 27).

2529   Tue Feb 11 18:21:17 2020 anchalDailyProgressPMCCan't keep my mind of this; Why PMC Error signal has these oscillations?

The PMC error signals have some weird broadened oscillations in them:

Above, pink is North and green is South PMC Error Signal taken from Mixer Out port.

### Spectrum measurement:

• I took the spectrum using SR785 of the Mixer Out port of the two PMC loops.
• Next, I measured the cross-spectrum density of the two error signals.
• I wasn't sure though if the peak in the cross-spectrum is just because higher magnitude signal on both error signals, so I divided it by the magnitude of measured FFT of the two signals to get a unitless cross spectrum in the third plot.
• I'm not sure though how to interpret the cross-spectrum in either case.
• The oscillations could be a manifestation of some common-mode noise on the table (maybe some mechanical oscillation).
• Or there could be a systematic bad UGF around 480 Hz.
• In any case, can we actually relate the bump seen in beatnote noise near 400 Hz to these oscillations?

Data

Attachment 2: PMC_Error_Signal_Analysis.pdf
2530   Wed Feb 12 10:01:04 2020 anchalNotesBEATIncreased sampling rate to 125kSa/s; lowest noise in higher frequencies

I have increased the sampling rate of moku to 125 kSa/s (fastest allowed) for the frequency-time series acquisition. After reading matermost chat of Sean Leavey etc, I felt I might have downconverted aliased noise as earlier sampling rate was just 15.625 kSa/s. This didn't change the noise below 1 kHz but we see some improvement above 1 kHz so I'll keep this from now on. The measured time series files are not around 900 Mb, so I'm not saving them and only keeping the psd calculated using modifiedPSD.py script.

Latest BN Spectrum: CTN_Latest_BN_Spec.pdf

Daily BN Spectrum: CTN_Daily_BN_Spec.pdf

Relevant elog post:

CTN:2521 and replies: Beanote Spectrum vs FSS Gain Values

CTN:2528: Cleaned up table; Installed hex beam dumps

2531   Thu Feb 13 15:27:18 2020 anchalDailyProgressBEATFurther iterated back and forth to optimized FSS Gains.

I took beatnote measurements and spanned gain values in the PMC to see if stability issues in PMC loops can be affecting the FSS downstream.

### Method

• I used BNspec.py with default settings (10 kHz bandwidth at 15.625 kSa/s for 60s) while spanning the parameter space of PMC gains.
• The beatnote frequency was robustly within 2 kHz of 27.34 MHz during this whole measurement.
• The overall experiment was run by spanParamSpace.py script.
• First I spanned North gains using the last optimum values found for South Side and tried to span the area towards which we found minima earlier.
• Next, I used the new minima found for North side and spanned region in South side.
• This time, I used sum of the ASD of beatnote frequency between 300 to 600 Hz to determine improvement in noise overall.
• Still, I would say, the improvement is hardly more than 5% over large areas of parameter space, meaning mostly BN noise is independent of these gains.

### Final result

North South
COM Gain (dB) 12 22
FAST Gain (dB) 10 17

Data

Attachment 1: FSS_Gain_Span.pdf
2532   Thu Feb 13 16:35:32 2020 anchalDailyProgressBEATBN Detector was saturated. Reduced laser powers.

I found that the beatnote detector was actually saturating and the output was not a good pure sinewave. I've reduced the laser powers reaching the intensity to avoid that so that 20 dB coupled output of beatnote remains around 200 mVpkpk. Following is the summary of changed settings:

North South
Before locking reflected power (mW) 4.97 5.92
After locking reflected power (mW) 1.32 0.96
After locking transmitted power (mW) 2.96 3.38
After locking total accountable power (mW) 4.28 4.34
Estimated power loss (mW) 0.69 1.58

However, the beatnote did not change because of these changes showing that moku is strictly sensitive to zero crossings of the acquired signal rather than its shape near the edges.

2533   Thu Feb 13 18:00:30 2020 anchalDailyProgressscatterblocked NF1811 with hex beam dump

I have blocked the unused output port of the beam splitter before our beatnote detector with a hex beam dump at (13, 19). This was being used for broadband detector NF1811. We don't need it now.

2534   Thu Feb 13 18:06:00 2020 anchalDailyProgressNoiseBudgetAdding dark noise measurement of beatnote detector SN101

I measured dark noise of the beatnote detector reaching moku and its effect on measured beatnote frequency noise.

### Measurement

• I used Andrew's low noise preamplifier to measure this noise.
• I first took the transfer function through the preamplifier so that everything later can be referenced back to its input.
• I measured measurement setup noise by shorting the input to the pre-amplifier.
• Then I measured dark noise after the 20 dB coupler (the cable that I connect to moku) when the detector was completely blocked with a beam dump.
• The noise was converted to precieved beatntoe frequency noise by using peak to peak voltage of beatnote signal. (Noise*freq*2*pi/Vsigpkpk)
• The noise came a little less than Tara's measurement. I'm updating my noise budget model with this now.

Data

Attachment 2: BNDetDarkNoise.pdf
2535   Fri Feb 14 10:59:13 2020 anchalDailyProgressBEATBeatnote noise lowered in low frequency region

With some of the changes done recently, the beatnote noise has lowered in the low-frequency region indicating reduction in scatter.

Latest BN Spectrum: CTN_Latest_BN_Spec.pdf

Daily BN Spectrum: CTN_Daily_BN_Spec.pdf

Relevant elog post:

CTN:2530 : Increased sampling rate to 125kSa/s; lowest noise in higher frequencies

CTN:2533 : blocked NF1811 with hex beam dump

CTN:2535 : BN Detector was saturated. Reduced laser powers.

2538   Fri Feb 14 17:40:27 2020 anchalDailyProgressISSInstalled ISS on both paths using SR560s

I've installed ISS on both paths using 3 SR560s each. Preliminary feedback is setup to get a stable loop. More optimization with TF analysis can be done further.

### Path changes:

• I replaced the unmarked waveplate in the north path right after the EOAM at (112, 39) with a zeroth-order quarter waveplate.
• I adjusted these quarter waveplates at (112, 39) and (90, 24) to the half transmission through the PBS when EOAM inputs are shorted.
• I followed the notes in Antonio's post (See CTN:1593 and CTN:1604).

### ISS:

• ISS is formed with three SR560s in series.
• First SR560 is AC coupled, has a 2nd order zero at 1 Hz and gain of 100 at Low noise Mode.
• The second SR560 is DC coupled and has a pole at 1 kHz (North) and 3 kHz (South) with gain 1 and High Dynamic Reserve setting.
• The third SR560 is also DC coupled and has a pole at 300 kHz with gain 1 and High Dynamic Reserve setting.

### Measurement:

• I took the measurement of the spectrum from the output of transmission photodiodes.
• I used the calibrated coupling factor I measured earlier to convert the measured voltage to watts of power right after the cavities. (See CTN:2528).
• Measurement configuration files is attached.
• We see about a factor of 10 improvement in the intensity noise between 100Hz to 1 kHz.
• Note that the bad spikes notes in the beatnote spectrum at 480 Hz and around 900 Hz are present in the north transmission spectrum as well.
• Effect on the beatnote noise would be evident after tonight's measurement at 3 am. In my preliminary measurements, I see some reduction in noise below 200 Hz but nothing much.

Data

Attachment 2: ISS_Effect.pdf
2540   Mon Feb 17 16:08:23 2020 anchalDailyProgressBEATBeatnote noise lowered in low frequency region with ISS

After installing the preliminary ISS, which I'll change tomorrow as per Rana's suggestions, we see some reduction in the beatnote noise in the lower frequency region. I think I should also have an estimate curve for the coupling of laser intensity noise into the final result. I can maybe make some sort of transfer function measurement from actuation on intensity to the beatnote frequency itself using moku.

Latest BN Spectrum: CTN_Latest_BN_Spec.pdf

Daily BN Spectrum: CTN_Daily_BN_Spec.pdf

Relevant elog post:

CTN: 2538 : Installed ISS on both paths using SR560s

CTN:2539 : Rana's suggestion on ISS.

Attachment 1: CTN_Noise_Budget_Effect_ISS_ON.pdf
2543   Fri Feb 21 16:22:04 2020 anchalDailyProgressBEATBeatnote noise lowered in low frequency region with ISS

After installing the new ISS, the noise is even lower in lower frequencies. Still no change in the noisy peaks at 400 Hz and around 800 Hz. There is also no difference in noise floor above 200 Hz.

Latest BN Spectrum: CTN_Latest_BN_Spec.pdf

Daily BN Spectrum: CTN_Daily_BN_Spec.pdf

Relevant elog post:

CTN: 2542: Installed New ISS on both paths using SR560s

Attachment 1: CTN_Noise_Budget_Effect_ISS_ON_New.pdf
2545   Fri Feb 28 14:00:11 2020 anchalDailyProgressISSInstalled New ISS on both paths using SR560s - PROBLEM

I'm currently struggling with a problem in North ISS which I think needs to be documented here. Here's the synopsis:

### The problem:

• After running for the weekend with high gains and gain peaking at ~45 kHz, I found this week that the North ISS was overloading.
• The power level reaching the cavities change over the course of the day by +- 20-30% usually (still a mystery to me), so I assumed this could just be because of increased laser power saturating the SR560.
• But what I have found is that except for some random instances, I can't keep the gain high enough in this ISS path.
• At the start, I thought this problem is at least reset-able by unlocking the North FSS and locking the cavity back. But later, I've found that it is not working either now.
• The defining feature is, as I slowly increase the gain (giving ample time for the loop to settle), at the first point where I see any sign of overload, it happens at a rate of ~300 mHz, that's once every 3  seconds!
• I checked at the photodiode output at this gain setting and I see the loop gets unstable and for about half a second every 3 seconds or so.
• On increasing the gain further, this overload just becomes a constant condition.
• My first guess is that somehow the lower unity gain frequency is becoming unstable. But the mystery is:
• Why did this not happen before?
• Why is this not seen in the South Path at all? It is healthy as ever.
• What really changed during the weekend?
• And why once in a while, it was actually working?
• The only difference in the SR560 settings for the two loops is that the South path has 'invert' ON while North path does not. At first, I was perplexed by this too. South needed 'invert' to be stable and the North did not. Maybe this is the reason?
• I have ruled out any failure in SR560 by switching it with another one and seeing the exact same phenomenon.
• Remaining suspects are a damaged BNC cable/connector, damaged photodiode or EOAM (I wish not). However, some bad control loop configuration is my primary suspect.

I would like it if anyone has any comments on any of the points above or suggestions to tackle this problem. I can make some measurements and post them on requests.

2547   Mon Mar 2 16:48:12 2020 anchalDailyProgressISSFollow-up, porbalem stopped happening, OLTF plot.

The issue isn't visible right now. I'm not sure what changed but with a similar power level, I'm able to increase the gain on North ISS to much higher than before. Currently, I'm taking RIN measurements (still using in-loop detector) to update the noise budget plot which is taking some time as I'm trying to measure the uncertainty in the measurement as well. So, the step response plot would come later.

### OLTF

• Open-loop transfer function of the North ISS loop was measured by exciting at port B in SR560 and using mode A-B.
• Measurement was taken as Input A / Output which is equivalent to open loop gain divided by the transfer function of SR560.
• I measured the transfer function of SR560 separately and hence obtained the OLTF.
• The first measurement is actually product of Actuator TF, Plant and Detector and I have plotted it as well.
• This plot shows that a pole of 20 kHz is coming through the plant. It can't be detector or actuator as they have very high bandwidths.
• At the lower end, we couldn't reach to the point where AC coupling sets in. SR560 manual (at page 12) says it is 30 mHz.
• Lower UGF is roughly 5 Hz, which the gain margin from lower ugf must be more than 20 dB.
• Since the power level of laser changes somehow  over day, keeping this OLTF is not really fixed and moves up and down. Hopefully, in the present setting, it doesn't hit oscillations.

### Witness detector?

The beatnote detector SN101's DC output has electronically railed. This might be due to a disconnection inside which may or may not be intentional. I don't think I should meddle around with that detector so close to the result. I will instead replace the other 1611 detector in the blocked port of beam splitter (11,19) with a thorlabs PDA10CS and use that for RIN measurement. But since, the loops were in modd of being stable today, I decided to go ahead and take the measurement with current settings. This will also work as in-loop measurement for comparison later.

Data

Attachment 1: NISS_OLTF.pdf
2548   Tue Mar 3 10:02:12 2020 anchalDailyProgressBEATLowest ever beatnote spectrum today!

Measurement at 3 am in the morning today has been the lowest ever recorded beatnote noise. The lasers have been locked for more than a week and the temperature of the cavities is also very. The ISS gains were increased yesterday to 5x1000 on each loop. I've also added RIN measurement and implied photothermal noise.

Latest BN Spectrum: CTN_Latest_BN_Spec.pdf

Daily BN Spectrum: CTN_Daily_BN_Spec.pdf

Attachment 1: CTN_Noise_Budget_March03-2020.pdf
2549   Tue Mar 3 11:49:16 2020 anchalDailyProgressISSTransmitted Laser Relative Intensity Noise

Attached are the latest transmitted RIN measurements.

### Measurement:

• These are taken with in-loop photodiodes of ISS and hence are mostly Fake News. I'll do an out-of-loop measurement as well.
• The measurement is done by running SR785 repeatedly with no averaging and saving all the curves. 238 measurements were taken this way.
• The median, lower bound (15.865% percentile (lower 1-sigma for Normal Distribution)) and upper bound (84.135% percentile (upper 1-sigma for Normal Distribution)) are calculated.

Data

Attachment 1: CTN_Trans_RIN_measurement.pdf
2551   Sat Mar 7 09:50:31 2020 anchalDailyProgressISSTransmitted Laser Relative Intensity Noise - Out of loop

Attached are the latest transmitted RIN measurements.

### Measurement:

• I setup a Thorlabs PDA10CS photodiode at the dumped end of beatnote (15, 13).
• Then, I took out-of-loop RIN measurement of transmitted light by blocking either the North or the South path falling on the beatnote beamsplitter (11, 19).
• I found out that, when the cavity transmission photodiodes are connected to Acromag input, the acromag input injects some noise into the ISS.
• On disconnecting the acromags, I was able to increase gains in ISS loops to 20000.
• A major issue was to measure DC power together with the spectrum to calculate the RIN. I first connected the measured photodetector to Acromag Cavity Transmission Channels (which are empty now) and measured the ratio of DC level with the cavity reflection photodiode.
• Later, using the cavity reflected dc level and this ratio, I was able to estimate the dc level of the spectrum I'm measuring. This way, I ran RIN measurements sequentially for North and South transmitted light.
• The measurement is done by running SR785 repeatedly with no averaging and saving all the curves. 500measurements were attempted, few of them had file transfer errors and were ignored.
• The median, lower bound (15.865% percentile (lower 1-sigma for Normal Distribution)) and upper bound (84.135% percentile (upper 1-sigma for Normal Distribution)) are calculated.

### Inference:

• Higher gain improved the noise a little bit more upto 30 kHz.
• Notice that there are 60 Hz harmonics (both odd and even) in the South RIN, while they are not present in the North RIN.

Data

Attachment 1: CTN_Trans_RIN_measurement.pdf
2553   Wed Mar 11 13:16:09 2020 anchalDailyProgressISSTransmitted Laser Relative Intensity Noise - Out of loop

These measurements take a long time as I take a median over 500 single measurement instances. I was able to increase gain later after having taken in-loop noise measurement, so I didn't repeat it. But what I can do is a take a quick in-loop measurement today with simple averaging and no error bars and post it here for comparison. If we are really interested, I can run overnight measurement for in-loop at this gain as well.

 Quote: weird - why is the Gain different for in loop and out of loop ?

2555   Wed Mar 11 16:43:55 2020 anchalDailyProgressISSComparison between Out of loop vs In loop RIN

I took spectrum of Out-of-loop (OOL) photodiode and In-loop (IL) photodiodes with transmitted light from the cavities when ISS is on in both paths at gain value of 2x10000.

### Measurement:

• In-loop photodiodes of ISS have been disconnected from Acromag cards as this was injecting noise into the loops.
• So, without witnessing the DC level, we can increase the ISS loops' gain to 2x1000 with HF and LF both set at 300 Hz with 6 dB/octave roll off.
• A major problem in this measurement is the slow drift of DC power level during the time of measurement.
• I was measuring DC value of photodiode averaged over 10 s with an oscilloscope just before the measurement was taken.
• In fact, one can see that between two measurements of the different spans, the DC value has changed enough to show mismatch in South In-Loop RIN.
• The out-of-loop photodiode was measured with sequentially blocking either the north laser or the south laser.
• The same issue with knowing the exact DC level persists here as well.

### Inference:

• As expected the out-of-loop noise is higher than the in-loop noise, but I did not expect a factor of ~5 difference.
• However, this method of measuring RIN isn't very faithful.
• Overall for the experiment, it is clear that increasing the gain of ISS further did not lower the beatnote spectrum much.
• This suggests that we have damped down photothermal noise in the experiment below other fundamental noise sources.
• My future efforts would be focused solely to get rid of the peaks in the range of 200 Hz to 1 kHz.
• I see that the sharp peak at 480 Hz has changed into a tri-peak around the same frequency without any changes in the experiment other than ISS.
• This atleast indicates that this peak was not undefeatable, and so must be the remaining ones.

Data

Attachment 1: CTN_Trans_RIN_OOLvsIL.pdf
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