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.
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:
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.
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.:
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.
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.
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.
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 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.
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:
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.
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.
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:
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.
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.
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.
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.
North Laser is hereby acquitted.
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.
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.
Rana's question about PMC:
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?
Edited on Mon Dec 23 15:21:50 2019 .
For the convenience of others, I'm summarizing the open questions I asked on elog in December. Comments on the posts, advice or answers to my questions would be nice.
Data and Analysis
I repeated this measurement to compare later after changes to RFPDs.
I made the following changes to SN009:
Parallel relevant threads:
CTN:2516 : Notch improved on FSS RFPDs. Gain values increased as well.
CTN:2517: TTFSS OLTFs with Maximum Gain
Repeated these measurements after changes to the RFPDs (Notch improved).
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.
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
Repeated these measurements with maximum possible gain values in the FSS loops.
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.
Relevant elog posts:
CTN:2384 : Mentioned that something is wrong with the boost stage. But It was just the issue of measuring it wrong.
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.
Relevant elog post:
CTN:2470 FSS Diagnostics - RFPD RF Ouput under inspection
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.
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.
Relevant Elog Post:
CTN:2522: Beanote Spectrum vs FSS Gain Values
CTN:2518: NFSS Boost Stage
CTN:2514: SN010 (South RFPD) Notch Improved
CTN:2512: SN009 (North RFPD) Notch Improved!
Yesterday I took beatnote measurements and spanned gain values (COM and FAST) to see the variation in beatnote with them.
I took beatnote measurements and spanned gain values (COM and FAST) on South side to see the variation in beatnote with them.
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.
Today I cleaned up the table, removed Scott's RFAM measurement setup and installed hex beam dumps on the input rejection of faraday isolators.
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.
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.
CTN:2521 and replies: Beanote Spectrum vs FSS Gain Values
CTN:2528: Cleaned up table; Installed hex beam dumps
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:
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.
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.
I measured dark noise of the beatnote detector reaching moku and its effect on measured beatnote frequency noise.
With some of the changes done recently, the beatnote noise has lowered in the low-frequency region indicating reduction in scatter.
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.
CTN:2531 : Further iterated back and forth to optimized FSS Gains.
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.
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.
CTN: 2538 : Installed ISS on both paths using SR560s
CTN:2539 : Rana's suggestion on 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.
CTN: 2542: Installed New ISS on both paths using SR560s
I'm currently struggling with a problem in North ISS which I think needs to be documented here. Here's the synopsis:
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.
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.
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.
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.
Attached are the latest transmitted RIN measurements.
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.
weird - why is the Gain different for in loop and out of loop ?
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.