40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  PSL, Page 4 of 52  Not logged in ELOG logo
ID Date Author Type Categoryup Subject
  861   Sat Mar 3 17:50:45 2012 taraDailyProgressBEATbeat is back

NO!! no dance party for you because you didn't return our label maker.



We expect it to be stable by tomorrow afternoon.

 What about my dance party?! 



  862   Sat Mar 3 18:07:15 2012 ZachDailyProgressBEATbeat is back

And I stole one of your air dusters :-)

There used to be TWO label makers in the ATF---one of which looked like that one. What ever happened to those?!


NO!! no dance party for you because you didn't return our label maker.




  863   Sun Mar 4 00:45:26 2012 frank, taraDailyProgressBEATbeat is back

Beat is back.

The beat signal( with the new softer springs) is measured and compared with the previous result (unfloated table). 

several notes about this measurement:

  • The resonant peaks from the stack show up as expected at 10, and 35Hz.
  • I have not added the contribution from seismic in the nb yet since TF between beat signal and seismic is not measured. We will measure that once the temperature settles.
  • Beat noise at high frequency  and  PLL readout noise do not match. This is probably because the gain setup on PLL changes and the readout noise changes with the gain, see psl:816 .
  • The table is not floated. If we float the table,seismic noise at frequency above 3-4Hz along with acoustic noise around100- 1kHz should decrease, and we might have a good chance to measure thermal noise around 100 Hz.
  • In loop noise for RCAV is around 50nV/rtHz instead of 10 (we could go down to this level before, see psl:855). The gain on TTFSS loop cannot be increased to what it was before (Common/Fast=920/850)We are checking what are the problems.
  • A quick measurement for ACAV in loop noise gives flat noise floor of ~40nV/rtHz. The slope from ACAV's error signal is 52kHz/V. Thus, the frequency noise floor from ACAV is ~ 2mHz/rtHz. Although it is higher than the designed value  (smaller than shot noise level), it is certainly not the current limiting noise source. 



  864   Mon Mar 5 16:29:50 2012 frank, taraDailyProgressBEATbeat is back

As mentioned in the previous entry that the in loop noise is higher than before. We investigate what might be the causes. We suspect that mode hopping or relaxation oscillation might be the case, but they are not.

The loop gain for TTFSS cannot be increased up to the usual setup, one thing we notice is that the signal from error point is quite large ( the input range selected on SR785 cannot go down below -30 dBVpk, it used to be -48 or -50 dBVpk)and oscillating at 200kHz. So we are also looking for what causes the oscillation at this specific frequency as well.

1)Checking for mode hopping:      Mode hopping might produce unexpected behavior of the laser. We want to make sure that the   usual SLOW_DC value is not close to where mode hop occurs.

 method: scan PMC and look at the signal from reflected power. The dip from transmitted TEM00 should become shallower when  mode hop occurs because some of the power is lost to other TEM00 mode.

 result: we are no where near mode hopping. The SLOW_DC coarse knob can be adjusted between 200 - 700 easily without seeing mode hopping. The usual set value is around 500 +/-20.


2) Relaxation oscillation: (cf siegman, laser, ch 25)

   Since we are looking for what cause the oscillation at 200kHz, relaxation oscillation is a possible candidate. We use a PDA10CS to measure the spectrum of the laser power. We use the beam reflected from the first Faraday isolator just after the NPRO. A broad peak at 462 kHz is observed, so this is unlikely to cause the oscillation.


fig1: relaxation oscillation is measued (green) with the frequncy at 462kHz. The yellow line is the noise floor of the PD.

    To sum up, we are looking into what degrade the performance on RCAV loop. We ruled out two possibilities which are laser mode hopping and relaxation oscillation.

  870   Tue Mar 6 20:48:47 2012 frank, taraDailyProgressBEATbeat is back

After debugging the RCAV loop, we measured the beat signal again.  We still cannot reach coating thermal noise, the noise floor now seems to be mostly seismic related.



 Fig1: Beat measurement, grey trace shows the beat measurement from 2012-02-22, floated table. After that we changed the springs on seismic stack. Blue is the beat signal, unfloated table, and Pink is the beat signal with floated table.


     The new beat signal has more mechanical peaks around 300 Hz - 1kHz than the old one does. This is probably because I do not properly damp a few mirrors. The input range used in today's measurement is 2kHz. It does not change between 1 or 2kHz input range, so we probably sits on other technical noise sources.

     In conclusion, with the new springs, we can reduce seismic noise in 5Hz- 100Hz bandwidth (see grey and pink traces in the plot). Next step, we will work on damping mechanical peaks in the signal properly, and adding air spring for the vacuum chamber.

    Note: The setup was optimized for unfloated table. The blue trace was measured after we optimized everything, from polarization to beam alignment, both before and after the cavities. When I floated the table, the beam was barely aligned and could not be locked. I realigned the beam with periscopes, and adjusted the beam alignment on beat RFPD before measuring the pink trace.

  878   Sat Mar 10 18:42:42 2012 taraDailyProgressBEATbeat- with air spring isolation

 Beat measurement with new air springs is measured with unfloated table (the N2 supply is empty). Good improvement can be seen at frequency above 20Hz.


The data is compared with beat signal before the air springs, unfloated table on 2012_03_03.  Once we float the table we should see no seismic/mechanical peaks around 100Hz. Then effects from RFAM/RIN couplings might reveal themselves.

Note about the setup:

  • Power into both cavities are ~ 1mW (I also adjust the power for beams to RFAM measurement PDs to be ~0.8 mW).
  • I replace the plastic legs for RCAV RFPD, so that the height is 3 inch, the reflected beam is dumped properly.
  • The measured Marconi frequency noise from Marconi in this nb is not updated yet. I'll update that soon.
Attachment 1: beat_2012_03_10.png
  882   Mon Mar 12 23:31:38 2012 frank, taraDailyProgressBEATbeat with floated table, air spring.

We measured beat signal with the upgraded seismic isolation system. It seems that we are sitting on some scattering noise source at low frequency.


      Most of the mirrors mount in beat path, are damped with rubber cones. A lead block is put on the board for beat setup to damp down any resonant peaks. With air springs and floated table, seismic noise is significantly reduced. From the shape of the bump around 70 Hz, we suspect that it is the scattering noise. That will be investigated further.


Fig1:  In grey: beat signal after softer RTV springs were used. In pink: beat signal with air spring, unfloated table. In blue: beat signal with air spring, floated table.

  900   Fri Apr 6 01:03:01 2012 frank, taraDailyProgressBEATground loop problem

We fixed the ground loop problem in ACAV setup and got rid of most of the harmonic lines in beat signal.

    The ground loop problem in our current setup comes from the common ground of the two Marconis. The first one is for 14.75 MHz EOM. Its signal is split, and sent to demodulate the signal from ACAV RFPD. The second Marconi is for driving the AOM around 80 MHz for frequency lock. By using a transformer between the mixer (for PDH demodulation) and the LO signal from the first Marconi, we broke the ground loop, hence got rid of the harmonic lines in the signal.


  929   Sat Apr 21 00:55:00 2012 frank, taraDailyProgressBEATBeat measurement update

We measured the beat after a few changes in the setup (modulation index, air spring, small enclosure around the beat). We update the noise budget with suppressed laser frequency noise and Marconi's frequency noise in ACAV loop as well. There is not much improvement in the overall beat noise.

A few changes in the setup:

  •  Air Springs: We activated the air springs (it was off because we though it causes the vacuum chamber to move too much during the work), so there is one more seismic isolation stage. The peak at 58 Hz disappears.  However, there are peaks around 6-12 Hz showing up this time. They are probably from the table and the legs that are not set to float the table properly*.
  • Modulation index: now the modulation index is 0.18. I measured the power in the carrier and sideband to calculate the mod index, see PSL:855 . This is the maximum we can have with the 13dBm from Marconi with 4-way splitter (-6dBm),-0.5 dBm loss from the cable, so the power to EOM is 6.5dBm.  With the new mod index, the frequency discriminator gain (inverse) for RCAV/ACAV become 22 and 31 kHz/V.
  • A prototype enclosure box around beat setup: PSL:925.


Updated Traces:

  •   Suppressed Laser frequency noise: Since we changed the OLG of TTFSS by changing the modulation index, we remeasured the OLG of TTFSS, applied to the laser frequency noise (estimated to be 1e4/f Hz/rtHz). Good news, we have enough gain in RCAV loop to suppress the laser noise to coating level. The gain setup for common/fast is 750/750
  •  Frequency noise of Marconi for ACAV: We had not done this so far, so we wanted to make sure that it would not be the limiting source. This is done by dividing the frequency noise of marconi by ACAV OLG TF. We assume that the frequency noise is the same as we measured from the marconi for PLL plotted in the noise budget.
  • Shot noise is updated with the current modulation index (0.18) and cavities visibilities (~80%). It is not shown in the nb because I included it in the PDH detection noise.

* The table legs have small leaks, so we have air compressor to keep the pressure high, PSL:880. It works once every hour when the pressure is low.  The change of pressure can screw up the beam alignment a lot (for example, DC readout from RCAV RFPD can be varied from 100mV - 1.2V, with 1.7V maximum value (off resonance). I think it affects the alignment to AOM and change the diffraction efficiency/ beam shape as well.  It also changes the seismic isolation property. The peaks around 10 Hz show up when the pressure is low.  I'll find new legs from New Port and ask for the quote.


==Plan for the next few days==

1) Try using DAQ to measure beat signal, see if we can get rid off the frequency noise from PLL

2) Find out about the new legs(prices/availability)

3) Make the acoustic enclosure box around the whole setup

4) Work on the design for new vacuum windows.


  946   Fri Apr 27 22:37:54 2012 taraDailyProgressBEATreplacing one mirror mount in beat path

One of the mirror mount in beat path was not properly mounted on the board because of the limited space. I changed that mirror mount with a block mount (similar to the one we use for the beam recombining beam splitter). The acoustic coupling is getting better.



fig1: the mirror with the mount similar to that of the BS, see psl:818.


fig2: beat signal, comparison between before and after the mount replacement.

I'm not sure why the beat signal at 2kHz and above does not match. It might be that I did not align the beat well enough, or the alignment in front of the cavities changed. However, there is a significant improvement in beat signal, except the new mechanical peak around 1.2kHz, it might be from the new mirror mount.


Note: I turned off the air compressor switch after I measured the beat before the mount replacement to make sure that the seismic isolation for both measurements will be similar.



Attachment 1: IMG_0841.jpg
  947   Mon Apr 30 01:41:18 2012 taraDailyProgressBEATnoise due to air leaking from legs

I suspected that the air leaking from the legs might cause  noise in acoustic bandwidth, so I measured the beat signal with unfloated /floated table. The beat signals from the two look similar. Air leaking from legs does not cause any extra acoustic coupling from 100Hz - 3kHz in the beat signal. 



fig1: Beat signal between floated and unfloated table. There is no significant difference between the two.

       On Friday evening, I turned off the air compressor. When I came back on Sunday night, the table was not floated. I measured the beat signal, then turned on the compressor, realigned the beam, then measured the beat again. The results were similar in 100Hz- 6 kHz band. Floating the table with these leaking legs will not add any extra acoustic noise to our signal (at least, at this level).

      Since the result looked nice with fewer mechanical peaks at night, I took a chance to check if it was limited by frequency noise of marconi in PLL loop or not. I changed the input range from 1kHz to 200Hz (their frequency noise level should be different and observable at 100-1kHz bandwidth, see here), but there was not different between the two input range, see fig2 below.


fig2: beat with different input range. There is no significant change in the results.

    It is likely that the flat level we are sitting on are detection noise + shot noise. This will be carefully checked next.

  966   Fri May 18 03:56:22 2012 taraDailyProgressBEATbeat measurement update

The beat signal is getting better at lower frequency, there is no obvious scattering bump around 80 Hz anymore. It is possible that the previous legs + leveling valves were bad and caused extra vibration.

After legs and leveling valves replacement,  the beat signal has better stability at low frequency. There is no bump due to scattered light around 80 Hz anymore. Plus, acoustic coupling (400-600Hz)f is significantly reduce without any improvement on the acoustic shield. Thus, it is very likely that acoustic coupling is related to legs' isolation performance.,Wrong statement, see PSL:970.(Mon May 21 21:13:11 2012 )

 Mechanical peaks around 25-50 Hz are probably from horizontal, or tip/tilt motion of the seismic stacks.


fig1: beat measurement. floated table, floated air spring. 1mW input power for each cavity, 1kHz input range on PLL.

 The result around 80-200 Hz has slope  f^(-0.5). Alas the span is only ~100 Hz bandwidth, with hideous unidentified peaks around 240 Hz. Otherwise we can be more certain that we reach thermal noise of something even though it is ~ a factor of 1.5 above the upper limit.


fig2: close up plot of the above figure, around 80-300 Hz. The measurement result is about a factor of 1.5 above the upper estimated level.


fig3: comparison with f^-0.5 line (cyan).



  967   Fri May 18 18:44:38 2012 taraDailyProgressBEATbeat measurement update

I'm trying to fix the mechanical peaks in the beat signal. The work is still in progress.

There are several mechanical peaks around our most sensitive band (60 Hz - 600 Hz). It is important that we damp these to improve the beat signal. 

One of unidentified peaks are around 240 Hz , which might be originated inside the vacuum chamber. I tried tapping the chamber, and saw the peaks went up, but I could not pinpoint to where(stack/heat shield/ cavity motion) exactly.

There are peaks around 500Hz that come from QWPs behind the cavities. I checked their components individually to pinpoint where was the part that produced 500 Hz peak.  Each QWP was mounted on a rotatable square mount that are screwed down to a solid aluminum block.

  1. I inserted a sheet of rubber between the two mounts to damp any possible motion from the structure, but there was no improvement.
  2. I replaced the Al block with a steel 1" post.-> The peaks still there. So,
  3. I replaced the rotatable square mounts to the circular desing, on a steel post, and made sure that the QWPs were secured. -> The peaks still there
  4. I tilted the wave plates a little bit to prevent any possible back reflection, still no improvements.

It might be the motion of the plate itself. If this is the case, we have to have a better acoustic insulation box. The current one for the beat path has to many holes.

  978   Mon Jun 4 22:38:44 2012 taraDailyProgressBEATlow frequency beat with 32kHz ADC

I tried to measure beat signal without using PLL, the FFT of the demodulated signal does not look good. We probably need to do a code for software PLL to improve the signal for this method.

==Why not PLL?== ,

     as we can see in the previous beat measurement with PLL readout technique, frquency noise of the oscillator dominates the signal at high frequency (500Hz and above. Its frequency noise is dependent on the setup (carrier frequency and f modulation range). By turning off the modulation range, and demodulating the beat signal with a fixed frequency signal, we hope to reduce the frequency noise of the oscillator a by little bit.

==what I did==   

     We have a cable from PSL to ATF that goes to DAQ. The channel name is C2:ATF-PSL2_DAQ_OUT. 

  • Check DAQ: By sending in a sinewave signal, I can check that DAQ is working properly. [fig1, FFT of sinewave 15 Hz]. 
  • To calibrate the signal, I used DAQ to measure the signal at V feedback to VCO, then get the plot from diaggui in fb0, and scale it so that it is comparable with the calibrated beat measurement.The calibration is ~0.45  times the result from FFT, see the figure below.


  • Turn off the frequency modulation on the vco, then measure the demodulated signal at mixer out, use the calibration from (2) to convert to absolute frequency



     The FFT results from the fix demodulated signal are not very stable. Thus, the line width is quite large and depends on the number of average and time of the measurement we choose. The plot shows the FFT results from two different average, 200 and 100, (and one more 100 from different time).  We might need a code to track the center frequency of the time series data and then FFT it to get a valid result for this technique.




  980   Tue Jun 5 23:30:47 2012 ranaDailyProgressBEATlow frequency beat with 32kHz ADC



     The FFT results from the fix demodulated signal are not very stable. Thus, the line width is quite large and depends on the number of average and time of the measurement we choose. The plot shows the FFT results from two different average, 200 and 100, (and one more 100 from different time).  We might need a code to track the center frequency of the time series data and then FFT it to get a valid result for this technique.

 Yes, absolutely. If the frequency has some low frequency wander, there's certainly no way to measure the frequency noise with a straight FFT. Instead, you make sure that the beat frequency is up at ~5-10 kHz and then record a long time series (100 - 1000 s). You then use a software PLL to estimate the frequency noise.

Make sure the beat shows up as at least 10000 counts peak.

  990   Tue Jun 19 00:08:19 2012 taraDailyProgressBEATlow frequency beat with 32kHz ADC

I'm using simulink to do software PLL. 

As Zach suggested that using simulink to do the PLL might be easier than using his codes, I looked up this  example and trying to build a model for PLL. It's not working yet, I'm still trying to get FFT from the feedback signal to the vco.




     The FFT results from the fix demodulated signal are not very stable. Thus, the line width is quite large and depends on the number of average and time of the measurement we choose. The plot shows the FFT results from two different average, 200 and 100, (and one more 100 from different time).  We might need a code to track the center frequency of the time series data and then FFT it to get a valid result for this technique.

 Yes, absolutely. If the frequency has some low frequency wander, there's certainly no way to measure the frequency noise with a straight FFT. Instead, you make sure that the beat frequency is up at ~5-10 kHz and then record a long time series (100 - 1000 s). You then use a software PLL to estimate the frequency noise.

Make sure the beat shows up as at least 10000 counts peak.


  991   Wed Jun 20 16:06:50 2012 taraDailyProgressBEATlow frequency beat with 32kHz ADC

Koji suggested I try to measure the frequency noise of the demodulated beat this way. By using I and Q signals of the demodulated beat signal, I can plot the signal of I and Q, which should be a circle. From that I can get dphi/dt which is the instantaeneous frequency, then FFT to get the beat noise. 

To do that I need:

  • Power splitter with 90 degree phase shift - ZMSCQ-2-180, datasheet.
  • another cable for DAQ (one for I, one for Q)
  992   Wed Jun 20 17:50:59 2012 ranaDailyProgressBEATlow frequency beat with 32kHz ADC

 This is overdoing it. Please just post the existing beat data somewhere and I can show you how to do it easily with a few lines of matlab code. Then you can go back to your usual noise hunting.

  993   Wed Jun 20 20:48:16 2012 SarahDailyProgressBEATfrequency noise of demodulated beat

 To avoid doing PLL with Simulink, Tara and I measured the Q and I signals for the demodulated beat frequency to measure the frequency noise. We used the following setup:


The measured demodulated signal was around 1kHz. From the visio, the signal was split, and thus the power went from 13 dBm to 10dBm. The mixers used were intended for a power of 13dBm, not 10dBm, but since we were not worried about the signal to noise ratio it was okay. The function generator was set at 172MHz, because that is the frequency corresponding to a 90 degree phase shift. Additionally, the SR560 was set to a gain of 20 to avoid saturating the ADC. I am currently processing the data, which includes plotting Q vs I, finding delta_phi/delta_t to get the instantaneous frequency, and taking a FFT to get the frequency noise.

== the cable==

we calculated the length of the cable for 90degree phase shift around 170MHz to be ~0.3m (speed of the signal in the cable is 2e8 m/s), so we picked a cable with the length close to our estimation and measured the TF to check the phase delay. The chosen one has the following TF, so we chose the frequency around 170 MHz for 90degree delay.



== explanation for this method==



== demodulated beat frequency ==

After evaluating the instantaneous demodulated beat frequency by dphi/dT, I plotted the result of demod freq vs time to check if it agrees with what we saw on the scope or not(~1kHz). The result looks good, the frequency fluctuates around 1khz as intended.



== FFT of the demodulated frequency==

 Then I tried to find the PSD of the above plot, and compare with the measured frequency noise from a Marconi with 100kHz input range. The results are not in good agreement. The result from ADC is about a factor of 10 lower than the measured frequency noise. The result from ADC agrees more with Marconi frequency noise with10kHz input range It might be possible that I made a mistake by choosing 10kHz instead of 100kHz when I setup the input range for the marconi.  The shape of the signal from ADC drops at higher frequency probably because of the anti aliasing filter in ADC.


  994   Thu Jun 21 16:04:39 2012 taraDailyProgressBEATlow frequency beat with 32kHz ADC


 This is overdoing it. Please just post the existing beat data somewhere and I can show you how to do it easily with a few lines of matlab code. Then you can go back to your usual noise hunting.

 Here is the demodulated beat signal, with 32kHz sampling rate, 120 second time strecth. I used SR560 to amplify the demod signal so that pk-pk value is ~10 000 counts. The data is store in demod.data with a signal column .

Attachment 1: demod_2012_06_21.mat
  995   Fri Jun 22 03:35:54 2012 taraDailyProgressBEATfrequency noise of demodulated beat

I added the result from the I-Q measurement method in PSL:993.

  1009   Sat Jun 30 05:01:04 2012 taraDailyProgressBEATbeat measurement with 2mW input power

I changed the power input from 1mW to 2mW for both cavities and measure the beat signal. From the measured results, there is no difference between the beat signal at different power input levels. This means that the current signal is unlikely to be sitting on shot noise or electronic noise that depend on power level.


     With the current laser (500mW), we can increase the power to both cavities up to 2mW. With higher input power, shot noise and electronics noise can be brought down. Even though the noise budget suggests that we are not limited by shot noise and electronic noise, I think it is a good idea to verify it . If we are limited by the shot noise due to low input power or electronic noise due to low frequency discriminator gain, we can see the change if we increase the power.


  1. The power into both cavities are increase to 2mW.
  2. For RCAV, common/fast gain are 630/750.
  3. For ACAV I used an ND filter to reduce the power incident on RFPD. I chose the one that reduce the power roughly in half.

==results and comments==


I plot the beat measurement with  1mW (black) and 2mW(pink) power together to show that there is no apparent improvement. Note that the noise budget is corrected for 2mW input power for both cavities. (I have not include the noise due to power fluctuation yet, I'll add that soon). 

 The slope around 50 Hz to 200 Hz is strongly go with 1/f^0.5 which is the slope of Brownian noise. However, it is ~ a factor of 1.2 above the upper limit of our estimation. I will check with Peter if he still has some information about the coating or not, and also the parameters in the noise budget calculation (for example, the loss in SiO2 for spacer, substrate to see if the values I use are sensible or not.

  1039   Fri Aug 10 19:50:37 2012 taraDailyProgressBEATcode schmidtt trigger for beat

I used Schmidtt trigger process to track frequency of beat measurement. This is a first step for digital PLL.


      we are trying to do offline PLL digitally, so we can avoid extra frequency noise from the LO used in PLL. The first step is to track down frequency of the beat measured by the PD.

==the code==:

       I use Schmidtt trigger algorithm to covert analog signal to digital (the plot below show (-1,1) instead of (0,1) for easier comparison with the analog signal). The data below is taken from beat measurement  with +/- 5000 count. The level is set to +/- 0.2 from amplitude of +/-1. Then I record how long the digital signal stay at 1, or 0 before the signal flip, then use that time to calculate the frequency of half cycle and plot it in the below figure.



Plot: Above, measured signal from daq (blue) and digitized signal via Schmidtt trigger (Green). Below, frequency of beat as obtained by the calculation from the digitized signal. Note the different time span between the two plots.



     I have not FFT the frequency drift in time series yet because I just realize that the way I collect the frequency drift vs time might be a problem. The time step for frequency drift would be varied from point to point depending on the current drift frequency. For example,sat at 1Hz, the signal crosses zero twice per second, and twenty times per second at 10Hz. This means the data density (point per time) between the two frequencies are different, see the below zoomed picture. And it might cause a problem when I do FFT with varied dt size.   To fix this, I 'll try to assign constant frequency to fill in the space. Once the problem is fixed, I can just FFT the signal. I'll think about using PLL code as well and compare the two methods.


plot2: zoom in of the first figure.



 This is overdoing it. Please just post the existing beat data somewhere and I can show you how to do it easily with a few lines of matlab code. Then you can go back to your usual noise hunting.

 Here is the demodulated beat signal, with 32kHz sampling rate, 120 second time strecth. I used SR560 to amplify the demod signal so that pk-pk value is ~10 000 counts. The data is store in demod.data with a signal column .


Attachment 3: schmidtt.zip
  1041   Thu Aug 30 02:52:01 2012 taraDailyProgressBEATcode schmidtt trigger for beat

I'm trying to check if the schmidtt trigger algorithm will work as our beat readout or not (observe beat from dc to 1kHz), I also revisit IQ readout technique that we tried before as well. I'm analyzing the data and found that some data was not taken carefully (from chosen the wrong time). Here I'll just explain my plan and setup:

Objective: Check if readout methods (Schmidtt PSL:xxx, IQ PSL:xxx readout) are suitable for beat measurement or not. Do they provide a valid result?,

Method:  Compare the result obtained from the mentioned method with a reliable result. My plan is to use PLL to measure the frequency noise of a Marconi at various setup and use it as our reliable result. The setup is similar to what I did in PSL:Xxx, but I did not use 10MHz standard frequency input for Marconi.

   Setup 1: simple demodulated signal.  [add fig] . Both marconis' carrier frequencies were ~ 160MHz. Then the two signals were demodulated down to 1kHz and sent to DAQ (@32kHz). Three levels of frequency deviation (10kHz, 1kHz, 100Hz) were set on Marconi. The data will be used for testing Schmidtt trigger technique



fig2: setup for measurement 2 and 3.

   Setup2:  IQ technique.  With this setup, I measured I-Q signals with three different setting The signals were also demodulated to 1kHz and the frequency deviation were chosen to three levels (10kHz, 1kHz, 100Hz)as well.

  Setup3:  This setup was similar to that of setup 2, but the setting was different. I kept the frequency deviation at 100kHz, and varied the demodulated frequency instead. I chose 1 kHz, 3kHz, and 9 kHz. This data will be used for checking if our beat frequency drift around 9 kHz, will the read out still ok or not.

  1042   Fri Aug 31 00:48:00 2012 taraDailyProgressBEATcode schmidtt trigger for beat

 I compared results from three different readout techniques, it seems that my Schmidtt algorithm does not work at frequency above 10Hz for our required sensitivity, but IQ readout is very promising.

First, to check if I can produce the same results from the same data analyzed by two different technique  (Schmidtt, and IQ). I used 2 data set (chI and chQ) for IQ read out technique, then for Schmidtt technique, I used only chI data. Then I compared the results with what I got from PLL (I used the old data because I have not measured the new one yet, but it should give a rough idea).


fig1: comparison between IQ readout and Schmidtt technique. Black line is the old measurement for Marconi noise, 10khz input range.


The signal was demodulated down to ~1kHz and obtained by 32kHz DAQ. The data was taken with 150s time stretch. It turned out that the Schmidtt method is not sensitive enough.  IQ readout seems to be senstive down to 2 x10^-2 Hz/rtHz. The mismatch between the IQ and PLL was probably the setup between the two are different. (I have not measured the noise level of the current setup with PLL yet).


So I moved on to use IQ readout with other data. I chose marconi input frequency modulation range to be 1kHz and 100Hz. IQ method can measure down to 3x10^-3 Hz/rtHz. (I'll still have to verify this with PLL, but from a quick look it seems that the results are very reliable (I don't use any delay line in IQ read out at all and I can get sensitivity better than 10^-2 Hz/rtHz). It seems to be very promising for our beat readout.

I will try to compare this will results from PLL, if they agree. I'll move on to use this method to measure the noise level of marconi with frequency modulation function off (we can not do that by PLL technique). The result will give us an upper limit of the Marconi noise plus the readout technique noise which can be used in the noise budget.


fig2: Marconi noise at different settings, measured by IQ methods. The demodulated frequency is 1kHz. This plot is intended to show the sensitivity of IQ method which can be used to measure coating noise upto 1kHz (if the noise from an oscillator used for demodulation is not too noisy).  I have not plot the results from PLL on the same plot yet, but I have attached Marconi noise measured by PLL in fig3 below (data from 2010).


fig3: Marconi noise at different setting (Carrier @160MHz). The noise level for 1kHz and 100 Hz input range are almost the same and agree with what I got from IQ readout.



  •  I will check how sensitive IQ read out be (by compared it with PLL). However, I also notice one possible problem with this technique. The peaks around 1kHz, from demodulated signal, and its harmonics can be seen clearly in the plot . (there is also this mysterious peak at90 Hz as well, I'm not sure yet where it comes from). It means that if our beat frequency drifts around, the psd of the beat will have weird bumps all over those frequencies.
  • Think about digital delay line, I have not succeeded yet. I'm still confused with the calibration of this technique, but if IQ readout is working ok, I can use this method for beat measurement.


Attachment 2: compare_IQ_schmidtt.fig
Attachment 4: IQ_compare.fig
  1043   Mon Sep 3 01:02:46 2012 taraDailyProgressBEATcode schmidtt trigger for beat

I used IQ readout method to measure the frequency noise of Marconi. The sensitivity was good, it was lower than 10^-3 Hz/rtHz up to a few hundred Hz.


From previous entry, I finished up the measurement and analyzed the data. I did two things:

  • Compared IQ read out with PLL technique, (green and red against black and dark green): The measurements agree well so I think IQ method is reliable.
  •  Measured Marconi noise( carrier@160MHz, modulation off) when the marconis were locked and not locked with one Rb clock( pink and neon green). When the modulation is off, we cannot use PLL to measure marconi's frequency noise. So I used IQ to measure the noise.  



From the measurements, I can conclude that IQ method has enough sensitivity at least down to 10^-3 Hz/rtHz (might be up to 1kHz) (neon green).  One thing we should keep in mind is that the frequency noise from marconi is still quite high at 1kHz, even with modulation frequency is off. So we still have problem with measure noise up to 1kHz .

Anyway, I have not determined the noise floor of IQ method yet. I will think about it. If we can keep the beat frequency drift small enough, IQ methods should be ok for beat measurement.

Attachment 2: IQ_compare.fig
  1115   Tue Mar 12 20:51:00 2013 ranaNotesBEATMode matching plan for beat breadboard


 Its good that the beams can be matched, but there are a few more considerations to take into account:

* How does this optimize the geometry to make the readout more insensitive to vibrations? Will it all fit inside a well insulated plastic box?

* How does the final beam size depend on the physical parameters (i.e. path length, lens Radius error, etc.)? i.e. we need a sensitivity analysis.

* The divergence angle of the beam at the detector determines, in part, how sensitive this setup is to backscatter from the RFPD. How to minimize this? How are the specular reflections from the PD face dumped?

  1229   Tue Jul 9 14:47:44 2013 EricaNotesBEATWriting progress report
July 7, 2013
Took pictures of the setups for mode matching and recombining the beams. I added beam lines. I will be putting these in my report.

I need to basically overhaul my report. Here is an outline. My first draft was essentially just the introduction. I need to add more of what I did, basically making it like a lab report.

Here are also notes on reading the beat frequency.

  1256   Fri Jul 26 11:07:44 2013 EricaDailyProgressBEATcircuit for measuring temperature fluctations on CTN table
July 23, 2013
Went to a lecture Alan gave for the CGWAS on data analysis in the morning at Cahill (http://www.cgwas.org/index.php/Caltech_Gravitational-Wave_Astrophysics_School_2013). I had more data, signal from the recombined beam over different time periods that I took so I put those into graphs.

I practiced soldering stuff.
Wet the sponge below the iron. You can test to see if iron is hot if you hear the sizzle when you touch the iron to the sponge. A good idea is to cover the iron w/ new solder, since the old solder on the tip has oxidized.
Solder will go where it is hot, so you need to heat both the board and the wire to get a good connection. Good joints look like volcanoes.

Circuit design:
Evan figured out a circuit to for the AD590, as seen below.

The 20k resister determines the voltage that goes to the rest of the circuit. A high pass filter follows, with a capacitor on the order of 100 uF and resistor about 1 M ohm. This takes out the DC signal and AC couples the circuit. The filter will also ignore fluctuations that are slower than 100s. The two resistors connected to the op amp have a gain of 100. The op amp can only have an output up to 15V so with a gain of 100x, it can only take in 0.15 V before it saturates.

We used a breadboard where we can just plug in the components to see if the circuit works like it want it to. For the high pass filter, we used two 22 uF capacitors in parallel = 44 uF (22uF is the largest WIMA capacitor - film capacitor; anything higher will be ceramic and have a lot more noise; also they may be polarized which is a bit more hassle when wiring stuff up) and two 1 Mohm resistors to make a 2M ohm resistor.

We connected the circuit to a function generator and oscilloscope. The function generator was also connected to the oscilloscope (using T connector).
There was a 100x gain, as expected.
Note: make sure the oscilloscope is DC coupled, or else another capacitor will be put into the circuit in the oscilloscope and you won't get the correct signal.
Also, be careful about making the amplitude too large because that can saturate the op amp. If you do both of these things, then you get this weird signal that is trying to be a square wave but failing.

Note: AD590 has a polarization. The pin with the little bump sticking out should be connected to the positive side.
Attachment 1: P1030056.JPG
  1257   Fri Jul 26 11:22:45 2013 EricaDailyProgressBEATcircuit for measuring temperature fluctations on CTN table
July 24, 2013

Took the tour to JPL today. Got to see the twin of Curiosity that they assembled before the real one so they could adjust procedures for assembly and the Mars Yard where they drive the rover over various types of terrain. There is also the Scarecrow which is essentially just the frame and wheels, which is used to simulate the smaller gravity on Mars.

Soldered parts to the circuit board. I'll be putting both circuits onto the same board, using about half of the board total, so that we can attach more components later, if needed.
Only one side of the board has metal around the holes so we placed the components on the non-metalized side, and had the connects protruding to the opposite side.

Used scrap wire that was in the base of the stand to connect various joints that were close to each other. Also folded over excess wire from components to make connections. Any longer wires that weren't used were cut off.

Using red for positive, black for negative, and green wire for ground, as is standard.
  1258   Fri Jul 26 11:45:28 2013 EricaDailyProgressBEATcircuit for measuring temperature fluctations on CTN table
July 25, 2013

finished building the circuit today. Had the positive, negative, and ground wires running above the board, while the one jumper wire from the output to the negative input under.
Twisted the positive, negative, and ground wires together using a drill, as well as the positive and negative wires that will connect to the AD 590. We made these longer so we can connect to the power supply and place the AD590 at opposite ends of the table.

Tested the circuit, used an amplitude of 0.01Vpp and 0.1 Hz for frequency to drive circuit, which was what we did on Monday. The output signal is a square wave which was strange but found out the problem: the bnc cable driving the circuit was put in the sync output of the function generator, instead of the function output.
Fixed this, and the circuit behaves as we expect.

Discovered that I used a 36 kohm resistor instead of a 33 k ohm resistor, so now we have a gain of about 110, which is close to that of 100.

We used aluminum tape to connect the AD590 and insulating tape to prevent shorting. The output signal was at some DC voltage, which we expect at first due to the power supply turning on, and it should die away, but it didn't, or was very very slowly. So Evan placed a 15k Ohm resistor in parallel w/ the 2M ohm resistor in the high pass filter to lower the time constant, which brought the signal close to zero. Once he took the resistor away, then the signal would drift up to the previous DC level. The circuit was responding as expected when he placed a cooler object by it, so the signal went down, and the signal went up when we held the AD 590.

We tried this in the CTN lab but it didn't seem to work; there was a lot of noise. I'll test it again tomorrow.
A possibility is the power supply could be noisy.
  1261   Mon Jul 29 13:30:26 2013 EricaDailyProgressBEATadding resistor, capacitor, and sockets
July 26, 2013

Was able to place the 24k ohm resistor in parallel with the 2Mohm resistor to make the time constant shorter, and see the response on the oscilloscope. The signal is way way high, because even w/ the shortened time constant, it takes awhile for the signal to actually reach zero.

Took out the 36k resistor. Attached sockets to the circuit board so we can easily remove and replace resistors. The new one we used was 1 kOhm resistor.

Had crazy noise - with spikes, due to extra capacitance from the additional sockets. We decided to add a capacitor to filter out the high frequency noise from feeding back to the negative input of the op amp.

Added more sockets so we can easily remove and attach the capacitor .
I tried out different capacitors, centered around 1 nF but this kind of helped. There were no longer giant spikes. However, now the signal in general is noisier.

Note: if soldering something to another component that is only attached at that 1 pin, make sure you hold that piece so that it doesn't drop. OR else, you need to solder that one-piece again, while making sure the other component gets connected to it as well.
  1265   Tue Jul 30 12:23:41 2013 EricaDailyProgressBEATaddressed noise problem, DC signal
july 29, 2913

Worked on progress report due Friday.

Tried using the power supply from the electronics lab and that was much better. Switched back to the old one and used shorter wires to connect the ground, so random magnetic fields don't induce current in the wires and this helped immensely. There were still random little noise but not like before. This was tested using the function generator giving a sinusoidal wave.

With the AD 590 connected, like before, the signal was well above 0V. However, it did not saturate, even when it not on the CTN table, but out in the open air.
The signal was pretty linear so we looked at the slope over a region where there were three peaks and calculated the corresponding input. This gave 1.39 uA, which is much smaller than the 13mA required just for the wire to change one wavelength.

We're not sure where the DC signal is coming from because the high pass filter is supposed to filter it out. This has been done with only one circuit so we don't have much to compare to. Perhaps need to rethink a different circuit...

Also, I put insulating foam underneath the exposed fiber. Tara has ordered more, which are wider, so that will cover the whole exposed area of the fiber.
  1274   Thu Aug 1 21:19:57 2013 taraDailyProgressBEATsearching for beat

I locked both cavities and trying to search for the beat signal, I have not succeeded yet.

I used lenses that could get the two transmitted beam to be close and small enough for the beat PD (new focus 1811) (we ordered  what we need but they are not here  yet).

I locked ACAV at a fixed SLOW DC level (1.207 V), and varied RCAV's SLOW DC level from 1.199V, 0.33V, -0.554V, -1.477V (1FSR ~ 4GHz is about 1 V). The slider for RCAV slow is set to +/- 2V so I have not tried other values yet. It can be changed to -2V to 9 V, but I have to restart the crate which will disturb the temperature servo, so I'll try to adjust RCAV slow value using a voltage calibrator instead.

I talked to Evan about the beat measurement in GYRO lab, the SLOW DC for both lasers can be different up to 6 V (for ~100MHz beat). see gyro1832

I varied RCAV's SLOW DC first because this path does not have a PMC, so I don't have to worry about locking the PMC.

From PSl:1124 ,the beat frequency should be ~60-100 MHz, without the heater on any cavity.  I'll try the same method to check the beat frequency between the two cavities one more time. If it is still ~ 100 MHz, I'll increase the range of SLOWDC, and see if the beat will show up of not.  The setpoint was not changed that much (31.2 to 31.25), So I expect the beat frequency should still be close.

If the beat still not show up, I'll try to realign the beam.


Current setup

Vac chamber Setpoint = 31.25

Vheat for RCAV =  0

Vheat for ACAV = 0



  1275   Fri Aug 2 12:18:16 2013 taraDailyProgressBEATsearching for beat

Found the beat @ 116 MHz. RCAV SLOW =5.762V, ACAV SLOW = 1.209 V.



beat 1kHz input range, calibration  = 718 Hz/V


above, beat signal with 1kHz input range on Marconi.

Plenty of things that I need to optimize and add:

input optics (ACAV/RCAV):

  • beam alignment
  • optimizing quarter wave plates in front of the cavities.
  • block all the reflected beams properly
  • fixing the back reflection from vac window for ACAV.
  • measure error point noise from both servos and compare them with beat
  • optimizing TTFSS servo gain

Beat setup:

  •  mode matching lens
  • power on beat PD
  • optimizing PLL servo
  • implementing ISS

Seismic isolation

  • new table legs ( I have not ordered the new set yet). The current set is broken
  1305   Fri Aug 16 22:05:27 2013 taraDailyProgressBEATnoise hunting

 Noise hunting is in progress, I checked the error noise from ACAV and RCAV loops and compared them to the beat. The beat is about an order of magnitude higher than the sum of error noise.

 NOte: slope of error signal RCAV = 1.57 MHz/V (13 dBm from Marconi, throug 4-way splitter, to BB EOM, 1mW input power).


ABOVE: beat signal in comparison with noise at error points from ACAV and RCAV loops. The beat signal is about an order of magnitude higher than the error noise.


I'm working on optimization and noise characterization of the setup. Before measuring the beat I have to make sure that:

  • The beams to the cavities are aligned
  • The power input is 1mW for both cavities
  • I aligned the polarization of the beams into EOM for side band ( minimizing RFAM)
  • The gains for TTFSS are adjusted and recorded
  • Beams in the beat setup are aligned, and dumped properly.
  • The PD is not saturated.
  • PLL is setup properly.

I think the gain in the TTFSS is the problem. For ACAV, the scattered light from the window interferes with the main beam and causes the loop to oscillate when the gain is up. For RCAV, the EOM is a broadband one and does not have enough gain. The bump in the frquency lower than 100Hz is probably the contribution from scattered light. I have not properly dumped all beams yet.


Also I noticed that the beat signal has weird sidebands at +/- 100kHz, 200kHz, and 300kHz, see the figure below. I'm not sure why, I have not seen it before. I might saturate the PD making it distorted from a perfect sine wave. I'll investigate this.


Attachment 2: nb_short_cav.fig
  1307   Tue Aug 20 20:10:01 2013 taraDailyProgressBEATnoise hunting

Noise hunting is in progress. Today I identified that scattered light from the window is one of the problem.

I spent sometime making sure that all the beams in the input optic and the beat areas were dumped properly. I also tightened all the screws on the optics and the mounts on the table.

I mentioned in the previous entry that for RCAV, the reflected beams from the cavity and the vacuum window overlapped a little bit. The window beam was much smaller and actually closer to the edge of the main beam, so I used an iris to remove the outer path, and let only the beam in the center area go through to the RFPD. With that I could increase the gain in RCAV loop to Common/Fast = 624/750, where they used to be ~ 600/600 before. The iris might introduce some extra scattered lights, since it clips a part of the beam.

The scattered noise around DC to 100 Hz is reduced a bit, see the below figure. However, not much improvement in the flat region (100Hz and above). Plus, some mechanical peaks around 1kHz appear with higher level than before.


I expected the scattered noise will be even lower if the cavities are tilted a bit to avoid the beams overlapping. At higher frequency, it might be the gain limit from RCAV loop where the modulation depth is very small.

Next thing to do is to increase more power in the modulation depth for RCAV.


I found out that the sidebands in the beat signal mentioned in the previous entry changed with the gain of the TTFSS (both ACAV and RCAV). With higher gain, the sidebands are suppressed more. It might have to do with the PZT resonant of the NPRO. 

  1316   Wed Aug 28 11:13:07 2013 EvanDailyProgressBEATIntensity-to-frequency transfer function

[Tara, Evan]

Last week we tried measuring a transfer function which takes intensity fluctuation induced at the south EOAM and returns frequency fluctuation as read out by Tara's beat setup. This is therefore meant to be the measurement corresponding to Tara's code farsi.m (on the SVN at CTNLab/simulations/misc).

We used the SR785 in swept-sine mode. As measured previously, the EOAM response is 3×10−5 W/V (although I think this number should be rechecked, since we've fooled around with the EOAM in the meantime). The beat PLL readout is 7.1 kHz/V (when the Marconi is set to its 10 kHz setting). [Edit, 2013–10–18: by comparing with the newer measurement in PSL:1368, I think the Marconi must have actually been at the 1 kHz setting, so the conversion factor is 710 Hz/V and the transfer function measurement below is a factor of 10 too high.] These two numbers give the conversion factor necessary to convert from V/V into Hz/W.

The attached plot shows the measurement and the expectation from Tara's code. Here I'm using the version of the code as checked out last night, and in my local copy of the code I changed the cavity length to 1.45″ and the input power to 1 mW. Below 200 Hz, the agreement in magnitude is good: the overall shapes agree the values are within a factor of 3 of each other. The phase also appears to be good below 40 Hz or so. Above 200 Hz the transfer function is apparently dominated by some other effect.

Attachment 1: intensity_to_frequency.pdf
Attachment 2: intensity_to_frequency_code.zip
  1361   Tue Oct 8 23:01:56 2013 EvanDailyProgressBEATAttempts at new beat measurement

[Tara, Evan]

Having successfully floated the table yesterday, we attempted a new beat measurement in the hopes that the large shelf below 100 Hz had disappeared. Unfortunately, this appears to not be the case. Additionally, many of our signals are plagued by unusually large, slow drifts. We're hoping that they're just thermal transients caused by all the work on the table over the past 12 hours, and that by tomorrow things will have settled down. We'll see if that's the case.

Anyway, we did the following things today:

  • We reconnected cables that come in from off the table and go onto cameras, PDs, etc., paying special attention to strain relief and vibration isolation since the table now floats.
  • We redid the alignment to recover ~90% visibility; this required only touching the periscope mirrors (somewhat surprising considering what we subjected the table to in order to switch out the legs).
  • We got the cavity PDH loops up and running again. The control signals show unusually large drifts. We also noticed this while sweeping the lasers to align the cavities; the resonance for north in particular would wander out of the sweep range every 30 seconds even though the laser was being driven at 10 Vpp from an SRS function generator.
  • We spent some time trying to null (what we assume is) RFAM-induced offset in the PDH error signals. We did this by adjusting the HWPs before each cavity EOM and nulling the offset on TTFSS common OUT1. The south cavity already had a small offset, so no adjustment was required. On the north cavity, there was a noticeable offset (~20 mV, compared to an error signal pk-pk of 220 mV), so Tara nulled it. We then found that we could get a stable lock with the laser PZT actuator alone, and that adding the EOM actuator caused the loop to oscillate (almost as if the EOM actuator was driven with the wrong sign). So we looked at the error signal again, and unexpectedly found another ~20 mV offset. Tara nulled it again and this time the lock was stable; in fact, we were able to get the common and fast gain knobs up to 1000 and 1000 (compared to 800 and 800 earlier in the day). No idea what the problem is here; possibly it drifted between the successive adjustments.
  • We looked at the beat. It appears to be not much better than with the table unfloated.
  • We took a measurement of RIN-induced disturbance in the beat by driving the south EOAM with a sine from the SR785 and taking the TF that takes transmission PD intensity to beat fluctuation. Unfortunately, this measurement is not consistent (in magnitude or phase) between successive sweeps. It seems to be due (at least in part) to DC drift in the beat.
  • We tried turning on the crude south ISS, but it made the beat more noisy.


  1368   Thu Oct 17 21:50:28 2013 EvanDailyProgressBEATNew intensity-to-frequency TF

[Tara, Evan]

We took another RIN-to-beat transfer function, as in PSL:1316. This time we've directly measured the conversion factor between power transmitted through the south cavity and voltage put out by the transmission PD. To do this, Tara purposefully misaligned the alignment into the cavity in order to get several different transmitted powers. For each misalignment, we measured the power immediately after the vacuum can using the power meter as well as the voltage out of the south PDA10CS with its gain at 30 dB. The result is (1.78 ± 0.05) V/W, and the fit is shown in the first attachment.

The second attachment shows the transfer function. I've applied the PD conversion factor mentioned above, as well as the Marconi actuation conversion factor (710 Hz/V for the 1 kHz FM setting). The red and green traces were taken with the table not floated, and the blue traces were taken with the table floated (we originally took all the traces with the table not floated, but the SR785 decided to write an empty data file and we didn't realize it until after we floated the table).

Also, I think I must have applied the wrong calibration (7.1 kHz/V) in PSL:1316; at low frequencies, the TF there is almost exactly a factor of 10 higher than the TF here.

Attachment 1: trans_pd_cal.pdf
Attachment 2: rin_to_beat.pdf
Attachment 3: ctn_rin_beat_2013-10-17.zip
  1369   Mon Oct 21 14:04:04 2013 ranaDailyProgressBEATNew intensity-to-frequency TF

 Even though it shouldn't be there, there is probably a coupling directly from RIN to the reflected PDH signal on each cavity. So you should measure all 3 terms (RIN 2 PDF for each cavtiy, and RIN 2 Beat).

  1373   Sat Oct 26 15:35:42 2013 EvanDailyProgressBEATBeat measurement with ISS

Summary: No good so far. Engaging the ISS seems to have basically zero effect on the beat. The beat overall looks worse than it did a month ago, and the shape seems to mimic the shape of the north cavity RIN. More optimization of the north EOAM is necessary.

Details: Having set up the north EOAM on Thursday (PSL:1372), I spent most of yesterday trying to get a RIN-suppressed beat measurement.

The continual drift of the laser frequency control signals was irritating, so I spent some time getting the slow digital PID controls for the lasers back up and running. At first only KP seemed to have no effect on the laser control signals; it turns out this is because the PID Perl scripts that run on the Sun machine rescale the KI and KD coefficients by a timestep variable, which had been set to zero. I've set it to 1. I've chosen KP = KD = 0 and KI = 0.0002 (with appropriate choice of sign for the two loops). The system is probably overdamped, but it manages to integrate the control signals down to zero in a resonable amount of time (<30 s) and I don't think it's a high priority to optimize it right now.

The south PDH error signal has noticeable 250 kHz oscillations which get worse as the common TTFSS gain is increased. The north PDH error signal is much quieter. Are we perhaps hitting a mechanical resonance of the EOM crystal? Or (dare I say it) do we have the wrong sign for the common path of the PDH loop?

I took out the hand-soldered integrating board that I built for the ISS loops; it was railing too often. The ISS setup for each path is now as follows: each ISS PD goes into the A input of an SR560, and a programmable voltage reference (Calibrators Inc. DVC–350A) goes into the B input. The voltage is chosen to match the dc voltage from the ISS PD. The SR560 is dc coupled and set to take the difference A − B. The gain is set to 5×103 V/V, with a single-pole low-pass at 1 kHz. The output from the SR560 is fed into the EOAM.

The suppressed and unsuppressed RIN measurements are given in the first two plots. Evidently, these simple ISS loops are able to suppress the RIN by a factor of 50 or so. Also, the north RIN is much worse than the south RIN, and the hump from 100 Hz to 10 kHz is reminiscent of a poorly aligned EOAM (as seen in PSL:1311, for example). So I'd like to spend some more time fiddling with the north EOAM to see if I can improve the RIN suppression. Alternatively, perhaps we are suffering because the north path has no PMC to stabilize the pointing into the EOM, EOAM, etc.

Anyway, I pressed ahead and looked at the beat. To convince myself of the repeatability of the setup, I took a measurement with the ISS loops on, then a measurement with the ISS loops off, and then a measurement with the ISS loops on again. The result is given in the third plot. Below a few hertz, the ISS may have a positive effect. Above this, there is either no effect or a small worsening effect.

Note that the shape of the beat follows the shape of the north cavity RIN. I think we should spend a little time noise hunting and optimizing on the north path to see if we can make this go away. Note also that the beat is worse than it was back in September (PSL:1321). Two immediate culprits that I can think of are (a) the installation of the EOAM or (b) the fact that the vacuum can is no longer floated. But it could just as well be that there's something else (e.g., PDH offsets) that I neglected to optimize.

Attachment 1: north_iss.pdf
Attachment 2: south_iss.pdf
Attachment 3: beat.pdf
  1375   Mon Oct 28 22:55:11 2013 EvanDailyProgressBEATBeat measurement with ISS

I've taken Tara's farsi.m and changed the values of finesse F and absorption α in order to fit the magnitude of the TF measurement in PSL:1368. I've chosen 7500 for the finesse and 5 ppm for the absorption, although for this calculation they are degenerate (entering into the TF as F/α).

Using this, I've taken the RIN measurements from Friday and used them to estimate the induced frequency fluctuation in the beat readout, assuming a transmitted power of 1 mW from each cavity.

In the case when the ISS is off, the estimated effect of RIN on the current beat is significant only below 10 Hz. When the ISS is on, the RIN is insignificant over the entire measurement range. This perhaps explains the observed reduction in the beat PSD below 10 Hz when the ISS is on.

Attachment 1: rin_to_beat.pdf
Attachment 2: expected_rin-to-beat.pdf
Attachment 3: expected_rin-to-beat_iss.pdf
  1382   Tue Nov 5 10:34:09 2013 EvanDailyProgressBEATNew beat measurement

[Tara, Evan]

Yesterday, we did a few final bits of optimization and then re-measured the beat spetrum.


  • Tara tweaked the north RFPD cable to symmetrize the north PDH error signal.
  • For each path, we hooked the RF output of each RFPD directly into the HP4395A and minimized the RFAM at 14.75 MHz (the PDH frequency).
    • For south, the initial RFAM was −107 dBm, and by adjusting the HWP before the resonant EOM, Tara was able to get it down to −112 dBm. This resulted in no visible change to the error signal offset (10 mV, compared to 300 mV peak-to-peak).
    • For north, the initial RFAM was −105 dBm, and by adjusting the HWP before the broadband EOM + resonant EOM (there is no intervening waveplate), Tara was able to get it below the noise floor of the spectrum analyzer (i.e., < −125 dBm). This resulted in a change to the error signal offset from 28 mV to 19 mV (compared to 300 mV peak-to-peak).
  • We measured the slopes of the error signals in order to get the calibration from voltage to frequency.
  • We locked the cavities with the slow digital control engaged. The south TTFSS gain was 712 slow and 796 fast, and the north TTFSS gain was 900 common and 900 fast. The south TTFSS control signal still has strong (~ 2 V peak-to-peak) oscillations at hundreds of kilohertz.
  • We measured the beat, the PDH error signals, and the RIN spectra. The modulation setting on the Marconi was 1 kHz, so the conversion factor is the usual 710 Hz/V.
  • After measuring the beat, we swapped out the SHP–150 on the RF input of the mixer for an SHP–100. The beat is currently at 120 MHz, and with the SHP–150 we were throwing away something like 60% of our power. Attenuation from the SHP–100 appears negligible when viewed on a scope.

The beat spectrum is attached, along with the expected coating Brownian noise estimate. I will post the estimates of the PDH and RIN contributions later.

Attachment 1: beat_2013-11-04.pdf
  1438   Thu Jun 26 17:10:09 2014 EvanDailyProgressBEATBeat

South laser slow at 1.234 V, north laser slow at 5.558 V, beat is 120(1) MHz at +5.5(2) dBm. South and north alignment has not yet been tuned up.

SR785 appears to have broken screen.

  1447   Wed Jul 9 22:31:59 2014 Emily, EvanNotesBEATfiber phase noise measurement

Installation of optics for fiber phase noise measurement


After light passes through the AOM, it is reflected back through the AOM and into the fiber.  We installed a 50/50 beamsplitter, quarter wave plate, mirror, lens and photodiode to do the beat measurement.  It is required that the beam spot size is 1/3 the diameter of the photodetector.  We installed a lens at the appropriate distance to obtain a waist that is roughly 50 microns.  We hooked up the photodiode to an oscilloscope and found that the voltage fluctuates between 100-500 mV.  We are not sure why the voltage is fluctuating, but we will continue to investigate the cause.  



  1492   Fri Aug 29 12:35:54 2014 EvanDailyProgressBEATBeat breadboard in place

Beat breadboard is slid back into place. North transmission appears on north camera. Still need to do south transmission.

  1493   Fri Aug 29 15:35:55 2014 EvanDailyProgressBEATMode-matching for beat

I predict (via alm) that the spot size on the diode (z = 991 mm) is 79 µm in the current configuration.

Attachment 1: ctnbeat_algaas.pdf
Attachment 2: ctnbeat_algaas_alm.zip
  1494   Fri Aug 29 19:56:06 2014 EvanDailyProgressBEATBeat breadboard in place


Beat breadboard is slid back into place. North transmission appears on north camera. Still need to do south transmission.

Tara has found south transmission on camera. I steered the transmitted beams onto the beat PD and then made the k-vectors as parallel as I could as seen on an IR card.

The DC voltage on the PD is okay (ca. 50 mV from each beam), but I cannot see a beat note on the AC path using the HP4395. Tara will give a temperature kick which hopefully will bring the beat note within the range of the 1811.

  1496   Tue Sep 2 11:06:54 2014 EvanDailyProgressBEATNo beat

Searched around over various axial modes in order to find a beat.

I fiddled a bit with the output QWPs in order to get the polarizations to match. Because of the birefringent coatings, light transmitted through the cavity is not circular, and the polarization state will depend on which of the two modes we lock to. In case, the original QWP angles were 202° for north and 19° for south.

ELOG V3.1.3-