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Entry  Tue Oct 25 15:56:11 2016, gautam, Update, General, PRFPMI locked, arms loss improved PRFPMIlock_25Oct2016.pdfITMYwoes.png
    Reply  Tue Oct 25 18:07:28 2016, Koji, Update, General, PRFPMI locked, arms loss improved 
       Reply  Thu Oct 27 12:06:39 2016, gautam, Update, General, PRFPMI locked, arms loss improved PRG.pdf
          Reply  Thu Oct 27 13:48:20 2016, Koji, Update, General, PRFPMI locked, arms loss improved 
             Reply  Thu Oct 27 23:29:47 2016, ericq, Update, General, PRFPMI locked, arms loss improved TRY_REFL_2d.pngAS.pngREFL.pngseis_sub.pdf
                Reply  Fri Oct 28 01:44:48 2016, gautam, Update, General, PRFPMI model vs data studies 6x
                   Reply  Fri Oct 28 19:13:57 2016, rana, Update, General, PR gain 
Message ID: 12586     Entry time: Fri Oct 28 01:44:48 2016     In reply to: 12585     Reply to this: 12588
Author: gautam 
Type: Update 
Category: General 
Subject: PRFPMI model vs data studies 

Following Koji's suggestion, I decided to investigate the relation between my Finesse model and the measured data.

For easy reference, here is the loss plot again:

Sticking with the model, I used the freedom Finesse offers me to stick in photodiodes wherever I desire, to monitor the circulating power in the PRC directly, and also REFLDC. Note that REFLDC goes to 0 because I am using Finesse's amplitude detector at the carrier frequency for the 00 mode only. 


Both the above plots essentially show the same information, except the X axis is different. So my model tells me that I should expect the point of critical coupling to be when the average arm loss is ~100ppm, corresponding to a PRG of ~17 as suggested by my model.

Eric has already put up a scatter plot, but I reproduce another from a fresh lock tonight. The data shown here corresponds to the IFO initially being in the 'buzzing' state where the arms are still under ALS control and we are turning up the REFL gain - then engaging the QPD ASC really takes us to high powers. The three regimes are visible in the data. I show here data sampled at 16 Hz, but the qualitative shape of the scatter does not change even with the full data. As an aside, today I saw the transmission hit ~425!


I have plotted the scatter between TRX and REFL DC, but if I were to plot the scatter between POP DC and REFL DC, the shape looks similar - specifically, there is an 'upturn' in the REFL DC values in an area similar to that seen in the above scatter plot. POP DC is a proxy for the PRG, and I confirmed that for the above dataset, there is a monotonic, linear relationship between TRX and POPDC, so I think it is legitimate to compare the plot on the RHS in the row directly above, to the plot from the Finesse model one row further up. In the data, REFL DC seems to hit a minimum around TRX=320. Assuming a PRM transmission of 5.5%, TRX of 320 corresponds to a PRG of 17.5, which is in the ballpark of the region the model tells us to expect it to be. Based on this, I conclude the following:

  • It seems like the Finesse model I have is quite close to the current state of the IFO 
  • Given that we can trust the model, the PRC is now OVERCOUPLED - the scatter plot of data supports this hypothesis
  • Given that in today's lock, I saw arm transmission go up to ~425, this suggests that at optimal alignment, PRG can reach 23. Then, Attachment #1 suggests the average arm loss is <50ppm, which means the average loss per optic is <25ppm. I am not sure how physical this is, given that I remember seeing the specs for the ITMs and ETMs being for scatter less than 40 25ppm, perhaps the optic exceeded the specs, or I remember the wrong numbers, or the model is wrong

In other news, I wanted to try and do the sensing matrix measurements which we neglected to do yesterday. I turned on the notches in CARM, DARM, PRCL and MICH, and then tuned the LO amplitudes until I saw a peak in the error signal for that particular DOF with peak height a factor of >10 above the noise floor. The LO amplitudes I used are 

MICH: 40

PRCL: 0.7

CARM: 0.08

DARM: 0.08

There should be about 15 minutes of good data. More impressively, the lock tonight lasted 1 hour (see Attachment #6, unfortunately FB crashed in between). Last night we lost lock while trying to transition control to 1f signals and tonight, I believe a P.C. drive excursion of the kind we are used to seeing was responsible for the lockloss, so the PRFPMI seems pretty stable.

With regards to the step in the lock acquisition sequence where the REFL gain is turned up, I found in my (4) attempts tonight that I had most success when I adjusted the CARM A slider while turning up the REFL gain to offload the load on the CARM B servo. Of course, this may mean nothing... 

Attachment 1: loss.pdf  13 kB  | Hide | Hide all
Attachment 2: REFLDC.pdf  14 kB  | Hide | Hide all
Attachment 3: CriticalCoupling.pdf  13 kB  | Hide | Hide all
Attachment 4: PRFPMI_Oct282016.pdf  480 kB  | Hide | Hide all
Attachment 5: PRFPMI_scatter.pdf  340 kB  | Hide | Hide all
Attachment 6: 1hourPRFPMILock.png  40 kB  Uploaded Fri Oct 28 02:45:32 2016  | Hide | Hide all
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