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Entry  Thu Dec 1 20:20:15 2016, gautam, Update, IMC, IMC loss measurement plan IMG_3471.JPG
    Reply  Fri Dec 2 16:40:29 2016, gautam, Update, IMC, 24V fuse pulled out 
       Reply  Mon Dec 5 01:58:16 2016, gautam, Update, IMC, IMC ringdowns IMCringdown.pdfIMCringdown_2.pdf
          Reply  Mon Dec 5 15:55:25 2016, gautam, Update, IMC, IMC ringdowns 7x
             Reply  Mon Dec 5 19:29:52 2016, gautam, Update, IMC, IMC ringdowns REFLanomaly.pdf
                Reply  Tue Dec 6 00:43:41 2016, gautam, Update, IMC, more IMC ringdowns 
                   Reply  Wed Dec 7 11:52:48 2016, ericq, Update, IMC, Partial IMC ringdowns IMCpartial.pngIMCpartial.zip
                      Reply  Thu Dec 8 19:01:21 2016, rana, Update, IMC, Partial IMC ringdowns 
                   Reply  Sun Dec 2 17:26:58 2018, gautam, Update, IMC, IMC ringdown fitting tauTheoretical.pdfringdownFit.pdfringdownScatter.pdfcavPole.pdf
                      Reply  Fri Dec 7 12:51:06 2018, gautam, Update, IMC, IMC ringdown fitting time_reflAndTrans.pdfcorner_transOnly.pdfcorner_reflAndTrans.pdf
Message ID: 12655     Entry time: Thu Dec 1 20:20:15 2016     Reply to this: 12660
Author: gautam 
Type: Update 
Category: IMC 
Subject: IMC loss measurement plan 

We want to measure the IMC round-trip loss using the Isogai et. al. ringdown technique. I spent some time looking at the various bits and pieces needed to make this measurement today, this elog is meant to be a summary of my thoughts.

  1. Inventory
    • AOM (in its new mount to have the right polarization) has been installed upstream of the PMC by Johannes. He did a brief check to see that the beam is indeed diffracted, but a more thorough evaluation has to be done. There is currently no input to the AOM, the function generator on the PSL table is OFF.
    • The Isogai paper recommends 3 high BW PDs for the ringdown measurement. Souring through some old elogs, I gather that the QPDs aren't good for this kind of measurement, but the PDA255 (50MHz BW) is a suitable candidate. I found two in the lab today - one I used to diagnose the EX laser intensity noise and so I know it works, need to check the other one. We also have a working PDA10CF detector (150 MHz BW). In principle, we could get away with just two, as the ringdown in reflection and transmission do not have to be measured simultaneously, but it would be nice to have 3
    • DAQ - I think the way to go is to use a fast scope triggered on the signal sent to the AOM to cut the light to the IMC, need to figure out how to script this though judging by some 2007 elogs by rana, this shouldn't be too hard...
  2. Layout plans
    • Where to put the various PDs? Keeping with the terminology of the Isogai paper, the "Trans diode" can go on the MC2 table - from past measurements, there is already a pickoff from the beam going to the MC TRANS QPD which is currently being dumped, so this should be straightforward...
    • For the "Incident Diode", we can use the beam that was used for the 3f cancellation trials - I checked that the beam still runs along the edge of the PSL table, we can put a fast PD in there...
    • For the "REFL diode" - I guess the MC REFL PD is high BW enough, but perhaps it is better to stick another PD in on the AS table, we can use one of the existing WFS paths? That way we avoid the complicated transfer function of the IMC REFL PD which is tuned to have a resonance at 29.4MHz, and keeps interfacing with the DAQ also easy, we can just use BNC cables...
    • We should be able to measure and calibrate the powers incident on these PDs relatively easily.
  3. Other concerns
    • I have yet to do a thorough characterization of the AOM performance, there have been a number of elogs noting possible problems with the setup. For one, the RF driver datasheet recommends 28V supply voltage but we are currently giving it 24V. In the (not too distant) past, the AOM has been seen to not be very efficient at cutting the power, the datasheet suggests we should be able to diffract away 80% of the central beam but only 10-15% was realized, though this may have been due to sub-optimal alignment or that the AOM was receiving the wrong polarization...
  4. Plan of action
    • Check RF driver, AOM performance, I have in mind following the methodology detailed here
    • Measure PMC ringdown - this elog says we want it to be faster than 1us
    • Put in the three high BW PDs required for the IMC ringdown, check that these PDs are working
    • Do the IMC ringdown

Does this sound like a sensible plan? Or do I need to do any further checks?

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