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Entry  Mon Oct 26 19:10:59 2015, gautam, Update, Green Locking, AUX PDH loop characterization 
    Reply  Tue Dec 8 23:24:08 2015, gautam, Update, Green Locking, Y end laser (Lightwave) PZT calibration Ycalib_8Dec.pdf
       Reply  Sun Dec 13 21:55:28 2015, gautam, Update, Green Locking, Y end laser (Lightwave) PZT calibration IMG_5972.JPGServoY_TF_13Dec2015.pdfDatanCode.zipPSD_916Hz.pdf
          Reply  Mon Dec 14 16:27:11 2015, gautam, Update, Green Locking, Y-end AUX PDH noise breakdown YAUX_NB_Dec2015.pdfPDH_errSig_Calib.pdf
             Reply  Mon Jan 4 16:45:11 2016, gautam, Update, Green Locking, Y-end AUX PDH noise breakdown ErrSigBreakdown.pdfcontrolSigBreakdown.pdfYEnd_PDH_OLTF.pdf
          Reply  Mon Jan 4 16:09:54 2016, gautam, Update, Green Locking, Y end laser (Lightwave) PZT calibration choosingExcFreqs.pdflaserPZTcalib.pdf
Message ID: 11877     Entry time: Sun Dec 13 21:55:28 2015     In reply to: 11865     Reply to this: 11879   11906
Author: gautam 
Type: Update 
Category: Green Locking 
Subject: Y end laser (Lightwave) PZT calibration 

Summary:

After the discussions at the Wednesday meeting, I redid this measurement using a sinusoidal excitation summed at the error-point of the PDH servo as opposed to a DC offset. From the data I collected, I measured the actuator gain to be 2.43 +/- 0.04 MHz/V. This is almost half the value we expect, I'm not sure if I'm missing something obvious.


Details:

  1. Attachment #1 is a sketch of the measurement setup and points at which signals are measured/calculated. Some important changes:
    • I am now using the channel C1:ALS-Y_ERR_MON_OUT to directly measure the input signal to the servo. In order to get the calibration constant for this channel from counts to volts, I simply hooked up the input to the channel to an oscilloscope and noted the amplitude of the signal seen on the scope in volts. The number I have used is 52uV/count (note that the signal to the ADC is amplified by a factor of 10 by an SR560).
    • I measured the transfer function from the input to the servo (marked "A" in the sketch) to the output of the Pomona box going to the laser PZT (marked "B" on the sketch) using an SR785 - see Attachment #2. This allowed me to convert the amplitude of excitation at A to an amplitude at B, which is what we need, as we want to measure C/B.
  2. The measurement itself was done by locking the arms to IR, running ASS to maximize IR transmission, setting up a green beat note, and then measuring the two channels of interest with the excitation to the error-point on. 
  3. I was initially trying to use time-series plots to measure these amplitudes - Koji suggested I use the Fourier domain instead, and so I took FFTs of the two channels we are interested in (using a flat-top window with 0.1 Hz BW) and estimated the RMS values at the frequency at which I had injected an excitation. Data+code used is in Attachment #3. In particular, I was integrating the PSD over 1Hz centered at the excitation frequency in order to calculate the RMS power at the excitation frequency - it could be that for C1:ALS-BEATY_FINE_PHASE_OUT_HZ, the spectral leakage into neighbouring bins is more significant than for C1:ALS-Y_ERR_MON_OUT (see Attachment #4)?
  4. With the amplitudes thus obtained, I took the ratio C/B (see sketch) to determine the MHz/V actuator gain. I had injected excitations at 5 frequencies (916Hz, 933Hz, 977Hz, 1030Hz and 1067Hz, choses in relatively "quiet" parts of the spectrum of C1:ALS-Y_ERR_MON_OUT with no excitations), and the result reported is the average from these five measurements, while the error is the standard deviation in the 5 measurements.
  5. Unrelated to this meaurement - while I had the SR560 hooked up to the input of the PDH box, I inverted the mixer output to the servo input, as I thought I could use this method to estimate the modulation depth. I did so by locking the Y arm green to the sideband TEM00 mode, and comparing the green transmission in this state to that when the Y arm is locked to a carrier TEM00 mode. I averaged C1:ALS-TRY_OUT for 10 seconds in 3 cases: (i) Carrier TEM00, (ii)sideband TEM00, and (iii) shutter closed - from this measurement, I estimate the modulation depth to be 0.209 +/- 0.006 (errors used to calculate the total error were the standard deviations of the measured transmission). 

Next steps:

  1. Check that I have not missed out anything obvious in estimating the actuator gain - particularly the spectral leakage bit I mentioned above.
  2. If this methodology and measurement is legitimate, repeat for the X end, and complete the noise budgeting for both AUX PDH loops.
Attachment 1: IMG_5972.JPG  1.492 MB  | Hide | Hide all | Show all
IMG_5972.JPG
Attachment 2: ServoY_TF_13Dec2015.pdf  92 kB  | Hide | Hide all | Show all
ServoY_TF_13Dec2015.pdf
Attachment 3: DatanCode.zip  288 kB
Attachment 4: PSD_916Hz.pdf  50 kB  | Show | Hide all | Show all
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