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Entry  Mon Dec 19 22:05:09 2016, Koji, Summary, IOO, PMC, IMC aligned. The ringdown PD/Lens removed. 
    Reply  Wed Dec 21 21:03:06 2016, Koji, Summary, IOO, IMC WFS tuning DSC_0142.JPG
       Reply  Thu Dec 22 18:39:09 2016, Koji, Summary, IOO, IMC WFS tuning WFS_error_noise.pdfWFS_error_noise_chans.pdf
          Reply  Fri Dec 23 20:53:44 2016, Koji, Summary, IOO, IMC WFS tuning DSC_0144.JPGDSC_0145.JPG
             Reply  Fri Dec 23 21:05:56 2016, Koji, Summary, IOO, IMC WFS tuning IMC_WFS_segment_TF.pdfIMC_WFS_channels_TF.pdfIMC_WFS_161221_table1.pdfIMC_WFS_161221_table2.pdfIMC_WFS_161221.xlsx.zip
                Reply  Sun Dec 25 14:39:59 2016, Koji, Summary, IOO, IMC WFS tuning 
                   Reply  Mon Dec 26 12:45:31 2016, Koji, Summary, IOO, IMC WFS tuning IMC_REFL_TRANS_26hrs.pngIMC_TRANS_P_Y_26hrs.png
          Reply  Thu Dec 29 13:22:21 2016, rana, Summary, IOO, IMC WFS tuning 
             Reply  Thu Dec 29 16:52:51 2016, Koji, Summary, IOO, IMC WFS tuning 
                Reply  Thu Dec 29 21:35:30 2016, rana, Summary, IOO, IMC WFS tuning Screen_Shot_2017-01-03_at_5.55.13_PM.png
                   Reply  Tue Jan 24 01:04:16 2017, gautam, Summary, IOO, IMC WFS RF power levels MC1_misalignment.png
                      Reply  Fri Jan 27 00:14:02 2017, gautam, Summary, IOO, IMC WFS RF power levels 
Message ID: 12748     Entry time: Tue Jan 24 01:04:16 2017     In reply to: 12690     Reply to this: 12759
Author: gautam 
Type: Summary 
Category: IOO 
Subject: IMC WFS RF power levels 


I got around to doing this measurement today, using a minicircuits bi-directional coupler (ZFBDC20-61-HP-S+), along with some SMA-LEMO cables.

  • With the IMC "well aligned" (MC transmission maximized, WFS control signals ~0), the RF power per quadrant into the Demod board is of the order of tens of pW up to a 100pW.
  • With MC1 misaligned such that the MC transmission dropped by ~10%, the power per quadrant into the demod board is of the order of hundreds of pW.
  • In both cases, the peak at 29.5MHz was well above the analyzer noise floor (>20dB for the smaller RF signals), which was all that was visible in the 1MHz span centered around 29.5 MHz (except for the side-lobes described later).
  • There is anomalously large reflection from Quadrant 2 input to the Demod board for both WFS
  • The LO levels are ~-12dBm, ~2dBm lower than the 10dBm that I gather is the recommended level from the AD831 datasheet

We should insert a bi-directional coupler (if we can find some LEMO to SMA converters) and find out how much actual RF is getting into the demod board.


I first aligned the mode cleaner, and offloaded the DC offsets from the WFS servos.

The bi-directional coupler has 4 ports: Input, Output, Coupled forward RF and Coupled Reverse RF. I connected the LEMO going to the input of the Demod board to the Input, and connected the output of the coupler to the Demod board (via some SMA-LEMO adaptor cables). The two (20dB) coupled ports were connected to the Agilent spectrum analyzer, which have input impedance 50ohms and hence should be impedance matched to the coupled outputs. I set the analyzer to span 1MHz (29-30MHz), IF BW 30Hz, 0dB input attenuation. It was not necessary to turn on averaging to resolve the peaks at ~29.5MHz since the IF bandwidth was fine enough.

I took two sets of measurements, one with the IMC well aligned (I maximized the MC Trans as best as I could to ~15,000 cts), and one with a macroscopic misalignment to MC1 such that the MC Trans fell to 90% of its usual value (~13,500 cts). The peak function on the analyzer was used to read off the peak height in dBm. I then converted this to RF power, which is summarized in the table below. I did not account for the main line loss of the coupler, but according to the datasheet, the maximum value is 0.25dB so there numbers should be accurate to ~10% (so I'm really quoting more S.Fs than I should be).

WFS Quadrant Pin (pW) Preflected(pW) Pin-demod board (pW)

IMC well aligned

1 1 50.1 12.6 37.5
2 20.0 199.5 -179.6
3 28.2 10.0 18.2
4 70.8 5.0


2 5 100 19.6 80.0
6 56.2 158.5 -102.3
7 125.9 6.3 11.5
8 17.8 6.3


WFS Quadrant Pin (pW) Preflected(pW) Pin-demod board (pW)

MC1 Misaligned

1 1 501.2 5.0 496.2
2 630.6 208.9 422
3 871.0 5.0 866
4 407.4 16.6


2 5 407.4 28.2 379.2
6 316.2 141.3 175.0
7 199.5 15.8 183.7
8 446.7 10.0 436.7


For the well aligned measurement, there was ~0.4mW incident on WFS1, and ~0.3mW incident on WFS2 (measured with Ophir power meter, filter out).

I am not sure how to interpret the numbers for quadrants #2 and #6 in the first table, where the reverse coupled RF power was greater than the forward coupled RF power. But this measurement was repeatable, and even in the second table, the reverse coupled power from these quadrants are more than 10x the other quadrants. The peaks were also well above (>10dBm) the analyzer noise floor 

I haven't gone through the full misalginment -> Power coupled to TEM10 mode algebra to see if these numbers make sense, but assuming a photodetector responsivity of 0.8A/W, the product (P1P2) of the powers of the beating modes works out to ~tens of pW (for the IMC well aligned case), which seems reasonable as something like P1~10uW, P2 ~ 5uW would lead to P1P2~50pW. This discussion was based on me wrongly looking at numbers for the aLIGO WFS heads, and Koji pointed out that we have a much older generation here. I will try and find numbers for the version we have and update this discussion.


  1. For the sake of completeness, the LO levels are ~ -12.1dBm for both WFS demod boards (reflected coupling was negligible)
  2. In the input signal coupled spectrum, there were side lobes (about 10dB lower than the central peak) at 29.44875 MHz and 29.52125 MHz (central peak at 29.485MHz) for all of the quadrants. These were not seen for the LO spectra.
  3. Attached is a plot of the OSEM sensor signals during the time I misaligned MC1 (in both pitch and yaw approximately by equal amounts). Assuming 2V/mm for the OSEM calibration, the approximate misalignment was by ~10urad in each direction.
  4. No IMC suspension glitching the whole time I was working today yes


Attachment 1: MC1_misalignment.png  50 kB  | Hide | Hide all
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