Here is a plot of the data taken from the DCPDs up to 4MHz.  I've removed the 20dB of gain and the poles at 265kHz and 290kHz from D1700376 in this plot.  This was measured with our usual darm offset, restuling in 10mA on each of the DCPDs (we used one of the single PD outputs from D1700376).  We tried to repeat this measurement with a diode set up in the AS air path yesterday, 54636, but the 3.125MHz peak was only 20dB above the noise level, so we weren't able to see any of the other noise apparent in this measurement.  

Thanks, this is useful for ongoing CLF work.

For reference, Koji's measurements of the transimpedance are below. You can see that the rolloff flattens a bit in the MHz, so I'm less certain how real the bump is.

https://nodus.ligo.caltech.edu:8081/OMC_Lab/236

https://nodus.ligo.caltech.edu:8081/OMC_Lab/235

does your RF equipment have the ability to make a decently long cross correlation? That is surely what we need if we can conveniently get equipment to do it. Alternatively, you may be able to take spectra simultaneously in a sum and null output configuration, and then subtract them. It won't be as clean as xcorr, but it will show excess in a way that is less succeptible to systematic errors from inverting the PD response. Sum can be picked off from the SQZ chassis, and null just needs the phase shift before an RF splitter used in reverse (depending on the phase convention of the splitter).

Update, I remembered that I had acquired the LISO model and fit it in IIRrational for just these kinds of occasions. These fits should be decent up to 10MHz where the simulation cut off.

scipy ZPK notation:

(array([-1.09367517e+06+9.16935186e+04j, -1.09367517e+06-9.16935186e+04j,
       -4.54321270e+01+3.94631525e-01j, -4.54321270e+01-3.94631525e-01j,
       -2.85835003e+07+0.00000000e+00j]), array([-1.12584306e+07+1.59926553e+07j, -1.12584306e+07-1.59926553e+07j,
       -3.11667017e+07+1.49827313e+08j, -3.11667017e+07-1.49827313e+08j,
       -4.41594538e+07+5.59042545e+07j, -4.41594538e+07-5.59042545e+07j,
       -1.51046460e+07+2.52263715e+07j, -1.51046460e+07-2.52263715e+07j,
       -1.02822684e+05+0.00000000e+00j, -9.54273138e+04+0.00000000e+00j,
       -5.08417603e+02+0.00000000e+00j, -4.91824766e+02+0.00000000e+00j]), 2.71124765615596e+56)

foton notation:
ZPK([
  -174063.80969071676 + 14593.476738924443*i; -174063.80969071676 - 14593.476738924443*i;
  -7.2307475830315395 + 0.06280755795247621*i; -7.2307475830315395 - 0.06280755795247621*i;
  -4549205.365087142;
],[
  -1791834.8749338891 + 2545310.1382306265*i; -1791834.8749338891 - 2545310.1382306265*i;
  -4960334.630691198 + 23845757.522465646*i; -4960334.630691198 - 23845757.522465646*i;
  -7028195.359231514 + 8897438.443450466*i; -7028195.359231514 - 8897438.443450466*i;
  -2403979.0673290133 + 4014901.7200727463*i; -2403979.0673290133 - 4014901.7200727463*i;
  -16364.738450407038; -15187.728699344743;
  -80.91717459920993; -78.27634271397135;
], 2.71124765615596e+56, "f")

Attached are the fits and the output of the LISO model. I think the model differs a touch from Koji's measurements on the exact 3MHz cutoff frequency.

Here is a plot (and the data used to make it) of the DCPD output measured with the analyzer in noise mode.  The first set of data (in the original log above) were taken in spectrum mode.  The dark noise in this new plot was measured durring Friday's EQ in Turkey, the "squeezer blocked" trace was taken durring the commisioning time Thursday (54681).   These are calibrated using the filters in D1700376 and Lee's estimate of the OMC DCPD transimpedance above.  Both spectra were taken with 30Hz resolution bandwidth and 3Hz video bandwidth.  A potential problem with this measurement could be that the squeezer came unlocked and the beam diverter closed partway through the measurement, although that didn't seem to have an impact on the spectrum here.  

It does seem suspicous that the quadrature difference, which should represent noise coming from the interferometer light, has a spectrum so similar to the dark noise. 

This appears qualitatively a bit different than the post-demodulation measurement of the spectrum at LLO50307. There the blocked and dark noise ASD's changed by a factor of about 1.5. This appears to be substantially less than that at 3.125MHz.

Here is one more plot of the DCPD spectrum up to high frequency, again.  In the original version of the attached plot in 54753  I made two mistakes in the calibration, which are fixed here..

• The field on the DCPDs could be written as E_carrier +E_clf + E_am where E_am is am sidebands from the interferometer at audio frequencies around 3.125MHz.  The amplitude noise spectrum we measure on the DCPD is E_carrier*E_am while the term that could cause audio frequency noise by downconverting with the CLF is E_clf*E_am  The requirement for the amplitude noise around 3MHz is that the ASD of the photocurrent at 3MHz should be smaller than our audio frequency photocurrent asd*abs(E_carrier/E_clf)  In other words, the amplitude noise requirement around the CLF (and RLF) fields is less stringent than that at DC by the amplitude ratio of the clf to carrier light.
• With 20mA total on the DCPDs, our shot noise limited sensitivity on each individual diode is 6e-11A/rtHz, without squeezing. 
• We have been running with 5uW CLF injected into the OPO, and about -29dBm of RF power at the 3.125MHz demodulator since Nov 26th, before that we used -19dBm.  These powers are measured after the 20dB of gain in D1700376. This means we get 10uW of RF power out of the transimpendance amp, which is 92uA of photo current from the beat note of the 3MHz sideband with the DC power (20mA, 23mW). This means that we were using about 0.5uW of CLF on the DCPDs for the first part of O3, when we had noice from the CLF just below DARM.  Edit: The filter cavity design document T1800447 states on page 24 that there will be 10uW of CLF and RLF combined on the DCPDs OMC, which means about 0.1uW on the DCPDs, so this is similar to our current operating power. 
• This means that to determine if ampltude noise down-converted by the CLF can explain the observed CLF noise, we would need to measure the spectrum of the DCPDs a factor of a few below 1.7e-8A/rtHz on a single diode.  To allow for 20 times more CLF power reaching the DCPDs with the filter cavity, we'd need to measure noise below 3e-9A/rtHz. 

The attached plot shows, in addition to the dark noise and locked spectrum, the level of amplitude noise which I think would be needed to create downconverted noise about equal to the current shot noise limited sensitivity.  One conclusion is that the dark noise of the DCPDs at 3MHz is too high for us to measure the level of amplitude noise that we need to measure to be sure that we can turn up the CLF power to the level required for the filter cavity controls. However, the difference between the in lock and the dark noise spectrum suggests that the current level of amplitude noise is well above this level, such that it should be dominating the noise in DARM.  So something seems to be wrong either with the measurement or with my projections.

One could worry that there might be significant variation between the individual transimpednce amplifiers at 3MHz.  We have a measurement made at 3.12MHz with the installed amplifier: 47540  which is roughly consistent with the one that Koji measured and Lee's fit above.  Another worry could be that we are running with a different transimpedance than these measurements were taken with, but we are running in high Z which is 400 Ohms at DC according to D060572, My plot of Lee's fit above gives a DC transimpedance of 200 Ohms, but it is ~220 Ohms at 3.125 MHz, so I think it is close enough.