This is just a quick report about one of today's calibration activities.
I made a measurement to help understanding the OMC DCPD response by having intensity modulated light on OMC in a simple configuration. At the beginning I was going to use ISS PD arrays to calibration the intensity noise, but it turned out that they were not accurate enough to get 1 % accuracy due to mismatched dewhitening filters in the digital system. Seeking for alternatives, I finally ended up with ASC AS_C. Data and analysis to come later.
The frequency response of OMC DCPDs (A and B) are checked using ASC-AS_C as a intensity noise monitor in the single bounce configuration. Even though the response of ASC-AS_C is not well known, the result shows an almost flat response as expected with a deviation of 6% at most which is encouraging.
In order to improve the accuracy of the measurement, we should prepare a well-calibrated photo detector, probably placed at ISCT6, and make the same comparison measurement with the OMC DCPDs.
Method:
The interferomter was in a single-bounce configuration where the beam bounces off of ITMX and goes to the AS port without any recycling. Also ETMs are misaligned as well in order to avoid flash from the arm cavities. The PSL power was intentionally set to the maximum in order to get the maximum signal to noise ratio everywhere. The OMC is locked to the carrier 00 mode with a OMC-LSC_GAIN of 20. I forgot the UGF of the OMC length loop, but it was on the order of 100 Hz, I believe. The OMC alignment was done by the QPD loops with a overall gain of 0.1. For some reason, I needed to engage DC centering loops for the AS WFS A and B, otherwise it would saturate the OMC suspension. The photo current on each OMC DCPD was about 17 mA, which is a bit too high compared with the nominal full lock photo current of 10 mA.
I injected swept sine signal to the ISS inner loop from 7 kHz to 10 Hz. Then I measured a relative response between ASC-AS_C segment 3 and two OMC DCPDs. One trick in doing this is that I had to use an IOP signal to measure the response of ASC-AS_C because the ASC front end runs at only 2 kHz. Also, since I used the IOP signal, I was not able to grab summed QPD signals, and that's why I ended up with one of the QPD segments. Segment 3 happened to receive the highest power and therefore I chose this for the final analysis.
The dtt file and its ascii formated data are checked into the SVN at
/CalSVN/aligocalibration/trunk/Runs/ER8/H1/Measurements/OMCDCPDs/2015-08-24_omc_dcpd_and_asc_pd.xml/CalSVN/aligocalibration/trunk/Runs/ER8/H1/Measurements/OMCDCPDs/2015-08-24_asc_to_omc_dcpds_tfs.txt/CalSVN/aligocalibration/trunk/Runs/ER8/H1/Measurements/OMCDCPDs/2015-08-24_asc_to_omc_dcpds_coh.txtAlso the analysis code can be found in SVN at
CalSVN/aligocalibration/trunk/Runs/ER8/H1/Scripts/OMCDCPDs/analyze_asc_to_omc_dcpds.mResults:
The result is shown in the attached pdf. It is relative gain (or transfer function) between OMC DCPDs and ASC-AS_C. The red dots are for DCPD A and blue for DCPD B. The following frequency responses are taken into account:
Note that since AS_C is acquired at 64 kHz without any downsampling, I did not apply any digital low pass for it. Aside from these known parameters, I had to make some assumptions as follows.
Obviously, the key points to success this method is to reduce the number of the assumptions, but this time I simply attempted with ASC-AS_C, which I had to make multiple assumptions, to get some idea of how the measurement would go.
As shown in the upper panel, the absolute magnitude is almost flat with a trend in which the DCPD response higher at high frequencies. The error bars are placed by using the usual coherence technique (see alog 10506). The low frequency part below 20 Hz is clearly limited by low coherence, but it seems to consistently show lower response at low frequencies. If we take a peak to peak of the variation, it is going to be roughly 1.04 / 0.94 ~ 10 %. Looking at the phase, shown in the lower panel, the phase seems to diverge as it goes to high frequencies such as AS_C response. I don't think it can be explained by mismatch in the analog anti-aliasing filters because they are usually well matched. At this point, we can not really say if the high frequency deviation is from the real DCPD response of some artifact from unidentified uncompensated AS_C response.