The fringes seen on ITMY must be due to fringes of the OpLev laser with an OpLev ghost beam coming off CPY.  

In the first attached screenshot I am driving CPY in L with 600 counts at 0.1 Hz in R0 DAMP L.  You can see that the fringing got worse then better as I moved CPY to improve the camera image (36936), then I moved the CP to maximize the fringes.  I went to the floor and unplugged the OpLev fiber, and the fringing went away, so this is not from IFO beams. 

Turned back on, because LLO is down for the night anyway. 

EDIT: The calculation of the modulation depth was corrected in response to Daniel's comment (shown below).

[Summary]

• Modulation depth@118MHz ~ 1 mard 0.2 mrad.
• Signal-to-noise ratio of the demod signals is worse than those of AS36 by some factor (depending on the PSL power).
• With this SNR, the SRC alignment can be sensed in frequencies up to 5 Hz which should be good enough to make a slow ASC control loop.

[Setup]

In the CER, we have two IFR units for this measurement (36354). One is configured for 118 MHz, providing the modulation, and the other is configured for 72 MHz, providing the demodulation RF source. We unplugged the RF source for the 90 MHz system in the ISC R3 rack (the one by HAM6) and plugged the 72 MHz source. This means that we use the 90 MHz demodulation system for demodulating the RF components at 72 MHz. This allows us for a minimum hardware modification at the cost of losing the ASAIRB_RF90 signals. Currently, ASAIRB_RF90 is used only once in the locking sequence where the ISC_DRMI guardian checks whether the DRMI is locked on a hopped mode or not. During this test, we have commented out this action from the ISC_DRMI guardian so that it doesn't stop at this point.

We cranked up the RF power of the 118 MHz unit from -10 dBm to 10 dBm. Note that the RF amplifier that is directly connected to this IFR unit has a input rate of +10 dBm. According to the previous measurement (36354), this should provide 30 dBm at the input of the EOM. The RF power of the 72 MHz unit was set to 13 dBm which gave reasonable RF powers in all the 90 MHz demodulator systems.

BTW, the Triplexer doesn't give appreciable loss at 72 MHz (see table.2 of E1600027).

[Measurement concept: frequency-offset demodulation]

Today we introduced an intentional frequency offset of 205 Hz to the 72 MHz unit. This results in the demod signals rotating in the I-Q phase space at the offset frequency. The main motivation for doing this frequency-offset technique is that it allows us for relatively accurate measurement of the relative RF phase between the two IFR units and the main RF source (Wentzel OCXO).

Looking at the raw RF signals with an oscilloscope, we confirmed that the RF phase of the IFR units with respect to the Wentzel OCXO slips at a rate of roughly 2 pi / 10 seconds even though the IFR units are synchronized to the 10 MHz source which is locked to the GPS 1 pps signal. Not surprisingly the relative phase between the two IFR units don't seem to slip by appreciable amount. In future, we will get an harmonic generator so that the 72 and 118 MHz RF signals are synchronized to the main OCXO.

Regardless of whether the RF phase slips or not, the idea is to have a look at the ASC-AS_WFS_A(B)_RF118 signals in frequency domain so that the peak height at the offset frequency gives us a measure of the modulation depth.

[Modulation depth]

We estimated the modulation depth of the 118 MHz RF sidebands to be 1 mrad 0.22 mrad, much smaller than the others (15661).

Our measurement is essentially comparison of the signal strength between the 36 and 72 MHz demodulated signals given that we know the modulation depth for 9 MHz. The below shows the derivation.

• DRMI was locked with both arm cavities set to off resonant point. PSL was set to 2 W.
• Measuring the rms of the 205 Hz peak produced in the ASC-AS_A_RF90_I1_OUT, we determined the rms to be about 7 counts.
  • This corresponds to an amplitude of 10 counts or peak-to-peak amplitude of 20 counts at 205 Hz
• Measuring the DC value of AS_A(B)_RF36_I(Q)_SUM, we found the quadrature sum  to be about 2000 counts.
• Taking out the gain difference (a factor of 40 from the whitening gain, a factor of 1/5 1/16 from PD response and difference in the interferometer transmissivity by a factor of 11) from the 72 MHz calculation, we get a signal amplitude in the unit equivalent to the 36 MHz to be 5 cnts ( = 10 cnts x 4 segments x 1/40 whitening gain x 5 PD response 16 PD response x 1/11 ifo trans).
• Therefore Gamma_118 / Gamma_9 = 1.45 cnts/ 2000 cnts.
• Assuming Gamma_9 to be 0.3 rad, we get Gamma_118 = 0.75 mrad ~ 1 mrad 0.22 mrad.

[Signal to noise ratio]

In the current freqeuncy-offset scheme, the ASC signals are upconverted to frequencies around 205 Hz. Looking at the spectrum of the demodulated signals when the interferometer was locked with a 2W PSL, we saw that the signal-to-noise ratio of the 72 MHz signals are worse than that of the 36 MHz by a factor of 10. See the attached dtt session.

After we reached a state equivalent to NOMINAL_LOW_NOISE, we took a look at the spectrum again to see how much the signal-to-noise ratio has improved. It seem to have improved by a factor of 5 or so even though the PSL power increased by a factor of 15  (2 W -> 30 W). See the second attachment. Some fraction can be explained by the 3 dB reduction of 45 MHz RF source power that takes place during the locking sequence.

It seems that we can sense the ASC signals up to 5 Hz before diving into some white noise.

[Future direction]

• If allowed, modify h1asc and place a PLL block for measuring the RF phase.
• If allowed, place a phase correction functionality for individual segments of AS118.
• If allowed, record some relevant channels.

The difference between the transimpedance gains at 36 and 72 MHz is taken from LIGO-T1300488. Looking at Figs. 5 and 6 on page 3, we read the response as +2 dB at 36 MHz (Q3 trace in Fig. 5) against –22 dB at 72 MHz (Q3 trace in Fig. 6). The gain difference is then estimated to be 24 dB, or a factor of ~16.

The transmission ratio towards the antisymmetric port between the 9 and 118 MHz sidebands is around ~1/10, see Hang's presentation LIGO-T1700215.

The PcalX is still fully functional and hardware injection can be carried out if needed. The WS_PD channel through which the excitation channel is routed is a spare channel that is only used during the end-station PD calibration.

Summary:

We have established that most of these down converted lines (the one that hurts us)  reported in alog 36959 arise from the aliasing of the harmonics of the injected high frequency calibration lines. The lines that have the possibility to show up in the DARM are the ones that lie between 10 Hz to few hundred Hz. We can avoid these lines by selecting the high frequency calibration lines such that  the aliased lines are not in the given low frequency region.

Details:

We ran into issues with low frequency lines (in 100 Hz range) showing up in DARM when we were running high frequency calibration lines in the region of 5.5 - 6 kHz. This was first noticed in this LHO alog 36959 and LLO alog 34346. To understand this problem we wanted to find where these lines were being produced, photon calibrator itself or the data acquisition system. We disconnected the cable that inputs the excitation to the Pcal electronics and used a DB9 breakout box to acquire the excitation signal so that we were getting these signal before it enters the Pcal system. We rerouted this signal through the H1:CAL-PCALX_WS_PD channel which was one of the unused Pcal channel. 

The first plot shows the spectrum with a 5036 Hz excitation line (in blue) and without an excitation (in red). Some of the extra lines seen in the  blue spectra are the result of aliasing of the  higher order harmonics  of the injected as well as the imaged lines (imaging happens when going from 16khz to 64 kHz). In this particular case the 68 Hz line is the aliasing of the 13th order harmonic of 5036 Hz injected line. The second plot shows the predicted aliased lines (based on harmonics and upsampling) that would be produced for 5036 Hz injected line. Some of the predicted lines show up in the actual spectrum but not all and not all line that are present in the spectra are predicted. However, the lines that show up above few hundred Hz will be well below the DARM signal as the actuation transfer function goes as 1/f^2. Shivaraj, at LLO has taken some additional measurement to see if we can find the source of all the aliased lines. Report to follow.

Tagging DetChar and CDS on this, so that the CW group can follow up with them to understand how to flag the data for this known issue.

RickS, SudarshanK,

The breakout box used to acquire the analog excitation signal on X-end Pcal has been removed and the Pcal configuration is now back to normal. During the visit, Rick repeated some measurement that reproduced the 86 Hz line when a Pcal line was injected at 5950 Hz with an excitation amplitude of 25000 cts. This measurement was made at the DAC (analog) output using SR785. The 86 Hz line is about 70dB below the injected line. This still does not explain why we don't see 86 Hz line in analog output in the measurement (LLO alog 34495) that Shivaraj made using Agilent signal analyzer. :(

On Jeff's request, I am putting down the empirical formula that predicts the lowest frequency down-converted lines for each high frequency excitation.

DCF =  2^16 - EF * n, where DCF is the down converted frequency, EF is the excitation frequency and n is the harmonic number. For frequencies around 4-6 kHz, the harmonic number that produces the line in the bucket are between 11 and 13.

I don't have a physical intuition on why this happens. If we were looking at the resampled 16 kHz signals, the presence of these lines would not be surprising because these would be produced because of aliasing but we see these low frequency lines  on the DAC output, measured using the analog spectrum analyzer. Plots showing the same are attached.

Was associated with FRS Ticket 8335.
Now associated with IIET Ticket 10318.

This happened again, this time I just let the measurement finish and when it was ramping off the interferometer unlocked. Screenshots of the two incidents are attached.

VerbalAlarms has been modified to look for new events from these new channels. Operators may get alarms with "VIRGO" at the beginning of the notification.

The corrected filter file needed for LHO Work Permit Number 7047 had already been staged on the DMT on June 14. Thus, this restart of the calibration code picked up the corrected filter file, and this completes LHO Work Permit Number 7047.

Correction to this alog: the pipeline restart does not affect the version of gstlal-calibration running, since version 1.1.7 was already running at Hanford. However it does pick up corrected filters, but the file pointing to those filters had the same name so no command line change was needed. I apologize for the confusion.

As Keita pointed out, the plots in the previous post were wrong, due to a missing factor of 1/(2*pi)^2 when converting from acceleration to displacement. I attach the same plots with the correct factor, and five new plots that use a window of 1 second around the time of the event instead of the window of 4 seconds that was used before