Calibration Team

Summary:

We did some Pcal trend study from the data taken during the first week of ER7 (03 June to 08 June).  Different comparisons were made between PCAL amplitude and DARM amplitude at these calibration frequencies.  We also used the data to estimate the Actuation stregth of ETMY ESD.

Details:

We use Spectral Line Monitoring Tool (SLMT) to  retrieve the data from the LIGO data frames and calculate the amplitude and phase by performing 60 seconds FFT on the time series. After that  we slected only the lock-stretch data (Guardian state vector greater than 500).  We use CAL-DELTAL_EXTERNAL_DQ channel for DARM readout and and respective PD channels for PCAL readout .

Plot 1-4:

We then plotted DARM/PCAL (Amplitude Ratio) at each calibration frequency.  The DARM/PCAL ratio varies at an average of about 10-20%  at higher frequency calibration lines(~500 Hz)  and slighlty more at lower frequencies (~30 Hz).  The first subplot is the DARM/PCAL Ratio. We also looked at the PCAL RxPD and TxPD reading along with DARM Amplitude and Phase.

• The Pcal PD at  Y-arm varies  only by +- 0.1% however the RxPD at X-end varies by  about +- 1.5 % while TxPD stays flat within what is seen at Y-arm. This is a known issue and we think there is somne kind of clipping on the return beam at X-end. We will look into it after the ER7.
• The amplitude of the PCAL line in the DARM varies by about +- 6-7% at higher frequencies and this variation is much more bigger at lower frequencies. This is also the  major contributor to DARM/PCAL Ratio variation. This is attributed to the change in DARM optical gain.
• The last plot on the subplot is the phase of the PCAL line in the DARM. Calculation of Cavity pole using this phase information is reported in alog 19301.

PLot 5-6:

We also calculated the actuation strength of ETMY ESD using the PCAL calibration lines. Knowing the calibration coefficient of Pcal in [N/V] we use the DARM as an intermediary to calculate the ESD Actuation strength in [N/cts] of ESD excitation using the following equation:

[N]/ESD_EXC[cts] = PCAL_CALIBRATION_FACTOR[N]/[V]*(PCAL_PD[V]/PCAL_DARM[m])*(ESD_DARM[m]/ESD_EXC[cts])

Ideally this coefficient should be same at all frequencies but we see almost an order of magnitude different between the high frequency calculation and low frequency calculation. Don't know why.

P.S. Kiwamu pointed out CAL-DELTAL_EXTERNAL_DQ may not be an ideal chnanel to conside for this comparison because of multiple loop involved between injection and read out. I will followup by comparing this calculation using LSC-DARM_IN_DQ channel.