I ran the DARM offset step code starting at:
2024 Jun 13 16:13:20 UTC (GPS 1402330418)
Before recording this time stamp it records the PCAL current line settings and makes sure notches for 2 PCAL frequencies are set in the DARM2 filter bank.
It then puts all the PCAL power into these lines at 410.3 and 255Hz (giving them both a height of 4000 counts), and measures the current DARM offset value.
It then steps the DARM offset and waits for 120s each time.
The script stopped at 2024 Jun 13 16:27:48 UTC (GPS 1402331286).
In the analysis the PCAL lines can be used to calculate how the optical gain changes at each offset.
See the attached traces, where you can see that H1:OMC-READOUT_X0_OFFSET is stepped and the OMC-DCPD_SUM and ASC-AS_C respond to this change.
Watch this space for analysed data.
The script sets all the PCAL settings back to nominal after the test from the record it ook at the start.
The script lives here:
/ligo/gitcommon/labutils/darm_offset_step/auto_darm_offset_step.py
The data lives here:
/ligo/gitcommon/labutils/darm_offset_step/data/darm_offset_steps_2024_Jun_13_16_13_20_UTC.txt
See the results in the attached pdf also found at
/ligo/gitcommon/labutils/darm_offset_step/figures/plot_darm_optical_gain_vs_dcpd_sum/all_plots_plot_darm_optical_gain_vs_dcpd_sum_1402330422_380kW__Post_OFI_burn_and_pressure_spikes.pdf
The contrast defect is 0.889 ± 0.019 mW and the true DASRM offset 0 is 0.30 counts.
I plotted the power at the antisymmetric port as in this entry to find out the loss term between the input to HAM6 and the DCPDs, which in this case is (1/1.652) = 0.605 with 580.3 mW of light at the AS port insensitive to DARM length changes.
From Jennie's measurement of 0.88 mW contrast defect, and dcpd_sum of 40mA/resp = 46.6mW, this implies an upper bound on the homodyne readout angle of 8 degrees.
This readout angle can be useful for the noise budget (ifo.Optics.Quadrature.dc=(-8+90)*np.pi/180) and analyzing sqz datasets e.g. May 2024, lho:77710.
Table of readout angles "recently":
|
|
|
alog | ||
| O4a | Aug 2023 | 46.6 mW | 1.63 mW | 10.7 deg | lho71913 |
| ER16 | 9 March 2024 | 46.6 mW | 2.1 mW | 12.2 deg | lho76231 |
| ER16 | 16 March 2024 | 46.6 mW | 1.15 mW | 9.0 deg | lho77176 |
| O4b | June 2024 | 46.6 mW | 0.88 mW | 8.0 deg | lho78413 |
| O4b | July 2024 | 46.6 mW | 1.0 mW | 8.4 deg | lho79045 |
##### quick python terminal script to calculate #########
# craig lho:65000
contrast_defect = 0.88 # mW # measured on 2024 June 14, lho78413, 0.88 ± 0.019 mW
total_dcpd_light = 46.6 # mW # from dcpd_sum = 40mA/(0.8582 A/W) = 46.6 mW
import numpy as np
darm_offset_power = total_dcpd_light - contrast_defect
homodyne_angle_rad = np.arctan2(np.sqrt(contrast_defect), np.sqrt(darm_offset_power))
homodyne_angle_deg = homodyne_angle_rad*180/np.pi # degrees
print(f"homodyne_angle = {homodyne_angle_deg:0.5f} deg\n")
##### To convert between dcpd amps and watts if needed #########
# using the photodetector responsivity (like R = 0.8582 A/W for 1064nm)
from scipy import constants as scc
responsivity = scc.e * (1064e-9) / (scc.c * scc.h)
total_dcpd_light = 40/responsivity # so dcpd_sum 40mA is 46.6mW