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Reports until 09:45, Friday 31 January 2025
H1 ISC (DetChar, SQZ)
jennifer.wright@LIGO.ORG - posted 09:45, Friday 31 January 2025 - last comment - 09:45, Friday 31 January 2025(82555)
Heating up then cooling back down OM2 during commissioning

Jennie W, Elenna, Camilla, Mayank

Today we had a longer commissioning period so we could heat up the TSAMS on OM2 without causing glitches during oberving as the heat up happens, see alog #78618 for details of these.

 

Method

17:20:16 UTC Camilla and Mayank started heating up OM2 after losing lock. This prevented us from taking darm offset step measurements beforehand.

We took the first darm offset step at 19:24:10 UTC about 1 hr 41 mins into lock.

We were going to take a squeezed data set of FIS, FIAS and No SQZ but we lost lock while in FIS.

After we lost lock we started cooling down OM2 to its nominal state at 19:49:21 UTC.

Then we took the second DARM offset step measurement when we were almost at the nominal cool state, around 31 mins into lock.

 

DARM Offset Step Method

This measurement script turns all the PCAL lines off except 410 and 255 Hz and increases their amplitude so they show up very strongly in DARM.

Then the fringe offset is varied in the DC readout loop to change the controlled power level on the output DCPDs (which is nominally set to 40mA during NLN).

The two PCAL lines allow us to measure the optical gain of the interferometer as we change this output power and thus the carrier light coming from the arms to the output of the IFO.

 

Results

The data for the hot state is saved in /ligo/gitcommon/labutils/darm_offset_step/data/darm_offset_steps_2025_Jan_30_19_24_10_UTC.pkl

The analysed pdf showing the plot of DCPD sum power vs. optical gain, optical gain vs. darm offset, and power into HAM6 vs. DCPD sum power is saved in /ligo/gitcommon/labutils/darm_offset_step/figures/all_plots_plot_darm_optical_gain_vs_dcpd_sum_1422300272_Jan_30th__OM2_Hot_state_1_hr_41m_into_lock_.pdf

The contrast defect (taken from the first graph) averaged between the two calculations for each line is (0.211 + 0.177) / 2 = 0.194 +/- 0.064 mW.

The equation for anti-symmetric port power taken from the third graph

P_AS = 1.485*P_DCPD + 688.147 mW

 

The data for the cold state is saved in /ligo/gitcommon/labutils/darm_offset_step/data/darm_offset_steps_2025_Jan_30_21_30_34_UTC.pkl

The analysed pdf showing the plot of DCPD sum power vs. optical gain, optical gain vs. darm offset, and power into HAM6 vs. DCPD sum power is saved in /ligo/gitcommon/labutils/darm_offset_step/figures/all_plots_plot_darm_optical_gain_vs_dcpd_sum_1422307856_Jan_30th__cold_OM2_31m_into_lock.pdf

The contrast defect averaged between the two calculations for each line is(0.213 + 0.224) / 2 = 0.219 +/- 0.088 mW.

And the equation for anti-symmetric port power is

P_AS = 1.533*P_DCPD + 669.724 mW.

 

Analysis

This contrast defect of 0.2 mW is much lower than the previous measurement from October 2024 with OM2 cold which was around 0.7 mW. The contrast defect does not seem to change much by heating up OM2, this makes sense as the curvature of OM2 should not change the wavefront matching between the arms. I am unsure why our arms are better mode-matched to each other compared to October last year though.

The HAM 6 throughput for the TM00 mode in the hot state is 1/1.485 = 67.3 %, for the cold state it is lower at 65.2 %. Does this mean the interferometer mode-matching to the OMC is better in the hot state? Compared to the throughput calculated for the October measurement which was 82%, this implies the IFO/OMC mode-matching has decresed since October. The junk light at the anti-symmetric port is now around 679 mW which is an increase of (((688+670)/2)  - 656 ) / 656 = 3.5% compared to October. Further thinking/analysis needed.

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Comments related to this report
jennifer.wright@LIGO.ORG - 16:51, Thursday 30 January 2025 (82557)

I compared the alignment of the SRM and SR2 mirrors at the hot state and cold state.

SRM_P did not change. SRM_Y moved down by 3 mircoradians. SR2_P decreased by less than a microradian. SR2_Y decreased by about 2 microradians.

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jennifer.wright@LIGO.ORG - 17:01, Thursday 30 January 2025 (82559)

kappa_C increased by 0.018/0.976 = 1.8 % from the hot state to the cold state. The first cursor in the attached photo is just before we did the DARM offset step in the hot state, the second is in the cold state a similar amount of time into lock ( 1hr 40 mins) as the first cursor.

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