Abstract:
I investigated the IFO conditions associated with two recent DARM sensitivities.
The analysis suggests that intentionally reducing the power recycling gain (PRG) by changing the beam spot position may improve the binary neutron star (BNS) range.
Details:
On December 1st (UTC), we observed a DARM sensitivity where the frequency noise was larger than usual, yet the sensitivity between 100–200 Hz was improved, as shown in the upper-left panel of Fig. 1.
The BNS range with this sensitivity curve (magenta) was slightly larger than that of the other sensitivity curve (blue), primarily due to the significant reduction of the 116 Hz and 127 Hz peaks.
To understand the underlying causes, I examined various IFO conditions, with parameters summarized in the following list from the attached Fig. 2, and the summary page.
The two sensitivities are referred to as S1 (blue) and S2 (magenta).
| S1 | S2 | Diff. | |
| REFL PDA1 DC | 3.0 mW | 2.2 mW | 0.8 mW |
| Normalised arm transmission | 130-140 | 120-130 | 10 |
| ITMX Pitch | -8.4 urad | -7.3 urad | 1.1 urad |
| ITMX Yaw | -0.6 urad | -0.6 urad | smaller than the fluctuation |
| ITMY Pitch | -17.1 urad | -16.5 urad | 0.6 urad |
| ITMY Yaw | 8.5 urad | 8.2 urad | smaller than the fluctuation |
| IMMT2 Pitch | 42.8 urad | 40.3 urad | 2.5 urad |
| IMMT2 Yaw | -13.5 urad | -11.7 urad | 1.8 urad |
| PRM Pitch | 24.3 urad | 21.9 urad | 2.4 urad |
| PRM Yaw | -53.9 urad | -48.7 urad | 5.2 urad |
Discussions:
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Frequency Noise
The frequency noise in S2 was larger than in S1.
Notably, while the CARM in-loop spectra (PDA1) were nearly identical, the out-of-loop spectra (PDA3) were different at high frequencies.
This suggests that the CARM optical gain in S2 was lower, leading to a reduction in the open-loop gain and increased residual frequency noise.
This issue could be resolved by increasing the CARM control gain under S2-like conditions. -
IFO Alignment and PRG
The IFO alignment differed between S1 and S2, particularly for IMMT2 and PRM.
These differences likely caused variations in the beam spot position on the ITMs, reducing the normalized arm transmission and, consequently, the PRG in S2. -
The 116 Hz and 127 Hz Peaks
The significant reduction of these peaks in S2 compared to S1 could stem from the different beam spot positions on the ITMs, altering the coupling of input jitter.
Another possible explanation is the reduction of the CARM sensing noise, as reported in klog:31667, due to lower carrier reflectivity from the IFO.
A reduced PRG indicates that the IFO was closer to a critically coupled cavity, thereby decreasing the REFL power.
Achieving zero REFL carrier with a PRG of 10 (normalized arm transmission of 100) could, in principle, eliminate such CARM sensing noise.
Future Direction:
I propose intentionally modifying the beam spot position on the ITMs to reduce the PRG to approximately 10, minimizing the carrier reflectivity.
This adjustment may mitigate input jitter coupling and CARM sensing noise, ultimately improving the sensitivity in the 100–200 Hz range and enhancing the BNS range, even if we will make the shot noise slightly worse.
