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takafumi.ushiba - 20:28 Friday 11 April 2025 (33372) Print this report
REFL PD power up to check CARM shot noise effect at high frequency

I increased REFL PD power from 5.5 mW to 16.7 mW while keeping CARM UGF around 50 kHz (fig1).
Figure 2 shows DARM spectrum when the REFL power was 5.5 mW (blue) and 16.7 mW (red).
At high frequency region, there is no significant reduction of the noise, which implies the current shot noise level doesn't contaminated by CARM shot noise.

Note:

I keep the REFL power at 16.7mW until lockloss happened.
After the lockloss, HWP angle will automatically go back to the nominal one and REFL power will be 5.5mW from the next lock.
To measure the spectrum I used the time from 11:02:00 UTC for 5.5mW lock and 11:12:00 UTC for 16.7mW lock.

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Comments to this report:
kentaro.komori - 22:07 Saturday 12 April 2025 (33381) Print this report

Abstract:

The CARM sensing noise is probably not CARM shot noise.

Detail:

To investigate the mystery reported in klog:33372, I compared the in-loop (PDA1) and out-of-loop (PDA3) CARM spectra at both measurement times, as shown in the lower panel of Fig.1.
Please note that this comparison was performed before resolving the PDA3 saturation issue reported in klog:33373.

It is reasonable that the DARM sensitivity at high frequencies remained unchanged, as the out-of-loop CARM spectrum at high frequencies also showed no variation (blue and black).
The coherence was approximately 0.1 and the same before and after increasing the power at PDA1, as shown in the upper panel of Fig.1.

These results suggest that the CARM sensing noise may be subdominant at high frequencies; however, it is likely to be limited not by CARM shot noise but other CARM sensing noise.
Further investigation is needed to identify the origin of this CARM sensing noise.
For example, only PDA3 might be limited by other sensing noises.

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