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MIF (ITF Control)
kentaro.komori - 2:44 Wednesday 28 May 2025 (33923) Print this report
Changing reference of ITM's BPC from IX to IY

[Tanaka, Ushiba, Komori]

Abstract:

We successfully changed the reference of the ITM's BPC from IX to IY in the 10-W RF-locked state.
The next step is to achieve the same change in the DC readout state.

Detail:

To improve sensitivity in the 60–100 Hz range, we plan to center the beam spot on IX to reduce pitch thermal noise.
On the other hand, we must avoid placing the beam spot on a region of high birefringence on IY, in order to maintain the current low jitter coupling and high power recycling gain.
As a preparatory step for these tasks, we started to change the BPC reference for the ITM from IX (currently in use) to IY.

First, we copied the BPC demodulation settings from IX to IY.
Then, we searched for appropriate values of P2L and Y2L in the IY drive alignment matrix that yield zero BPC error signal at the conventional beam spot position in the 10-W RF-locked state.
The resulting values were –2.64 for P2L and –0.45 for Y2L.

Next, we adjusted the demodulation phases to roughly maximize the BPC error signal.
The demodulation phases were set to 160° for IY pitch and 113° for IY yaw.

Following these adjustments, we successfully closed the new BPC loop with IY as the reference in the 10-W RF-locked state.
These settings have been implemented in the ASC_LOCK guardian.

The next step is to close the BPC loop in the DC readout state.
However, despite performing the same procedure just before engaging the BPC in the DC readout state, a sudden large drift occurred when the IFO was unlocked.
We plan to investigate this issue tomorrow.

Comments to this report:
kentaro.komori - 0:00 Thursday 29 May 2025 (33932) Print this report

[Ushiba, Tanaka, Komori]

Abstract:

We successfully closed the BPC loop with IY in the DC readout state.

Detail:

The issue reported in the previous post was a sudden drift that occurred immediately after closing the BPC loop from IY to PRM.
Upon investigation, we found that a large offset existed even when the beam spot was positioned at the point where both pitch and yaw error signals should be zero.
Moreover, these offsets were not proportional to the CLK gain.

Ushiba-san pointed out that the original CLK gain of 50 was too high when no attenuation was applied in the TM dither filters.
A factor of 0.1 is applied in the TM dither filters for IX and EY, but this was missing in the IY filter, resulting in saturation of the BPC modulation.
This saturation was the cause of the large offset.

We decided to implement a guardian to set the CLK gain to 5 for both IY and EY, and we removed the attenuation factor from the EY dither filters.

Additionally, we observed an exponential drift in the PRM pitch.
We reversed the control sign and confirmed that the error signal converged.

After these modifications, we successfully closed the BPC loop with IY in the DC readout state.

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