Abstract:

Power at AS can reach more than 4W even with 1W operation when the lockloss happens.
Due to the saturation, it is difficult to confirm how much power is going to AS when lockloss happens.

Detail:

To estimate the power at AS when lockloss happens, I calibrated K1:LSC-AS_PDA1_DC_OUT_DQ to OMC transmission.
Figure 1 shows the signals when OMC was locked with single XARM lock.
When OMC transmission was about 35.7mW, the value of K1:LSC-AS_PDA1_DC_OUT_DQ was 0.055, so the calibration factor from K1:LSC-AS_PDA1_DC_OUT_DQ to the power at OMC is 650 mW/cnt.

Figure 2 shows an example of lockloss with 1W operation.
It takes 20ms to increase the signals at AS from 0.2 to 6.2 (saturation), which are corresponding to 130mW and 4W.
After saturation, signals keep more than 6.2 cnts for 17.5ms but I'm not so sure the power to OMC keeps morethan 4W for 17.5ms because the signals had already saturated.

Since current PD gains at OMMT2T and POS are too high and they saturate before the saturation of AS PDs, it is necessary to reduce the PD gain to confirm the real trend of AS power.

Images attached to this report

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Comments to this report:

takafumi.ushiba - 9:12 Thursday 21 November 2024 (31705)

I checked the signals last night and confirmed that OMMT2T trans DC PD was not saturated when lockloss happened.
Figure 1-3 shows the signals when the lockloss happened in this morning.
The first peak power is about 19W and the second is about 4W (fig1).
FWHM of the first peak and second is about 15ms (fig2) and 5ms (fig3), respectively.

After achieving PRFPMI_RF_LOCKED, maximum power at AS is about 500mW (fig4), so if we set the power treshold at AS as 1W or something, the trigger seems to work only when lockloss happens.
In addition, speed of power increase from 1W to 19W is about 20ms (fig5).

Images attached to this comment

31705_1732146449_Screenshot from 2024-11-21 08-45-55.png

31705_1732146723_Screenshot from 2024-11-21 08-47-24.png

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31705_1732147970_Screenshot from 2024-11-21 09-04-36.png

yuta.michimura - 10:28 Thursday 21 November 2024 (31711)

The lock loss blast with a peak height of 19 W and FWHM of 15 msec gives integrated energy of roughly 19 W * 15 msec = 0.3 J.
Intra cavity power with 1 W input for each arm is Pcav = Pin * PRG * 4/T_ITM / 2 = 1 W * 15 * 4/0.4% / 2 = 7.5 kW.
This means that total energy stored in XY arms is Ecav*2 = Pcav * 2 * 2 * Larm / c = 0.3 J.
Amazingly consistent (see also JGW-T2416173).
To have less than 30 mJ at OMC PDs (total of two PDs) when the input power is 10 W, we need to reduce the OMC duty factor to less than 30 mJ / (0.3 J * 10) = 1%.
Continuously sweeping OMC with a triangular wave of peak-to-peak of 1 FSR gives (effective) duty factor of 1/Finesse = 1/800 = 0.125%.
So, sweep of 0.125 FSR peak-to-peak would be enough.
Using 100 Hz triangular wave, time to sweep the OMC resonance peak will be
1 / 100 Hz / 2 * (1/Finesse) / 0.125 = 50 usec
Even the OMC resonance peak happens to align with the peak of the lock loss blast, the energy deposited on OMC PDs (total of two PDs) for 10 W input will be
10 * 19 W * 50 usec = 9.5 mJ
This is smaller than the 30 mJ requirement. So, sweeping with 0.125 FSR peak-to-peak at 100 Hz will be good.

tomotada.akutsu - 11:04 Thursday 21 November 2024 (31714)

Too consistent, isn't it...?

takafumi.ushiba - 13:05 Thursday 21 November 2024 (31715)

>Even the OMC resonance peak happens to align with the peak of the lock loss blast, the energy deposited on OMC PDs (total of two PDs) for 10 W input will be
>10 * 19 W * 50 usec = 9.5 mJ

If we consider the speed of scan (5ms), OMC psses on the resonance 3 times within FWHM (15ms).
Even in that case, obtained energy of PD is less than 9.5mJ * 3 = 28.5 mJ, which satisfy the requirement (< 30mJ).