The goal of the intervention was to measure the locking bandwidth of the SL laser, increase it by increasing the power at its input and see its effect on the phase margin of the IMC loop.
We started by measuring the OLTF of the IMC loop (see figure 1) 82 kHz of UGF and 17 ° of phase margin.
We measured the SL bandwidth by measuring the linear region of the carrier error signal and calibrating it thanks to the sideband that are 2 x 14.480 MHz away one from the other.
It is difficult to do an accurate measurement, we checked both with the oscilloscope and with the dataDisplay (figure 2 for example). From the measurement we did this morning it seems that the measuremnts were more consistent with the oscilloscope and we will report only those ones here below.
With the preamplifier at 24.6 dBm we measured a locking bandwidth of 1.84 MHz (10:37 UTC) which is slightly less than what has been measured in December 2023: 1.9 MHz (https://logbook.virgo-gw.eu/virgo/?r=62699).
Given the unprecision of the measure, it is not easy to get a clear indication from this measurement but this decrease could suggest a slight worsening of the coupling of the ML in the SL.
We increased the power of the premaplifier of 1 dB (25.6 dBm, 10:56 UTC), the power seen by the PSL_ML_DC photodiode situated on the laser bench increased from 0.194 W to 0.244 W as expected.
We measured the locking bandwith 1.94 MHz which is an increse of about 1.05. This is less than what we were expecting: sqrt(24.4/19.4) = 1.12.
This may be due to the (un)precision of the measurement itself.
We increased the power of the premaplifier of 1 more dB (26.6 dBm, 11:10 UTC), the power seen by ML DC increased from 0.244 W to 0.303 W as expected.
We measured the locking bandwidth and found 2.05 MHz which is again an increase of 1.05 while we were expecting about 1.11.
We relocked everything but noticed that there was a loop oscillation at about 480 Hz on the PMC. By decreasing the gain of 2 dB it disappeared.
We measured the OLTF of the IMC. We could see some structures both in the gain and in the phase (figure 3 and zoom figure 4) and so no particular improvements.
We tried to decrease the gain of the preamplifier to intermediate step of 25.6 dBm but observed the same structures (figure 5).
We went back to the inital situation: 24.6 dBm on the preamplifier and 2 more dB on gain of the PMC loop.
The OLTF of the IMC had still a quite strange behavior but it seems that it was stabilizing in time and came back to relatively normal values (figure 6)
Conlcusion :
Increasing the power from the premaplifier had an effect on the locking bandwidth of the Slave laser but less than expected.
However we could not see any effect on the transfer function of the IMC. We could even see a worsening of the situation.
This is not clear why, we may have to wait for the premaplifier to stabilized somehow. To be investigated.
