Carl, Terra, Ross, Tega
Tonight we used the freshly installed LVLN ITMX ESD driver to ring up and damp two mechanical modes of ITMX, 15063 Hz and 15077 Hz.
After sorting out some phase settings, we drove the ITMX ESD close to saturation in a differential drumhead pattern. Negative gain rang up 15063 Hz. Flipping the gain sign to postive then damped this mode and rang up 15077 Hz. The amplitude plateaued as a the saturated drive signal approched a square wave. Figure below tracks amplitude of 15063 Hz and 15077 Hz (seen in OMC trans), with gain sign flip occuring around the 0.15 time mark.
Also attached is a spectrum of H1:OMC-PI_DCPD_64KHZ_A_DQ during no gain, negative gain, and positive gain times, i.e. on either side of the 0.15 time mark from the plot above.
At about 0.23 hours the gain was turned off and the mode rangdown. The fit to this ringdown indicates the mode has a Q factor of (omega_o)/(2alpha) = 1.2 million.
Settings: Power 1.9 W, DC bias 100k, butterworth BP filter, iWave bypassed, -60deg damp filter, damp gain 300,000.
Such a wonderful conclusion to the installation and commissioning of this system. Much thanks for the great support I received from all involved.
I suspect that 15077 Hz mode is an aliased 48923 Hz mechanical mode (64 - 15.077) kHz. The FEA gives an interesting mode at 48944 Hz (mode shape attached). Observation of the analog channel PSD on transmission is required to confirm if this is a case. The 15077 Hz mode is only 14 Hz above known 15063 Hz mode. I am not very familiar with linetracking filter but I assume that the two resonant lines 15063 HZ and 15077 Hz cannot be sufficiently separated and the signal with higher resonant peak will be eventuality phase-locked. It may be interesting to observe the transition from one mode to another. If this all is true you measured the Q-factor of one of the modes in the range were PI may also show up at higher circulating power than during O1. It looks to me that the 48923 Hz mode is very sensitive to off-center position of optical TEM00 (that's probably why can be seen on OMC) and can be used for centering of TEM00 on ITMs.
In the Q estimate 'f' was used in place of 'omega_o' introducing a 2 pi error. The corrected estimate of the Q factor of the 15077Hz mode is 7.4million.