alberto.stochino@LIGO.ORG - posted 18:47, Tuesday 24 July 2012 - last comment - 23:24, Tuesday 24 July 2012(3575)
Reference cavity locked with different settings
[Alberto, Bram]
Today we locked the reference cavity with some optimized settings. We set the knobs in the TTFSS with these values:
COARSE: 690; FINE: 610; COMM: 988; FAST 520; OFFSET 294.
The laser temperature diplay read: 41.064 degrees.
We measured a UGF of 172 kHz.
Two peaks appeared initially in the loop gain measurements at about 2.4 Hz and 4 Hz. We found that the first was due to the cleam room HEPA filters and the second one was the cavity motion due to us touching the table.
I measured the transmitted beam of the RefCav which is monitored by a thorlabs PD (also it hits a Watec CCD, and of course into the fiber). the PD is in Beckhoff world and will soom be available in EPICS. At the moment we have an EVO stream of the TV and oscilloscope!
Attached are three plots, the first two with the same frequency span ~0.05 Hz to 50 Hz and the third plot from ~100 Hz to 112kHz. All recorded with an SR785.
The first plot is the transmitted power readout form the Thorlabs PD, with the HEPA filters ON and OFF. There is a reduction of 2 of the 2.75 Hz peak as well as no harmonics. There is a 4 Hz spike appearing. You can see the beam jiggling on the TV monitor.
The second plot is the TTFSS mixer output (on the interface chassis), indicating that these spikes are not seen in reflection.
The third plot is a large frequecy span, of both the transmission PD and the TTFSS mixer output. There are spikes visible which are not in each others signals.
In summary, if we get the cavity locking again, then the 2.75 Hz peak should be smaller with 'no' harmonics. Although a 4 Hz peak will appear ...
The fourth file is a zip of the data and matlab script for the interested ones.