Following on from last week's entry about the power stabilisation photodiodes
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=27054
It took advantage of the laser being down to measure the output of the photodiodes
when laser was off. The two measurements are quite different. Not just in
level but also spectral shape. The spectrum posted earlier is observed when one
tees the signal off and inputs it into an oscilloscope.
One observation is that the isolated LEMO connectors on the photodiode box
are not isolated from the table surface. Neither are the photodiode housings
within the box.
For what it's worth, attached is the free running RIN of high power laser measured by the first loop diodes (blue, green) and FSS transmission (red) as of now.
Even though Peter and Jason decreased the light on the PDs, I had to increase the diffraction to 15+%, otherwise the PDA and PDB "AC" still rails once in a while and ruin the spectrum.
Also plotted is the dark noise of PDA projected onto RIN. This "dark" spectrum was from last evening when the laser was shut down. Since the only PDA and PDB signals we record on the frame are REL channels (i.e. dewhitened AC divided by DC), I had to look at the dark offset of PDA from last evening:
RIN projection = REL spectrum * dark offset / 2.3V
where 2.3V is the "DC" output when the red, blue and green traces were taken. I couldn't do this to PDB as that guy has almost zero dark offset.
Anyway, at least when the laser was shut down and nobody was in the room, the dark noise looked fine.
