Louis, Naoki, Craig

Yesterday we measured some spectra and open loop transfer functions of the FSS, using the IN1, IN2, and EXC cables attached to our TTFSS.
We changed the gain sliders to four levels
1) Common 11 dB, Fast  8 dB (Jenne's latest setting)
2) Common 13 dB, Fast  8 dB
3) Common 15 dB, Fast 11 dB (old nominal)
4) Common 15 dB, Fast  8 dB

I believe that our TTFSS is the second generation version, or version 1 here: https://dcc.ligo.org/LIGO-D040105-v1

OLGs
----
Plot 1 shows the four OLGs taken at each gain state.
All show messy behavior in the 160 kHz and 2.5 MHz region, as well as at 770 kHz.
The notch at 1.8MHz shows up clearly here.

It looks as though we get another unity gain crossing at 2.5 MHz with the high common gain setting.
At high common gain, we actually lose some phase somehow.

Spectra
----
Plot 2 shows the four stitched spectra taken with the slow SR785 and fast AG4395a at TTFSS IN1, so the FSS error spectra.
Plots 3 and 4 are the same, just zoomed in at the 160 kHz and 2.5 MHz peaks of interest.

1) The Fast gain does appear to lower the error signal as expected, and makes a sharper feature with higher noise at 20 kHz (green vs red).

2) The old 14 kHz peak we thought was from the reference cavity longitudinal mode is not present, unlike in an IMC REFL I spectra from earlier this year.  This could be because the loop completely suppresses this signal onto the laser, making it invisible in the error spectrum.

3) Focusing on the 163 kHz peak, we see that lowering the fast gain by itself reduces this peak, but then lowering the common gain brings it back even worse.  The nearby peaks mirror this.

4) Focusing on the 2.5 MHz peak, lowering the common gain does indeed make this peak completely disappear.  It is extremely sensitive at this region, just 2 dB of gain increase make the peak come back, and 4 dB makes it very strong.  The Fast gain appears to have no effect (as expected at this frequency regime).


alogs:
------
Vicki finds the high 2.5 MHz peak in the SQZ laser alog 64649
Daniel tracks it to the PSL alog 64653
Jenne lowers FSS gain to make it disappear alog 65200
Jenne touches the resonant EOM on the FSS pickoff path in the PSL alog 65220
Jenne touches the broadband EOM for frequency stabilization in the PSL alog 65236