Daniel, Dhruva, Aaron Jones, Vicky

Summary: Daniel found a weird 2.5 MHz (drifts +/-20Hz) tone in SQZ TTFSS, and maybe this excess tone is why we haven't been able to engage TTFSS's EOM path properly. This 2.5MHz seems to be from the PSL, not the sqz laser. At psl racks, we see the 2.5MHz on PSL FSS, and IMC REFL DEMOD I, but not the IMC VCO output.

TTFSS PZT "fast" path problem: The weird tone at 2.5MHz is likely limiting the TTFSS-PZT fast path UGF to ~5kHz (down from ~25kHz in o3, see LHO:46651). We can't increase the FAST PZT path UGF past ~5-7kHz by turning up the fast gain, which doesn't make sense. Looking on a scope, Daniel found that turning up the fast path gain past ~10dB basically just distorts this 2.5MHz tone (which is order ~1V) from a sine. We suspect this tone is saturating the fast path electronics, which is why increasing its gain doesn't increase its ugf.

As a result of the low fast path ugf, TTFSS EOM RMS is big ~0.5-0.7. Nutsinee O3 alog this looks more like ~0.006. Looking in-loop at the EOM monitor's spectra, there seems to be lots of low-freq (< 10kHz) noise, which nominally the fast path PZT would suppress with its 25kHz ugf. Daniel has better pics, but you can kinda see that the pzt path suppresses noise below ~5kHz, but there is lots of noise between 5-15kHz left in the eom loop, likely holding up the eom loop rms. At least, the EOM path can finally engage without railing, if common gain is low enough, ~12dB (down from 20dB) is the highest it can go with the eom working. Hopefully if the weird 2.5MHz can be resolved, the fast path can reach its ~25khz ugf and suppress this low-freq noise from the eom loop, allowing the eom path to engage consistently with lower in-loop noise.

This weird 2.5MHz seems to be from the PSL rather than SQZ laser, which we checked by chopping on/off power from the two lasers to ttfss. We see the 2.5 MHz both around the 160MHz optical PSL-LO beatnote, and around DC. Looking at the TTFSS PD around DC, the 2.5MHz disappears when we unplug the PSL fiber from sqzt0, but it is still there when we turn off the SQZ laser. This sent us back to the PSL racks to continue tracking it down.

At the PSL racks, we did not find the source of this 2.5 MHz, but we see it on IMC REFL's DEMOD I MON and the "FSS_TF_IN2" cable from the psl enclosure, and we do not see it on the IMC VCO output (from the unused splitter port). TBD where it is coming from.

As a totally separate note, yesterday when the IFO was fully locked, we saw strange 160kHz sidebands around the 160MHz beatnote, Daniel/Camilla have pics of this 160kHz on sqz ttfss. Yesterday we tracked down the 160kHz beatnote to the IMC VCO output, but we did not see it today; does this 160kHz possibly depend on ifo/imc lock status? We've seen this 160kHz on all of our sqz loops recently, e.g. LHO:64518 for the 160kHz lines in the LO/CLF loops. Very unclear whether the 160kHz and 2.5Mhz tones are related problems or not.