aLIGO LHO Logbook

SQZ with SRCL offset at -306, FC detuing at -28

Sheila, Camilla, Jennie

This morning we changed SRCL offset from -191 to -306 and FC de-tuning from -34 to -28, as discussed in 83570. Took some SQZ data here as we were interested if we could get FIS SQZ lower than No SQZ ~100Hz and below, Sheila's models (e.g. 83572) suggest we should but it looks like there's a low frequeceny noise source (in FIS not FDS) in our data sets preventing us from getting down to the modeled level of SQZ.

Sheila turned OPO trans setpoint up from 80uW to 95uW to increase NLG from 11 to 19 (similar to what we had earlier in O4). Measured NLG with 76542. OPO gain left at -8. Turned off SQZ ASC.

opo_grTrans_ setpoint_uW	Amplified Max	Amplified Min	UnAmp	Dark	NLG (usual)	NLG (maxmin)	OPO Gain
95	0.0176	0.000279	0.00002	0.00094	19.1	20.0	-8
110	0.03315	0.000269	0.000879	-0.00002	35		-8

Starting FC2 misaligned offsets in M1 TEST were 100 and 200, Sheila increased to 200 and 400. So that we know FC2 is really misaligned. Saw no difference at SQZ or ASQZ. So this low fruecny noise in FIS is not due to FC backscatter.

Data attached and saved at camilla.compton/Documents/sqz/templates/dtt/20250331_SRCL_neg306.xml

Type	Time (UTC)	Angle	Notes	DTT Ref
No SQZ	03/29	N/A		ref 0
FIS SQZ		171	Angle tuned for FDS (maybe thermalized since)	ref1
FIS SQZ	17:05:00	154	Ang tuned for FIS	ref2
FIS Mid(ish)	17:15:00	101	Little better than no SQZ at 60Hz	ref3
FIS Mid(ish)		92		ref4
ASQZ FIS		68		ref5
ASQZ FIS -10deg	17:24:00	58		ref6
ASQZ FIS +10deg		78		ref7
FIS Mid(ish)	17:31:30	115		ref8
FIS Mid(ish) other side	17:43:00	27		ref9
FIS Mid(ish)	17:45:30	82	Check data doesn't include a glitch	ref10

Then changed the SRCL de-tuning back to -191 (still in FIS so FC de-tuning doesn't matter). Comparison is attached.

This SRCL offset change didn't effect the level of low frequency noise.

Type	Time (UTC)	Angle	Notes	DTT Ref
FIS ASQZ +10deg	17:53:00	82	Plot seems similar with same ang, different SRCL offset	ref 11
FIS ASQZ	17:56:00	72		ref12
FIS ASQZ -10deg		62		ref13
FIS Mid (ish)		104	Can see that rotation is a little different with SRCL de-tuning different but low freq noise level is the same.	ref14

Kept SRCL de-tuning at -191 but increased NLG to 35.

This was interesting for mid-SQZ values as the level of the low freq noise increased with the higher NLG but was the same at different SQZ angles (112 and 100deg). Plot attached. Compare grey and pink (NLG 35) to blue and brown (NLG19) <70Hz. So the low frequency FIS noise is NLG dependent.

Type	Time (UTC)	Angle	Notes	DTT Ref
Mid SQZ		112	Interesting data here. Low freq noise higher than with NLG 19.	ref 15
ASQZ	18:20:00	70		ref16
MidSQZ	18:22:30	100		ref17

Sheila turned OPO trans back to 96uW so expect NLG to be 19 going into Observing, larger than normal but closer to the value uses before the last OPO crystal move. SQZ angle servo off and angle set back to 171. ADF left on.