Elenna, Vicky
Operating with high generated sqz level ~ 16.5 dB, with IFO at 76W input, ~435 kW circulating, Elenna changed the SRCL offsets and we measured coherent SQZ rotations based on SRCL detuning.
Some takeaways:
- SRCL detuning + filter cavity detuning compound as coherent sqz rotations. Mis-rotations between FC + SRCL hit hardest in the bucket. With high generated squeezing levels, this matters more, because the cavities rotate in the elevated anti-squeezing noise. We will need to pick a generated SQZ level for O4, which optimizes quantum noise in the bucket.
- FC is too narrow (~70 Hz FWHM) to un-rotate both the SRCL and RPN rotations of squeezing and anti-squeezing. FC is designed to un-rotate RPN (radiation pressure noise), largely below 100 Hz.
- We have sometimes had to move the FC detuning to counter-act SRCL detuning, but this strategy has limited efficiency b/c FC is too narrow for SRCL, where the detuning is ~100-200 Hz around ~450 Hz.
- Mode-mismatch possibly adds into this, but very different PSAMS configurations seem to produce only slightly different sqz phase rotations.
- The DARM spring seems to evolve in the first 1-2 hours of IFO thermalization, we will try using the ADF to track this sqz angle rotation.
- At present, more "negative" SRCL filter bank offsets are better for SQZ, in that they do not induce higher frequency early-onset of anti-squeezing.
Here, at each SRCL offset, we rotate the SQZ angle to be ~0 deg (aka phase sqz) at high frequencies > kHz. And so, we can see how SRCL acts as basically a broadband filter cavity to coherent rotate the sqz angle around the coupled cavity pole. The "SRCL offset" in the plot legend refers to the arbitrary filter bank offset counts, H1:LSC-SRCL1_OFFSET. We did not what physical SRCL detuning (in e.g. degrees) this corresponds to during this squeeze mearement, but this has been measured before at this circulating power. I also tried to explore whether we can see the compounded effects of coherent sqz angle rotations from both SRCL detuning + mode-mismatch together, but this mode-mismatch rotation seems much smaller an effect than SRCL detuning rotations.
Kevin Kuns, Vicky -- The way we see SRCL acting as a broadband filter cavity for squeezing is consistent w/calculations of how SRCL detuning changes sqz angle.
This is not necessarily a problem, as 0 detuning seems good for calibration and us, but it's kinda an interesting effect to see. The SRCL filter cavity action adds coherently with the FC detuning. I'd guess if we're in a bad spot with a big detuning, we could lose some range (guessest'd < 5 Mpc?) due to this misrotation of quantum noise in the bucket. Mostly, it does seem like a large SRCL detuning (like > 0.5 deg) might be non-ideal for squeezing, because FC is too narrowband to undo both significant SRCL detuning and RPN.
See here: as IFO thermalizes for 2 hours to 430 kW, the amount of high-frequency squeeze angle change (~17 deg) during the first ~2 hours (left) is consistent with Kevin's calculations of sqz angle rotations at various SRCL detunings (right).
- The observed range of squeeze angle rotation with thermalization is physically compatible with the range of SRCL detunings we've explored with the various H1:LSC-SRCL1_OFFSET filter bank offsets.
- From our FIS measurements yesterday, we rotated the angle to set high-frequency FIS --> 0 at each SRCL offset. Compared to Kevin's plots on the right, this is equivalent to rotating the kHz squeezing back to "0", and observing how SRCL rotates low-frequency squeezing into anti-squeezing below the cc-pole ~450 Hz.
- For sqz, a large SRCL detuning leads to the (non-ideal) blue trace in both kevin's calculations and the freq-indep sqz spectra from yesterday.
Over thermalization, our squeezer band-limited RMS monitors (SQZ BLRMS = H1:SQZ-DCPD_RATIO_*_DB_MON) monitors show drifts of:
BLRMS_3 @ 325 Hz (yellow, below darm pole) ~ 1dB
BLRMS_4 @ 750 Hz (green) ~ 2dB
BLRMS_5 @ 1700 Hz (blue) ~ 3.2dB, ~34/2 = 17 degrees sqz rotation to recover high-frequency squeezing before/after thermalizing. SQZ rotation can be seen on ndscope, last column, 3rd from top (*_PHASEDEG ~ 2x sqz angle).
BLRMS_6 @ 4700 Hz (purple) ~ 2.8dB
High-freq sqz above the DARM pole (~450Hz) sees more rotation as expected, while sqz angle in the bucket is pretty stable w.r.t srcl detuning. We'll generally leave the squeeze angle optimized for the bucket, and let high-freq squeeze thermalize down. So, the sqz angle might look wrong at the start of locks (by ~15 deg, or ~30 deg on our slider).
Interesting alogs related to recent SRCL detunings:
- late-Feb 2023. LHO:67613 has great plots at various SRCL offsets, with a very helpful table from Jeff comparing the "counts" of offset in H1:LSC-SRCL1_OFFSET to physical SRCL detunings in nm and degrees. His log 67613 describes fully, but some relevant points takeaways::
- SRCL offset counts going from 50 --> -250 changed SRCL detuning by ~0.9 deg, from +0.15 --> -0.76 degrees. This range is calculated in Kevin's plots.
- From Jeff's alog, quoting here: "calibration of the SRCL1_OFFSET as 3.04e-3 deg/ct or 9.00e-3 nm/ct."
- From Kevin's calculations -- a SRCL detuning of about [-0.75, 0.15] degrees rotates the high-frequency SQZ angle from about [+15, -3] degrees.
- Note: we have been setting the SQZ angle to optimize quantum noise reduction in the bucket ~100 Hz, which BNS range is most sensitive to. Conveniently, SQZ angle is pretty unaffected by SRCL detuning in the bucket, and only the high-freq (kHz+) squeezing angle is strongly swung around by the SRCL detuning (order ~10-20 deg rotation throughout thermalization), so this sqz thermalization shouldn't create too much range drift.
- 68554 4/10, New SRCL offset at 78W, changed filter bank offset from -175 (set at 60 W) to -165. This is where we've been sitting now. Good list of other relevant alogs.
Plot/file attached here in meters/rtHz, sligthly more intuitive to compare with calculations for radiation pressure noise.
