Naoki, Dhruva, Nutsinee
Yesterday we had only 3dB squeezing at IFO so we checked squeezing at homodyne. Although the visibility is good (98.5%), the squeezing was only 4.5dB at homodyne with 6dBm CLF6. We reduced the CLF power and recovered 8dB squeezing as shown in the attached figure.
The CLF6 was reduced from 6dBm to -42dBm and the 8dB squeezing was obtained with -38dBm CLF6. The CLF6 between -38 and -20 dBm gave similar squeezing so we set it at -24dBm, which is similar to O4a value and corresponds to 8uW CLF_REFL_LF_OUTPUT. The squeezing at IFO is recovered to 4.5dB with -24dBm CLF6.
Note that the LLO also saw the better squeezing at IFO with less CLF power in LLO70072.
We had 6.5dB squeezing at homodyne in 76040 and 4.5dB squeezing at IFO in 76226 with 6dBm CLF6 before. The question is why we lost squeezing at homodyne and IFO this week with same CLF power? The commissioning list in 76369 might give us a clue.
At high CLF power we have about 12 uW of light for CLF and RLF after the OPO. Each homodyne PD has 0.5 mW of power, or 1 mW total. This yields a CLF/RLF to LO ratio of 0.012. Using the equation Gamma^2/2 to get the modulation index, we obtain an estimate of 150 mrad for the maximum phase modulation. In reality, it will be somewhat smaller since some of the power will be in amplitude modulation. This will limit the maximum amount of achievable squeezing on the homodyne. But, this has no bearing on the DCPDs, since the CLF/RLF to LO ratio there is approximetaly 5000 times smaller.
A bit more details on this.
The power ratio between SB and CR of each sideband is approximated to be ~ (gamma/2)^2 where gamma is modulation depth in radian. Add the two sidebands together you get (gamma^2)/2.
A total power transmits through the OPO during high CLF case was 12uW. A total LO power hitting the HD was 1mW. So the phase noise contribution to HD sqz was
(gamma^2)/2 = 12uW/1mW
High CLF phase noise (gamma) at the homodyne = 154mrad (max)
A total power transmits through the OPO during low CLF case was 0.6 uW. A total LO power hitting the HD was 1mW. So the phase noise contribution to HD sqz was
Low CLF phase noise (gamma) at the homodyne = sqrt(2*0.6uW/1mW) = 35mrad (max)
These number include amplitude modulation. It's the worse case that could possibly happen. Using 45 mrad of phase noise and *16dB of asqz fits the high CLF squeezing of 6.5 dB in the homodyne. 15 mrad and 16 dB of squeezing fits the low CLF squeezing of 8 dB in the HD.
For the IFO case we've only injected sqz using low CLF so far. All the sideband power gets attenuated by the OMC (a factor of 5000). The LO (IFO carrier) is ~40mW. Phase noise contribution from CLF/RLF sidebands is 0.08 mrad, which is negligible. Even for high CLF power (RF6 = 6 dBm) this phase noise would be 0.3 mrad. That number is still negligible so there's no reason why we shouldn't be able to see good squeezing with high CLF. Using **15dB of aSQZ and a loss of ***30% we have 5 dB of squeezing as observed on Friday.
*ASQZ trace overlapped with 16 dB aSQZ reference in HD https://alog.ligo-wa.caltech.edu/aLOG/uploads/73562_20231018171710_8dB_hd_sqz_2023Oct18.png
**15 dB of aSQZ in DARM https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=76426
*** If I'm reading this noise budget correctly the inferred loss in the IFO is 30%.
