Fundamental noises for the current configuration was estimated (Attachment #1).
All the codes live in
/users/Commissioning/data/NoiseBudget/Spectra/2024/1121/Fundamentals

Thermal noise calculations:
 - Calculations are based on kagra_sensitivity.py but modified for O4 (kagra_O4_sensitivity.py).
 - Following parameters were used.
 TM temperature: 260 K (estimated from IM temperature)
 IM temperature: 260 K (measured)
 Sapphire blade spring frequency: 25 Hz (eyball fitted to align with the peaks)
 Sapphire blade spring loss: 3.6e-05 (see JGW-G1910180)
 Sapphire fiber loss: 1.0e-04 (see klog #26113)
 TM Q value: 1.0e+06 (see JGW-L2315445)

 - Sapphire parameters were extracted from arXiv:2005.0557. If temperature is below 100 K, default fitting equations are used.
    kappa20=(15700./15880)*((5*Tm**2.75)**(-4./5)+(10**10.25*Tm**(-3.8))**(-4./5))**(-5./4) #thermal conductivity
    Cth20=(0.69/0.80)*Tm**3.14/3.8/rhom  #specific heat per unit mass
    alpha20=(5.6/5.496)*Tm**2.99/10**12.15  #thermal linear expansion
 - For temperatures above 100 K, data in arXiv:2005.0557 was interpolated. See Attachemnt #2 for the data, fitting function, and interpolated curve.
 - Now kagra_O4_sensitivity.py can calculate thermal noise at any temperature.

Quantum noise calculations:
 - Calculations are based on kagra_sensitivity.py but modified for O4 (kagra_O4_sensitivity.py).
 - Following parameters were used.
 Power at BS: 19.60 W  (1.4 W input times PRG of 14; assuming all the power from IMC couples to the IFO)
 IFO to PD loss: 18 %  (Rough estimate; see below)
 - IFO to PD loss was estimated from the sum of the following losses
    OFI: 5% [JGW-G1809012, OFI wiki]
    OSTM: 0.89% [klog #30229]
    OMC: 5% [klog #30229]
    DC PD: 7% [From spec quantum efficiency of Excelitas C30665GH]
 - This is also consistent with klog #21397 with in the error bar.
 - Measured sensitivity is roughly 50% higher than the calculated shot noise at 1 kHz.

PD dark noise:
 - To see if shot noise calculations are correct, measured DC PD spectra are compared with dark noise measured in klog #31616 and shot noise calculated from K1:OMC-TRANS_DC_(A|B)_OUT_DQ, which is calibrated in mW.
 - See Attachment #3 for the spectra. Attachment #4 is the zoomed version.
 - A and B are unbalanced by 3%. A=7.4 mW B=7.6 mW.
 - Shot noise was calculated with P_shot = sqrt(2*h*nu*P_PD/eta) where eta=0.93 is the quantum efficiency.
 - Measured spectrum is 14% higher for A and 7% higher for B than shot+dark spectrum at 1 kHz.
 - Measured spectrum is 35% higher for A and 33% higher for B than shot noise at 1 kHz. This is not so consistent with 50% from quantum noise calculations.
 - If we believe in the DC PD power based calculations here, IFO to PD loss is estimated to be ~40%.
 - This sounds a bit too large, but there might be large misaglinment to OMC. If this is not the case, this could mean that there is error in K1:CAL-CS_PROC_DARM_DISPLACEMENT_DQ by ~20%, or there are some underlying additional noise that increases the noise at 1 kHz by ~20%, as indicated from AxB correlation measurements (klog #31577). Note that error in DC PD calibration into mW is not relevant for explaining the descrepancy.
 - See Attachment #5 for the dark noise contributions to DARM.

Next:
 - Check mode-matching of the beam into PRM.
 - Check OMC alignment.
 - Measure total optical loss from BS to PD using ITM single bounce.
 - Check DC PD A and B calbrations. The calibration factors in FM8 of K1:OMC-TRANS_DC_(A|B) seem to be the same, but A and B might be different, as indicated by 7.4 mW and 7.6 mW unbalance.
 - Check DARM calibration.