Since we found the IMC transmitted power changed when the beam position on MCe changed (klog29133), I remeasured the IMC cavity pole frequency after the beam position adjustment on MCe to maximize the transmitted power in the same manner in klog29071. Fig.1 shows the measurement and fitting results. (This time, I used the gain data from 2 kHz to 20 kHz for the fitting in the same manner in klog29071.) As you can see, the pole frequency lowered to (5.82 +/- 0.06 ) kHz. Also, we obtained the finesse of IMC = 483.4 +/- 4.9 from this cavity pole frequency. This finesse value seems to be consistent with the previous measurement's result before O4a.
I attempted to re-estimate the round trip loss of the IMC using the same method in klog:29090.
However, this approach did not yield accurate results due to the closeness of the cavity to critically coupled conditions and imperfect mode matching or alignment, as observed this time.
Therefore, I relied on the measured MCI/MCO transmission of (0.64 ± 0.01)% from klog:29090 and conservatively calculate the upper limit of the round trip loss (L_loss) and the lower limit of the throughput (T_th) as follows:
L_loss < 580 ppm
T_th > 91%
Current mode matching seems to be around 90%, so the actual throughput might be around 80%.
I tried to fit the data as a complex function. Fig. 1 shows the results. From the fitting with the complex function, the cavity pole is (5.74 +/- 0.04) kHz. the finess is 490 +/- 4. There seems to be no significant difference between the previous result and the result at this time.
