Abstract:
We summarize our discussion on the loss estimation.
We conclude that the calculated increase of loss is plausible and point out the possibility that this increase originates from molecular layer formation on the mirror surface.
Detail:
Figure attached to this report shows the round-trip loss before the signal reduction in the previous report, with an additional black dashed line.
Since we consider a model that assumes the round-trip loss increases in both the X and Y arm, the portion of the red line located in the upper right region, bounded by the black dashed line, represents the possible range of losses after the signal reduction.
We examine whether this range is reasonable in terms of the finesse.
Here, we add the assumption that the reflectivity of ITM does not change.
When considering the situation where only the X arm round-trip loss increases, the intersection of the horizontal black dashed line and the red line corresponds to the maximum loss.
At this point, T_ETMX = 83.3 ppm.
If T_ITMX = 0.4520 % remains the same before and after the signal reduction, the finesse becomes F_x = 1362.0 because T_ETMX increases from 75.0 ppm to 83.3 ppm.
The finesse value is feasible considering the error.
Likewise, for the Y arm, if T_ITMY = 0.4851 % remains the same, the finesse becomes F_y = 1269.5 due to the increase of T_ETMY from 77.4 ppm to 86.3 ppm.
The finesse value is also plausible within the measurement error.
We also examine whether the change in round-trip loss obtained from the calculation can be explained from the perspective of losses caused by molecular layer formation on the mirror surface, as reported in the PhD thesis.
The measured speed of the molecular layer formation was 25.5 nm/day.
According to klog #34664, the signal reduction was observed for about 20 days.
Therefore, the thickness of the molecular layer would increase by approximately 0.5 um.
The measured imaginary part of the refractive index of the molecular layer was (2.18 ± 1.81) * 1e-7, approximately equal to that of N_2 (2e-7) as shown in Fig 5.3.
Assuming this value, the decrease of the ETM reflectivity with the refractive index of N_2 in Fig 5.3 of this thesis shows the increase of loss by a few ppm.
Thus, our loss estimation seems consistent with the results of this thesis.
The measurement results of the signal reduction during the observation runs can be consistently explained if the PRC loss increases from 3 % to 3.26 %, the round-trip loss simultaneously increases from 76.2 ppm (X arm: 75.0 ppm, Y arm: 77.4 ppm) to 80.5 ppm (X arm: 80.0 ppm, Y arm: 80.9 ppm).
