Summary

We found that the power budget with SRY on resonance is not so weird after all. We misunderstood what is the 'reflection light' from SRY.

Detail

As reported shortly in this post, we observed an unfamiliar power change when SRY flashed. Fig.1 shows the DC power signal in several ports: REFL, AS, POP, POP s-pol, POP p-pol, and POS (newly installed on Monday). Surprisingly, the power at both REFL and AS ports increased. Another concern was that the buildup evaluated at POS was smaller than expected from the mirrors' reflectivity: it should be ~7, but it was just 1.6.

Hiroki pointed out that the REFL port in SRY (also SRX) is actually not the cavity reflection in the usual term. Fig. 2 shows the cavity mirror layout, and thinking of it carefully, the 'normal' cavity reflection for SRY is the beam going to ITMX (for SRX it's ITMX), and the REFL port is one of the cavity transmissions. In that sense, REFL and AS are equivalent for SRX/Y, and we should see the built-up power at both ports.

The buildup ratio at REFL and AS is 5.2 and 12, respectively. It seems consistent (but a bit low) at REFL, but at AS it is overestimated for some reason. On the other hand, the other ports show smaller buildup: 1.6 at POS (as mentioned above), 1.7 at POP, 1.5 at POP s-pol, and 2.3 at POP p-pol. We don't have any clear answer for them now.