I briefly ran an Optickle simulation to see if we can explain the observed low recycling gain by the 750 ppm Y arm intracavity loss.

According to the result, I am concluding that the loss is the cause of the current low recycling situation.

(Some details)

I set loss of ETMY to 750 ppm such that it does not ruin the SB build up in the DRMI. For the X arm, I assumed a round trip loss of 60 ppm for now. Then I ran the same code as the one I did the day before yesterday (alog 15389).

According to the resultant plots (see the attached), both TRX and TRY went up to only 166 which is close to what we observed yesterday i.e. 140-ish. This corresponds to a recycling gain of roughly 5. Even with this amount of loss, REFLDC dropped to 35% which does not contradict what we observed i.e. 50-ish%. Due to the low TRX and TRY, the CARM signal synthesized by sqrt(TRY +TRY) can be a good linear signal only up to an offset of 15 or so at which point sqrt(TRX+TRY) starts loosing the sensitivity because we are running into the plateau region of TRX and TRY. Indeed we needed to hand the CARM from the sqrt signal to REFL9 at an offset of -15 yesterday.

Interestingly, ASDC does not increase so much between off-resonance and on-resonance points even though we, at the beginning, speculated that differential losses would show up at the dark port as a large contrast defect. So it means that the light at the dark port is still dominated by the 45 MHz SBs which qualitatively agrees with the observed ASDC from yesterday. I am guessing that the reason why the carrier defect is so small is that the power recycling gain is already so low in the first place that the whatever carrier components at the dark also result in a low power.

Sorry, Evan. Fig files again.