Dave O. Elli, Daniel, Kiwamu,
We briefly checked how lossy our arm cavities are by locking the individual arm without recycling.
For the X arm:
ASAIR_A_LF = 1180 cnts when unlocked.
ASAIR_A_LF = 1155 cnts when locked.
For the Y arm:
ASAIR_A_LF = 1180 cnts when unlocked.
ASAIR_A_LF = 970 cnts when locked.
We made a corase estimation of intra cavity loss (or a.k.a round trip loss) for the y arm, which is estimated to be about 750 ppm (!). In the calculation, we did not take a mode-mismatch or RF sidebands into accout. We need a closer look at this arm cavity to see why it is so lossy.
If beam mis-centering on the TMs is the cause of all the extra loss, it will have to be quite a big mis-centering. I did a quick calculation, and to get 750ppm loss at the ETM, the beam has to be offset from the center by about 6cm (roughly one beam size). The attached plots show the intensity spill over for a 62mm radius beam on a 163mm radius coating, with no offset and with a 6cm offset. The proportions of beam power outside the coating, with no offset and 6cm offset, are 0.991ppm and 762ppm respectively.
Just in case someone wants a plot ...
Dave has a calculation which makes some assumptions about mode-mismatch and sideband power for the X arm.
Suppose the power in the sidebands is 6% of the incident beam, and 15% of the incident carrier doesn't enter the cavity because of mode matching issues; i.e., about 20% of the light is nonresonant. Then the equivalent power reflection is (1155 - 236) / (1180 - 236) = 0.974. This gives a loss of 108 ppm in the X arm.