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AdV-COM (AdV commissioning (1st part) )
mwas - 8:13 Monday 12 November 2018 (43524) Print this report
Comment to flat noise hunting - OMC1 alignment (43516)
Note that an 8% reduction in OMC losses should actually correspond to a 4% increase in optical gain (see computation below).
So the result are consistent with the OMC alignment being actually improved in dark fringe as much as on single bounce, and it is just the height of the DARM line in the OMC1 reflected light that we don't understand.

The power on the B1 photodiodes can be written as
P_B1 = A L^2
where L is the dark fringe offset and A is a normalization that includes power in the arms, detection losses, etc.
The optical gain is the derivative of this
OG = 2 A L
Given that we keep the B1 power constant the dark fringe offset is
L = sqrt(P_B1/A)
which makes the optical gain proportional to the square root of the detection efficiency
OG = 2 sqrt(P_B1) sqrt(A)
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