The idea is that fluctuations above the cavity pole are promptly reflected by the interferometer, whereas the DC field will be resonant and have a reflectivity smaller than unity. In our case, where we are close to critically matched but still over-coupled, the reflected DC field also picks up a minus sign. Any field that is not mode matched will simply be prompt reflected and not acquire a minus sign. The later is true for both the fluctuations and the DC field.
Including the total reflected power, one can then invert the equation and determine both mod matching and reflectivity.
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