  As a future plan, an advanced gravitational-wave detector will employ an
optical configuration of resonant sideband extraction (RSE), achieved with an
additional mirror at the signal-detection port of the power-recycled
Fabry-Perot Michelson interferometer. To control the complex coupled cavity
system, one of the most important design issues is how to extract the
longitudinal control signals of the cavities. We developed a new
signal-extraction scheme which provides an appropriate sensing matrix. The new
method uses two sets of sidebands: one of the sideband components satisfies the
critical coupling ondition for the RSE interferometer and reaches the
signal-extraction port, and the other sideband is completely reflected by the
Michelson interferometer. They provide a diagonalized sensing matrix and enable
the RSE control to be robust.
