The signal recycling mirror reflects the signal fields induced by gravitational waves and leads them back to the interferometer. It increases or decreases the effective finesse for gravitational wave signals, depending on the interference condition of the optical cavity created by the signal recycling mirror. The increase of effective finesse results in higher sensitivities to gravitational waves at low frequencies, while leading to a narrower observation bandwidth [15]. The decrease of effective finesse results in the opposite effects and this variation of the scheme is called resonant sideband extraction [16].