Besides the seismic noise, the sens

Besides the seismic noise, the sensitivity of the interferometer is fundamentally limited by the thermal noise and the quantum noise (see figure 1.8). The goal of the vibration isolation system is suppressing the seismic noise level well below these noise levels in the detection band of gravitational waves. More specifically, the requirement for the seismic noise level is to be lower by a factor of 10 in amplitude spectrum densities than other noise levels in the frequency region above 10 Hz. Required displacement noise level of the core optics of the KAGRA interferometer in two different detection modes is shown in figure 4.1 [69]. KAGRA has two optional detection modes called BRSE (Broadband Resonant Sideband Extraction) and DRSE (Detuned Resonant Sideband Extraction) [32]. These two modes can be switched by changing the resonant condition of the signal recycling cavity. The DRSE mode has better sensitivity around 100 Hz, for detecting gravitational waves from binary neutron stars, but less in the high frequency region.