Introduction

Why good broadband seismic isolation is an essential design feature for laser interferometric antennas

One of the key features of laser interferometric detectors is the potential wideband nature of their operation. Proposed long baseline detectors are intended to achieve sensitivities in the region h ∼ 10–21 to 10–22 or better over a range of frequencies f from a few tens of hertz (possibly as low as 10 Hz) to a few kilohertz in a bandwidth Δf ≈ f / 2. If the performance of such detectors is limited by photon shot noise in the output light, for constant light power the effect of this noise source decreases towards lower frequencies for a constant light intensity, when the detectors are operated in searches for burst sources or a stochastic background. However, other sources of noise have spectra which rise towards lower frequencies. These include thermal noise from the pendulum suspensions of the masses, and, more particularly, seismic noise. In fact it is likely that the extent to which these detectors can be operated with reasonable sensitivity at the lower end of the frequency spectrum will depend crucially on the level of seismic and mechanical isolation achievable. Since there are interesting sources of gravitational waves in the region of ten to a few hundred hertz, such as fast pulsars and coalescing compact binary systems, it is advantageous to incorporate as much seismic isolation as practicably possible into the design of these detectors.