Double Michelson/Fabry-Perot interferometer for laser- and displacement-noise-free gravitational-wave detection

In this paper we demonstrate that a double Michelson interferometer with Fabry-Perot cavities in its arms is able to perform laser- and displacement-noise-free gravitational-wave (GW) detection if certain model assumptions are met. Assuming the input mirrors of a single Michelson/Fabry-Perot interferometer can be rigidly attached to beamsplitter on a central platform one can manipulate with interferometer's response signals in a way to cancel laser noise and displacement noise of all test masses except the cental platform. A pair of symmetrically positioned Michelson/Fabry-Perot interferometers with common central platform can be made insusceptible to the later then, thus allowing complete laser- and displacement-noise-free interferometry (DFI). It is demonstrated that the DFI response to GWs of the proposed interferometer is proportional to $f^2_{\textrm{gw}}/\gamma$, where $\gamma$ is the cavity half-bandwidth, that is the strongest DFI response allowed by general relativity.