Reduction of coating thermal noise by using an etalon

Reduction of coating thermal noise is a key issue in precise measurements like a gravitational-wave detector, where each mirror is coated by a few tens of dielectric layers to achieve high reflectivity while the noise level increases with the number of layers. One way to realize the reduction is mechanical separation of the first few layers from the rest. Using an etalon with a few layers on the front and more on the back surface is a way to realize such a system. We perform a noise analysis of an etalon using the Fluctuation–dissipation theorem with probes on both sides.