Description
Third generation of Gravitational Wave detectors like the Einstein Telescope or the
Cosmic Explorer will be Michelson interferometers with Fabry-Perot cavities in the arms,
using mirror test masses with diameter at the limit of technical feasibility. Unlike other
detectors, the Einstein Telescope will have a 60° angle between the arms. Because of its
larger incidence angle, at any given beam size, it would require beam splitters almost double
in size and much heavier than the 90° case. It is proposed here to install beam expander
telescopes with angled mirrors located inside the Michelson interferometer between the
Fabry-Perot cavities and the beam splitter. Beyond reducing the beam sizes and the
beam splitter to manageable sizes, the proposed solution allows to bring the optimal
recombination angle to 90°. The proposed geometry offers a natural way to separate the
beam splitters of different detectors into individual, smaller and more stable caverns, thus
improving observatory observation-time efficiency, to provide needed beam diagnostic
points and convenient degrees of freedom for beam alignment into both the Fabry-Perot
cavities and the beam splitter, as well as to provide a method for maintaining optimal mode
matching of the two arms onto the beam splitter without thermal compensation plates.
