Angled beam expander telescopes for the Michelson beams in third generation gravitational wave observatories

The 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 degrees 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 degrees 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. In addition to reducing the beam sizes and the beam splitter to manageable sizes, the proposed solution allows to bring the optimal recombination angle to 90 degrees. The proposed geometry also 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.