Beam expander telescopes for the Michelson beam splitters in third generation Gravitational Wave Observatories

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Abstract:: The third generation of Gravitational Wave detectors, the Einstein Telescope or the Cosmic Explorer, will still be Michelson interferometers with Fabry-Perot cavities in the two arms. They will need the widest possible stored beams to reduce thermal noise, using mirror test masses with diameter at the limit of technical feasibility. A serious problem is how to feed the beams of the two arms into the beam splitter for recombination without clipping. The problem is worse in Einstein Telescope, where the 60o angle between the arms would require beam splitters double in size and 8 times in weight than the 90o case. It is proposed here to move the beam expander telescope inside the Michelson, between the Fabry-Perot cavities and the beam splitter. In addition to allowing use of smaller beam splitters, the proposed solution allows change of the recombination angle from 60o to 90o, offers additional and easy degrees of freedom for beam alignment, a method for optimizing the match of the modes of the two arms on the beam splitter and a natural way to separate multiple detectors housed in the same tunnels in separate beam splitter halls.

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LIGO Document P2000130-v3