LIGO Document P1400191-v1

Investigating the Effect of Lenses in a Long Interferometer Arm

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Current gravitational wave interferometers are approaching fundamental limits to their sensitivity. In order to build more sensitive gravitational wave detectors, significant changes must be made to the current advanced LIGO design. One simple way to increase the sensitivity of a next generation detector is to increase the length of its arms. However as the length of the arm increases, the beam width also expands. A larger beam would require larger optics and a larger beam tube, both of which are costly and can be difficult to manufacture. In this paper, I examine the possibility of placing lenses within the beam tube of a 40km long interferometer in order to maintain a narrow beam throughout the entire detector arm. I first examine different configurations of lenses that could achieve this and then investigate how motion of the lenses can add noise to the detector to propose limits on the required seismic isolation of the lenses. I also show these lenses can be created from flat pieces of fused silica through thermal lensing and thermal expansion effects.
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