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Following a decade of unprecedented success through LIGO and Virgo’s observations of compact binary coalescences, gravitational wave astronomy is now recognized as a key tool in our continued efforts to understand the Universe and our place within it. Far from resting on their laurels though, the gravitational wave community is forging ahead with major plans for the future. The proposed “ultimate terrestrial gravitational wave detector facility” Cosmic Explorer recently received a boost with significant funding from the NSF to proceed with a conceptual design. This paper surveys the current state-of-the-art ground-based gravitational wave detector facilities, and their planned near-term upgrades. After motivating the next-generation Cosmic Explorer concept with a discussion of the key science targets, this paper describes some of the unique technical challenges it faces, including a focus on the ongoing optical design of Cosmic Explorer’s 40 km-scale laser interferometers. 

Odd-indexed higher-order Hermite–Gauss (HG) modes are compatible with four-quadrant segmented mirrors due to their intensity nulls along the principal axes, which guarantees minimum beam intensity illuminating the bond lines between the segments thus leading to low power loss. However, a misplaced HG beam can cause extra power loss due to the bright intensity spots probing the bond lines. This paper analytically and numerically studies the beam displacement tolerances on a segmented mirror for the$\mathrm{H}{\mathrm{G}}_{3,3}$mode. We conclude that for ‘effective’ bond lines with 6 µm width, and the$\mathrm{H}{\mathrm{G}}_{3,3}$beam size chosen to guarantee 1 ppm clipping loss when centered, the beam can be rotated by roughly 1^∘or laterally displaced by 4% of its beam size while keeping the total power on the bond lines under 1 ppm. We also demonstrate that the constrained beam displacement parameter region that guarantees a given power loss limit, or the beam displacement tolerance, is inversely proportional to the bond line thickness.

 

This paper analytically and numerically investigates misalignment and mode-mismatch-induced power coupling coefficients and losses as a function of Hermite–Gauss (HG) mode order. We show that higher-order HG modes are more susceptible to beam perturbations when, for example, coupling into optical cavities: the misalignment and mode-mismatch-induced power coupling losses scale linearly and quadratically with respect to the mode indices, respectively. As a result, the mode-mismatch tolerance for the${\mathrm{H}\mathrm{G}}_{3,3}$mode is reduced to a factor of 0.28 relative to the currently used${\mathrm{H}\mathrm{G}}_{0,0}$mode. This is a potential hurdle to using higher-order modes to reduce thermal noise in future gravitational-wave detectors.

 

This paper describes a novel, to the best of our knowledge, approach to build ultrastable interferometers using commercial mirror mounts anchored in an ultralow expansion (ULE) base. These components will play a critical role in any light particle search (ALPS) and will also be included in ground testing equipment for the upcoming laser interferometer space antenna (LISA) mission. Contrary to the standard ultrastable designs where mirrors are bonded to the spacers, ruling out any later modifications and alignments, our design remains flexible and allows the alignment of optical components at all stages to be optimized and changed. Here we present the dimensional stability and angular stability of two commercial mirror mounts characterized in a cavity setup. The long-term length change in the cavity did not exceed 30 nm and the relative angular stability was within 2 µrad, which meet the requirements for ALPS. We were also able to demonstrate$1\mathrm{p}\mathrm{m}/\sqrt{\mathrm{H}\mathrm{z}}$  length noise stability, which is a critical requirement for various subsystems in LISA. These results have led us to design similar opto-mechanical structures, which will be used in ground verification to test the LISA telescope.

 

Small, highly absorbing points are randomly present on the surfaces of the main interferometer optics in Advanced LIGO. The resulting nanometer scale thermo-elastic deformations and substrate lenses from these micron-scale absorbers significantly reduce the sensitivity of the interferometer directly though a reduction in the power-recycling gain and indirect interactions with the feedback control system. We review the expected surface deformation from point absorbers and provide a pedagogical description of the impact on power buildup in second generation gravitational wave detectors (dual-recycled Fabry–Perot Michelson interferometers). This analysis predicts that the power-dependent reduction in interferometer performance will significantly degrade maximum stored power by up to 50% and, hence, limit GW sensitivity, but it suggests system wide corrections that can be implemented in current and future GW detectors. This is particularly pressing given that future GW detectors call for an order of magnitude more stored power than currently used in Advanced LIGO in Observing Run 3. We briefly review strategies to mitigate the effects of point absorbers in current and future GW wave detectors to maximize the success of these enterprises.