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1. Substrate-transferred GaAs/AlGaAs crystalline coatings for gravitational-wave detectors

   https://doi.org/10.1063/5.0140663  

   Cole, G. D. ; Ballmer, S. W. ; Billingsley, G. ; Cataño-Lopez, S. B. ; Fejer, M. ; Fritschel, P. ; Gretarsson, A. M. ; Harry, G. M. ; Kedar, D. ; Legero, T. ; et al ( March 2023 , Applied Physics Letters)

   In this Perspective, we summarize the status of technological development for large-area and low-noise substrate-transferred GaAs/AlGaAs (AlGaAs) crystalline coatings for interferometric gravitational-wave (GW) detectors. These topics were originally presented as part of an AlGaAs Workshop held at American University, Washington, DC, from 15 August to 17 August 2022, bringing together members of the GW community from the laser interferometer gravitational-wave observatory (LIGO), Virgo, and KAGRA collaborations, along with scientists from the precision optical metrology community, and industry partners with extensive expertise in the manufacturing of said coatings. AlGaAs-based crystalline coatings present the possibility of GW observatories having significantly greater range than current systems employing ion-beam sputtered mirrors. Given the low thermal noise of AlGaAs at room temperature, GW detectors could realize these significant sensitivity gains while potentially avoiding cryogenic operation. However, the development of large-area AlGaAs coatings presents unique challenges. Herein, we describe recent research and development efforts relevant to crystalline coatings, covering characterization efforts on novel noise processes as well as optical metrology on large-area (∼10 cm diameter) mirrors. We further explore options to expand the maximum coating diameter to 20 cm and beyond, forging a path to produce low-noise mirrors amenable to future GW detector upgrades, while noting the uniquemore »requirements and prospective experimental testbeds for these semiconductor-based coatings.« less
2. Calibrating gravitational-wave detectors with GW170817

   https://doi.org/10.1088/1361-6382/ab2142  

   Essick, Reed ; Holz, Daniel E. ( May 2019 , Classical and Quantum Gravity)

   The waveform of a compact binary coalescence is predicted by general relativity. It is therefore possible to directly constrain the response of a gravitational-wave (GW) detector by analyzing a signal’s observed amplitude and phase evolution as a function of frequency. GW signals alone constrain the relative amplitude and phase between different frequencies within the same detector and between different detectors. Furthermore, if the source’s distance and inclination can be determined independently, for example from an electromagnetic (EM) counterpart, one can calibrate the absolute amplitude response of the detector network. We analyze GW170817’s ability to calibrate the LIGO/Virgo detectors, finding a relative amplitude calibration precision of approximately20% and relative phase precision of(uncertainty) between the LIGO Hanford and Livingston detectors. Incorporating additional information about the distance and inclination of the source from EM observations, the relative amplitude of the LIGO detectors can be tightened to  ∼%. Including EM observations also constrains the absolute amplitude precision to similar levels. We investigate the ability of future events to improve astronomical calibration. By simulating the cumulative uncertainties from an ensemble of detections, we find that with several hundred events with EM counterparts, or several thousandmore »events without counterparts, we reach percent-level astronomical calibration. This corresponds to  ∼5–10 years of operation at advanced LIGO and Virgo design sensitivity. It is to be emphasized that directin situmeasurements of detector calibration provide significantly higher precision than astronomical sources, and already constrain the calibration to a few percent in amplitude and a few degrees in phase. In this sense, our astronomical calibrators only corroborate existing calibration measurements. Nonetheless, it is remarkable that we are able to use an astronomical GW source to characterize properties of a terrestrial GW instrument, and astrophysical calibration may become an important corroboration of existing calibration methods, providing a completely independent constraint of potential systematics.

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3. Reducing control noise in gravitational wave detectors with interferometric local damping of suspended optics

   https://doi.org/10.1063/5.0144865  

   van Dongen, J. ; Prokhorov, L. ; Cooper, S. J. ; Barton, M. A. ; Bonilla, E. ; Dooley, K. L. ; Driggers, J. C. ; Effler, A. ; Holland, N. A. ; Huddart, A. ; et al ( May 2023 , Review of Scientific Instruments)

   Control noise is a limiting factor in the low-frequency performance of the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO). In this paper, we model the effects of using new sensors called Homodyne Quadrature Interferometers (HoQIs) to control the suspension resonances. We show that if we were to use HoQIs, instead of the standard shadow sensors, we could suppress resonance peaks up to tenfold more while simultaneously reducing the noise injected by the damping system. Through a cascade of effects, this will reduce the resonant cross-coupling of the suspensions, allow for improved stability for feed-forward control, and result in improved sensitivity of the detectors in the 10–20 Hz band. This analysis shows that improved local sensors, such as HoQIs, should be used in current and future detectors to improve low-frequency performance.
4. Observing Scenarios for the Next Decade of Early Warning Detection of Binary Neutron Stars

   https://doi.org/10.3847/1538-4357/ac7f33  

   Magee, Ryan ; Borhanian, Ssohrab ( August 2022 , The Astrophysical Journal)

   We describe representative observing scenarios for early warning detection of binary neutron star mergers with the current generation of ground-based gravitational wave detectors as they approach design sensitivity. We incorporate recent estimates of the infrastructure latency and detector sensitivities to provide up-to-date predictions. We use Fisher analysis to approximate the associated localizations, and we directly compare to Bayestar to quantify biases inherited from this approach. In particular, we show that Advanced LIGO and Advanced Virgo will detect and distribute ≲1 signal with signal-to-noise ratio greater than 15 before a merger in their fourth observing run provided they maintain a 70% duty cycle. This is consistent with previous early warning detection estimates. We estimate that 60% of all observations and 8% of those detectable 20 s before a merger will be localized to ≲100 deg². If KAGRA is able to achieve a 25 Mpc horizon, 70% of these binary neutron stars will be localized to ≲100 deg²by a merger. As the Aundha–Hanford–KAGRA–Livingston–Virgo network approaches design sensitivity over the next ∼10 yr, we expect one (six) early warning alerts to be distributed 60 (0) s before a merger. Although adding detectors to the Hanford–Livingston–Virgo network at design sensitivity impacts the detectionmore »rate at ≲50% level, it significantly improves localization prospects. Given uncertainties in sensitivities, participating detectors, and duty cycles, we consider 103 future detector configurations so electromagnetic observers can tailor preparations toward their preferred models.

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5. Demonstration of dynamic thermal compensation for parametric instability suppression in Advanced LIGO

   https://doi.org/10.1088/1361-6382/ab8be9  

   Hardwick, T. ; Hamedan, V. J. ; Blair, C. ; Green, A. C. ; Vander-Hyde, D. ( September 2020 , Classical and Quantum Gravity)

   Advanced LIGO and other ground-based interferometric gravitational-wave detectors use high laser power to minimize shot noise and suspended optics to reduce seismic noise coupling. This can result in an opto-mechanical coupling which can become unstable and saturate the interferometer control systems. The severity of these parametric instabilities scales with circulating laser power and first hindered LIGO operations in 2014. Static thermal tuning and active electrostatic damping have previously been used to control parametric instabilities at lower powers but are insufficient as power is increased. Here we report the first demonstration of dynamic thermal compensation to avoid parametric instability in an Advanced LIGO detector. Annular ring heaters that compensate central heating are used to tune the optical mode away from multiple problematic mirror resonance frequencies. We develop a single-cavity approximation model to simulate the optical beat note frequency during the central heating and ring heating transient. An experiment of dynamic ring heater tuning at the LIGO Livingston detector was carried out at 170 kW circulating power and, in agreement with our model, the third order optical beat note is controlled to avoid instability of the 15 and 15.5 kHz mechanical modes. We project that dynamic thermal compensation with ring heatermore »input conditioning can be used in parallel with acoustic mode dampers to control the optical mode transient and avoid parametric instability of these modes up to Advanced LIGO’s design circulating power of 750  kW. The experiment also demonstrates the use of three mode interaction monitoring as a sensor of the cavity geometry, used to maintain theg-factor product tog₁g₂= 0.829 ± 0.004.

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