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1. Point absorbers in Advanced LIGO

   https://doi.org/10.1364/AO.419689  

   Brooks, Aidan_F; Vajente, Gabriele; Yamamoto, Hiro; Abbott, Rich; Adams, Carl; Adhikari, Rana_X; Ananyeva, Alena; Appert, Stephen; Arai, Koji; Areeda, Joseph_S; et al (April 2021, Applied Optics)

   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. 

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2. 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)

   Abstract 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 heater 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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3. Prospects for neutron star parameter estimation using gravitational waves from f modes associated with magnetar flares

   https://doi.org/10.1093/mnras/stae1987  

   Ball, Matthew; Frey, Raymond; Merfeld, Kara (August 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Magnetar vibrational modes are theorized to be associated with energetic X-ray flares. Regular searches for gravitational waves from these modes have been performed by Advanced LIGO (Laser Interferometer Gravitational-wave Observatory) and Advanced Virgo, with no detections so far. Presently, search results are given in upper limits on the root sum square of the integrated gravitational-wave strain. However, the increased sensitivity of current detectors and the promise of future detectors invite the consideration of more astrophysically motivated methods. We present a framework for augmenting gravitational-wave searches to measure or place direct limits on magnetar astrophysical properties in various search scenarios using a set of phenomenological and analytical models. 

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4. 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. 

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5. Improving earthquake monitoring for gravitational-wave detectors with historical seismic data

   Soltero, Sky; Coughlin, Michael (July 2018, LIGO Laboratory Summer 2018 Undergraduate Research)

   The LIGO detectors are susceptible to high magnitude teleseismic events such as earthquakes, which can disrupt proper functioning, operation and significantly reduce their duty cycle. With advanced warning of impeding tremors, the impact can be suppressed in the isolation system and the down time can be reduced at the expense of increased instrumental noise. An earthquake early- warning system has been developed relying on near real-time earthquake alerts provided by the U.S. Geological Survey (USGS) and the National Oceanic and Atmospheric Administration (NOAA). The alerts can be used to estimate arrival times and ground velocities at the gravitational-wave detectors. We use machine learning algorithms to develop a prediction model and control strategy has to reduce LIGO downtime. 

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