Sensitivity and Performance of the Advanced LIGO Detectors in the Third Observing Run (2008.01301v2)

Abstract: On April 1st, 2019, the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO), joined by the Advanced Virgo detector, began the third observing run, a year-long dedicated search for gravitational radiation. The LIGO detectors have achieved a higher duty cycle and greater sensitivity to gravitational waves than ever before, with LIGO Hanford achieving angle-averaged sensitivity to binary neutron star coalescences to a distance of 111 Mpc, and LIGO Livingston to 134 Mpc with duty factors of 74.6% and 77.0% respectively. The improvement in sensitivity and stability is a result of several upgrades to the detectors, including doubled intracavity power, the addition of an in-vacuum optical parametric oscillator for squeezed-light injection, replacement of core optics and end reaction masses, and installation of acoustic mode dampers. This paper explores the purposes behind these upgrades, and explains to the best of our knowledge the noise currently limiting the sensitivity of each detector.

Sensitivity and Performance of the Advanced LIGO Detectors in the Third Observing Run

This paper provides a detailed examination of the progress and evaluation of the Advanced LIGO (aLIGO) detectors during their third observing run (O3), which lasted from April 2019 to March 2020. The authors report on advancements in sensitivity, stability, and the ongoing search for gravitational waves, contributing to the astronomical data collection endeavor. This analysis spans the technical details of instrumental upgrades, their implications on performance, and underscores persistent challenges in gravitational wave detection.

Context and Overview

Since the initial detection of gravitational waves in 2015 from a binary black-hole merger, the Advanced LIGO detectors have undergone substantial upgrades to improve sensitivity and operation time. O3 marked the most sensitive search for gravitational waves thus far, with collaboration extending to the Advanced Virgo detector. The paper explores several technical advancements, including increases in laser power, the integration of squeezed-light injection, and key component replacements. These improvements have resulted in enhanced detection capabilities, exemplified by higher duty cycles and the detection of a significantly increased number of gravitational-wave candidates.

Technical Enhancements

Several noteworthy upgrades are highlighted:

1. Increased Laser Power: To reduce shot noise and improve sensitivity at high frequencies, laser power circulating in the arm cavities was increased to approximately 190 kW at LHO and 240 kW at LLO. This was facilitated by replacing high-power oscillators, which diminished beam jitter noise, and allowed testing under higher power conditions.
2. Squeezed-Light Injection: The implementation of an in-vacuum optical parametric oscillator promoting the injection of squeezed vacuum resulted in reduced shot noise above 50 Hz, enhancing sensitivity by 2.0 dB at LHO and 2.7 dB at LLO. This upgrade is pivotal in elevating the binary neutron star inspiral range significantly.
3. Core Optic Replacement: Improving the harmonics and reducing scatter loss in core optics positively impacted lock acquisition robustness and optical build-up of the cavity powers.
4. Detection Strategy: A variety of noise sources including seismic, thermal, and electronic noise were analyzed, mitigated, and corrected with innovations such as new signal-recycling mirrors and adjustments to test mass electrostatic drives.

Challenges and Solutions

Despite the success of O3, the paper reflects on remaining challenges, especially in frequency regions below 100 Hz where noise dominates due to complexities such as non-linear coupling and environmental factors. Techniques targeting stochastic noise reduction, refined digital control systems, and changes in control loop bandwidths and stability were employed. Continued efforts to identify noise sources, streamline calibration, and expand the capacity for handling environmental disturbances are forecasted as crucial for future detector operations.

Implications and Future Work

The practical enhancements furnished during O3 extend theoretical implications, such as improved detection volume and refinement of gravitational wave data processing, thereby supporting ongoing discoveries in astrophysical phenomena. The paper suggests potential avenues for further advancements, including the pursuit of frequency-dependent squeezing for quantum noise reduction and improved mechanical control for minimizing nonlinear noise couplings. The authors advocate ongoing collaboration with international partners to escalate detector performance ahead of upcoming observation periods.

Conclusion

The insights presented in this paper underscore an evolutionary leap in gravitational wave detection capabilities. Strategic enhancements to the Advanced LIGO detectors have not only increased sensitivity and duty factor but also broadened our cosmological understanding through the discovery of numerous gravitational-wave signals. Nevertheless, tackling remaining noise interference while embracing future upgrades promises even greater detection prowess in subsequent observing runs, fostering a deeper comprehension of the universe's structure and dynamics.