Review of the Advanced LIGO gravitational wave observatories leading to observing run four

TL;DR

This paper reviews the advancements and upgrades in the Advanced LIGO gravitational wave observatories, focusing on improving sensitivity for the upcoming fourth observing run and summarizing recent detector performance and noise limits.

Contribution

It provides a comprehensive overview of LIGO's detector technology, noise sources, and the A+ upgrades aimed at enhancing gravitational wave detection capabilities.

Findings

Ninety confident detections by 2021

Details of the optical configuration and noise limits

Implementation status of A+ upgrades

Abstract

Gravitational waves from binary black hole and neutron star mergers are being regularly detected. As of 2021, ninety confident gravitational wave detections have been made by the LIGO and Virgo detectors. Work is ongoing to further increase the sensitivity of the detectors for the fourth observing run, including installing some of the A+ upgrades designed to lower the fundamental noise that limits the sensitivity to gravitational waves. In this review, we will overview how the LIGO detectors work, including their optical configuration and lock acquisition procedure, discuss the detectors' fundamental and technical noise limits and compare to the current measured sensitivity, and review the A+ upgrades currently being installed in the detectors.