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

Understand

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.