"It added an additional three-kilometer-long ring between the main interferometer and the detector to circulate the light before it exited the interferometer’s

"It added an additional three-kilometer-long ring between the main interferometer and the detector to circulate the light before it exited the interferometer’s arms." Isn't that a delay line? The benefit being that when the undelayed and delayed signals are mixed, the phase shift you're looking for is amplified.