Low phase noise squeezed vacuum for future generation gravitational wave detectors

Abstract

Squeezed light has become a standard technique to enhance the sensitivity of gravitational wave detectors. Both optical losses and phase noise in the squeezed path can degrade the achievable improvements. Phase noise can be mitigated by having a high bandwidth servo to stabilize the squeezer phase to the light from the interferometer. In advanced LIGO, this control loop bandwidth is limited by the 4 km arm cavity free spectral range to about ~15 kHz. Future generation gravitational-wave detectors are designed to employ much longer arm cavities. For cosmic explorer, a 40 km arm length will limit the bandwidth to ~1.5 kHz. We propose an alternative controls scheme that will increase the overall phase noise suppression by using the in-vacuum filter cavity as a reference for stabilizing the laser frequency of the squeezed light source. This will allow for rms phase noise of less than a milliradian - a negligible level for all future generations of gravitational-wave detectors.