Simulating tunable coherence for scattered light suppression in laser-interferometric gravitational wave detectors

Abstract

Ground-based gravitational wave detectors use laser interferometry to realize ultra-precise displacement measurements between free-floating test masses to detect signals in the frequency band of 10 Hz to 10 kHz. Light scattered out of and back into the main beam path creates noise in these detectors that non-linearly up-converts low-frequency, out-of-band signals into the measurement band. This scattered light induced noise becomes more relevant as the sensitivity at lower frequencies is improved in current and future detectors. To suppress this noise source, we study how the strong spatial coherence of the laser light can be reduced to a few centimeters by introducing high-speed pseudo-random noise phase modulations into the Michelson interferometer topology. We simulate the interferometer signals in the presence of scattered light in the time domain. Our simulations show that tuning of the coherence can reduce the coupling of scattered light by orders of magnitude, and that the phase modulations are, in principle, compatible with resonant cavities that are employed in the complex interferometer topologies of modern detectors to achieve ultra-low quantum noise levels. We outline the currently expected limitations of this approach and discuss the applicability to detectors.