Reduction of noise due to scattered light in gravitational wave antennae by modulating the phase of the laser light

In long-light-path laser interferometry, as used for gravitational wave detection, scattered light is a major noise source. Interferometers that employ optical delay lines are particularly susceptible to noise from scattered light. The authors present a method of suppressing such noise signals by using fast phase modulation of the laser light. It is shown that all scattered all contributions from the delay line may be cancelled, up to arbitrarily long path differences. This can be achieved with an appropriately structured square-wave modulation function which switches the phase of the light by pi .