A method to blot out scattered light effects and its application to a gravitational wave detector

For the interferometric detection of extremely small displacements one of the principal limiting factors can be scattered light, having a certain path difference against the main beam, in combination with the inevitable frequency fluctuations of the light source. This paper presents a method to suppress the effect originating from the co-operation of these two deficiencies. The idea is to blot out the scattered light effect by deliberately phase-modulating the light with appropriate frequency and phase swing. A theoretical treatment is given for the principles of this novel method. The paper discusses possible sources of scattered light in a gravitational wave detector and the application of the method to blot out the effect caused by the scattered light. It furthermore describes a method to measure the different contributions of scattered light and presents first experimental results for their suppression. Suggestions and calculations for further improvements are given.