Laser frequency stabilisation for LISA: Experimental progress Original Research Article

In all space-borne laser interferometric gravitational wave detectors, inevitable differences in the lengths of the interferometer arms will potentially allow laser frequency noise to give rise to apparent gravitational wave signals. Active frequency stabilisation to a reference optical cavity and use of a frequency noise correction algorithm are both required to allow the target gravitational wave sensitivity to be achieved. Here we report on an experiment to frequency stabilise a laser over timescales of several hours, and to measure the excess noise using an independent analysing cavity. The residual frequency noise, at the spectral frequencies of interest to LISA, is shown to be significantly above the desired specification. However, in this relatively simple experiment it appears that the residual frequency noise can largely be accounted for by thermal drifts in the frequency discriminator cavity pathlength. Thermal stabilisation of the reference cavity can be expected to yeild a major improvement in absolute laser stability.