Adaptive thermal compensation of test masses in advanced LIGO

As the first generation of laser interferometric gravitational wave detectors nears operation, research and development has begun on increasing the sensitivity of the instrument while utilizing the existing infrastructure. In the laser interferometer gravitational wave observatory (LIGO), significant improvements are being planned for installation around 2007, increasing strain sensitivity through improved suspensions and test mass substrates, active seismic isolation and higher input laser power. Even with the highest quality optics available today, however, finite absorption of laser power within transmissive optics, coupled with the tremendous amount of optical power circulating in various parts of the interferometer, results in critical wavefront deformations which would cripple the performance of the instrument. A method of active wavefront correction via direct thermal actuation on optical elements of the interferometer is discussed. A simple nichrome heating element suspended off the face of an affected optic will, through radiative heating, remove the gross axisymmetric part of the original thermal distortion. A scanning heating laser will then be used to remove any remaining nonaxisymmetric wavefront distortion, generated by inhomogeneities in absorption of the substrate, thermal conductivity, etc. A proof-of-principle experiment has been constructed at MIT, selected data of which are presented.