Research on the Thermal Correction System and Thermal Coating Noise

Advanced LIGO is a gigantic laser interferometer that is likely be the first instrument to detect gravitational waves. Opening this completely new window to the universe is one of the most anticipated scientific events in the near future. But Advanced LIGO will only be successful if it is possible to compensate and correct all static and dynamic radii of curvature errors in all optical components inside the instrument. These curvature errors would otherwise reduce the contrast inside the interferometer, increase the losses and the noise, and limit the reach of Advanced LIGO. These curvature errors will be caused by manufacturers who have limited control over the surface figure during polishing and coating of the optics and by thermal gradients inside the optics which will be caused by residual laser beam absorption. The thermal compensation system (TCS) is responsible for measuring and correcting these errors. The TCS uses ring heaters around each mirror to change their radii of curvatures and a laser projection system to write a specific heating pattern into the optic to compensate the thermal lens inside the substrate. This award supports a small testbed to develop and test the TCS, analyze the requirements to be imposed on the TCS, and train students on this important subsystem. Advanced LIGO will be limited by the Brownian motion of the atoms which form the dielectric coatings of the mirrors inside the interferometer. Although the theory of coating thermal noise is well established, the magnitude of such noise depends on several less well understood material parameters. Direct measurements of the noise are rare, hardly systematic, and have never been done across the Advanced LIGO frequency band. This award will also support an experiment dedicated to measure thermal coating noise for different coatings, substrates, and different beam sizes to study thermal coating noise in great detail and across the entire Advanced LIGO frequency band.

The importance of the detection of gravitational waves for astrophysics and astronomy can hardly be overestimated. These waves are caused by moving masses which change the curvature of spacetime. This change propagates through space in the form of gravitational waves. These waves are a direct consequence of the speed limit in the universe and their exact form is described by Einstein's theory of general relativity. Opening this completely new window to the universe will allow study of the physics behind many interesting astronomical objects such as neutron star or black hole binaries or supernovae. However, the experimental challenges to reach the expected sensitivity present an ideal training ground for graduate students and young scientists. These students will gain valuable skills by working on a small but challenging laboratory experiment within the context of Advanced LIGO.