Characterization of dielectric mirrors

The performance of the mirrors used in the LIGO interferometric system is limited by both thermal noise and light scattering that are a result of the manufacturing process. The study of these scatterers is fundamental in the development of better mirrors. We attempt to measure the distribution of light scatterers within the layers of coating witness samples. Mapping them both depth-wise and laterally using microscopy and fitting procedures. Last year we demonstrated the feasibility of identifying the depth of small features with a few nm precision. Building on that experience we now try to identify the size and location of a scatterers inside real coatings, using CCD microscope camera, 3-axis stepper motor stage and advanced cleaning techniques to eliminate spurious signals by traces of dirt. Currently another feasibility study is being conducted to see if it is possible to determine weather scatters are within 3 microns of each other on a lower magnification microscope. If indeed they are its possible to determine if scatterers reside in the roughly 3 micron thick deposition of a LIGO test mass or witness sample. If successful this may become a standard inspection technique to monitor and guide the development of lower scattering and thermal noise.