Towards a darker dark port
At the output (dark port) of the GEO600 gravitational-wave detector we have a small amount of light that originates in imperfections of the mirrors. These are, e.g., small differences in the horizontal and vertical radii of curvature of these mirrors. This light can disturb the precise measurement of the tiny expected length fluctuations caused by gravitational waves.
In order to reduce the amount of disturbing additional light at the interferometer output, we have installed a compensation system. It can match the radii of curvature of a mirror in horizontal and vertical direction. The system employs heating devices which heat a mirror from the sides. As a result we can reduce the power at the dark port and even out its distribution as shown in the image.