Construction of interferometric gravitational wave antennas has led to paying attention to ultralow-loss mirrors. The loss of these mirrors reaches a level of ppm (at 1.06 μm) nowadays. The development of these kind of mirrors is based on the technology for mirrors of Ring Laser Gyroscope (RLG). Such ultralow-loss was realized by the hypersmooth polishing technology with the accurate measurement of surface roughness and by the advanced ion beam sputtering deposition technology. This paper reviews current progress in ultralow-loss mirrors through the development of RLGs, gravitational antennas and soft x-ray optics.
