Scattering and mechanical loss of ultra-low loss laser coatings

Ultralow-loss laser coatings have important applications in precision measurement fields such as gravitational wave detection，optical atomic clocks，and optical cavity ring-down spectroscopy. The cavity length stability and total optical loss of the laser resonator determine the sensitivity and signal-to-noise ratio of the measurement system. With the development of thin-film materials，fabrication processes and detec⁃ tion techniques，significant progress has been achieved in thin-film optical loss and thermal noise research. As regards optical loss，the absorption of the thin film can be controlled at the sub-ppm level，and the scat⁃ tering of the thin film has become the main contributing factor of optical loss. This paper focuses on defect-induced scattering and interface scattering and presents the research ideas and achievements of thin-film scattering control. The interface scattering of thin films is reduced through optical factor design and inter⁃ face power spectral density control. The theoretical analysis model of nodule defect-induced scattering is es⁃ tablished，and the physical mechanism and the control technology of the defect-induced scattering are pro⁃ posed. With regard to thermal noise research，this paper primarily introduces the physical mechanism of thin-film mechanical loss，presents the mechanical loss reduction of reflective coating through thin-film ma⁃ terial optimization，and the corresponding improvement in the characterization method of mechanical loss. © 2022 Chinese Academy of Sciences. All rights reserved.