Dynamic photothermal effects in different optical cavities

Ultra-stable lasers with excellent long-term stability are increasingly demanded in fields like gravitational wave detection and optical frequency standards. The dynamic photothermal effect, arising from local temperature fluctuations in cavity mirrors, is a significant limiting factor. We conduct precise measurements of the dynamic photothermal responses in various Fabry-Pérot (FP) cavities, and systematically investigate both the contributions from different materials and the characteristics of spacer photothermal noise. For ultra low expansion glass (ULE) cavities with fused silica (FS) mirror substrate and all-ULE cavities, thermoelastic noises from the substrate and coating contribute the most, respectively. In cryogenic sapphire cavities, spacer thermoelastic noise dominates and causes a more significant low frequency dynamic photothermal influence. A modulation experiment is conducted to verity the measured response coefficients. This work provides a general guidance for understanding the dynamic photothermal effect and aids in designing cavities with optimized sensitivity, advancing ultra-stable lasers with superior long-term frequency stability.